1976 , volume , issue april-1976 - about hp labs | hp ... frequencies are approximately 75 khz,...

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© Copr. 1949-1998 Hewlett-Packard Co.

Electronic Total Station Speeds Survey Operations This new e lect ron ic survey ing ins t rument measures s lope d is tance and zen i th ang le s imul taneous ly , then computes and displays horizontal or vertical distance in feet or metres. I ts base measures hor izonta l angle.

by Michael L . Bul lock and Richard E. Warren

IN 1970, WITH THE INTRODUCTION of the 3800A Distance Meter, Hewlett-Packard launched

into the electronic measurement of distance. Since that time HP has become a major supplier of electron ic distance measuring equipment (EDM). Typically, EDM has been used in conjunction with some angle measuring device such as a transit, because the posi tion of one point with respect to another is usually de scribed by a horizontal distance, a vertical distance, and a horizontal angle from a known bearing or line. Now both distance and angle measuring capability are available in a new instrument â€” the 3810A Total Station (Fig. 1).

In the past, horizontal distance was measured by taping the distance, being very careful to maintain the tape in a horizontal plane, or by measuring the slope distance (typically with EDM equipment), measuring the zenith angle (using a theodolite or transit), and then computing the horizontal distance on a calcula tor. The 3810A Total Station is capable of measuring slope distance and zenith angle simultaneously and then calculating the horizontal distance or vertical distance for immediate display. This horizontal dis tance capability, combined with a base that measures horizontal angle, makes the total station a powerful instrument for subdivision layout, as well as for many other surveying applications (see page 6).

A glance at the total station's control panel (Fig. 2) shows not only its simplicity of operation but also the variety of measurement options selectable by the operator. The total station measures and displays either vertical, horizontal, or slope distance, or zenith angle. It gives a single reading to the full resolution of the instrument or continuously updated readings with reduced resolution. Readings are in feet or metres for distances and degrees/minutes/seconds or grads for angle. The operator simply preselects the desired function and units, aims the instrument at the target

point using the integral telescope and aiming tangent screws, and presses the MEASURE button. The instru ment then automatically controls its own measure ment cycle to produce the desired readout. All para meters other than the selected one are also measured or computed and are available for immediate recall. The continuously updated reading is especially use ful for such things as laying out a certain distance.

The total station measures distances up to 1.6 km (1 mile) with resolution as fine as 1 mm. Measurements are accurate within 5 mm + 10 ppm.

C o v e r : W i t h i t s b u i l t - i n calculator, angle transducer, a n d m o d u l a t e d - l i g h t - b e a m distance measur ing system, th i s one ins t rumen t , Mode l 3810A To ta l S ta t i on , g i ves t h e o p e r a t o r a d i r e c t r e a d o u t o f d i s t a n c e t o a p o l e - mounted re t rore f lec tor he ld

b y h i s r o d m a n . T h e d i s p l a y e d h o r i z o n t a l d i s t ance , co r rec ted fo r re f rac t i on and the ea r th ' s cu rva tu re , i s au tomat i ca l l y de r i ved f rom s lope d is tance and ver t ica l angle.

In this Issue: Elec t ron i c To ta l S ta t i on Speeds Su r vey Operat ions, by Michael L. Bul lock and Richard E. Warren page 2

D e s i g n i n g E f f i c i e n c y i n t o a D i g i t a l Processor for an Analytical Instrument, by John S. Poo le and Len B i len . page 13


Fig. 1 . distances 38 1 0A Total Station measures horizontal, vertical, and slope distances and vertical angle, snap-in automatical ly. I ts cal ibrated base measures horizontal angle. A snap-in battery pod

el iminates cables.

Distance Measuring Technology The total station measures distance using ampli

tude modulation of a light beam (910 nm wavelength) from a GaAs diode. The wavelength of the modula tion envelope (A. m) is chosen to be consistent with the requirements of the measurement.

The modulated light is transmitted through a trans mitter optics assembly and downrange to the end of

Resolution Switch 0.000 Measure (One Reading) 0.0 Track (Updating)

Sighting Readout (Digital)

Off/On/Self Check Switch

Mode Switch (Vertical, Horizontal and

Slope Distance; Zenith Angle)

Degree- Min-Sec/

Grad Switch

Environmental Correction

Input

F i g . 2 . 3 8 1 0 A c o n t r o l p a n e l . O p e r a t o r s i m p l y s e l e c t s t h e func t i on and un i t s , a ims the i ns t rumen t a t t he ta rge t po in t us ing the in tegra l te lescope, and presses the MEASURE but ton. He can also select a single reading with ful l resolut ion or a cont inuously updated readout wi th reduced resolut ion.

the line being measured, where a retroreflector sends the beam back to the instrument. A receiver optics as sembly focuses the beam on a photodiode detector/ mixer, which produces an electrical signal that has the characteristics of the received modulated light enve lope. Ideally this signal is identical to the modulation signal except for a displacement or phase shift pro portional to the measured line length.

The phase shift between the transmitted and re ceived signals is a consequence of the finite velocity of the signal envelope, which is essentially equal to the speed of light. Fig. 3 shows how this phase shift is proportional to the distance being measured. The light beam actually travels the measured distance twice, once going out and once coming back, and Fig. 3 shows the light path "unfolded" to illustrate more clearly the effect of distance on phase. A measured distance of V2Xm is equivalent to one complete modu lation wavelength, or 360Â° of phase shift. Typically, a phase measurement cannot distinguish between 0Â° and 360Â° of phase shift, thus leading to a repetitive phase-versus-distance characteristic, as shown in Fig. 4. In the total station, two different modulating fre quencies are used alternately, a lower frequency to determine the basic range, and a higher frequency for resolution. The frequencies are approximately 75 kHz, which provides a 2000-m measurement interval, and 15 MHz, which provides a 10-m interval.

In this idealized amplitude modulation system, the output signal of the detector is compared with the


Reflector

T r a n s m i t t e r I D e t e c t o r

90

180

270

01 a 3 6 0 / 0

9 0

V Â » V t % 1 / 2 5 / 8

Distance to Reflector (Am)

F i g . 3 . T h e t o t a l s t a t i o n m e a s u r e s d i s t a n c e b y m e a s u r i n g the phase shi f t between the modulat ion envelopes of two l ight beams. One is a reference l ight beam and the other is a l ight beam tha t has t raversed the unknown d is tance and been re f lected back to the inst rument .

signal driving the modulator to determine phase shift. In practice, modulators and detectors introduce phase shift. If this phase shift were constant, it could be taken into account in the measurement. However, it can vary considerably with time and temperature and can therefore introduce measurement errors.

Our solution to this problem is to generate a refer ence signal that has been exposed to the same variable phase shifts (except that proportional to the distance being measured) as the transmitted-received signal. This is accomplished by alternately directing the output of the amplitude modulator to the transmitter optics and through an internal reference path to the detector. This guarantees that any phase shift intro duced by the modulator and detector is present in both the external signal and the internal reference, so any differential phase shift between these two signals is proportional to the distance being measured.

Distance Measur ing Circui ts A block diagram of the distance measurement por

tion of the 3810A Total Station is shown in Fig. 5. The basic resolution and accuracy are determined by the accuracy and stability of the 15-MHz oscillator in the transmitter, and these are determined by the tempera ture and time stability of the oscillator crystal. Typi cal long-term stability of the crystal is 2-3 ppm/year. The temperature coefficient of the oscillator is <Â±10 ppm over the entire environmental range of the in strument {-20Â°C to +55Â°C).

The transmitter provides the drive signal to the emitting diode. It divides the 15-MHz signal digitally to provide the second modulation frequency of 75

kHz and a 3.75-kHz square wave electrical reference signal. The transmitter also provides 15-MHz and 75-kHz signals to the receiver.

The transmitter diode produces a modulated light beam under the control of the transmitter drive sig nal. The chopper, a blade rotating at a 10-Hz rate, al ternately routes the diode output either through the transmitter optics (external path] or through the variable optical attenuator (internal path). The block labeled "optics" sends the light beam toward the target reflector and focuses the returned and refer ence beams on the detector diode.

The receiver and phase-lock circuit provides the local oscillator drive to the photodiode detector. The local oscillator drive is always 3.75 kHz above the modulation frequency currently being transmitted. The receiver has automatic gain control to maintain a constant output level regardless of input level. It also filters the detector output to eliminate all but the 3. 75-kHz component.

The limiter takes the 3. 75-kHz sine wave (IF) and produces a square wave output (IFL). The limiter is also an important part of the automatic balance fea ture of the 381 OA. Under control of the micro processor, the limiter detects the difference between the internal path signal amplitude and the external path signal amplitude, and adjusts this difference to zero by adjusting the variable attenuator in the inter nal path. When the two paths are balanced this fact is communicated back to the microprocessor via the flag line. Control lines from the phase detector tell the limiter when a reading is being taken and enable the beam break circuit on the limiter. The beam break cir cuit detects whether the IF signal is below or above preset levels. Data collected under such conditions is questionable, so the microprocessor eliminates it from the measurement process.

The phase detector and accumulator circuit makes the actual distance measurement by measuring the phase difference between the IFL signal and the 3. 75-kHz reference. The internal path is measured

% 1 1 V 4

Measured Distance (Xm)

Fig. 4. Phase shif t is a l inear function of distance within one- ha l f wave leng th o f t he l i gh t -beam modu la t i on s i gna l . Two modulation frequencies are used: 75 kHz for the basic 2000-m measurement interval and 15 MHz for a 1 0-m interval and bet ter resolution.


75 kHz or 15 MHz Modulat ion Frequency J - L T U T -

Retroreflector

F i g . 5 . T o t a l s t a t i o n d i s t a n c e m e a s u r i n g c i r c u i t s . L i g h t a t a wavelength of 910 nm is produced b y a G a A s d i o d e . T h e a c c u m u l a t o r c o u n t s 1 5 - M H z p u l s e s f o r 1 0 0 c y c l e s o f t h e 3 . 7 5 - k H z r e f - ence, f irst for the internal path and then for the externa l path ; i t then holds the average over 100 cycles o f t he phase d i f f e rence be tween internal and external paths. This is t ransfer red to the microprocessor for analys is and d isp lay.

first. The phase difference between IFL and the 3.75-kHz reference is determined by opening a gate on the leading edge of the 3.75-kHz reference signal and closing it on the leading edge of the IFL signal. While the gate is open, 15-MHz pulses enter the ac cumulator and count it upward. This is repeated for 100 cycles of the 3.75-kHz reference. At the end of 100 cycles the accumulator holds a number that repre sents the average phase difference between the 3.75-kHz reference and the IFL signal (internal path) over those 100 cycles. Next, the external path signal is selected, and a similar measurement is done, except that the accumulator is counted down. The accumu lator then holds the average over 100 cycles of the phase difference between internal and external paths. Measured data is then transferred to the micro processor for analysis and display.

Angle Measuring Technology Fig. 6 is a block diagram of the angle measuring

system of the 3810A Total Station. The system is di vided into two sections, analog and digital. The analog section does the actual angle measurement, while the digital section controls the timing of the measurement and interfaces the angle system to the rest of the instrument.

The operation of the analog section is serial in na ture. The 2.5-kHz oscillator is the source of the drive signal that is used to make the angle measurement.

The active attenuator divides this signal by a factor of almost 20, and the transducer driver provides an in verted version of this signal to the angle transducer.

Because of the precision needed in the angle mea surement, the effects of electronic noise must be kept to a very low level. This is done by starting out with an oscillator that has a good output signal-to-noise ratio. Noise components in the output of the trans ducer driver are more than one million times smaller than the desired signal.

The transducer is the heart of the angle system. De tails of its construction are discussed in the box on page 10. Electrically, the transducer can be repre sented by the circuit shown in Fig. 7. Two of the "re sistors", RL and RR, are functions of tilt angle 0 and another parameter * , while RA is a function only of * . The parameter * includes the effects of geometry, physical properties, temperature, and other para meters. The transducer sense amplifier senses tilt angle as shown in Fig. 8. Notice that the effects of the non-angle-related parameters are cancelled, leaving only a function of tilt angle 9 and some constant fac tors that are calibrated out.

In the conversion from ac to dc, differences can creep into the various signal paths and cause errors. For this reason the next three blocks in Fig. 6 are tied together in an automatic zeroing loop. The signal select block, under control of the digital section timing sig nals, connects the rectifier block to one of three pos-


How the Total Station Is Used The pr imary apl icat ion for the 381 OA Total Stat ion is layout.

A typ ica l layout appl icat ion might be a subdiv is ion of 40 acres div ided into 160 indiv idual lots. The drawing of th is subdiv is ion (Fig. and has al l the points located by hor izontal d istances and horizontal angles from other points. With a 381 OA and the draw ing, the points can easi ly be set out wi th a lmost no f ie ld ca lcu la t ions or i terat ions, s ince the tota l s tat ion d isp lays hor izonta l d is tance d i rect ly . I f the crew has t ransposed these rectangular coord ina tes in to po la r coord ina tes abou t con t ro l po in ts , then e v e n f a s t e r l a y o u t c a n b e a c c o m p l i s h e d b e c a u s e t h e i n s t r u ment can be set up at only a few locat ions and a l l o ther points la id ou t f rom there . In lay ing ou t a po in t , the cont inuous ly up da ted read ing becomes very use fu l , he lp ing the opera to r te l l the rod man when he is c lose to the des i red spot . Then the in strument is swi tched to make an accurate reading and the f inal few cent imet res can be se t w i th a pocket tape.

r

Fig. and Typical subdiv is ion layout . Hor izonta l d is tances and ang les are impor tant measurements .

In laying out along a l ine, there is st i l l the problem of gett ing the rod man on l ine. To speed up this step, an accessory cal led a l ine an can be mounted on top of the tota l s tat ion. This is an optical device that tel ls the rod man whether he is to the r ight or the left of the l ine by showing him different colors when he looks back at the inst rument . Wi th a tota l s tat ion and the techniques

described here, t ime savings of 30-60% over other methods are easi ly obtained in layout s i tuat ions.

Another type of appl icat ion that the total stat ion does wel l is deta i l and locat ion survey. S ince the ins t rument has both d is tance and angle capabil i ty, a survey for topographic information can be done wi th th is one ins t rument (F ig . 2 ) . By se t t ing the target on the range pole at the same height as the instrument, d i f fe rences in e leva t ion can be read d i rec t l y . The hor izon ta l locat ion of each point can also be establ ished using horizontal d is tance and hor izonta l angle.

Topographic Survey

Fig. 2 . Topographic survey is another app l ica t ion for which the 381 OA Total Station is well suited.

An app l i ca t i on apa r t f r om land su rvey ing i s i n t he hyd ro - g raph ic area, fo r example p lo t t ing a pro f i le o f a r iver o r lake bottom. The total stat ion can quickly determine the posi t ion of a boat tha t i s equ ipped wi th sound ing equ ipment to measure depth . The resu l t ing da ta can be cor re la ted la te r to ob ta in a prof i le . Again, the updated reading is especia l ly usefu l in th is appl icat ion.

The versat i l i t y o f the to ta l s ta t ion shou ld a lso he lp i t meet the requirements of many other appl icat ions, such as bui ld ing foundat ion layout , u t i l i t y and/or p ipe l ine layout , t raverse sur veys, volume est imat ing, and general h ighway layout .

sible voltages. The rectifier provides full-wave rec tification of its input signal in synchronism with the 2.5-kHz VRef signal. The active low-pass filter converts the full- wave rectified signal to a dc level for use in the A-to-D converter. During the autozero por tion of an angle measurement the signal select block grounds the input to the rectifier and causes the re sulting system offset voltages to be stored on a capaci tor so their effect can be subtracted from the integrator input voltage.

During a measurement the output of the active low- pass filter is a dc level alternately proportional to the reference voltage or to the unknown angle-dependent voltage. The dual-slope integrator operates on these two voltage levels. It "ramps up" on the unknown

voltage for a fixed time, then "ramps down" on the known reference voltage until the ramp crosses through zero. The ramp-down time is proportional to the unknown angle. This time is measured in the digi tal section by counting pulses while ramp-down is in progress.

The digital section provides all the timing signals for angle measurement and accumulates the actual angle measurement. The digital section is controlled by an ASM (algorithmic state machine). The qualifier multiplexer allows various signals to affect the value of the state counter only when they are intended to do so. The state counter keeps track of which phase of the measurement is currently taking place. The state counter decoding logic generates the control signals


Active Attenuator

^ _ A n g l e T r a n s d u c e r ! ! T r a n s d u c e r

D r i v e r E d

Transducer Sense

Amplif ier

Fig. analog the station zenith-angle measuring circuits. The analog section excites the angle trans ducer and conver ts i ts output to a dc level that is measured by the dual -s lope in tegrat ion tech

n ique. The d ig i ta l sect ion prov ides t iming and cont ro l .

needed to operate the analog section and to control the six-decade counter. The six-decade counter gener ates the timing for auto-zero and ramp-up operations and serves as the reading accumulator during ramp- down. The zero-detect logic senses when the ramp crosses through zero, thus terminating the accumula tion of ramp-down counts. The scale factor and offset logic generates signals under processor control to

RL=f<e,*)

RR=f (e , *

R A = f ( * )

Fig. 7. Angle t ransducer equivalent c i rcui t . Q is the unknown angle and * is a parameter that inc ludes the ef fects of geom etry, physical propert ies, temperature, and other factors.

allow readout of the PROM that calibrates the angle system. The angle data multiplexer routes the various data sources to the processor for combination and dis play. The angle system controls itself during a read ing cycle, thereby freeing the microprocessor to con trol other instrument functions while an angle read ing is being taken.

Processor Technology The 3810A Total Station uses the same micro

processor as the HP-35 hand-held calculator.1 The microprocessor provides not only calculating ability but also complicated control functions that would be practically impossible by any other technique. Fig. 9 shows the processor system and its interfaces to the rest of the instrument.

The microprocessor determines when an angle measurement is called for and, at the proper time, sig nals the angle logic sequencer to proceed. The micro processor continues with its calculations until the angle flag signal goes high, at which time four data


RL=f(e,*)

F ig . 8 . The t ransduce r sense amp l i f i e r cance l s t he e f f ec t s of non-angle-related parameters, leaving only a funct ion of t i l t ang le Q and some constant fac tors tha t a re ca l ib ra ted out .

strobe lines are properly sequenced to read out the data from the angle system via the data bus. The microprocessor corrects for angle scale factor and offset as follows:

Corrected angle = (raw angle X scale factor) + offset

All of the angle system readings are in decimal de grees referenced to level. The microprocessor con verts the units to either degrees-minutes-seconds or to grads and changes the reference from level to zenith.

The distance measuring system interface exempli fies the deeper levels of control the microprocessor

exerts upon the system. Fig. 10 shows this interface in greater detail.

Most of the interface between the distance measur ing system and the microprocessor consists of the phase detector interface, since the phase detector is the last link in the distance measuring system. The control lines to the phase detector control block deter mine when a reading is initiated, select initial phase detector operating mode, and modify the operating mode to account for existing conditions as shown by the initial readings.

The phase detector flag signals the processor when a phase detector cycle is completed. A typical dis tance measurement consists of many such phase de tector cycles. At the end of each cycle the processor examines the good flag line to determine if a beam break condition existed any time during the last phase detector cycle. If a beam break did occur the processor ignores the data taken during that cycle and initiates a new phase detector cycle.

In addition to controlling the phase detector to measure distance, the processor can switch the phase detector input to the output of a one-shot delay cir cuit. The delay is controlled by the position of a vari able resistor located on the front panel of the instru ment. The processor requests that the phase detector measure the phase corresponding to this delay, and displays the phase as a number between +110 and -40. This reading is taken repetitively; it allows the operator to dial in a parts-per-million correction factor to be applied to the distance measurement. The correction factor, a function of air temperature and pressure, is necessary to compensate for changes in the velocity of light caused by changes in the index of refraction of air.

Display Drivers

Angle System Interface

Distance Measuring System Interface

F ig . 9 . The m i c rop rocesso r p ro v i des soph i s t i ca ted con t ro l and computa t iona l capab i l i t i es . Th is s i m p l i f i e d d i a g r a m s h o w s t h e microprocessor and i ts interfaces to the angle and distance circuits.


Distance Measuring System Processor

BCD Line (Data to

Microprocessor)

F ig . 10 . A more de ta i l ed l ook a t t he i n te r face be tween the microprocessor and the d istance measur ing c i rcui ts . In a d is t ance measu remen t t he p rocesso r hand les a l l con t ro l and computation functions, including statist ical analysis of the raw data to detect marginal measur ing condi t ions.

The lines labeled "four data strobes" in Fig. 10 con trol the data multiplexer, which in turn routes data from several sources onto the data bus for entry into the microprocessor. Data basically comes from three sources: phase measured data comes from the ac cumulator, distance offset information is read from a programmable PROM structure, and front-panel status information is read from the front-panel controls.

The microprocessor also performs other control functions in the distance measuring system. The operation of the automatic balance circuit is con trolled by two lines. The time during which the re ceiver AGC is allowed to track deviations in received signal amplitude is controlled by the AGC sample/ hold line. The selection of which modulation fre quency, either 15 MHz or 75 kHz, is being transmitted is controlled by the frequency select line. The test line indicates to the system that the operator has request ed a self-test.

The processor performs many calculations in the course of each reading. Statistical analysis of the raw distance data is performed to determine if the var iance, cr2, lies within predetermined limits based

upon the instrument specification. If the variance ex ceeds these limits more data is taken until the var iance is within limits or a maximum number of data samples has been taken. If this maximum number has been reached and the variance still exceeds the limits the average of these data samples is displayed in a flashing manner to warn the operator that the measur ing conditions were marginal.

The processor also applies the dialed-in ppm cor rection factor, performs offset calibration, corrects the raw angle data for scale factor and offset, and cal culates horizontal and vertical distances corrected for refraction and earth curvature. The equations used in this last calculation are:

H . D . = S . D . C o s 9 - [ l - S - D - g n 9 ' ] 2RF

V.D. = S.D. Sin 0'

where H.D. is horizontal distance displayed, V.D. is vertical distance displayed, S.D. is slope distance measured, RE is the radius of the earth, and 6'is cor rected vertical angle:

6' = 9 + 13.9 arc-seconds per 1000 metres of slope distance.

Finally, the processor does unit conversions to pro vide distances in metres or feet and angles in degrees- minutes-seconds, grads, or percent grade.

Front-Panel Sel f -Test When the operator selects the -888 position on the

front panel the processor lights all segments of the display to show that all are working. This results in a -8888.888 display. When the operator presses the MEASURE button with the switch in the - 888 position the processor commands 20 different internal tests. If all tests are completed successfully, â€”8888.888 is displayed to indicate that the instrument is electroni cally sound with a high degree of confidence. If any test fails the instrument flashes "0" to indicate a prob lem. Self-test is very valuable for checking out an in strument before it is carried out into the field for use, an operation that may involve a crew of two or three persons traveling considerable distance. The test is not a 100% test, so it does not give 100% confidence, but it does test all functions that can be tested internally.

There are also thirteen additional test modes that are accessible only at the factory or service center. These tests help the service technician to determine the source of a problem and to verify proper operation when a repair has been made. For example, the front- panel self-test can also be accessed from a test key-


Angle Transducer I n p r inc ip le , the ang le t ransducer i s a ve ry s imp le dev ice .

Basical ly, i t is a resist ive component wi th electrodes separated by an e lect ro ly te (see Fig. 1) . The e lect rode areas covered by electrolyte represent the angle to which the unit Â¡s t i l ted. I f the r ight and lef t e lectrodes are covered equal ly , then the uni t has zero output . As the un i t Â¡s t i l ted , one s ide gets covered less whi le the other is covered more.

Side View of Toroidal Angle Detector

RL, R

Common

Area of Submersed E lect rodes = f (O)

Fig. 1. 3870/4 angle t ransducer is a resist ive component wi th electrodes covered by an electrolyte. Relat ive coverage of the two e lect rodes is a measure of t i l t angle.

Al though the concep t i s s imp le , t he re a re many sub t l e t i es tha t requ i re cont ro l in the fabr ica t ion o f the par ts and in the i r assembly to obtain an accurate device. The materials used have to be very stable with t ime and temperature. Machining and mask i ng to le rances a re mos t c r i t i ca l . The assemb ly mus t be com p le te l y sea led as we l l as vo id o f any impur i t i es . F ina l l y , t he assemb ly mus t be packaged in a manner tha t m in im izes tem pe ra tu re g rad ien t s ac ross t he l i qu id . I n teg ra ted c i r cu i t t ech nology is employed in manufactur ing the electrodes, and a very stable ceramic mater ia l is used as the substrate. The parts are

fritted together and a glass tube is welded shut to finish the seal ing af ter the device Â¡s f i l led. Two sets of thermal shie lds are added and the result ing device Â¡s a very rel iable and accurate gravity sensing angle transducer. Gravity sensing Â¡s desirable because it means that only one sensor is needed to measure the

20 T

10

- 2 0

Angle (degrees) 1 0 2 0 3 0

â € ¢ I 1 H

Fig. 2. Typical l inear i ty error of ver t ica l angle t ransducer

2 3 4 5 A n g l e ( d e g r e e s )

- 1 0 - L

Fig. 3. Typical l inear i ty error for smal l angles

5 r

f O

- 5 J -

- 4 - 3 - 2 - 1 2 3 4 5

A n g l e ( m i n u t e s )

Fig. 4 . Typica l micro l inear i ty o f angle t ransducer

vertical angle and reference it to gravity. The sensor is sensitive only to the di rect ion of gravi ty, not to i ts magni tude.

The accuracy of the angle transducer is shown in Figs. 2, 3, and 4. Fig. 2 shows a typical l inearity error plot over the entire work ing range of Â±30Â°. Figs. 3 and 4 show the same type of errors a t reduced ranges.

board. The identical tests are performed, but when a failure occurs the display shows a flashing code from 1 to 20 instead of a flashing "0". The service techni cian can then consult a table that describes the test that is failing, and the type of failure can be found using other available tests. The service tests also allow display of all raw distance and angle param eters in all combinations of operating mode, allow direct readout of calibration constants, test all processor controls by operating them in continuous

"signal generator" modes, provide known test condi tions for checking auto-balance operations, and pro vide dynamic testing of flag return signals on a con tinuous basis. These added test modes mean easier, faster, and less costly production and repair.

Optical System The infrared lens system of the 3810A Total Station

is one of the key elements in obtaining the range and accuracy of the distance meter portion of the

10


1 Transmitter

Fiber Optics Reference

Path

Diode Transmitter

i L e n s

Mirror Reflector

External Unknown Path

F i g . 1 1 . T o t a l s t a t i o n o p t i c a l system produces a beam angle o f 3 arc-minutes . A l ignment o f t rans mi t ter and receiver d iodes is care f u l l y d o n e t o a s s u r e a c c u r a c y o f Â±(5 mm + 10 ppm).

instrument. A schematic view of the infrared op tical system is shown in Fig. 11. The aperture of the lens system was designed to obtain the desired range. The focal lengths of the lenses and the sizes of the transmitter and receiver diodes are optimized to obtain the desired beam angle of 3 arc-minutes. Since only one wavelength is used (910 nm), color correction is not necessary. Spherical aberrations are corrected by using plano-convex doublet lenses.

Proper diode alignment and stability of this align ment are essential for obtaining and maintaining range and accuracy. Alignment is done by viewing the diodes with the same wavelength of light that is

F ig . 12 . Ho r i zon ta l base has a l eas t coun t o f 20 seconds w i th es t imat ion to f i ve seconds o f a rc . The read ing here i s 269Â° 29 '50".

transmitted and aligning the two diodes optically to the reticle of the telescope within 12.5 /u.m. Stability is assured by lens, mirror, and diode mounting tech niques. Thermal matching and/or thermal compensa tion by selection of materials is used in as many loca tions as possible. The surfaces that mount the optical elements are manufactured to precise tolerances so as not to distort the optical surfaces.

Horizonta l Base and Telescope The horizontal angle base of the 3810A Total Sta

tion is designed according to conventional practices used in modern theodolites. Horizontal angle is mea sured by accurately scribed lines on a glass circle. The operator uses a microscope and optical micro meter to observe the lines and interpolate between them while the circle is rotated about an accurate bearing/shaft arrangement.

The horizontal base, custom built for the total sta tion, has a least count of 20 seconds with estimation to 5 seconds of arc (Fig. 12). This means that each minor division accounts for 20 seconds of arc but be tween these divisions interpolation to 5 seconds of arc is possible. The bearing/shaft design is cylindrical. Maxi mum radial clearance is approximately 0.6/u.m. Support is provided by ball bearing thrust members. Upper and lower halves of the base both move, allowing the operator to obtain more resolution by measuring the angle a number of times and dividing the sum of the readings by the number of readings.

To measure angle accurately one must be able to sight the object in question. The total station em ploys an 18 x erect-image telescope for acquiring tar gets. Much design effort went into making this tele scope stable and accurate, because the ability to mea sure angle is only as good as the sighting telescope. To assure stability, each assembly is thermally cycled ten times between -40Â° and +160Â° F. Accuracy is ob tained by close control of the machining and as-

11


RANGE: On* Ã±ute (1 6 dm) with a tr iple pnsm assembly under average condit ions Average condit ions are those tourtd during the day when moderate heat shimmer is evident

ACCURACY: S lope D is lance Â±( 016ft + 01 H per 1000 ft) m.s.e.@ - 15CF 1o â€¢ 105CF r ( 5 m m * 1 0 m m p e r k m ) m s e . @ 1 C T C t o + 4 ( T C Â±( 030H - 03 H pe r 1000 f t ) mse @ -5Â°F to -15sFand * 105 " F t o *13 f fF Â±)lOmm + 30mm pÂ«f km) m.s.e @ -20Â°C lo -10=0 and + 4<rC to -55=C

ACCURACY: Zen i t h Ang le

S P E C I F I C A T I O N S H P M o d e l 3 8 1 0 A T o t a l S t a t i o n

PRECIS ION RATIOS FOR HORIZONTAL DISTANCE: The p rec i s ion ra l i o o f t he hor izontal d istance is dependent on the angular and slope distance accuracy of the HP vert ical Total Stat ion For any combinat ion of s lope distance and vert ical angle to the r ight of Curve I. the precision rat io of the horizontal distance wil l be

to the r ight of curves II I and IV the precision rat io wi l t be 1 25,000 o: i 50 000 or be l ie f respect ive ly

UNIT OF DISPLAY: 001 t t Or 1 sec or 10CC Zenith Angle

DISPLAY RATE: Track Mode 2 secreading - s lope d is tance

001 r

Temperature F)

1 0 5 F t o - 1 3 0 - F

S l o p * D i s t a n c e ( M e t r e s )

2 5 0 5 0 0 7 5 0 1 0 0 0 1 2 5 0 1 5 0 0

3 3 g < Â ¡ o - 2 2 g g

- 2 2 g to â€¢ 33 g

1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0

S l o p e D i s t a n c e ( F e a t )

m e t o - 4 0 X - 2 0 - c i o - i t r i c - 4 < X C t o - 5 5 C

Tempera ture ( C)

'; sec.read.ng â€¢ zen.lh angle U N I T O F d e g r e e s s e l e c t a b l e i n e i t h e r t e e t o r m e t r e s a n d e i t h e r d e g r e e s

T I L T R A N G E : - 3 0 " TELESCOPE: Inlernal locus erect image, 18 â€¢ P O W E R V d c o p t i o n a l r e c h a r g e a b l e b a t t e r y p o d o r e x t e r n a l 1 2 V d c DIMENSIONS: 13 - 103 â€¢ 58 (330mm â€¢ 262mm â€¢ 147mm) WEIGHT: Total Station w.o Battery. 26 2 Ir j ( 1 1 9 Kg)

Snap-m Bat le fy Pod. 23 Ib (1 0 kg)

H o r i z o n t a l A n g l e B a s e HORIZONTAL ANGLE CIRCLE: 2 95 inch (75mm) diameter glass circle graduated

to 1 degree Micrometer scale reads direct to 20 seconds or 5QCC with estimation to 5 seconds or 10

OPTICAL MICROMETER READING:Honzontal angle circ le readings are obtained

LEVEL VIAL: p late level v ia l sensi t iv i ty 30 seconds per 2mm

EQUIPMENT SUPPLIED: P in and hex ad justment wrenches BASE OPTIONS:

OPTION 01 1 Degree graduat ion wi t t i Wi ld Inter lace OPTION 021 Grad graduat ion wi th Wi ld In ter face

NOTE OPTION 031 3810A wi thout hor izonta l ang le base SERVICE: One year war ran ty on ma lena l and workmansh ip Serv ice con t rac ts

avai lable alter warranty period PRICE IN U.S.A. : $9250 MANUFACTURING DIV IS ION: C IV IL ENGINEERING PRODUCTS DIV IS ION

815 F th Stre Loveland Colorado 80537 U S A

sembly processes and tolerances and by using a high- resolution reticle. The resulting scope, which focuses from three metres to infinity, is repeatible within four seconds of arc.

Acknowledgments Many people contributed to the final realization of

the HP 3810A Total Station. Appreciation is due these key contributors: Perry Wells for providing the initial angle electronic design as well as fathering the angle transducer development. Ron Klein for devel opment of the limiter/automatic balance functions and environmental test coordination. Dave Smith for development of the angle electronics as well as the power supply and transmitter design and develop ment. Jerry Bybee for his optics mounting, telescope design and development, and mechanical design ef forts. Arnold Joslin for his contributions as both in-

Michael L. Bul lock Mike Bul lock was eng ineer ing group leader for the 381 OA Total Stat ion. He was born in Dal las, Texas and a t t ended t he Un i ve r s i t y o f Texas a t A r l i ng ton , g radu ating in 1 969 with a BSME degree. Joining HP that same year, he con- t r ibuted to the mechanica l des ign of the 3800A Distance Meter, then served as mechan ica l p ro jec t leader for the 3805A Dis tance Meter . He 's now 381 OA produc t ion eng ineer . Mike and h is w i fe and two chi ldren l ive in Berthoud, Co lo rado and p roduce mos t o f

the i r own food by ra is ing an imals and a garden. Mike is a lso in terested in so lar energy and Bib le s tudy, and en joys go l f , h ik ing and vol leybal l .

dustrial designer and mechanical engineer. Tom Christen, Ken Gilpin, Gary Peterson, and Hal Chase for their efforts in making the angle transducer pro ducible. The members of the 3805 and 3800 distance meter design teams for their invaluable help, espec ially Dick Clark, Dave Lee, Alfred Gort, Billy Miracle, Claude Mott, and Charles Moore. Wilbur Saul for tooling design on all three projects. Finally, lab mana ger Bill Smith, for his efforts as instigator and prime mover, whose ideas, energy, and constant search for better answers made the 3810A a reality. E

Reference 1. T.M. Whitney, F. Rode, and C.C. Tung, "The 'Powerful Pocketful': an Electronic Calculator Challenges the Slide Rule," Hewlett-Packard Journal, June 1972.

Richard E. Warren Rick Warren received his BSEE degree from the Universi ty of Nebraska in 1968 and his MSEE from the University of Southern Cali fornia in 1970. In 1972, after four years of spacecraft control system design, he jo ined HP's Civ i l Engineer ing Products Div i sion. He developed the processor and sof tware for the 3805A Dis tance Meter and the phase detec tor, processor, and software for the 381 OA Total Station. He was electr ical project leader for the 381 OA. Rick is a member of IEEE.

Born in Scottsbluf f , Nebraska, he now l ives in Loveland, Colorado. He 's marr ied and has a 5-year-o ld daughter . A s p o r t s t o h e p l a y s c i t y - l e a g u e b a s k e t b a l l a n d l i k e s t o dr ive his sports car or go cross-country ski ing in the Colorado mountains. He a lso p lays gui tar .

12


Designing Eff iciency into a Digital Processor for an Analyt ical Instrument Hardware contro l o f the I /O system el iminates excessive overhead in the architecture of a digital processor used in a gas chromatograph, lead ing to s ign i f icant improvements in operat ing convenience.

by John S. Poole and Len Bi len

THE CURRENT RUSH TOWARDS applying microprocessors to all kinds of control tasks

has not escaped the notice of those designing analyti cal instruments for chemical laboratories. In fact, a processor-based gas chromatograph, HP Model 5830A (Fig. 1), was announced two years ago,1 and an advanced version, Model 5840A (Fig. 2), with 30% more program memory and a magnetic card recorder/ reproducer that simplifies the entering of routine set-up instructions and calibration information was recently placed in production. The advantages that built-in digital control gave these instruments has

gained wide acceptance for them by the chemical industry.

Basic Considerat ions How can a digital processor best be utilized in a gas

chromatograph? In its simplest form, a gas chromato graph analyzes complex mixtures of organic com pounds, separating them into individual molecular compounds by passing a sample in vaporized form through a long, narrow tube that places a "drag" on the chemical components. The drag is a function of the molecular mass and chemical composition of

F i g . 1 . M o d e l 5 8 3 0 A R e p o r t i n g Gas Chromatograph has a bui l t - in digi tal processor that operates the i n s t r u m e n t t h r o u g h o u t a n a n a l y t i c a l r u n f o l l o w i n g i n s t r u c t i o n s en te red th rough the keyboard . I t a lso monitors the detector output, i d e n t i f y i n g s a m p l e c o m p o n e n t s a n d c o m p u t i n g t h e i r c o n c e n t r a - t ions, and generates a chromato- g r a m t h a t i n c l u d e s a c o m p l e t e analyt ical report.

13


â€¢f:.- HP im mm â€¢â€¢â€¢ â€¢ Â«Â» â€¢â€¢ â€¢â€¢â€¢â€¢

F ig . 2 . Mode l i ns t ruc t i ons can be en te red by way o f t he keyboard o f t he new Mode l 5840A Gas Chromatograph, then ed i ted and recorded on a magnet ic s t r ip for la ter re-ent ry (F ig . 3) . W i t h 3 0 % o p m e m o r y t h a n M o d e l 5 8 3 0 / 4 , M o d e l 5 8 4 0 A c a n b e p r o g r a m m e d t o c h a n g e o p

era t ing cond i t ions between runs to accommodate sample- to-sample d i f fe rences.

each component, so the lighter components tend to elute first from the tube, or column as it is commonly known, followed in time by the heavier components.

A detector at the column exit responds to the presence of components as they pass, tracing corre sponding peaks on a strip-chart recorder. The result is a chromatogram, as shown in Fig. 4. Each component is identified by the time delay between sample inser tion and detector response, called retention time. The area under the curve of each peak is proportional to the amount of that component in the sample.

Measuring the retention times and the areas of the peaks yields the specific results desired. In the past, the level of sophistication used in evaluating this data was the primary limit on the accuracy of analyses. Be cause manual methods have obvious limitations, the use of electronic data handling devices grew rapidly as the art of chromatography evolved. Initially there were hardware integrators that reported the time and calculated the area of each peak. Then came systems that used analog-to-digital converters to supply the data to computers for application of more sophisti cated means of recognizing and evaluating peaks.

Obviously, these computations are tasks suitable for a built-in digital processor. The first such efforts at using built-in digital processors, however, were in stand-alone microprocessor-controlled integrators, such as HP's Model 3380A,2 that used many of the

computer-based concepts to evaluate peak area. If used properly, all these methods reduce data an

alysis errors to negligible levels but they require the use of a relatively expensive auxiliary device with the chromatograph. They also contribute nothing to wards improving the primary source of information, the chromatograph itself. Although built-in data an-

F i g . 3 . M a g n e t i c c a r d s a r e l o n g , n a r r o w s t r i p s o n w h i c h s e t - u p i n s t r u c t i o n s a n d c a l i b r a t i o n i n f o r m a t i o n c a n b e r e corded and used to set up the chromatograph at a later t ime exact ly as before.

14


alysis is the most visible and perhaps most significant difference between the Model 5840A and conven tional analog chromatographs, the contribution that the digital processor makes to better chromatograph operation is also of major importance.

To understand this contribution, it is helpful to review the relationship of the processor to the hard ware. In a conventional chromatograph, each func tion and feedback control system has separate elec tronic circuits and if several detectors are used, each of them also has individual circuits for processing its signal. All of these require individual range or setpoint settings. A digital processor, however, is fast enough to handle several signals simultaneously by digital multiplexing so it can service the detector signals while simultaneously carrying out, by means of software algorithms, all the control functions. This automatic control of all aspects of gas chromatograph operation gains a considerable advantage in both cost and performance that alone would justify the use of a digital processor. Since there is sufficient processor time remaining to do data analysis, the analysis turns out to be a bonus obtained essentially free, except for the investment in software development.

The improvement that the digital processor brings to gas chromatograph operation can be illustrated by the autoranging electrometer for the flame ionization detector. With a conventional chromatograph, the oper ator has to select an electrometer range setting by means of front-panel switches. Selection of a too-sen sitive range can result in flat-topped peaks caused by electrometer saturation whereas a not-sensitive- enough range may result in failure to detect trace- concentration components. With autoranging under processor control, the detector is always operated on an appropriate range so very low trace concentra tions can be analyzed during the very same run as major components.

Designed-in Digital Control The goal sought in the design of the digitally-

controlled gas chromatographs was to provide auto matic data reduction and printout of the results along with elimination of as many operator errors as possible by the use of autoranging detectors and automatic selection of integrating parameters. Also, the digital processor could schedule oven tempera ture changes, switch the column effluent to other detectors, and do many of the other chores that had kept a chemist tied to his chromatograph during lengthy procedures, some of which can go on for hours.

The most economical way to integrate all these functions into a single instrument that met our perfor mance standards was to use a central processing unit (CPU) with computer-like architecture. The heart of the system that evolved is a 16-bit, serial-oriented

r

Fig. 4 . Re la t ive ly s imple chromatogram made by a manual ly operated gas chromatograph. To analyze resul ts , the chemist must der ive the t ime delay between sample insert ion (marked by the nega t i ve sp i ke a t l e f t ) and each peak , and the a rea bounded by each peak and the basel ine. Special calculat ions determine areas of peaks that overlap, l ike those shown here. Chmmatograms o f comp lex m ix tu res such as c rude o i l may be 10 t imes as long and have hundreds o f peaks .

CPU like that used in the HP 9800-series Calculators.3 Besides the 16-bit word size, a clock rate of around 5 MHz was needed to perform all the desired opera tions in a reasonable length of time, requirements that could not be met by the microprocessors available at the time. Hence, MSI computer circuits were used.

All operating parameters are entered through the function-oriented keyboard (Fig. 5), eliminating the many control knobs that a sophisticated gas chroma tograph can have. The parameters are stored in a read/ write memory as digital numbers, allowing the oper ating values to be entered with much greater resolu tion than is economically possible with the rotary, slide, or pushbutton switches commonly used. By pressing the LIST key, all variables under processor control relating to the chemical analysis are recorded automatically on the chromatogram, giving all the data pertinent to the analysis on a single sheet of paper.

Organiz ing the Processor Processor organization is shown in Fig. 6. Each

system parameter that operates under processor con trol is treated as a peripheral to the CPU, and is ac cessed through the I/O bus. For example, each heated zone has a unique electrical address, and temperature control is executed by a particular algorithm stored in a read-only memory (ROM). Besides economy, this technique provides several advantages over previous

15


mm â€¢â€¢) mm mmm â€¢ â€¢ mm mm mm mim m fj

m m m m m m 1 S

Fig. 5. Through the keyboard, a l l aspec t s o f t he ana l ys i s a re con trolledâ€”the column oven tempera ture program, the temperatures of o the r hea ted zones , t he i n teg ra t ion parameters , the ca l ib ra t ion , a n d t h e t y p e o f c o m p u t a t i o n . Operat ion o f backf lush va lves, a c h a n g e i n r e c o r d e r s p e e d , a c h a n g e i n d e t e c t o r s , a n d o t h e r pa ramete rs can be p rog rammed t o o c c u r a t s p e c i f i c t i m e s f o l lowing the start of a run.

methods of hardware control, such as enabling the listing of setpoint and the actual value of any heated zone as an integral part of the final printout of results.

The key to efficient utilization of the processor system was recognition of the fact that the peripherals connected to the I/O bus need constant attention and therefore should not be accessed by the usual I/O routines, that is with a request for interrupt followed by a CPU poll and so on. It was decided to establish a regular, periodic interrupt routine controlled by

hardware. This allows each device to be serviced on a known schedule so module operation can be synchronized to have data ready when serviced.

This scheme is implemented by using three of the bus lines for hardware-controlled addresses. Ad dresses are placed on these lines by square-wave div ider circuits such that a 40-Hz square wave appears on the first line, a 20-Hz square wave on the second, and a 10-Hz square wave on the third, as shown in Fig. 6. Each time a transition of the 40-Hz square wave

Read/Write Memory

Automatic â€¢ Automatic â€¢ Auxi l iary â€¢ Magnetic â€¢ Optional V a l v e L i q u i d I H e a t e d I C a r d â € ¢ I n t e r f a c e

S w i t c h i n g I I S a m p l e r I I Z o n e s I I R e a d e r I I P o s i t i o n

Read-Only Memory Print

and Plot

Data Bus â€” Bidirectional

Detector (2 Max)

â „ ¢ " ^ ^ ^ â „ ¢ ^ ^ ^ ^ â „ ¢ IF IF Injection

Port Heated Zone

Column Oven

Heated Zone

Detector Heated Zone Fig. 6. Organizat ion of the digi tal

p r o c e s s o r i n t h e M o d e l 5 8 3 0 A Gas Chromatograph.

16


1 0 H z J

2 0 H z j

4 0 H z

T i m e S l o t T O T 1 T 2 T 3 T 4 T 5 T 6 T 7 T O

I n t e r r u p t

Fig . 7 . An in te r rup t occurs on each t rans i t ion o f the 40-Hz square wave. The states of the square waves at each interrupt define a 3-bit address for the routines to be performed during that interrupt.

occurs, an interrupt is generated and the present states of the three lines determine the I/O address for that interrupt. This is the only interrupt in the system, and it occurs 80 times a second. All eight addresses are thus cycled ten times a second.

A -software counter in the CPU, synchronized to the hardware system, addresses a section of ROM for determining the servicing routine for each interrupt slot. The ROM programs cause the processor to ad dress four other lines on the data bus called quali fier lines. One of these determines the direction of data flow and the other three address various func tions. During any one interrupt time slot, up to eight functions may be addressed sequentially by the quali fier lines. Consequently, with these seven lines the processor has the capability of addressing up to 64 lo cations 10 times a second, and to determine the direc tion of data flow at each location.

Address Organization The address locations are listed in the table below.

Note that the printer/plotter is addressed during every interrupt. This enables the printer/plotter to access data 80 times a second for simultaneous real-time

Time Slot and Qualifier Address Function Table

plotting and/or printing. A brief description of the addresses will aid in un

derstanding how the processor is used. Addresses 00, 01, 02, 40, and 41 are 32-bit transfers of data from the detectors to the processor. The instrument han dles up to five detector data signals on the bus. Speci fic addresses are allocated for the detectors com monly used in GC work, i.e. the flame-ionization de tector (FID) and the thermal conductivity detector (TCD), and a third address is allocated on each of the chromatograph's two channels for any special detec tors that the chromatographer may wish to use.

Addresses 04, 14, 24, 34, and 44 are concerned with temperature control in various parts of the chromatograph. The read/write qualifier is used in conjunction with these addresses to enable data transfer in either direction. First, a digital word describing the actual temperature of the particular zone is transferred from detectors in the heated zone to the processor. An algorithm stored in ROM compares the actual temper ature to the setpoint previously entered in the read/ write memory from the keyboard. The algorithm then calculates a duty cycle for the triac supplying power to the zone heater and transfers the value of the duty cycle to the zone controller. Note that the tempera tures of all these zones are sampled and corrected 10 times a second.

Addresses 31 and 32 are concerned with column oven temperature. A combination of resistance heat ing, cryogenic cooling, and ambient air mixing is used, permitting the setpoint for the oven tempera ture to be set anywhere between â€” 50Â°C and +400Â°C.

Address 50 is for the keyboard. When any key is pressed, its identity is stored in a register. Address 50 transfers the contents of this register to the CPU for in terpretation.

Address 51 is for control of external devices such as valves for sample injection or column switching. Be cause the processor has a master clock, a list of time- dependent variables such as these valves may be stored in a software table and executed as a function of time elapsed since sample injection. Keyboard en tries define the function that is to occur and the time of occurrence. Time programming is available for time-dependent variables within the instrument as well as for the external devices.

Address 52 is for an automatic liquid sampler (HP Model 7671A). The actual injection sequence for the mechanism is stored in a programmable read-only memory in the chromatograph. The sequence can be actuated by the CPU during address 52 or infor mation concerning the status of the sequence can be transferred to the CPU.

Addresses 54 and 64 are for transmitting digital readings from the electronic flow sensors in columns A and B. Address 60 is reserved for future options.

1 7


Time slot 7 is used by the CPU for synchronizing the hardware/software system.

When the processor completes its housekeeping functions in each interrupt time slot, it returns to the background program until the next interrupt occurs. Everything described so far requires only about 30% of the processor time, leaving plenty of time for back ground programs such as the integrating algorithms and calculations.

Printing with a Plotter The operation of the printer-plotter is another ex

ample of the contribution that digital control can make. The recording "stylus" is a small ceramic rectangle on which a single vertical row of seven dots is formed by thick-film techniques. Each dot is a resis tor that reaches a temperature of 200Â°C in about 4 ms when 100 mA at 20 V is applied. Power is maintained for about 6 ms, enough time for the heat to change the color of the heat-sensitive paper where touched by the dot.

The dots are selectively pulsed by the CPU as the column of dots is moved linearly across the page. This generates characters with the equivalent of a 5X7 dot matrix.

To trace the chromatogram, one of the dots is turned on continuously at reduced power. The detec tor output, which is converted to digital words 10 times a second for transmission to the CPU, is recon verted to an analog voltage for driving the servo me chanism that positions the print head. To achieve high resolution, a constant-density trace is main tained by modulating the power to the print head as a function of stylus slew rate. The CPU calculates the slew rate 80 times a second and a digital word describing the required power level is transmitted to the printer-plotter module.

At the same time, the CPU controls the stepping motor that drives the recorder chart. Either a step or a no-step pulse is transmitted to the motor during each interrupt. The chart speed is thus determined by the rate at which step pulses occur, which is calculated by the CPU according to the chart speed entered on the keyboard. This technique permits the chart speed to be set from 0 to 10 cm/min in 0.01-cm/min incre ments. As with the zone temperature controllers, this fine resolution is obtained with no increase in hard ware costs.

The ability to intermix printing and real-time plot ting without causing discontinuities in the plot is an other advantage derived from driving the chart under CPU control. When a retention time is to be written adjacent to a peak, the chart drive is stopped and subsequent realtime data points stored while the printed information is being generated. Plotting re sumes when the printing operation is complete, but it

Retention Time Identifies Peak in Report Section

Marks Start of Peak Area Integration

Marks End of Integration

j. 1 1 2. S3 Ã¯. ft,

1S15BU

2 5 3 6 B B

Â¿5 .5 .30 3 B . 8 B 9 4 3 . U 4 S

F i g . 8 . T h e p r i n t e r - p l o t t e r t r a c e s a c h r o m a t o g r a m a s a n analysis proceeds and prints pertinent data on the same sheet of paper. In th is example, calculat ions determine the percen tage of each peak with respect to the total area of al l peaks. The basic data is retained in memory so the operator can cal l for addi t ional reports calculated in other ways.

goes at an increased rate until the stored data points are all plotted. The chart speed then returns to normal and real-time plotting resumes.

With its ability to both print and plot, the printer- plotter greatly simplifies record keeping by enabling the chromatogram, operating parameters, and the cal culated results to all be recorded on the same piece of paper, as shown in Fig. 8.

From Detector

F ig . 9 . The e lec t romete r fo r t he F ID uses the exponen t ia l re la t ionship between the emi t ter -base vo l tage and co l lector current of a transistor. In the feedback path of the input ampli f ier, and derives a logari thmic relat ionship between input and output (Q1 ). In the next stage (Q2), i t l inearizes the amplif ier ou tpu t . Rang ing is accompl ished by chang ing the base vo l tage, and hence the ampl i f icat ion factor , of t ransistor Q2.

18


Invisible Autoranging With the digital processor integrated into the gas

chromatograph, it was possible to incorporate autoranging into the detectors in a way that made the operation invisible to the user.

The electrometer used with the flame ionization detector (FID) is based on the electrometer developed for the HP Model 5700A Gas Chromatograph.4 This electrometer eliminated many of the problems of ear lier designs by using a logarithmic amplifier that can accommodate input signals over a wide dynamic range followed by an exponential amplifier that lin earizes the output of the logarithmic amplifier (Fig. 9). Range switching occurs at a low-impedance point where it can be executed without introducing the kind of transients that occur when the high-impe dance circuits of a conventional electrometer are switched.

The digital-to-analog converter that follows the exponential amplifier is a conventional dual-slope converter as found in many digital multimeters. The clock rate for the converter is 10 MHz, enabling a complete, high-resolution conversion in less than 1 ms. For each reading, 100 conversions are added, thereby giving an averaged value that has improved signal-to-noise ratio.

Ranges are switched by comparators that look at the output of the analog-to-digital converter, upranging if the accumulating clock pulses reach a full count before the measurement cycle is complete, or down- ranging if the count does not reach 20% of maximum by the end of the measurement cycle. Since a range change occurs within 1 ms of the start of a measure ment cycle, the system is able to track peaks that have very fast leading edges.

The range-to-range ratio was made 16 to 1 so a range change can be introduced into the binary output simply by shifting it four places. Thus the CPU always sees a number corresponding to the absolute value of the electrometer output over a wide dynamic range without recourse to a separate range indication. The only range change visible to the user is the user's own selection of a suitable recorder sensitivity.

Similar arrangements for digital readout and autoranging are used with the other types of detectors. A major benefit of autoranging is the elimination of errors resulting from undetected overranging. Some detector systems do not clip a peak overload but round the top of the peak, giv ing it the appearance of a normal peak. It thus happens that the detector amplifier can be oper ated in a non-linear region, causing errors in the interpretation of the chromatogram, without the user's being aware that an overrange condition exists. This does not happen with the digitally- controlled chromatograph.

Safe Unattended Operat ion Because the use of the digital processor with its

stored programs made long periods of unattended operation possible, the design of the entire gas chro matograph had to be made fail-safe with considera tion given to many areas unrelated to chemical per formance. For example, the memory is protected against loss of its contents by a standby battery power supply that can support the memory for the duration of a typical power failure. In the event that the power is off longer than the standby power supply can sup port the memory, the system automatically enters de fault values when power returns so no harm is caused the system on restart.

The power supplies are protected against both overvoltage and overcurrent such that an orderly shutdown occurs before any part of the instrument is damaged. The heated zones are protected by an inde pendent software-controlled detection circuit that shuts down secondary 115V power to the zones and the column oven before damage can occur.

The concern for fail-safe operation also extended to non-electronic parts of the system. For example, hy drogen gas, sometimes used as a carrier gas and com monly used in the FIDs, could fill the column oven and be ignited by the resistance heaters if there were a leak. If this should occur, the resulting violent explo sion must be contained within the shell of the instru ment to prevent injury to nearby personnel. This places quite a burden on the designer because it is also desirable to keep the mass of the oven low to give fast thermal response. The design of the oven in the Models 5830A/5840A is such that it successfully withstands multiple test explosions.

Acknowledgments Doug Smith and Ivan Crockett generated the orig

inal concept of a processor-based gas chromatograph and developed the architecture of the instrument. Ruder Schill was responsible for the man/machine in terface, defining the keyboard, data handling meth odology, and final report format. E

References 1. G.V. Peterson and J.S. Poole, "Design Concepts of a Processor-Based Gas Chromatograph," American Labora tory, May 1974. 2. A. Stefanski, "Deriving and Reporting Chromatograph Data with a Microprocessor-Controlled Integrator," Hewlett-Packard Journal, December 1974. 3. HJ. Kohoutek, "9800 Processor Incorporates 8-MHz Microprocessor," Hewlett-Packard Journal, December 1972. 4. D.H. Smith, "High Performance Flame-Ionization De tector Systems for Gas Chromatography," Hewlett-Packard Journal, March 1973.

19


Len Bilen Born Â¡n Sysekil, Sweden, Len B i l en g radua ted f rom the Cha l mers Univers i ty o f Technology, Sweden, in 1965 wi th a degree in phys ics. Pr ior to jo in ing HP, he spent three years a t Saab, where he was involved wi th the centra l processor of the Viggen a i rcraf t , and a year and a ha l f a t Genera l

* Dynamics in Rochester, New York, " w i th respons ib i l i t y fo r p rocessor -

contro l led test s tat ions on the F1 1 1 B aircraft. At HP he had major sof tware responsibi l i ty for the

5830A Gas Chromatograph and was p ro jec t leader fo r the 5840A. He is marr ied and has three chi ldren. Church act iv i t ies and main ta in ing a house c la im most o f Len 's spare t ime.

John S. Poole Before joining HP in 1967, John Poole worked for six years on satell i te instrumentation at the U.S. Naval Research Labs in Washington, D.C., where he was involved in the design, launch, t racking, and data reduct ion aspects of spacecraft operat ion. He has a BSEE degree from the Universi ty of Delaware and has completed work towards an MSEE degree there. At HP, he was project leader on the 7670A Auto-

Â¡ mat ic Sampler and was manu factur ing engineer ing manager for a year and a hal f before assuming project leadership of the 5830A Gas Chromatograph. Marr ied, and with three chi ldren, his act ivi t ies include boating, water sk i ing, and swimming.

B A S I C I N S T R U M E N T S P E C I F I C A T I O N : P r o c e s s o r - b a s e d , k e y b o a r d - c o n t r o l l e d .

o n e 0 1 t w o d e l e c t o r ( s ) . s i n g l e - o r m u l t i p l e - c o l u m n g a s C h r o m a t o g r a p h w i t h

i n t e g r a t i o n , l i m e - p r o g r a m m a b l e f u n c t i o n s , c a r r i e r f l o w r a t e p r i n t o u t t e m p e r a t u r e p r o g r a m m i n g , a n d A r e a % a n d m e t h o d s c a l c u l a t i o n s

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l l a m e a n d t h e r m a l c o n d u c t i v i t y d e t e c t o r s a n d a u x i l i a r y c i r c u i t

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m a t i c a l l y i d e n t i f i e d b y r e l a t i v e r e t e n t i o n . I n E S T D a n d N O R M m e t h o d s , i d e n t i f i

c a t i o n i s a b s o l u t e r e t e n t i o n t i m e o c c u r s a u t o m a t i c a l l y i l r e f e r e n c e p e a k i s n o t

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a l l c a l i b r a t e d p e a k s i n E S T D a n d N O R M m e t h o d s

P R I N T E R / P L O T T E R :

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s t o p c a n b e m a n u a l o r a u t o m a t i c

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a v a i l a b l e i n s p e c i a l k e y b o a r d c o n t r o l l e d v e r s i o n o l M o d e l 7 6 7 1

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o v e r a m b i e n t r a n g e o l 1 5 - 4 5 C . 0 - 9 0 % r e l a t i v e h u m i d i t y r a n g e S I Z E O v e n m o d u l e 2 1 v , H " 2 1 V Ã D * ; M i n W ( 5 4 6 - 5 4 . 6 " 8 6 . 4 c m ) , t e r m i n a l

8 H â € ¢ 2 2 V j D - 1 7 V , i n W ( 2 0 3 Â « 5 7 2 * 4 4 5 c m ) W E I G H T 1 6 0 I b s ( 7 3 k g )

P O W E R 1 2 0 V I - 5 . 1 0 % ) 5 8 , 6 2 H z . 2 2 0 V ( - 5 . - 1 0 % ) 4 8 / 6 2 H z s i n g l e o r s p l i l

p h a s e , 2 4 0 V ( - 5 , - 1 0 Â ° a ) 4 9 / 6 2 H z s i n g l e o r s p l i t p h a s e .

P R I C E I N U . S . A . : B e g i n s a t Â £ 1 1 , 5 2 5 f o r a c o m p l e t e s y s t e m

M A N U F A C T U R I N G D I V I S I O N : A V O N D A L E D I V I S I O N R o u t e 4 1 & S t a r r R o a d

A v o n d a l e . P e n n s y l v a n i a 1 9 3 1 1 U . S . A .

l o r y = 1 i t i n s t a l l e d , o t h e r w i s e = 0

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