ce-1 on off temp cntrl

8/13/2019 CE-1 on Off Temp Cntrl

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1HEM \ON-OFF TEMP. CONTROLLER

HEM ELECTRONIQUES MIDC MIRAJ (M.S.)INDIA

ON/OFF TEMPERATURE CONTROLLER

INTRODUCTION:

On/Off controller or two position controller is one of the various

controller modes, which shows discontinuous changes in the controller

O/p as contro lled variable error occurs.

Although an on-off type control does not quali fy as a cont inuous cont rol

,it is used in numerous residential ,commercial, and industrial

applications. It is commonly used in control of home appliances ,hot

plate and oven-temperature control, refrigerator temperature control

and dishwasher water level control .It is by far the method of choice

for environmental temperature control of all residential and commercialbuildings. Some may even argue that a digital controller is nothing but

an extremely fast and fancy On/Off controller.

TYPES OF ON/OFF CONTROL.

On-off contro llers can be operated in three different ways.

(strict)on-off control

Two position control

Floating control

(strict)On-off control:- In this method of control ,controller O/Pchanges at the instant the error changes from posit ive to negative or

vice versa. Whenever the controlled variable is below the set point

(positive error) the controller O/P is at 100%,and if the controlled

variable is higher than the set point (negative error) the controller

O/P drops to 0%.

A Str ict On-Off method of control is not pract ical one, because in the

majority of applications ,controller O/P would constantly be

switching between full-scale (100 %)and zero (0 %)O/P .thus ,a strict

On-Off method of control is seldom used. A modified form of on-off

control, two pos ition control, is more commonly used.

Processes with small time constants:-

On-Off control cannot be used for process with a small time constant

,such as a DC motor with a light load. if an attempt is made to control

the motor speed through a switching relay, the relay will be

constantly chattering (switching On/off )and will not last very long.


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Processes with Large Time constants:-

On-off control can be used in the processes where the process

inertia is very large compared to the energy supply unit, such as

when a large building is heated with a relatively small or moderately

sized furnace .On-off control can also be used in the situations

where measuring inst ruments have built in hysteresis.

TWO POSITION CONTROL

Two position control is an improvement over the basic on-of f .The

controller O/P stil l swings between 0% and 100 % ,but an addit ional

hysteresis element (neutral zone) is now i ncorporated.

NEUTRAL ZONE

The neutral zone is the region of error over which the control ler doesnot change its O/p.

The neutral zone is also known by the terms dead band and error

band .Generally,the neutral zone is symmetrical about 0% error,but

there is no rigid rule.

FIELD OF APPLICATIONS:-

Generally two posi tion control ler mode is best adapted to large scale

systems,with relatively slow process rates.Thus in either form,heating or airconditioning system,the capacity of the system is very

large in the terms of air volume and overall effects o f the heater or

the cooler is relatively slow. Sudden large scale changes are not

common to such systems .the process under two position control

must allow continued oscillation in the dynamic variable .Because by

its very nature ,this mode of control always produces oscillations.

For large systems, these oscillations will be long duration, which is

partially a frequency of the natural zone size.

CIRCUIT OPERATION:-

PT-100(Platinum resistance temperature Detector )is used as a

sensor for measuring the temp. in the range 0-200 Degree Celsius.

TEMPERATURE MEASURMENT: RESISTANCE TEMPERATURE

DETECTOR(RTD)


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INTRODUCTION: (refer fig. A,B, &C)

The principle of operation of resistance temperature detectors (RTD) is

based on the fact that the electrical resistance of many metals increases

almost directly with temp. And is reproducible to high degree of

accuracy. The term used to express this characteristic is well known

temperature coefficient of resistance and is defined by the appropriate

formula:

Rt = Ro (1 + t) where-Temp coeff icient of resistance for the metal used.

Ro - Resistance of the element at 0 degree Celsius

t - Temperature of the element in degree Celsius.

Platinum ,Nickel and copper are generally used as basic materials for

RTD.

We may note the following as regards the RTD as transducer for temp.

measurement.

1. The resistance of R.T.D. increases as the temp. increases. The

resistance and temp. are linearly related over a wide temp. range.

2. In general, resistance thermometers are larger and less convenient

to apply than the thermocouple. They are massive and hence exhibit

poor time response characteristics.

3. They require bridge balance circuitry and have rather restricted

upper temp. range. They exhibit higher accuracy, reliability than

thermocouples.

Circuit analysis:In this ci rcuit RTD is made an arm of a simple bridge circuit . The RTD

exhibits a change of about 39 % in resistance from 100 ohms at 0

degree centigrade to 139 ohms at boiling point of water (100 degreeC) . So in case RTD is made part of a bridge circuit with equal arms ,

there will be a non linear relationship between resistance change and

the bridge output vol tage.Typically when X = 20 % (where X is

fractional change in resistance ) then error will be about 9% , where

as for X = 30% , error will be 13 %.

In order to overcome this non linearity problem,the resistance ratios of

the top elements to the bottom bridge elements are made large. This

process however reduces the bridge sensitivity. But the sensitivity

problem can be readily taken care of by using amplif ier. Refer figure .

In this arrangement differential output from the bridge is amplified byan operational amplifier. The top arms of the bridge are about 8 kohm in

value and the lower arms are around 100 ohms. The MIN and MAX

controls are shown in the figure. We shall develop our understanding

about linearity of the bridge output with respect to changes in

resistance.

Case 1: (fig A)

R3= 100 ohms i.e. RTD at 0 degree C. Vout = 0.0mV = Va - Vb


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Case 2:(fig B)

R3= 139 ohms i.e. RTD at 100 degree C,we have ,

Va = R3 /( R1 + R3) * 5000milivol t where bridge excitation is 5 volts.

Therefore Va = 139/7939 *5000milivolts = 87.54 milivo lts.

We have

Vb = R4 / /( R2 + R4) * 5000milivolts = 63.29 milivolts. Hence for 100

degrees change in temperature

Va - Vb = 87.54 63.29 (mil ivolt ) = 24.25 milivolt .(1)

Case 3: We can check for intermediate values of temperature of 30

degrees centigrade. We know that resis tance of RTD R3 wil l be 112

ohms at 30 degree C temperature. So by similar calculation we have,

Va = (112/7912) * 5000mV =70.77mV where as

Vb = 63.29mV (of course unchanged)

Va - Vb = 7.48 mil ivo lt .(2)

For linearity for 100 degree centigrade

Va - Vb = 24.25milivolt.

Therefore for 30 degree centigrade

V0 = 24.25 * 0.3 = 7.27 mil ivolt .(3)

So comparing equation No 2 and 3 for full scale output of 24.25

milivol ts , percentage error at 30 degree centigrade

=(7.48mV 7.27mV) /24.25 * 100 = 0.21 /(24.25) *100

= 0.86%

which i s much better than equal arm arrangement.

In this set-up MIN and MAX controls are provided in terms of TRIM

pots located on the PCB for signal conditioning .(PCB near the front

panel.)MIN control gives adjustment fo r 0 degree centigrade(100 ohm

for RTD) and MAX control gives adjustment for 100 degree

centigrade (139 Ohms for RTD)

The O/P from signal conditioning is provided to the I/P of On-Off

controller P.C.B. alongwith set point adjustment POT P1 on the front

panel. IC 1 forms the error amplifier (Summation block).Its O/P isgiven as error voltage E = SP (vol tage ) PV (voltage)

IC 2 acts as two position controller .Voltage Vr at the non-inverting

terminal is governed by the saturation voltage ,Vsat of IC 2 and two

resistors R2 and R3.It varies between two limits (error bands ) and

can be determined from the following expression.

Vr = +- (R3/(R2 + R3) )* Vsat


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= + - Beta * Vsat

Vref is +ve when IC 2 O/P Vsat is Positive and is negative when IC2

O/P is at Vsat ve value .

The controller O/P switchs state is based upon its two I/P , error

vol tage E and Ref vol tage Vref.

Error vol tage E is measured at TP1 and Vref is measured at TP2 W.r.t.

ground at rear panel.The total dead band (Neutral zone for the circuit

is ,

Dead Band = 2*beta*Vsat (Beta is variable with the help of pot P2

on the front panel.)

OPERATING INSTRUCTIONS.

1) Connect the RTD sensor PT-100 two the binding post

provided on the front panel.

2) Connect the heater cable on the rear side socket.

3) Keep rotary switch on the front panel in 0 degree posi tion.

And switch on the supply.

4) The D.P.M. should ind icate 0.0 degree centigrade.

5) When select switch is taken to 100 Degree position ,DPM

should indicate 100 degree C.This completes calibration

check and ensures that MIN and MAX controls on the PCB are

properly adjusted .

6) Keep select switch in SET posi tion and adjust SET TEMP.(P1)

pot to 55 degree C.Keep Dead-band pot to most C.C.W.

position.(MIN)

7) Take select switch to RTD posit ion.Now you can observe that

RTD temp. goes on increasing and the controller keeps the

temperature of the process around the set point depending

on Dead-Band adjustment.

8) You make take temp. readings at regular interval of 10 Sec.or 15 sec. and plot a graph fo r temp. Vs time readings.

9) You may adjust dead band to most C.W. positi on (MAX) and

repeat the above process again.

10) If you keep a fan near the process model ,you may again take

another set of readings and observe oscillations in the temp.


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PRECAUTIONS:

1) Operate the select rotary switch carefully.

2) RTD connections should be firm.

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ce-1 on off temp cntrl

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