LVDT Interface Box

Trans-Tek

Connor Bailey, Manuel Medeiros, Aaron Ciardullo

Team 1607

1

Table of Contents

● Summary - 2

● Background - 2 ○ Overview - 2

○ Theory - 2

● Solution - 3 ○ Theory - 3

○ Approach - 4

○ Preliminary Experimental Results - 6

○ Updated Results - 7

○ Differential Amplification - 8

○ Power Circuitry - 9

● Timeline and Projections - 12

● Budget - 15

● Collaborators - 16

Summary:

This document will cover the proposal of the design of a sinusoidal oscillator and demodulator

unit that will be used as a standalone unit to drive a linear variable differential transducer (LVDT). This

unit has been requested by LVDT manufacturer Trans-Tek incorporated, who will also be collaborating

with our team throughout the project. This proposal will include a few different possible design ideas and

routes that can be taken to achieve our overall goal.

2

Background:

Overview:

A linear variable differential transducer (LVDT) is primarily used as a position sensor for various

applications from robotics, to manufacturing. The problem with LVDTs is that to run them they require

that one use a signal generator to drive the primary coil as well as custom circuitry that can analyze the

output sinusoidal waveform that the secondary coil generates. These supporting devices can be relatively

large and make the implementation of a LVDT more difficult due to size limitations. It would be much

easier to be able to have a device that only requires a DC power supply and some circuitry that can read a

DC voltage, which most small microcontrollers can do easily. This is where our device comes into play.

Our device is going to essentially be a LVDT driver box that will only need a DC power supply to

function. The device will create a sinusoidal wave form to excite the primary coil on the transducer and

then demodulate the sinusoidal from the secondary coil to a DC voltage that can be read. This makes

implementing an LVDT much easier for system designers because they now have a much smaller device

to work with.

Theory:

“LVDT stands for Linear Variable Differential Transformer. An LVDT is also referred to as a

linear displacement transducer, or linear position transducer. This sensor device measures linear

displacement (or linear position) very accurately. The typical LVDT sensor consists of a primary coil and

two secondary coils wound on a coil form. A ferromagnetic core links the electromagnetic field of the

primary coil to the secondary coils. Differencing the output of these coils will result in a voltage

proportional to the relative movement of the core versus the coils.” (retrieved from Trans-Tek

http://transtekinc.com/what-is-an-lvdt/ ) This is shown in figure 1.

3

FIGURE 1 LVDT DIAGRAM. DIGITAL IMAGE. N.P., N.D. WEB.

<HTTP://WWW.MACROSENSORS.COM/>.

Solution:

Theory:

The current design for the interface circuit uses all discrete components; as a result, the design is

not very flexible different types of linear variable differential transformers (LVDT). Using a Digital

signal processor, allows us to reprogram the device to meet the requirements for many more LVDTs. The

Digital signal processor can also output a digital signal instead of an analog signal which would allow a

customer to implement Trans-Tek products in different configurations.

FIGURE 2 BLOCK DIAGRAM OF BASIC OPERATION OF DSP BASED DEVICE

Figure 2 shows the basic operation of a DSP device. The DSP will generate a sine wave that

would then go through some signal conditioning so that it meets the requirements to be feed into the

4

primary side of the LVDT. The secondary side is then taken from the LVDT and goes through signal

conditioning so that the signal can be read by the onboard analog to digital converter on the DSP. The

DSP then demodulates the signal to provide an output which could be either digital or analog.

Approach:

There are two ways to create a sinusoid using a digital device. One method would be to use direct

digital synthesis and the other would be to write different levels in a digital to analog converter. In the

direct digital synthesis, the DSP creates a sinusoidal pulse width modulation signal (SPWM) which is

then passed through a low pass filter to provide a sinusoid fundamental signal. The reason that we use

SPWM is so that we push the third harmonic to higher frequency which then makes it easier to filter. This

method is used on microcontrollers that do not have a full DAC installed. The problem with this method

is that we must create a very high frequency PWM signal to be filtered which is intensive for the

controller. The other method is very straightforward; a sinusoid table is created in the DSP. Then each

value in the table is written to the DAC sequentially. The only thing that limits this processes is how long

it takes the DAC to write.

FIGURE 3 SIGNAL CONDITIONING FOR DAC APPROACH

Figure 3 is a differential amplifier with a push-pull amplifier in series. The DAC can only create

positive voltages and therefore we use the differential amplifier to remove the DC offset and set the

voltage gain. Since the output current is quite small from the op-amp we need a current amplifier in order

to support the load from the LVDT.

5

FIGURE 4 H-BRIDGE APPROACH FOR DIRECT DIGITAL SYNTHESIS

Figure 4 shows the signal conditioning for the direct digital synthesis approach. In this method

the DSP provides a PWM signal for each of the four switches. The H-bridge then inverts the DC Bus at

high speed which creates a PWM centered at zero. This PWM signal is then filtered to create the sine

wave. This approach is good as it the switches can be large to support any load, however since the LVDT

is an inductive load each switch would need a snubber and the DSP would have to provide high speed

PWM signals.

Preliminary Experimental Results:

We explored three different types of chips the TMS320F2877S, MSP430 and an Arduino DUE

with the AT91SAM3X8E.

MSP430- We first tried to create a sinusoid using Direct digital synthesis on the MSP430.

However, the fastest PWM that could be created was around 7KHz, which would not meet the 10 KHz

specification and would be very hard to filter as it could not utilize SPWM.

TMS320F2877S- For this chip we started to use the example code provided by Texas instruments

to create a sinusoid using direct digital synthesis. However, when loading the code into the

microcontroller we had some problems having it to output. Therefore, we need to do more testing in

order to use this microcontroller.

6

Arduino DUE with AT91SAM3X8E- On this chip we used the DAC to produce a sine wave with

2V peak to peak with a 1V offset. THD for the sine wave was less than 5% and the maximum frequency

that we could produce was 5KHz. We believe with some code optimization we should be able to produce

much higher frequency sine waves.

FIGURE 5 ARDUINO CODE FOR SINE WAVE UP TO 5KHZ

We also started to write the demodulation code to run with schedule (Arduino multithreading)

however we have not tested it with the sine wave running at the same time.

Updated Results

Through further experimentation we realized that we needed to increase the resolution and

number of datapoints in out sinewave lookup table. To do this we use 360 points to get a full sine wave

cycle with values ranging from 0 to 212 (4096). The reason for this max value selection is due to us using

a 12 bit DAC.

7

FIGURE 6 ARDUINO CODE FOR SINE WAVE WITH DAC

In figure 6 you can see in the beginning of the code comments relating to the step size variable

and its corresponding frequency. This is the method used to increase and decrease the sine wave output

frequency. By stepping through at certain intervals you can skip some values to make the output run

faster, this can then be filtered at a later stage to make sure that the output waveform is clean. In the final

design these values will be assigned with pin change interrupts when the user sets the DIP switches to

their desired frequency.

Differential Amplifier

Through experimentation we verified that our differential amplifier approach was valid. We have

successfully set up a differential amplifier with a -1V offset that will amplify the sinewave from the

microcontroller and also act as a buffer for when an LVDT load is connected to it. The only issue with the

op amp itself is that it does not provide enough current to drive the LVDT. To remedy this we have

8

connected a two transistor push-pull amplifier to the op amps output. This amplifier is connected directly

to both the -10 and 10V rails in order to provide more current for the driving signal.

In the prototyping phase we used a simple LM741 op amp to test with, however in the final design we

will utilize a ready made differential op amp IC. The reasoning behind this is that in order to change the

gain of the amplifier to match the user's input selection we need to change two of the gain resistors at the

same time. By using a ready made IC we can make this easy, since we only need to change one resistor

Rg to adjust the gain. This is also a benefit because too can throw on a digital potentiometer and have the

microcontroller control the gain through software.

Power Circuitry

From Transtek’s specification sheet the LVDT interface circuitry must be able to be powered

from 11 to 40 Volts. The microcontroller, op amps and current amplifiers require different voltages in

order to operate, therefore we must produce these voltages from the ones supplied. The three man

voltages that we need are 3.3V for the microcontroller, 10 Volts for the Vin+ and -10 volts for the Vin-.

In order to save space and limit the power circuitry noise the 10 V (IFX25001) and the 3.3 V (LD1117)

will be produced using a linear regulator. Since the LD1117 maximum input voltage is 15 volts the

regulator needs to be connected to the output of the IFX2001. In order to produce the negative voltage we

need to introduce a switching regulator. One option is to use the ICL7662EPA which is a charge pump.

The chip operates by taking two capacitors and charging one up and then flipping polarity to charge up

the other. This chip has very poor voltage regulation and is very noisy because the switching frequency is

5 khz. This makes the regulator very noisy for us as the frequency is in the center of the output range of

the DAC and we are unable to filter the switching noise out. However this charge pump is currently used

by Transtek and therefore readily available in stock.

Another solution for the inverting rail is to use an inverting buck switching regulator. We are

currently testing the MC34063 which is a switching regulator that can be configured for buck, boost and

inverting. Since the MC34063 runs at 100 khz, the switching frequency can be filtered out easily.

9

Figure 7: MC34063 Inverting Converter Application

The Figure above is from the MC34063 data sheet. In order to use this chip, an inductor, diode,

capacitors and resistors need to be size to the current and voltage ripple specifications. The chip operates

by charging up the inductor by turning on Q1. The switch is then turned off and since current can’t stop

instantously the current is forced through ground to charge up the capacitor. This inverts the voltage. The

disadvantage of using this chip is that we need to have an input voltage of more than 15V or else the

current becomes too large for the chip to handle. The other disadvantage is the board space required to

implement the chip. Since an inductor, and capacitor is needed the the footprint of the switching regulator

is high, therefore we need to determine if this regulator is necessary or if we can filter and shield the

switching noise from the charge pump.

10

Figure 8 Block Diagram of Our Current Oscillator System

11

Timeline and Projections:

FIGURE 9 TIMELINE OF OUR PROGRESS UP TO PRESENT DATE 12/6

The team was formed in the first week of September and spent the first meeting getting to know

each other. Later that week we set out to meet with our sponsor company, Trans-Tek, to discuss

specifications and objectives. After meeting with Trans-Tek and getting a tour of their facility we

discussed and reviewed the specifications they expected of us. We spent a lot of time considering the

specifications in order to fully understand what the design was required to do.

In the last week of September, we began to research possible solutions and started completed our

project statement. Into the first weeks of October we continued to research and debate possible options to

implement in our design. We decided to order two microcontrollers and one digital signal processor

(DSP) to tamper with and see if we could meet the specifications. In the second week of October we

attained the schematic of a similar oscillator-demodulator design from Trans-Tek, which also drives an

LVDT. Through the remainder of October, we simulated and tried to decipher the workings of this unit, in

order to come to a better understanding of how to drive an LVDT. The circuit and its simulated results are

given below:

12

FIGURE 10 TRANS-TEK OSCILLATOR CIRCUIT

FIGURE 11 OSCILLATOR OUTPUT TO LVDT

13

FIGURE 12 TRANS-TEK DEMODULATOR CIRCUIT

FIGURE 13 DEMODULATOR OUTPUT

14

We built prototypes for both approaches and strenuously tested them throughout the month of

november. From the testing results, we decided to move forward with the DAC and filtering approach for

our design.

Figure 14 Projected Timeline for Senior Design II

The team will continue to test the oscillator circuitry and begin designing the demodulator over

the winter break. We intend to have a working prototype by mid February and will then begin designing

the PCB layout, which puts us ahead of schedule. This means that if obstacles arise and we need to re-

design some aspects of the system, PCB layout design can be pushed back.

Budget:

Trans-Tek has graciously offered to cover all of our R&D expenses throughout this project. This

includes any supplies we need for our design such as microprocessors, digital signal processing (DSP)

chips, and other circuitry components including resistors, capacitors and op-amps. We anticipate the most

expensive parts of our design to be the processors and PCB manufacturing. Even though a specific

number has not been specified for this project, we are working closely to make sure that we are getting

the best performance to cost ratio that we can with our components. This includes evaluating smaller and

cheaper microcontrollers to complete our design rather than the bigger more expensive DSP chips if

possible. We are also looking at different PCB manufactures to minimize the cost of developing prototype

PCBs where we do not need the highest quality possible since issues can arise that might impact the

layout of the PCB. We are also minimizing the amount of circuitry needed for our design and trying to

15

utilize the advanced software techniques that have come out over the recent years that allows us to do

most of the signal conditioning through software rather than dedicated analog hardware.

Collaborators:

Manuel Medeiros:

Education:

University of Connecticut, Storrs, CT

Bachelor of Science in Engineering: Major in Computer Engineering; Major in Electrical Engineering

Bachelor of Arts: Major in German Studies; Minor in Mathematics

Graduation Expected December 2017

Experience:

Transtek Incorporated, Ellington, CT

Intern, Summer 2014-Present

UTC Aerospace Systems, Windsor Locks, CT

Intern, Summer 2013

MAHLE Behr GmbH & Co. KG, Stuttgart, Germany

Intern, March 2015-August 2015

Connor Bailey:

Education:

Bachelor of Science in Engineering: Major in Electrical Engineering, Minor in Mathematics

Expected Graduation: May 2016

Experience:

BAE Systems, Nashua NH

Intern, Summer 2015

Aaron Ciardullo:

Education:

University of Connecticut

16

Bachelor of Science in Engineering: Major in Electrical Engineering

Anticipated Graduation Date May 2016

Experience:

General Dynamics Electric Boat, New London, CT

June 2015 – August 2015

APEDL, Storrs, CT

August 2014 – December 2014

General Dynamics Electric Boat, New London, CT

July 2014 – August 2014

Experis at Carbonite, Cambridge, MA

July 2013 – August 2013