Active Sensors

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The Active Sensor ProjectEE271: Electronics and Electrical Techniques and Design 2

Jeswin Mathew Louise Morran Richard Morrison Ewan Moyes

1:1:1:1

Group EME 7

Completed under the supervision of Prof G. Stewart

Date Completed: 20/1/2011


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Declaration

We declare that the contents of this report are our own work under the supervision of Prof George Stewart

Signed Date

................................ Richard Morrison ......................................

................................. Jeswin Mathew .......................................

................................ Louise Morran ..........................................

.................................. Ewan Moyes ............................................

Abstract

The primary objective of this team project was to design and construct an electronic hardware to

measure the thickness of a material when a sensor was placed on the surface; the material focussed on primarily

was mild steel and the measurement circuitry was configured accordingly. In order to make distance

measurements, firing ultrasound waves through the top surface and measuring the time taken to retrieve the

pulses was found to be a viable solution and using the velocities of sound in different materials, the thicknesswas measured. The project was undertaken by a team of four over a five week period: each member

contributing to different parts of the circuitry that performed different tasks.


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Table of Contents

Declaration 1

Table of Contents 2

Introduction 3

Transmitter 4

Generation of a pulse of a given width 4

Increasing the amplitude of the signal 6

Receiver 7

Amplifier

Receiver 8

Comparator 9

Building the amplifier 9

Building the rectifier 10

Final Design 11

Timing and Control 12

Programmable counter 12

Distance measurement circuitry 13

Operation of the circuitry 14

Counter module 15

Testing and analysis 16

Conclusion 18

References 19


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Introduction

Ultrasound technology is widely employed in science and engineering particularly in the non-destructive

evaluation (NDE) of engineering structures and also for imaging in both technology and medicine. In the NDEinstrumentation, the sensors can operate in a variety of configurations: The pulse echo and the dual element. In

the former configuration a single transducer transmits and receives the ultrasound signals and the latter

constitutes a single transducer that only transmits the signal into the object but the instrumentation harbours a

separate device to receive the signal. The configuration used in this project was pulse echo.

The objective of this project was to design, construct and test a circuit that can fire ultrasound energy

pulse into a block of mild steel metal and derive the thickness of the block by using a circuitry that can translate

the time taken for the pulse to return into a numerical distance value displayed on 7-segment LED displays. The

entire circuit consisted of three parts contributing different functionalities to the complete circuit: The

Transmission, the receiver and the distance measurement; these circuits are discussed in detail in the latter part

of the report. The paramount goal was to design a simple NDE hardware that will be able to detect cracks in mildsteel; a conceptual representation of the desired functionality of the hardware is portrayed below in Figure 1.

As mentioned before. The hardware consists of three circuits linked together:

1. The transmitter

2. The Receiver

3. The timing and control

The objective of the transmitter was to fire the ultrasound energy, the receiver was to receive the partial

(most of the energy is dissipated) echo of the pulse, process it and supply to the timing and control. The timing

and control uses the signals from the transmitter and receiver and converts it into a measure of distance

(thickness).

Hardware

Value of the thickness till

the crack in mm

Pulse echo

transducer

Figure 1. Desired functionality of the hardware


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Transmitter

Generation of a pulse of a given width

The transmitter was designed in a way which could produce a signal that can be sent through the steel,

giving time for the reverberating energy to completely decay before the next pulse was to be sent. It also had to

be able to deliver certain parts of the signal to the clock timer and the receiver, in order to synchronise the whole

circuit. The transmitter was also required to produce a pulse with a chosen period and frequency, produce a

positive clock edge pulse from this wave and finally invert and amplify the signal. The system reset output, upon

leaving the circuit, was sent through to the reset input of the counter module, thus resetting the counter module

so that it can register the new value of the thickness.

The pulse repetition frequency (PRF) must lie between 100 Hz and 1000 Hz1. An astable multivibrator

was used to produce a pulse of a chosen frequency and pulse width and a Schmitt trigger oscillator circuit toproduce the specific pulse width on positive clock edges. In order produce the pulse at greater amplitudes, a

switching circuit had to be used. The circuit was built used a switching field effect transistor to switch on the

circuit every time a pulse was transmitted.

Through research it was found that the speed of sound through steel is approximately 5900m/s2. A 1ms

delay was required1 in the astable multivibrator in order for the previous pulse to die down before the next one is

sent. This means that the period of the pulse produced must be this length of time as shown below in Figure 2.

The relationship between frequency and period is f = 1/P= 1 kHz. Since this is known, the desired

period of the square wave was obtained by setting up the multivibrator to produce a signal at around 1 kHz.

Figure 3 shows the layout of the multivibrator, assembled using a HEF40106 chip.

Figure 2. Period of pulse

Figure 3. Layout of multivibrator


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The value of R and C were investigated, and the results are compounded in Table 1.

Resistor, R () Capacitor, C (nF) Frequency (Hz)

1,000,000 1.5 582

820,000 1.5 704

560,000 1.5 1090

The circuit was designed to give a monostable pulse on every positive clock edge. Figure 4 shows the

circuitry used to produce a pulse on every clock edge using the 74121 chip.

Using the oscilloscope, the output monostable pulse was monitored. The graph output is shown below

in Figure 5.

Table1. Results of multivibrator

Figure 4. Schmitt trigger one-shot

Figure 5. Voltage Graph


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The monostable circuit was connected through the output shown in Figure 3, it was noted that the

frequency dropped to 850 Hz; however this frequency is more suitable as it lies between the recommended 100-

1000Hz1. The measure and pause function on the oscilloscope were used to accurately measure the pulse

width, which was found to be 1 s.

When the output voltage from the 74LS00 NAND gate chip was measured, a small voltage of 900mVwas found to be the amplitude of the signal. It was then observed that the chip was not supplied, and by doing

this the amplitude was increased to 3.92v: the pulse width was then measured to be 4.02s.

Increasing amplitude of the signal

In this part of the circuit a BS108 transistor chip was used, as shown in Figure 6, which has a threshold

value of 0.5v, and a maximum switching value of 1.5v. This means the amplitude of the signal entering the base

of the transistor must deliver a pulse to the value of the amplitude within this region.

Figure 6. Amplifier circuitry


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Receiver Circuit

The role of the receiver is to supply the appropriate information needed for this end result to be reached. To

achieve this it must take all the reflected ultrasound pulses and convert them into a TTL pulse (Transistor-

Transistor Logic pulse) which can then be processed by further circuitry to calculate the distance travelled by thewaves and ultimately display these results on the LED display. The proposed circuit is shown in Figure 71.

Figure 6 Suggested design of receiver circuit

Amplifier

Figure 8 -The amplifier circuit of the receiver

Figure 8 shows, the AD797 chip would be used to implement the amplifier circuit. The purpose of the

amplifier is to take an input voltage of fairly small amplitude and multiply or increase this voltage by a desired

magnitude. This magnitude can be varied to the correct size by changing the two resistors in the circuit. This is

because the closed loop gain (measure of the ratio of V out to Vin) on the amplifier is equal to the sum of the

resistors divided by the resistance of resistor R21 as shown in Equation 1.


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Receiver

Figure 9 - The rectifier circuit of the receiver.

For this part of the circuit, the TL081 chip was used. The ultrasound pulse given into the receiver circuit

will be that of a sine wave; the purpose of the rectifier is, as its name suggests, to rectify this wave. This means

that it must take the negative parts of the sine wave and invert them to the positive Y axis. This is achieved by

connecting the chip to two resistors and an OA47 diode as shown in Figure 9. The effects of the rectifier are

seen in Graph 1 and Graph 2 below:

Graph 1 - Sine wave Graph 2 - Rectified wave

Equation 1. Closed loop gain


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Comparator

Figure 10 - Comparator circuit of the receiver

The LM311 chip was used for this stage of the circuit. The purpose of the comparator, shown in Figure

10, was to take the new rectified wave and produce it in the form of a TTL pulse (Transistor-Transistor Logic

pulse). The reason for this is because the end stage of the full circuit is an LED display which (from digital

electronics) we know must have a signal with a sharp edge in order to trigger a clock pulse. Graph 3 shows the

effect of the comparator on the wave.

Graph 3- effects of comparator

Construction of the Circuit

Building the Amplifier Stage

It was found that the circuit should have an input voltage of approximately 5mV with a required output

voltage of approximately 5V1. By using Equation 1, the gain of the circuit was found to be 1000 at these values:


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Given this ratio of 1000, R1 was thought then to be approximately 1000 times greater than R2,

suggesting values of perhaps 1 and 1K. This would be perfect in theory, however inefficiencies within the

circuit components had to be taken into consideration at later stages it was found that the input must be changed

from 5mV to 500mV which took the previously calculated ratio from 1000 to 10. This was not a hugely

problematic issue as it simply meant R1was given a value of 10K while a resistor value of 1k was used for

resistor R2. The completed circuit is shown in Figure 11. As required, when tested, the output voltage wasindeed found to be 5V when the input RX was a 500mV sine wave.

Figure 11 - Our completed amplifier circuit

Building the Rectifier

This stage of circuit proved to be very demanding. Taking the ratio of resistors to remain the same for the

rectifier as the amplifier, it was found the design of the circuit should have been as shown in figure 12.Unfortunately on testing this design, a sine wave was continuously found at the output and although many

various solutions were tried and tested this problem was unable to be resolved. Time restraints played a large

factor in this problem as adverse weather conditions meant I was unable to have the necessary time in the

laboratory to appropriately investigate the issue.

Figure 12 - Proposed Rectifier circuit


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Final Design

Using some research and advice from the laboratory teaching assistants a different, slightly simpler,

receiver circuit was derived shown below in Figure 13.

Figure 13 - Final receiver circuit design

This simplified design still uses the basic properties of the receiver circuit and uses the AD797 chip for

the amplifier and LM311 chip for the comparator as before. In place of the TL081 chip however a simple OA47

diode was used. This newly derived circuit was fully tested and found to produce the desired TTL pulse from a

sine wave. The design could not be built to suit exact specification however as when using the input voltage of

500mV, the input frequency had to be altered to 1 KHz which meant that technically the wave was not an

ultrasound wave, even though the receiver circuit did serve its main purpose. The results produced when testingthe circuit are very similar to those which were expected only with differing amplitudes and frequencies.


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The Timing and Control

The timing and control incorporates inputs from the receiver, transmitter and the counter module, and is the

most vital part of the circuit as it translates the signals received from the transmitter and the receiver into a

magnitude of distance on the LED displays: once the transmitter sends a pulse to the timing and control, itbegins to count at a clock rate equivalent to the velocity of sound in the relevant materials (the reason will be

discussed later) and terminates the counting once the processed pulse from the receiver is retrieved and the

counter module converts the binary code to decimal and displays it on the LED display. This circuit consisted of

three different units performing different tasks:

1. The Programmable counter

2. Distance Measurement circuit.

3. The Counter Module

The Programmable Counter

The programmable circuit shown in Figure 14 was assembled, and used in accordance with the

distance measurement circuit. It was this set up that processed the time, and converted it into a distance. It was

made up of a crystal oscillator, in which the output was divided to produce two synchronous waveforms which

are rated to both the sound in mild steel, and in water.

Time was converted to frequency using Equation 2. This was derived from the pulse width being equal

to the distance travelled, divided by the velocity. The minimum distance to be travelled was given as 1mm, this

is half the given value in order to take the echo into consideration, and the velocity of this in steel is 5900 m/s.

This brought us to a pulse width of 1.675x10 -7. The inverse of this was then taken, which gave the frequency of

5.9 MHz; for every second the pulse travelled, the counter module counted up to 3x10 6 mm.

The distance was divided by two, as this eradicates the time period for the echo to return. It was due to

this reasoning that the frequency was divided by two.

n/c +5v+5v

Oo Steel

OW 1/8 Water

HEF 40406 MHz

Oscillator

Module

Figure 14. Programmable counter

Equation 2. Pulse Width


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Distance Measurement Circuit

The distance measurement unit consists of 7473 Dual J.K flip flops IC and a SN74121 IC, a mono

stable device known as the one-shot: it receives a HIGH trigger and outputs a conditioned pulse with a

wavelength that can be configured by manipulating its analogue components. These chips and their functions in

the circuit is discussed in the succeeding paragraphs. The circuit also employs the 74LS00 quad NAND IC for avery insignificant purpose compared to the whole circuitry.

The 7472 Dual J.K IC has two negative edge triggered J.K flip flops, with a clear input, embedded into

the IC. The chip architecture for the aforementioned IC is shown below. It also has a Vcc and a ground input like

any other IC chip. Although two J.Ks were available in the chip only one was required for the circuitry; the J.K

incorporating the inputs from pins 1,2,3,12,13 &14 was used. The clock input was supplied from the

programmable counter, the circuit Figure 15.

As mentioned earlier the 74121 IC is a mono stable device i.e. it only has a single stable state and will

transition to its unstable state when a trigger is applied to it5. Once its been triggered the one shot stays in its

unstable state for a definite period of time and this duration determines the output pulse width, is dependent on

the time constant RC; the trigger(Pin 5) is supplied by the receiver circuit. The one shot used in this circuitry has

a special kind of trigger input called the Schmitt-Trigger; this prevents any erratic switching between states due

to a slow changing voltage level at the trigger3. The chip architecture of the 74121 IC is shown below. The pulsewidth of the output is manipulated by altering the Rext and Cext and the formula to calculate the pulse width for this

kind of one shot is shown below in Equation 33.

But the pulse width was not calculated in this project because a graph of Pulse Width vs Resistancewas given. The value pulse width aimed for was 1s and the corresponding resistor and capacitor values chosen

were 15 K and 100 pF1. The chip architecture of the 74121 is shown in Figure 16.

Figure 15. Chip Architecture of the 7472 Dual JK

Equation 3. Pulse Width from the 74121 IC


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The complete circuitry of the distance measurement with all the connections is shown in Figure 17

below. Pin 6 of the SN74121 is the output pulse given by the one shot; Pin 12 from the 7473 is connected to the

enable of the Counter Module and pin 1 is connected to the programmable counter. Also the output of the

receiver is inputted into the trigger input of the 74121 IC.

The Operation of the Circuitry

1. When the transmitter fires an ultrasound pulse into the object, it also sends a trigger into the J input of

the multi-vibrator in the circuitry. This sets the J.K i.e. Q output goes to logic one and the connected

counter enable input also transitions to logic one. At the same time as this it also renders a logic one at

the reset input of the counter module.

2. The counter enable starts counting at the frequency of the same clock supplied to the distance

measurement circuitry and records the thickness of the object.

3. Once the ultrasound is echoed back, the receiver circuitry converts the ultrasounds analog signal to aTTL pulse of a certain pulse width. This TTL signal is supplied into the Schmitt-trigger one shot,rendering an output pulse of 1s.

4. The output is fed into the K input of the J.K, the logic 1 at the K input resets the system i.e. Q goes LOW

and it disenables the counter module.

Figure 16. 74121 IC4

Figure 17. Distance Measurement Circuitry


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5. The same output from the 74121 IC is supplied to the latch of the counter module but with a delay usingtwo NANDs from the 74LS00 quad NAND IC; the reason for this delay was to allow the counter moduleto register the counted value and display the value on the LEDs before latching the information.

6. The cycle reiterates itself with the transmission circuitry again firing ultra sound energy into the object.

The Counter module

The operation of the counter module is synchronised by the distance measurement circuitry shown in

Figure 18. It consists of a reset, enable, clock and a latch input. The 4518 ICs are 4 bit synchronised 4-bit

counters controlled by the same clock input. This module was already supplied prior to the beginning of the

project and therefore was not constructed on a breadboard.

Operation of the Module

The reset input was supplied from the System Reset output of the transmitter circuitry; the circuit sends

a logic one into the reset at the same time it is firing the ultrasonic energy into the object: a logic 1 at the reset

will clear all the counters and any numbers displayed on the LEDs will transition to zero and thus will be able to

record the new value of the thickness after the pulse that was fired earlier is retrieved.

A HIGH at the enable input will permit the counter module to start counting and the clock input

determines the frequency at which the module is counting; the frequency of the clock is the same as the clock in

the measurement circuit.

Figure 18. Counter Module


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Once the processed signal from the receiver was retrieved by the distance measurement circuitry the

74121 IC further manipulated the pulse width and send it to the K input; the J.K resets making the enable to a

logic 0. The same input that was wired to the K was also wired to the latch but with a propagation delay of

around 20ns with the assistance of the NAND gates as seen in the circuit diagram; the reason for this was to

permit the counter module to register the counted value and display the value on the LEDs before latching the

information.

Analysis and Testing

The Clock Input

After the construction of the circuit, it was necessary to test the functionality of the timing and control

circuitry. This was done was testing each input and output separately. Firstly the clock input was tested and the

timing diagram obtained from the oscilloscope is shown below in Figure 19.

The graph indicates a clock rate of 3.03MHz; which is the required frequency to facilitate accurate

measurement in steel. The calculations employed to determine the value of the required frequency were

discussed earlier.

Figure 19. Clock Input


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The Counter Enable Input and the Firing pulse

The upper timing diagram, Figure 20, represents the input of the firing pulse into the J input of the flip

flop. Once the transmitter fires a pulse into the object it also sends a trigger into the J input. This sets the Q

output to HIGH which subsequently sets counter enable to a HIGH until the conditioned receiving pulse is

applied to the K input of the multi-vibrator, at which point it transitions back to LOW and the counter terminates

the counting. This is depicted in the lower timing diagram in Figure 20.

The Receiving and Firing pulse Inputs

The timing diagrams below shows the inputs from the firing pulse and the 74121 IC i.e. the conditioned

receiver output: the upper and the lower diagrams. The time difference between them is the duration of time the

enable stays HIGH as seen in the previous timing diagram shown in Figure 21.

Figure 20. Firing Pulse and Enable

Figure 21. Transmitter Pulse and 74121 IC Pulse


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The Receiver Pulse and the Latch

The input to the Latch is the same as the K-input but with a delay in the signal. The upper timing

diagram shows the pulse from the 74121 IC and the lower diagram is the voltage level taking at the node after

the first NAND gate (pin 6 of the Quad NAND); as predicted the output is inverted and the propagation delay

between the output and input is evident. This signal is inverted back to its original state by another NAND gatebefore supplying it into the latch input, thus doubling the propagation delay measured in the diagram below,

Figure 22.

The testing indicated that the circuitry was functioning as the designs intend and it was concluded that

this hardware will provide an accurate value of the thickness of the object.

Conclusion

The laboratory project was overall a success. It helped the group to facilitate the necessary skills such

as teamwork, technical thinking and sourcing information, which are all extremely paramount for future

endeavours.

Unfortunately the design brief given in the notes was unable to be carried out specifically due to a few

contributing factors, the main of which being time constraints. However the circuit was successfully designed to

give a needed TTL wave from an input sine wave but given that the input wave did not qualify as an ultrasound

wave it would unfortunately not work when connected up.

If the laboratory was to be repeated, as a team, improvements could be made with regards to

timekeeping. This could be improved by setting dates in the first week of the laboratory, in which each group

member has a target to achieve, thus giving us more time to analyse the overall outcome of the project, and

each individual part.

Figure 22. Pulse from 74121 IC, and the Output of Pin 6


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References

[1] Electric and Electronic techniques and design course notes.

[2] http://www.library.thinkquest.org/19537/physics4.html, date accessed: 17/1/11

[3] Digital Fundamentals: Pearson Education; 9 edition (1 May 2005) Floyd

[4] - http://www.ti.com/ , Date accessed: 21/1/11

[5] - http://www.national.com/analog, Date accessed: 21/1/11
http://www.national.com/analoghttp://www.national.com/analog

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