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Digital Caliper Design Utilizing an

Optical Distance Measuring Sensor

Calvin A. Sessions Christopher Wittmier

Jonathan Law Instrumentation Design Team Project

December 1, 2004

Western Washington University Electronics Engineering Technology

ETEC 377, Professor Grady

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1

INTRODUCTION

Overall Objective

The overall objective of this instrumentation team project was to gain

research and design experience while further developing and enhancing

cooperative teamwork skills. In the span of two months, this project allowed our

members to step through the actual design process from conception and design,

to prototype development.

Project Purpose

In particular, our instrumentation design team developed a digital caliper

utilizing an optical distance measuring sensor. This method was chosen as a

unique and interesting technique to process information measured by a caliper.

The function of the digital caliper was designed to be able to optically measure

the dimensions of small objects, electronically eliminating human error

generated from analog/mechanical calipers or other measuring tools. It was

decided that the design had to be portable, inexpensive, and perform with

accurate precision. Its use would be best suited for a laboratory environment;

therefore it was appropriate that our design utilized standard SI units: 0mm –

99.9mm range.

FUNCTIONAL DESCRIPTION

As shown in the system block diagram in Figure 1, there were four main

procedures conceptualized in the digital caliper design. First, a physical process

had to be changed into some type of electronic signal. In this project a

transducer was implemented to convert the physical process into a voltage.

Then the analog signal obtained from the transducer output had to be

conditioned and scaled through signal processing circuitry. The processed

analog signal was then digitized with an A/D converter, and finally displayed.

Figure 1: Digital Caliper System Block Diagram

The Sharp GP2D12 distance measuring sensor was implemented as the

transducer in the electronic design. Characteristics of this particular sensor will

be discussed in great detail later in this document. Because the output produced

from the GP2D12 is an analog, nonlinear voltage, the last three stages of the

system block diagram shown in Figure 1 required the most attention.

2

3

Detailed Hardware Description

Figure 2 displays a basic sketch of the digital caliper design. A standard

vernier caliper was mounted on the chassis housing the transducer and signal

conditioning circuitry. The physical process for this system mentioned in Figure

1 is the variable distance adjustment of the movable caliper arm. As depicted in

the diagram, the other caliper arm is mounted and serves as a reference.

Figure 2: Digital Caliper Design Sketch

The distance measuring sensor emits a beam of light in which reflects off of a

surface connected to the adjustable arm of the vernier caliper and is processed by an

onboard photodetector. Upon signal conditioning, the distance information is digitized

and displayed on three seven segment LED modules in millimeters. A 9-volt battery

powers the entire system.

4

Distance Measuring Sensor

Figure 3 displays images and dimensions of the Sharp GP2D12 distance

measuring sensor. This device contains both an infrared light source and a

photodetector.

Figure 3: Sharp GP2D12 Images Figure 4: GP2D12 Measuring Function

According to the Robotic Society of Southern California (RSSC) website,

light is emitted from the infrared LED through a lens which focuses the beam on

a small spot of the measured object. The light then reflects off of the object to the

position sensitive device (PSD), as shown in Figure 4. This is referred to as the

optical triangulation principal. Also stated on the RSSC website, as the distance

to the sensed object changes, the spot of light moves on the position-sensitive

detector, and a different distance is reported.

5

Note in the internal circuitry diagram in Figure 5 that the GP2D12 has

three connections: ground, 5 volt supply, and an analog output. This analog

output is a nonlinear curve as displayed in Figure 6. Also note in the graph that

the reflectivity is highest when a white surface is implemented; therefore white

paper was used to cover the reflective surface on the adjustable caliper arm.

These figures were taken from the sensor data sheet notes appended to this

document. Because the workable range of the GP2D12 is 10cm to 80cm, an offset

greater than 10cm was created, as shown in the digital caliper sketch in Figure 2.

Figure 5: GP2D12 Internal Circuitry Figure 6: GP2D12 Analog Output Vs. Distance

6

Signal Processing

The output of the analog distance measuring sensor signal resembles a

decaying logarithmic function. To correct this nonlinear characteristic, a

logarithmic amplifier was constructed a stage after the transducer. This was

performed by placing a diode in the feedback of an operational amplifier.

Though the output signal was more linear, a decaying output with respect

to the increasing distance was present. Therefore a summing amplifier was

constructed. This summing amplifier consisted of amplifying the negative

linearized output and adding it to an offset voltage. The output of the summing

amplifier was designed to yield a range from 0 volts for 0mm to .100 volts for

100mm.

Because the output of the summing amplifier yielded a negative voltage,

an inverting amplifier with a gain of 10 was implemented. This gain was

necessary because the A/D converter and output circuitry required a 0 to 1.0 volt

working range. Figure 7 depicts a general block diagram of the signal

conditioning procedures.

Figure 7: Signal Processing Block Diagram

7

A/D Conversion and Output Display

Range: 0mm-99.9mm Units: milimters

Figure 8: 7 Segment LED Module Display

Having previously linearized and scaled the input voltage, the function of

the A/D section was essentially that of a voltmeter. Intersil’s CA3162E dual slope

A/D converter was chosen for its appropriate range and precision, featuring

three onboard multiplexing lines designed specifically for use in three-digit

displays. These lines were used to enable three 7-segment LED modules, as

pictured above, driven sequentially by a single 74LS247 BCD - Seven segment

display driver. PNP transistors were used as multiplexing switches, and the

74LS247’s open collector outputs were pulled up with a resistor network. The

converter operates at 4Hz, using the built in timing of the CA3162E in low speed-

mode. Although the display’s tenth of a millimeter resolution is somewhat

useless given the overall caliper error of over one millimeter, this provides for

later improvement of transducer linearization, were more time allotted.

8

PROTOTYPE DEVELOPMENT

Distance Measuring Sensor

Upon receiving the Sharp GP2D12 distance measuring sensor, the device

was tested and analyzed to inspect its functionality and for general planning

purposes. A test fixture was first constructed, as shown in Figure 9. The sensor

was mounted on a metal plate while an additional metal plate covered with

white paper was set at variable distances marked on a piece of plywood.

Figure 9: GP2D12 Test Fixture Setup

The following data displays sensor output voltage characteristics with respect to the variable distance. The following page displays a graphical representation of Table 1.

Table 1: Distance Vs. Voltage Out

Distance (cm) Voltage Out (V) Distance (cm) Voltage Out (V) 0 0.029 16 1.710 1 0.702 17 1.609 2 1.18 18 1.526 3 1.145 19 1.470 4 1.805 20 1.392 5 1.847 21 1.317 6 2.01 22 1.268 7 2.577 23 1.223 8 2.751 24 1.166 9 2.66 25 1.129

10 2.508 26 1.091 11 2.334 27 1.052 12 2.176 28 1.014 13 2.033 29 0.975 14 1.904 30 0.956 15 1.795

9

Graph 1: Distance Vs. Voltage Out

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 300

0.5

1

1.5

2

2.5

3Distance vs. Voltage Out

Distance (cm)

Vol

tage

Out

(V)

Vo

distance

Signal Processing and Power Supply:

The following table displays the sensor output using a logarithmic

amplifer with respect to a varying distance:

Table 2: Distance Vs. Voltage Out – w/ log amp

Distance Voltage Out Distance Voltage Out 8 0.653 20 0.626 9 0.653 21 0.625

10 0.651 22 0.623 11 0.648 23 0.621 12 0.645 24 0.619 13 0.643 25 0.617 14 0.638 26 0.615 15 0.636 27 0.614 16 0.635 28 0.612 17 0.633 29 0.611 18 0.631 30 0.610 19 0.629

10

Graph 2: Distance Vs. Voltage Out w/ Log Amp

8 9 10 11 12 13 14 15 16 17 18 19 20 21 221.29

1.28

1.27

1.26

1.25

1.24

1.23

1.22Distance vs. Voltage Out

Distance (cm)

Vol

tage

Out

(V)

V−

distance

The output of the sensor signal using a logarithmic amplifier was

recorded and graphed with respect to the varying distance. As shown in the

graph, the curve produced was not completely linear. Using the line of best fit

for the signal conditioning was the only reasonable option without implementing

a computer or microcontroller in the design. More about this issue will be

further discussed in the Discussion Section of this document.

11

+3

-2

V+

4V

-11

OUT 1

U1A

TL084

+5

-6

V+4

V-

11

OUT7

U1B

TL084

+10

-9

V+

4V-

11

OUT8

U1C

TL084

D1D1N4148

00

0

0

IN1

OUT2

GN

D3

U2

Sharp GP2D12

R1

1k R2

4.99k

R3

20k

R4

20k

R51.02k

R61k

R7

2k

R8

20kR9

150

R10150

C110u

-5V

-5V

-5V

+5V

+5V

+5V

+5V

+5V

0

C210u

0

Signal Conditioning

IN1

OUT2

GN

D3

U4 GL7805

-+

+-

U5

TC7662A

C610u

C710u

0

C810u

0

0

V29Vdc

0

-5V

+5V

Charge Pump

Voltage RegulatorPower Supply

The following schematic diagram and signal conditioning equations were

implemented, shown in Figure 2. This circuit was described in the Detailed

Hardware Description. The transducer, signal processing circuitry, and power

supply circuitry was soldered onto a single protoboard. The power supply circuitry

is also displayed below.

Schematic 1: Signal Conditioning Schematic

Summing Amplifier/Signal Conditioning Calculations Distance (cm) Sensor Output w/ Log Amp (volts)

0 cm - 0.626 volts 10 cm - 0.600 volts

Equation 1: 0 = - 0.626 m + Voffset Equation 2: 0.1 = -0.600 m + Voffset m = 3.84615 GAIN Voffset = 2.40769 volts

Schematic 2: Power Supply Schematic

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Image 1: Power Supply and Signal Processing Circuitry

Image 2: Power Supply and Signal Processing Assembly

13

A/D Conversion and Output Display

Illustrated in the schematic below, the complexity of the A/D converter and

display design was greatly reduced by the CA3162E’s built-in features. Initially, an

AD0804 converter was considered which would have provided an 8-bit binary output.

This converter was dropped, as 3 cascaded ‘185 decoders (or a PLD) would have been

required to produce a displayable decimal output. In determining conversion speed, the

CA3162E offered two built in speed modes. High speed mode (96Hz) would have

produced unreadable display flicker, leaving 4Hz low-speed mode. Ideally, an even

lower speed would be used, as there is still some degree of annoying flicker. To achieve

this, a sample-holding pulse circuit was designed and constructed using a 555 timer.

Although this pulse provided the variable sampling intended, the timing could not be

adjusted much below 4Hz before appearing very sluggish. Thus, the circuit was excluded

in the final prototype as the minor benefits provided did not seem to justify the additional

hardware.

Schematic 3: ADC and Digital Output Schematic

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Image 3: ADC and Digital Output

Caliper and Housing Fixture

For the prototype, scrap metal found in the Electronics Engineering Technology

Research Laboratory in the Ross Engineering Technology building was used to construct

the digital caliper housing. The vernier caliper purchased from Sears was mounted firmly

onto the housing as described earlier in the Hardware Description. The overall outcome

of our working prototype met specifications previously defined in our Project Plan

Document.

Image 4: Final Prototype

15

Image 5: Final Prototype

DISCUSSION

Given the small amount of time to design, test, and construct the final

instrumentation prototype, we as a group are satisfied with the outcome our project.

However, there are a significant amount of aspects in the design project that can be

improved or even performed differently.

The final prototype display flickers between a few millimeters due to noise issues.

From our observations, the GP2D12 generates a noisy analog signal, therefore a 10uF

capacitor was placed in between the output signal and ground. The noise that does not

get filtered becomes amplified after the various stages of operational amplifiers. Two

other 10uF capacitors were utilized to filter the final analog output signal in the signal

conditioning stage and in between the 5 volt and ground buses. It was also observed that

more noise is introduced as the 9 volt battery drains. From this, we noted that our design

consumes a lot of power.

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A large issue that determines the accuracy of the digital caliper is the linearization

technique used to condition the sensor signal. As shown in the Prototype Development

Section of this document, the output signal of the sensor after 10 cm is not exactly

logarithmic. It was later discovered in the GP2D12 Application Notes that the output is

more like a 1/X function. We attempted to create a dividing amplifier utilizing two

logarithmic amplifiers, a difference amplifier, followed by an antilog amplifier, however

this process failed. Perhaps if this issue came to our attention sooner, a dividing

amplifier IC could have been ordered.

Another technique that could have corrected the linearization problem was

implementing a microcontroller or computer. A look-up table would be created to match

the correct distance output with the sensor signal. This idea was first discussed during

the project conceptualization phase, but it prevented our team from learning about the

various stages involved in analog signal processing and digitizing the output.

This was our first attempt to design and build a working electronic

instrumentation project on a design team. Further developing our teamwork skills in a

cooperative environment has better prepared us for industry.

17

PROTOTYPE BILL OF MATERIALS

Description Part # Source Price ($) Quantitiy 9V Battery 9VDC Haggen 4.95 1 5V Regulator GL7805 lab free 1 Charge Pump TC7662A lab free 1 Capacitor C lab free 7 Resistor R lab free 14 Rheostat R_var lab free 4 Range Finder GP2D12 Junun.org 8.25 1 Quad JFET Op-Amp TL084CN lab free 1 diode 1N4148 lab free 1 ADC for 3-Digit Display CA3162 lab free 1 BCD to Seven Segment 74LS247 lab free 1 Seven Segment Display 7540 lab free 3 PNP Transistor 2N3906 lab free 3 Vernier Caliper - Sears 9.99 3