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National University of Singapore Department of Electrical and Computer Engineering

MCH5002 Applications of Mechatronics

E2: INDUSTRIAL SENSORS 1. OBJECTIVES

This is a hands-on session to explore the operational principles and characteristics of common industrial sensors. To benefit more fully from this session, students should read the manual before going to the laboratory.

2. APPARATUS

A single panel Transducer and Instrumentation Trainer, the DIGIAC 1750 will be used. The unit provides examples of a full range of sensors and actuators, signal conditioning circuits and display devices. The unit is self-contained and enables the characteristics of many individual sensors to be investigated, building to form complete closed-loop systems. A layout diagram of the DIGIAC 1750 unit is shown below in Figure 1.

Figure 1: Layout of the DIGIAC 1750

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3. TEMPERATURE SENSORS

In this part, we will look at a platinum RTD resistance transducer. The construction of the transducer is shown in Figure 2. It consists of:

• A thin film of platinum deposited on a ceramic substrate • Gold contact plates at either end

Figure 2 : RTD Resistance Transducer 3.1 Operational Principles The operational principles are briefly given below:

• The platinum film is trimmed with a laser beam to cut a spiral for a resistance of 100Ω at 0oC.

• The resistance of the film increases with temperature. It has a positive temperature coefficient (p.t.c)

• The increase in resistance is linear, the relationship between resistance change and temperature rise being 0.385Ω/oC.

tRRt 385.00 +=

where Rt = resistance at temperature t oC, R0 =100Ω = resistance at temperature 0oC .

• Normally, the unit would be connected to a DC supply via a series resistor and the voltage developed across the transducer is measured, which represents the measurement temperature

.

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3.2 Characteristics

Figure 3: Connection for RTD experiment

• Set the slider of the 10kΩ carbon resistor to mid-way and connect the circuit as

shown in Figure 3, with the digital multi-meter set to its 200mV or 2V DC range. • Switch ON the power supply and adjust the slider control of the 10kΩ resistor so that

the voltage drop across the platinum RTD is 108mV as indicated by the digital multi-meter.

This calibrates the platinum RTD for an assumed ambient temperature of 20°C, since the resistance of the RTD at 20°C will be 108Ω. Note that the voltage reading across the RTD in mV is the same as the RTD resistance in Ω, since the current flowing must be 108/108=1mA. Note: If the ambient temperature differs from 20°C, the voltage can be set to the correct value for this ambient temperature if desired: 1. Set the voltmeter to its 20V range and measure the INT output from the IC

Temperature Sensor to obtain the ambient temperature in °K by multiply the output with 100 (°K = output of INT × 100), then °C=°K-273. This provides the reference temperature.

2. RTD resistance = 100 + 0.385 * °C. Set the voltage drop across the RTD for this value.

• Connect the +12V supply to the Heater Element input and note the values of the

voltage across the RTD with the voltmeter set to its 200mV (this representing the RTD resistance) and the output voltage from the IC Temperature Sensor with the voltmeter set to its 20V range, (this representing the temperature of the RTD) at the time set in Table 1.

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• Convert the two voltage readings to RTD Temperature (°K) and RTD Resistance (Ω)

and record the values in Table 1.

Table 1: Resistance-Temperature relationship

Time (minutes) 0 1 2 3 4 5 6 7 8 9 10 °K RTD

Temperature °C RTD Voltage (mV) RTD Resistance (Ω) IC Voltage (V)

• Convert the RTD Temperature into °C(°K-273) and add to Table 1. • Plot the graph of RTD resistance (Ω) against temperature (°C) on the axes provided

below (Graph 1). Extend your graph down to cover 0°C.

Graph 1

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3.3 Questions 1. Enter the total change in the resistance of the RTD Transducer over the temperature

range 20-50°C in Ω. 2. Is the resistance/temperature characteristic linear? 3. Enter your estimated (extrapolated) resistance of the RTD Transducer from the graph

at 0°C. 4. Calculate the power dissipation in the RTD Transducer at a temperature of 50°C

when the standard circuit current of 1mA flows in it: µW. Switch OFF the power supply.

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4. LIGHT SENSORS In this part, we will look at a photovoltaic and a photoconductive cell, and their applications to light measurement. 4.1 Photovoltaic Cell A photovoltaic cell is a two-layer silicon device which generates an EMF (electromagnetic force) when light falls onto it. The structure is shown in Figure 4.

Figure 4: Photovoltaic cell

4.1.1 Operational Principles The operational principles for a photovoltaic cell are briefly described below: • One of the PN regions is made very thin (1 micron). Light can easily pass through

this without much loss of energy. • When light reaches the depletion layer, it is absorbed and the released energy creates

electron-hole pairs which diffuse across the junction. • A voltage is set up and a current will flow if a resistance is connected across the

terminals. • When used as an energy source, they are known as solar cells.

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4.1.2 Characteristics

Figure 5: Connection for photovoltaic cell

• Connect the circuit as shown in Figure 5 with the digital multi-meter (ammeter) on

the 2mA range to measure the short circuit current between the Photovoltaic Cell output and the ground. Fit an opaque box over the clear plastic enclosure to exclude all ambient light.

• Switch ON the power supply and set the 10kΩ wire wound resistor to minimum for

zero output voltage from the power amplifier. • Take readings of Photovoltaic Cell short circuit output current as indicated on

the digital multi-meter as the lamp voltage is increased in 1V steps. Record the results in Table 2.

• Switch OFF the power supply, set the multi-meter as a voltmeter to read the open

circuit output voltage. Switch ON the power supply and repeat the readings, adding the results to Table 2.

Table 2: Current/Voltage-Light relationship

Lamp filament Voltage (volts)

0 1 2 3 4 5 6 7 8 9 10

Short Circuit Output Current

(µA)

Open Circuit Output Voltage

(V)

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• Plot the graphs of Photovoltaic Cell short circuit output current and open

circuit output voltage against the Lamp Filament voltage on the axes provided (Graph 2).

Graph 2

4.1.3 Questions 1. From the graph, estimate and enter the short circuit output current in µA when the

lamp filament voltage is 7.5V. 2. Are the graphs linear? Switch OFF the power supply.

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4.2 Photoconductive Cell The basic construction of a photoconductive cell is shown in Figure 6, consisting of a semiconductor disc base with a gold overlay pattern making contact with the semiconductor material.

Figure 6: Photoconductive cell

4.2.1 Operational Principles The operational principles of a photoconductive cell are briefly described below: • The resistance of the semiconductor material between the gold contacts reduces when

light falls on it as electron-hole pairs of charge carriers are created. • The change in resistance can be detected via an electrical circuit. • Another name for this device is Light Dependent Resistor (LDR).

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4.2.2 Characteristics

Figure 7: Connection for photoconductive cell

• Connect the circuit as shown in Figure 7 and set the 10kΩ carbon slider control to

setting 3 so that the Photoconductive Cell load resistance is approximately 3kΩ as shown in the equivalent circuit in Figure 8.

Figure 8: Equivalent circuit of the photoconductive cell connection

• Connect the digital multi-meter on the 20V DC range to measure the

Photoconductive Cell output voltage. Fit the opaque box over the clear plastic enclosure to exclude all ambient light.

• Switch ON the power supply and set the 10kΩ wirewound resistor to minimum for

zero output voltage from the power amplifier.

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• Take readings of Photoconductive Cell output voltage as indicated on the digital multi-meter as the lamp voltage is increased in 1V steps. Record the results in Table 3.

Plot the graph of Photoconductive Cell output voltage against the lamp filament voltage on the axes provided in Graph 3.

Table 3: Voltage-Light relationship

Lamp filament Voltage (volts)

0 1 2 3 4 5 6 7 8 9 10

Photoconductive Cell Output (V)

Graph 3 From the graph, estimate and enter the Lamp Filament voltage when the circuit output voltage is 3V. Switch OFF the power supply.

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5. LINEAR POSITION/FORCE SENSORS In this part, we will focus on a Linear Variable Differential Transformer (LVDT) and a strain gauge and investigate how they may be applied to linear position and force measurements respectively.

5.1 Linear Variable Differential Transformer (LVDT)

The construction and circuit arrangement of an LVDT are as shown in Figure 9. It consists of three coils mounted on a common former and having a magnetic core that is movable within the coils.

Figure 9: LVDT 5.1.1 Operational Principles The operational principles of a LVDT are briefly described below:

• Center coil is the primary and is supplied from an AC supply. The coils on either side are secondary coils and labeled A and B.

• Coils A and B have equal number of turns and are connected antiphase in series so that the output voltage is the difference between the voltages induced in the coils.

• Output voltage changes in amplitude which increases with the movement of the core from the neutral position to a maximum value. The phase changes according to the direction of movement.

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5.1.2 Characteristics

Figure 10: Connection for LVDT

In this exercise, you will measure the rectified output using the digital multi-meter on the 20V DC range and also amplify and measure it using the M.C. analog meter, as this gives a better impression of the variation of output voltage with core position. • Connect the circuit as shown in Figure 10 with the digital multi-meter on the 2V DC

range to monitor the output of the Full-Wave Rectifier. Switch ON the power supply.

• Set the A.C. Amplifier gain to 1000. • Set the GAIN COARSE control of Amplifier #1 to 100 and GAIN FINE control

to 0.2. Check that the OFFSET control is set for zero output with zero input and adjust if necessary.

• Adjust the core position by rotating the operating screw to the neutral position. This

will give minimum output voltage. Note the value of this voltage from the digital multi-meter and record in Table 4.

• Rotate the core control screw in steps of 1 turn for 4 turns in the clockwise direction

(when viewing the control from the left-hand side of the D1750 unit) and record your results in Table 4. Then turn the control screw in the counter clockwise direction, again recording the results in Table 4.

Table 4: Position-Output Voltage relationship

Core position (turns from neutral) -4 -3 -2 -1 0 1 2 3 4 Digital meter Output

Voltage (V) Analog meter

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• Plot the graph of output voltage from the analog meter readings against core position on the axes provided.

Graph 4

Switch OFF the power supply.

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5.2 Strain Gauge Transducer Figure 11 shows the construction of a strain gauge transducer, consisting of a grid of fine wire or semiconductor material bonded to a backing material.

Figure 11: Strain gauge transducer

5.2.1 Operational Principles The operational principles are briefly described below:

• The unit is attached to the beam under test and is arranged so that the variation in length under loaded conditions is along the gauge sensitive axis

• Loading the beam increases the length of the gauge wire and also reduces its cross-sectional area. Both of these effects will increase the resistance of the wire.

• The force on the beam can be detected through the change in resistance. 5.2.2 Characteristics

Figure 12: Connection for strain gauge transducer

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You will have 5 similar weights to increase the force loading on the strain gauge in regular steps. • Connect the circuit as shown in Figure 12 and set Amplifier #1 GAIN COARSE

control to 100. • Switch ON the power supply and with no load on the strain gauge platform, adjust the

offset control of Amplifier #1 so that the output voltage is zero. • Place all five of your weights on the load platform and adjust the GAIN FINE

control to give an output voltage of 7.0V as indicated on the moving coil meter. Note that this value of output value should cover all ranges of coins within the setting of the GAIN FINE control.

• Place one weight on the load platform and note the output voltage. Record the value

in Table 5. • Repeat the process, adding further weights one at a time, noting the output voltage at

each step and recording the values in Table 5.

Table 5: Loading(Force)-Voltage relationship

Number of Coins 0 1 2 3 4 5 Output Voltage(V)

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Plot the graph of output voltage against number of coins on the axes provided in Graph 5.

Graph 5 Enter the output voltage obtained with four coins on the platform.

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6. ROTATIONAL SPEED AND POSITION SENSORS In this part, we will explore a reflective opto transducer and a hall effect transducer for rotational speed and position measurements. 6.1 Reflective Opto Transducer Figure 13 shows the construction of a reflective opto transducer, consisting of an infra-red LED and phototransistor.

Figure 13: Reflective Opto Transducer

6.1.1 Operational Principles The operational principles of a reflective opto transducer are briefly described below:

• The beam from the LED is reflected back to transistor if a reflective surface is placed at the correct distance on the rotating disc. A non-reflective surface breaks the beam.

• In this case, we have 3 LEDs and phototransistors, each corresponding to a ring of the disc. The rotating disc is overlaid with Gray-coded adhesive as shown in Figure 14.

• Therefore, from the output of the transistor, the positional zone and rotational speed can be derived.

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Figure 14: Gray-coded disc 6.1.2 Characteristics

Figure 15: Connection for reflective opto transducer

• Connect the circuit as shown in Figure 15 with the digital multi-meter on the 20V DC

range. • Switch ON the power supply and rotate the drive shaft by hand to alter the LED

states. • Rotate the shaft until it is in the position with all LED’s OFF. Use the digital multi-

meter to measure the voltage at each of the outputs and record in Table 6.

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• Turn the shaft until all LED’s are ON and repeat the readings, recording the results

again in Table 6.

Table 6: Output Voltage

Output Voltage (V) Output LED OFF LED ON

A B C

• With the shaft initially in the position with all LED’s OFF, rotate the shaft

counterclockwise, when looking at the coded side of the disc, and note the state of the LED’s at each change of state.

Denote an LED OFF as logic state 0 and LED ON as logic state 1.

• Record the values in Table 7.

Table 7: LED States

Position C B A 0 1 2 3 4 5 6 7

Switch OFF the power supply.

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6.2 Hall Effect Transducer Figure 16 shows the layout of the Hall effect transducer.

Figure 16: Hall effect transducer 6.2.1 Operational Principles The operational principles of the Hall effect transducer are briefly described below:

• When current flows through the flat slice of semiconductor at right angles to a magnetic field, there is a force on each electron which tends to move it in one particular direction (the motor principle)

• The current is pushed to one side of the slice. The surplus of electrons on one side means that this side is negatively charged, resulting in an EMF across the slice (the Hall voltage VH) which is at right angles to both the current and the magnetic field.

• The value of this voltage is directly proportional to the strength of the magnetic field.

• By monitoring the value of this voltage, the sensor can thus be used for proximity and speed measurement.

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6.2.2 Characteristics

Figure 17: Connection for Hall effect transducer • Connect the circuit as shown in Figure 17. Set the Amplifier #1 GAIN

COARSE control 10, GAIN FINE to 0.8 and the motor drive voltage to zero. Switch ON the power supply.

• Set the drive shaft position so that the magnet in the Hall effect disc is horizontal (to

one side) so that there is no magnetic field cutting the Hall effect device. • Adjust the OFFSET control of Amplifier #1 for zero output indication on the

Moving Coil Meter. • Note the output voltage from the – and + output sockets of the Hall effect device with

the digital voltmeter directly on the Hall Effect Sensor panel and the also from the Moving Coil Meter. Record the results in Table 8.

Table 8: Output voltage

Digital Multi-meter Magnetic Field Output Voltage (-)(V) Output Voltage (+) (V)

Moving Coil Meter(V)

None Maximum

• Rotate the disc so that the magnet is directly above the Hall effect device. This

position will be indicated by the maximum output voltage. • Note the voltages again and record in Table 8

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These readings illustrate the basic characteristics of the Hall effect device and indicate its application to proximity detection. It is also suitable for speed measurement applications. • With the output of Amplifier #1 connected to the Counter/Timer input, set

the controls for COUNT and 1s (1 second).The counter wil then count for one second and the count value is “frozen” at the end of one second to show the speed in rev/sec.

• Transfer the digital multi-meter to the output of the Power Amplifier and apply an

input voltage of 2V to the motor so that the shaft rotates slowly. Press the Counter RESET button and note the displayed value, this representing the shaft speed in rev/sec. Record the result in Table 9.

Table 9: Shaft speed

Motor Voltage Shaft Speed (rev/sec)

2V 4V 7V 10V

Hall Effect Transducer

• Repeat the procedure for the other values of motor drive voltage given in Table 9 for

Comparison. Switch OFF the power supply. Hall effect devices are available for proximity detection, linear or angular displacement, multiplier and current or magnetic flux density measurement applications.

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7. SOUND SENSOR The construction of an ultrasonic transmitter/receiver for sound measurement is shown in Figure 18. The receiver and transmitter are almost identical and consist of a slice of ceramic material with a small diaphragm fixed to it, inside the case of the unit.

Figure 18: Ultrasonic Transmitter/Receiver 7.1 Operational Principles The operational principles of the ultrasonic transmitter/receiver are briefly described below:

• Certain ceramic materials produce a voltage when they are stressed (piezoelectric principle).

• Vibration of the diaphragm stresses the ceramic material and produces an output voltage.

• The reciprocal applies to the transmitter. An applied alternating voltage produces stress which causes the ceramic slice to vibrate.

• The dimensions of components are arranged so that there is resonance (best response) at 40kHz. This is above audible range and is therefore referred to as ultrasonic.

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7.2 Characteristics

Figure 19: Connection for ultrasonic transmitter/receiver • Connect the circuit as shown in Figure 19. Set the AC Amplifier gain control to

1000 and Amplifier #1 GAIN COARSE control to 1 and GAIN FINE to 0.5. Switch the Low Pass Filter time constant to 100ms.

• Switch ON the power supply and adjust Amplifier #1 OFFSET to give zero

output on the Moving Coil Meter. • Note the bargraph display as you move your hand or any other object over the

ultrasonic devices. The display should respond, indicating the receipt of a signal of frequency 40kHz by the ultrasonic receiver.

• Place a small book (approximately 6 inches (15cm) x 4 inches (10cm)) or other flat

object 3 feet (90cm) above the ultrasonic transducers. Slowly move the object closer to the transducers, watching the output reading on the bargraph display, until the object is covering the transducers.

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• At which of the following positions is the maximum output obtained? (Choose one)

(a) Object in contact with the Ultrasonic Transducers. (b) Object 4 inches (10cm) above the unit. (c) Object 12 inches (30cm) above the unit. (d) Object 3 feet (90cm) above the unit.

Remove any other equipment from the vicinity so that you have free access to the ultrasonic transmitter/receiver area. • Increase the Amplifier #1 GAIN FINE control to 1.0. Hold a thin object such

as a pencil approximately 6 inches (15cm) above the ultrasonic transducers, move it horizontally and vertically and note the effect on the output response. This indicates how critical the direction angle is for the device.

• Is the position of the reflector critical? • Put a sheet of paper over the ultrasonic transducers to intercept the path and move

your hand up and down above the transducers. • Does the beam pass through a piece of paper? In this exercise, the received signal has been amplified rectified, filtered (to remove all unwanted frequencies) and then amplified again to operate the display. Pulsed ultrasonic devices can be used for distance measurement to reflecting surfaces by measurement of the time between the transmission and return of the pulsed signal.

8. REPORT The report should log the results from the experiment with your own interpretations, observations and conclusions. You should try to answer all questions in the manual.