exercise 1: introduction to transducers - lab-volt manual facet by lab-volt 11 transducer...

Student Manual

10 FACET by Lab-Volt

Familiarization Transducer Fundamentals

Exercise 1: Introduction to Transducers

EXERCISE OBJECTIVE

When you have completed this exercise, you will be able to describe the basic operation of transducer

devices. You will verify your results by measuring basic transducer output parameters.

DISCUSSION

Different input transducers have different outputs. Some of the more common output types are shown.

Some transducers have a voltage output that is a function of the input parameter. For example, the

thermocouple on your circuit board is a temperature transducer whose output is a voltage that is a

function of temperature.

Other transducers may output a current that is a function of the input parameter. An example of this type

is the IC temperature transducer on your circuit board.

Other transducer types output a resistance change that varies with the input parameter. Examples on

your circuit board include the resistance temperature detector (RTD), thermistor, and strain gauge.

When this type of transducer is connected in a circuit, the transducer appears electrically as a

a. voltage source.

b. current source.

c. variable resistor.

Student Manual

FACET by Lab-Volt 11

Transducer Fundamentals Familiarization

Some transducers output a capacitance change that is a function of the input parameter. When connected

in a circuit, this device appears as a variable capacitor.

The CAPACITANCE SENSOR circuit block on your circuit board uses this type of transducer.

Depending on the transducer type, its output can be used in different ways.

For example, you can use an ohmmeter to directly read the output of the resistive transducer shown.

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Alternatively, you can connect the transducer to additional circuitry that can perform one or more of the

following functions.

• amplify the signal

• scale the signal

It is often useful to have a convenient relationship of input parameter to output signal

(e.g., 1°C = 10 mV).

• convert the signal to another form

For example, you can convert a resistive output to voltage to drive a meter or other display device.

• convert the measuring units

Other considerations in the selection and use of transducers include the linearity of the response and

whether the output parameter increases or decreases with respect to the input parameter.

the most linear response?

a. thermocouple

b. RTD

c. thermistor

d. IC transducer

temperature increases?

a. thermocouple

b. RTD

c. thermistor

d. IC transducer

PROCEDURE

In this PROCEDURE, you will become familiar with the transducer devices on your TRANSDUCER

FUNDAMENTALS circuit board and how to measure their basic output parameters.

On the IC TRANSDUCER circuit block, place the shunt on the TEMP header in the 40 C

position. Insert a two-post connector in the OVEN ENABLE position.

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These connections tell the oven controller circuitry to regulate the oven temperature at 40 C.

Since you started at room temperature (typically 25 C), the oven needs a minute or two to rise to

40 C.

Complete the following steps as you wait for the oven to reach the new temperature.

Set your multimeter to measure Vdc and connect the leads to TP1 (+) and TP2 (–).

The IC transducer outputs a current that is a function of temperature. The 1 k resistor (R1)

between TP1 and TP2 is in series with the transducer, and therefore has the same current

By measuring the resistor voltage, you can calculate the transducer current (IT) as follows.

IT =VR1

Observe the voltmeter for several seconds. What is happening as the temperature is rising

to 40 C?

a. Voltage increases.

b. Voltage decreases.

c. Voltage remains about the same.

Because the voltage is increasing, the current is also increasing. A temperature transducer

whose output parameter increases as temperature increases is said to have a positive

temperature increases.

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You can determine when the oven temperature has stabilized by watching the OVEN ON LED,

which indicates when power is applied to the heater resistors.

the oven temperature approaches 40 C, the heater will cycle on and off to maintain that

temperature.

The temperature is now stable at 40 C.

Record the voltage across R1.

VR1 = mV (Recall Value 1)

Calculate the current out of the IC transducer.

IT = VR1

= mV (Step 5, Recall Value 1

= A (Recall Value 2)

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A. Is your

calculation of A (Step 6, Recall Value 2) close to this value?

a. yes

b. no

Remove the meter connections and set the meter to read ohms.

Make sure there are no two-post connectors in the THERMISTOR circuit block. Connect the

meter leads as shown.

The thermistor is a temperature transducer whose resistance is a function of temperature. With

C.

RTH = (Recall Value 3)

at 40 C. Is your measurement of

(Step 9, Recall Value 3) close to this value?

a. yes

b. no

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In the IC TRANSDUCER circuit block, move the TEMP shunt to the 45 C position to

increase the oven temperature.

Observe the thermistor resistance on the meter for several seconds.

a. positive.

b. negative.

Return the TEMP shunt to the 40 C position.

Make sure there are no two-post connectors in the RTD circuit block.

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Remove the ohmmeter leads from the THERMISTOR circuit block and connect them across

the RTD as shown. Make sure the negative meter lead is in the OUT jack.

The RTD (resistance temperature detector) is a transducer whose resistance is a function of

temperature. What other temperature transducer has a temperature-dependent resistance

output?

a. IC transducer

b. thermistor

c. thermocouple

Observe the OVEN ON LED and allow it to complete several cycles to make sure the oven

temperature has returned to 40 C.

Record the RTD resistance at 40 C.

RRTD = (Recall Value 4)

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Why must the resistance of the thermistor and the RTD be measured without two-post

connectors in the circuit?

a. The resistance of the connector can cause measurement errors.

b. The transducer must be isolated from other components that would affect the measurement.

c. Both of the above

Remove the two-post connector in the IC TRANSDUCER circuit block to turn off the oven.

The STRAIN GAUGE circuit block allows you to measure stress and strain that is applied to a

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The strain gauge is a transducer whose output is a resistance that is a function of the strain on

the surface of the object to which it is attached.

Make sure there are no two-post connectors in the STRAIN GAUGE circuit block. Connect

the ohmmeter across the transducer as shown.

The adjustment knob allows you to bend the beam upward or downward to create compression

or tension, respectively.

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Turn the knob fully counterclockwise (CCW) and measure the strain gauge resistance.

RCCW = (Recall Value 5)

Turn the knob fully clockwise (CW) and measure the resistance.

RCW = (Recall Value 6)

Because there is a very small resistance change, the strain gauge is a good example of a

transducer whose output needs to be increased for practical applications.

voltage swing for the range of force measured.

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The variable capacitor in the CAPACITANCE SENSOR circuit block is connected to an RC

In this case, a variable capacitance is converted to a variable frequency.

Insert a two-post connector in the BLOCK ENABLE position. The block enable connection

supplies power to the circuit block.

Connect oscilloscope CH 1 to OUT and ground the probe to the GND jack.

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Adjust the scope controls to view a square wave output from the CAPACITANCE SENSOR

circuit block.

Slide the movable plate up and down while observing the scope.

What happens to the waveform?

a. The frequency changes.

b. The amplitude changes.

c. Both of the above

Insert a two-post connector in the BLOCK ENABLE position of the ULTRASONIC

TRANSDUCERS circuit block.

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Connect the channel 1 oscilloscope probe to the left pin of the transmitter. Ground the scope

probe to the GND jack.

What type of signal appears on the scope?

a. square wave

b. dc level

c. short bursts of pulses

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You are viewing the ultrasonic bursts that the transmitter sends out to be echoed back to the

receiver.

Remove the two-post connector from the ULTRASONIC TRANSDUCERS circuit block and

insert it into the BLOCK ENABLE position of the INFRARED CONTROLLER circuit block.

The INFRARED CONTROLLER circuit block has both an input and an output transducer. The

infrared LED is an output transducer that converts electrical pulses into light pulses.

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The light sensor is an input transducer that senses infrared light pulses and converts them to

electrical signals.

Observe the DATA LEDs in the RECEIVER section as you perform the following steps.

Set the DIP switches in the TRANSMITTER section to 0000 (all switches down) and press

the XMT button.

Set the DIP switches to 1111 (all switches up) and press XMT. You can see that each time

you press XMT, the DIP switch code is sent to the DATA LEDs.

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sensor.

Set the DIP switches to 0000 and press XMT. What can you conclude from the DATA LEDs?

a. The code was received by the RECEIVER section.

b. The code was not received.

c. Only part of the code was received.

Remove the BLOCK ENABLE two-post connector from the INFRARED CONTROLLER

circuit block.

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CONCLUSION

• Input transducers (sensors) convert a physical quantity into a proportional electrical signal.

• Output transducers convert an electrical signal into a physical quantity that can be detected or used

externally.

•

• Transducers often require additional circuitry to amplify the output or to convert the output parameter

to another form.

REVIEW QUESTIONS

1. A transducer converts

a. temperature to resistance.

b. force into current.

c. position into voltage.

d. one form of energy to another.

2. Which circuit block on the TRANSDUCER FUNDAMENTALS circuit board shows a schematic symbol

of a transducer whose output is a change in resistance?

a. THERMISTOR

b. RTD

c. STRAIN GAUGE

d. All of the above

3. Which device on the TRANSDUCER FUNDAMENTALS circuit board is an output transducer?

a. strain gauge

b. infrared LED

c. RTD

d. thermistor

4.

a. A

b. B

c. C

d. D

5. Which circuit block senses the position of an object by emitting pulses and measuring the time it takes

for the pulses to echo back?

a. INFRARED CONTROLLER

b. CAPACITANCE SENSOR

c. ULTRASONIC TRANSDUCERS

d. STRAIN GAUGE

exercise 1: introduction to transducers - lab-volt manual facet by lab-volt 11 transducer...

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