transducer characteristics

18
Transducer Characteristics and Application Summary Sensors or Transducers are devices that convert physical phenomenon into a measurable quantity. They are known as transducers because they convert energy from one form to another. With respect to data acquisition, transducers are used as sensors that output an electrical signal relative to changes in a physical phenomenon. Actuators use an electrical signal to produce a change in a physical phenomenon. The change can be an increase or decrease in the physical phenomenon. Here we will look at some examples of physical phenomena. We will discuss the use of sensors and actuators and how they can be used to measure and control physical phenomena. In this experiment, the characteristics of two temperature transducers are determined and compared. Objectives Define the terms physical phenomenon, transducer, actuator Determine the response time, sensitivity, and calibration equation of a transducer. Determine the accuracy and precision of a thermal data acquisition system. Background Information Physical Phenomena Before explaining sensors and actuators, it is important to look at physical phenomena. Examples of physical phenomena are force, temperature, acceleration, light, pressure, or distance. The units of a physical phenomenon are important. For example, distance may be measured in mm or km. The valid units should be identified and used. Transducers There are many different types of transducers, which are used to measure different types of physical phenomena. Thermal transducers for example, output a signal relative to changes in temperature. A thermocouple outputs a voltage signal, whereas the resistance of a thermistor changes with the temperature.

Upload: mwanarusi-mwatondo-njira

Post on 16-Apr-2015

84 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Transducer Characteristics

Transducer Characteristics and Application

Summary

Sensors or Transducers are devices that convert physical phenomenon into a measurable

quantity. They are known as transducers because they convert energy from one form to

another. With respect to data acquisition, transducers are used as sensors that output an

electrical signal relative to changes in a physical phenomenon.

Actuators use an electrical signal to produce a change in a physical phenomenon. The change

can be an increase or decrease in the physical phenomenon.

Here we will look at some examples of physical phenomena. We will discuss the use of

sensors and actuators and how they can be used to measure and control physical phenomena.

In this experiment, the characteristics of two temperature transducers are determined and

compared.

Objectives

• Define the terms physical phenomenon, transducer, actuator

• Determine the response time, sensitivity, and calibration equation of a transducer.

• Determine the accuracy and precision of a thermal data acquisition system.

Background Information

Physical Phenomena

Before explaining sensors and actuators, it is important to look at physical phenomena.

Examples of physical phenomena are force, temperature, acceleration, light, pressure, or

distance. The units of a physical phenomenon are important. For example, distance may be

measured in mm or km. The valid units should be identified and used.

Transducers

There are many different types of transducers, which are used to measure different types of

physical phenomena. Thermal transducers for example, output a signal relative to changes in

temperature. A thermocouple outputs a voltage signal, whereas the resistance of a thermistor

changes with the temperature.

Page 2: Transducer Characteristics

2

By taking readings of the signal from a transducer for different values of the physical

phenomenon, we can find the Calibration Equation and Sensitivity of the transducer. This

is covered in detail in experiment procedures.

Actuators

Actuators are used to change the physical phenomenon. To increase temperature, the obvious

actuator which is commonly used is a heater. Any heat producing device can be used as an

actuator. Conversely, to decrease the temperature a cooling device such as a refrigeration

system or an air-conditioning system.

A motor can be used to control rotational speed or by connections to a mechanical system, the

rotation can be further converted to distance.

Other physical phenomena can also be controlled using other electrically powered actuators.

If we desire the physical phenomenon to reach a certain constant value, we can set the

required power being supplied to the actuator using the Open Loop Equation. This will be

covered in detail in experiment 5.

Examples of Transducers

Thermocouple

A thermocouple is a temperature transducer

consisting of two metals joined at one end.

The device is based on the fact that an

electromotive force exists across the junction

of two dissimilar metals. This force produces

a small voltage, which varies linearly with

temperature.

Thermistor

Another type of temperature transducer is the

thermistor. The thermistor is a

semiconductor, which changes resistance in

relation to changes in temperature. It is a

non-linear device in that the resistance of a

thermistor decreases exponentially as the

temperature increases.

Since there is more than one transducer to measure temperature, the engineer considers the

transducer cost, precision, and response time when choosing which to use. For example, if the

design requires fast temperature measurements, the engineer uses the transducer with the

smallest response time. If the design requires very precise measurements, then the engineer

chooses the transducer with the smallest least count.

Voltage (mV)

Temperature

Figure 1: Thermocouple

Cable

Resistance (KΩ) Temperature

Figure 2: Thermistor

Page 3: Transducer Characteristics

3

Light intensity transducer

To measure light intensity an optical transducer is mounted on a wire and outputs Ohms in

relation to light intensity (units of light intensity are Candles or Lux).

A similar light intensity transducer produces a voltage or current for different levels of light.

This is similar to the solar cell, which is not used to measure the light intensity but to capture

the energy in the light.

Strain Transducer

A strain gauge transducer is attached to a cantilever metal bar with a mass, as shown in the

figure. This can be used to measure masses hanging from a mass hanger attached to the metal

bar.

Clamp

Strain Gauge

Metal Bar

Mass

Hanger

& Masses

DMM

Figure 3: The use of a strain transducer as a sensor to measure mass.

The output of the transducer is in Ohms and is measured with the DMM.

Ultrasonic transceivers for distance measurement

This is made up of two devices which can be used to make or receive ultrasonic sound. The

receiver measures a signal from the transmitter in Voltsp-p the distance between the two

devices. A device similar to this is used to measure distance in cars, to sense if the car is too

close to a wall. This triggers an alarm to help prevent accidents.

Page 4: Transducer Characteristics

4

Opto-Switch Transducer

This sensor measures a light signal that is going on and

off. When the light is blocked, the signal has a low value

(usually, voltage) and when the light can pass, the signal

increases. This value can be measured.

Also, if the signal is changing from high to low quickly

enough, the frequency can be measured. This can be

related to the speed switched on and off. A device such as

this is found inside a computer mouse.

As mentioned in the summary, a transducer converts physical phenomenon into a measurable

quantity. There are many types of transducers, which are used to measure different types of

physical phenomena. Thermal transducers output a signal relative to changes in temperature.

In the procedures, we shall be using the two types of thermal transducer described earlier.

Transducer

Transducer Output

Physical

Phenomenon

Calibration

Equation

Measurement

of

Physical

Phenomenon

Physical

Phenomenon

(Transducer)

(Input)

The calibration equation is the best

mathematical formula which relates

the transducer input to its output.

Figure 5: Calibration and its use in a data acquisition system.

The mathematical relationship between the physical phenomenon (input) and transducer

output is called the calibration equation. Some transducers output a signal that is

proportional to changes in the physical phenomenon. These transducers are linear devices as

the transducer calibration equation, which relates the output to the input, may be described by

a first order equation: y = ax + b. Other transducers do not have a proportional relationship

between the output signal and changes in the physical phenomenon. These transducers are

non-linear devices. The output units of a transducer vary according to the device and may

have the units of Voltage, Amperes or Ohms.

Figure 4. Opto-Switch inside a mouse

Page 5: Transducer Characteristics

5

Calibration is the process by which a mathematical equation describing the relationship

between the transducer output and physical phenomenon is found, as displayed in figure 1.

Specifically, calibration is achieved by measuring the transducer output at known phenomena

quantities. Then the best mathematical function relating the input to the output is determined.

After calibration, the transducer output and calibration equation may be used to measure

unknown phenomena quantities.

The sensitivity of a transducer is the ratio of the change in its output over the change in the

physical phenomenon. For example, if the output of a temperature transducer changes 5Ω for one degree change in temperature, then the sensitivity is 5Ω/°C. Using the calibration equation, the sensitivity would be the reciprocal of the slope.

The sensitivity of a transducer is used to determine the data acquisition system’s measurement

least count. The least count is defined as the smallest measurable quantity of the system and

relates to the system precision. For example, if a system is using a temperature transducer with

a sensitivity of 5Ω/°C and is using an Ohmmeter with a least count of 1Ω, then the equation below determines the temperature least count.

Temperature Least Count C2.0

C

5

1

ySensitivitTransducer

CountLeastOhmmeter O

O

=

Ω

Ω=

=

Time is required for a transducer to respond to changes in the physical phenomenon. If the

output signal of the transducer is increasing, the rise time is the time required for the

normalized output to change from 10% to 90% of the maximum, as shown in figure 2.

Normalized

Transducer output

Rise

TimeFall

Time

Time

0

0.1

1.0

0.9

Figure 6: Rise time and fall time of a transducer.

Similarly, the fall time is the time required for the normalized output to decrease from 90% to

10% of the maximum, also shown in figure 1. The response time is the average of the rise

time and fall time.

Page 6: Transducer Characteristics

6

Normalization is accomplished by subtracting the minimum data point from all the data and

then dividing by the resulting maximum value. For example, if the transducer output changes

from 5mV to 15mV, the normalized output is found by first subtracting 5mV (the minimum)

from the data, which now ranges from 0mV to 10mV. Then, the data is divided by 10mV, to

produce an output that ranges from 0 to 1, with no units.

Often there is more than one type of transducer that may be used to measure the same physical

phenomenon. For example, in this experiment two thermo-transducers are used to measure

temperature.

Examples of Actuators

Motor

A motor is supplied with power and produces rotational motion. The rotational motion can be

linked to a spindle to provide linear movement.

Light bulb

A common actuator in all homes is a light bulb. When power is supplied to the bulb it

produces light. If various amounts of voltage are provided the amount of light produced will

change. Also, a light bulb heats up when power is supplied, so (in experimental set ups) it can

be used as a heater.

Pump

In essence a pump is a motor in a sealed compartment which can move fluids from one side of

the pump to the other. This may be used to pump water or oil. An example is at the petrol

station. Petrol is pumped through the pipe and nozzle and into the car’s petrol tank. The

pump is powered by electricity. The amount of fluid transferred depends on the amount of

power provided to the pump.

Heater

Examples in the home include the oven and the water heater. Power is supplied to the heater

and it produces heat. In the lab, a device such as a bulb or a power resistor can be used as a

heater.

Refrigerator

This actuator cools things down. Examples in the home are air conditioning and fridges. The

power which is supplied to the refrigerator in reality goes to two actuators, a pump to move a

liquid through a network of pipes and a motor attached to a fan to blow air across the pipes to

create a cooling effect. The liquid used has a low boiling point and is known as a refrigerant,

for example Freon or CFC-12.

Page 7: Transducer Characteristics

7

Use of Actuators and Sensors

Figure 7 shows how connections can be made to measure and control a heating system. A

transducer is used to measure the physical phenomenon and the actuator changes it.

Figure 7: The use of a sensor and an actuator in a control system

The table below gives some examples of Physical Phenomena and related Sensors and

actuators

Physical Phenomenon Example

Sensor or Transducer

Example

Actuator

Light Opto-transducer Light Bulb

Speed Opto switch Motor

Temperature Thermocouple Heater (Heating system)

Temperature Thermistor Refrigerator (Cooling system)

Flow Flow rate transducer Pump

Most transducers produce an electrical signal, although mechanical transducers are also

available. Here the output units of a transducer vary according to the device and may have the

units of Voltage, Amperes or Ohms.

Figure 4 shows another system where a transducer is used to measure the speed of a motor and

then an actuator increases or decreases the physical phenomenon.

Figure 8: The use of sensors and actuators in a system to control motor speed

Page 8: Transducer Characteristics

8

Procedure

In this experiment, two temperature transducers are to be compared. Specifically, the

calibration equation, least count, and response time of each are measured.

Thermometer

Power

Resistor

Power Supply

V+= 8VDC

DMM

Thermocouple

Figure 9: Procedure 1 equipment setup.

1. Thermocouple Calibration

1.1. Connect the power resistor to the power supply. This resistor is used as a heat source.

Also, connect the thermocouple to the DMM, according to the equipment setup in

figure 4.

1.2. Calibrate the thermocouple by heating the power resistor and measuring the

thermocouple output at various temperatures.

Page 9: Transducer Characteristics

9

Specifically, begin with the power resistor at room temperature, place 8V across the

resistor, and then record the thermocouple output voltage and thermometer reading

approximately every 5OC until a maximum of 75

OC is reached.

Thermocouple

Output (Units)

Thermometer

Reading (Units)

1.3. Enter the thermocouple and thermometer values of the table in 1.2 in an EXCEL

spreadsheet, plot the calibration curve.

From the curve, determine the best calibration equation. In your report, comment on the

linearity of the calibration equation and record the transducer’s average sensitivity.

Thermocouple Calibration Equation, T(V) Sensitivity (mV/

OC)

1.4. Use the transducer’s average sensitivity and DMM least count to determine the

temperature measurement least count.

Thermal Data Acquisition Least Count

1.5. Write a LabVIEW VI to measure the thermocouple output and uses the calibration

equation to compute the power resistor temperature. The VI continues operation until

stopped by a Boolean push button.

1.6. Complete the following table to determine the temperature measurement accuracy at

several different temperatures. Use the thermometer reading as a standard.

Compute the average of the accuracy calculations as an approximation of the LabVIEW

data acquisition system accuracy.

Page 10: Transducer Characteristics

10

Thermometer

Temperature

LabVIEW

Temperature

% Accuracy

Average

2. Thermistor Calibration

2.1. Similar to procedure 1, calibrate the thermistor by heating the power resistor and

measuring the transducer output at various temperatures.

Thermistor

Output (Units)

Thermometer

Reading (Units)

2.2. Enter the thermistor and thermometer values of the table in 2.1 in an EXCEL

spreadsheet, plot the calibration curve.

From the curve, determine the best calibration equation. In your report, comment on the

linearity of the calibration equation.

Thermistor Calibration Equation, T(Ω)

2.4. Write a LabVIEW VI to measure the thermistor output and then use the calibration

equation to compute the power resistor temperature. The VI continues operation until

stopped by a Boolean push button.

2.5. After the calibration equation has been implemented by a LabVIEW VI, complete the

following table to determine the temperature measurement accuracy at several different

temperatures. Use the thermometer reading as a standard.

Page 11: Transducer Characteristics

11

Compute the average of the accuracy calculations as an approximation of the LabVIEW

system accuracy.

Thermometer

Temperature

LabVIEW

Temperature

% Accuracy

Average

3. Response Time Comparison

3.1. Heat the power resistor to a high temperature by applying 8VDC Wait at least 10 minutes

for the power resistor temperature to stabilize.

3.2. Write a LabVIEW VI to record and plot the thermocouple output voltage as a function

of time.

• The VI continues operation until stopped by a Boolean push button.

• The sampling time of this system is 1 second. In other words, the DMM measures

the thermocouple voltage every second.

• The VI measures the thermocouple output in millivolts.

• The measured value is output on a waveform chart.

• The time and millivolt data is saved to an Excel spreadsheet for later analysis.

3.3. Run the VI and place the thermocouple junction inside the power resistor. After the

thermocouple voltage stabilizes, remove the junction from the power resistor and wait

for the voltage to return to the original value. Then quit and save the data.

3.4. Use Excel to analyze the data by normalizing the data, plotting the data, and computing

the rise time, fall time, response time.

Thermocouple Parameter Seconds

Rise time

Fall time

Response time

Page 12: Transducer Characteristics

12

3.5. Adjust the LabVIEW VI to record and plot the thermistor output resistance as a function

of time.

3.6. Run the VI and place the thermistor junction inside the power resistor. After the

thermistor resistance stabilizes, remove the junction from the power resistor and wait for

the resistance to return to the original value. Then quit and save the data.

3.7. Use Excel to analyze the data by normalizing the data, plotting the data, and computing

the rise time, fall time, response time.

Thermistor Parameter (Units)

Rise Time

Fall Time

Response Time

3.8 Compare the two thermal transducers by completing the following table.

Parameter Thermocouple Thermistor

Response Time

Least Count

System Accuracy

Page 13: Transducer Characteristics

13

Presentation Question 1

An engineering company has contracted your group to write a LabVIEW VI to determine the

direction a satellite dish is pointing. An angle transducer will be attached to the base of the

dish and a LabVIEW VI will show the angle in degrees from 0 - 180o as the dish turns. The

output of the transducer is in Ohms. The first stage of this project is the calibration of the

angle transducer. The company will supply your group with the transducer.

The company wants your group to perform the following experiments, and answer these

questions about the project.

• Explain how the transducer is connected to the lab equipment.

• Collect data on the transducer output against the required range of inputs. The company expects your group to repeat the data collection procedure at least three times to reduce

random error.

• Use Excel to determine the calibration equation.

• Implement the calibration equation in LabVIEW. What are the details of the LabVIEW VI? How does it work?

• Display lights on the front panel to indicate when the angle is less than 20o and another for more than 140

o

• Once the system is calibrated, determine the average accuracy and precision of the transducer.

• Were there any problems faced in writing the program?

After the presentation, the company may have a few additional questions for you.

Page 14: Transducer Characteristics

14

Presentation Question 2

An engineering company has contracted your group to write a LabVIEW VI for use with an

electronic caliper they have designed. A linear displacement transducer will be attached to the

jaws of the caliper and a LabVIEW VI will show the length in millimeters as the jaws move.

The output of the transducer is in Ohms. The first stage of this project is the calibration of the

linear transducer. The company will supply your group with the transducer.

The company wants your group to perform the following experiments, and answer these

questions about the project.

• Explain how the transducer is connected to the lab equipment.

• Collect data on the transducer output verses input for the required range of inputs. The company expects your group to repeat the data collection procedure at least three times to

reduce random error.

• Use Excel to determine the calibration equation.

• Implement the calibration equation in LabVIEW. What are the details of the LabVIEW VI? How does it work?

• Display lights on the front panel to indicate when the displacement is less than 10mm and another for more than 25mm

• Once the system is calibrated, determine the average accuracy and precision of the transducer.

• Were there any problems faced in writing the program?

After the presentation, the company may have a few additional questions for you.

Page 15: Transducer Characteristics

15

Presentation Question 3

An engineering company has contracted your group to write a LabVIEW VI in order to

measure light intensity. The optical transducer is mounted on a wire and outputs Ohms in

relation to light intensity (Lux). Your group is to calibrate the transducer using a light source

and light meter. The light brightness may be adjusted using the power supply (0 - 12V).

The company wants your group to perform the following experiments, and answer these

questions about the project.

• Explain how the transducer is connected to the lab equipment.

• Explain the procedure used to determine the transducer calibration equation.

• Implement the calibration equation using LabVIEW. What are the details of the LabVIEW VI? How does it work?

• Display a light on the front panel if the brightness is above 10 Lux.

• Once the system is calibrated, determine the average accuracy of the transducer.

• Were there any problems faced in writing the program?

After the presentation, the company may have a few additional questions for you.

Page 16: Transducer Characteristics

16

Presentation Question 4

An engineering company has contracted your group to write a LabVIEW VI for use with an

electronic scale they have designed. A strain gauge transducer is attached to a cantilever metal

bar with a mass, as shown in the figure. The LabVIEW VI uses the transducer information to

determine a measure of the mass in grams. The balance is able to measure in the range of

0 - 900 grams.

Clamp

Strain Gauge

Metal Bar

Mass

Hanger

& Masses

DMM

The output of the transducer is in Ohms and is measured with the DMM. The first stage of the

project is the calibration of the transducer. The company will supply your group with the

transducer and balance. Your instructor will assist you in setting up the equipment.

The company wants your group to perform the following experiments, and answer these

questions about the project.

• Explain how the transducer is connected to the lab equipment.

• Collect data on the transducer output verses input for the required range of inputs. The company expects your group to repeat the data collection procedure at least three times to

reduce random error.

• Use Excel to determine the calibration equation.

• Implement the calibration equation in LabVIEW. What are the details of the LabVIEW VI? How does it work? Arrange for the system to measure the mass every 2 seconds.

• Once the system is calibrated, determine the average accuracy of the transducer.

• Were there any problems faced in writing the program?

After the presentation, the company may have a few additional questions for you.

Page 17: Transducer Characteristics

17

Presentation Question 5

An engineering company has contracted your group to write a LabVIEW VI for use with an

electronic scale they have designed. The balance is based upon a spring and potentiometer

(variable resistance). The LabVIEW VI uses the resistance transducer to determine a measure

of the mass in grams. The balance measures grams in the range of 0 - 200 grams.

The output of the transducer is in Ohms and is measured with the DMM. The first stage of the

project is the calibration of the transducer. The company will supply your group with the

transducer and balance. Your instructor will assist you in setting up the equipment.

The company wants your group to perform the following experiments, and answer these

questions about the project.

• Explain how the transducer is connected to the lab equipment.

• Collect data on the transducer output verses input for the required range of inputs. The company expects your group to repeat the data collection procedure at least three times to

reduce random error.

• Use Excel to determine the calibration equation.

• Implement the calibration equation in LabVIEW. What are the details of the LabVIEW VI? How does it work? Arrange for the system to measure the mass once every 5

seconds.

• Once the system is calibrated, determine the average accuracy and precision of the transducer.

• Give examples of transducers which are used to measure mass in your home.

After the presentation, the company may have a few additional questions for you.

Page 18: Transducer Characteristics

18

Presentation Question 6

An engineering company has contracted your group to write a LabVIEW VI for use with new

distance measurement system. The system consists of two ultrasonic transceivers, which

means the device can be used to make ultrasonic sound or receive ultrasonic sound. For this

system, one device is connected to the function generator with a sine wave, amplitude of

20VPP, frequency of 40KHz, and no offset voltage.

The receiver is connected to the oscilloscope and the peak-to-peak voltage is measured and

calibrated against the distance between the base blocks. The system should be able to measure

from 0 to 200mm.

The company wants your group to perform the following experiments, and answer these

questions about the project.

• Explain how the system is connected together.

• Collect data on the transducer output verses input for the required range of inputs. The company expects your group to repeat the data collection procedure at least three times to

reduce random error.

• Use Excel to determine the calibration equation.

• Implement the calibration equation in LabVIEW. What are the details of the LabVIEW VI? How does it work? The VI should display a warning light if the distance is below

20mm.

• Once the system is calibrated, determine the average accuracy of the transducer. What uses could be made of such a system?

After the presentation, the company may have a few additional questions for you.