Alciatore D.G., Histand M.B. Introduction to Mechatronics and Measurement Systems
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Creating terminal blocks
1 . Select the wire spool icon from the Tools palette (or the automatic icon).
Right-click on the rate input (arrow on the side of the block) on the DAQ Assis-
tant block and select create > control. A block labeled rate should appear with
a wire connected to the DAQ Assistant block.
2 . Repeat this to create a control for the number of samples input. These two con-
trols will appear in the Front Panel window.
3 . Activate the Front Panel window and open the controls palette if it is not open.
From the Front Panel window, under the View menu, select Controls Palette to
open the Controls palette.
4 . From the Controls palette, select modern > graph and drag the Waveform
Graph icon onto the Front Panel window. A block labeled “Waveform graph”
will appear in the Block Diagram window.
5 . Right-click on the graph and select properties. Under the Scales tab select
Amplitude (Y-axis) in the top pull-down menu. Deselect Autoscale and set the
maximum and minimum to the values used for the signal input range on the
DAQ Assistant block. Click Ok to close the properties window.
6 . Select the Block Diagram window and select the wire spool icon
(or auto-
matic icon) on the Tools palette. Click on the data output of the DAQ Assistant
block and then on the Waveform Graph block. A wire will now connect the two
blocks and it should look like the window below.
Sampling an analog signal
1 . Connect an analog signal to the USB 6009. The positive side is connected to
screw terminal 2 (AI0), and the negative side is connected to screw terminal 1
(GND).
8.6 Practical Considerations 371
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372 CHAPTER 8 Data Acquisition
2 . Select the Front Panel window and select the operate value
icon from the
Tools palette (or automatic icon).
3 . Set the rate and number of samples controls to appropriate values.
4 . Under the Operate menu, select run to run the program. A waveform
should appear on the Waveform Graph. A picture of the waveform
can be saved to a file by right-clicking on the waveform and selecting
Data Operations > Export Simplified Image . . . .
Sampling music
1 . Connect the output of an audio device (e.g., speaker wires from an amplifier, or
the headphone wires of an MP3 player) to the inputs of the USB 6009.
2 . Select a sampling rate of 44,000 kHz (the standard high-fidelity music sam-
pling rate) to capture all audio frequencies without aliasing. Because human
hearing is generally limited to the 20 Hz to 20 kHz range, any rate less than
or equal to 40,000 (from Shannon’s Sampling Theorem) can result in aliasing
(poor fidelity).
3 . Add the Play Waveform block to the block diagram, which can be found in the
Functions palette: Programming, Graphics and Sound, Sound, Output. Press
OK when the configuration dialog window appears. Wire the data input of the
Play Waveform block to the data output of the DAQ Assistant block. Create a
constant for the timeout input of the DAQ Assistant (using the same method
used to create a control for the rate input) and set it to 30 (the letter icon will
need to be selected from the Tools palette). Setting the timeout to 30 allows
up to 30 seconds of music to be recorded. The block diagram should now look
like the following figure.
4 . Sample the music and listen to the recorded waveform.
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Video Demo 8.5 demonstrates all of the steps in the process, including the play-
back of sampled music at different sampling rates. Video Demo 8.7 illustrates the
effects of sampling in more detail and also demonstrates the effects of reduced reso-
lution. Excellent online tutorials dealing with various aspects of LabVIEW can be
found at Internet Link 8.6.
QUESTIONS AND EXERCISES
Section 8.1 Introduction
8.1. Why is it not possible to connect sensors such as thermocouples, strain gages, and
accelerometers directly to a digital computer or microprocessor?
8.2. Assuming that the bandwidth of an audio music track is 15 kHz, what is the mini-
mum sampling rate necessary to digitize the track with high fidelity for storage on an
audio CD?
8.3. Suppose you want to digitize the raw signals coming from the following sources:
a. thermocouple sensing room temperature
b. stereo amplifier output
What is the minimum sampling frequency that you would choose in each of
these cases? Be sure to state and justify any assumptions you make.
8.4. Plot two periods of the function V ( t ) ⫽ sin( t ) ⫹ sin(2 t ) at time intervals correspond-
ing to 1/3, 2/3, 2, and 10 times the Nyquist frequency. Comment on the results.
8.5. Generate plots for the function in Example 8.1 with sample intervals of 0.5 sec,
1 sec, and 10 sec. Explain the results in light of the sampling theorem.
8.6. Document a complete and thorough answer to Class Discussion Item 8.2.
Section 8.2 Quantizing Theory
8.7. Given a 12-bit A/D converter operating over a voltage range from ⫺ 5 V to 5 V, how
much does the input voltage have to change, in general, in order to be detectable?
8.8. A signal’s values have a range of ⫾ 5 V, and you wish to make measurements with
an analog quantization size no more than 5 mV. What minimum resolution A/D con-
verter is required to perform this task?
8.9. If an 8-bit A/D converter with a 0 to 10 V voltage range is used to sample an analog
sensor’s voltage, what digital output code would correspond to each of the following
sensor values:
a. 0.0 V
b. 1.0 V
c. 5.0 V
d. 7.5 V
8.5
LabVIEW data
acquisition and
music sampling
8.7Music
sampling rate and
resolution effects
Video Demo
Questions and Exercises 373
In
te
rn
et
Lin
k
8.6LabVIEW
Graphical
Programming
online course
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374 CHAPTER 8 Data Acquisition
Section 8.3 Analog-to-Digital Conversion
8.10. How much computer memory (in bytes) would be required to store 10 seconds of
a sensor signal sampled by a 12-bit A/D converter operating at a sampling rate of
5kHz?
8.11. Derive Equations 8.6 and 8.7 from the truth table in Table 8.1.
8.12. Generate a plot similar to Figure 8.10 for a 5-bit converter if the input signal is
2.25 V and the input range is ⫺ 5 V to 5 V. What is the correct digital output?
8.13. As a mechatronic designer moves progressively from an 8-bit to 10-bit to 12-bit A/D
converter, what problems might be incurred in the design of the data acquisition
system?
8.14. Visit Microchip’s website ( www.microchip.com ) and find specifications for the
MCP32X series of A/D converters. What is their resolution? What type of architec-
ture is used in obtaining the binary representation of the analog value?
8.15. Using the Internet, look up the specifications for the National Semiconductor
( www.national.com ) ADC0800 8-bit A/D converter. Determine the maximum sam-
pling rate and the method for performing the conversion. Also define each of the
inputs and outputs.
Section 8.4 Digital-to-Analog Conversion
8.16. Prove Equation 8.13.
8.17. Prove Equation 8.14.
8.18. Prove Equation 8.15.
8.19. Document a complete and thorough answer to Class Discussion Item 8.5.
8.20. Document a complete and thorough answer to Class Discussion Item 8.6.
BIBLIOGRAPHY
Datel Intersil , Data Acquisition and Conversion Handbook , Mansfield, MA, 1980.
Gibson, G. and Liu, Y., Microcomputers for Engineers and Scientists , Prentice-Hall, Engle-
wood Cliffs, NJ, 1980.
Horowitz , P. and Hill , W. , The Art of Electronics , 2nd Edition, Cambridge University Press,
New York, 1989.
O’Connor , P. , Digital and Microprocessor Technology , 2nd Edition, Prentice-Hall, Engle-
wood Cliffs, NJ, 1989.
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375
CHAPTER
9
Sensors
T
his chapter describes various sensors important in mechatronic system
design. ■
INPUT SIGNAL
CONDITIONING
AND INTERFACING
- discrete circuits
- amplifiers
- filters
- A/D, D/D
OUTPUT SIGNAL
CONDITIONING
AND INTERFACING
- D/A, D/D
- PWM
- power transistors
- power op amps
GRAPHICAL
DISPLAYS
- LEDs
- digital displays
- LCD
- CRT
SENSORS
switches
potentiometer
photoelectrics
digital encoder
strain gage
thermocouple
accelerometer
MEMs
ACTUATORS
- solenoids, voice coils
- DC motors
- stepper motors
- servo motors
- hydraulics, pneumatics
MECHANICAL SYSTEM
DIGITAL CONTROL
ARCHITECTURES
- logic circuits
- microcontroller
- SBC
- PLC
- sequencing and timing
- logic and arithmetic
- control algorithms
- communication
- amplifiers
system model dynamic response
CHAPTER OBJECTIVES
After you read, discuss, study, and apply ideas in this chapter, you will:
1. Understand the fundamentals of simple electromechanical sensors, including
proximity sensors and switches, potentiometers, linear variable differential
transformers, optical encoders, strain gages, load cells, thermocouples, and
accelerometers
2. Be able to describe how natural and binary codes are used to encode linear and
rotational position in digital encoders
3. Be able to apply engineering mechanics principles to interpret data from a
single strain gage or strain gage rosette
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376 CHAPTER 9 Sensors
4. Be able to make accurate temperature measurements using thermocouples
5. Know how to measure acceleration and understand the frequency response of
accelerometers
6. Understand what a microelectromechanical (MEM) system is
9.1 INTRODUCTION
A sensor is an element in a mechatronic or measurement system that detects the
magnitude of a physical parameter and changes it into a signal that can be processed
by the system. Often the active element of a sensor is referred to as a transducer.
Monitoring and control systems require sensors to measure physical quantities such
as position, distance, force, strain, temperature, vibration, and acceleration. The
following sections present devices and techniques for measuring these and other
physical quantities.
Sensor and transducer design always involves the application of some law or
principle of physics or chemistry that relates the quantity of interest to some measur-
able event. Appendix B summarizes many of the physical laws and principles that
have potential application in sensor and transducer design. Some examples of appli-
cations are also provided. This list is useful to a transducer designer who is searching
for a method to measure a physical quantity. Practically every transducer applies one
or more of these principles in its operation.
Internet Link 9.1 provides links to numerous vendors and online resources for
a wide range of commercially available sensors and transducers. The Internet is
a good resource for finding the latest products in the mechatronics field. This is
especially true for sensors, where new technologies and improvements evolve
continuously.
9.2 POSITION AND SPEED MEASUREMENT
Other than electrical measurements (e.g., voltage, current, resistance), the most com-
monly measured quantity in mechatronic systems is position. We often need to know
where various parts of a system are in order to control the system. Section 9.2.1
presents proximity sensors and limit switches that are a subset of position sen-
sors that detect whether or not something is close or has reached a limit of travel.
Section 9.2.2 presents the potentiometer, which is an inexpensive analog device for
measuring rotary or linear position. Section 9.2.3 presents the linear variable differ-
ential transformer, which is an analog device capable of accurately measuring linear
displacement. Finally, Section 9.2.4 presents the digital encoder, which is useful for
measuring a position with an output in digital form suitable for direct interface to a
computer or other digital system.
Because most applications involve measuring and controlling shaft rotation
(e.g., in robot joints, numerically controlled lathe and mill axes, motors, and gen-
erators), rotary position sensors are more common than linear sensors. Also, linear
Internet Lin
k
9.1Sensor online
resources and
vendors
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Figure 9.1 Various configurations for photoemitter-detector pairs. (Courtesy of Banner Engineering,
Minneapolis, MN)
9.2 Position and Speed Measurement 377
motion can often be easily converted to rotary motion (e.g., with a belt, gear, or
wheel mechanism), allowing the use of rotary position sensors in linear motion
applications.
Speed measurements can be obtained by taking consecutive position measure-
ments at known time intervals and computing the time rate of change of the position
values. A tachometer is an example of a speed sensor that does this for a rotating
shaft.
9.2.1 Proximity Sensors and Switches
A proximity sensor consists of an element that changes either its state or an analog
signal when it is close to, but often not actually touching, an object. Magnetic,
electrical capacitance, inductance, and eddy current methods are particularly suited
to the design of a proximity sensor. Video Demo 9.1 shows an example application
for a magnetic proximity sensor. A photoemitter-detector pair represents another
approach, where interruption or reflection of a beam of light is used to detect an
object in a noncontact manner. The emitter can be a laser or focused LED, and
the detector is usually a phototransistor or photodiode. Various configurations for
photoemitter-detector pairs are illustrated in Figure 9.1 . In the opposed and ret-
roreflective configurations, the object interrupts the beam; and in the proximity
configuration, the object reflects the beam. Figure 9.2 shows a commercial sensor
that can be used in the retroreflective or proximity configurations. Video Demo 9.2
shows an interesting student project example of the proximity configuration. Com-
mon applications for proximity sensors and limit switches are detecting the pres-
ence of an object (e.g., a man in front of a public urinal), counting moving objects
(e.g., passing by on a conveyor belt), and in limiting the traverse of a mechanism
(e.g., by detecting the end of travel of a slider or joint).
There are many designs for limit switches, including pushbutton and levered
microswitches. All switches are used to open or close connections within circuits.
As illustrated in Figure 9.3 , switches are characterized by the number of poles (P)
Video Demo
9.1Magnetic
pickup tachom-
eter used in a PID
speed controller
test-stand
9.2Automated
laboratory rat
exercise machine
with infrared
sensor and
stepper motor
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Figure 9.2 Example of a photoemitter-detector
pair in a single housing. (Courtesy of Banner
Engineering, Minneapolis, MN)
Figure 9.3 Switches.
SPST
NC
SPDT
NO pushbutton
NC pushbutton
NO
378 CHAPTER 9 Sensors
and throws (T) and whether connections are normally open (NO) or normally
closed (NC). A pole is a moving element in the switch that makes or breaks con-
nections, and a throw is a contact point for a pole. The SPST switch is a single-pole
(SP), single-throw (ST) device that opens or closes a single connection. The SPDT
switch changes the pole between two different throw positions. There are many vari-
ations on the pole and throw configurations of switches, but their function is easily
understood from the basic terminology. Figure 9.4 and Video Demo 9.3 show vari-
ous types of switches with the appropriate designations. Video Demo 9.4 shows an
interesting example of a normally open mercury switch that is used to turn on or off
an air conditioning or heating unit when a bimetallic strip coil (see Section 9.4.2 )
rotates a certain amount.
When mechanical switches are opened or closed, they exhibit switch bou-
nce, where many break-reconnect transitions occur before a new state is estab-
lished. If a switch is connected to a digital circuit that requires a single transition,
the switch output must be debounced using a circuit or software as described in
Section 6.10.1.
Video Demo
9.3Switches
9.4Thermostat
with bimetallic
strip and mercury
switch
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Figure 9.4 Photograph of various types of switches.
NO
SPST
reset
switch
SPST
toggle
switch
NO
DPST
(dual SPST)
pushbutton
switches
SPDT
microswitches
NO
SPST
pushbutton
switch
9.2 Position and Speed Measurement 379
■ CLASS DISCUSSION ITEM 9.1
Household Three-Way Switch
A household three-way switch is used to allow lights to be controlled from two loca-
tions (e.g., at the top and bottom of a stairwell). Note that the term three-way refers
to the number of terminals (3) on each switch and not the number of switches (2).
A three-way switch is the SPDT variety. Draw a schematic of how AC power and
the two switches can be wired to a light fixture to achieve the desired functionality,
where either switch can be used to turn the light on or off.
9.2.2 Potentiometer
The rotary potentiometer (aka pot ) is a variable resistance device that can be used
to measure angular position. It consists of a wiper that makes contact with a resis-
tive element, and as this point of contact moves, the resistance between the wiper
and end leads of the device changes in proportion to the angular displacement.
Figure 9.5 illustrates the form and internal schematic for a typical rotary potenti-
ometer. Figure 9.6 shows two common types of potentiometers. The one on the left
is called a trim pot. It has a small screw on the left side that can be turned with a
Figure 9.5 Potentiometer.
wiper
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Figure 9.6 Photograph of a trim pot and a rotary pot.
380 CHAPTER 9 Sensors
screwdriver to accurately make small changes in resistance (i.e. “trim” or adjust the
resistance). On the right is a standard rotary pot with a knob allowing a user to eas-
ily make adjustments by hand. Through voltage division, the change in resistance
of a pot can be used to create an output voltage that is directly proportional to the
input displacement. This relationship was derived in Section 4.8.
9.2.3 Linear Variable Differential Transformer
The linear variable differential transformer (LVDT) is a transducer for measuring
linear displacement. As illustrated in Figure 9.7 , it consists of primary and secondary
windings and a movable iron core. It functions much like a transformer, where volt-
ages are induced in the secondary coil in response to excitation in the primary coil.
The LVDT must be excited by an AC signal to induce an AC response in the second-
ary. The core position can be determined by measuring the secondary response.
With two secondary coils connected in the series-opposing configuration as
shown, the output signal describes both the magnitude and direction of the core
motion. The primary AC excitation V
in
and the output signal V
out
for two different
core positions are shown in Figure 9.7 . There is a midpoint in the core’s position
where the voltage induced in each coil is of the same amplitude and 180 ⬚ out of
phase, producing a “null” output. As the core moves from the null position, the out-
put amplitude increases a proportional amount over a linear range around the null as
shown in Figure 9.8 . Therefore, by measuring the output voltage amplitude, we can
easily and accurately determine the magnitude of the core displacement. Internet
Link 9.2 points to an interesting animation that illustrates how the output voltage of
the LVDT changes with core displacement.
To determine the direction of the core displacement, the secondary coils can be
connected to a demodulation circuit as shown in Figure 9.9 . The diode bridges in this
circuit produce a positive or negative rectified sine wave, depending on which side of
the null position the core is located (see Class Discussion Item 9.2).
Internet Lin
k
9.2Animation of
LVDT function
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