Alciatore D.G., Histand M.B. Introduction to Mechatronics and Measurement Systems
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Figure 7.25 Example battery discharge curve.
Time
Cell Voltage
mid-point voltage
end of life voltage
7.10 Practical Considerations 341
As shown in Table 7.8 , the chemistry of the cell will determine its open circuit
voltage. High peak-current-demand devices are good candidates for lead-acid and
nickel-cadmium (NiCd) batteries. If a device is in storage most of the time, alkaline
batteries are appropriate. Because batteries may be the heaviest component of a mecha-
tronic system design, the very light lithium-ion (Li-ion) and lithium-polymer chemis-
tries may be good candidates. Lithium chemistries provide the highest energy per unit
weight (specific energy) and per volume (energy density) of all types of batteries.
Most rechargeable batteries will function well even after hundreds of charge-
discharge cycles. They are significantly more expensive than primary cell batter-
ies. Nickel-metal-hydride (NiMH) batteries should be deep-discharged several times
Table 7.8 Characteristics for various types of batteries
Type Voltage
(open circuit)
Type Typical Ah Capacity R internal
(W)
9 V (heavy duty) 9 V primary 0.30 @ 1 mA 35
0.15 @ 10 mA
9 V alkaline 9 V primary 0.60 @ 25 mA 2
9 V lithium 9 V primary 1.0 @ 25 mA 18
alkaline D 1.5 V primary 17.1 @ 25 mA 0.1
0.95 @ 80 mA
alkaline C 1.5 V primary 7.9 @ 25 mA 0.2
alkaline AA 1.5 V primary 2.7 @ 25 mA 0.4
alkaline AAA 1.5 V primary 1.2 @ 25 mA 0.6
BR-C PCMF-Li 3 V primary 5.0 @ 5 mA
CR-V3 MnLi 3 V primary 3.0 @ 100 mA
NiCd D 1.3 V secondary 4.0 @ 800 mA 0.009
3.5 @ 4 A
NiCd 9 V 8.1 V secondary 0.1 @ 10 mA 0.84
Lead-acid D 2.0 V secondary 2.5 @ 25 mA 0.006
2.0 @ 1 A
NiMH AAA 1.2 V secondary 0.55 @ 200 mA
NiMH AA 1.2 V secondary 1.3 @ 200 mA
NiMH C 1.2 V secondary 3.5 @ 200 mA
NiMH D 1.2 V secondary 7.0 @ 200 mA
NiMH 9 V 8.4 V secondary 0.13 @ 200 mA
ML2430 MnLi 3 V secondary 0.12 @ 300 mA
Lithium Ion 3.7 V secondary 0.76 @ 200 mA
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342 CHAPTER 7 Microcontroller Programming and Interfacing
when put into service, for best performance. NiCd batteries can suffer from an effect
called “memory” where the battery capacity can diminish over time. This is caused
by shallow charge cycles where the battery is only partially discharged and then fully
charged repeatedly. Sometimes, you need to give the battery a deep discharge peri-
odically for best performance.
Excellent resources for battery information can be found online at Internet
Links 7.20 and 7.21. Also, Internet Link 7.22 offers good practical advice for using
batteries effectively.
7.10.4 Other Considerations for Project Prototyping
and Design
For basic prototype circuit assembly and troubleshooting advice, see Section 2.10.2.
For advice concerning debugging circuits with PIC microcontrollers, see Section
7.10.1 . Here are some other practical suggestions when prototyping and designing
projects:
■ When ordering ICs, make sure you specify DIP (dual in-line packages) and
not surface mount packages (e.g., SOP). DIP chips are well suited to use in
breadboards and protoboards. Surface mount ICs require printed circuit boards
(PCBs) and special soldering equipment.
■ Make sure your power supply can provide adequate current for the entire
design. If necessary, use separate power supplies for your signal and power
circuits.
■ Use breadboards with caution and care because connections can be unreliable,
and the base plate adds capacitance to your circuits. Hardwired and soldered
protoboards or printed circuit boards (PCBs) can be much more reliable (see
Section 2.10.4 for more information). For soldered prototypes, be sure to
use sockets for all ICs, to prevent damage during soldering and to allow easy
replacement of the ICs. Also, if you have a working breadboard circuit, it is
advisable to use duplicate components (where possible) for the soldered board
(i.e., don’t cannibalize components from a working prototype circuit, in case
something goes wrong or gets damaged when soldering your board).
■ Use a storage capacitor (e.g., 100 μf) across the main power and ground lines
of a power supply that does not have built-in output capacitance (e.g., batteries,
wall transformers, and regulated voltages) to minimize voltage swings
during output current spikes. Also, use “bypass” or “decoupling” capacitors
(e.g., 0.1 μf) across the power and ground lines of all individual ICs to
suppress any current and voltage spikes.
■ Be careful with grounding and electromagnetic interference (EMI). Section
2.10.6 presents various methods to reduce EMI, specifically using opto-
isolators, single point grounding, ground planes, coaxial or twisted pair cables,
and decoupling capacitors.
■ Don’t leave IC pins floating (especially with CMOS devices). In other words,
connect all functional pins to signals or power or ground. As an example, do
Internet Lin
k
7.20Battery
University
7.21Battery
information
resource page
7.22All about
circuits—batteries
and power
systems
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Questions and Exercises 343
not assume that leaving a microcontroller’s reset pin disconnected will keep a
microcontroller from resetting itself. You should connect the reset pin to 5 V
for an active-low reset or ground for an active-high reset, and not leave the pin
floating where its state can be uncertain.
■ Be aware of possible switch bounce in your digital circuits and add debounce
circuits or software to eliminate the bounce (see Section 6.10.1).
■ Use flyback diodes on motors, solenoids, and other high-inductance devices
that are being switched (see Section 3.3).
■ Use buffers, line drivers, and inverters where current demand is large for a
digital output.
■ Use Schmitt triggers (see Section 6.12.2) on all noisy digital inputs (e.g., a
Hall-effect proximity sensor or photo-interrupter).
■ Use a common-emitter configuration with transistors (i.e., put the load on the
high side) to avoid voltage biasing difficulties and emitter degeneration (see
Section 3.4.2).
■ Be careful to identify and properly interface any open-collector or open-drain
outputs (see Section 6.11.2) on digital ICs (e.g., pin RA4 on the PIC16F84).
■ For reversible DC motors, use “off-the-shelf” commercially available H-bridge
drivers (e.g., National Semiconductor’s LMD 18200) instead of building your
own. For more information, see Internet Link 10.3.
QUESTIONS AND EXERCISES
Section 7.4 Programming a PIC
7.1. Document a complete and thorough answer to Class Discussion Item 7.2.
Section 7.5 PicBasic Pro
7.2. Write an assembly language program to turn an LED on and off at 0.5 Hz. Draw the
schematic required for your solution.
7.3. Write a PicBasic Pro program to turn an LED on and off at 1 Hz while a pushbutton
switch is held down. Draw the schematic required for your solution.
7.4. Write a PicBasic Pro program to perform the functionality of the Pot statement (see
Class Discussion Item 7.7).
7.5. Write a PicBasic Pro subroutine to provide a software debounce on pin RB0 (see
Class Discussion Item 7.8). The code should wait for the pushbutton switch to be
pressed and released while ignoring the switch bounce.
7.6. Microcontrollers usually do not include D/A converters, but you can easily create a
crude version of one using a single digital I/O pin. This can be done by outputting
a variable-width pulse train to an RC circuit. See the documentation for the PWM
statement in the PicBasic Pro compiler manual for more information. Write a
PicBasic Pro subroutine that uses pin RA0 to output a constant voltage (between
0 and 5 V) that is proportional to the value of a byte variable called digital_value,
Internet Lin
k
10.3H-bridge
resource
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344 CHAPTER 7 Microcontroller Programming and Interfacing
whose value can range from 0 to 255. The subroutine should hold this voltage for
approximately 1 sec.
7.7. For the security system in Example 7.5 , explain what would happen if a burglar
closes an opened door or window after setting off the alarm? Consider all pertinent
operating states. How could the design be improved to overcome any perceived
limitations?
Section 7.6 Using Interrupts
7.8. Write a PicBasic Pro program to implement an interrupt-driven solution to the
security system presented in Example 7.9.
Section 7.7 Interfacing Common PIC Peripherals
7.9. Write a PicBasic Pro program to display the value of a potentiometer as a percentage
on an LCD display. The message should have the form pot value X%, where X is
the percentage value ranging from 0 to 100. Draw the schematic required for your
solution.
7.10. Write a PicBasic Pro program that allows the user to enter a set of a multidigit
numbers (up to five digits) on a numeric keypad. Have the # key serve as an enter
key; and when a number is entered, it should appear on a two-line LCD display.
The first number should appear on the first line, the second number should appear on
the second line, and subsequent numbers should appear on the second line with the
previous number moving up to the first line. Draw the schematic required for your
solution.
7.11. In Figure 7.13 , explain why there is a 1 k resistor attached from 5 V to RA4. How
does this interface differ from connecting RA4 directly to the V
e e
pin of the LCD
(with no resistor or 5 V connection)?
Section 7.9 Method to Design a Microcontroller-
Based System
7.12. Apply the procedure presented in Section 7.9 to solving the problem stated in
Question 6.44. Perform every step in detail. Do as much as possible in software
(e.g., instead of using flip-flops).
7.13. Solve Question 7.12 based on the revised requirements in Question 6.58.
7.14. Using a PIC16F84 and two 7447 display decoders (see Design Example 7.1), write a
PicBasic Pro program to implement a counter with a two-digit display controlled by
three pushbutton switches: one to reset the count to 0, one to increment the count
by 1, and the third to decrement the count by 1. If the count is less than 10, the first
digit should be blank. Incrementing past 99 should reset the count to 0, and decre-
menting below 0 should not be allowed. Make sure you debounce the switch input
where necessary and prevent repeats while the pushbutton switches are held down.
Use the design procedure presented in Section 7.9 and show the results for each step.
7.15. Visit Microchip’s website and select a flash-memory PIC microcontroller available
as a DIP package that has at least 16 lines of digital I/O and 2 A/D converters. Select
the lowest cost model that meets these requirements, and list the approximate cost,
the number of I/O pins, the number of A/D converters (along with their resolutions),
and the total number of pins on the package.
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Bibliography 345
7.16. Draw a detailed flowchart illustrating all of the functionality of Threaded Design
Example A.
7.17. Draw a flowchart illustrating the functionality of the main program (not including
details for the subroutines) for Threaded Design Example B.
7.18. Draw a detailed flowchart illustrating all of the functionality of the move subroutine
in Threaded Design Example B. Include details for the move_steps and step_motor
subroutines.
7.19. Draw a detailed flowchart illustrating all of the functionality of the speed subroutine
in Threaded Design Example B. Include details for the get_AD_value subroutine.
7.20. Draw a detailed flowchart illustrating all of the functionality of the position subrou-
tine in Threaded Design Example C. Include details for the get_encoder subroutine.
7.21. Draw a detailed flowchart illustrating all of the functionality of the slave PIC code in
Threaded Design Example C.
BIBLIOGRAPHY
Gibson , G. and Liu , Y. , Microcomputers for Engineers and Scientists, Prentice-Hall ,
Englewood Cliffs, NJ, 1980 .
Herschede , R. , “Microcontroller Foundations for Mechatronics Students,” master's thesis,
Colorado State University, summer 1999 .
Horowitz , P. and Hill , W. , The Art of Electronics, 2nd Edition, Cambridge University Press ,
New York, 1989 .
Microchip Technology, Inc., www.microchip.com , 2010 .
Microchip Technology, Inc., PIC16F8X Data Sheet, Chandler, AZ, 1998 .
Microchip Technology, Inc., MPASM User’s Guide, Chandler, AZ, 1999 .
Microchip Technology, Inc., MPLAB User’s Guide, Chandler, AZ, 2000 .
microEngineering Labs, Inc., www.melabs.com , 2010 .
microEngineering Labs, Inc., PicBasic Pro Compiler, Colorado Springs, CO, 2008 .
Motorola Technical Summary, “MC68HC11EA9/MC68HC711EA9 8-bit Microcontrollers,”
document number MC68HC11EA9TS/D, Motorola Advanced Microcontroller Divi-
sion, Austin, TX, 1994 .
Peatman , J. , Design with Microcontrollers, McGraw-Hill , New York, 1988 .
Predko , M. , Programming and Customizing PICmicro Microcontrollers, McGraw-Hill , New
York, 2001
Stiffler , A. , Design with Microprocessors for Mechanical Engineers, McGraw-Hill , New
York, 1992 .
Texas Instruments, TTL Linear Data Book, Dallas, TX, 1992 .
Texas Instruments, TTL Logic Data Book, Dallas, TX, 1988 .
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346
CHAPTER OBJECTIVES
After you read, discuss, study, and apply ideas in this chapter, you will:
1. Understand how to properly sample a signal for digital processing
2. Understand how digitized data are coded
CHAPTER
8
Data Acquisition
T
his chapter presents the concepts involved with converting between analog
and digital signals. This is important when interfacing digital circuits to ana-
log components in mechatronic systems. The concept of virtual instrumen-
tation is also introduced along with LabVIEW software examples from National
Instruments. ■
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
- system model - dynamic response
DIGITAL CONTROL
ARCHITECTURES
- logic circuits
- microcontroller
- SBC
- PLC
- sequencing and timing
- logic and arithmetic
- control algorithms
- communication
amplifiers
The authors express their thanks to Jim Cahow of National Instruments (Austin, TX) for providing
resources and support for the editing of this chapter.
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8.1 Introduction 347
3. Know the components of an A/D converter
4. Understand how A/D and D/A converters function and recognize their
limitations
5. Be aware of commercially available hardware and software tools for data
acquisition and control
6. Understand the basics of LabVIEW programming and data acquisition
7. Understand the effects of sampling rate and resolution on music sampling
8.1 INTRODUCTION
Microprocessors, microcontrollers, single-board computers, and personal computers
are in widespread use in mechatronic and measurement systems. It is increasingly
important for engineers to understand how to directly access information and analog
data from the surrounding environment with these devices. As an example, consider a
signal from a sensor as illustrated by the analog signal in Figure 8.1 . One could record
the signal with an analog device such as a chart recorder, which physically plots the
signal on paper, or display it with an oscilloscope. Another option is to store the data
using a microprocessor or computer. This process is called computer data acquisition,
and it provides more compact storage of the data (magnetic, optical, or flash media vs.
long rolls of paper), can result in greater data accuracy, allows use of the data in a real-
time control system, and enables data processing long after the events have occurred.
To be able to input analog data to a digital circuit or microprocessor, the analog
data must be transformed into coded digital values. The first step is to numerically
evaluate the signal at discrete instants in time. This process is called sampling, and
the result is a digitized signal composed of discrete values corresponding to each
sample, as illustrated in Figure 8.1 . Therefore, a digitized signal is a sequence of
numbers that is an approximation to an analog signal. Note that the time relation
between the numbers is an inherent property of the sampling process and need not
be recorded separately. The collection of sampled data points forms a data array, and
although this representation is no longer continuous, it can accurately describe the
original analog signal.
Figure 8.1 Analog signal and sampled equivalent.
analog
signal
digitized signal
voltage
time
sampled
point
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348 CHAPTER 8 Data Acquisition
An important question is how fast or often the signal should be sampled to
obtain an accurate representation. The naive answer might be “as fast as you pos-
sibly can.” The problems with this conclusion are that specialized, high-speed hard-
ware is required and a large amount of computer memory is needed to store the data.
A better answer is to select the minimal sampling rate required for a given applica-
tion that retains all important signal information.
The sampling theorem, also called Shannon’s sampling theorem, states that we
need to sample a signal at a rate more than two times the maximum frequency compo-
nent in the signal to retain all frequency components. In other words, to faithfully rep-
resent the analog signal, the digital samples must be taken at a frequency f
s
such that
f
s
> 2 f
max
(8.1)
where f
max
is the highest frequency component in the input analog signal. The term f
s
is referred to as the sampling rate, and the limit on the minimum required rate (2 f
max
)
is called the Nyquist frequency. If we approximate a signal by a truncated Fourier
series, the maximum frequency component is the highest harmonic frequency. The
time interval between the digital samples is
Δt = 1 ⁄ f
s
(8.2)
As an example, if the sampling rate is 5000 Hz, the time interval between samples
would be 0.2 ms.
If a signal is sampled at less than two times its maximum frequency component,
aliasing can result. Figure 8.2 illustrates an example of this with an analog sine wave
sampled regularly at the points shown. Twelve equally spaced samples are taken over
10 cycles of the original signal. Therefore, the sampling frequency is 1.2 f
0
, where f
0
is the frequency of the original sine wave. Because the sampling frequency is not
greater than 2 f
0
, we do not capture the frequency in the original signal. Furthermore,
the apparent frequency in the sampled signal is 0.2 f
0
(2 aliased signal cycles for ten
original signal cycles). You can think of this as a “phantom” frequency, which is an
alias of the true frequency. Therefore, undersampling not only results in errors but
also creates information that is not really there! Video Demo 8.1 demonstrates the
concepts of sampling rate, Nyquist frequency and aliasing, applied to a sine wave
signal, and Video Demo 8.2 provides an interesting visual example of aliasing.
8.1Sampling
rate, Nyquist
frequency, and
aliasing
8.2Tuning fork
super-slow-motion
video aliasing
Video Demo
sampled point
original signal
aliased signal
Figure 8.2 Aliasing.
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The method of Fourier decomposition presented in Chapter 4 provides a means
to determine the frequency components of an arbitrary analog signal. Representing a
signal in terms of its frequency components allows us to identify the bandwidth and
correctly apply the sampling theorem.
■ CLASS DISCUSSION ITEM 8.1
Wagon Wheels and the Sampling Theorem
Relate the sampling theorem and its implications about correct and incorrect data
representation to the use of a movie camera to film a rotating wagon wheel in a
Western movie. The camera shutter can be considered a sampling device, where the
shutter speed is the sampling rate. Typically, the sampling rate is 30 Hz to provide
a flicker-free image to the human eye. What effects would you expect in the movie,
when viewed, if the wagon wheel is turning rapidly?
Sampling Theorem and Aliasing
EXAMPLE 8.1
Consider the function
Ft() at()sin bt()sin+=
Using a trigonometric identity for the sum of two sinusoidal functions, we can rewrite F ( t )
as the following product:
Ft() 2
ab–
2
----------- -
t
⎝⎠
⎛⎞
cos
ab+
2
------------
t
⎝⎠
⎛⎞
sin⋅=
If frequencies a and b are close in value, the bracketed term has a very low frequency in
comparison to the sinusoidaal term on the right. Therefore, the bracketed term modulates
the amplitude of the higher frequency sinusoidal term. The resulting waveform exhibits what
is called a beat frequency that is common in optics, mechanics, and acoustics when two
waves close in frequency add. For more information and an audio example of beat frequency,
see Internet Link 8.1 and Video Demo 8.3.
To illustrate aliasing associated with improper sampling, the waveform is plotted in the
following figures using two different sampling frequencies. If a and b are chosen as
b = 0.9aa 1
Hz 2π
rad
sec
--------
==
then to sample the signal F ( t ) properly, the sampling rate must be more than twice the high-
est frequency in the signal:
f
s
2a> 2 Hz=
Therefore, the time interval between samples (plotted points) must be
Δt
1
f
s
---
0.5
sec<=
8.3Beat
frequency from
mixing signals of
similar frequency
Video Demo
(continued )
In
te
rn
et
Lin
k
8.1Beat
frequency
8.1 Introduction 349
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Lab Exercise
Lab 12 Data
acquisition
MathCAD Example
8.1 Sampling
theorem, aliasing,
beat frequency
350 CHAPTER 8 Data Acquisition
Lab Exercise 12 explores the effects of using different sampling rates when digi-
tizing a sine wave signal and audio from a music player. Video Demos 8.4 and 8.5 dem-
onstrate the results. Video Demo 8.6, which contains only audio, and Video Demo 8.7
illustrate the effects of sampling rate on various tones and music recordings. For
The first data set is plotted with a time interval of 0.01 sec (100 Hz sampling rate),
providing an adequate representation of the waveform. The second data set is plotted with a
time interval of 0.75 sec (1.33 Hz sampling rate), which is less than twice the maximum fre-
quency of the waveform (2 Hz). Therefore, the signal is undersampled, and aliasing results.
The sampled waveform is an incorrect representation, and its observed maximum frequency
appears to be approximately 0.4 Hz because there are approximately four cycles over 10 sec.
You might argue that neither plot is perfect. However, unlike the second plot, the first
plot retains all the frequency information in the analog signal. MathCAD Example 8.1 con-
tains the analysis used to generate the plots above. You can easily edit the file to see the
effects of different sampling rates.
(continued )
F(t
0.01
)
t
0.01
F(t
0.75
)
t
0.75
40
■ CLASS DISCUSSION ITEM 8.2
Sampling a Beat Signal
What is the minimum sampling rate required to adequately represent the signal in
Example 8.1 ?
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