Alciatore D.G., Histand M.B. Introduction to Mechatronics and Measurement Systems

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■ CLASS DISCUSSION ITEM 8.5

Bipolar 4-Bit D/A Converter

If V

⫽ 10 V and the ground reference is replaced by a ⫺ 10 V reference, what out-

put voltage would correspond to each possible binary input applied to a 4-bit D/A

R -2 R ladder network?

THREADED DESIGN EXAMPLE

DC motor power-op-amp speed controller—D/A converter interface A.5

The figure below shows the functional diagram for Threaded Design Example A (see

Section 1.3 and Video Demo 1.6), with the portion described here highlighted.

The D/A used in this design is an external 8-bit TLC7524 converter. Detailed information

about the device can be found in the data sheet at Internet Link 7.15. As shown in the figure

on the next page, the PIC interfaces to the D/A converter via two control lines and 8 data lines

(the entire PORTB register). One control line is the chip select pin that is used to activate the

D/A converter. The other is a write line that signals the D/A when to perform the conversion

and update the output voltage. Both lines are “active low.”

The code required to initialize the PIC outputs and to activate the D/A converter follows:

da_cs Var PORTA.3 ' external D/A converter chip select

(low:activate)

da_wr Var PORTA.4 ' external D/A converter write (low:

write)

TRISB ⫽ 0 ' initialize PORTB pins as outputs

High da_wr ' initialize the A/D converter write line

Low da_cs ' activate the external D/A converter

1.6 

DC motor

power-op-amp

speed controller

Video Demo

7.15 TLC7524

D/A converter

Internet Lin

potentiometer

PIC microcontroller

with analog-to-digital

converter

power

amp

motor

A/D D/A

light-

emitting

diode

digital-to-

analog

converter

(continued )

8.4 Digital-to-Analog Conversion 361

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362 CHAPTER 8 Data Acquisition

Figure 8.14 illustrates the roles that A/D and D/A converters play in a mecha-

tronic control system. An analog voltage signal from a sensor (e.g., a thermocouple)

is converted to a digital value, the computer uses this value in a control algorithm,

and the computer outputs an analog signal to an actuator (e.g., an electric motor) to

cause some change in the system being controlled. This topic is explored more in the

next section and in Chapter 11. Sensors and actuators are the topics of the next two

chapters.

The code required to update the voltage output from the D/A converter follows. The

voltage is stored as a scaled value in the PIC in a byte variable ( ad_byte ). The value ranges

from 0 (corresponding to 0 V) to 255 (corresponding to 5 V). The entire byte (all 8 bits)

is transmitted in parallel to the D/A converter through PORTB. Note the small pause

after the write command to ensure that the conversion has completed before the write is

disabled.

' Send the potentiometer byte to the external D/A

PORTB ⫽ ad_byte

Low da_wr

Pauseus 1 ' wait 1 microsec for D/A to settle

High da_wr

PIC16F88

RA3

RA4

PORTB

TLC7524C

D/A converter

4–11

13–6

2, 3

5 V

1, 14

REF

voltage

output

A/D

converter

D/A

converter

digital

signals

analog

signal

analog

signal

sensor

computer

actuator

Figure 8.14 Computer control hardware.

(continued )

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8.5 VIRTUAL INSTRUMENTATION, DATA

ACQUISITION, AND CONTROL

A virtual instrument consists of a personal computer equipped with data acqui-

sition hardware and software to perform the functions of traditional instruments.

Stand-alone traditional instruments such as oscilloscopes and waveform genera-

tors are very powerful, but they can be very expensive and sometimes limiting. The

user generally cannot extend or customize the instrument’s functionality. The knobs

and buttons on the instrument, the built-in circuitry, and the functions available

to the user are all specific to the instrument. Virtual instrumentation provides an

alternative.

Software is the most important component of a virtual instrument. It can be used

to create custom applications by designing and integrating the routines that a particu-

lar process requires. The software also allows one to create an appropriate user inter-

face that best suits the purpose of the application and the users who will interact with

it. The LabVIEW software from National Instruments (see Internet Link 8.4) is an

example of an easy-to-use application development environment designed specifi-

cally for creating virtual instruments. LabVIEW offers powerful features that make

connecting to a wide variety of hardware and other software easy.

One of the most powerful features of LabVIEW is its graphical programming

environment. Custom graphical user interfaces can be designed to allow a user to

■ CLASS DISCUSSION ITEM 8.6

Audio CD Technology

A compact disk (CD) stores music (an analog signal) in a digital format. How is this

done? How is the digital data reconverted to the music you hear? Given that audible

frequencies range from 20 Hz to 20 kHz, what is an appropriate sampling frequency?

For this sampling rate, how many bits must be stored on a CD to produce 45 minutes

of listening pleasure, assuming the music is stored uncompressed (e.g., as a WAV file

and not as an MP3 file)? Assume just the raw data is stored, without the sophisticated

error correction and compensation schemes described in Internet Link 8.5.

■ CLASS DISCUSSION ITEM 8.7

Digital Guitar

A digital guitar is a standard electric guitar equipped with additional components that

send MIDI signals to a digital synthesizer. MIDI stands for musical instrument digi-

tal interface, and a MIDI signal consists of digital bytes containing amplitude and

frequency codes for musical notes. What system components are required to do this?

Lin

8.5Audio

CD format

information

Lin

8.4 National

Instruments’

LabVIEW

software

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364 CHAPTER 8 Data Acquisition

operate the instrumentation program, control selected hardware, analyze acquired

data, and display results graphically, all on the PC screen. One can customize virtual

front panels with knobs, buttons, dials, and graphs to emulate control panels of tra-

ditional instruments, create custom test panels, or visually represent the control and

operation of processes. Figure 8.15 shows an example graphical user interface that

can easily be created. Figure 8.16 shows the block diagram used to create the user

interface and to perform the acquisition, analysis, and display functions. LabVIEW

is known as a visual programming environment, because to create a custom appli-

cation and user interface, you simply drag, drop, and connect icons on the screen.

Computer programming skills are not required.

LabVIEW, in addition to its virtual instrumentation and data acquisition capa-

bilities, provides many tools for model-based control system design. Tools are

available for characterizing (modeling) a system through data acquisition of test

data (a process called system identification ), designing a controller for a system,

simulating how the system will respond to various control inputs, and embedding

the controller into hardware to perform real-time control. Video Demo 8.8 shows

a demonstration of how these tools can be used to develop a speed controller for a

simple DC motor system. LabVIEW is used to send a command signal to the DC

motor and to monitor the resulting motion by acquiring a feedback signal from a

tachometer. Video Demo 8.9 provides much more background and detailed demon-

strations of the individual steps in the process. Also, the basics of control theory are

presented in Section 11.3.

Figure 8.15 Example custom graphical user interface created with LabVIEW.

8.8 National

Instruments DC

motor demo

8.9 National

Instruments

LabVIEW DC

motor controller

design

Video Demo

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Lab Exercise

Lab 12Data

Acquisition

8.6 PRACTICAL CONSIDERATIONS

This section introduces detailed procedures required to use the LabVIEW software

and a USB data acquisition card to sample and display analog signals. Specific tools

for sampling, displaying, and replaying audio signals are also presented. Detailed

instructions are provided for the National Instruments USB 6009, a specific USB data

acquisition module, to show how the software is used in practice. Lab Exercise 12

provides even more detail and explores the effects of sampling rate.

8.6.1 Introduction to LabVIEW Programming

LabVIEW is a user-friendly graphical programming environment. It has many built-

in features for data acquisition and works well with many commercial DAC cards. A

brief description of the basics of LabVIEW is presented below.

There are two primary windows in LabVIEW, the Block Diagram and Front

Panel window (see Figures 8.17 and 8.18 ). The Block Diagram window contains

the graphical program that you create, and the Front Panel window contains the user

interface. The user interface is used to input control parameters, run the program,

and visualize the results (e.g. the plot of a waveform).

Additional windows called palettes contain the libraries of built-in LabVIEW

functions. The Functions palette is a library of blocks that can be used in the Block

Diagram window (it is only available when the Block Diagram window is active).

The Controls palette is a library of the functions available for the front panel

Figure 8.16 Example LabVIEW block diagram.

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366 CHAPTER 8 Data Acquisition

(it is available only when the Front Panel window is active). The Tools palette is

used to modify the function of the cursor. Different tools are used to perform dif-

ferent functions. For example, connect wire is used to connect blocks, and

operate value

is used to used to change the value of a control (described below).

Alternatively, the automatic tool selection will automatically change

which tool you are using depending on the location of the cursor.

A LabVIEW VI (pronounced “vee eye”) file is made up of objects (or blocks)

with connections between the objects. There are two types of objects: nodes and

terminals. The nodes perform functions such as acquiring a digital signal from a

data acquisition card, multiplication, and signal processing. Terminals are the con-

nections between the block diagram and the front panel. Everything in the front panel

appears as a terminal on the block diagram. Every object has inputs, outputs, and

parameters that determine its function. For example, an analog-to-digital conversion

block will have an analog signal as its input (hardware input), a digital-value signal

as its output, and parameters such as sampling rate. The output of an A/D conversion

block can be an input to a block that graphically displays the waveform (terminal).

The parameters of a block can be set in different ways. One way is to open the prop-

erties window for the block (by right-clicking and selecting properties) and enter the

values for the parameters. Some of the parameters cannot be changed independently

of other parameters (e.g., parameters that define the configuration or mode of the

block) and can be set only within the properties window. The parameters that can be

Figure 8.17 Example block diagram.

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set independently of the others (such as sample rate and number of samples for an

A/D converter block) may be set using inputs. This can be done using constant or

control terminals. A constant is set in the block diagram, and a control is set in the

front panel. Figure 8.17 contains control blocks on the left labeled # periods to dis-

play, input frequency, and sample rate. Figure 8.18 shows the corresponding controls

in the front panel. These figures define the VI file to perform A/D conversion using

the National Instruments USB 6009 DAC card.

8.6.2 The USB 6009 Data Acquisition Card

The National Instruments USB 6009 is a typical small external data acquisition

card that is connected to a computer through a USB port. The device is pictured in

Figure 8.19 . It has A/D conversion capabilities as well as D/A conversion, digital

I/O, and counters/timers. The I/O lines are connected with wire (16-28 AWG) to the

detachable screw terminals.

As shown in Tables 8.2 and 8.3 , screw terminals 1–16 are used for analog I/O,

and terminals 17–32 are used for digital I/O and counter/timer functions. Notice

that the analog terminals are different depending on which mode the device is in,

single-ended mode (also known as RSE) or differential mode. In single-ended mode

the positive voltage signal is connected to an AI terminal and the negative voltage

or ground signal is connected to a GND terminal. This mode uses two terminals,

Figure 8.18 Example front panel.

8.6 Practical Considerations 367

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368 CHAPTER 8 Data Acquisition

meaning there are eight analog inputs available (AI0–AI7). The maximum voltage

range in this mode is –10V to 10V. Differential mode can be used to get a larger volt-

age range. This mode measures the difference between two signals, AI ⫹ and AI ⫺ ,

each referenced to GND. A voltage range of ⫺ 20V to 20V can be achieved, but the

maximum voltage on one pin (AI ⫹ or AI ⫺ ) referenced to ground is ⫾ 10V. Differ-

ential mode uses one more wire than single-ended mode, so only four analog inputs

are available in this mode.

Figure 8.19 The USB 6009 connected to the computer with the screw terminals attached.

123456789

10 11 12 13 14 15 16

17181920212223242526272829303132

Module Terminal

Signal,

Single-Ended Mode

Signal,

Differential Mode

GND

AI 0 AI 0+

AI 1+

AI 2+

AI 3+AI 3

AI 2

AI 6

AI 5

AI 4

AI 1

AI 7 AI 3−

AO 0

AO 1

AI 2−

AI 1−

AI 0−

GND

GNDGND

GND

AO 0

AO 1

GND

Module Terminal Signal

P0.0

P0.1

P0.7

P1.0

P1.1

P1.2

P1.3

PFI 0

+2.5 V

+5 V

P0.5

P0.2

P0.4

P0.3

P0.6

GND

Table 8.2 Analog (1–16) and Digital (17–32) pin assignments of the USB 2009

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Table 8.3 Signal descriptions for the USB 6009

Signal Name Reference Direction Description

GND — — Ground—The reference point for the single-ended AI measurements,

bias current return point for differential mode measurements, AO

voltages, digital signals at the I/O connector, ⫹5 VDC supply, and the

⫹2.5 VDC reference.

AI <0..7 > Varies Input Analog Input Channels 0 to 7—For single-ended measurements, each

signal is an analog input voltage channel. For differential measurements,

AI 0 and AI 4 are the positive and negative inputs of differential analog

input channel 0. The following signal pairs also form differential input

channels: <AI 1, AI 5 > , <AI 2, AI 6> , and <AI 3, AI 7> .

AO 0 GND Output Analog Channel 0 Output—Supplies the voltage output of AO

channel 0.

AO 1 GND Output Analog Channel 1 Output—Supplies the voltage output of AO

channel 1.

P1.<0..3>

P0.<0..7>

GND Input or

Output

Digital I/O Signals—You can individually configure each signal as an

input or output.

⫹2.5 V GND Output ⫹2.5 V External Reference—Provides a reference for wrap-back

testing.

⫹5 V GND Output ⴙ5 V Power Source—Provides ⫹5 V power up to 200 mA.

PFI 0 GND Input PFI 0—This pin is configurable as either a digital trigger or an event

counter input.

8.6 Practical Considerations 369

Another difference between differential and single-ended mode is the resolution

of the analog inputs. Differential mode has a resolution of 14 bits whereas single-

ended mode has a resolution of 13 bits.

The analog input converter type is successive approximation, and the maximum

sampling rate is 48 thousand samples per second (kS/s). The device contains one

analog-to-digital converter that is multiplexed to each input.

8.6.3 Creating a VI and Sampling Music

This example is based on LabVIEW version 8.0, but should be fairly compatible

with previous or later versions. The procedure also assumes that the USB 6009 is

already set up to interface with the computer according to the instructions that come

with the device.

Opening a blank VI ﬁ le

1 . Start LabVIEW with Start > Programs > National Instruments > LabVIEW 8.0 >

LabVIEW.

2 . Click Blank VI to start a new project. The Block Diagram window and the

Front Panel window should appear. If only one is open, then under the Win-

dows menu click Show Block Diagram or Show Front Panel. Some other small

windows may also be open.

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370 CHAPTER 8 Data Acquisition

3 . Open the Functions palette if it is not open. From the Block Diagram window,

under the View menu, click Functions palette to open the Functions palette.

Creating node blocks

1 . From the Functions palette, select Measurement I / O > NI - DAQmx. Drag the

DAQ Assist icon onto the Block Diagram. A DAQ Assistant window should

appear.

2 . Connect the USB 6009 device to the computer. The green light should start

blinking. From the DAQ Assistant window, select Analog Input > Volt-

age > ai 0 > Finish. If the ai0 does not appear, then press the plus next to “Dev1

(USB-6009)” to display the available analog input channels.

3 . A new window will open displaying the properties of the DAQ Assistant block

(see below).

a .

4 . Under Settings, set the maximum and minimum values for the Signal Input

Range based on the amplitude of the input and the desired quantization size.

5 . Under Settings, set the Terminal Configuration to RSE (single-ended mode) .

6 . Under the Task Timing tab, set the acquisition mode to N samples. The wiring

diagram can be view by selecting the Connection Diagram tab towards the

bottom of the window, assuming an appropriate range was selected (max ⱕ 10

and min ⱖ ⫺ 10).

7 . Select Ok to close the DAQ Assistant properties window. This may be opened

later by right-clicking on the DAQ Assistant block.

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