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Transcript
```DIGITAL INSTRUMENTATION PRINCIPLES
WHY DIGITAL?
- Transducers: analog output.
- Analog to digital:
- precision
- values not changed during processing
- signal processing using micro-processing.
Digital instrumentation principles
1
- Analog signal is converted into a digital signal: sampling and hold.
Digital instrumentation principles
2
• Switch is closed (sampling phase), delay
• Switch is opened (holding phase), delay
Digital instrumentation principles
3
• Delay on switching from hold to sample (acquisition time).
• Binary digits 1 or 0,
o '1' (high): Vcc down to +2V,
o '0' (low): 0V up to +0.8V
Digital instrumentation principles
4
THE SAMPLING THEOREM
“A continuous signal can be represented completely by, and reconstructed
from, a set of instantaneous measurements or samples of its voltage which are
made at equal spaced times. The interval between such samples must be less than
one-half of the period of the highest-frequency component in the signal”.
Convert an analog signal into digital samples and convert them back into
original signal provided the number of samples per second is more than twice
the highest frequency in the signal.
Digital instrumentation principles
5
Suppose the signal contains a higher frequency than we expected, so that in
fact the number of samples per second is less than twice that frequency. We
then get what is known as aliasing.
Digital instrumentation principles
6
• Instrumentation signal is a varying voltage (not a regularly repeating
waveform).
• By Fourier analysis, any such shape can be shown to be a summation of a
constant term (DC voltage) and sine waves of various amplitudes and
frequencies.
Digital instrumentation principles
7
ANALOG TO DIGITAL CONVERSION
1. Single-slope
or single-ramp A/D converter
. Simple
. Cheap
. Comparatively slow
. Inaccuracy
Digital instrumentation principles
8
2. Dual-slope A/D converter
Cancels out inaccuracy due to variations in clock frequency or ramp slope by
using two ramps in a count-up, count-down process.
Digital instrumentation principles
9
. Accuracy is unaffected by drift (upward and downward ramps are affected equally).
. High-frequency noise disappears in the integration
. Changes in the analog input signal during the integration period are averaged out.
. Slow speed (digital voltmeters and applications does not require fast conversion).
Digital instrumentation principles
10
3. Successive-approximation A/D converter
Faster than the ramp type
- Set 1 starting from MSB,
- Compare: < analog, retain 1, else =0,
- Continue to lower bits
Digital instrumentation principles
11
4.
Parallel, simultaneous or “flash” A/D converter
Fastest converter of all
Digital instrumentation principles
12
. n-bit converter requires (2n-1) comparators (additional bit doubles
comparators).
. Single integrated circuit chip.
. Limit is 11 bits (12-bit resolution=two 11- bit chips in series)
. 8-bits is a more usual size (255 comparators, 20 million A/D conversions per
second.
Digital instrumentation principles
13
A/D Consideration:
• Saturation error (full scale, upper/lower limits)
• Resolution and quantization error
full scale/2n
• Conversion errors (non-linearity, zero-offset, scale error)
• Sample rate
• Signal conditioning for A/D conversion (prevent aliasing: filter removes >fs/2)
Digital instrumentation principles
14
DIGITAL TO ANALOG CONVERSION
Digital instrumentation principles
15
Digital instrumentation principles
16
. types
. resolution
. speed (settling time: time it takes for a full scale input to be converted to within
half of the least significant bit (1/2 LSB)).
. The number of bits also determines the resolution.
. Each additional bit in the binary number doubles the number of steps in the
graph, and so halves the (theoretical) maximum error in the output voltage.
. The actual maximum error depends also on the stability of the reference
voltage applied to the chip and the stability of the resistors in the switching
circuit.
Digital instrumentation principles
17
A COMPLETE DIGITAL SYSTEM
. First LPF filters out high-frequency noise and prevent aliasing,
. Digital filter between the A/D and D/A converters
. A digital filter more precise than the analog version
Digital instrumentation principles
18
. Never get absolutely perfect reproduction of the input signal at the output
. Process of sampling changes a smooth analogue curve into an approximation
composed of steps made up of horizontal and vertical lines
. Superimpose the approximation onto the original curve will show the divergence
between them is a maximum at the sharp corners of the steps
. By working from sample voltages, noise is introduced (quantization noise)
. Lower quantization noise: reduce the height of the steps by increasing the number
of voltage levels at which the LSB changes by 1, and this will require more digits
in the digital conversion of the voltage samples.
Digital instrumentation principles
19
Example:
1. Question 4: (20 marks) – Final 2001
Consider the circuit in a temperature measurement below. The A/D is a 5-bit
successive-approximation A/D converter type with an analog span of 0 to 10V, find
the input voltage of the A/D converter. The thermistor, RT, has a resistance of 2K at
20OC and the coefficient β is assumed to be constant at 1650, find temperature of the
thermistor.
R T = R 0e
1 1
β( − )
T T0
12V
10M
4.7K
4.7K
1M
V1
V2
_
5-bit
A/D
0-10V
+
1M
4.7K
MSB
RT
tOC= ?
Digital instrumentation principles
10M
LSB
1
0
0
1
1
?
20
Data-acquisition systems (DAS)
• Measurement system quantifies and stores data.
• Dedicated microprocessor systems continuously perform their programming
instructions to:
measure,
store,
interpret,
provide process control without any intervention.
Digital instrumentation principles
21
• Microprocessors have I/O ports to interface with other devices to measure and to
output instructions.
• Programming allows for operations such as, which sensors to measure, and when
and how often, and for data reduction.
• Programming can allow for decision-making and feedback to control process
variables.
Signal flow scheme for an automated data-acquisition system
Digital instrumentation principles
22
• Personal computer-based data-acquisition systems:
♦ Hybrid systems combining a data-acquisition package with both the
microprocessor and human interface capability of a personal computer
(PC).
♦ Using a dedicated microprocessor (handling repetitive tasks involving
measurement, recording, and control and portable.)
♦ Direct serial or parallel communication with a host computer is possible.
♦ Availability and flexibility
♦ Data can be recorded
♦ Computer can be used to interface with control equipment, make
programmed decisions to control the measured process, to send data over a
network line, to reduce the data into results, and to write the final report.
Digital instrumentation principles
23
DAS Components:
1. Signal conditioning:
a. Filters: analog or digital
b. Amplifiers or attenuators:
Digital instrumentation principles
24
c. Shunt circuits
2. Analog multiplexers
Digital instrumentation principles
25
3. A/D converter
4. D/A converter
5. Digital Input-Output:
1 or 0 , level voltages depend on interfaces (TTL, CMOS: 1.8V,
3.3V, 5V, ...)
6. Central Processing Unit: microporcessor
Speed (clock and bus size)
Stand alone (direct application uP)
7. Memory
RAM, ROM, external memory
Digital instrumentation principles
26
8. Central Bus
9. Buffers
Small amounts of pre-assigned RAM.
Holding tank (between 2 devices operating at different speeds).
Digital instrumentation principles
27
Data conversion in a process control system
Digital instrumentation principles
28
Analog meets digital
Digital instrumentation principles
29
1. Data-Acquisition boards
Digital instrumentation principles
30
2. Single- and Differential-Ended Connections
3. Special signal conditioning modules
4. Data-acquisition triggering
5. Data transfer
Digital instrumentation principles
31
A strain-gauge interface (on-board Wheatstone bridge).
Gauges are connected by wires to the interface
Digital instrumentation principles
32
A 16-Bit, 200-kHz PCI Data Acquisition Board
•
16-bit, 200-kHz A/D converter
•
100% digital calibration
•
DMA bus mastering for synchronous analog I/O, digital I/O, and counter inputs
•
•
•
•
Up to four 16-bit, 100-kHz analog outputs with infinite continuous waveform
output capability
40 digital I/O lines, can be scanned synchronously or asynchronously with
Four counter/pulse input channels can be scanned synchronously or
Two timer/pulse output channels
Digital instrumentation principles
33
•
The PC
•
DAQ Hardware
•
Software
•
Signal Conditioning
•
Transducers
Digital instrumentation principles
34
DAQ Hardware:
o Number of Channels
o Sampling Rate
o Resolution
Digitized Sine Wave with a Resolution of Three Bits
o Range
o Settling Time (changing between channels, signal strength, …)
o Noise (proper shielding, layout technique,…)
Digital instrumentation principles
35
- Analog outputs (D/A):
o Settling time
o Slew rate
o Resolution
- Digital I/O (communications)
- Timing I/O:
o Counter resolution (number of bits)
o Clock frequency
Digital instrumentation principles
36
DAQ Software:
Software transforms the PC and the DAQ hardware into a complete data
acquisition, analysis, and display system (LabVIEW)
National Instruments created VI Logger application software to aid users in data logging
applications
Digital instrumentation principles
37
Automobile Lubricant Test Application showing a SCXI Chassis and LabVIEW running
on a Macintosh
Digital instrumentation principles
38
Digital Input-Output Communications
• Standards to communicate between digital devices
• Serial (bit by bit) or parallel (byte by byte)
• Handshake (interface procedure)
• Signal levels
Digital instrumentation principles
39
1. Serial Communications RS-232C
• Protocol allows two-way communication by using two single-ended signal (+)
wires: TRANSMIT and RECEIVE, between data-communications equipment
(DCE) and data-terminal equipment (DTE).
• A signal GROUND wire allows signal return (-) paths.
• The remaining wires in the original standard are used to access the state of the
telephone lines, if needed.
Digital instrumentation principles
40
• Minimum number of wires required between DTE and DCE equipment is the
three-wire connection (TRANSMIT, RECEIVE, and GROUND lines)
Standard RS-232C assignments to a 25-pin connector
Digital instrumentation principles
41
• Communication between similar equipment, DTE to DTE or DCE to DCE,
requires only the nine lines
Standard serial connections between DTE and DTE or DCE to DCE equipment
Digital instrumentation principles
42
• Data are sent in successive streams of information, 1 bit at a time.
• Value of each bit is represented by an analog voltage pulse (1 and 0)
distinguished by two equal voltages of opposite polarity in the range of 3-25V.
• Communication rates are measured in baud, (number of signal pulses per
second).
• A typical transmission is 10 serial bits:
♦ 1 start bit, 7-or 8-bit data stream, 1 or no parity bit, terminated by 1 or 2
stop bits.
• The start and stop bits form the serial "handshake"
• The start bit allows for synchronization of the clocks of the two communicating
devices
• The parity bit allows for limited error checking.
• Asynchronous transmission: information may be sent at random intervals.
Digital instrumentation principles
43
Digital instrumentation principles
44
2. RS-422A/423A/449/485
• 37-pin connector and differential-ended connections to reduce noise,
• Rates up to 2-M baud
• Distances up to 1000 m.
• +5V TTL pulse signal to distinguish between a 1-bit (+2 to +5.5V) and a 0-bit (0.6 to +0.8V).
• RS-485 protocol allows for multidrop (allows up to 32 to 255 devices on one
line) operation that is well suited to local area networks (LANs).
Digital instrumentation principles
45
RS449 / V.11
RS-449
Digital instrumentation principles
46
3. Universal Serial Bus
•
Permit peripheral expansion for up to 128 devices
•
Support low to medium transfer rates from 1.5Mbs up to 12Mbs.
•
Supports a "hot swap": allows to plug in without reboot.
Digital instrumentation principles
47
4. Parallel Communications GPIB (IEEE-488)
• General purpose interface bus (GPIB)
• High-speed parallel interface
• 16-wire bus with a 24-wire connector.
• 8 data lines
• 8 lines for bus management and handshaking (two-way control
communication)
• 8 lines are used for grounds and shield.
• Bit parallel, byte serial communication at data rates up to 1 Mbyte/s
• GPIB devices: Listeners, Talkers, and/or Controllers
Digital instrumentation principles
48
Digital instrumentation principles
49
Digital instrumentation principles
50
Km/h
2. Example:
?
0
10
t(s)
Velocity
Display
100K
5µ
µF
Accelerometer
8-bit
A/D,
0-10V
_
A
+
Gain=100
?
?
?
MSB
.
.
.
.
µP
and
D/A
LSB
Input
display
?
6.6V
0
10
t(s)
The above arrangement is used to measure the velocity of a moving vehicle. The
waveform shown at the input display is the output of the D/A converter (data from
the A/D connects directly to the D/A). Ignore quantization error, find the A/D output
word. Sketch analog input voltage waveform at the input of the A/D converter, the
input of amplifier A and the accelerometer output. The accelerometer has an
inversion factor of 0.25V/m/s2 (i.e., 250mV corresponds to 1m/s2). Find the final
velocity of the vehicle if its initial velocity is 100Km/h and sketch the velocity
display of the vehicle.
Digital instrumentation principles
51
```
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