Download DIGITAL INSTRUMENTATION PRINCIPLES

DIGITAL INSTRUMENTATION PRINCIPLES
WHY DIGITAL?
- Transducers: analog output.
- Analog to digital:
- precision
- values not changed during processing
- signal processing using micro-processing.
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- Analog signal is converted into a digital signal: sampling and hold.
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• Switch is closed (sampling phase), delay
• Switch is opened (holding phase), delay
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• 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
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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.
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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.
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• 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.
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ANALOG TO DIGITAL CONVERSION
1. Single-slope
or single-ramp A/D converter
. Simple
. Cheap
. Comparatively slow
. Inaccuracy
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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.
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. 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).
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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
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4.
Parallel, simultaneous or “flash” A/D converter
Fastest converter of all
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. 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.
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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)
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DIGITAL TO ANALOG CONVERSION
R-2R ladder
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. 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.
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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
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. 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.
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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= ?
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10M
LSB
1
0
0
1
1
?
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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.
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• 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
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• 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.
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DAS Components:
1. Signal conditioning:
a. Filters: analog or digital
b. Amplifiers or attenuators:
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c. Shunt circuits
2. Analog multiplexers
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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
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8. Central Bus
9. Buffers
Small amounts of pre-assigned RAM.
Holding tank (between 2 devices operating at different speeds).
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Data conversion in a process control system
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Analog meets digital
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Analog Input-Output Communications
1. Data-Acquisition boards
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2. Single- and Differential-Ended Connections
3. Special signal conditioning modules
4. Data-acquisition triggering
5. Data transfer
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A strain-gauge interface (on-board Wheatstone bridge).
Gauges are connected by wires to the interface
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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
analog inputs
Four counter/pulse input channels can be scanned synchronously or
asynchronously with analog inputs
Two timer/pulse output channels
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•
The PC
•
DAQ Hardware
•
Software
•
Signal Conditioning
•
Transducers
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DAQ Hardware:
- Analog inputs:
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,…)
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- 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
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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
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Automobile Lubricant Test Application showing a SCXI Chassis and LabVIEW running
on a Macintosh
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Digital Input-Output Communications
• Standards to communicate between digital devices
• Serial (bit by bit) or parallel (byte by byte)
• Handshake (interface procedure)
• Signal levels
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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.
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• 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
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• 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
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• 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.
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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).
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RS449 / V.11
RS-449
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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.
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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
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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.
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