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EE-208 APPLIED INSTRUMENTATION
Unit-3
Measurement of Speed
and Torque
DEPARTMENT OF ELECTRICAL ENGINEERING
IIT ROORKEE, ROORKEE-247667
2/17/2015 1
Reference List
1. C. D. Johnson, “ Process Control Instrumentation Technology, Prentice Hall of
India Private Ltd., 2008
2. Deoeblin E.O. “Measurement system,” Tata McGraw-Hill Publishing Company
Ltd, 2008.
3. Rangan C.S., Sarma G.R., Mani V.S.V “Instrumentation Devices and
Systems,Tata McGraw-Hill Publishing Company Ltd, 2008.
4. A. Kutiyanawala,
mechatronics.ece.usu.edu/.../ali_Tachometers%20%20An%20Overview, Utah
State University.
5. https://uqu.edu.sa/files2/tiny_mce/.../Speed_measurement.pptx
Applications
Any linear/rotary position/velocity sensing
DC Motor control – robotics/automation
Mechanical computer mouse
Digital readouts for measurement gauges
Tachometers- planes, trains and automobiles
4
Tachometer, What’s That?
Tachometer is used for measuring angular speed of rotating shaft.
Can also be used to measure flow of liquid by attaching a wheel
with inclined vanes.
It is used as a sensor/transducer for measurement of rotational
speed.
Tachometer
A tacho-generator is a small AC or DC generator.
5
6
Types of Tachometer
Classified on the basis of data acquisition –
• Contact or Non-contact types
Classified on the basis of measurement technique –
• Time based or Frequency based technique of
measurement
Classified as Analog or Digital type
Classified as A.C. or D.C type
7
Comparison Between Analog and
Digital Tachometers
Analog Tachometer
Has a needle and dial type of
interface.
No provision for storage of
readings.
Cannot compute average,
deviation, etc.
Digital Tachometer
Has a LCD or LED readout.
Memory is provided for
storage.
Can perform statistical
functions like averaging, etc.
8
Digital Tachometers
9
Classification Based on Data
Acquisition Technique
Contact type – The wheel of tachometer needs to be
brought into contact with the rotating object.
Non-Contact type – The measurement can be
made without having to attach the tachometer to the
rotating object.
10
Classification Based on Measurement
Technique
Time Measurement – The tachometer calculates
speed by measuring the time interval between the
incoming pulses.
Frequency Measurement – The tachometer
calculates speed by measuring the frequency of the
incoming pulses.
11
Comparison Between Contact and
Non Contact Tachometers
Contact Type
Tachometer has to be in
physical contact with the
rotating shaft.
Preferred where the tachometer
is generally fixed to the
machine.
Generally, optical encoder/
magnetic sensor is attached to
shaft of tachometer.
Non Contact Type
Tachometer does not need to be
in physical contact with the
rotating shaft.
Preferred where the tachometer
needs to be mobile
Generally, laser is used or an
optical disk id attached to
rotating shaft and read by a IR
beam or laser
12
Comparison Between Time & Frequency
Based Measurement
Time Based
Tachometer calculates speed by
measuring time interval between
pulses.
More accurate for low speed
measurement.
Time to take a reading is
dependent on the speed and
increases with decrease in speed.
Resolution of the tachometer is
independent of the speed of the
measurement.
Frequency based
Tachometer calculates speed by
measuring the frequency of pulses.
More accurate for high speed
measurement.
Time to take a reading is
independent of speed of rotation.
Resolution of the tachometer
depends on the speed of the rotating
shaft.
13
Typical Specifications of a Non-Contact
Type Tachometer
Display 5 digits large LCD
Range: 2.5 - 99,999 RPM
Distance: 50 to 1,000 mm; 12 to 40 inches.
Resolution: 0.1 RPM<1000 RPM (2.5 to 9,999 RPM)
1.0RPM >1000 RPM
Measurement angle: at less than 120 degrees.
Range selection: Auto
Laser Output Power: <1mW class II
Sampling Time: 1.0 seconds (over 60 RPM)
Memory: Last value, Max Value, Min. Value
Time base: Quartz crystal
Circuit: Exclusive one-chip LSI circuit
Battery: 4 X 1.5V AA
Weight: 300g/0.65lb
Size: 190 X 72 X 37 mm
14
Typical Specifications of a Contact
Type Tachometer
Display 5-digit LCD Display
Range selection Automatic range selection
Time Base 4MHz Quartz Crystal
Sampling Time 1 second (>60 rpm); >1 second (10 to 60 rpm)
Accuracy ± (0.1% of reading + 2 digits)
Photo Tachometer Distance 2 to 12” (5 to 30cm)
Operating Temperature 32 to 122oF (0 to 50oC)
Operating Humidity 80% RH Max.
Power supply 9V Battery
Battery Life 40 hours (approx.)
Applicable standards EN 50081-1/1992 (EN 55022) EN 50082-1/1997 (EN 55024)
Dimensions 461700: 4.9 x 2.0 x 1.3” (124 x 51 x 33mm)
Weight 461700: 4.0 oz. (114g)
15
Block Diagram of a Digital Tachometer
Optical / Magnetic
Sensor/Encoder
Signal
ConditioningMicrocontroller
Memory
Display
External
Port (to
controller)
16
Optical Sensing
It is used to generate pulses proportional to the speed of the
rotating shaft.
Can be achieved by the following ways:
Attaching a disk, which has an alternate black and white pattern, to
the shaft and reading the pulses by a IR (Infrared) module pointed
towards it.
Using a slotted disk and a U shaped IR emitter detector pair to
generate waveforms.
17
Magnetic Sensing
Hall effect sensors – These make use of the Hall effect
to generate pulses proportional to the speed of the shaft.
Passive magnetic sensors – These make use of variable
reluctance to generate pulses.
What is an Encoder?
2/17/2015 18
Any transducer that changes a signal into a coded
(digital signal).
Optical Encoders
Use light and photo sensors to produce digital code.
Can be linear or rotary.
Type of Encoder
19
Two types of Optical Encoders:
1. Incremental
Measure displacement relative to a reference point.
2. Absolute
Measure absolute position.
Advantages – A missed reading does not affect the next reading.
Only needs power on when taking a reading.
Disadvantages – More expensive/complex. Cost/complexity
proportional to resolution/accuracy.
Optical Encoder Components
20
Light source(s)
LEDs or IR LEDs provide light source.
Light is collimated using a lens to make the beams parallel.
Photo-sensor(s)
Either Photodiode or Phototransistor.
Opaque disk (Code Disk)
One or more “tracks” with slits to allow light to pass through.
Optical Encoder Components
21
Optical Encoder Components
22
Motion
Photo sensor
Index grating
Fixed scale
gratingLight source
to counter
Other Component of Optical Encoder
23
Stationary “masking” disk
Identical track(s) to Code Disk
Eliminates error due to the diameter of the light beam being
greater than the code disk window length.
Signal amplifiers and pulse shape circuitry.
Quadrature
24
Two tracks (A & B) at 90
degrees offset.
Provide direction
information.
Provides up to 4 times
resolution.
Encoder Disks
25
Absolute Disk Codes
26
Example: 3 bit binary codeAngle Binary Decimal
0-45 000 0
45-90 001 1
90-135 010 2
135-180 011 3
180-225 100 4
225-270 101 5
270-315 110 6
315-360 111 7
Problem with Binary Code
One angle shift results
in multiple bit
changes.
Example: 1 => 2
• 001 (start at 1)
• 000 (turn off bit 0)
• 010 (turn on bit 1)
Angle Binary Decimal
0-45 000 0
45-90 001 1
90-135 010 2
135-180 011 3
180-225 100 4
225-270 101 5
270-315 110 6
315-360 111 7
Problem with Binary Code
One degree shift
results in multiple bit
changes.
Example: 1 => 2
001 (start at 1)
000 (turn off bit 0)
010 (turn on bit 1)
It looks like we went
from 1 => 0 => 2
Angle Binary Decimal
0-45 000 0
45-90 001 1
90-135 010 2
135-180 011 3
180-225 100 4
225-270 101 5
270-315 110 6
315-360 111 7
Gray Code
One bit change per angle change.
Angle Binary Decimal
0-45 000 0
45-90 001 1
90-135 011 2
135-180 010 3
180-225 110 4
225-270 111 5
270-315 101 6
315-360 100 7
Bit 0
Bit 0
Bit 1
Bit 1
Bit 2
Bit 2
Converting from Gray Code to Binary
Code
1. Copy MSB.
2. If MSB is 1, write 1s until next 1 is met.
If MSB is 0, write 0s until next 1 is met.
3. When 1 is met, logically switch what you are
writing (1=>0 or 0=>1).
4. Continue writing the same logical until next
1 is met.
5. Loop back to step 3.
Converting from Gray Code to Binary
Code
Encoder Reliability and Errors
Resolution
Incremental where N= # of windows.
Resolution can be increased by reading both rising and
falling edges ( ) and by using quadrature ( ).
Absolute where n = # of tracks.
N
360
n2
360
N2
360
N4
360
904
360
Encoder Reliability and Errors
Encoder errors
1. Quantization Error - Dependent on digital word size.
2. Assembly Error - Dependent on eccentricity of
rotation (is track center of rotation = center of
rotation of disk)
3. Manufacturing tolerances - Code printing accuracy,
sensor position, and irregularities in signal
generation.
Encoder Reliability and Errors
Comment on pulse irregularity
It is a result of noise in signal generation, variations in
light intensity, and imperfect edges.
It can be mitigated using a Schmidt Trigger, but this
can lead to hysteresis.
Using two adjacent sensor will negate this problem.
35
Analog Tachometers
36
Analog Tachometers
These are generally the ones that display the speed of your car.
The interface is needle and dial arrangement.
Generally speed is converted to voltage through the use of an
external frequency to voltage converter.
Tachometer can also act as a generator and produce a voltage that
is proportional to the speed of the shaft.
This voltage is then displayed by an analog voltmeter.
37
D.C. Tachometer
D.C. generator converts a shaft rotating into an electrical signal
which is proportional to speed.
Where, Total no. of conductor of armature
Number of field pole
Total magnetic flux per pole (Web.)
Speed of rotation (rad/s)
Number of para
g
NPE
A
N
P
A
llel paths in armature windings
38
D.C. Tachometer
To increase sensitivity, impedance of winding kept as low as
possible.
Commutator has large number of segments to provide smooth
output voltage.
A light weight wire brush assembly is used for good contact and
minimum friction and noise.
5% ripple content and sensitivity 5V to 20V per 1000 rev/min. is
observed.
Linearity ± 0.01% is achievable.
39
A.C. Tachometer An A.C. two phase induction motor having a squirrel cage
type rotor can be used as a.c. tachometer by exciting one
phase with ac supply & utilizing second phase winding to
provide o/p voltage related to speed.
40
A.C. Tachometer
41
A.C. Tachometer
Rotation causes dynamically induced emfs in the rotor, which are
in phase with Ød and are proportional to the speed of rotation.
Phase reversal by 1800 occurs, when the direction of rotation is
reversed.
Normally excitation voltage frequency is in between 50Hz to
400Hz.
Sensitivity is less than the DC tachometers i.e. 2.5V per 1000
RPM.
42
How To Choose a Tachometer?
Accuracy
Precision
Range
Acquisition Time
Contact type / Non Contact type
Portable / Fixed
Digital / Analog
Cost
43
Calibration
Why calibrate? Wrong calibration = Wrong readings
Calibration compensates for ageing, wear and tear and
other degrading effects.
How to calibrate? Calibration is done by comparing the reading from
tachometer to a standard speed.
Necessary changes are made so that the actual reading
matches the desired reading.
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