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

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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.

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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

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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.

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Digital Tachometers

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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.

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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.

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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

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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.

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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

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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)

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Block Diagram of a Digital Tachometer

Optical / Magnetic

Sensor/Encoder

Signal

ConditioningMicrocontroller

Memory

Display

External

Port (to

controller)

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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.

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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?

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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

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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

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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

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Optical Encoder Components

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Motion

Photo sensor

Index grating

Fixed scale

gratingLight source

to counter

Other Component of Optical Encoder

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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

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Two tracks (A & B) at 90

degrees offset.

Provide direction

information.

Provides up to 4 times

resolution.

Encoder Disks

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Absolute Disk Codes

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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.

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Analog Tachometers

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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.

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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

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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.

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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.

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A.C. Tachometer

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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.

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How To Choose a Tachometer?

Accuracy

Precision

Range

Acquisition Time

Contact type / Non Contact type

Portable / Fixed

Digital / Analog

Cost

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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.