N O U M A N M U S H TAQ - 1 2 0 0 2 0 0 1 0 1 6S A J I D N A D E E M - 1 2 0 0 2 0 0 1 0 0 4

TACHOMETERS – AN OVERVIEW

RPM-METERS IN AIRCRAFTS

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TACHOMETER, WHAT’S THAT?

• To measuring rotational speed.• To measure speed of a rotating shaft (in

cars etc. )• To measure flow of liquid by attaching a

wheel with inclined vanes.

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DIFFERENT TYPES OF TACHOMETERS:

• Tachometers can be classified on the basis of data acquisition – contact or non contact types.

• They can also be classified on the basis of the measurement technique – time based or frequency based technique of measurement.

• They can also be classified as analog or digital 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

SOME FEATURES OF DIGITAL TACHOMETER

• Accurate in all conditions

• Easy to install

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CLASSIFICATION BASED ON DATA ACQUISITION TECHNIQUE

• Contact type – The wheel of the 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• The tachometer has

to be in physical contact with the rotating shaft.

• Preferred where the tachometer is generally fixed to the machine.

Non Contact Type• The tachometer does

not need to be in physical contact with the rotating shaft.

• Preferred where the tachometer needs to be mobile.

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COMPARISON BETWEEN TIME AND FREQUENCY BASED MEASUREMENT

Time Based• The tachometer

calculates speed by measuring the 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.

Frequency based• The 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.

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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• Range selection: Auto

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TYPICAL SPECIFICATIONS OF A CONTACT TYPE TACHOMETER

• Display 5-digit LCD Display• Automatic range selection• Operating Temperature 32 to 122F• Power supply 9V Battery• Battery Life 40 hours (approx.)• Weight 461700: 4.0 oz. (114g)

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BLOCK DIAGRAM OF A DIGITAL TACHOMETER

Optical / Magnetic Sensor

Signal Conditioning

Microcontroller

Memory

Display

ExternalPort (to controller)

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OPTICAL SENSING

• It is used to generate pulses proportional to the speed of the rotating shaft.

• no physical contact between the tachometer and the engine

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MAGNETIC SENSING

To generate pulses proportional to the speed of the shaft.

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MICROCONTROLLER

• Compute the speed• Can store the readings• Can output values to a display unit• Give out warning signal when speed

reduces / increases beyond set margins• Transfer data to external controller

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DISPLAY UNIT

• Used to output the values to the operator.

• Can be used to view the stored values.

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ANALOG TACHOMETERS

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ANALOG MEASUREMENT TECHNIQUES

• Generally speed is converted to voltage through the use of an external frequency to voltage converter.

• This voltage is then displayed by an analog voltmeter.

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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 is accurate.

ANY QUESTIONS !