ContentsWhat is tachometer??Uses.Types.Working Principle.Comparison Between Analog and Digital

Tachometers.Calibration.

Contents 2

What is tachometer….???A tachometer is an instrument designed to

measure the speed of an object or substance. A tachometer (also called a revolution-counter).

The word is formed from Greek roots: tachos, meaning speed, and metron, meaning measure.

What is tachometer 3

History…….The first mechanical tachometers were

based on measuring the centrifugal force, similar to the operation of a centrifugal governor.

The inventor is assumed to be the German engineer Dietrich Uhlhorn; he used it for measuring the speed of machines in 1817. Since 1840, it has been used to measure the speed of locomotives.

History 4

Tachometer, UsesTachometer is used for measuring rotational

speed.Can be used to measure speed of a rotating

shaft.Can also be used to measure flow of liquid by

attaching a wheel with inclined vanes.

Uses 5

Tachometer, UsesEven medical science uses tachometers. If a turbine like small device is placed in the

vein or on an artery, the doctor can make use of the tachometer to measure the blood flow rate from the spinning speed of the turbine in the device.

Uses 6

In automobiles,trucks,tractors and aircrafts…..Tachometers on automobiles, aircraft, and other

vehicles show the rate of rotation of the engine's crankshaft, and typically have markings indicating a safe range of rotation speeds.

In vehicles such as tractors and trucks, the tachometer often has other markings, usually a green arc showing the speed range in which the engine produces maximum torque, which is of prime interest to operators of such vehicles.

Uses 7

Working Principle……..Every engine spins within the limits of its

design.The pistons inside the engine pump the crankshaft to spin.This spinning crankshaft sends horsepower to the street.

A tachometer counts the number of rotations the crankshaft is making a minute(RPM).

Working principle 8

Working principle…….If the RPM of an engine is allowed to go

outside of specific limitations,moving parts begin to generate enough heat and friction to become damaged.

Use a tachometer to know when to shift and when to stop pushing a motor for more.

Working principle 9

What Are the 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 types.

Types 10

Comparison Between Analog and Digital TachometersAnalog TachometerHas a needle and

dial type of interface.

No provision for storage of readings.

Cannot compute average, deviation, etc.

Digital TachometerHas a LCD or LED

readout.Memory is provided

for storage.Can perform

statistical functions like averaging, etc.

Comparison 11

Comparison Between Time and Frequency Based Measurement

Time BasedThe tachometer

calculates speed by measuring the time interval between pulses.

More accurate for low speed measurement.

Time to take a reading is dependant on the speed and increases with decrease in speed.

The resolution of the tachometer is independent of the speed of the measurement.

Frequency basedThe 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.

The resolution of the tachometer depends on the speed of the rotating shaft.

Comparison 12

Comparison Between Contact and Non Contact Tachometers

Contact TypeThe 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 TypeThe 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 is attached to rotating shaft and read by a IR beam or laser.

Comparison 13

…………. 14

Classification Based on Data Acquisition TechniqueContact 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.

Classification 15

Classification Based on Measurement TechniqueTime 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.

Classification 16

Typical Specifications of a Contact Type TachometerDisplay 5-digit LCD DisplayRange selection Automatic range selectionTime Base 4MHz Quartz CrystalPhoto Tachometer Distance 5 to 30cmOperating Temperature 0 to 50oCOperating Humidity 80% RH Max.Power supply 9V BatteryBattery Life 40 hours (approx.)Dimensions 461700: (124 x 51 x 33mm)Weight 461700: (114g)

Specifications 17

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. Measurement angle: at less than 120 degrees. Range selection: Auto Laser Output Power: <1mW class II 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

Specifications 18

Block Diagram of a Digital Tachometer

Block Diagram 19

Optical / Magnetic Sensor

Signal Conditioning

Microcontroller

Memory

Display

ExternalPort (to controller)

Optical SensingIt 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 module pointed towards it.

Using a slotted disk and a U shaped IR emitter detector pair to generate waveforms.

Optical Sensing 20

Magnetic SensingHall 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.

Magnetic Sensing 21

Signal ConditioningThe output of the sensors may be noisy.

The output may have to be amplified.

It has to be digitized.

Signal Conditioning 22

MicrocontrollerNot essential, but is generally the norm to

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

reduces / increases.Transfer data to external controller.

Microcontroller 23

Display UnitUsed to output the values to the operator.

Can be used to view the stored values.

Display Unit 24

………… 25

Analog TachometersThese are generally the ones that display the

speed of your car.

The interface is needle and dial arrangement.

Analog Tacho… 26

Analog Measurement TechniquesGenerally speed is converted to voltage

through the use of an external frequency to voltage converter.

The 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.

Technique 27

How To Choose a Tachometer?AccuracyPrecisionRangeAcquisition TimeContact type / Non Contact typePortable / FixedDigital / AnalogCost

How to choose 28

CalibrationWhy calibrate?

Wrong calibration = Wrong readingsCalibration compensates for wear, 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.

Calibration 29