OVERVIEW• Tachogenerator• Optical Encoder :

i. Incrementalii. Absolute

Tachogenerator • An electromechanical generator is a device capable of producing electrical power

from mechanical energy, usually the turning of a shaft. • They are the measurement devices for measuring the shaft speed in mechanical

equipment.• One of the more common voltage signal ranges used with tachogenerators is 0 to 10

volts. • Tachogenerators are frequently used to measure the speeds of electric motors,

engines, and the equipment they power: conveyor belts, machine tools, mixers, fans, etc.

Principle of Operation

The major principle for tachogenerator is that the relative motion between a magnetic field and a conductor results the voltage in that conductor.

There are mainly two types of tachogenerator:• A.C. Tachogenerator• D.C. Tachogenerator

DC Tachogenrator

Construction• An Armature is rotating type & this magnet is fixed type.• Armature is coupled to a machine whose velocity is to be measured. • It consists of commutator and brushes is connected to armature.• Output is connected to Moving Coil (MC) type Voltmeter.Operation• When the armature is stationary there is no relative motion between the magnetic field and

the armature winding.• Hence the output voltage is Zero• As the armature speed increases the relative motion also increases.• The output voltage is induced in the armature winding.• The magnitude of this voltage is proportional to the speed of armature.• A commutator and brush are connected in the armature to give the DC output voltage.• This output voltage is measured with the help of moving coil voltmeter calibrated in terms of

speed.

Optical Encoders • An optical encoder is an electromechanical device which has an electrical output in

digital form proportional to the angular position of the input shaft. • It is used to measure rotational movement precisely.• Information obtainable from an optical encoder includes direction, distance, velocity

and position.

There are two types of encoders:1. Absolute Encoders2. Incremental Encoders

Absolute Encoder• It provides a unique binary word coded to represent a given position of the object.• It has a light source that emits a beam of light onto photoelectric sensor called

photo detector.• The photo detector converts the receiving light into an electric signal.• A coded wheel is placed between the light source and photo detector.• The coded wheel has alternating opaque and transparent sections.

• As the disk rotates, these patterns interrupt the light emitted onto the photo detector, generating a digital or pulse signal output.

• The result obtained is a series of signals corresponding to the rotation of coded wheel.

• A counter is used to count these signals to determine how far the wheel is rotated.

Encoding Techniques

There are two types of encoding techniques:• Standard Binary Encoding• Gray Encoding

Incremental Encoder

• Incremental encoders are commonly used than absolute encoders because of their simplicity and lower cost.

• It consists of three parts: Rotary disc ( with alternate opaque and transparent slots) A light source A photo detector

• Incremental encoders are used for both velocity and position measurement and is most reliable and inexpensive.

• For angular measurement, it consists of a sensing shaft attached to a disk that is divided into equal number of sectors on circumference.

• In linear type, there are equal segments along the length of travel.• The readings are sensed by direct electrical contact with a brush or wiper or

optically slit gratings.• It counts the lines on disk. More lines, higher resolution.• This is expressed as pulses per revolution.

• There are two types of incremental encoders: i. Tachometer Type ii. Quadrature Type

• Tachometer type has one output channel and is used for velocity measurement. This is done by looking at the pulses during a certain interval of time.

• However, the output of the single-channel encoder does not indicate direction. • To determine direction, a two-channel, or quadrature, encoder uses two detectors

and two code tracks.• Quadrature type have dual channels A and B; they are arranged in such a way that

they are 90 degrees out of phase with each other. • If A leads B, for example, the disk is rotating in a clockwise direction. If B leads A,

then the disk is rotating in a counter-clockwise direction. • Therefore, by monitoring both the number of pulses and the relative phase of signals

A and B, you can track both the position and direction of rotation.