DC Tachogenerator, Absolute Optical Encoder and Incremental Optical Encoder

1. OVERVIEW
• Tachogenerator
• Optical Encoder :
i. Incremental
ii. Absolute

2. 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.

3. 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

4. DC Tachogenrator

5. 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.

6. 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 Encoders
2. Incremental Encoders

7. 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.

8. • 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.

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

12. 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.

13. • 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.

14. • 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.