Encoder

 

Working principle

 

An encoder is a rotary transducer that converts an angular movement into a series of electrical digiyal pulses. If associated to racks or endless screws, these generated pulses can be used to control angular or linear movements. During rotation, electrical signals can be elaborated by numerical controls (CNC), programmable logic controls (PLC), control systems, etc. Main applications of these transducers are: machinery, robots, motor feedback, measure and control devices. In M.D. Micro Detectors encoders the angular movement transduction is based on the photoelectric scanning principle. The reading system is based on the rotation of a radial graduated disk formed by opaque windows and transparent ones alternated. The system is perspendicularly illuminated by an infrared light source. The light projects the disk image on the receivers surface which are covered by a grating called collimator having the same disk steps. The receivers transduce the occurring light variations caused by the disk shifting and convert them into their corresponding electrical variations. Electrical signals, raised to generate squared pulses without any interference, must be electronically processed. The reading system is always carried out in differiantial modality, that is comparing different signals nearly identical but out of phase of 180 electrical degrees. That in order to increase quality and stability of output signals. The reading is performed comparing the differencce between the two channels, to remove the noise known as "common mode", because signals are overlapped in equal way on each wave.

 

 

 

Incremental encoder

 

The incremental encoder usually gies two types of squared waves out of phase of 90 electrical degrees. They are usually called channel A and B. The first channel gives information about the rotation speed while the second, basing on the state sequence produced by the two signals, provides the direction of rotation. A further signal, called Z or zero channel, is also available. It gives the absolute zero position of encoder shaft. The signal ia a squared pulse with phase adn width centered on A channel.

 

The incremental encoder accuracy depends on mechanical and electrical factors. These errors could be: grating divisions, disk eccentricity, bearings eccentricity, electronic reading and optical inaccuracy. The measurement unit to define encoder accuracy is the electrical degree. It determinates the division of the impulse generated by the encoder: 360 electrical degrees correspond to the mechanical rotation of the shaft which is necessary to carry out a complete cycle. To know how many mechanical degrees the following formula has to be applied:

The encoder division error is given from the maximum shifting shown in the electrical degrees of two consecutive edges. This error exists in any encoder and is due to the above mentioned factors.
On M.D. Micro Detectors encoders pulse error is ± 18° e max. on full operating range, which corresponds to a ± 10% from nominal value.
Regarding the 90 electrical degrees phase relation between the two channels, it differs in ± 35 electrical degrees max which corresponds to ± 10% respect to signal period.

 

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