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Encoder Controllers

A rotary encoder is a device which not only measures angular position, but provides this information as an output signal. The encoder therefore needs to be connected to another device, let us call it a controller. This controller could be just a display (showing position as a direct resolution, as batches or scaled in some way) and/or a device that has pre-set outputs, for example a relay output which operates when a given value is reached. In this basic idea we connect one encoder to one controller using a multi-core cable, most likely over short runs.

But the "controller" could be a PLC, which in turn can have multiple counter/timer/output functions. Again we can connect one encoder direct to one PLC using multi-core cabling. In each of these illustrations the "controller" or PLC has to be able to read the data coming from the encoders; which in the case of Hengstler Incremental encoders will be in QUADRATURE, i.e. A & B channels plus N for the number of turns. In essence this is how an incremental encoder will most frequently be used. The limit of incremental encoders assuming one running at the design maximum speed of its mechanical bearings and maximum resolution, is the frequency of the data being generated. A controller must be able to read and decode the incoming data. This limit is aside from the loss of position knowledge on loss of power.

 

Hengstler Absolute encoders are defined as ones which "know" absolute angular position, irrespective of power on/off. They transmit data digitally (lets us say in strings of data bits or telegrams). In these applications a common way or protocol for the encoder to "talk" to its "controller" is SSi (Standard Synchronous Interface) or more recently BiSS. So now the controller or PLC reading the encoder output has to have an SSi (or BiSS) interface. A marked benefit is that cabling between encoder and controller is now greatly simplified, basically 2 wire. Communication is more secure, and can be read more quickly than with QUADRATURE. Speed and resolution limits are therefore greater.

This digital data exchange between absolute encoder and controller has been further developed with the advent of Fieldbus systems in industrial and process automation. We introduce Fieldbus systems with an overview elsewhere on this site. By using a popular Fieldbus protocol (say PROFIBUS) at the digital output interface of the the absolute encoder, a single encoder can communicate through an entire 2-wire network to a remote PLC, which in turn can interpret the data from many encoders (and other sources for that matter). In this case an absolute encoder is known as a "slave" and its controlling PLC as a "master".