Absolute interface

Now there is an easier way to interface PLCs to absolute encoders

If you interface encoders to PLCs, you will have noticed that while most PLCs provide an interface to incremental encoders, many do not provide an easy interface to absolute encoders. In the main, designers are left with the job of using a parallel interface to the PLC from the encoder and using one input per bit. When using a single or multi-turn unit with 25 or more bits, this has meant that a lot of expensive I/O is used and a good deal of PLC programming is involved.

That in itself is a pity, since absolute encoders offer some real benefits over their incremental counterparts, not in the least that unlike incremental encoders, the absolute encoder will retain positional information even when power goes down.

Now, a German company, T R Electronic has developed a solution to the problem. The idea itself is quite simple. The company has developed a new output configuration for its range of absolute encoders called the Incremental Serial Interface (ISI) that replicates the output signals from an incremental encoder. To the PLC, the encoder then looks like an incremental, rather than an absolute encoder.

Because of this, the output can be interfaced directly to any high-speed counter card or servo controller incremental interface card. And all PLCs and servo controllers equipped with such an interface can accept the ISI inputs, because the format of the ISI output is exactly the same as that of an incremental encoder.

When a powerdown situation occurs with an incremental encoder, the positional information is lost, while with the ISI encoder the position is retained within the encoder. When power is re-established, and the machine is ready to accept information, the PLC sends a `load input’ signal to the encoder (Figure 1). The encoder then transmits a stream of pulses (at up to 125kHz) proportional to the absolute position, and when this is complete, it sends an `End of load’ signal to the PLC, confirming that the position has been re-established.

Because the encoder is a true absolute unit, even if machinery is moved, or if machine speed exceeds the speed of data gathering of the PLC, the true subsequent position information is sent.

David Hampden-Smith of Sensor Consultants, distributors of the new encoders here in the UK, says that they have already found acceptance from users of Allen Bradley, Mitsubishi, Omron and Modicon PLCs – systems that do not provide dedicated absolute encoder interfaces – as well as servo controllers such as those supplied by Trio Motion Technology.