Absolute Position Control

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Thread Starter

Jeremy Richard

What defines a system as having absolute position control? Yes, I know the simple definition is: having closed loop position feedback and the ability to know the exact position after a power down/failure. What I want to know is what are the options for achieving this? I have been told the only true "absolute position control" is done using a absolute encoder/resolver. My arguement is that the same end result can be achieved using other means. An absolute encoder decodes sine/cosine waves (via onboard electronics) and can output this result as BCD and other formats. This result is remembered/stored by the encoder upon power down/failure. If I use an incremental encoder with a high speed counter PLC module (encoder mode) and store the counter value in non-volatile PLC memory doesn't this achieve the same result as an absolute encoder. The HSC module counts pulses up/down depending on direction given by encoder signals.
 
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Peter Nachtwey

> What defines a system as having absolute position control? Yes, I know the simple definition is: having closed loop position feedback and the ability to know the exact position after a power down/failure. What I want to know is what are the options for achieving this? I have been told the only true "absolute position control" is done using a absolute encoder/resolver.

No. Check

http://www.stegmann.comhttp://www.balluff.comhttp://www.temposonics.com
just to start.

Most of the work I do is with the above absolute encoders or transducers and not one is a resolver.

> My arguement is that the same end result can be achieved using other means.

Yes

> An absolute encoder decodes sine/cosine waves (via onboard electronics) and can output this result as BCD and other formats.

Binary and Gray code are more common.


> This result is remembered/stored by the encoder upon power down/failure.

The counting feature must be always powered with an encoder. This is not necessary on a true absolute encoder.

> If I use an incremental encoder with a high speed counter PLC module (encoder mode) and store the counter value in non-volatile PLC memory doesn't this achieve the same result as an absolute encoder.

No, the PLC will not count while power is off.

> The HSC module counts pulses up/down depending on direction given by encoder signals.
 
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Curt Wuollet

The fly in the ointment is if the position changes while the electronics are down. Now, you only think you know where you are. Absolute encoders avoid this and other quite possible
nasties.

Regards

cww
 
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