Controlling a DC motor with a PLC

Nov 04, 2004 4 Replies
D

Hello,



Is it possible to control a DC motor + encoder with a PLC (mitsubishi FX1N) with analog outputs, high speed digital inputs (counters), and a separate PWM unit + power supply.



The specs are matched for each other so there shouldn't be a problem putting it all together.



My main question is however, is it possible to implement a simple PID controller in a PLC program? What difficulties are to be expected? Is a PLC to slow to do this (DAC outputs take 4ms to convert)? The motor does not need to be controlled highly dynamic. The gap in all of this is that I know very little about PLC's so I'm not sure this is doable and will yield a decent solution. The goal is relatively accurate speed control of the motor. Any feedback is greatly appreciated. Thanks in advance. Sincerely,



Diederik


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W

Diederik,

It is quite common to implement PID controllers in PLCs. This is a common function that is provided by the vendors. A PID controller can operate any speed. The issue is -- what speed does your process need? A 4 msec DAC time implies a loop dead time of perhaps 12 msec for input/process/output. Is that fast enough to keep your motor under control?

Large process controls may operate on cycle times as slow as 1000 msec but that would definitely be too slow to fly an aircraft, for example.

Walter.

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T

And I'd be pretty nervous flying in an aircraft that flew on PLCs! How fast can they sample, anyway?

A general rule of thumb is to set your sample rate equal to 10 times the highest frequency of interest, which is usually more or less one over the desired settling time.

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D

Fortunately my highest frequency of interest is not very high. If I can drive the motor every 0.1 s, it's still good enough. Thank you both for your answers.

Diederik

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C

An alternative could be the Aromat FP0 PLC with motion control built in:

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