Experimental Physics and
Industrial Control System

Brian Bevins <[email protected]> · Thu, 15 Jun 2000 10:23:43 -0400

Ben,

Having a deadband field is definitely a good idea. The cpid record
already implements this, but in a somewhat different way. The DMIN field
can be set to prevent the cpid from writing out small changes. If the
change calculated from the cpid algorithm is smaller then DMIN, then the
output doesn't change. This provides a sort of deadband on the output
rather than on the input. There is also an analogous DMAX field that
sets an upper limit on the rate of change of the output. This is useful
in protecting sensitive devices from sudden drastic changes (like sensor
failures).

We use cpid's to run a lot of DC motors (among other things) and we find
that a non-zero DMIN greatly extends the life of the motors.

I agree that a deadband on the input side would also be useful.

--Brian Bevins

Benjamin Franksen wrote:
> Since it looks as if we are going to replace the PID anyway, I have
> another proposal: an additional field for an error deadband. Whenever
> the (absolute value of the) error in the controlled variable is smaller
> than this deadband value, the PID record should assumes the error is
> zero. So differences below this value will never lead to a change of the
> correction value, even if KI is non-zero.
> 
> This would be very useful in situations where the resolution of the
> controlled variable is low and corrections are supposed to occur as
> seldom as possible (for instance, the controlling device is a motor and
> you want to reduce wear and tear). An output from the PID record that
> tries to correct an error which is caused by the limited measurement
> resolution of the controlled variable is nonsense anyway and causes
> unnecessary and easily avoidable oscillation.
> 
> Ben
> --
> The Notorious Neb Nesknarf

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System