Re: [CAD_CAM_EDM_DRO] Comments from expert PID programmers?

A more mathematical answer is that backlash represents
a step function in the control loop. A step function
contains frequency elements from DC to infinity.
You simply cannot create a compensating control
function in the real world that handle that wide
a frequency range.

Pete.

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, August 05, 2006 12:47 AM
Subject: Re: [CAD_CAM_EDM_DRO] Comments from expert PID programmers?


> Dennis Schmitz wrote:
>
>>Regarding tuning:
>>
>>We've found several approaches to tuning and after having checked them
>>out, they all look like kludges to me. Every one we've looked at has
>>been a trial and error kind of thing with no real analytical solution.
>>
>>
>>Did I mention "rock solid backlash comp"? This potentially means
>>replacing expensive zero-backlash stuff with less expensive backlash
>>prone stuff, doing more with less, etc.
>>
>>
>>
> I have a couple problems with all this. In some machines, wear has
> caused friction to
> vary with position, in others, changes in the balance of overhanging
> tables does the
> same. The torsional spring in small leadscrews varies with the length
> between
> motor and nut. Some of this could severely affect a system that was
> finely tuned
> at some different position. Finally, you have to account for
> intermittent and
> unexpected impacts from the cutting teeth of the machine. This is in no
> way a
> steady-state system, and any steady-state tuning will fall flat.
>
> Second, backlash can't be "compensated", due to the finite time it takes
> the motor
> to get to the other side. if you don't know the amount of free play at
> that position,
> then the motor slams into the end of the backlash at high velocity, and
> you have a
> severe hammering. Even if the code knows the amount of backlash to be
> compensated
> out, the motor still has to accelerate like mad, and then brake so as to
> just nudge the
> machine from the other direction. Immediately after the nudge, it then
> has bumped the
> table in that direction, and may have to run to the other side to nudge
> it back. There's no
> free lunch here, no matter how much digital signal processing you throw
> at it, hardware can't
> be "in two places at the same time", unless the backlash is already very
> nearly zero.
>
> Jon
>