On Sat, 22 Dec 2012, Steve Stallings wrote:
> Date: Sat, 22 Dec 2012 17:52:19 -0500
> From: Steve Stallings <steve...@newsguy.com>
> Reply-To: "Enhanced Machine Controller (EMC)"
> <emc-users@lists.sourceforge.net>
> To: "'Enhanced Machine Controller (EMC)'" <emc-users@lists.sourceforge.net>
> Subject: Re: [Emc-users] Pid saturated, was: Servo error
>
> It is evident that discussing servo tuning is going to
> generate lots of interest and comments. Stephen and
> Peter both have lots of academic and theoretical
> background as well as practical experience to go
> with it. Many of the rest of us only have casual
> exposure to the concept of the response and tuning
> of servo loops. In order to help keep this discussion
> on track and meaningful for those of us in the casual
> category I would like to suggest that the systems be
> described completely when making statements about how
> they behave.
>
> As I understand things, Peter is referring to control
> of brushless DC permanent magnet servo motors with
> encoders that can provide only shaft position feedback.
> Velocity, if used, is computed from position. I do not
> know if the control circuits in Peter's drivers are
> current mode, voltage mode, or some hybrid of the two.
> I suspect that they are current mode with the current
> loop controlled in the drives.
>
> Stephen's mill I think has classic +/- 10 volt DC
> controlled PWM type servo drivers running brush type
> permanent magnet servo motors with both encoders for
> position and tachometers for velocity. Stephen
> stated that they are presently configured as voltage
> mode so the +/- 10 volts DC signal adjusts the PWM
> duty cycle and thus the apparent voltage seen by the
> servo motor. The motor current is not being controlled
> other than a protective limit on maximum current. This
> classic type of driver also closes the velocity loop in
> hardware, so if the motor voltage derived from just the
> +/- 10 VDC input does not result in the specified speed,
> then the tachometer feedback will alter the voltage
> (PWM duty cycle) in an attempt to get the specified
> speed. This is a servo loop in hardware and the driver
> should have electronic control adjustments independent
> of the PID in the LinuxCNC software. I am going to
> guess that these controls address P (control voltage
> gain), a gain adjustment for the Tachometer feedback
> that, and an adjustment related to the time variant
> response to changes in the control voltage and the
> tachometer feedback voltage. This last one may be
> similar in effect to a D term, but is likely not
> a true D term. As best I know the driver does not
> have an I term. These types of drivers usually also
> have an offset or nulling term that may be similar
> to FF0. All of this is happening in the servo driver,
> not in the PID software in LinuxCNC, so it is an
> "inside" loop.
>
> So now, how do we talk about apples to apples comparison
> of these two control systems? In both cases the PID in
> LinuxCNC receives only a position feedback and is the
> outermost loop of the control system that is used to
> control position. Perturbations to the PID control can
> come from both changes in the requested position and
> from changes in the mechanical response to the control
> system. As I understand it there is a 90 degree phase
> shift in the response of the position loop between the
> types of loop control, current-torque, or voltage-velocity
> and this alone keeps me confused about where the response
> poles are and how to adjust for them.
>
> While it is natural to describe servo systems mathematically,
> please try to include intuitive descriptions for those of
> us who are mathematically challenged.
>
> Thanks,
> Steve Stallings
Thats basically right. I think we are arguing about nothing really.
Stevens tuning method is appropriate for first order systems (velocity mode
servos or voltage to current loops in motor controls or spindle speed loops)
Tuning second order systems (and the bare hBridge systems like the 7I39
approximate a second order system) are tuned differently and depend on the D
term for stability.
Peter Wallace
Mesa Electronics
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