Re: Digest Number 98

Andrew - here are a few comments on your comments.

Anyone who knows better what is involved please jump in!

----- Original Message -----
From: Andrew Werby <drewid@...>
To: <CAD_CAM_EDM_DRO@onelist.com>
Sent: Sunday, July 18, 1999 4:30 AM
Subject: Re: [CAD_CAM_EDM_DRO] Digest Number 98


> From: Andrew Werby <drewid@...>
>
> Message: 16
> Date: Sat, 17 Jul 1999 22:10:01 -0700
> From: "Andy Olney" <aolney@...>
> Subject: Re: EMC and PIc-Servo
>
> Andrew -
>
> I have talked with both Fred Proctor and Jeff Kerr (picservo designer)
about
> what it would take to make EMC run a picservo.
>
> [Thanks for running this down, Andy. I'm going to need a little help
> translating some of this, though, so bear with me...]
>
> Here is Fred's reply:
>
> Andy,
>
> I was forwarded your question on the possibility of adapting the EMC
> code to interface to a servo board. This is quite possible. I prefer
> using dumb I/O cards that have DACs and encoder counters, using the CPU
> for all calculations, but if you have an I/O card already then it makes
> sense to keep that and avoid paying for an I/O card.
>
> [Are these features (DACs -whatever they are- and encoder counters)
> standard features on I/O cards? If they had them, would they still be
> "dumb"? I've put an I/o card in my computer when I needed an extra
parallel
> port, but I didn't think they did much more than route signals to and fro.
> Is Fred referring to the pic-servo card as an I/O card?]

The dumb i/o card Fred is referring to is the $880 Servo To Go card which
can read (in hardware)
8 encoders and do a digital to analogue conversion of EMC's control output.
This analogue control output is sent to a servo amplifier which powers DC
servo motors.

>
> You described taking the voltage output, which is proportional to the
> desired velocity, and feeding this to the servo board. The board would
> then close the loop on velocity, which is probably not what you want.
> You can take the *input* to the PID calculations,
>
> [What are PID calculations?]
Proportional, Intregral, Derivitive calcs are essentially a way to make a
servo follow a commanded position with the minimun undershoot or overshoot
and max smoothness.

>
> and pass this directly
> to the board. There are no extintf.h functions for this since I don't do
> this now, but they could look like this:
>
> extern int extPosWrite(int axis, double pos);
>
> This would be called instead of the PID calculations done in
> pidRunCycle, e.g.,
>
> // assume 'softwareServo' is set somehow to non-zero for usual, 0 for
> hardware boards
> if (softwareServo) {
> emcmotStatus->output[axis] =
> pidRunCycle(&emcmotStatus->pid[axis],
> emcmotStatus->input[axis],
> jointPos[axis]);
> }
> else {
> emcmotStatus->output[axis] = 0.0; // FIXME-- could read DACs on board
> and fill this in
> extPosWrite(axis, jointPos[axis]);
> }
>
> I haven't tested this but it's close to what will work.
>
> --Fred
>
> [Uh, are these commands I'd have to give EMC, or is this a summary of the
> programming work that would have to be done?]

He is describing where to alter the EMC code to talk to the Pic Servo.
The function "extern int extPosWrite(int axis, double pos);" would have to
be written to talk to the picservo through a serial port. I am guessing you
would want to access the serial port through the RTLinux subsystem to be
sure of communicating in a timely manner.


>
>
> Jeff Kerr also said that he was able to talk to 2 axis at about 400
> commands per second:
>
> [But I need to talk to 3 axes, I think.. Would that slow things down, or
be
> impossible, or what?]

Hopefully three axes could be done at a slightly slower rate.

>
> > Regarding the NIST EMC software, the stepper control may be the easiest
> > to implement, although inelegant. We have implemented some contouring
> > schemes reading the positions and commanding velocities and have gotten
> > good results down to about 30 commands/second. Another alternative it
> > to use two serial ports with two controllers on each port. This would
> allow
> > you to issue raw positions commands (Using the STOP command) for two
> > axes at a rate of about 400 commands per second. This would probably
> > be smooth enough, and is much simpler to implement.
> >
>
> [I don't know how I'd generate all these STOP commands. I was hoping to be
> able to use standard G-code.]

The Picservo has a special STOP command which in effect lets you issue a
stream of position commands without stopping at all. The function "extern
int extPosWrite(int axis, double pos);" would issue these commands under
EMC, which is interpreting the G-codes.


>
>
> To get EMC talking to a Picservo it looks like at least the following
would
> have to be done:
>
> 1. Write Fred's external interface.
>
> [Who would be the one to do that- Fred?]

Good question. I think I know what it needs to do, but it would take be a
long time to learn how to write, compile, debug etc.

>
> 2. Figure out how to make a serial port talk to the picservo from within
RT
> linux ( There is some code for a serial port interface on the RT linux
> site).
>
> [Is this still a problem then? Is this something that hasn't been done
> before? Would the serial port be used to convey the positional feedback
> information to EMC while the parallel port is issuing the directional
> commands?]

There is a stepper interface to the PicServo ( another little circuit
board ) but the goal would be to talk entirely via serial port, which
implies a serial connection.

>
> 3. Write the standard external interfaces to setup and initialize the
> picservo boards as well as read the limit swithces etc.
>
> [Since these are standard, could they be copied from somewhere? Is this up
> to me to do (as you can probably tell, I'm hardly qualified for the job)
or
> is this an easy task for Jeff Kerr?]

I was referring to EMC's external interface functions, which have to be
customized for each set of hardware.


> I sure there is more to it as always. I am not much of a C programmer
> understatement !) but I would like to get these little servo boards going
> with EMC also.
>
> Andy Olney
>
> [Since this all sounds extremely daunting (not to mention the "more to it"
> part), is there any servo card that IS currently supported by EMC? What
> about the Servo-to-go? ]
>

I am interested in the Pic Servo because is looks to be a cost effective way
to add closed loop motion control to my mill.

Andy