RE: Servo-PWM-DSP

I am considering putting the control functions in the interface which will eliminate the need for an analog board. Of Course, when the encoder is mounted with the motor, rather than on the table itself, the encoder will supply the tach (speed) data as a part of the position data. I understand your sugustion to consider a tach with the motor as it applies to the case where you mount the encoder on the table. I appreciate your suggustion and think it is a good one. To reduce the difficulty of locating a tach, I believe a stepper motor may be used as a tach by reading it like a generator. No one seems to have any difficulty finding stepper motors. A small stepper is all that is required and I've seen them for under two dollars.

I beleive stepper motors may also be used as encoders in a similar manner. To overcome the possibility of loosing possition while they are at rest (not generating any signals), I have a scheme that works pretty well on paper, but I don't believe it is a good idea because there are plenty of encoders around, and encoders are already encoders and don't need a scheme. Why climb out the window where there is a door.

The embeded control firmware person I may employ in this potential effort, is not well experienced in DSP. He is good though and given a clear understanding and an envirnoment to test the code in, he never fails. To make sucess likely, my thinking is that the project performance is 3/4 of the way up from a stepper, but not equal to an off the shelf servo system. I hope to keep the cost under control for one thing. It may be difficult to develop something which is stable across many applications and super high performance at the same time.

What do you think about getting say 3 times the speed of a stepper system? 4 times the speed? Steppers are slow; they have resonance; and some discussions I've read suggust that you can tell by looking at the tooled part if a CNC system was controlled by a stepper based or servo based controller.
My thinking is that smooth, fast, quiet, and reasonable cost are major bullet items?

Thaks for some good information.

From your sugustion, it is easy enough to have both an encoder input for position, as well as an optional input for the motor speed (with a second encoder or tach) when the position encoder has backlash involved. The PIC family of microcontrollers has the required PWM and A/D inputs to reduce the component count (and cost). It's really not all that scary of a project. With a multi-channel parallel port board for a PC (which is very fast), some DC motors, some encoders, and some details it should be easy enough to out pace stepper systems. I'm am hopeful some one can spell out the software side of the concept before I smile that smile.

David Howland

-----Original Message-----
From: Bertho Boman [SMTP:boman@...]
Sent: Friday, September 10, 1999 3:52 PM
To: onelist.com, CAD_CAM_EDM_DRO
Subject: [CAD_CAM_EDM_DRO] Servo-PWM-DSP

From: Bertho Boman <boman@...>

I have been thinking about stirring up this issue for a while and then I
saw David Howland's nice post about an I-O board so I guess it is time
to do it.

A "traditional" servo system has a "controller" that outputs typically
+/-10 VDC to the analog servo Amplifier. The servo amp internally
monitors motor current (torque) and gets a velocity and acceleration
feedback from a tachometer attached to the motor. Internally in the
servo amp, the feedback loops are tweaked for stability and accuracy.
The actual position of the motor or table is fed back to the
"controller" from an encoder (shaft or linear).

When designing a hobby system, the "controller" is the easy part. A PC
with an analog output board and an input card to read the encoders will
basically meet our requirements. There are lots of surplus brush DC
motors around that can be attached to a ball screw or other loads.
Similarly, attaching a shaft encoder or a linear DRO is straightforward
and mostly a question of finding the right surplus one and how to
mechanically mount it.

I see very little discussion in the mail list about using tachs. They
are little harder to find but again but that is not a problem and the
mounting is a mechanical issue again. But why so little discussion?
Further, I have seen no comments at all about servo amplifier design and
tuning. Has everyone found a perfect sources and there are no
problems???? Or maybe everyone is using steppers??

After this long introduction, I finally have arrived at the meat of the
subject:

A proper servo amp is difficult to design, difficult to tune and
requires a tach for best performance.

With today's very fast processors, it is possible, and it is being done,
to measure the time between each encoder pulse, calculate the actual
speed and use that value in a DSP filter for the servo amp. That
removes the need for the tach. With DSP, the servo tuning can be done
in software instead of with screwdrivers.

Now for the final step: Why not go digital all the way? If we used a
switching amplifier, class-D or PWM, there would be no need for the
analog difficult portion of the servo amp, just a smoothing filter on
the out put to keep the RFI out of the motors. (I forgot about the
current sense, but that is not a big deal to add a little A-D to measure
it.)

A DSP-PWM servo amp basically would have a small digital control section
with a pair of heavy duty output switching devices. This might be
reasonable as a hobby project and it would be a lot easier than an
analog servo amp.

How does EMC fit into this. It is my impression that it will put out
the +/- voltage in servo mode and read the encoders through an I-O
card. Is it actually using the DSP calculations internally and if so,
what specs are required for the servo amps since if that case, the
internal amplifier feed back loop is not needed?

Comments?

Bertho Boman
Vinland Corporation


--------------------------- ONElist Sponsor ----------------------------

ATTN ONELIST USERS: stay current on the latest activities,
programs, & features at ONElist by joining our member newsletter at
<a href=" http://www.onelist.com/subscribe/onelist_announce ">Click</a>

------------------------------------------------------------------------
Welcome to CAD_CAM_EDM_DRO@...,an unmoderated list for the discussion of shop built systems in the above catagories.
To Unsubscribe, read archives, change to or from digest.
Go to: http://www.onelist.com/isregistered.cgi
Log on, and you will go to Member Center, and you can make changes there.
For the FAQ, go to http://www.ktmarketing.com/faq.html
bill,
List Manager