Re: Low cost Servo Controller

I am happy to see that you are going the digital way for the servo amp. Please keep us informed on your progress and thoughts.

You mentioned about using start-end positions and calculating speed for velocity feedback. I do not think that will work
because you need continuos and "instantaneous" velocity feedback. If there is a delay in the velocity feedback signal the servo
loops will be all messed up.

The software tachs go to great lengths to QUICKLY calculate the velocity from the time between two encoder pulses. There has
been a lot of research on how to do that fast enough that the introduced delay is not significant to the servo loop.
Preferable, the delay should also be independent of the value converted. There was an article in one of the IEEE magazines some
years ago about the theory and software. With all this discussion, it is only a question of time before I have to dig it
out. Today's DSP should fly through the time to velocity calculations.

I did such a calculation 22 years ago on a MC6800 for a EKG & pacemaker tester. It had to display both Beat per Minute and time
in ms between pulses. Since the software also was used for timing, the math loops duration had to be independent of value or
the timing would have been messed up. I did it all in hand assembly! (No PCs at that time.... only 4 & 8 bit Intel's and
Motorola's 8-bit 6800 available) It makes me feel old thinking back that far.

Bertho
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Dean Franks wrote: <dean@...>

>
> True, I was assuming from the previous conversation that the system would be
> built using both linear encoders for position detection, and rotary
> encoders. The rotary encoders would be just for motor velocity generation.
> On each cycle, an difference error would be generated between the current
> position, and the previous cycle goal. This error signal would be used to
> throttle the feed rate when tracking errors start to occur. The system
> would then generate target velocities for the current cycle goal, and send
> them to the 629's. This is would not be the most accurate way to do things,
> I suspect that the positioning error inside a single cycle will be
> unacceptable for moderate to high precision applications.
>
> I am not advocating this approach, I believe that it would be much easier to
> develop an integrated system using the TI motion DSP's (TMS320 series). The
> chips are cheaper, the hardware simpler, and the development exercise
> becomes almost totally a software one. It's much easier to do basic
> research (incremental design) in software than hardware (particularly when
> you have DSP's with flash memory).
>
> I am building a servo/encoder based system. I have built a table-top test
> x-y table (using threaded rod and joiner nuts), and am developing the
> software in x86 assembler (software encoder decode and pwm generation). The
> x86 is too slow for real machine control, but it provides an opportunity to
> solve all the problems I will face with a full side/ball screw machine.
> Once the software is right, I will port it to the TMS320x and build an EMC
> interface.
>
> The application is a gantry wood router.
>
> Dean
>