Re: EHP

Hi Dave..
Thanks for the input... my comments between your text

daveland@... wrote:

> From: daveland@...
>
> I've been lurking so I'll give my 2Cents worth on the external
> hardware interface. I see two camps and I think both have different requirements.
> I also believe that the interface (parallel, serial, usb...etc) is less important
> than what you want to offload from the PC's CPU. Define that first and then
> pick an interface!!
>

Well, not as I see it.
RS232 is ubiquitous, whereas the prarallel port normally need specail drivers, and may not even be bi-directional.

>
> Servo interface
> ----------------
> 1) THE STG board is too expensive. We need a 4 axis $100-$200 alternative.

>
>
> 2) The existing EMC servo routines are plenty fast enough in a pentium class
> computer to get very good feed rates. I fact motor size and amplifier design
> is more important that update rate. Update rate just tries to minimize following
> errors.
>
> 3) I think most people underestimate the number of parameters that must be "tweaked"
> in the servo system. These must be downloaded from some GUI or file into the
> hardware prior to running. this may seem trivial, but it is tough work to make
> sure 20-30 parameters are all properly downloaded into hardware. EMC DOes this
> already. THIS is PER AXIS as well!!!

So, that's not a lot of data, you you did nto specify a time frame for it. !

>
>
> 4) ISA slots are disapearing, but they haven't gone away yet. I belive that
> 2 slots would be needed for a 4 axis machine. one for a 4 D/A card and one for
> 4 axis encoders. We should be able to find these for several years to come.
>

Going faster than you think !

>
>
> Stepper hardware
> ------------------
> 1) serial and other interfaces can work but the interpoaltion math MUST be done
> in the PC. Microcontrollers take 100us to 1mS to perform the math calculations
> needed for rudamentary interpolation. I did some math benchmarking and found
> that a PC does arctans in 4-5 us and multiplies 1-2us. This contrasts with
> the same calculations taking 100's of usec in a 24Mhz 8051 controller. Note
> that this is floating point NOT integer math. There are ways to "integerize"
> the problem but these will constrain the distances and feed rates so that the
> solution is not as general. I believe that lots of the dedicated controls do
> this, but then it only has to work with their software.
>

Right, When I said make the hardware as autonomous as possible it would have top be within working limits. If that mean a simple
PWM unit
then so be it... I want away from the Parallel port and dependency on one OS ....

>
> 2) DSP's are based upon integer math (at least the cheap $10 to $20 ones). They
> also are not geared to perform transcidental functions. So circular interpolation
> might be difficult. The current ones are optimized for signal processing and
> not bit twiddleing. Steppers seem to need fine resoltion PWM and bit twiddling.
> The TI parts sound interesting if they solve the bit twiddle problems and trancedental
> math. I imagine though that they are optimized for the PID algorithms and NOT
> for the implimentation of the linear and circular interpolation stuff. They
> were designed to replace the analog parts of the servo speed control loop. They
> are still integer DSP's and as such they will have a finite resolution. this
> requires programming finesse to handle 3.3333333333"
>

Amen.

>
> 4) External pulse generation is the best bet. A good fine reslution PWM generator
> is needed. In fact one for each axis would be ideal. Then the PID loop could
> adjust the stepper rate just like a servo system. I'll look around for one.
>

At last ! Thanks

Regards
Hans Wedemeyer

>
> dave
>
>
>
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