Re: stepper pulse rate idea

[Robin Szemeti <lists-at-redpoint.org.uk>]

On Friday 14 March 2003 00:51, Tim Goldstein wrote:
> > 3 - c  Ray needs to reduce his caffeine intake?
>
> This one gets my vote.
>
> How many machines would it actually help considering the hardware and
> software work required? Most of the machines that I am aware of
> requiring a higher pulse rate than EMC can provide are using 20 TPI
> screws direct drive on 10 micro step drives. 

Think about it this way:

Most modern stepper motors produce 70% torque at 2K or more steps/second 
(Sanyo H series for example)

The (now ubiquotous) Gecko drives are 10:1 microstepping. You'll need to 
provide 20Khz into the drive to get maximum speed out of the motor

If youve designed the machine correctly, at your peak cutting speed you'll 
hit 20khz on the drive pulses, and 2K steps/second on the motor ... 

Before someone says 'but I can still get all the speed I need and the force I 
need by going up to 500 steps/second (5Khz drive)  ... say you bought a new 
car and it came with only one gear, the engine revved nicely to 5K rpm but 
the designer had geared it to hit 55mph at 1Krpm ... would you consider the 
gearing to be optimal, and the best use to be made of the motor?

The long and short of it is ... if you have a modern stepper motor and a 10:1 
microstep drive, you need 20Khz of drive or you are not using the full range 
of the motor, which means your gearing is wrong.

Anyway .. I think rays idea is interesting, but I'm not sure if it will fly 
;) ... it *should* fly .. but it has many new ways of crashing.  Ive seen 
similar things done in some drives to improve torque at high speeds .. they 
keep a constant 10:1 microstepping ratio but as the speed increases they 
chnage from a sine wave to a square wave for the output drive waveform, 
giving the smoothnes of microstepping at low rpm and the improved torque of 
full step drive at high rpm, the changeover being progressively made as pulse 
rate increases.

I'd be more interested in seeing more development done on the pulse 
multiplier board (and the desing published open source ;) or a cheap and open 
source interface to a conventional motion control card (a cheap one mind!) 
that could provide pulses at the rates that are required by modern drives.

Of course, if Ray can get it to fly thats great ... but I feel it might not 
be the right approach. Its novel, but the chances of accumulating pulse 
errors as the switch ocurs are great.

Having said that ... If you think about it in the following terms: 

a pulse on wire A means +1 microstep
a pulse on wire B means +10 microsteps

then the responsibility for keeping count of where the drive is in its along 
its sine wave would be delegated to the drive, and you no longer have the 
'what happens if I switch pulse modes at 5 microsteps in' problem
so long as the drive can count up to say 100 in a ring counter then its not 
too difficult at all.
-- 
Robin Szemeti

Redpoint Consulting Limited
Real Solutions For A Virtual World