Re: 5-axis machine controller

["Lawrence Glaister" <lg-at-jfm.bc.ca>]

If you have lots of power supply and big resistors, the original old l/r
drive system is quite easy to implement for 6 lead stepper motors. You need
a big resistor from each coil centertap to the positive supply. Example:
40vdc supply, 2v 4 amp stepper coils. You need 2 resistors/stepper and 4
fets/stepper.
the resistor needs to drop 38volts at 4 amps... r=v/i = 38/4 = 9.5ohms,
now hold onto your hat... P = i^2R = 4 * 4 * 9.5 = 152 watts
The fets are connected from each of the 4 remaining stepper wires to ground.
A standard fet will need about 10 volts gate drive to insure full turn on.
Try something like a IRF540. These are pretty cheap $.60 to 1.95 depending
on quantity. I have seen several circuits that use an rs232 driver chip to
drive the fet gates ( like a max233). You will need a small diode from the
gate to ground to prevent the negative drive from getting to the fet gate.
The resistors can be expensive... you may want to find an appliance repair
shop and see if you can get some bulk nichrome wire to wind your own on a
chunk of ceramic floor tile or brick.
=======================================================
Lawrence Glaister VE7IT             email: lg-at-jfm.bc.ca
1462 Madrona Drive                  http://jfm.bc.ca
Nanoose Bay BC Canada
V9P 9C9
=======================================================
----- Original Message -----
From: D.F.S. <dfs-at-xmission.com>
To: Multiple recipients of list <emc-at-nist.gov>
Sent: Thursday, December 21, 2000 8:08 AM
Subject: Re: 5-axis machine controller


>
> >
> >
> > Hi Robert,
> > I have a phase drive system running with EMC. The electronics package is
> > from TMG (The Motion Group) and is a small box with power supply and 3
2A
> > stepper drivers. In the cvs code, I wrote an alternate phase drive
> > subroutine that could be used to drive unipolar stepper phases through a
> > logic level fet and current limiting resistor. The code I wrote is very
> > simple and can be changed from 2 outputs/motor to 4 outputs/motor quite
> > easily. The output bit patterns are looked up in table. You could also
used
> > a decoder driver chip to convert the 2 bits/phase to the 4 bits of logic
> > level fet drive you are considering. You may want to have a look at the
code
> > in emcmot.c around line 3500. Let me know if you need some help
modifying
> > the emc code. I am starting on a drive system for a new machine and need
to
> > run 4 unipolar steppers at 4.7 amps/phase. I am still looking for the
> > easiest/cheapest way to get the job done. I have not decided if I will
use
> > the direct phase drive method with resistors or try something that has a
> > switched mode current source for better efficiency.
>
> Well, I bought a batch of misc stuff maybe a year ago when there was talk
of
> doing a design here for an open-source type design for a driver board.
>
> Some of the things I got were 3 amp outut op-amps, 3, 5 & 10 amp Fets,
> Bi-polar? 'Regular old' 10 amp Metal Can Transistors, Opto-isolators,
> a slew of regular old op-amps and a ton of LEDs, I already have ~50
> thousand Misc TTL parts...
> The last time this came up after lots of arguments the project appeared to
> disappear although it looks like one LPT interface may have been built...
>
>
> In any event, could you elaborate on the "Fet & Resistor" scheme please?
>
> Where and why are you using the resistors?
>
> The FETs I have are not logic level, but that should be easy enough to
> work around I presume.
>
> Marc
>
>
>