Re: 5-axis machine controller

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

Speed wise, you will probably notice something like a 10-1 difference
between driving the coils at close to their rated voltage as opposed to
using a higher drive voltage. I have seen rule of thumb to use 10 - 20 times
the coil voltage. If you have a slow speed requirement, by all means use a
lower drive voltage. Yes, the need for chopper drives becomes quite evident
when one looks at how much power can be wasted in the resistors. I have been
trying to dream up a 2 terminal cct that could be used in place of the
resistors. Basically a constant current source chopper. This would let one
use older unipolar motors without too much wasted power without having to
build a full blown dual bridge chopper cct for each axis. 1488 should be
fine... you may even want to experiment with them connected to a single
supply as the negative drive voltage is not required or desired.... not sure
how they would react.
have fun and keep us informed of your progress
=======================================================
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 1:48 PM
Subject: Re: 5-axis machine controller


>
> >
> >
> > 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
>
> OK, unless I'm missing something the only reason to raise to voltage and
> add the resistor is to speed up the rise time but not toast the coils.
> Right?
>
> As long as I keep the power dissipated in the coil to 8 Watts I'm OK?
>
> So I could also use 12V input to the coils, a 2.5 Ohm Resistor -at- 40 Watt
> OR 5V .75 Ohm and 12 Watt?
>
> As such the trade-off is one of torque and speed VS Input Voltage and
> power Dumped by the Resistor?
>
> How much of a disadvantage are we talking about?
>
> Keeping in mind this Machine is designed for building exactly this
> kind of circuit board. I'd much rather get a setup that works although
> at 1/4 speed and 1/2 touque but is dirt simple and cheap THEN use it to
> build a GOOD controller for itself.
>
> I have some great PCB design software to plot out the boards, I have the
> mount already built to hold the drill, I have the drill bits, I have
> The etching process down for the boards. The only thing missing is the
> PC->Machine interface.
>
> You example clearly shows the advantage of chopper drives and the second
> cut at this will surely be of such a design.
>
> > 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
> Now THAT is a good idea. The fact I have about 800 MC1488 RS-232 Line
Drivers
> I'll ever need, that is a killer use for them.
>
> The max 232 has all the crap required for the charge pumps and run about 2
> bucks by the time all that is set up.
> The 1488 when I will have the higher voltages handy on the board are a
great
> option, that is a REALLY good idea.
>
> I think these little suckers are good to at least +/-30 volts, I'll just
> feed them whatever is required to fully enable the FETs.
>
> > You will need a small diode from the
> > gate to ground to prevent the negative drive from getting to the fet
gate.
> I assume with the 1488 that requires external sources for both voltages
> I can simply ground the negative voltage input, end of problem.
>
> > The resistors can be expensive...
>
> All the more reason to build that chopper drive ;-).
>
> Thanks a LOT.
>
> That RS-232
>
> Marc
>
>
>