Re: PWM stepper driver with external bridge

> Message: 1
> Date: Sat, 6 Jan 2001 19:45:31 -0800
> From: "Lawrence Glaister" <lg@...>
> Subject: PWM stepper driver with external bridge

Hey, neighbour .. a few comments here ...

1. You may be reinventing the wheel, as there are several good similar designs
already on the market.

Dan Mauch's design, for example, uses an F84 for logic, will run up to 40
volts and gives 1/4 step resolution. While rated for 5 amps, it could
likely run close to 10 with just a wee bit of tweaking.

2. You cannot expect that the "inbetween" steps in a microstepping controller
to be at all accurate and as the number of intermediate steps goes up, so does
the error. ( Ericcson/NJR has a useful app note describing the problem )
If your aim is higher resolution, you would do better to use 1/2 or 1/4 step
and different pulleys to gain the increase.

3. The a3955 IS interesting in that it mixes current decay modes automatically,
but that's about all the chip has going for it as a conventional controller.

It is ABSOLUTELY UNUSEABLE for your application, as it is designed to drive a
motor ( and hence has two outputs ) whereas a bridge driver has FOUR outputs.

4. Since you're already planning on using a PIC, use it for ALL the functions.
( I've designed such a circuit, so I know it's possible ).

PIC -> Bipolars -> FETs OR

PIC -> IR drivers ( as mentioned previously ) -> FETs

Add a dual comparator and a resistor ladder of some sort and you're done.

Here's the advantages of such a system. You now CAN go to 1/8 or 1/16 step (or
even further) with accurate step sizes by tailoring the reference voltage
for each step IN BOTH DIRECTIONS of each motor using a table inside the PIC.

You would start with the classic sin(a)/cos(b) current relationship, but then
measure the ACTUAL step size and adjust the currents accordingly. These new
values are then programmed into the PIC and you have a controller customized
for your particular motor.

The PIC is certainly able to perform all the on/off logic and since it knows
about the current levels ( having output VRef itself ), the PIC can also
decide upon a current decay mode strategy.

The design of the bridge should present no particular problem, if it is
understood that a relatively high current capacity totem pole must be used to
drive each base. ( You must charge and discharge that GATE cap quickly or
your maximum step frequency suffers and the FETs get hot from living too long
in the linear region ) There ARE totem pole driver chips out there to ease the
design complexity, but they start to drive the price of the project up.


Hope this helps.

Alan


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Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
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