Bipolar vs unipolar stepper drivers

[Bassnut (Fred)]

This a bit of a long shot, but someone may know off hand.

Ive nearly finished a timelapse controller that has been designed around PICAXE (PIC CPU) and bipolar stepper motors using a standard L297/L298 chip set for driving. The L298 has 4 H bridge outputs for each side of 2 bipolar motor windings, works a treat. They are 6v driven from 12v but current limited per step..

I need to now drive 12v unipolar steppers, 4 windings with a common B+ to all 4 at the other end. Easy I thought, the step sequence is the same, just connect the same 4 outputs to the 4 coils, and common the other ends to 12v.

It works, but only slowly and no guts.

Typical unipolar driver outputs are single ended not H bridge, but I wouldve thought that wouldnt make a difference, the top H bridge tranny just shorts to + 12v instead of O/C (maybe thats the prob?).

Is there some big difference driving unipolar over bipolar?, I cant see anything on the interweb that tells me anything on the diff.

Im not looking for advice on a total redesign, just a clue on the diff.

Im wondering if a 12v stepper windings driven from single ended 12v driver is way less powerfull than H bridge 6v motors current limited?, given H bridge effectively swings bipolar windings 24v.

[Cloudyagain (Neale)]

Hi Fred,

The explanation at the start of the attached re bi and unipolar motors may help.

It sounds as if your unipolar motors are 5 wire with the "centre taps" of each pair of coils connected internally. I am not sure if you can drive these with a bipolar driver due to this internal connection. If you have 6 or 8 wire unipolar motors though that is different and you just need to connect them so you effectively have one coil on each side.

Hope this helps as I am no great expert.

Neale

[alistairsam]

Hi,

you can't drive a unipolar motor with a bipolar driver. its to do with the fact that current flows in both directions in the bipolar motors and only one direction in the unipolar. hence the names.

quote
"Unipolar steppers are easier to drive because the center tap supplies the +V, and the individual coils are energized by grounding one of the coil ends in a sequential fashion (single stepping). This can be done by using four MOSFETs or Darlington transistors.

With a bipolar stepper motor, you need to reverse the direction of current through the coil. Therefore, each coil must be controlled by an H-bridge configuration; you need two complete H-bridges to drive a bipolar stepper motor. "


understand you're not after redesigns, but some info below.

open source unipolar driver. http://www.piclist.com/techref/io/st...step/index.htm

versatile bipolar microstepping driver : a4983 board from pololu. for $14, unbeatable value cause it does 1/16th microstepping, and just needs usual step, dir, enable, etc..
i'm experimenting with a chip from TI, the DRV8824. this does 1/32 microstepping and is pretty good.
thought it might help.

[Cloudyagain (Neale)]

"you can't drive a unipolar motor with a bipolar driver. its to do with the fact that current flows in both directions in the bipolar motors and only one direction in the unipolar. hence the names."

This is not completely true, you can drive 6 and 8 wire unipolar motors with a bipolar driver by ignoring the centre tap and using the 2 coils on each in series to effectively create 1 coil a side.

See the attachment to my first post and I have since found another article confirming this and also confirming 5 wire motors cannot be driven by a bipolar driver.

http://schmalzhaus.com/EasyDriver/

Neale

[bojan]

Unipolar motors CAN be driven by bipolar driver (but bipolar motors can't be driven with unipolar driver).
You just have to use the central tap and one end of coil (if you use the "wrong one" motor will step in other direction).
Connecting the whole winding (leaving central tap "in the air") effectively increases coil inductance, hence reducing torque at higher speeds ("no guts")

[mswhin63 (Malcolm)]

The main choice for which motor or motor configuration is Torque at the required speed.

[Bassnut (Fred)]

Thanks for the feedback Guys, that Linistepper and easydriver PCBs looks very interesting, at least I would only need to drop in another PCB without a full redesign, and it does micro stepping. I wasnt aware of these, will look at them.

As Bojan says, you CAN use a bipolar driver on unisteppers, I just connected the central taps to B+, that was easy. The use of an H bridge doesnt matter, its just the high side drivers arent used (or that IS a problem, when the high side driver is on and the bottom one off, that coil is not now powered, but it is shorted, this doesnt happen with single ended drivers,mmmm).

Im still none the wiser then, perhaps the sequence is wrong, needs more current, high side drivers shorting, use of a 12v motor on 12v supply, dunno, will experiment more.

[bojan]

Originally Posted by Bassnut

... I just connected the central taps to B+, that was easy. .

Fred, that was a reason why there were no guts.
The way you connect the motor, when one side of the coil is energised, the other one is shorted, so the magnetic field can't build up.
Use only one half of the coil and you will see difference.

[Bassnut (Fred)]

Originally Posted by bojan

Fred, that was a reason why there were no guts.
The way you connect the motor, when one side of the coil is energised, the other one is shorted, so the magnetic field can't build up.
Use only one half of the coil and you will see difference.

aha , doh, got it now, thanks Bojan.

[Bassnut (Fred)]

On a L 297 datasheet reread, its a bit obscure, there is a control mode selection for chopper control to operate on the ABCD phase lines (instead of inhibit), using the Hbridge, for unipolar motors, looks like thats the go, should have noticed that, ill give it a go.