[yngndrw]

I know I'm about a month late to this thread, but I'm just wondering why all of this complicated FOC stuff is even needed. Here's how I see a stepper-servo:

Given a 200 step per revolution stepper motor and an encoder which has a multiple of 200 counts per revolution. (Let's use an 800 pulse per revolution encoder.) The multiple between the encoder resolution and the stepper resolution becomes the "Motor direction resolution" - 4 in this case.

So, as for the drive - Let's split it into two parts:
1) The PID controller - A simple controller which takes a step / direction input, an encoder input and outputs a direction and power level that it wants the motor to respond to. Nothing new, non-standard or complicated here so I don't go into the implementation here.
2) The motor driver - This is the new part which takes a direction / power level, an encoder input and outputs the current levels for each winding.

I see #2 as doing the following:
Let's consider 4 full-steps of the stepper motor in terms of degrees 0 - 360 degrees, as we normally would for a stepper driver. We have an encoder which has 4x the "Motor direction resolution" steps over this range.

We know that full torque is generated when the winding is driven 90 degrees from the stepper motor's current position, so depending on the direction / power level requested, we just drive the windings between -90 and 90 degrees of the current position using a sine / cosine lookup.

Am I missing something here because what seems to be needed in my head is a lot simpler than what is being mentioned in this thread and I very much doubt Mariss would go to all this trouble if it wasn't needed ?

[H500]

I think the problem is that without the FOC calculations, it is difficult to keep the force vector at 90 degree at the higher RPMs where the winding current lags. Much of the benefits of closed loop would not be realized. It would be hard to justify the cost of an encoder if you end up getting hardly any noticeable improvement over an open loop driver.

When I have time, I'm planning to experiment with both approaches to find out for myself.

[KIMFAB]

I haven't been on here much lately and I might be confused about what you are talking about but I happened to mention this post to my son, the electronics parts peddler and he came up with this:

Infineon Technologies

Dunno if it is even relevant but I thought I'd throw it out there.

[Mariss Freimanis]

I appreciate the posts here. The Infineon part is interesting for low-pole count motors (BLDC aka PMSM) but not useful at all for high pole-count PMSM motors better known as step motors.

There is very little difference between step motors and Brushless DC motors. For my personal entertainment and knowledge, I set up a drive (I lobotomized a perfectly innocent G201X) to act as a 3-phase step motor drive.

The motor ran as expected and just like a step motor at 12 full-steps per revolution. Low torque and endless RPM (>6,000 RPM).

It means the FOC equations used for BLDC motors will apply to step motors. There is no substantive difference between them. My experiment validated what I believed.

Mariss

[harryn]

Thanks for all of your work in motor and motion control.

If you could somehow use this concept to produce closed loop linear positional feedback rather than the feedback of motor rotational position, that would be very useful. In the end, that is really what a person wants on their cnc machine.

If it can be done at hobby / low end commercial equipment price points, it would be amazing.

Some of the linear encoder tapes are getting pretty reasonably priced, so perhaps this is becoming viable ?

Good luck with your experiments and projects.

Harry

[PEU]

I view most of your posts as a cross between a textbook, an open-ended word problem and a crime scene. Your should edit your sig to say “… and from this point the solution is trivial and should be obvious to a skilled practitioner in the field. The working out of the remaining details has been left as an exercise to the student.”

Fermat!!

This is the only reference I understood from this technical discussion :rainfro: nevertheless I enjoy every post of it.

thseng: thanks for choosing this thread to make your first 3 posts after 8 years of registering

Mariss: Hope you can release it this year!


Pablo