Stepper: Distribution of Microsteps within a full step

Question

I am currently working on a project to precisely move a XY-table using stepper motors in microstepping mode. The goal is to reach 1/16 microstepping and there is an encoder with 4000 pulses per resolution to account for missed steps.

In full-step mode the steppers work correctly, but in microstepping mode the motor steps only every n-th step, e.g. every fourth in 1/4 microstepping and every eighth in 1/8 microstepping mode. This was tested by stepping at 1Hz and observing the motor.
I expected the microsteps to be approximately equally distributed within one full step (i.e. 1/4 microstepping = 1.8° / 4 per step = 0.45° microsteps), is this a misconception? If so, what is the point of microstepping if you can still only reach full steps accurately?

Several motors of different makes where tested, with the load of the table aswell as without it, but they all show this behaviour.

Hardware used is:
Stepper Motor Controller: Texas Instruments DRV8711
Stepper Motor: Nanotec ST4118S1404-B
Pulsegeneration: STM32F446ZET
Evaluationboard: Pololu High-Power Stepper Motor Driver 36v4

Any tip is helpful, as the project is missing its point without sufficiently accurate microstepping.

Currently my guess is that the stepper-drivers circuit is faulty or it is misconfigured. To address the circuitry issue we ordered an evaluation board which we will test this week.

Answer 1

The assumptions in your question are correct. In microstepping mode, each step is divided into a number of smaller steps. Hence, 8 microsteps mean, that a full step is divided into 8 equally long smaller steps. The limit is the mechanics. Up to 16 microsteps, you can expect the microsteps to be equidistant. After that, the mechanical deviations of the motor result in the steps being of slightly different size.

I advice you to have a look into the data sheets of Allegro Microsystems A3977 or A3979 stepper motor drivers. They have a rather detailed and easy to understand description of how microstepping works.

The pictures you mention in the comments indicate, that there is a problem with the driver. Measure the current of the motor coils. That is done by inserting a resistor and measuring the voltage across this resistor, not coil to GND. If you do that for both coils simultaneously, be aware that the GND of the probes make a shortcut. So, use DIFFERENTIAL probes. For 4/8/16 microsteps (only half-step is not enough, better use 1/16-steps mode as you intend straightaway), you should see a screen like "Figure 16. Microstepping Drive Current" in the data sheet. If this is not the case, there is a problem with the driver. As long as this is not clear, it is rather useless to guess about possible further sources of error.

The current through the two motor windings have 90° phase shift. This is absolutely essential. This is the reason, why you should check both windings. Unfortunately, the DRV8711 has no HOME-output, which is useful as a trigger for measuring the motor currents separately. If you can, get at least one differential probe and measure the currents simultaneously.

It is also essential that the windings are connected in the same polarity. The data sheet of the motor gives you details of how to wire the motor properly.

Last personal hint: Nanotec is crap! I have never met anybody in more than 30 years in electronics now, who spoke in favor of Nanotec. Having just checked the web-site of Nanotec, this was confirmed in the usual unpleasant way, Nanotec is known for. Real stepper motors are made by Oriental Motors or SANYO DENKI. Use one of these. Oriental optimizes their steppers especially for microstepping.

Answer 2

To answer the question myself: The problem was not in the DRV8711 parameters, microstepping mode or devices used, but rather a layout issue. In Fig. 12 (p. 20) of the DRV8711 datasheet a filter is suggested to be placed in the current-sensing path. Against the suggestions given in the datasheet this lead to errors in measuring the current and thus incorrect regulation of the motor phases.

Taking out the capacitor (and deactivating the filter) fixed the issue, and microsteps up to 1/32 have been tested to be sufficiently reliable in Auto-Mixed Decay-Mode, with Torque adequate to the R_ISENSE and motor current. The filters frequency was in the order of ~3MHz, so the time constant is ~333 ns, which from the datasheet seems to have been to high. For now there sadly is no more time to investigate whether the suggested 50 to 60ns will have a positive impact on the signal or show the same behaviour.