StepStuck

When I built my Mendel I used A3977 stepper drivers. Before that I did some maths to show that the component values need to be carefully selected to match the motor in order to achieve 8× microstepping. Makerbot produced a board with four potentiometers and I published settings for motors popular at the time.

Since then Pololu stepper drivers have become popular (and the StepStick clone), but they only have one thing that you can adjust: the current. They also have 16× microstepping, which makes the range of component values that work even smaller. I was always pretty sure the off-time would be wrong for the motors we use and while commissioning my second Mendel90 I could hear that it was wrong, so I decided to look into it.

When stepping one motor at a constant speed you should hear a single pitch at the step rate. If the off-time is too short then the lowest current microsteps cannot be achieved, the motor pauses twice every 16 microsteps so you hear a lower pitch sound as well.

If you step the motor very slowly (G1X10F1) you can hear a sequence of steps with a pause.

The reason for this is that the lowest current step when ×16 microstepping is 9.8%. If the current is set to 1A then that is only 98mA. The minimum on-time for the chip is fixed at 1μs and my formula predicts the off-time needs to be at least 54μs with 1.65Ω motors. That would require a 47k resistor but the value fitted is only 10K. That gives an off time of 12μs which isn't even long enough for 8× microstepping. The situation is even worse on the Z axis with two motors in parallel.

The problem with increasing the resistor to 47k is that the switching frequency drops to 14kHz, which is audible. So my conclusion is that the A4983 is not really suitable for driving such low resistance motors. The A3977 allows you to control the minimum on-time so you can avoid the switching frequency becoming too low.

Later Pololus and some StepSticks use the A4988 chip. That has an interesting section in the datasheet: -

Low Current Microstepping. Intended for applications where the minimum on-time prevents the output current from regulating to the programmed current level at low current steps. To prevent this, the device can be set to operate in Mixed decay mode on both rising and falling portions of the current waveform. This feature is implemented by shorting the ROSC pin to ground. In this state, the off-time is internally set to 30 μs. 

Conceptually an easy mod to do, simply short out R4, but due to the size and location of the resistor and the age of my eyes it was not at all easy. I applied the mod to a StepStick and it worked, the steps are now regular, no missing beats. Running is a bit quieter but I think the motors are more noisy when stationary. More investigation is needed.

What to do with my A4983 Pololus? Well if I increase the current to 1.3A and change the resistor to 36K then the minimum frequency is 17kHz, which is ultrasonic to me nowadays due to the age of my ears. Alternatively switching to 8× microstepping and using a 22K resistor keeps it above 30kHz and the current can be 1A.

I don't think constant off-time choppers are the best idea. The current range is too limited and the switching frequency varies wildly. As the two halves of the chip run at different frequencies they can generate beat frequencies in the audio band.

The other thing I don't like is that they regulate the peak current so there is an offset of half the ripple current which can make the first step inaccurate.