DRV8825 in 32 micro step mode

Hi Richard,

What decay mode are you using? If you are on slow decay, can you try mixed or fast?

Best regards,

Jose Quinones

Hi Jose,

I have tried both fast and slow decay. I cannot see any difference.

It's almost like the motor jumps 2 steps at the end of the 32 step sequence.

You can actually hear the steps running. You get a 31 regular steps then the 32 step is louder.

The motor is driving a CCTV camera and the jump on every 32nd step is quite obvious when viewing the camera video.

Hi Richard,

I would say then you are facing a problem with the zero crossing distortion. If you have a current probe you should be able to see it.

The problem with microstepping is that as current becomes smaller and smaller, it gets harder to read it, so there is a higher current regulation error percentage. Also, the MIN TIME ON will cause a small deformation around the zero crossing point.

A suggestion would be to add some series resistance, or decrease the input voltage to limit the current charge rate and diminish the zero crossover distortion.

What voltage is your application? If you can operate at less than 11V, then I recommend trying the DRV8834 which has dynamic TBLANK designed in place especifically to decrease the zero crossover distortion.

Hope this info helps. Best regards,

Jose Quinones 

Hi Jose,

I'll try running at lower voltage to see if there is any difference. Currently 24v.

I don't have a current probe but I'll manually step the drive to identify the position where the motor jumps and see what I can measure with my scope.

We need quite a bit of power, so i don't think the 8834 will cut it.

I don't see much difference when lowering the input voltage.

In 32 step mode, step 18 seems to be a big step and the following 4 or 5 steps don't create any movement.

Same in 16 step mode. 9th step is a big step. 10th and 11th steps don't create much movment of the rotor.

Hi Jose,

Still not getting smooth movement in 16 and 32 step modes.

Anything else i can check?

Hi Richard,

One thing that I forgt to ask is what kind of motor you are trying to drive? can you share its part number or related information? You can send to my personal email if you don't want to make it public, at jquinones@ti.com.

Best regards,

Jose Quinones

Hi Jose,

The motor is a KA50 from NIDEC Servo Corp.

The exact model is KA50 KM2-552

Spec is:

KA50KM2-552

degree/step 1.8
V/Ø  5.1
A/Ø  1
Ω/Ø  5.10
mH/Ø  10

holding torque
mN·m 520  
oz·in  74

Detent torque
mN·m  20
oz·in  2.8

Rotor interia
g·cm2  100
oz·in2 0.5

Problem solved!

DRV8825 decay pin wasn't getting set correclty.

Once set to FAST mode, drive runs nice and smooth.

I found this by monitoring the Sense resistor and looking at the voltage across it. In mixed mode I was getting a missing quadrant. In Fast mode I see a complete wave shape.