DRV8428: Micro stepping a Permanent Magnet Stepper Motor

Hi Nivethitha,

Permanent stepper motors usually are small, low power, short life and cheap motors used in large quantities in home appliances like gas water heaters. Step usually is in the range 7.5 - 30 degrees. Hybrid stepper motors have higher torque for the same size, have long life, can be quite powerful and are more expensive. Their step is usually 1.8 degrees and because of that they are much more precise. They are commonly used in printers, also 3d printers and industry machines. Hybrid motors can be microstepped and I am quite sure permanent stepper motors can be microstepped as well.

Regards,

Grzegorz

Hi Grzegorz,

   Thanks for your reply Grzegorz.

   If the permanent magnet stepper motors can be microstepped, will there be any modifications to be made in the motor driver? Can we use the same driver we use for hybrid stepper motor?? 

Regards,

Nivethitha.D

Hi Nivethitha,

Permanent stepper motors usually are small, low power, short life and cheap motors used in large quantities in home appliances like gas water heaters. Step usually is in the range 7.5 - 30 degrees. Hybrid stepper motors have higher torque for the same size, have long life, can be quite powerful and are more expensive. Their step is usually 1.8 degrees and because of that they are much more precise. They are commonly used in printers, also 3d printers and industry machines. Hybrid motors can be microstepped and I am quite sure permanent stepper motors can be microstepped as well.

First question :Can this  DRV8428PRTER be used for micro stepping a Permanent Magnet Stepper Motor? Will it be precise?

You would need to know motor parameters like current, nominal voltage etc. permanent stepper motors can have very low current and microsteeping them even with 1A driver can be not very efficient. Accuracy - it depends what kind of accuracy you need.

Regards,

Grzegorz

I do not think any changes will be needed, the most important thing is to couple motor with suitable driver.

Grzegorz

Hi Grzegorz,

       Thanks Grzegorz.

      1) Do we need drivers with high current for driving permanent magnet stepper motor to make it efficient?

      2)How much higher the peak current current of driver should be, than the Motor current rating?

      3) Will the accuracy be affected if we use permanent magnet stepper motor in place of Hybrid stepper motor?

       4)Can the accuracy of hybrid stepper motors achieved by permanent magnet stepper motors? Is it possible?

Thanks in advance!

Regards,

Nivethitha.D

Hi Nivethitha,

1. I think motor current for microstepping should have nominal current around 40%-71% of driver IC full scale current. If driver has current capacity much more than 200% of motor current its microstepping becomes less precise (falling precision of driver current regulation, especially those ones with integrated current sensing) but with driver IC load close to 100% of its full scale come higher power losses and driver design becomes more demanding.

2. I would not use peak current capability for driver IC, I would rather use Irms or full scale current. I keep space between full scale current and peak current as a safety margin in case of resonances etc. when current can sometimes be a bit higher.

3. Yes, probably it will decrease a few times.

4. Rather not, unless you have some very special permanent magnet motor or use feedback device like encoder but price of such design would be very high. The main advantage of permanent stepper motors is their low price.

I just happen to work for permanent stepper motor manufacturer and I might be able to evaluate if there is a chance to use permanent stepper motor in your application at all.

Regards,

Grzegorz

Hi Grzegorz,

       1) Why should the motor current for microstepping should have nominal current around 40%-71% of driver nominal current (full scale)?

Is it only in the case when we are using permanent magnet stepper motor or it is for Hybrid stepper motor also?

Thank you for your prompt responses and Thank you for offering to help me out !!

Regards,

Nivethitha.D

Hi Nivethitha,

Lets say motor nominal current is 0.5A, for microstepping you can usually drive it with current up to 0.5Arms where peak of that current will be 1.41x0.5A = 0.705 A and driver should be able to deliver at least 0.705A so its full scale current should be at least 0.705A. 0.5/0.705 gives around 71%. There is some chance that driving stepper motor with current higher than its nominal current (0.705A instead of 0.5A while microstepping) its linearity will worsen because of magnetic saturation. If you want to use driver IC close to its full scale current for a time longer than a few seconds its power losses become quite high and heat management is very important during PCB design.

If you use driver that its full scale current is much higher than motor nominal current microstepping accuracy will worsen because driver current regulation accuracy falls with lower currents. "current trip accuracy" on page 7 of DRV8428 datasheet. Drivers with separate sense resistor have usually better accuracy over broader current range but may come with some additional problems during design process. Integrated drivers like DRV8428 are the easiest to implement.

I use that range for hybrid motors in full step and microstepping but for permanent magnet motors it will be similar.  It all depends on application and duty cycle. I used permanent magnet motors just a few times but in full step mode.

If you have any more questions I should be available in a few hours.

Regards,

Grzegorz

Hi Grzegorz,

         Thanks for your reply

Hi Nivethitha,

I edited my last two posts and changed current naming to be more relevant to driver ICs with integrated Mosfets like DRV8428. Current levels like "nominal" and "peak" can be confusing and mean something different for drivers with external Mosfets or ready to use drivers.

Regards,

Grzegorz

Nivethitha,

The Permanent Magnet Stepper Motor may need a non-sinusoidal waveform. So, the host controller can control the VREF pin voltage to get different output current setting.

I agree with Grzegorz's comments.

Regards,

Wang Li