Linear Stepper Motor Driver?

[TeleFunKen]

Greetings all,

I need to find a way to power stepper motors without the typical chopping method used in most drivers, such as the A4988. Or more to the point, get rid of the noise that is emitted from the motors being powered by them.

To be more precise, I am working on a robotic application that involves a microphone, and the squeal from the stepper motors that comes while sustaining hold torque is a problem. I've read about stepper motor drivers that use resistors for current limiting, but can't seem to find a decent schematic of one.

My other option was to possibly increase the chopping frequency to be above the audible range, but I'm not sure how to accomplish that with the A4988 carrier that I'm currently using.

Any other ideas would be welcome... I just need to be able to get rid of the squeal when maintaining hold torque. When the motor is running it can squeal all it wants.

Thanks!

[bdring]

Do you need a lot of hold torque? Could you jut short a winding or both for hold torque? Could you drop the voltage to where it does not chop?

Some drivers, like Geckos, are virtually silent. Sometimes a little capacitance on the motor or 5v quiets them down too.

[TeleFunKen]

I don't need too much hold torque, compared to some applications I guess. The motors I'm currently using are the Pololu 1208 bipolar stepper motors with the A4988 carrier board. I'm just now developing the first prototype, so I'm not sure if those motors are going to have sufficient torque anyway. So far they seem to, but with a lot of chopping going on. (at least from what I can hear)

I've wondered if there are better drivers out there that either chop at frequencies above 20khz, or at least have control of the switching frequency. I'll look into the geckos, I have a friend who has some on his Novakon Mill, driving significantly larger steppers.

I've wondered about the possibility of shorting a winding for hold torque, but wasn't sure how best to achieve that. Would that be a problem if I'm microstepping and holding in between whole steps?

Am I correct to understand if I lower the voltage to a point where it doesn't chop that I will see a significant decrease in hold torque? I'll try some experimenting tonight to see if I can find a happy medium.

A little capacitance on the motor? Across both windings? What kind of value would be a good starting point?

Thanks for your help! And thanks for designing the Makerslide system... It's making my life so much easier!

[bdring]

I have never put capacitance across the windings. That could annoy some drivers. Put it across the power supply as close to the drivers as possible.

[frob]

Well that's a complex problem but not an impossible one. there are a few different ways to approach it:

I don't need too much hold torque, compared to some applications I guess. The motors I'm currently using are the Pololu 1208 bipolar stepper motors with the A4988 carrier board. I'm just now developing the first prototype, so I'm not sure if those motors are going to have sufficient torque anyway. So far they seem to, but with a lot of chopping going on. (at least from what I can hear)
I've wondered if there are better drivers out there that either chop at frequencies above 20khz, or at least have control of the switching frequency. I'll look into the geckos, I have a friend who has some on his Novakon Mill, driving significantly larger steppers.

Yes its true these things generate a *ton* of switching noise. Trying to increase the frequency can help but its complicated by the fact that they don't work at a fixed frequency at all; the frequency is a factor or several things, mainly motor inductance, input voltage, the timing resistor on the IC (sets the off time), and the instantaneous current setting. Going to low inductance motors will increase the frequency but also likely increase the noise intensity. The other approaches will be easier and work better:
Most of the noise is radiated and to some degree caused by the cables between the driver and the motor. So a real effective solution is just to get rid of the wires entirely: ideally, mount the drivers right on the motors themselves. so cable are only a couple inches long. barring that, make them as short as possible, and use heavy gauge shielded /twisted pair cable.

Same thing applies to your microphone circuit. mic cables are at a high impedance so they pickup noise very easily. if its a long wire you're better off putting a preamp right at the mic and then cable the line level output from there instead. Or at least use very high quality balanced / shielded mic cable with an amp at direct earth ground, away from the motor system digital ground.

If you want to try a different driver circuit, i am preparing one such product that runs at a fixed frequency of 46kHz.
if all else fails, pm me and i'll see about getting you a beta sample to try out.

...Am I correct to understand if I lower the voltage to a point where it doesn't chop that I will see a significant decrease in hold torque? I'll try some experimenting tonight to see if I can find a happy medium.

Not exactly. if you lower the voltage to just that point you will get about the same torque. only if you keep lowering it further will torque start to decrease. This may be in fact the easiest thing to try. the downside is it will quickly limit the max speed you can attain with your motors, and torque will drop off very rapidly with increasing speed. this would work best if using only full steps or uncompensated 1/2-steps. if you have a variable supply you could adjust it dynamically to keep the voltage up high for active motion and then drop it down for "hold mode" when the motors are stopped.

A little capacitance on the motor? Across both windings? What kind of value would be a good starting point?

>>>WARNING: adding capacitance to the the output of a switching stepper driver, especially at the motor, will only make the problem much worse, overload the drivers, and likely destroy them. don't do it. this trick is good for DC brush motors *only*.

[TeleFunKen]

Wow frob, thanks for the awesome reply!

At the moment I have significantly longer wires going to the steppers than needed... So I'll definitely shorten them down to a couple of inches like you recommend (which in reality is all I need, I think). Then I'll start tweaking the voltage to the motors with a variable supply to see if that will quiet them down.

Your 46khz drivers may very well be a solution that will work for me, it's well above the audible range... PM me when you get a chance and let me know how to reach you to discuss purchasing a beta sample to try out.

Thanks again for your help! It is truly appreciated.

[KyleAdam]

A stepper motor is a brush less, synchronous electric motor that can divide a full rotation into a large number of steps.There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor: bipolar and unipolar. Unipolar motors.