Stepper motor driver with less noise?

Over time I've tried maybe half a dozen driver chips. They were all noisy. It's the current ramps where the problem is. SMPS' can be jittery at low currents and steppers are double worse, with two ramps at work and both throwing transients across to each other. Yes. Best to roll your own.

Don't know of any, but to me it would seem they should have added a way to filter the current sense node to provide the correct feedback stabilization when different motors and voltages are used. Won't a compact solution like this always be a overly worst case design?

Regards

Klaus

The data sheet talks about regulating the on and off times with a one- shot. All the monostable circuits I know about involve some kind of comparator looking at an analog ramp, and the comparator is obviously going to be sensitive to noise on the ramp and the voltage reference. You can minimise the external contributions to this noise, but with a single chip solution you are stuck with the chip-designers layout inside the chip - a chip that is switching the same drive current that is running through the motor.

I'd be much happier with a chip that used a digital delay generator ... Naturally, I've never heard of one.

-- Bill Sloman, Nijmegen

Adding a cap across the sense resistor was about the only thing the app engineer could imagine would help but it didn't. The feedback loop is all internal. According to him the other drivers from Allegro are all similar, the main difference being extra control features.

And yes, being a compact solution might just as well be the problem here. We were only drawing a few hundred mA but still there are eight large FETs on the same die as the control circuit. This is also a reason why I am not a fan of fully integrated switcher chips (plus the fact that there usually isn't a 2nd source).

Yup, that's about how they all operate. It can work but only if the chip designer is very experienced with noise mechanisms. Some things have to be done differentially and I am afraid they weren't. The mono-stable seems kind of ok but the comparators are probably the ones where their thresholds are flopping about.

On that project we'll have one pretty soon. It'll be quantized but that is much better than such gross jitter. I guess that once the client has seen the first "homemade" controller work they'll kick out all the other microstepping driver chips as well. All you really need is an FPGA with some space left on it and a few vacant pins. Or a uC that has an unused timer with at least two CC registers.

I guess you are right, there just may not be an easy way out. I wonder why the self-rolled SMPS or motor controllers are quiet and many chips aren't. Maybe because the power devices are one the same die.

OTOH those driver chips are not cheap so a roll-your-own session brings about a nice BOM cost reduction.

I can't remember the details, right now, but I had lots of trouble producing a stable, quiet operation from a two phase Allegro driver chip, once I increased the clock frequency high enough that the fundamental should have been inaudible. The problem centers around the low di/dt over the short current control cycle, so the regulation rattles all over the place and sub harmonic operation ensues. I solved the problem by impressing a small triangle wave on the current sense reference that had slope opposite to the motor current wave, so that, even though the motor current varied little each cycle, there were clear current limit signal crossings each cycle, keeping the current control stable. In effect, the impressed triangle wave lowered the gain of the current limit control loop. This eliminated all the sub harmonic jumping around and the motor went completely silent when holding position. If anyone is interested, I'll dig up the details.

I't doesn't lower the total cost so much.. Relatively cheap stepper motor controller costs only ~3e (4$) in reasonable quantitys. For example, the cheapest ones in New Japan Radio NJM377x-series.

And to do it in discrete components, you'd need.. Components for 2 H-bridges, their drivers, logic, switch mode current limit, etc. And the cost of pcb space and installing all those discrete components. Not to mention the designing cost.

As long as cheap controllers are suitable for the job, they seem to be the easiest and cheapest way to go. I know, there are much more advanced and a lot more axpensive chips around. And applications where cheaper controllers just aren't good enough.

However, as a hobby project, discrete stepper motor controller/driver would be great.

That is interesting. However, the ramp would have to be synchronous to the end of the off-time and my impression was that this driver did not always have them coincide for the two bridges yet there is only one ref pin. The internal schematic shows that the oscillator feeds both branches but the scope didn't corroborate that. Who knows.

In this case we'll just chuck them and roll our own. Then we know it'll work.

Yes, but the problem is that the noise pollution it is spewing around is so bad for this circuit that we must abandon it. If they would contain a proper PWM, meaning not much jitter, I'd agree.

You can use cheap gate drivers for H-bridges but I have suggested to that client to consider unipolar steppers. Then all you need is a bunch of As long as cheap controllers are suitable for the job, they seem to be the

And applications where even the "good" controllers aren't good enough. That is kind of my domain, where standard stuff doesn't cut it and a roll-your-own solution with better performance is required. IOW that's when my phone rings ;-)

Not just for hobby. In production there is a huge incentive to live with standard discretes. It greatly reduces the chance of a line stop if for some reason the fancy chip slips in delivery schedule.

I've used the NJP 3961/3770 's driving bipolar stepper motors in industrial printers (32 microsteps / fullstep) in production for years and the Allegro A3979SLP-T recently in the same application. No problems with FCC class A radiated or conducted. Allegro's are low cost, run cool, tiny, low parts count.

Just my 2 cents....

Bruce

Well, sure, because raggedy PWM cycles will distribute the EMI across a huge swath of spectrum. It works like artificial clock dithering in PCs to make them pass. But when you have to build a system that contains a sensitive receiver in a certain band this doesn't work.

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Interesting. I'm having trouble visualising it. A pic' would be worth a thousand words!.

A few more words, first:

At the beginning of each clock cycle, the the reference voltage is compared to the amplified current measurement, and if the current is less than the reference, the output switch is turned on, till the current exceeds the reference, at which time the switch is turned off. If the clock is run at a low frequency, relative to the di/dt for the motor and supply voltage, this results in a single voltage pulse per clock cycle, if there is enough voltage to pass through the setpoint current cleanly in half the cycle or less, with a nice big current sawtooth passing the reference twice per cycle. But with a high frequency clock (an ultrasonic frequency) the motor current changes almost none at all during a clock cycle, so you get multiple clock cycles of on time (with one truncated cycle at the end that is just noise), followed by multiple clock cycles of off time, as the current falls below the reference.

By adding a small clock cycle triangle to the reference voltage, so that the reference peaks high, just as the cycle starts, and falls through most of the cycle, a short and variable power pulse is produced, less than a clock period, that maintains the slowly changing motor current somewhere through the middle of that reference triangle wave. In effect, the gain is lowered from infinite (bang. bang control) to a lower proportional gain (inverse to the amplitude of the impressed triangle wave) that stabilizes the control loop, even though the motor current time constant is many clock cycles.

Oh, in that case it would make sense to make own controller. Or pay more for some advanced controller.. Most of the simple and cheap controllers use RC timing network, which isn't accurate.

With unipolar steppers, it is allmost too easy.. Too bad, they don't give the maximum power output from stepper motor.

RC networks can work well, that is how I do my own stepper circuits and switchers. The problem is their comparator for the current ramp that is flopping all over the place. Maybe because it has to share the die with the H-bridges. I guess it's like trying to listen to Mozart while in New York City during rush hour.

Mostly we don't need power. For really good and reliable torque without skips most integrated controllers aren't that good anyhow because you have to operate with variable clock. If you have to do that externally you might as well build it all, and properly. That's what we are going to do now.

Sort of like the slope compensation used in current mode converters to remedy the subharmonic problems and noise sensitivity at low ripple currents (see Unitrode U-97)

Regards

Klaus

(snip)

Yes, I think so. Thanks for pointing this note out.