stepper motor odd behaviour

[duncan webster 1Participant @duncanwebster1]

[duncan webster 1Participant @duncanwebster1]

I've been experiencing odd behaviour from a couple of steppers. They work perfectly when full stepping, but when I alter the code to get half stepping they seem to go sort of 1/4 then 3/4 if you get my meaning. I'm running them well below their rated current, so is possibly that the electromagnetic pull is insufficient to pull the rototr away from it's preferred postion determined by the permanent magnet? I've had this with both 4 wire and 5 wire motors, both 48 step

The one I'm suffering with at present is Mitsumi MS35P-9T. Can't find a data sheet for it, there is one for a 5 wire very similar, which quotes 400mA per phase, 13.5 ohm per phase, but this is a 5 wire unipolar motor, mine is 4 wire bipolar and has phase resistance 10 ohms. Can I just scale it on I^2R and use 400*(13.5/10)^2 = 730mA? This sounds a lot, I'm using 130mA at present.

[Neil WyattModerator @neilwyatt]

I had a similar issue when using a driver module, basically when run slow there wasn't enough back-emf to make the controller send enough current through at small displacements, meaning missed microsteps that would catch up with a jerk. here was a simple mod (replacing a resistor with a link) for my boards that gave much smoother motion by activating another mode on the driver chip.

Not sure if this is related to your issue, but it does seem that a 'simple' microstep drive won't always give smooth results.

Neil

[duncan webster 1Participant @duncanwebster1]

It's not driving from a clever microstep controller, it's an Arduino driving a SN754410 H bridge driver.

I've also realised my sum in the OP were wrong should be 400* sqrt(13.5/10) = 465mA

[John HaineParticipant @johnhaine32865]

How do you do microstepping with a bridge driver? You have to be able to vary the current in an analogue fashion to get the motor to stabilise between steps.

[TrevorGParticipant @trevorg]

If you are not using a clever microstep controller then your microcontroller will need to do the clever stuff instead. Unless you have access to suitable software then you will need to program it yourself. Which is why I wimped out and bought a controller.

You do not mention the voltage you are running on or how you deal with the effects of inductance. If you calculate the voltage from I*R this will work but the motor will only run at very low speed. Due to the time taken for the effects of motor inductance limiting the rate of rise of current.

In a commercial controller they run from a very high voltage and limit the current with pulse width modulation. Using current monitoring to regulate the current in each winding. This also has the effect of allowing the motor to run at a much higher speed.

It is difficult to see how you could achieve microstepping without doing something similar. The hardware you have is capable of this, once you have added in some current monitoring. The problem is in the complexity of the software needed.

[MuzzerParticipant @muzzer]

The TI app note for the SN754410 specifies how to achieve half step control for this device with a bipolar 4-wire stepper. That section is rather brief but it sounds as if it should be capable of it. I've no experience of it myself although it sounds as if the mode of operation is a bit of a compromise, giving large torque variation with position.

As John seems to imply, you may need a high frequency PWM current control to achieve reasonable microstepping without losing significant torque. Perhaps a TB6600 driver may work better in this application. There seem to be all-in-one drivers available at a low price.

Murray

[Les Jones 1Participant @lesjones1]

This link is a good source of information on stepping motors.

Les

[duncan webster 1Participant @duncanwebster1]

thanks for all the links, I'm trying to half step, not microstep, so clever pwm routines should not be required. This is only a small stepper running very slowly (about 4 rpm), the application doesn't justify buying a clever controller. However, having read the links, and looked at the data sheet again, I realise that what I've always though of as full stepping is called wave drive, and I could get 1.4 times the torque, and smoother, by using full stepping where 2 phases are energised at once. I've also realised that when I apply 5v to the driver chip it actually only feeds 2.9v to the motor, so I'm well down on the current I thought I was driving. I'll reduce the series resistors and report back.

[John HaineParticipant @johnhaine32865]

Given that modern microstep drivers are so cheap, like the one linked above or this one, is it worth using a kluge? Steppers work much better with a proper chopper drive, you can avoid the series resistor, get more torque and smoother operation.

[Shed HappensParticipant @shedhappens]

Hi you could try one of these …. http://www.hobbytronics.co.uk/a4988-stepper-motor-driver

I've used these connected to an arduino and they are only a fiver (probably less if you get from Hong Kong off ebay)

search for a4988 stepper

The driver features adjustable current limiting, overcurrent protection, and five different microstep resolutions. It operates from 8 – 35 V and can deliver up to 2 A per coil.

John

[MuzzerParticipant @muzzer]

The terminology is a bit confusing but I was simply referring to anything other than simple, whole steps being microsteps, ie half steps and smaller. The other advantage of current control is that the torque is less dependent on the back emf but I guess you knew that. Doesn't sound as if the solution here need be any more than reducing the resistor.

[duncan webster 1Participant @duncanwebster1]

Thanks again. If I hadn't already built it I'd probably go for the a4988 device, but I had the SN754410 in stock left over from another job, and being a Yorkshireman, 'only a fiver' doesn't compute!

Having read more of the links, I upped the current to ~150mA by reducing the series resistors, and it now purrs sweetly. Moral of the story is don't try to half step on very small current.

I probably won’t be buying any more SN754410 chips, but as they are only 37p each you never know, for an undemanding job they can do the trick

[ZebethyalParticipant @zebethyal]

The a4988 is currently £0.99 for one or £8.25 for 10 on the Bay with free P&P from China/Hong Kong, the first set of these I bought cost me nearly £8.00 each back in 2011.

Arduino compatible 4 axis shields for the a4988 drivers are also available for £1.53 with free P&P (search for CNC shield on the Bay) these are clones of the Protoneer board.

I have also built stepper drivers using the Toshiba 6560AHQ chip, using the PMinMO design – 36V and 3A capable with the right resistors, total cost was under £5.00 per driver.

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