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Topic: General question of microstepping a stepper motor? (Read 192 times) previous topic - next topic

kbrown

Hi,

I'm using a Pololu 1182 (An Allegro A4988 based) stepper motor driver which is capable of 16th microsteps. Everything works just fine but I have a generic question about micro steps as I'm fairly new to stepper motors in general: Let's say you drive your motor 5 microsteps (out of 16 possible) forward. What will happen if you cut off the current to the motor at this stage? Will it just return to the previous full step position as it didn't really reach half way of a full step yet? Or will it just stop where it is?

Cheers,
Kari
Cheers,
Kari

Peter_n

When the device is powered again, and the code starts again, it could start with the position at 5 micro steps. In theory the stepper motor would continue from where it was. However, I think that might not work. The stepper motor might change a little when not powered (it probably will) and I also doubt if the software and hardware can start with a micro step.

When the code starts with a full step, the stepper motor will move to the nearest full step.

To know the previous position when the devices is powered again, a positioning system would be needed that even can determine microsteps. I think that is not possible.

That is why a device returns to a 'home' position (where an optocoupler or switch indicates the 'home' position) after power up.

kbrown

Thanks Pete!

That's what I was suspecting too and it does make sense. The only reason why I asked was because my application is a battery operated one and I was wondering if I could save some amps by turning the stepper motor driver off when a movement is not required. But to hold a microstep driven position you need to keep the coils energized, I guess...
Cheers,
Kari

Peter_n

#3
Jun 18, 2015, 06:06 am Last Edit: Jun 18, 2015, 06:08 am by Peter_n
Turning (temporary) off the stepper motor is not okay. Perhaps it might work with a full step, but I doubt if that will be reliable. If it is not 100% reliable, then it is even bad for a full step.
The current can be reduced. It depends on the mechanical holding torque how much current is needed for that.

kbrown

The current can be reduced. It depends on the mechanical holding torque how much current is needed for that.
Thanks. I'll keep this in mind. I'm not too sure about the torque required yet as I haven't completed the mechanical bits yet.
Cheers,
Kari

jremington

Keep in mind that microstepping is for smooth operation, avoiding resonances, and does not provide accurate steps or constant torque.

kbrown

Keep in mind that microstepping is for smooth operation, avoiding resonances, and does not provide accurate steps or constant torque.
That's good to know, thanks. Accurate microsteps or constant torque may not be required in my application. I'm just driving a focuser on a telescope via a 3:1 gear ratio with it...
Cheers,
Kari

MarkT

The standard practice is to drop the current by a factor of two or so when the motor is stationary,
reducing motor power consumption/heating by four or so.  The stationary torque is always more than
dynamic torque, so this torque reduction is not normally a limiting factor.

Completely powering down means the motor loses authority (unless you have a worm drive to
hold position), and could move an arbitrary amount.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Peter_n

If you use 3:1 gear with 16 microsteps, can you use a 3*16=48 (or more) ratio gear with full step ?
A gear with 3:1 is only little for accurate things. Google returns many results for : stepper gear 100

kbrown

The standard practice is to drop the current by a factor of two or so when the motor is stationary,
reducing motor power consumption/heating by four or so.  The stationary torque is always more than
dynamic torque, so this torque reduction is not normally a limiting factor.

Completely powering down means the motor loses authority (unless you have a worm drive to
hold position), and could move an arbitrary amount.
Thanks. That's a good idea! The driver has an adjustable current limiting feature via the onboard potentiometer. Maybe I can replace that with a digital one so I can control it from arduino...

If you use 3:1 gear with 16 microsteps, can you use a 3*16=48 (or more) ratio gear with full step ?
A gear with 3:1 is only little for accurate things. Google returns many results for : stepper gear 100
I'm sure the 3:1 is plenty as the focuser itself has its own ratio too. Not sure how much it is but it doesn't take much force to get it moving... Also the stepper motor I have is probably a bit oversized for this application.
Cheers,
Kari

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