Driving PF M motor slowly with BuWizz or SBrick

[pdw]

I've got a PF M motor that I want to be able to move very small amounts.  The problem I've got is that whilst the motor will run slowly, getting it started requires quite a lot of power such that the shortest possible tap of a button moves it too far.

I've noticed quite a difference in the behaviour between an SBrick and a BuWizz running at the same voltage.  For example, where the SBrick takes about 85% power to reliably get it moving, but the BuWizz needs just 60%.

As you'd expect, running the BuWizz in high power mode reduces the percentage power needed to get it moving, but strangely, in ludicrous mode it's completely stalled at the same percentage and requires a much higher percentage power to get it moving.  My only theory is that the stalled motor is tripping the current protection at the higher voltage.

I've tried hacking the BrickController2 code to send the shortest possible burst of power to the device, but even that seems to be too long.  I've also tried a fake M motor which behaves differently again, but not usefully so.

Any other suggestions or experience?  Obviously the right answer is lower gearing, but that's not really an option here.

[pdw]

A bit of a breakthrough on this one:  I've discovered that SBricks allow you to vary the PWM cycle length, and if I use the maximum value for this, which works out at about 73Hz rather than the usual 1KHz, it seems to work beautifully.  The motor will reliably start at low power even when loaded, and I can reliably turn the motor fractions of a rotation by tapping the output.

Since discovering this, I've found out that PFx Bricks have a "torque compensation mode" which does exactly this - low PWM frequency drive.

At the moment I've just got a hacked version of BrickController2 for testing, but I'm planning to turn this into an option that can be set on individual outputs.

[Mr Jos]

Can't you just downgear or even use a worm gear to get the slow speed needed? It will have way more torque and still turn slowly than lowering voltage/PWM.

Edited January 20, 2021 by Mr Jos

[pdw]

It's already using a worm gear, but that's not sufficient.  It's the fork tilt control for this model.  I want to be able adjust the tips of the fork height by less than a stud so that I can get them aligned for sliding under a palette.  This is about 10° of fork tilt.  It's controlled by a worm gear driving a 20T gear, so a full motor rotation is 18°.  There's a little bit of scope for playing with the geometry of the connecting arm, but no space for any additional gearing.

The issue isn't running the motors at slow speed, it's getting them started at slow speed, and the lower PWM frequency seems to do this.