Re: Stepper Motors
 

More problematic than the motors are the controllers. You would need a source(s) stable enough to provide the same controller through one or multiple seasons for all the teams. The controller would have to interface with the roborio in such a way that the enable/disable is robust.

Re: Stepper Motors
 

I'll be honest, I've spent some time trying to source a brushless motor for some non FRC projects and it's mildly daunting to find a motor and a controller. And then interfacing with them is a whole other issue (though I've found one that I can press fit WCProducts pinions (for bag motors) onto and do decent reductions)

Re: Stepper Motors
 

I don't know, I've never had that problem. ESCs for standard brushless motors seem pretty easy to come by once you know what you're looking for. I'll second the gear problem though; you tend to have to use set screws or press fits to keep stuff on the shaft.
SDP-SI sells "fairloc" hubs for their gears, which are like integrated shaft collars that allow you to leave the shaft unmarred. Something like that would be good for interfacing a brushless motor.

Re: Stepper Motors
 

Low cost, sensored, robust, controllers that go both directions without having to come to a full stop first?

Re: Stepper Motors
 

The sensor part is really critical for brushless motors. Without a sensor, you get much less torque at low speeds, and you can sometimes stop in a place that you can't start from. I do think that brushless motors will eventually replace the motors we use now in FRC.

Re: Stepper Motors
 

Another way to use stepper motors is to put encoders on the stepper.
Then accumulated error is less and correction steps can be made.

That's basically what a closed loop MaxNC mill or router does on the axis drives:
MaxNC

If the goal is low RPM discrete movements over partial rotations steppers have a very valid use case.

If the goal is higher RPM discrete movements over higher numbers of rotations motors rigged to be servos make more sense.

In CNC case it is common that servo drives like those from Panasonic on my larger home brew tools are AC motors with encoders and for short low speed movements I need to use gearing or cog belts to get the speed down and the keep in the higher torque operational area of the servo. For occasional short movements it's okay to not gear down but honestly it is hard to just use the servo output shaft in most cases. Really same problem with FIRST drive trains and CIM motors. Stepper motors start to have decreased torque output the higher the output RPM where as servos tend to have higher torque in an RPM band higher up.

If my CNC work area is small, like say a 3D printer, it makes no sense to implement a full servo just so I can gear it down or do something with the electronics to push power into the motor and maybe blow past my intended target position. It makes more sense to use a stepper motor that tends to move slowly in discrete steps at higher power and catch the errors - with the extra bonus if a missed step is caught by the stepper with encoder it can be tried again which often works great with cutting tools pushing through materials.

Just to put it in perspective: servos with reductions can run at higher RPM with higher power than steppers so where professional larger size CNC is concerned (run time is directly proportional to production volume and money) I'd want the more expensive servos.

Course there's the short movement intermediate case of hobby servos. In the hobby servo case the target position and the desired kind of movement is more to be smooth and close enough. Also it was likely cheaper to make the plastic gear boxes in volume than create the electronics when those first appeared on the market.

Radio Control History

Re: Stepper Motors
 

Focusing only on the OP - Open loop control of a stepper will only be successful in those robot locations and during those match times where/when the stepper is always stronger than the varying loads it encounters. Otherwise the stepper gets stalled, and your count history becomes useless until you correct it using some known reference.

If you count the number of places on a typical (or close enough) FRC robot where the loads on motors are always low (or close enough), and/or fully predictable (or close enough), I'll bet you come up with a short list. Those are the places where reasonably-priced (likely to be donate-able) steppers and their controllers are going to be useful.

I suggest using the steppers on your demo machines and other eye-candy, and using the alternatives during your matches.

Blake

Re: Stepper Motors
 

I agree. Maybe I've led a sheltered life, but the only project anyone ever discussed with me that included steppers was ca. 1983 when the University of New Orleans automated the aiming of their 8" telescope. (With the light pollution, it had at least twice the light-gathering capability that could be justified. At least until 2005, when the lights went out for most of a year.) I can also imagine steppers being useful in machining applications. I really cannot come up with an on-field FRC application for a stepper that would not be more robustly met with a brushed motor and a high-count encoder.


On another subject I saw earlier in this thread: I have never paid much attention to brushless DC motors, except to plug a COTS fan into a computer I was building, the high-level theory as I was taking undergrad E&M, and a bit of subliminal/hindbrain work as I was reading "Prodigal Genius" about ten years ago. I was rather surprised to read that brushless motors could be left in a state that could not be electrically escaped. It seems to me that brushless motors should be built with N magnets on the rotor and N-1 electronically cycled coils on the frame/stator (or N+1 on the stator, but I prefer that the simpler solution be on the active side). In either of these configurations (assuming the smaller number is at least two), there would be no "trapped" states from which the motor could not start; essentially half of the coils would be available to pull the shaft around at any position. Did I miss a trick, or did the designers of brushless motors?

Re: Stepper Motors
 

This is one the reasons why the motors have multiple (most commonly three) phases. All typical industrial servo motors will be able to generate torque at any shaft angle.

Re: Stepper Motors
 

That is actually one of the more compelling reasons that they should be added to FRC! One of the most amazing things about FRC over the last decade or so is how VexPro, AndyMark and other vendors have stepped up to "democratize technology"... in the sense that it is so much easier now to source working gearboxes, chassis, control systems, and speed controllers.

If a particular brushless motor/controller combo were allowed how long do you think it would take before:

a) There would be a half dozen gearboxes designed to fit it available for sale at reasonable prices

and b) There would be suggestions for better motor/controller combinations?

I love the reliability and economy of the CIM... but the efficiency does leave a bit to be desired!

Jason

Re: Stepper Motors
 

That's the big issue. Once I knew what I was looking for in a controller and in a motor it became pretty easy to find things that would PROBABLY fit the requirements. [1] But a lot of the issues I ran into were because the specifications don't involve things like "torque" and "rpm" directly and instead involve new terms like "kv" and a handful of constants that I didn't know.



[1] Was looking sensorless brushless for cost reasons, ended up changing scale on the project and am using a pair of tiny brushed motors that, even with controller, come in cheaper than the motor.

Re: Stepper Motors
 

So not really knowing much about Brushless motors (other than the basics of operation), what do they do better than the brushed counterpart?

Re: Stepper Motors
 

More efficiency is a good one. Also more power density.

Re: Stepper Motors
 

It's easy to tell a brushless motor to go at an exact speed, or to configure it to have a flat speed/torque curve. They also are generally more powerful/efficient for the same weight/size.

Re: Stepper Motors
 

Yes and no.

It's not that they are inherently amazingly better for this, it's just that the typical control methodology required for brushless motors requires you to have your ducks in a row and already have the sensors/firmware required to do so.