Stepper Motors

[AdamHeard]

Originally Posted by GeeTwo

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?

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.

[slomobile]

Reading 2015 R42 Table 4-4, it is legal to power electrical solenoids with the PCM. What is a stepper but 4 solenoids arranged about a steel rotor?
If each coil of a stepper were connected to a solenoid channel, and the channels were activated and deactivated in the right sequence, the stepper would function correctly. It would not be fast or efficient code, but it would work, it would be safe, and as far as I can see, it would be legal. Getting a robot inspector to agree is a completely different matter.

The inherent current limitation of the PCM ensures that unsafe forces will not be developed.

Useful applications of steppers in FRC include camera turrets, gating, and autonomous aiming mechanisms where a device needs to be rotated to multiple positions with a high degree of repeatability. There are some applications where there just isn't room for multiple limit switches or the degree of movement required is so small that it simply cannot be achieved by brushed motors without overshoot.
All the warnings about becoming out of step mentioned earlier in the thread still apply, but a home limit switch can mitigate the effects.

[EricH]

It would also be useless. See, the limit on electrical solenoid actuators is 10W, 1" stroke, each. Therein lies the problem: converting that 1" stroke into a stepper motor isn't going to be easy... (For that matter, I've only ever seen 1 team that I know for sure used electrical solenoid actuators. 1. And I've been doing this for... well, let's not go into that.)

Meanwhile, the folks who used a servo or three are going to be done already and busy tracking. (And have probably rigged a window motor with PID to act as a really big servo...)

[Alan Anderson]

Originally Posted by slomobile

Reading 2015 R42 Table 4-4, it is legal to power electrical solenoids with the PCM. What is a stepper but 4 solenoids arranged about a steel rotor?

The 2015 list of legal motors includes Electrical solenoid actuators with a specific limited power and a specific limited stroke length. I am absolutely certain that stepper motors are not, nor do they contain, solenoid actuators.

If I were inspecting a robot and found a stepper motor, I would not consider it legal. If a team tried to argue that stepper motors are just solenoids in a certain arrangement, I would tell them they are entitled to present their case to the Lead Robot Inspector.

If I were the LRI and a team tried to make that case to me, I would give them a yellow card for egregious attempted lawyering.

[jman4747]

So I think, other than the brushless discution, we are somwhat off track. I Think the main point of the OP was to find resions why stepper motors could not be legalized but this has become more of a discussion on whether or not they are useful to a particular team. I think thay can and shold be allowed with very simple restrictions that are easy to verify. It won't hurt you if you don't touch them.

[techhelpbb]

Originally Posted by jman4747

So I think, other than the brushless discution, we are somwhat off track. I Think the main point of the OP was to find resions why stepper motors could not be legalized but this has become more of a discussion on whether or not they are useful to a particular team. I think thay can and shold be allowed with very simple restrictions that are easy to verify. It won't hurt you if you don't touch them.

I think someone would have to demonstrate to FIRST headquarters that: your stepper motor control was cost effective, open-source and meets the safety requirements to get started down this path.

As noted previously then they might need to donate a bunch to FIRST to evaluate.

Good news is thanks to RepRap printers there is a big DIY community for NEMA17/23 steppers and the parts needed to control them. Thing is the end product will probably have to be FIRST specific or have FIRST specific options.

So yes someone could drive this change: but there's some costs and some lobbying involved that will probably take this into late next year at the very earliest - if they even agree to consider it.

Also I've picked that NEMA size stepper for a better reason than merely the availability of cheap control parts.
You want steppers that will work within reason at the battery voltages of the robot.
CNC controls for steppers often use more than 24V even more than 48V for the stepper power supplies.
So realistically we should consider the practical limitations: FIRST is not likely to start letting you add whole batteries.

[jman4747]

Originally Posted by techhelpbb

I think someone would have to demonstrate to FIRST headquarters that: your stepper motor control was cost effective, open-source and meets the safety requirements to get started down this path.

As noted previously then they might need to donate a bunch to FIRST to evaluate.

Good news is thanks to RepRap printers there is a big DIY community for NEMA17/23 steppers and the parts needed to control them. Thing is the end product will probably have to be FIRST specific or have FIRST specific options.

So yes someone could drive this change: but there's some costs and some lobbying involved that will probably take this into late next year at the very earliest - if they even agree to consider it.

Also I've picked that NEMA size stepper for a better reason than merely the availability of cheap control parts.
You want steppers that will work within reason at the battery voltages of the robot.
CNC controls for steppers often use more than 24V even more than 48V for the stepper power supplies.
So realistically we should consider the practical limitations: FIRST is not likely to start letting you add whole batteries.

That assumes we use the same rule that only specific controllers are allowed. A 12v to 24v step up converter isn't necessarily all that hard to find. I Don't however know about the required current capacity for a typical size stepper you could likely see on an FRC robot.

[techhelpbb]

Originally Posted by jman4747

That assumes we use the same rule that only specific controllers are allowed.

How else would you implement the field safety features such that the stepper can not be moved while not enabled?

Originally Posted by jman4747

A 12v to 24v step up converter isn't necessarily all that hard to find. I Don't however know about the required current capacity for a typical size stepper you could likely see on an FRC robot.

Yes you can certainly boost voltage like FIRST did with the cRIO.
Something like this would do it.
Course at 24V@3A you are just scratching the surface of the power requirements for a single stepper.
As an inductive load your switching DC-DC converter will need to be slightly overrated.

A A4988 for example can handle 1A coils with minimal heatsinking, 2A if you fully heatsink and force air cool. So if your RAMPS 1.4 PCB is configured like a Prusa I3 then you have: X, Y, Z split to 2 stepper drivers, extruder single stepper. Assuming the coils are all in parallel it's easy to pull 1.4A or more to each stepper while moving full steps if the current limit set on the A4988 allows it. It is quite possible the maximum torque will limit below 200oz-in at the lowest RPM, more over even below 100oz-in with some stepping motors. This assumes of course that the voltage of the stepper is such that the robot battery voltage is sufficient to move it the full stepping and micro-stepping modes. So the voltage of the stepping motor should be below 12V to come off a FIRST style battery. (This is safe because this A4988 driver is a current limited chopper driver: set the current limit wrong smoke the stepper motor AKA safe unless you do it wrong.)

To put this in perspective: a <12V NEMA23 stepper with this A4988 on a 12V battery is going to be about the force of a general case hobby servo.
A RepRap generally needs a fraction of this force to move the mechanics if built properly.

[Andy A.]

I'd like to see steppers included in the rule set. Surely there are cases where they'd be useful, and it'd be a neat way to introduce students to a different sort of motor and control scheme.

I recall a turret on a robot from a few years ago that'd have been a perfect candidate for a stepper, as lost steps seem unlikely and the speed/torque requirements strike me as having been within reason for the smaller NEMA size motors out there. It's been a while, but I'm pretty sure it'd have worked nicely. A closed loop brushed motor achieves the same results but in this case the stepper strikes me as the more elegant answer.

As a practical matter, I suspect it'd take a motor/controller combination that can achieve the same sort of holding torque that a window motor currently produces, and which is also reasonably compatible with FRC's electrical and cost limitations. I have no idea if such a thing exists, but it's probably out there.

Whether the benefit, which is mostly the novelty factor, justify the headache of sourcing them and the rule changes is another story.

[techhelpbb]

Originally Posted by Andy A.

As a practical matter, I suspect it'd take a motor/controller combination that can achieve the same sort of holding torque that a window motor currently produces, and which is also reasonably compatible with FRC's electrical and cost limitations. I have no idea if such a thing exists, but it's probably out there.

Whether the benefit, which is mostly the novelty factor, justify the headache of sourcing them and the rule changes is another story.

Oh you want more power eh:
Same idea with external power MOSFETS
On a PCB

In fairness many window motors use a worm drive.
So the holding torque is really about how strong the gearing is.
As long as the stepper has the torque required to turn the worm gear.

Another way is to use a something like a dog gear to lock the mechanism at the stop point.
The advantage with the dog is that you don't inherit the high worm gear ratio because as steppers move faster they operate with lower torque.

Then again could just use a clutch.

[JamesTerm]

Originally Posted by Oblarg

Do you think there's any chance we'll ever see steppers on the list of FRC-legal motors?

I don't know much about steppers but I'd like to ask a question... if it were legal, how would it be wired to the roboRIO? What is its voltage and current specs?

One mystery I've been tracking deals with the loss of the Vex 393 being legal, and while I have yet to find the official answer why it was pulled... I'm guessing its electrical needs are not quite a good fit for the roboRIO. I'm still working out the amp draw from the roboRIO vs. the digital side car.

As I recall there were several rules about the wiring that prohibit any special wiring needs to manage the voltage... so for example if I wanted to use the 393 I could either provide a separate 7.2 power source, or wire my own voltage regulator. (these things would be discouraged). So I'm curious how good of a fit these stepper motors would be in terms of wiring.

[techhelpbb]

Originally Posted by JamesTerm

So I'm curious how good of a fit these stepper motors would be in terms of wiring.

Well if we used standard 1.8 or 0.9 degree NEMA23 steppers like a RepRap with 1.2-1.4A per phase at around 3.5 to 4.2Ohms.
We could use the same Allegro Semiconductor A4988 drivers as the RepRap RAMPS 1.4.
That would get a STEP+DIR interface to the stepper motor for a 5V TTL signal level and take the motor power from either the battery directly or some DC/DC conversion.

An Arduino can obviously control this with a RAMPS 1.4 board on a RepRap 3D printer with several steppers per MEGA2560 so there' s no reason someone couldn't hack up either an Atmel based stepper controller or just use the digital I/O.

The carrier board based A4988 drivers can often be found for the $4-$8 range each.
If you look around you can find a cheap Arduino in the $5 range.
Even if you forked over for the whole RAMPS 1.4 board with the Arduino you can get that whole package for around $75 from SainSmart.
You may also want to get some small aluminum heatsinks for the A4988 they can overheat near stall.

So one could create some custom RAMPS 1.4+Arduino MEGA2560 firmware and have a bunch of these steppers running from a RoboRio with that as a coprocessor drawing power from the robot PDP breakers. Seems like $75 + $10-$20 a stepper motor. Cost wise it's easily comparable to cost of a Talon and a CIM or a bunch of hobby servos. Course this has the issue of how to integrate the field safety shutoffs, however I can see quite a few ways to make that work.

Speaking purely as a matter of possible - not as a matter of FRC legal.