RS775 Gearboxes

Why, in the age of quadrature encoders and with targets at a set height, would you be subjecting your lifting arm to human control (beyond 'this is the height I want)?

Why subject your robot to a potential sensor failure when you can train a monkey (or a student, if monkeys are lacking) to put the tube at the right height every time?

Because I trust software about as far as I can throw it, and since it has almost no mass, I can’t throw it very far. :slight_smile:

I know there are a lot of great programmers in FIRST. I also know that teams are always pushed right up to a deadline, and the software teams get very little time to test. I also know a slow arm is a lot easier to stop, and if your arm is going 180 degrees/second and you miss a limit switch for some reason, something is going to break. There are a lot of great teams in 2007 that didn’t actuate at anything close to 180 degrees/second. There probably are some that did too, but I’m a lot more comfortable with a slow arm that I can fall back on a human to control, than needing a relatively complex piece of software to control.

Why would you use an encoder? It is simpler to use a potentiometer since your are limited to probably less than one revolution, no?

A. Trimpots drift. Encoders don’t.
B. The same idea could be replaced with “why do this in the age of the limit switch?” and the objection is the same (though with slightly different hardware and software, without quite so much elegance).
C. With 20 minutes of LabView training, I was able to bring an arm up to whatever height I wanted to by pushing the appropriate button.
D. If you haven’t made things autonomous, then you can’t score in autonomous…

Banebot just emailed me back:


The new CIM-U-LATOR gearbox and P60 mount for the RS775s will be available for order Wednesday.

Thank you,

BaneBots Sales

Jason Law

On the BaneBots page, it says they’ll be offering a 64:1 and a 256:1 for the 775. Really? Nothing in between those ratios?

Can you explain (in non-EE terms, preferably) how big of a problem this is in FRC style applications? In my six years of robot building we used potentiometers many times in the feedback loop for rotation, and many more times in our Operator Interface. We never had a problem, and none of my EE mentors ever mentioned it as a potential issue in our many hours of gremlin chasing. Did we dodge a bullet, or is this a case of “it only really matters when you’re building spacecraft”? Thanks! :slight_smile:

The one year we used a trimpot, we found drift values of 5-10%. At one point it drifted sufficiently so that it was still driving while stalled.

We were the “oooh, neat, fire!” robot that year.

I would imagine that they may be the only packages for the RS775 that they offer off the shelf, but in years past swapping motors between BB transmissions was just a matter of ensuring you have the correct Mounting Block for the Motor and series of Transmission - so you should be able to buy a P60 with the desired ratio and swap the mount, right?

Actually, on the planetary page for the P-60 for the RS-775, it says they have multiples of 4.

4:1, 16:1, 64:1, and 256:1.

What Tom said. Looks like they’re initially only offering the 4:1 stages for the RS-775s. If the RS775 4:1 stages are identical to the RS-540 4:1 stages, then you could buy some of the 5:1 stages from the P60 Gearbox Parts pages and swap them in to make up your own ratios. I suspect they’re not going to have a 5:1 pinion for the motor itself, so you’d be swapping in at the 2nd stage or later, which is a little less than ideal… But you’d still get the different ratios you’re looking for. 4:5:4 = 81:1, 4:5:5 = 103:1, which gives you a few more options between 64 and 256. Granted that you still have the big jump up from 103 to 256, but unless BB comes out with some 3:1 stages, that’s always gonna be a sizable jump.

RS775 18V Motors… Before jumping to the motor with the larger number, check out the Motor Specs to each of the available Motors. In my eyes, that RS550 Motor in most ways is superior and now one of our fellow FIRST Team members spoke to Banebots this morning about the Availability of the different versions of the gearboxes for the RS775 Motor… More options going with the lower motor. Plus more Torque.

Can you explain what you mean by this? From the specs I have, it looks like the RS 550 outputs 21 fewer Watts at peak power (266W vs. 245W). To me, this is strictly inferior (although not by much). Is there some other factor you are taking into account? I have also heard that because the motor is designed for 18V, it runs more reliably at 12.

We are considering using one RS 775 for each of the strafing wheels in our holonomic drivetrain. Will the load from a drivetrain be too much for these motors and the gearboxes on them? There seems to be some concern about max output torque. Would the RS 550 be a comparable alternative, in light of the greater availability of those gearboxes?

Plus more likely to catch fire under stall load!
I’ll stick to the 775’s and their greater robustness…

Which specs are you looking at when comparing those two motors? I see two conflicting sets of numbers when I look at the motor curves on the KOP website and then compare to the yellow BaneBots Motors page from the kit. The motor curves give lower numbers than the BaneBots sheet. Here is what that sheet lists for RS-775:
No Load: 13000 RPM, 1.8 A
Peak Efficiency: 119 W, 11360 RPM, 12.5 A, 14.2 oz-in.
Stall: 86.7 A, 110.8 oz-in.
Then you go to the site listed on the yellow sheet,, and you find a third set of numbers.

Does anybody know which numbers are right? The motor curve on FIRST seems wrong.

Seconded! RS-550’s and FP’s fail spectacularly under heavy loads. For the uninitiated, these motors have nice little plastic impeller fans to blow air across the coils and cool them. But the more you load them, the slower they turn, the hotter they get… This continues for a short while till you melt the little plastic fan. At that point, the motor’s toast because it’ll never be able to adequately cool itself. And that’s assuming it hasn’t managed to catch fire or short itself out before the breaker trips.

I am using the numbers from the Banebots site for the 18V motor, then bringing them down according to the Voltage difference, giving the performance at 12V.

These are:
Free Speed - 13000 rpm
Stall Torque - .7833 Nm
Stall Current - 86.67 A
Free Current - 1.8 A
for the 18V RS 775 running at 12V.

There was another thread about this, and banebots posted some official numbers for all the KOP motors, including the 775-18 running at 12V.

P60 with RS775, P80 with RS775 and CIM-U-LATOR should be up on the web site by the end of the week and available for order at the start of next week.

Links to data sheets for the BaneBots motors found in the KOP:

Note the RS775 data sheet is for the 18V motor in the KOP at 12V


FIRST used the numbers for a different BB RS-775 motor that’s specifically a 12V motor. The correct numbers and curve were posted by BB in a different thread:

In 2007, 1075 built a multi-position pneumatically powered arm (a single cylinder drove it, under control of 3 air valves [supply valve, exhaust valve, and direction valve]) It was able to stop anywhere in its travel, and used PID control to stop where we wanted it to (at the three heights, plus the floor) using a potentiometer. It was FAST. I would not be at all surprised if it approached 180deg/sec. It never seemed in danger of tearing the robot apart, to me at least.

It did however have PID issues where it would oscillate around the set point occasionally, particularly if air pressure fell below 60psi and the valves stopped operating consistently.

It was a much-improved derivative of our 2004 off-season arm, which was also multi-position pneumatic, (and as far as I know, one of the earliest examples of multi-position pneumatics using KOP valves in FRC [at the time, you were restricted to using the valves that came in the KOP, which were standard single and double solenoid valves, without an 3rd setting to stop the cylinder])