Motor Used for Shooting

We are using 3:1 versaplanetarys with 775pros on a greenhorn style 2 wheel horizontal variable angle shooter.

Cims and mini Cims do decent at 1:1 bags don’t work at all unless you go for a 5:1 or higher gear ratio.

If you are operating the wheel at max motor speed there is no headroom to do closed-loop control.

So if you have a 775Pro geared at 3:1 and want to control the wheel speed at 6000 RPM you won’t have a lot of battery voltage left to work with.

I’m not the one behind the controls for the robot, but I believe we’re more interested in using the encoders for 1) making the two sides of the shooter differ by set amount to see how spin will affect trajectory and 2) monitoring when the flywheels get up to speed so that we can move the ball into the shooter the moment it’s up to speed.

We’re planning to pull our catapult with an AM PG71 (motor is a 9015), taken down another factor of 3.6 (at initial design) with sprockets and #25 chain. The encoder is already in place on the COTS assembly.

We’ll probably use another PG-71 (no encoder, just limit switches) for our mechanism to lift the ball over the bumpers and into the launcher.

Roller pickup TBD.

I have seen you posting the same thing in a few other threads. I was wondering if you have a minute to explain/have a link to a thread that explains how operating voltage is tied to using control loops, to someone who knows very little about such things?

The control works by varying the voltage supplied to the motor. The motor controllers cannot put out a higher voltage than they take in from the battery. Without getting into PID theory, essentially if it’s going too slow, the voltage is increased; if it’s going too fast, the voltage is decreased. If you’re running the motor at top speed (giving it full voltage), you can only adjust down. Far better to have your nominal operating speed at say 75% of top speed, especially later in the match when the battery voltage is reduced.

We have been using bb550 motors on versa planetary 3:1 gearboxes with a four inch wheel. It has been working great for us. We have used these motors in the past with great success and I am surprised i haven’t seen anybody else using these motors.

Our team has tested both the miniCIMs and the CIM’s. We are using two CIM’s to “launch” the boulder. Good luck to all teams this year!

We’re also looking at using a 775pro at a 3:1 but I’m having trouble figuring out how to attach a longer shaft to the VersaPlanetary Gearbox shaft.

I figure in order to gain efficiency it would be better to drive the shooter wheels directly from the gearbox, without adding a second set of gears, belt/pulley or sprocket/chain set-up. We don’t have a lathe, so is our only option to not directly drive the shooter wheel?

For a shooter it really doesn’t matter too much. The main thing that matters is your RPM and compression, I’m not convinced torque has a large impact on the shot as much as it has on spin up time / slow down after a ball is shot (which doesn’t matter in this game). You want to get the wheels going at a specific repeatable speed to get an accurate shot - generally between 3000-8000 RPM with a 4" wheel(s) and depending on the distance / angle etc.

The advantage to a Mini CIM is that you can direct drive and be at around the right RPM, the disadvantage is that you can’t really go any faster than free speed unless you gear up - which is silly. The Bag, 775, or 550 all have high free speeds and require some gearing down, but a versa planetary box makes that really easy. 550s are a little weird this year since you can’t get them from VP or AM. Also consider packaging and where your motor / shooter will live inside the robot.

We’re using 775Pro’s through a Versaplanetary, geared to appropriate reductions for the wheel size (still not determined). Other than power:weight as has been stated, I like the open air cooling that’s perfect for a high duty cycle use case like what a shooting wheel does. For shooters that use low compression on the ball, it’s perfect.

What Gus said:

If you still have questions, please do ask.

Since the target is a hole in the wall and not a hoop. I think getting your shooter to have a fairly straight trajectory is needed. Let the motor rip and throw a fastball into the hole. It’s all about aiming.


So what’s your plan if your battery runs low, or (hopefully not) you forget to switch it between rounds? Pray that it still makes it?

“Letting the motor rip” could still mean controlling for voltage fluctuations. Just gear your shooter such that at 80%-85% speed you’re still shooting much harder than you need to.

We have have exceptional results with our prototype using a single RS 775-18 geared 2:1 driving a pair of 4" wheels. We run the motor anywhere between 8.5v and 10v.

Our intent is to run a pair of 775pro’s geared 3:1 (60:19 to be exact) and (This is for you, Ether) Bang-Bang to control the rate of the wheel.
Our plan is to use a single retro reflective section and just measure the period to determine the rate the wheel is spinning. We may end up with two sections to preserve balance in the wheel.

Cool Thanks!

We have plenty left over from previous years so it is not a problem for us. However you can order the 550’s directly from banebots.

We have done some more testing of the 550’s on a 3:1 with 4 in wheels and are able to get a very straight shot at full speed.


Vex Pro has the solution. Hex shaft coupler.