I was wondering if using two 775pros each on a 2:1 belt reduction to the flywheel would be enough power to shoot the balls
That depends on a lot of factors, I’d build a test rig so you can test lots of variables at once.
The answer is an obvious “yes” – two 775 pro shooters can spin up a flywheel to shoot balls. In theory, so can one 775 pro, or one snowblower motor…
Can they spin the flywheel up to speed in a time that makes you happy? Can they recover fast enough to shoot the balls as quickly as you want to shoot them? That’s going to depend quite a bit on what you’re willing to settle for.
In my personal experience, it’s also quite easy to smoke 775 pros on a flywheel. The initial “oomph” while you’re still at low speed draws a lot of current, and 775 pros are not current-tolerant motors, so teams who have had success with two (or three or four or six) 775 pros on a flywheel shooters have had to incorporate hardware design features and/or software to make sure that their motors do not cook.
We had something pretty similar to this on our robot, of course there are variety of factors in shooting but my answer would be: yes you can use it.
However, we found that the motors would get hot really quick, it was a big problem in the pits and they had a tendency to lose momentum as well. If your programming has good pids and probably an inertia wheel too you should be fine in the latter regard. Still I’d recommend trying to use a motor that doesn’t have the same sort of issues, we’re swapping those 775s out with falcons but
they’re out of stock right now so I don’t know if you guys would have access to them or not.
Do some testing and research and you can figure out if there’s a better alternative for you guys but TLDR 775s can work fine if the programming is tuned properly, there are better alternatives though.
we used 2 neos and they worked fairly well with minimal software tuning, since falcons are out of stock this might be a good option.
The answer is likely “Yes”. But, to prevent disappointment, I do recommend we go deeper. Think about:
- How much does your rotating member (‘flywheel’) weigh? What are its physical dimensions? This will impact how much energy it stores while spinning at a given speed, which in turn impacts how much energy is imparted to the balls.
- Which balls are we talking about? Different years of FRC have very different gamepieces that needed different shooting mechanisms. I assume we’re talking 2020, but can you confirm?
- How far do you need to be able to shoot the balls?
- How long between each shot are you willing to wait? More motors is often needed if you want to shoot quickly and not wait for the shooter to get back up to speed?
- (comment) - have you considered prototyping with a few different “backing” materials on the hood? Our experience from 2020 showed that various yoga mat and teflon materials made a big difference in the consistency of the shot. While getting the distance first is the key, your very next urgent task is getting the distance with enough accuracy to make the shooter worthwhile.
- Are you locked into using 775’s? Any ability to use other motors?
We started our prototyping for a hooded shooter using 2 775pro motors with 3:1 reduction and ended up using 3 of them with a 1:1.67 reduction. That worked for us, but it also worked because of the other things we had in our design - high compression, the low friction plastic hood, the flywheel etc. The only way to see if this works for your specific demands and robot design you would have to test it, we went through a lot of effort to making it work and we still thought we should’ve used Falcons in retrospect.
As others have said, you can certainly design a shooter that will work fine with 2x 775pros. The trick is finding the proper combination of the thousand or so other variables that allow that shooter to perform well. In theory, with the right compression, backing, surface speed, inertia wheel, gear ratio, etc. and a very long spin-up/recovery time, you could power your flywheel with a single throttle motor.
Power is the change in energy dived by time. There are two main cases where you’re putting energy into the flywheel: to accelerate it from a standstill and to bring it back up to speed after shooting. To figure out the power needed to accelerate from standstill, you have to calculate the kinetic energy of the flywheel at the speed you want then divide that by your maximum acceptable spin-up time. To figure out the power needed to bring the flywheel back to speed after shooting, you have to take the difference between the kinetic energy in the flywheel at full speed and the kinetic energy at the minimum speed the shooter reaches after shooting, then divide that by the amount of time between shots. The bigger of those numbers will give you a ballpark estimate of how much power you need, and therefore how many motors you need.
Edit: Also, I wouldn’t be too worried about burning the motors. If you’re running at least one of the motors off a Talon SRX or SPARK MAX to take advantage of the fast internal closed-loop control, you can add a software current limit with two lines of code. Limit it to something reasonable and you’ll have very little chance of burning the motors. We only burnt 775pros in testing when we were using a PWM speed controller and servo tester; as soon as we switched to a real control system we didn’t have any more problems.
my team used two with 1.5 in compression (no flywheel and a 9:1 reduction and we were getting 30-40 ft shots on test runs before covid happened
I am not a programmer but we burnt 775 pros last year. Where can I find examples of codes?
Look for “current limiting” in the Talon SRX or SPARK MAX documentation
Edit to add: I also don’t do any programming myself in FRC (just giving advice to the programming students) so I don’t have exact links or code snippets to share offhand
Overall, it’s a workable / proven viable design.
Team 2992 used a shooter design (almost) exactly like this. 2 x Redline motors (instead of 775 pro), 1:2 reduction (Vex 12T GT2 pinion pulley to 24T 1/2 hex pulley), 4 inch WCP solid roller wheels (4 x 1" wide). Additionally, we had pre-feed wheels (2" Andymark Stealth 50A) belted to rotate at same RPM to pre-accelerate PC. Approximately 2" of compression. We had originally planned on additional flywheel (stainless steel machined discs), but had to remove them due to weight issues.
We were easily able to shoot from the trench. Spinup time is about 2 - 2.5 seconds. Recovery time between shots is approximately 350ms.
We are using current limiting to protect motors, and closed loop PID in Talon motor controller, along with Rev hex encoder. Have not had any issues with overheating motors or motor failure.
Free speed of shooter wheel is above 8000 RPM measured. Shooting from trench, we are tuned to about 5700 RPM which produces a fairly line drive shot.
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