Like all engineering there are many solutions that will work for you and it’s more about execution. I understand where you’re coming from on that design philosophy especially if you’ve been burned. I think there are several things teams can do to make the 775 more robust in these applications that may still be lighter than switching to a miniCIM. If you have a solution that works no reason to change, but just some thoughts for the future / others reading this thread.[/quote]
So, I went back and re-read my post and I feel that I may have come across overly critical of the 775s. That was not my intent. Last night, my team pointed out that I had neglected to talk about several applications that we would definitely select the 775s as the preferred motor such as shooters.
My post also generated a decent discussion among our team members (which is great) about what we know and what we don’t know about how to utilize these motors with minimal risk of failure. I am very grateful for threads like this where teams share their experiences and we all learn a few more things that can help us get better at picking the correct engineering solution.
I agree with you that each application requires a level of detailed engineering evaluation to pick the right solution and there is no “one size fits all” here. (My team also reminded me about all the benefits of the 775s in terms of small size, weight and other factors that we consider when making our design choices).
With proper current protection in the talon this is doable, but I somewhat agree with you here.
Stalling arms / wrists doesn’t seem like the right control solution for most motors. Use a limit switch or two? BAGs are really good here though.[/quote]
Our gear intake in 2017 was a motor driven arm (I can’t remember which motor we used, but I don’t think it was a 775). The motor was sitting at stall most of the time when the intake was up. Most of the time it was only up for short periods of time. The only time any smoke came out was on the practice field where we left the robot initialized for several minutes with the intake in the up position (while we were designing a new auto routine and forgot to turn the robot off). So, it can be done. With the increased knowledge we have now about stalling these motors, I’m confident we could implement this design in a more motor-friendly way if we had to do it again.
This really depends on the weight of your lift and the friction in it. We only ran 1 775pro on our lift with no brake, it wasn’t the fastest lift in FRC, but we stalled it constantly at very low voltage to hold position and never had issues. It’s helpful to check current draw in the logs and see what it takes to hold your motor in place. If you aren’t satisfied with that current draw then gear down, add a motor, or add a brake.[/quote]
Seems like gearing down the lift to make it even slower would be a hard trade-off to justify versus switching to a miniCIM. Cycle time is a premium and unless we were really weight critical, I would be picking faster speed over lighter weight. Adding a motor is an option, if you catch it early enough in the design cycle to re-design the gearbox. Adding a brake sounds heavier than switching to a miniCIM.
But as I said above, every application needs to be thought through carefully.
Strongly disagree. 775s are the ideal motor for hanging because they’re high power, low weight, and in this application they don’t need to run long. You don’t need a ratchet, just need enough friction in the system that it doesn’t want to back drive when brake mode is on. We used ball screws this year and lead screws in 2016 for this. You can also put a brake on it.[/quote]
They may not need to run long (we typically try to climb in a few seconds), but they can potentially need to “hold” the hang for several seconds. Recall that the testing results that started this thread showed smoke after only a few seconds. If you start your climb too early and you need to hold for 15 or 20 seconds, you might smoke the motor. The brake mode on the controller worked great this year and now that we know how to use that, I think we might have more success with 775s holding a hang.
But, as it was, in order to get the climbing speed we wanted this year, and in order to lift 500 lbs (3 robots plus margin) we needed the power of 3 CIMs which allowed us to use a COTS, 3 CIM Ball Shifter. I’ve not done the math with 775s, but I expect we would have needed a bunch more motors and we may have exceeded our motor limit and also required a non-COTS gearbox (which we could have done, but elected to spend our resources elsewhere).
So again, it is about evaluating the design requirements, doing the math and figuring out the best solution for each application.
Anyway, apologies if it seemed I was ruling out 775s carte blanche. That is not how I feel about it (and after the discussion last night, clearly not how my team feels about it either).
I am looking forward to using 775s in the future and looking forward to applying all the knowledge that people are sharing here to reduce and hopefully eliminate all the drama from 2017.