775 vs CIM

The CIM is dead to me :wink:

Long live the 775Pro.

Google {“Dr. Joe J” 775Pro](https://www.google.com/search?q=“dr.+joe+j”+775Pro&rlz=1C1NHXL_enUS695US696&oq=“dr.+joe+j”+775Pro&aqs=chrome..69i57.11645j0j1&sourceid=chrome&ie=UTF-8)} to see my many posts singing the praises of the 775Pro.

You can’t abuse it as much as you can abuse the CIM but net-net, I am completely sold. The CIM is a great motor but I am going to use 775Pro as my go to motor (for every FIRST FRC application – it is that good).

Dr. Joe J.

On CD I read awhile ago a team did some testing and if I can remember correctly they found at 2 seconds of stall did permanent damage and like 1.5 seconds was still usable

Given how loud they are, you’ll have the added benefit of never being snuck up on by one of your robots!

This is stalling at 12 volts / 100% signal, to be clear. The motors last much longer stalled at various lower voltages.

This is solved by using 3mm GT2 belts for the first stage reduction, it’s remarkable the difference it makes.

-Aren

That was likely me, as those are the results we found in our limited testing :slight_smile: 2 seconds of stall at 12V caused noticeable change in the free speed current of the motor, and around 1.5 seconds of stall 10 or so times in a row (roughly 1 minute breaks in between) caused no noticeable change in free speed current. Vex’s motor data shows a more complete story though, telling at what point the motors fails to provide torque.

Just curious: What does that do to the rotational inertia seen by the motor?

It adds the belt inertia to it, but nothing worth writing home about?

-Aren

Probably, I don’t know, I was just wondering. How heavy is the belt (and pulley?) compared to the part(s) it replaces? Is it right at the motor shaft, where speeds are high?

It is a very small belt, Russ. 9mm wide, with teeth 3mm apart.

Here is an example.





We love 775s. We don’t use anything else anymore, except for when you really don’t need much power (turning the swerve modules or a turret, in which case a 9015 does the trick). Would highly recommend everyone making this switch.

I would advise against making this recommendation unless you share everything you’ve done differently from a traditional CIM drive system. Teams can throw a CIM drive together out of the box and never fry the motor. Use the same drivetrain with a 775 and may be replacing them every match.

How fast is your swerve geared for? Are you using 775 drive motors?

Please don’t use 775’s in your drive! If you are thinking about doing this, please take a step back and ask yourself:

“Why am I doing this?”
“What are the potential benefits?” (a smidge of space and weight?)
“What are the consequences?” (burned motors, dead robot, blown main breaker, a literal fire).

Our team has used a 2 CIM 1 775 setup the past several years. It has a noticeable difference vs. 2 CIM setup. We would rarely ever change out the 775 in previous years, only after a lot of miles doing many events and driver practice.
This year we went with the 775 Pro and there was noticeable fatigue in the motors after doing 1 whole event and after practice day the subsequent event. We sent ours back to VEX who want to see what’s happening with them.
We ended up swapping them out on our drive before each and every event towards the latter part of the season.
1st indication of it going bad was when our robot starting drifting slightly during auto modes doing front center peg.

I have a question about how your 2012 drive gearbox worked out. For those who do not know what I am talking about you can see it in this link. Did the combination of the 775 and two cims give the best of both worlds? Did you have issues with the 775 burning out? Did you have to limit the current draw from the 775 to prevent this from happening?

We had all manner of current limiting code, and we were also traction limited. I’ll try and upload the CAD soon. We had 8 775s, two per module, geared for 18ish ft/second on the swerve. The acceleration was tremendous. It took up too many pdp slots to shoot though :frowning: - our drive code will be released soon, it just needs a bit of cleanup (I’m told).

The programming was intensive, so it is not as simple as a cim in a gearbox. We wanted a high degree of mobility in this game given that we would be crossing the field for our cycles, then precisely placing a gear on a peg.

As awesome as it was, I don’t think a twelve motor swerve applies to many FRC games, with this year’s game, 2008 and 2011 I can see as exceptions.

Holy cow, that sounds like my dream setup. I toyed with the idea of shifting 2-775 swerve but dismissed it for the motor requirements. Good to know that worked well for you, especially with the 18fps.

775Pro
Benefits:

  • Low Weight (0.8LB)
  • Higher RPM (~18,000RPM!)
  • Slightly higher power (345W)
  • Lower cost if they need to be replaced. ($17.99)

Drawbacks:

  • Not durable, debris can stick inside and cause damage.
  • Stalling beyond a few seconds will wreck it.
  • Air cooled so needs to maintain a high RPM at all times
  • Low torque/high RPM means more gearing needed

CIM
Benefits:

  • Nearly indestructible.
  • Can run stalled for relatively long periods of time.
  • Sealed and can’t be damaged by loose debris
  • High torque

Drawbacks:

  • Rather heavy compared to 775 (2.8LB)
  • Slightly lower power (330W)
  • “Low” RPM (~5000RPM)
  • Slightly higher cost ($28), but rarely needs replacing.

Overall:
Unless you’re needing to pack so much stuff into your bot that that extra 2LB of weight per motor is going to make-or-break the build, there’s not much reason to switch from CIM to 775. You gain a tiny bit more power and lower weight in exchange for a much more fragile motor that has to be planned around carefully to avoid stall conditions. And in the end, the extra weight needed in gearing to get that 18k RPM free speed down to something usable nearly negates any weight savings.

The lower cost of the 775pro compared with the CIM is insignificant, once you factor in the cost of the extra stage of reduction( bearings, shafts, gears etc). The fact that more 775pros are needed to gain the same performance safely means a higher cost in motors and more importantly motor controllers( teams running custom 775pro drives are probably using either SP or SRX).