Quote:
Originally Posted by MichaelBick
There is a bit more that goes into this decision. If you look at the graphs 775Pros don't really mind stalling @4V. If your mechanism design/gear ratios don't need more than .143N*m (AKA .43N*m * 4V/12V) of torque while stalled, you should be good.
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Adding onto this, because the BAG motor and 775pro motor have the same mass, they theoretically should be able to sustain the same/similar current draw for the same period of time before burning out (assuming they're solid blocks of the same material, surface area, etc. which they are not). That means a 775pro stalled at 40 amps (or about 4v) is equivalent to stalling a BAG motor at 12v (also 40a). With that in mind, I try not to use BAG motors anymore because the 775pro also has the advantage of ball bearings and the integrated fan in the back, not to mention the higher stall torque/current if I want to use it.
It's not just stalling a motor that kills it; heat kills it, which is proportional to current. A 775pro stalled at 4v is the same as stalling a BAG at 12v in that regard. You can "stall" a CIM at 0.5v, but it's not going to kill the motor any time soon.
To the OP: 775pro > RS-775 for a variety of reasons, but you can make any motor work. If you want to save money you might be able to dig up a CIM (many teams have them in excess). 180:1 may not be high enough if you want to control it accurately. As a rule of thumb, I like to make sure I'm only running at 5-6% of a motor's stall current (or about 7 amps for a 775pro) to make something very easy to control. JVN Calculator 2016 makes these calculations a breeze. Then you can use a simple PID controller to control your position. This is very conservative, but for teams just starting out with encoders and controlling things accurately I like it.
For a low load like yours on an 18" arm, I'm getting a number around 600:1 for a speed of 0.5rps to maintain 5-6% stall current, but JVN calculator will help you narrow down your options.