Quote:
Originally Posted by Chak
90+ fps means 60+ mph. 
Just off the top of my head, I'm sure there's a reason robots don't go that fast. Something about tripping fuses and breakers? |
I started to answer this:
Quote:
Originally Posted by GeeTwo
With no gearbox, I get a ... likely torque-limited acceleration of about 5-6 fps/s (less than one-fifth gee) at 40A draw per CIM. Bursts of greater acceleration are possible without tripping breakers.
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5-6 fps / s is not very exciting. 6fps/s would result in a final speed at the end of a 54 foot run of 12 fps, so that 90+ fps would only mean anything if you had most of a mile of track. That's not the whole story - this was based on a 33 - 40lb total robot weight.
Each CIM produces a torque of 100 oz-in. There are two of them, working at a radius of two inches, so you have a maximum traction force at 40A of 100oz, or 6.25 lb. As described above, the 5-6 fps/s acceleration was based on a 33# to 40# gross weight. By the time you load up to a robot of, say, 130#, that 100 oz can only accelerate the robot (even at perfect efficiency) about 1.5 fps / s. Applying that acceleration from a dead stop at one end of a 54 foot field will yield a speed at the other wall of about 6.4 fps - before accounting for frictional losses.
The AA robot I spoke of above was geared down 3:1 from this - a CIMple box and 6" wheels. Even it was rather logy at getting up to any speed. Most FRC robots are geared down at least 3:1 for a fast robot and some are as high as 20:1 for low gear for a robot that needs plenty of pushing power or high acceleration. More typical for a single speed robot is probably in the range of 7:1 to 12:1.
And of course, you can double or triple the torque-limited acceleration without sacrificing free speed by using four or six CIMs.
Quote:
Originally Posted by Michael Hill
CIMs (as well as pretty much the rest of our motors) are fairly wimpy when it comes to torque, so while free speed may be high, once you put any load on it at all, the speed drops like a rock. That's why we use gearboxes to trade speed for torque.
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CIMs do have great torque - for motors. It's just not enough to accelerate a 100+# robot with any authority. Correct as to speed dropping - that's why I used the term "free speed". Most FRC robots have an actual top speed that's somewhere around 60-90% of free speed, depending on the gearing, alignment, friction, and ultimately air resistance. With less gear-down, expect poorer efficiencies because more stuff will be shaking and drawing air.