Help about motors

[luanrookie]

Good evening, I have some doubts about motors, is my second year as a member and I was researching how the teams select the right motor for every situation, this year when our shooter did not think the possibility of using other motors out instead of cim but there teams that made with other motors and have better results, as I discover the qualities of an motors? I found a list in my research:
http://www.usfirst.org/sites/default/files/uploadedFiles/Robotics_Programs/FRC/Game_and_Season__Info/2013/2013MotorInformation.pdf

but I do not really know about these items I know about rpm and torque but how can this help me decide on a motor in certain situations, there is a pattern as a kind of calculation or something that will help me in my research?

[Sparks333]

Hmmm... A very broad and rather vague question. Let's see what we can do.

Generally speaking, choosing a motor is a balancing act of maximum speed and maximum acceleration, as well as (for most of us mortals) finding a premade gearbox that will allow the performance you desire (you can't just arbitrarily say 'This works best with a 37.4432:1 gear ratio, so I'll go out and find a gearbox that does exactly that).

A few rules of thumb about DC electric motors - Though the spreadsheet shows maximum power, maximum speed, and maximum torque, you won't get all three of those at the same time - max speed is completely unloaded, max torque occurs at stall, and maximum power is (ideally) at half max speed (which also gets you half max torque - torque has a negative linear relationship with speed, with max torque at 0 RPM and 0 torque at max RPM).

Now, choosing a motor for a particular application - that is a tricky one. I'd recommend using Newton's equations of motion to calculate how much torque you want, and at what speed, then choose a gearbox and motor that will put you as close to half-max-speed as possible during normal operation. Keep in mind that what really drives the torque requirement is your robot's mass and the rate at which you'd like it to accelerate. It will be difficult to estimate the losses in most drivetrains, so don't expect that you'll come anywhere close to maximum speed on the motor - there will always be some losses that prevent the motor from reaching zero torque (though in a direct-connect to a shooter wheel, that's a fairly reasonable assumption).

Hope some of this is useful. Selecting motors is a bit of an art form, and a lot of teams have spent a lot of time experimenting to find what works.

[BBray_T1296]

That pdf (http://www.usfirst.org/sites/default/files/uploadedFiles/Robotics_Programs/FRC/Game_and_Season__Info/2013/2013MotorInformation.pdf) you linked too is interesting, particularly the stall torques:

Denso | 262100-3030 (Right) | 19
Denso | 262100-3040 (Left) | 21

I am presuming these are the window motors. Why does the left motor stall at a higher amperage than the right? I thought they were identical but mirrored?

[Al Skierkiewicz]

Brian,
The difference is that the left hand motor (drivers side) is assumed to need extra life and so is built for that extended use. I believe the difference is a larger gauge wire in the armature.

[Michael Hill]

Brian,
The difference is that the left hand motor (drivers side) is assumed to need extra life and so is built for that extended use. I believe the difference is a larger gauge wire in the armature.

That's...pretty interesting. I've always wondered that myself. They assume the driver wants to open their window more often?

[Al Skierkiewicz]

I would guess that statistically, the driver's side window gets 75% or more usage on average. (Based on my own experience.) Of course it is more likely that the lower usage motor reduces cost by designing for the lower life span. Say it cost 50 cents less per unit, after 1 million units, a lot of money has added up.

[Pault]

That's...pretty interesting. I've always wondered that myself. They assume the driver wants to open their window more often?

Probably has to do with the fact that there is always a person in the driver seat of the car (well, until the Google Car becomes commercially available) but I'm guessing the majority of the time there is nobody in the passenger seat.

[CENTURION]

Probably has to do with the fact that there is always a person in the driver seat of the car (well, until the Google Car becomes commercially available) but I'm guessing the majority of the time there is nobody in the passenger seat.

Also because you use the driver's window for toll booths, drive-thrus, parking lot gatebooths, etc.