Really, all seriousness aside, it depends on the total weight (not just the payload weight), the motion profile (how much accelerating/decelerating), the drivetrain friction (chains, belts, gearing, bearings), the rolling friction (what type of tires and what type of surface you are driving on), the efficiency of the power electronics, the wire gauges and lengths…
And what’s up with the 2 wheels? Is this a Segway?
This part of the design is not my specialty but calculating the wheel circumference I get about 1 meter. So to travel 3.74 m/s, wheel rpm is about 224 rpm (3.74rev/sec60sec) and motor rpm is 7630 (224rpm34). That is about 2000 rpm higher than the free speed of the CIM. So something needs to change.
There is someone who posted here on CD a few years back that had built a cheap segway of their own. You should search for that series and see if you gain any additional info.
The computer that i’m on has strict internet filter so I can’t access that calculator right now (I’m actually surprised I could get on this site) but if you guys approve of it, I can check it out later.
If it does run for 45min I probably should determine a way to cool the motors, I’m sure they will heat up. Especially if the robot is used on a hot summer day. So would you think a fan by each motor would be sufficient? I don’t have a fan in mind that I would use though
I’ve never seen any specs or data on how long you can drive a CIM at different power levels before it overheats and is damaged.
The CIM is a sealed motor; no ventilation. My understanding is that it was designed for intermittent use. It has sufficient mass to absorb a lot of heat for short periods of operation (like a 2-minute FRC match).
An external fan might help somewhat, but the real question is how hard are you going to be driving the motors.
I suggest you use an empirical method to solve this problem. Get a $25 flooded lead acid battery designed for riding mowers (I think the size is U1) or the sealed type of that size used for mobility scooters, and see how it does. If it’s too small, then get a normal marine battery, or run two of the U1 size in parallel.
I called my mentor and he said that the design requirments state that the robot should run for 45min but in all honesty the robot is only going to be used in short burst (~5min) but it should be able to handle this type of running for a long period of time on one battery charge.
Please keep in mind that in a two wheel configuration, the motors will need to be capable of accelerating to a higher speed than your target in order to accomplish balance. As I remember the Segway is driven via toothed belts and max speed on the police version is I believe 24 MPH.
@Eric: In other words, when you’re cruising along at your desired 3 m/s, you need some headroom to increase the voltage.
Suppose you leave 2 volts headroom, so you’re operating at 10 volts at 3 m/s (I have no idea if 2v headroom is enough or too much, this is just for sake of discussion).
CIM free speed at 10v is 4425 rpm. To go 3 m/s at that motor rpm with 12.5" dia wheels, you’d need a gear ratio of 24.5:1, not 34:1.
Of course, at free speed a motor is producing no torque output. So let’s say 90% of 10v free speed, or 3983 rpm CIM speed (again, I do not know if that is sufficient1). Now you need a gear ratio of 22.1:1.
1At 10v and 3983 rpm, each CIM produces about 28.6 ozin torque. Run this through the 22.1:1 geartrain and assume 10% torque loss in the drivetrain and you’ve got about 3 ft-lb torque at each wheel. For a 12.5: dia wheel this translates to about 5.7 lb motive force per wheel, or 11.4 lb total for both wheels. Is that enough to move your 100 lb robot+payload at 3 m/s up a 5% incline on a gravel surface?
Suppose all of these calculations are correct and work for the robot. How do I determine if the battery will last 45min?
PS: I’m an electrical engineer intern so I don’t really know a lot about the mechanical side of things so this might be an easy question for you to answer, but when you say 11.4 lb total motive force does that mean that the force would be able to move 11.4 lbs? so therefore not enough to move my 100lb robot+payload?
The motive force required to move a 100 lb robot is usually not 100 lbs, it’s some percentage of that. But it depends on rolling resistance, how much acceleration you want, whether you’re going up hill or down hill, if you’re turning, etc. And if it’s a balancing robot, you’re going to have a balancing load…you will probably be driving the motors in an oscillating manner to keep the robot upright, so you will be using power just to stand still.
This is very hard to figure out with numbers…which is why I suggested you do it empirically. Try it and see what happens.