We have been using two Fisher Price motors on our shooter, using a combination of an AM planetary and sprockets to gear the motor down and then up again. The final ratio is about 1.8:1 (reduction).
We are using the 0673 Fisher Price motors (the ones with the wattage similar to a CIM), and have been having lots of problems with overheating. We are getting mediocre performance from the shooter (about 13 feet), and have already fried at least one motor. Overheating really the problem, in my mind. We are running a motor that has much less thermal mass than a CIM around the CIM’s wattage. To me, its no wonder it is overheating.
Some more data: we were using an optical tachometer (the type used to measure the speed of a model airplane prop) and getting a speed spinning freely of about 4800 rpm. The motor is drawing about 37 amps accelerating and 27 steady state.
To me, these numbers show that our setup is very inefficiency, and we could significantly reduce our overheating issues by using a more efficient reduction. What is a good solution to our overheating/inefficiency problem?
I will hazard a guess without having all the information that you are running WAY too high a gear ratio.
I helped another team set up a shooter to shoot from mid court with 6 inch wheels and 2 banebots 775 (13000 rpm free speed). Their gear ratio once we did the math is around 2.5:1.
FP’s run around 18000 rpm. Which means to run the same setup, you should be running around 6:1 or 7:1. You’ve overheating because you’re on the wrong side of the motor curve, and are stalling the motors.
Fisher price are incredible motors, as long as you run them at high speed so their fan can cool them off a bit. That means a high gear ratio.
Cims are bulletproof and can be abused. FP’s take a bit more math and finesse.
Make sure you aren’t blocking any vents on the motors. I don’t know about the 673s, but similar motors have vents on the same side as the output shaft that are supposed to draw “cool” air over the electrical elements and out the back. Installing the motor in a gearbox will most likely block the vents. There is a thread describing how a team modified their gearbox to allow for cool air to enter at the vents.
I would suggest a higher reduction, really. We’re using the CIM-sim to direct drive 6" wheels and getting plenty of distance with our shooter. I think this is a case where less is more.
As far as increasing the distance of your shooter, it’s hard to say without seeing your set up. You could have too little compression on the ball, so you’re not transferring much energy to the ball. This is probably true if your wheels don’t slow down much after firing. You could have too much compression. If your wheels slow down a LOT, this could be why.
You might have too little engagement time with the ball if the wheels don’t slow down much. Or too much time if the wheels slow down a lot.
And then there’s the angle you’re firing at, and a dozen other variables, I’m sure.
We are using two belts, one on either side of the ball. The lower of the two rollers has a pitch diameter of 3.14in, the upper, around 1.69. As for the measured current, that was told to me by the programmers (I really just do mechanical stuff), I suspect off the dashboard. The motors are run by Jaguars with CANBus, and were run a 100% to get that speed and current.
Your suggestion about removing the second stage is interesting. If I was unclear before, we are getting that ratio with sprockets, so a 1:1 set up would be very possible. You are essentially saying that our motors are having trouble because they have too much load? It seems like that would improve overheating, although at the cost of range.
He provided enough information* (motor speed and amps) to determine that he is running the motors at about 8 volts. So they are gearing the motors UP and then lowering the voltage. I suggested they get rid of the gear-up portion and try that.
The motors are operating at a very unhappy torque & speed point.
It seems like that would improve overheating, although at the cost of range.
If you get rid of the gear-up you can increase the motor voltage to get the same wheel speed at a faster motor speed which puts the motors at a much happier operating point.
Motors don’t like to go slow. They want to go fast.
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What do you mean by running them at a different voltage? As I understand it, the current changes when more or less power is given to the motor, not the voltage.
Also, changing the ratio to be greater will make the motors run faster, not slower, to run the outputs at the same speed. I’m not quite sure why changing the gearing would change the motor speed. As I understand it, it would just make the belts spin slower. This would give them more torque, but the motors are overheating even without feeding balls through.
I think what is being said is that as you draw current from the battery, its voltage drops and decreases the speed of the motor further. If you run a lower gear ratio to get lower theoretical free speed on the belt, you may find that the lower current draw on the motor results in less voltage drop and therefore higher speed at the belt when all is said and done. Correct me if I’m wrong, though.
The command that you send to the Jaguar (or Victor) changes the duty cycle of the Jag’s (or Vic’s) PWM output, and thus changes the voltage being applied to the motor.
IF the data you posted is correct (4800rpm wheel speed, 1.8:1 gear ratio, 0673 motor drawing 27 amps), then you can use the motor curves to calculate the voltage the motor is getting. I explained this in post#2 in this thread. The answer was about 8 volts for the data you gave.
If you increase your gear-down ratio from 1.8:1 to 4:1 as I suggested, your motor will spin faster and your wheel will spin slower. You can then increase the voltage up from 8 volts to bring the wheel speed back up to where you want it. Your motor will now be spinning much faster than it was before, and with less torque load, and it will be much happier.
We are using those motors at 6:1 (ganged with some AndyMark motors running at 4.64:1) to a 8" wheel, the ball is going 24’ or so and the motors running cool. We are thinking we were a little conservative and and might try 4:1.
On behalf of DampRobot’s team (Team 100), thank you for that suggestion. we tried that and it ended up working great. The motors are much happier now, and our shooter is going at least as far as it was before.