Hello guys, recently i have been updating our competition robot, and trying to increase the efficiency
of the motors. When i was examining the hero bot of this year, moby, i found that the required torque for the moby arms is around 2.8 nm without the gears according to my calculations. A 200 rpm vex motor can produce maximum 1.05 by itself which 2 motors can produce 2.1 nm torque. So why moby uses 1/15 gear ratio? Is 1/5 ratio not enough for moby arm?
Second problem is about the efficiency about robot. When i was analyzing the torque and work graphs of vex v5 motor, i found that motor is most efficient at %60 of speed, which produces 11watt of work. But everynone knows that these motors are heating. How can i calculate the heating of the v5 motor for designing the gears for best performance? Thanks.
you would be correct that a 1/5 ratio could probably work for moby’s arm. My guess to why vex gave it a 1/15 ratio might be to introduce people to compound gear ratios, and to ensure that the arm would still work when not build very well, where the efficiency is low and friction is high.
I did a few experiments a while back, because like you, I’d seen that theoretically the v5 motors produce maximum power output at roughly 2/3 speed.
I built two testing rigs, one was a pulley being powered by a 200 rpm motor at 100% velocity, connected with a 1:1 ratio. The other rig was identical, except the motor was being run at 66% velocity, and the pulley was on a 3:2 ratio with the motor, so the output was running at 200 rpm, the same speed as the first rig. Both rigs used a sprocket ratio in order to have very similar levels of friction between them.
The idea was, that the second rig might be able to handle more load despite running at the same speed, because the motor was spinning at the velocity that, according to the graph, outputted near maximum torque.
But my findings didn’t support that, the rig with the motor running at maximum speed performed far better than the one running at 66% speed geared up. This aligns with my experience that the slower a robot is driving (as a fraction of it’s maximum speed), it will be overall slower (obviously) but also have overall less torque.
I might be misinterpreting what that graph means, but I’ve found that motors output the most power when at peak velocity.
If I’m interpreting your question right, you should just be able to print your motor temperature onto your brain screen.
Ex:
Brain.Screen.Print(motorName.temperature);
syntax might be a bit off though.
Yeah. I have my teams monitor motor temps for except for wheel motors.
Temps go in increments of 5 degrees… so it’s not real precise.
As far as torque calculations go… you are looking at peak torque. You have to remember that you must design for the lowest torque available… so it’s doesn’t stall at certain rpms.
Heating of the motor is caused primarily by the winding loss. Winding loss is the ohmic loss (Joule heating) in the motor winding, P = RI^2, that is, winding resistivity (~1R for V5 motor) times square of the winding current. Winding current is directly proportional to the torque the motor is producing (shaft torque + internal friction, but you can neglect the latter).
Now consider the experiment mentioned by @Xenon27: In one case, the motor had to produce 50% more torque (at lower speed) for the same output power at the same output speed. A motor running in such a mode would produce 125% more heat (losses) for the same output power, being significantly less efficient.
In other words, if possible, you want to design your systems (gears) to run the motor at 100% speed. At 100% speed, the motor uses highest voltage and lowest current for given output power, thus has the lowest ohmic losses, least heating.
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