Limitting motor torque

[JamesBrown]

Originally Posted by Hawiian Cadder

if you are not using CAN then the torque can be calculated with the voltage you are putting in and speed, this requires an encoder on the motor though. although instead of worrying about how to make do with less traction, i would focus on getting more traction. this years robot was geared for 14 feet per second, and before we switched to roughtop tread it was able to spin the wheels at stall, after the switch, we had no problems.

Why does it make a difference what kind of control you are using?

Increasing traction by increasing friction will also increase power draw which is not a good thing (for obvious reasons).

http://mizugaki.iis.u-tokyo.ac.jp/st...F/EV_Trans.pdf This paper originally posted to CD by Tom Schindler may be interesting to some of you. It explains how to implement traction control on an electric vehicle.

[EricVanWyk]

Originally Posted by JamesBrown

Why does it make a difference what kind of control you are using?

Jaguars using CAN report their current draw and support a current control mode.

If you using PWM, you don't have access to the internal current sensor and need to find torque by some other path, such as an external current sensor or `Hawiian Cadder`s method.

[JamesBrown]

Originally Posted by EricVanWyk

Jaguars using CAN report their current draw and support a current control mode.

If you using PWM, you don't have access to the internal current sensor and need to find torque by some other path, such as an external current sensor or `Hawiian Cadder`s method.

Ah, that makes sense, I was assuming he was saying you couldn't do it with CAN, my mistake.

[Hawiian Cadder]

Originally Posted by JamesBrown

Why does it make a difference what kind of control you are using?

Increasing traction by increasing friction will also increase power draw which is not a good thing (for obvious reasons).

http://mizugaki.iis.u-tokyo.ac.jp/st...F/EV_Trans.pdf This paper originally posted to CD by Tom Schindler may be interesting to some of you. It explains how to implement traction control on an electric vehicle.

increasing friction will increase the amount of power draw when the robot stalls. however that is not a bad thing, more grip = more pushing power. because the robots only run for 3 min at most, power draw, and efficiency dont really matter.

[Chris is me]

Originally Posted by Hawiian Cadder

increasing friction will increase the amount of power draw when the robot stalls.

No it won't. If the wheels are stalling, the motors are outputting stall current. There's no "super duper stall" or whatnot that's harder than that.

[Ether]

Originally Posted by Chris is me

No it won't. If the wheels are stalling, the motors are outputting stall current. There's no "super duper stall" or whatnot that's harder than that.

He said when the "robot" stalls, not when the "wheel" stalls.

[Ether]

Originally Posted by Hawiian Cadder

power draw, and efficiency dont really matter.

Unless you trip a breaker or the Jag's overcurrent protection, or brown-out the cRIO.

[JesseK]

Originally Posted by Hawiian Cadder

because the robots only run for 3 min at most, power draw, and efficiency dont really matter.

In many regional finals, you have a 6-minute match cycle time. If you have a high-current match, then your battery may not have enough time to fully recharge before the next time it is used. In the worst case, it could take 6 matches to make it to finals -- most teams (though I'm sure not all) I've seen at competition have between 4 & 6 batteries. The question then becomes, can your battery chargers properly keep up with your demand?

Match-for-match in elims (or anything in quals) where high-current situations aren't common, sure I'd agree with you in most* scenarios. Yet I'd hate for you to put yourself in a bad situation come competition day because you thought something was totally negligible.

* In situations where multiple mechanisms must perform quickly (i.e. high-power) and heavy game objects are involved (2008 is a perfect example), then electrical efficiency should be considered (IMO). Other considerations are PID-hold algorithms where an arm must move to a position and use back-drive current with extra current to hold itself in position. 2006 also saw at least one team implement a PID-hold software mechanism on their drive train in order to keep from being pushed while they shot at the goal (I'm not sure of the team, but it's in the "Behind the Design" book). All of the mechanisms on the robot can accrue large amounts of power draw (mAh) over the course of a match if you're not careful; so designing the drive train to use a little less current isn't a bad thing.

[Jared Russell]

Originally Posted by JesseK

In many regional finals, you have a 6-minute match cycle time. If you have a high-current match, then your battery may not have enough time to fully recharge before the next time it is used. In the worst case, it could take 6 matches to make it to finals -- most teams (though I'm sure not all) I've seen at competition have between 4 & 6 batteries. The question then becomes, can your battery chargers properly keep up with your demand?

Also remember that as your motors transform electrical power into mechanical power, they are generating heat due to inefficiency. The more inefficient your motors (e.g. the further in the power curve from the "max efficiency" point you are), the more heat they generate. What is truly insidious is that a hot motor is less efficient than a cool one - resistivity in metals increases with temperature. So the hot motors get hotter more quickly. In FRC terms, red hot motors will produce less mechanical power before tripping their breakers (or burning themselves out for good).

[Ether]

Originally Posted by Jared341

What is truly insidious is that a hot motor is less efficient than a cool one - resistivity in metals increases with temperature.

In addition to increasing the electrical resistance of the motor's coils, the heat also affects the motor's magnetic properties.

[Hawiian Cadder]

we put mountain dew cans over our cims, at first because it looked cool, but i think it did help with cooling. the cims in this years robot never seemed quite as hot as previous drive's.

[Ether]

Originally Posted by Hawiian Cadder

we put mountain dew cans over our cims, at first because it looked cool, but i think it did help with cooling. the cims in this years robot never seemed quite as hot as previous drive's.

Unless the dew cans were adorned with fins, I can't see why it should help with cooling. Black is a better radiator.

[JamesBrown]

Originally Posted by Hawiian Cadder

increasing friction will increase the amount of power draw when the robot stalls. however that is not a bad thing, more grip = more pushing power. because the robots only run for 3 min at most, power draw, and efficiency dont really matter.

I have seen robots that drain a battery in one match, power draw is something teams should think about.

Increasing friction also increases the amount of power that is required to turn, this can result in a large power draw. If there is a reasonable amount of performance increase that can be gained by minimizing slipping in software why not take advantage of it?