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 [DPRG] How to protect TETRIX motors Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] Previous message: [DPRG] How to protect TETRIX motors Next message: [DPRG] How to protect TETRIX motors Subject: [DPRG] How to protect TETRIX motors From: Chuck McManis cmcmanis at mcmanis.com Date: Wed Feb 20 00:32:21 CST 2013 ```This has been an interesting discussion for two reasons, one because the state of motor technology is still kind of sad, and two because there is a self destructing motor out there. On the motor, it fries, as many have observed, because at high currents it cannot dissipate the heat buildup in the wiring which subequently melts. The simplest way to keep it from smoking and dying is to put a resistor in front of the driver. As you would expect this lowers the voltage the motor sees as the resistance goes up. Measure the static wire resistance of the armature, pick a safe "current", You want the voltage to be 'zero' at that current, so apply ohms law as follows: E = I * R -> R = E/I, compute R total Now Rtotal = Rwire + Rlimit So your limit resistor is (Rtotal - Rwire) pick a resistor that is the next higher resistance (round up) and put that limiting resistor in series with your motor. Poof, no more melt downs. There are two downsides of course, the first is that torque is proportional to current and since the resistor will never let the current get too high, even for an instant, your motors will be less 'torquey' The other issue is that resistor will dissipate heat, I*I*Rlimit at the worst case (where I is the max current). Lets say you have a 5W power resistor as your current limiter because its 1 ohm and at 2 amps that is 4*1 or 4 Watts. That thing will get hot, and that heat is wasted battery power. The alternative is to build a chopper circuit. Use a much smaller resistor as a shunt (say .01 ohms) and an opamp to measure the voltage drop over that resistor, then when the current exceeds your threshold have it turn off the power (easy way is a FET in series with ground) Now when ever the motor trys to draw too much current, blam, the circuit turns it off. You can put an RC network on the output to limit the slew rate (which will let the motor go overcurrent briefly (based on the time constant) but will still protect it from extended current excursions. --Chuck On Tue, Feb 19, 2013 at 4:57 PM, Dick Swan wrote: > Several suggestions. > > Run the motors at 25 to 50% of max power if that is still sufficient to move > your arm. Or at least use this level for testing until you've got your code > functional. > > Probably not legal but run the motor power outlets through a limit switch > that mechanically disconnects the motor power when reached. > > Create a separate task that is monitoring the encoders and power level. If > motor encoder does not change for NN millisecond and high power is being > applied then motor is stalled. Have the task stop the motor! > > There may be an alternative source for the motors? The original motors were > same manufacturer part number as a motor that I think Lynxmotion (now part > of Robot Shop) used to sell at a lower price. In any case, the encoders > should be re-usable on a different motor. > > > _______________________________________________ > DPRGlist mailing list > DPRGlist at dprg.org > http://list.dprg.org/mailman/listinfo/dprglist ``` Previous message: [DPRG] How to protect TETRIX motors Next message: [DPRG] How to protect TETRIX motors Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] More information about the DPRG mailing list