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Give your Robot the Mobility Control of a real Mars Rover: Part 3

Bill Marshall
Engineer, PhD, lecturer, freelance technical writer, blogger & tweeter interested in robots, AI, planetary explorers and all things electronic. STEM ambassador. Designed, built and programmed my first microcomputer in 1976. Still learning, still building, still coding today.


August 28, 2018 10:00

I'm still struggling with going downhill - I've even swapped out the drive motors to pair of Ampflow A28-400-F48 with Vex victor BB controllers. I'm thinking it's either the 100:1 wormdrive gearbox or the fact I'm using a 2.2kw invertor generator and it objects to the back voltage. It does however work ok if I use a brake and drive the machine lightly against it. Any ideas?

0 Votes

September 6, 2018 07:56

@TegwynTwmffat I'm guessing that the motors speed up as you start to go downhill. From what you say, I think this is what's happening: Those VEX driver modules contain a PID control loop designed to maintain a set motor current and hence torque no matter what the load. It's meant to simulate the throttle action on a car. When the load reduces as the vehicle starts to go downhill, the motor current falls and the PID loop compensates by increasing the voltage and hence the speed. To maintain the speed constant, you must back off the throttle. The PID loop in my article does this for you because it measures speed not current. Your robot behaves like a basic IC-engined car; my robot behaves like the same car, but with cruise-control. I think that's why applying a mechanical (foot)brake works, by maintaining the torque level. It should be possible to put a tachometer-fed speed control loop based on a microcontroller around your driver modules. It may need some careful tweaking of K constants to avoid the loops 'fighting' each other however! Hope this helps.

September 5, 2018 09:08

@Bill Marshall Thanks for reply - it helps! I'm going t add an optical encoder to the machine and apply PID. I've further researched the problem and seems like it might be because the motor starts to act like a generator and send a higher voltage back to the VEX controller than it gives out. t might be cured by adding a small battery in parallel with the switching power supply which should absorb some of the spare electrons flying about.

September 6, 2018 07:40

@TegwynTwmffat You're right about the generator action, but that's how a PMDC motor works. A Back-EMF (Electromotive Force) develops across the motor as it speeds up. This opposes the input voltage, causing the current to fall until the speed stabilises when the motor torque matches the load. See Fig.1 above. The controller handles all this, including high-voltage spikes caused by the commutator action; all the power supply has to do is supply whatever current is demanded at, I assume, 48 volts. You could always replace, at least temporarily, the PSU with four 12V batteries in series if you think it can't supply all the current....

September 10, 2018 14:47

@Bill Marshall This is my proposed solution here: …. Not sure how to size the resistor Ohms - I've asked Ampflow, the suppliers of the motor, for some tips :)

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