By all means prove me wrong but there's a good reason that CNC machines have feedback on the axis. It's not that you can't keep good position with a stepper motor, but it's to ensure that if steps are missed or there's backlash, the system can still travel the correct distance. Different application, different motor, same idea.
Challenge accepted.
I've been looking at brushless drive too (Mr Mangle inspired me!) My biggest concern right now though is finding ESCs that will have good Fwd/Rev response, most truck ESCs have braking or a delay.
All brushless esc's have a delay, even the sensored ones. You can't trow a brushless in reverse like a brushed motor. It has to be stopped. The only thing you can have is a minimal delay.
It will take a lot of practise to get the best from the new tech.
You're right different application, different motor. Which is why this doesn't apply. CNC machines keep feedback because they need precision to fractions of a millimeter, a fighting robot does not, no matter how expert a driver your 'precision' does not warrant such control. Back EMF is a perfectly good way of measuring the motor. This isn't some few hundred steps a millimeter stepper motor, this is a brushless that probably has around 10-30 poles. The feedback in a fighting robot is not position tracking but ensuring that the motor RPM and ESC timing match to sustain torque. If you want position tracking in your robot feel free to drive with stepper motors.Originally Posted by typhoon_driver
Agree with the practice but I need that regardless but its a slight change in driving style
The availability of brushless motors and esc is improving. The accuracy that sensored brushless gives is not really necessary in our applications where some wheel slip is acceptable. Where sensored does score is the startup of the motor. Sensorless controllers have challenges during motor start-up, as no back EMF is present when the motor is stationary and therefore need to bump the motor initially to determine position on powerup. The motor can become out of sync when running with the position the ESC believes it in. Improvements in ESC s/w should minimise this.
Its going to take some experimentation in building drivetrains for fighting robots and testing them in competition.
Last edited by shakesc; 4th May 2014 at 22:26.
If there is a delay on my brushless drives, its too short to detect. I also find that the sensored brushless drives have better torque and control at low speeds than the brushed system I was using. Perhaps a short video will convince the doubters? There are also few problems with the sensors being delicate. The sensors are three small surface-mount hall switches on a well protected PCB; the only thing I worry about is the sensor cable and its tiny connectors.
BTW: the 2nd Hobbyking ESC mentioned looks ideal to me; I have two of the 150A controllers on the way for testing.
I tried the NTM28-36 motors. Start torque ain't what is needed. It drives, but not what we want.
What gear ratio did you use? With sensorless motors, you need plenty of reduction and/or a fairly large motor. By comparison the 540 size sensored motors I use have very similar performance to drill motors.
There is some testing I did with NTM 42-38 with a 12:1 gearbox (video description is wrong), 15kg of steel dumped on top.
I'm hoping to have some 19:1 gearboxes shortly to test with to see if the start up torque is enough to smoke the tires
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