brushless-dc-motormotormotor controller
For servos PWM controls angular rotation – for example 1ms width means 0 degrees and 2ms means 180 degrees (it is PWM width is proportional to angular rotation).
I am not sure what PWM width influence in brushless motors. Of course when I set PWM width to higher value motor runs faster, but is PWM width proportional to thrust, RPM, power or maybe something else?
DC motors don't work well at low RPMs. They stall and have horrible torque. (i.e. they can't turn very hard) So people have created gear motors: motors with integrated gearing. The result looks like a slightly bulkier motor, but one that has low RPMs and high torque. If you were to take apart a running a gear motor, you'd see the motor part actually runs at several thousand RPMs, but it's geared down to something like 60 RPM max.
A common specialized one is the standard hobby servo, which has some additional electronic bits but is fundamentally a gear motor. Check out any place that sells motors for robotics or surplus electronics and you'll see several different gear motors to choose from.
DC gear motors are controlled just like normal DC motors, so an Arduino motor shield works just fine with them.
So, if I want the speed to remain at, say 3000rpm, and increase the thrust from, say 400g to 500g, I increase the current through the motor. I do this by increasing the PWM duty cycle. But, this means the speed will also increase from 3000rpm to a higher value, right? How can this be maintained then?
You would only want to increase the torque (thrust) if the mechanical load were threatening to slow the motor down - by increasing the PWM to counter that increase in load torque you are largely keeping the motor at constant speed. Remember a simple DC motor (even a brushless type) has a very clear torque-speed characteristic that means, for a given speed at a given torque, an increase in torque of \$x\$ will reduce the speed largely by \$x\$ also.
It's a not a perfectly linear relationship and changes entirely when field windings are implemented but it's fairly reliable: -
Best Answer
For servomotors, the angular position setpoint is contained in the width of the PWM pulses, but there is no direct action on the motor: a servomotor is in reality a complete control system, with its own angular position sensors and control loops. PWM is convenient given that very often no controller is actually in the servomotor, but if there was one any other digital communication could do.
For brushless motors, PWM is not used a communications means, but actually as a technique to modulate the voltage sent to the brushless motor based on a single DC bus. The voltage applied is proportional to the duty cycle of the PWM, or timeON/period. You can safely assume a DC brushless behaves like a DC brushed motor, although inside it is actually an AC synchronous machine with "electronic brushes" (the phases are switched depending on the rotor position, sensed by hall effect sensors usually). Often, they are sold without the switching controller - but both of them are equivalent to DC brushed motors.
In brushed DC motors, the voltage applied to the motor is applied across the winding, and its resistance determines how much current it draws from the supply. Current is proportional to the torque provided by the motor. Then, if that torque (called stall torque) is above the torque applied by the load (friction or whatnot), the rotor will spin up and according to Faraday's law of induction, a voltage that is called "back electromotive force" will counteract the voltage applied. This back emf is proportional to the speed of the rotor, therefore there will come a point when the voltage across the winding resistor, and therefore the current, will decrease to a point where load torque and motor torque will be balanced. That's the operating point of the motor.
(note: the equivalent model includes an inductor, ignored here)
As mentioned above, a brushless motor behaves the same way, therefore the voltage applied acts on the torque and the speed of the motor at the same time, in the following way:
If the load torque is very low compared to the stall torque (when rpm=0), then you can assume voltage is approximately proportional to speed since the operating point will be at the very bottom of these torque-speed curves.