My question is at the end (to change the speed) we are controlling the input voltage of a DC motor in both PWM (pulse width modulation) and variable resistance cases. Is the only reason for choosing PWM to obtain a better precision or not consuming extra power? If it is the only reason it seems odd to use PWM equipment for simple demonstrations.
Electronic – Why is PWM used to control DC motor speed instead of using a variable resistance
dcmotorpwm
Related Solutions
It looks to me that your control loop is essentially:
if speed is too slow:
increase PWM duty cycle one unit
if speed is too fast:
descreate PWM duty cycle one unit
As you have observed, this doesn't work so well. Your control loop cyclically overshoots the target speed.
The canonical solution to this sort of problem is a PID controller. The concept is essentially the same, measure a thing and compare it to a target to calculate an error. Then adjust something (in your case, PWM duty cycle) based on the error.
However, a PID controller has three error terms:
- P: the current error
- I: the integral of the error
- D: the derivative of the error
For each of these terms, the controller has programmed some gain. It then multiplies each term by the respective gain to calculate how much the control input (PWM duty cycle) should be changed. Properly tuned, this allows you to get a feedback loop that at first ramps up the duty cycle, then as your vehicle approaches the target speed, backs off smoothly so you get a minimum of overshoot.
You will experience some significant performance issues if you were to use the EN for PWM.
If you look at the datasheet, especially the part you provided in the OP:
the EN pin doesn't just stop sourcing/sinking power (ie the FET's are not gated), it creates a "Fast motor Stop" condition which basically means BothTop or BothBottom switches are enabled. This essentially shorts out the machine windings and it rapidly stops the rotor.
As a result if you were to use such a pin for PWM you would be continuously stopping
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Best Answer
Power efficiency The induction of the motor will cause the current to average. At the same time the transistors in PWM mode have very low impedance and therefore a low voltage drop and low power dissipation. In case of a series resistor a lot of power is dissipated in the series resistor.
Speed control behavior With PWM the motor will 'see' a very low power supply impedance, even though the power supply is constantly switching between high and low voltages. The result is that the motor has a much higher torque. With a series resistance the motor will experience a very weak power supply and it will be easy to stall the rotor.
Control circuit For a control electronics (eg. a microcontroller) it is very easy to switch on/off transistors. Outputting an analog voltage or controlling a series resistor requires much more expensive circuitry and in turn will cause more power dissipation.