Electronic speed controls

October 21, 2007Dan No Comments »

My R/C car (after repair)

My boss is sorta into R/C cars and remote control vehicles in general. I brought my R/C car up here at least 2 years ago but it was in a state of disrepair and I hadn’t really done anything with it until just now. I started tinkering with it a couple of weeks ago, when I decided I really needed to do something with it or to get rid of it (and that was out of the question).

The biggest problem with it was the speed control. It used to be fairly hard (or just fairly expensive) to control the speed of a motor over a couple of amps. The stock option in the 80’s/early 90’s was very simple: just connect a resistor inline with the motor to lower the voltage that the motor receives. This is a horrible solution for 2 reasons. One, you are sinking all of the energy not used by the motor into heat. This is not efficient. Two, it’s not affordable to get a variable resistor to handle more than a couple of amps. So the stock solution was to use a whole additional servo to switch between maybe 2 different resistors to give you 3 speeds. This adds extra weight and complexity into the system.

Enter the late 90’s — high power MOSFETS go into mass production enough to be affordable. Electronic motor speed controls enter the hobby scene in full force. The electronic speed control generates a pulsed signal based on how much power you want to apply to the motor. Think of it as a square wave. With the throttle at 10%, a wave is constructed that spends 10% of its time in the ‘high’ position. The rest of the time it is low, or zero volts. If this cycle is repeated slowly, the motor would actually pulse forward very slowly. However, if you were to up the frequency of the wave so it is maybe hitting that ‘high’ mark a couple thousand times a second, you get something very useful. The signal is still only high 10% of the time, but it’s being switched on and off so fast it acts as a constant voltage to the motor. The motor actually responds like it is receiving a lower input voltage. Given an R/C scenario with a source of 7.2 volts and 10% throttle, the motor acts like it is receiving .1*7.2 volts = .72 volts. So the motor spins slowly. As I push the throttle open more and more, the ‘duty-cycle’ of this very high frequency wave becomes higher and higher (20%, 30%, 40%) until it is up to 100%, or full speed.

Now you know how ESC’s work. This post didn’t cover everything I wanted to discuss with my car and used a lot more words than I expected. Look forward to another R/C car post in the near future. At least you (hopefully) learned something. Danno out.

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