Electronic speed controls

Model Airplane News, Nov 1998 by McDonough, Tim

IN THE EARLY days of electricpowered R/C aircraft, the electric motor typically had two speeds: on and off. The on/off control was most commonly implemented by using a standard R/C servo and a switch. When the servo turned in one direction, the switch was moved to the "on" position. Moving the servo the opposite way would turn the switchand the motor-"off." In most cases, this meant that the pilot had the choice of either climbing or gliding.

This simple on/off control was certainly more desirable than letting the motor run at full speed until the battery pack was exhausted. However, it imposed limits on the kinds of maneuvers that could be flown and, in most cases, made scale-like flight difficult, if not impossible.

With the advent of the electronic speed control (ESC), electric flyers finally had fully proportional control of their electric motor speed. In the years since the advent of the ESC, we've come to take this essential component for granted. But just how does this little electronic wonder do its job, and are there things you should know about its care and feeding?

The specifics of how a DC motor works are beyond the scope of this article. Without going into a lot of detail, I'll just say that the speed of any particular DC motor is dependent on the voltage applied to the armature. Raise the voltage, and the motor turns faster. Lower the voltage, and it slows down. Remove the voltage altogether; it stops.

Ideally, then, the ESC would serve as a sort of valve that allowed you to control the amount of voltage applied to the motor. The trouble with building this ideal ESC is that regulating the motor voltage in a fully proportional manner is more difficult to do efficiently and economically than simply turning the motor voltage on and off.

The modern ESC is a compromise between the ideal ESC and the simplicity of an on/off control. By using a technique called pulse-width modulation (PWM) and relying on the momentum of the motor as it turns, the ESC controls the motor speed by changing the average voltage applied to the motor. The key to understanding all this is in seeing just what PWM does. PWM divides up some increment of time (called the "base period") into a given number of even steps, or increments, much as a minute is divided into 60 seconds. During each of these increments, the thing being controlled by the PWM-our motor voltage, for instance-is either on or off. The average voltage out of the control is a ratio of the "on time"-or how wide the "on" signal, or pulse, lasts-and the length of the total PWM interval. An example will make this clearer.

To make the math easy, let's assume the base period of the PWM signal is 10 seconds long. Also assume that the battery pack provides 20 volts. It's easy to see that if we never turn the voltage on, the motor will not run, and if we always have it on, the motor will run at full speed. But what if we turn on the voltage only for the first 5 seconds of each 10-second period? Half of the time, the full voltage would be applied; the other half, no voltage would be applied. The average voltage would be (5 10) x 20 = 10 volts. Similarly, if the voltage is turned on for the first 7 seconds out of each 10, the average voltage is (7 10) x 20 = 14 volts. Figure 1 illustrates PWM voltage control.

Given my example, you can see that this mythical ESC could supply an average voltage to the motor in increments of 10 percent from 0 to 20 volts. You are probably thinking that if the motor is turned on for 5 seconds and then left off for the next 5, it will have more than enough time to stop turning before the next PWM cycle starts, when the battery voltage is again applied. If you also thought that the motor would run at full speed for those first 5 seconds, you would be correct once again.

The key to successfully using PWM to control our motors is in the careful choice of the length of the base PWM period and the numher of discrete steps between off and full speed. We humans often think of time in terms of seconds, minutes and hours. The microprocessors used in most modern ESC designs can easily deal with time increments of 1//1,ooo second and less. The base period needs to be short enough for the rotating mass of the motor to prevent it from making noticeable starts and stops at low throttle settings. A base period that is too long can contribute to a condition called "cogging." This problem is particularly noticeable on geared systems. In severe cases at the lowest throttle settings and high gear ratios, you can actually see the prop turn in one step at a time instead of rotating smoothly! This can be very hard on gears and belts and can cause premature failure.

The more individual steps the PWM provides, the closer the output will be to our ideal control. In practice, however, there need only be as many individual steps in the PWM signal as you need to satisfy the pilot that he has smooth control of the motor. A typical modern ESC starts a new PWM cycle 2,500 times a second and provides 40 or 50 individual steps.