PROPORTIONAL CONTROL
Servos are controlled by the receiver in the airplane, which converts the transmitter’s signals into electrical pulses that are then sent to the appropriate servo for a specified control function. Inside the servo, an electric motor is connected to a feedback potentiometer (pot) and a control board. Basically, the servo’s circuitry allows the servo output arm to move to the required position and then keeps it there until commanded to go to a new position. The pot is primarily a variable resistor, and the motor and its drive gear train move the output shaft and the pot at the same time. When the servo is at idle, (no command input), the pot keeps the servo arm at its neutral (center) position. It will command the servo to stay at this position even when flight forces try to move the servo arm. The control circuit does this by constantly repositioning its center position relative to the stick position. Move the stick a little, and the pot moves the center position a little, and then the servo moves to the new center point. Move the stick a lot, and the center position is moved a lot, and then the servo arm likewise moves a lot. This feedback circuit is what gives us fully proportional control.
Regardless of the brand, RC servos don’t vary much in terms of function, and the electronics used are practically all the same. Depending on the size of the servo and its rated strength, the differences come from the internal mechanics: the motor, gear train, pot, and output shaft support. And, just like everything else, you get what you pay for. Inexpensive, budget servos won’t last as long or perform as precisely as more expensive servos. As the size and complexity of your models increase, so should the quality of your servos. Less expensive servos featuring plastic gear trains are fine for small to medium foam backyard flyers and ARFs, but larger, faster, and heavier planes should be equipped with the more expensive servos that have metal gear trains for more precision and durability.
Another consideration when choosing servos for your model is the type of output shaft support used. For sport flying, less expensive servos with plastic bushings will provide adequate support, but they will wear out more quickly. Servos with metal bushings will last much longer and will also provide superior support. For the best support, servos equipped with precision ball bearings are the preferred choice.
There are two types of servos available: analog and digital. For years, analog servos were the standard, but today, the digital servos have become the preferred choice. Analog servo circuitry regulates the motor speed by sending it electrical pulses of different widths, called pulse width modulation (PWM). Typically, the motor receives about 50 to 60 pulses per second. The longer the pulse width sent to the motor, the faster the motor spins and the higher torque it produces. Analog servos are still available and are less expensive than digital servos. They are very good for sport flying up to 60 size model airplanes, but they are not as precise as digital servos while reacting to smaller command inputs. If you are on a budget and fly small, light aircraft, then analog servos are hard to beat.
Digital servos, on the other hand, provide several hundred pulses per second, and this greatly improves motor reaction time and produces more torque with less response delay. Digital servos provide quicker servo speed and better holding power at any given servo position, including at neutral. You can easily feel the difference in position holding between analog and digital servos.
If there is any drawback to using digital servos, it’s that they have a higher power consumption rate compared to analog servos. To compensate for this higher power draw, RC pilots use larger capacity receiver battery packs, typically 2000mAh and 2500mAh and higher, depending on the size of the airplane.