About RC Servos

A servo motor is a motor that has feedback so that electronics can be used to control it. The servos used with Scon SB's are are small Radio Control (RC) hobby type servo motors. They are quite different than industrial servo motors of which there are several types.RC servo motors have built in electronics to drive and control the motor, plus gearing to give usable output shaft speed and torque. There is a huge selection of servos of various size, speed and power and from different manufacturers. Most RC Servos employ a similar control scheme. Unlike general purpose motors, most RC servos have a limited rotation of less than 180 degrees and normally about 60 degrees because of the limitations of the feedback potentiometer and the original intended application. This is sometimes changed to allow full rotation by removing the potentiometer and setting the feedback voltage at a fixed level. Most servos are designed to work well between 4.5 - 6 volts. There are three leads, one for power, one for common, and a signal connection.These servos are controlled by PWM (Pulse Width Modulation). The servo receives a continuous stream of pulses and rotates to a specific position in response to the width of these pulses. Most RC servos operate by a long time standard, however manufacturers may vary from the standard. The standard is that a pulse of approximately 1,500 us (1.5 ms) is the neutral point for the servo. Given the rotation constraints of the servo, neutral is defined to be the position where the servo has the same amount of potential rotation in the counter clockwise direction as it does in the clockwise direction. It is important to note that different RC servos will have different constraints on their rotation but they all have a neutral position, and that position is generally around 1,500 us. Most RC systems repeat the pulse for each servo at about 18-20 ms. While 18 ms may seem fast, it is the main limiting factor of smooth operation when using these servos in robotics. Slow movement may be jerky if the change in position occurs during the pulse off time. Jerky motion is also caused by the feedback gain and dead band in servos. Dead band is the range in rotation that the servo's internal control circuity considers to be the specified position. In other words if you send a pulse to a servo that gives a 30 degree rotation, the servo may have a range of 29.5-30.5 all of which it considers 30 degrees. Many new "digital" servos have better logic and have very narrow dead bands. Some new digital servos are actually worse than a quality analog servo.The generally accepted pulse width range is 1,000us to 2,000us for most servos, however many servos operate over much wider ranges. Care must be used not to send out of range pulses to servos. Most servos do not have electronic limits, and can be physically damaged be rotating past the end of their range.