If you’ve had to ever purchase a servo or replacement servo for your application, you may have stumbled in to these terms. What is the difference between an analog servo and a digital servo?
If I were to grab a bunch of digital and analog servos, ripped the labels off and asked for you to hand me only the digital servo’s, it would be very challenging. The reason is, there really isn’t a difference between digital and analog servos on the outside. In fact, there really isn’t much of a difference on the inside either. Both types of servos contain the same gear train, same 3 wire lead to the Rx, motor, case, and even potentiometer to determine the servos position!
Where the real difference lies is within the circuit board found in the servo. In a servo the signal sent from the receiver to the servo gets process by the circuit board. From here the signal is converted in to power pulses that are sent to the motor inside the servo.
The Analog RC Servo
The circuit board in an analog servo receives the signal from the receiver and then outputs a power signal to the servo motor. The frequency at which the output occurs is 50hz. This means in one second an analog servo would only apply an output every 20 milliseconds. It may seem like a very short period of time, however, 20 milliseconds (0.02 seconds) is a long time in any control system.
In order to create movement of the servo arm, the controller would apply full voltage to the servo motor. If there is little servo arm motion required, the circuit would turn the motor on only for a split second within this 20 millisecond period. However, if the servo arm would have to rotate a significant amount based on control input from your transmitter, the servo would apply a longer duration on time to accomplish this. The on time during each cycle is what determines the amount of power that the servo may actually output. Below is a list of advantages and disadvantages for an analog servo.
|Analog Servo Advantages||Analog Servo Disadvantages|
|Low Power Consumption||Large Deadband Zone|
|Low Frequency Audible Noise||Weak Holding Torque|
The Digital RC Servo
A digital servo receives a signal from the receiver onboard the RC vehicle and translates this in to a pulse sent to the servo motor. The rate at which pulses of power are sent to the servo motor are much higher. You can expect that a digital servo would send a pulse to the servo motor at a rate of 500hz. This is equivalent to every 0.002 seconds. The delay in time before the next pulse of power is much better than an analog servo. It is this very reason that provides a digital servo with many advantages. Below is a list of advantages and disadvantages for a digital servo.
|Digital Servo Advantages||Digital Servo Disadvantages|
|Excellent Holding Power||Higher Power Consumption|
|Minimized Deadband Zone||High Frequency Audible Noise|
Why do Digital Servos Whine?
Is it ok if my digital servo makes noise when there is no control input or external forces acting on the servo? The quick answer to this question is yes. It is quite normal for a digital servo to sing to you. The reason for this is because of the high frequency pulses that get sent to the motor within the servo. You are in fact hearing these high frequency pulses that the motor is receiving.
In the next article, we will look at the application of digital vs analog servos. We will also make some interesting comparisons of specifications.