How Does a Servo Motor Work?

Typical servo motor mechanism is not complex. The servo motor has control circuits and a potentiometer that is connected to the output shaft. The shaft, which is the output device, links to a potentiometer and control circuits that are located inside the servo. The potentiometer, coupled with signals from the control circuits, control the angle of the shaft – anywhere from 0 to 180 degrees, sometimes further. The potentiometer allows the control circuitry to monitor the current angle of the servo motor. If the shaft is at the correct angle, the servo motor idles until next positioning signal is received. The servo motor will rotate the correct direction until the angle is correct.

Each servo motor works off of modulation known as Pulse Coded Modulation, or PCM. The motor has a control wire that is given a pulse application for a certain length of time. The angular degree of the shaft is determined by the length of the pulses, which the servo motor anticipates every couple seconds. A normal servo is mechanically not capable of rotating further due to a mechanical stop built into the main output gear. The amount of power applied to the motor is proportional to the distance it needs to travel. So if the shaft of the servo motor needs to turn a large distance, the servo motor will run at full speed. If the servo motor needs to rotate only a small amount, the motor will run at a slower speed. This is referred to as Proportional Control. The servo motor expects to see a pulse every 20 milliseconds, (.02 seconds) and the length of each pulse will determine how far the servo motor will rotate.

How to Select a Servo Motor

The simplified definition of a servo system is that it consists of several components which together control or regulate speed/position of a load. The servo motor is one of these components in the system. When it comes time to select an appropriate servo motor for an application some people may be naïve in thinking that they can just check size the motor based on the horsepower rating of the presently installed motor, or exclusively based on the application’s torque requirements. The following factors must all be taken into account when selecting the appropriate motor: inertia ratio, speed, and max torque at desired speed.

Any rotating object has a moment of inertia which is a measurement of how difficult it is to change the rotating velocity of that object. Moment of inertia in a servo system can be divided into two parts; load inertia and motor inertia. The motor inertia is part of the servo design and is typically listed in the manufacturers’ specification sheet. Load inertia is more complicated because it involves every component that is moved by the motor, and is calculated using proper equations for each component. A typical inertia ratio for most applications is 5:1, but the lower the ratio is, the higher performance will be, and vice versa.

Since there may be a variety of servo motors that meet the required inertia ratio specifications, the next step is to find the smallest, most cost-effective servo motor that will meet the speed and torque demands. Servo motor manufacturers normally provide speed-torque curves for each series of motors, which illustrate several interesting points of the servo motor’s characteristics. The speed-torque curve contains two regions; continuous and intermittent, which can translate to correct match or incorrect match (respectively) for the application. If the speed-torque required for a specific application falls into the continuous region of the speed-torque curve, then that motor can produce that torque and speed without overheating. If the speed-torque required for the application falls into the intermittent region of the speed-torque cure, then that motor can only produce that speed and torque for a limited amount of time before overheating.