A servo motor is an extremely important unit in any physical automation process. From being used in industrial process automation, CD drives, to robots, servo motors have made their presence known everywhere. It is important for us to understand what a servo motor is and how it works if we wish to use them ourselves. Hence, we shall first look at the electromechanical aspects and then dive into the control aspects of the servo motor.
Servo motors, also called control motors, are motors with high torque capabilities and precise position control. Although servo motors work on the same electromechanical energy conversion principles like other motors, their design, construction, mode of operation and the area of application differ significantly. Servo motors come in various sizes starting from a fraction of a watt up to a few hundreds of watts. Before we proceed further, let's see how these motors differ from the other motors.
- They provide high torque at all speeds including when at rest.
- They are capable of holding a static position.
- Due to low moment of inertia, they can quickly switch their direction of rotation.
- They don’t overheat at lower speeds or in a static position.
- They can accelerate and decelerate quickly.
- They can move to any given position without any drift.
These motors are usually narrow (smaller in diameter but longer in length). This increases their torque capabilities and minimizes the moment of inertia which enhances its response time.
Servo motors are classified on the basis of electrical aspects as well as mechanical aspects. On the basic of the electric aspects:
1. DC Servo Motor
These motors are usually a permanent magnet dc motor for smaller sizes and separately excited dc motor for larger sizes. The stator (the stationary part) of the dc motor houses the field windings and the rotor (the rotating part) houses the armature winding. The field winding creates a magnetic field and as we know, a current carrying conductor in a magnetic field experiences a force. In this case, the armature, being a winding of current carrying conductors, experiences a force that makes it rotate. The control here can be achieved either by controlling the current through field winding or by controlling the current through the armature conductors. This basically implies that we are controlling the force experienced by the conductors, thereby controlling the dc servo motor.
2. AC Servo Motor
Most ac servo motors are basically two-phase squirrel cage induction type and are usually used for low power applications. Recently, three phase induction motor systems have been modified for high power servo applications replacing the high-power dc servos. The stator of this servomotor has two windings with a 90 degree phase shift between the electrical signals (the angle between a successive north and south pole is 180deg electrical angle). One winding is the reference winding which is supplied with a constant voltage source. The other winding is the control winding which is supplied with a variable voltage of the same frequency as that of the reference voltage, depending on the control strategy. The speed and torque of the motor are controlled by the phase difference between the reference winding and the control winding.
On the basis of mechanical aspects, the servo motors are categorized as positional rotation servos – which is also called a half circle servo i.e., its rotation is restricted to 180deg. The other one is the continuous rotation servo which is similar to the positional servo but rotates fully. Check out this tutorial on how to convert a positional rotation servo into a continuous rotation servo - https://www.circuitbread.com/tutorials/how-to-make-a-360-degree-continuous-rotation-servo-motor. There is also something called a linear servo which is basically a positional rotation servo fitted with a rack and pinion mechanism.
Although the construction and features of the above discussed servos are different, their control mechanism is pretty much the same. We call this “the servomechanism”.
Let’s take a look at the diagram below to get an understanding of how the servo “knows” where it is at all times and goes to where it’s “supposed” to be.
In this mechanism, a pair of potentiometers acts as an error measuring device as they convert the difference in the reference and output position to an electrical voltage that is proportional to it. The control input determines the position of the input potentiometer which is the reference position. The difference in the potentiometer voltages is the error voltage (ev = ec - er) which is amplified with a gain k and fed to the armature circuit of the motor. The motor develops torque in the presence of an error voltage and rotates the load in a way that reduces the error. Once the output load position is the same as the reference position, the error voltage is zero and the motor stops. This is the way that the servomechanism is used to control position.
Therefore, it is the servomechanism which is responsible for the precise control of the servo motor whereas the high torque capabilities is satisfied by good electromechanical design of the servo motor. The combination of good mechanical, electrical, and control design is what makes servos so versatile with a wide range of practical applications.