Basics of servo motor designScott Orlosky | December 11, 2023
Electric motors come in a variety of configurations depending on the needs of the application. However, with rare exception, they consist of:
- Stator: an iron or steel cylinder typically wrapped by copper windings; when AC power is supplied to the windings, it generates a rotating magnetic field.
- Rotor: a rotating shaft, in which torque is induced by its reaction to the stator's magnetic field. The shaft is bearing-mounted for precision rotation, with at least one end of the shaft extending outside the motor enclosure. It also provides a linkage point for mechanical power transmission.
- Motor controller: generates the commands that manipulate the timing of the current to control the speed and torque from the motor.
- Feedback device: ensures the actual motor movement matches with the commanded value.
The standard motor configuration uses a rotor suspended in the stator body by rotary bearings. The stator houses coils of wire and is mounted to a baseplate with an electrical breakout box on the side so an electrician can wire it up. In this configuration, if a feedback device is used, then it is mounted to the back of the motor. The motor controller is usually in a separate box often mounted alongside an electrical cabinet, which might be located 20 ft or more away. This configuration works fine for a lot of industrial applications, where there is plenty of room, and equipment stays in a fixed location.
What applications need
However, once electric motors begin to be integrated in complex machinery, such as in process automation or robotics, problems immediately start to arise with the “conventional” motor construct as described above. Each motor requires a feedback and controller. This requirement puts a premium on component size and reduces wiring complexity. Of course, if components get too small, they don’t have the torque required to do the work.
The result of this balancing act is a servo motor/controller package in which the motor housing (stator) becomes the framework for attaching feedback devices, printed circuit boards, wiring, a controller and sometimes a gearbox. Each axis of motion is self-contained with two cables — a larger one for the motor drive and a smaller one for feedback control — attached at each servo motor assembly.
Although motors used in this type of control scheme can be AC or DC powered, most often they are DC driven (12 V to 15 V is popular). Brushless DC motors can use magnetic commutation or even the on-board position feedback to commutate (i.e., switch the magnetic field on and off in sequence to turn the motor). Using DC voltages more closely matches the voltage required for the electronic control, producing less waste heat, and DC voltages tend to generate less electronic noise, improving the precision of control. Typical feedback controls close the loop on velocity, position and current in order to provide adequate control without overheating.
Balancing design trade-offs
There are other compromises that come with a compact design. As motors get smaller the amount of torque that they can produce is reduced. Since torque is the force required to rotate a robotic joint, it limits the amount of work the robot can do. The remedy is to use a gear reduction on the output end of the motor. Since torque • RPM is roughly constant, more torque is available at a lower RPM. Of course, a robot that moves too slowly is not useful.
At the other end of the spectrum, moving too fast has its own problems. Very rapid movements may require quick stops, that cause the end of the robot arm to bounce after it has traveled the required distance. Then the process has to wait until the bouncing stops before it can perform the desired action. The designer has to take all of these factors into account when designing a robot for a specific use.
Control and feedback
When using the term “motor” one typically thinks of a device that spins continuously when power is applied. Certainly, servo motors, when used in applications like driving conveyor belts, labeling machines or continuous lamination machines, will use their full 360° of rotation. However, when used in most robotic applications to control joint movement in a robotic arm for example, they don’t require the full 360˚ of movement. Usually, the total movement is limited to 180˚.
To achieve this type of position control, the typical control method is to use a pulse width modulated (PWM) signal, where the pulse duration represents the desired percentage of travel from 0˚ to 180˚. The way this works is that internal servo electronics sample the PWM signal periodically (usually every 20 ms) and calculate the difference between the actual value (as read from the feedback device) and the requested value. This difference is known as the error and is used to calculate which direction to move the motor and how fast. A large error uses a faster movement and as the error comes closer to zero, the movement slows. This is known as proportional control. When the error gets to a point within the allowed tolerance, it will “stop and hold” until a new target is set. This is a simplified explanation, but it covers the basic principle of PWM position control.
There was a time when were considered to be too light duty to be a viable solution for industrial applications. Advances in control electronics, thermal management, bearing manufacturing, rare Earth magnets and improved ICs have all contributed to the steady growth of “servos” as the go-to solution.
Servo motor solutions were driven, in part, by the needs of designers to embody the compact, high-torque solutions that are on the market today. In turn, the industry has responded by not only helping to create the need for these devices but by finding new uses to help expand their use further. As a result, servo motor applications of all types are one of the fastest growing sectors of the industrial component economy — and likely will continues for many years to come.