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HOME NEWS & ANALYSIS INDUSTRIES ELECTRONICS ARTICLE




BASICS OF SERVO MOTOR DESIGN

Scott Orlosky | December 11, 2023
Source: Windell Oskay/CC BY 2.0

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.

[Learn more about servo motor working principles on GlobalSpec]


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.

[Find servo motor suppliers on GlobalSpec]


LOOKING AHEAD

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.

To contact the author of this article, email GlobalSpeceditors@globalspec.com


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