What Exactly is a Servo Motor?

What Exactly is a Servo Motor?

At Rotalec we have been providing motion control solutions from the beginning.  We have a variety of solutions on offer from brush DC, Induction AC, coreless, ironless, BLDC, BLAC, linear, frameless, voice coil, stepper, synchronous AC, and many more.   What makes a motor a servo motor?

Currently the accepted norm of a “Servo Motor” is a brushless DC or AC permanent magnet synchronous 3 phase motor with some kind of feedback device.  While there is truth to this, it is a very narrow application of the term.  A servo motor can be any motor, rotary, or linear.  What makes a servo is not the motor type, but rather the feedback loop.   A controller commands an action from a motor that can be force/torque, velocity, or position.  The controller receives feedback from a device mounted to the motor that provides the actual value the motor responded with.   The controller calculates the error – difference in actual measured value vs. commanded value to determine the next command signal.  This is known as a negative feedback loop.   The simplest analogy to a servo process that we are all familiar with is the temperature controller in your home for heating and air conditioning.  There is a user set point temperature that defines what the output action is.  The feedback is provided by a temperature sensor.  The output is a simple ON/OFF signal.

Going back to motors.  

By having a cyclic feedback loop where we command, evaluate feedback and repeat very rapidly allows us precise control of motor attributes such as torque/force, speed, position.   This is known as a servo update rate.  Now it is not impossible to get loop updates 8KHz or faster. 

With modern DSP’s we are now able to offer full servo closed loop control of motors that were previously run as open loop only.  Enter the bipolar stepper motor.  Traditionally stepper motors have been used for open loop positioning to great success.  By placing a feedback device in the form of an encoder and closing the loop we can now take these 2 phase 100 pole AC hybrid permanent magnet motors and turn them into a SERVO MOTOR.  Doing so offers a number of advantages in many industrial applications where intermittent motion is required.   High speed pick and place, indexing are all great applications for a servo motor of this type.  The high pole count offers very high torque density per frame size.   One really interesting thing about using a stepper as the motor for closed loop servo system is their ability to run more efficiently and cooler than the standard 6-8 pole BLDC/BLAC PM motor.  This is particularly important for battery powered applications.

Quicksilver – one company focused on solutions for these motors has an interesting white paper titled “Motor Quality Factor” that compares a 100 pole hybrid motor to a low pole BLDC in areas such as efficiency, heating, power vs. speed.

This shows the total efficiency of a high pole motor exceeds that of a low pole motor over most of the real operating speed range.

Take a look at the Whitepaper from Quicksilver Controls Inc.

At Rotalec, we often find we are tasked to undertake a role as technology advisor for our customers.  Motion control is one area where there are many approaches you can take to solve a particular problem.  By having the exposure to many industries and experts in their respective fields, we have learned what works best.  Just because something works, doesn’t mean it can’t be optimised – reduced in size, reduced in price, increased in performance. 

Need a servo?  You need Rotalec.


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