This article is a brief introduction to servo motor control and explains the different components in a servo motor system and how each component works to create the entire motion control solution. An example system is shown here:

Servo Motor System Components

• Brush or Brushless Servo Motor
• Encoder / Feedback Device
• Servo Motor Amplifier / Drive
• Servo Motor Controller

Brush / Brushless Motor

The choice of what motor to use is usually an early step in the process of selecting a servo system.  The critical things to be aware of are the desired speed (RPMs or Revolutions Per Minute) and the desired torque.  Many times a speed-torque curve will be available for a motor that shows the amount of torque that a motor has at a specific speed.  Servo motors generally have a relatively flat speed torque curve in that the amount of torque the motor has available stays relatively constant up until you reach the maximum speed of the motor.  (A stepper motor on the other hand can have a significant ramp where the torque drops off considerably as the speed gets higher.).  A gear-head or mechanical gear-train can be used to increase the torque of a motor when necessary.

Encoder / Feedback Device

An encoder is a position sensor that outputs a signal telling the controller where the motor is and how fast it is moving.  Many servo motors come with an encoder built in however it is also possible to use an external encoder or feedback device to control a motor.  When using an external encoder, it is critical that there not be an excessive amount of compliance between the motor and the encoder.  Essentially, any change in the motor position should immediately be detected by the encoder.  A situation where the encoder is mounted after a belt drive will cause problems due to the compliance in the belt.  In a system where the position of the load is required, it is recommended to use two encoders - one directly on the back of the motor and a second encoder on the "load".  The Galil controller has built in functionally called "dual-loop" mode to control a system with two feedback devices.

Servo Motor Amplifier / Drive

A servo motor amplifier takes a control signal from the motor controller and amplifies it up to deliver a specific amount of power to the motor.  There are a number of different types of servo amplifiers out there but a common one is called a "torque-mode" amplifier.  This type of amplifier converts the command signal from the controller into a specific amount of current (in Amps) delivered to the motor.  In a rotary motor - current is directly proportional to torque - so the amplifier is actually directly controlling the amount of torque on the motor.  In a linear motor, current is proportional to force - so likewise the amplifier is directly controlling the amount of force coming out of the motor.  The amplifier closes a current loop around the motor at a very fast rate so that the motor controller's command signal closely matches the actual current that is delivered to the motor.

Servo Motor Controller

Up to this point, the components explained above can essentially take in a control signal and deliver a specific amount of current to a motor - the motor will then turn and the amount that it turns can be seen by the encoder.  The job of the servo motor controller (or commonly referred to as the motion controller) is to close the loop on the system by constantly looking at the encoder signal and applying a torque to the motor in order to control it.  The simplest form of this is to hold a specific position.  In this scenario, if a disturbance causes the motor to move off of a position - the encoder detects this change in position - creating an error signal.  This error signal is then translated into a commanded current by the controller in order to drive the motor back to the original position.  A more advanced case is when the controller wants to move the motor to a new position.  In this case the controller creates a desired motion profile using a specific acceleration, deceleration, and speed.  In a very fast loop, the controller commands a specific position for the motor at a precise time - this results in the motor rotating along the desired motion profile.

Conclusion

Galil offers a wide variety of motion controllers for servo systems.  To make things easy, Galil also offers a variety of optional plug-in amplifiers that mate up directly to the controller.  There are two series of motion controllers that offer this - one is the high-performance "Accelera" series shown here: http://www.galil.com/motion-controllers/multi-axis/dmc-40x0 and the other is the more cost-effective "Econo" series shown here: http://www.galil.com/motion-controllers/multi-axis/dmc-41x3