Servo Loop

The servo loop decreases the quantity of error that occurs between the feedback positions and velocities and the commanded positions and velocities.

The units of the servo loop output depend on the type of axis being controlled. Refer to the table that follows.

Table: Servo Loop Axis Type, Output, and Units

Axis Type	Servo Loop Output and Units
Piezo axis	Amplifier command in Volts.
Stepper axis	Commutation The action of steering currents to the proper motor phases to produce optimum motor torque/force. In brush-type motors, commutation is done electromechanically via the brushes and commutator. A brushless motor is electronically commutated using a position feedback device such as an encoder or Hall effect devices. Stepping motors are electronically commutated without feedback in an open-loop fashion. angle in microsteps.
All other axes	Current command in Amps. When the servo loop output is a current command, the current loop is used to generate the amplifier command in Volts. For more information, refer to Digital Current Loop Block Diagram.

 

Servo Loop Block Diagrams

• Servo Loop Block Diagram - Typical shows the typical use-case for controlling a mechanical device that contains a single feedback sensor.
• Servo Loop Block Diagram - Single Feedback PI is for spring-like systems such as flexure-based piezo stages and the Yaw (Theta) axis of a rigid H-bridge gantry.
• Servo Loop Block Diagram - Dual Feedback (Secondary I Only) shows the Secondary I method of dual feedback control. This method is more robust than the Secondary PI method and might supply better stability.
• Servo Loop Block Diagram - Dual Feedback (Secondary PI) shows secondary PI method of dual feedback control. This method is high-performing, but it requires a relatively high resolution Auxiliary feedback device with a tight structural loop.
• Servo Loop Block Diagram - Full shows the full servo loop for the Automation1 drives.