EPOS4: Dual loop control

Topic:

• What is a "Dual loop control"?
• What is the advantage of a "Dual loop control"?
• How does the EPOS4 "Dual loop control" work?
• How to configure a "Dual loop control"?

Basics about "Dual loop control"

Positioning of mechanical loads is typically processed by a PID position control loop with an encoder mounted on the motor shaft as the feedback sensor.

In cases when gears, spindles, or belt systems are used to transmit motor rotation to the load, the backlash or elasticity of these elements may negatively affect performance and position accuracy of the load or the load might even oscillate at its target position. Such impacts can be overcome for most system designs by a so called "Dual loop control". An additional encoder is added at the load side for direct measurement of the load position which is integrated in the control loop then.

Motor and load encoder are both used by a "Dual loop control" as feedback sensors to achieve improved performance and accuracy of the load's motion and position and to suppress negative effects (e.g. backlash, elasticity) caused by the imperfections of the mechanical system. 

There are actually three control loops present (incl. the "Current control loop"). The term "Dual loop control" just refers to the two control loops which use an encoder mounted at the load side (-> "Main loop") and an encoder mounted at the motor shaft (-> "Auxiliary loop").

A closer look into EPOS4's "Dual loop control"?

Please find more details about EPOS4's "Dual loop control" in following documents:

• Attached German and English version of a technical article.
• Chapter "10 Dual Loop Control" of the "EPOS4 Application Notes Collection".

Recommended encoder resolutions:

• Motor shaft encoder
  The encoder which has to be stiffly mounted on the motor shaft is in use for FOC (= field oriented control) in case of a brushless motor and as the feedback device of the inner auxiliary control loop of Dual Loop control. The general recommendation is to have an encoder mounted on the motor shaft with at least 500 cpt. There are even 1024 cpt. often in use as an standard.
  
  
• Output shaft encoder (of the load)
  The general recommendation is to use an encoder which has at least a 2 - 4 times higher resolution than the load's positioning accuracy required by the application.
  Example 1:
  If you expect a load side position accuracy of 0.1°, the encoder mounted on the load or output shaft of the drive train should have a resolution of at least 0.025°... 0.05° corresponding to an encoder incremental encoder in use of typ. 2048 or 4096 cpt. or an absolute encoder with 13 or 14 bits single turn resolution.
  Example 2:
  If you have a linear scale and want to achieve an position accuracy of 0.1 mm, the resolution of the linear scale should be typically at least 0.025 ... 0.05 mm.

Please find more information about encoder selection and additional related aspects by the following linked Support Center document:

• Which encoder resolution should be chosen?

Configuration of a "Dual loop system"

The configuration of the encoders and gear in use by a "Dual loop control" can be done easily by EPOS Studio's "Startup" wizard.

• The total gear ratio in between the motor shaft encoder and the load side encoder has to be configured accurately(!).
  
  Also take care if there is a change of direction present due to the mechanics and gear in between the two encoders and configure it accordingly.
  
  
• The feedback sensors in use mounted on the motor shaft and the load side (= "On gear") have to be configured properly (i.e. the encoder type, resolution, etc.).
  
  There can be "Digital incremental encoders", "Analog Incremental encoders (Sin/Cos)", or "SSI encoders" selected and assigned to the motor or load side.
  
  
• "Dual loop position controller" has to be selected.
  

Evaluation and tuning of "Dual loop control" parameters

The EPOS4 commissioning software “EPOS Studio” has an integrated "Regulation Tuning" wizard for full automatic evaluation and tuning of the complex "Dual loop control" algorithm parameters.

The recorded Bode diagrams of the drive train transfer function can be exported. This data can be used for expert analysis or be the base of manual parameter optimization focused on specific application demands.

Related documents dealing with "Dual loop" aspects too:

• Which encoder resolution should be chosen?