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Comparison of Motion Control Loops for Industrial Applications
High-performance AC and DC servodrives are widely used in motion control applications such as machine tools, packaging, printing, web handling, robots, textiles, and food processing. The motion control algorithms are based on the mechatronics assumption of nearly ideal electromagnetic torque control. This assumes ideal field orientation and current regulators of bandwidth considerably beyond the motion control bandwidths desired. Feedback devices, chiefly encoders and resolvers, are employed in these systems to sense motor position and to calculate the sample average motor velocity, albeit with significant average velocity resolution limitations. The vast majority of motion control algorithms employed in industrial applications are of two forms: 1) an average velocity loop is cascaded with a position loop, or 2) state variable loops (Proportional-Integral-Differential, PID position loops) are closed in parallel.
The state variable loops in parallel (PID position) configuration is known to completely separate command tracking tuning from disturbance rejection tuning. However, such motion controllers require an acceleration command which is often not provided in industrial servodrive controllers. This causes the tuning to again become cross-coupled and interdependent.
The cascaded loop topology is very commonly found in industrial servodrives and has a variety of adjustments to handle this (unwanted) cross-coupling of the tuning process.
The cascaded average velocity loop is usually a Type I (integrating) loop which is cascaded with a proportional position loop. In that case, there are two types of average velocity loops that are commonly employed: Proportional-Integral (PI) and Pseudo-Derivative Feedback (PDF). Both of these controllers offer different possibilities for handling the tuning cross-coupling.
This article focuses primarily on an alternative to PI and PDF which will be called here PI+ but is also sometimes called PDFF. PI+ will be shown to be a general controller within which PI and PDF are special cases. Further, PI+ will itself be shown to be a special case of the PID position loop.