Cascade Control

Nests several feedback loops, where the outer loop's output becomes the inner loop's target — like position over velocity over current.

Cascade control nests several feedback loops together, with the output of the outer loop serving as the target value for the inner loop. The most typical example, in a motor or joint module, is three loops: the outermost position loop compares the target angle against the encoder reading and outputs a target velocity; the middle velocity loop outputs a target current; and the innermost current loop regulates the winding voltage, with motor current roughly proportional to output torque. In the open-source driver ODrive, for example, the position loop is proportional control, while the velocity and current loops are both PI control; skipping the position loop gives velocity mode, and keeping only the current loop gives torque mode. The design principle is that each inner loop runs much faster than the loop outside it, so disturbances like a sudden load change or a power fluctuation get suppressed by the inner loop first, letting the outer loop treat the inner loop as close to an ideal actuator. What robotics calls position control, velocity control, and torque control essentially just names which of these three loops the command enters at; joint interfaces like MIT mode instead combine the PD terms for position and velocity with a feedforward torque into a single torque command.

ExampleA joint module receives 'turn to 90°': the position loop computes 'turn at 2 rad/s,' the velocity loop computes 'need 3 A of current,' and the current loop adjusts voltage to hold the current at 3 A; if the load suddenly increases, the current and velocity loops absorb the disturbance first, and the position loop barely notices.

Also called

Position/Velocity/Current Loops, Three-Loop Control, Cascaded Control

Related

Proportional-Integral-Derivative Control · Position Control · Velocity Control · Torque Control · Field-Oriented Control · MIT Mode

Sources

ODrive Documentation: Control Structure and Tuning
Wikipedia: Proportional–integral–derivative controller