Motion servo drives are generally three-loop control systems, consisting of a current loop, a speed loop, and a position loop from the inside out.
1. Current loop:
The input to the current loop is the output after the speed loop PID regulation, which we call the "current loop setpoint". Then, the difference between this current loop setpoint and the "current loop feedback" value is used for PID regulation within the current loop and output to the motor. The "current loop output" is the phase current of each phase of the motor. The "current loop feedback" is not the encoder feedback, but the Hall element (which converts magnetic field induction into current and voltage signals) installed inside the driver in each phase and fed back to the current loop.
2. Speed ring:
The input to the speed loop is the output of the position loop PID control and the feedforward value of the position setting, which we call the "speed setting". The difference between this "speed setting" and the "speed loop feedback" value is used for PID control of the speed loop (mainly proportional gain and integral processing), and the output is the "current loop setpoint" mentioned above. The speed loop feedback comes from the encoder feedback value, which is obtained by the "speed calculator".
3. Position ring:
The input to the position loop is an external pulse (except for servos that directly write data to the driver address). This external pulse, after smoothing and filtering and electronic gear calculation, becomes the "position loop setting." The setting and the pulse signal from the encoder feedback, after being calculated by the deviation counter, are then processed by the position loop's PID control (proportional gain control, no integral or derivative stage). The sum of this output and the position setpoint feedforward signal constitutes the speed loop setpoint mentioned above. The position loop feedback also comes from the encoder.
The PID constant of the servo current loop is usually set inside the driver and does not need to be changed by the user.
The velocity loop primarily operates on PI (proportional and integral) gain, so we need to adjust the velocity gain and velocity integral time constant appropriately to achieve the desired effect. The position loop primarily operates on P (proportional) gain. For this, we only need to set the proportional gain of the position loop.
There are no fixed values for adjusting the parameters of the position loop and speed loop. They must be determined based on many factors, such as the mechanical transmission connection of the external load, the motion mode of the load, the load inertia, the speed and acceleration requirements, and the rotor inertia and output inertia of the motor itself. A simple method of adjustment is to adjust the gain parameter from small to large and the integral time constant from large to small within a general range of experience based on the external load conditions, and set the steady-state value that does not cause vibration overshoot as the optimal value.
When adjusting the position loop in position mode, it is best to adjust the speed loop first (at this time, the proportional gain of the position loop is set to the minimum value of the empirical value). After the speed loop stabilizes, adjust the position loop gain, gradually increasing it. The response of the position loop should be slightly slower than that of the speed loop, otherwise speed oscillation is likely to occur.
