Three-loop Control System

The three-loop control system is a closed-loop control method in which the motor uses current loop, speed loop, and position loop from the inside to the outside in turn.

Ⅰ. Three-loop Control Principle

1. Current loop: The input of the current loop is the output after the PID adjustment of the speed loop, which is called "current loop setting". Then the difference between the setting of the current loop and the feedback value of the current loop is compared to the current loop for PID adjustment and output to the motor. The output of the current loop is the phase current of each phase of the motor. The feedback of the current loop is not the feedback of the encoder but the Hall element (magnetic field induced into a current and voltage signal) installed in each phase of the driver feedback to the current loop.
2. Speed loop: The input of the speed loop is the output after the PID adjustment of the position loop and the feed-forward value of the position setting, which we call "speed setting". The difference between the speed setting and the speed loop feedback value is compared with the speed loop PID adjustment (mainly proportional gain and integral processing) and the output is the "current loop setting" mentioned above. The feedback of the speed loop is obtained from the value feedback from the encoder through the speed calculator.
3. Position loop: The input of the position loop is an external pulse (usually, the servo that writes data directly to the drive address is an exception). The external pulse is processed by smoothing filter processing and electronic gear calculation as "position loop setting". This setting and the value of the pulse signal from the encoder feedback after the deviation counter is calculated, after the PID adjustment of the position loop (proportional gain adjustment, no integral derivative link), the combined value of the output and the feed-forward signal of the position setting constitutes the setting of the speed loop mentioned above. The feedback of the position loop also comes from the encoder.  

Ⅱ. Factors Affecting Control

1. The speed loop mainly performs PI (proportional and integral) adjustment, and the proportion is the gain, so we need to adjust the speed gain and the speed integral time constant appropriately to achieve the desired effect.
2. The position loop mainly performs P (proportional) adjustment. For this, we only need to set the proportional gain of the position loop. When you need to adjust the position loop in the position mode, it is best to adjust the speed loop first. There are no fixed values for the parameter adjustment of the position loop and speed loop. It should be determined according to many conditions such as the mechanical transmission connection mode of the external load, the movement mode of the load, the load inertia, the requirements for speed, acceleration, and the rotor inertia and output inertia of the motor itself. The simple method of adjustment is to adjust the gain parameter from small to large, and the integral time constant from large to small within the scope of general experience based on the external load, and set the best value at the steady state value without vibration overshoot.

Ⅲ. The Control Application of The Chassis in The Speed Closed Loop And Displacement Closed Loop

Under normal circumstances, the current closed loop function is completed by the motor drive module, inverter and other controllers in order to output a constant torque. The robot platform consists of motors, encoders, and controllers as power control outputs. The encoder collects the speed and displacement as feedback. Through the PID algorithm, the control speed is more precise and stable, and the change range is reduced, making the chassis travel more accurately. Under the speed closed-loop control, adding a position closed-loop can realize the precise navigation control of the robot chassis and reduce the displacement control error.
The application of three-loop control is more suitable for precise control of mechanical arms, steering gears and other equipment.