How to Implement Synchronized Multi-Axis Motion Control with STM32 and PID

Implementing Synchronized Multi-Axis Motion Control Using PID Algorithms with STM32 Timers and Incremental Encoders for a CNC Milling Machine

This tutorial will guide you through the process of implementing synchronized multi-axis motion control in a CNC milling machine using PID algorithms. We will utilize STM32 timers and incremental encoders for precise control. The focus will be on setting up the system, calibrating the PID parameters, and ensuring synchronized movement across axes.

Prerequisites

• Basic understanding of C programming
• Familiarity with STM32 microcontroller development
• Knowledge of PID control theory
• Access to a CNC milling machine with multiple axes
• Incremental encoders for position feedback

Parts/Tools

• STM32 microcontroller (e.g., STM32F4 series)
• Incremental encoders (suitable for your CNC machine)
• Motor drivers for controlling stepper or servo motors
• Development environment (e.g., STM32CubeIDE)
• Power supply suitable for motors and controllers
• Connecting wires and breadboard (for prototyping)

Steps

1. Set Up the Hardware
   • Connect the incremental encoders to the STM32 GPIO pins.
   • Wire the motors to their respective drivers and connect the drivers to the STM32.
   • Ensure all components are powered appropriately.
2. Initialize the STM32 Environment
   • Open STM32CubeIDE and create a new project for your STM32 microcontroller.
   • Configure the clock settings to ensure accurate timing.
   • Set up GPIO pins for encoder inputs and motor outputs.
3. Implement Timer Interrupts
   • Set up timer interrupts for periodic updates of the control loop.
   • Use the following sample code to initialize the timer:

   
   void TIM_Config(void) {
       __HAL_RCC_TIM2_CLK_ENABLE();
       TIM_HandleTypeDef htim2;
       htim2.Instance = TIM2;
       htim2.Init.Prescaler = 8399; // Adjust for 1ms tick
       htim2.Init.Period = 999; // 1 second
       HAL_TIM_Base_Init(&htim2);
       HAL_TIM_Base_Start_IT(&htim2);
   }
               

4. Read Encoder Values
   • Implement a function to read encoder values using interrupts:

   
   void HAL_GPIO_EXTI_Callback(uint16_t GPIO_Pin) {
       if(GPIO_Pin == ENCODER_PIN) {
           // Read encoder value
           encoder_position++;
       }
   }
               

5. Implement the PID Controller
   • Define the PID parameters and implement the control logic:

   
   float Kp = 1.0, Ki = 0.1, Kd = 0.01;
   float previous_error = 0, integral = 0;
   
   float PID_Controller(float setpoint, float measured) {
       float error = setpoint - measured;
       integral += error;
       float derivative = error - previous_error;
       previous_error = error;
       return (Kp * error) + (Ki * integral) + (Kd * derivative);
   }
               

6. Control Motor Movement
   • Use the output from the PID controller to adjust motor speed:

   
   void Control_Motors(float pid_output) {
       if (pid_output > 0) {
           // Move motor forward
       } else {
           // Move motor backward
       }
   }
               

7. Synchronize Multi-Axis Motion
   • Implement a master control loop that reads all encoders and uses the PID output to synchronize movements:

   
   void Control_Loop(void) {
       float setpoint_x = 100; // Desired position
       float setpoint_y = 100; // Desired position
       float measured_x = Read_Encoder_X();
       float measured_y = Read_Encoder_Y();
       
       float output_x = PID_Controller(setpoint_x, measured_x);
       float output_y = PID_Controller(setpoint_y, measured_y);
       
       Control_Motors(output_x);
       Control_Motors(output_y);
   }
               

Troubleshooting

• Encoder Not Reading: Check connections and ensure that the interrupt routine is correctly set up.
• Inconsistent Motor Movement: Adjust PID parameters (Kp, Ki, Kd) for better control. Use a tuning method (Ziegler-Nichols) if necessary.
• System Overheating: Ensure that the motors are not overloaded and check power supply ratings.

Conclusion

Implementing synchronized multi-axis motion control in a CNC milling machine using PID algorithms with STM32 is a complex but rewarding task. By following this guide, you should have a functional setup that allows for precise control of multiple axes. Continuous tuning and testing will ensure optimal performance.