Improve the Steering of a Two-Wheel Vehicle Using a Closed-Loop Control Algorithm
This example shows how to implement a closed-loop control algorithm to make a two-wheel LEGO® MINDSTORMS® EV3™ vehicle drive straighter.
In a vehicle using independent wheel control, applying the same power to each wheel generally does not result in the vehicle moving straight. This is caused by mechanical and surface differences experienced by each of the wheels. To reduce deviation in the vehicle heading, a better approach is to use a closed-loop controller which adjusts the power applied to two motors based on difference in their rotations. One such controller is a well-known proportional-integral-derivative (PID) controller.
PID control is a basic control loop feedback mechanism. The controller minimizes the difference between the measured and the desired value of a chosen system variable by adjusting the system control inputs.
This example demonstrates the control algorithm implementations, first with no feedback control (Open-Loop Control), and then with feedback control (Closed-Loop Control) based on P controller.
This example requires extra hardware:
- Two EV3 Large Motors
Task 1 - Set Up Hardware
1. Build a vehicle using two motors to control two independent wheels. Connect one motor to output port 'A' and the other to output port 'B'. For example, you can build a vehicle similar to the one described in the printed building instructions in the education core set.
2. Communicating with a moving vehicle is easier with a wireless connection than with a USB cable. Therefore, we recommend setting up WiFi or Bluetooth communications, as described in Getting Started with MATLAB Support Package for LEGO MINDSTORMS EV3 Hardware example.
Task 2 - Open and Run Open-Loop Control MATLAB Script
1. Open the open-loop control script template.
The code sets two motors to same speed and leave them unchanged during execution.
2. Run script.
Click Run button to run the open-loop control script. The execution time is 10 seconds defined in
EXE_TIME = 10
3. Observe deviation with the open-loop system.
The script specifies the same speed for both wheels. However, mechanical and environmental conditions make the wheels to rotate at different speeds, causing the vehicle to deviate from a straight path.
Task 3 - Open and Run Close-Loop Control MATLAB Script
1. Open the close-loop control script template.
The code reads the encoders in each wheel, calculates the proportional difference between the rotation speeds of each wheel, and compensates for that difference by adjusting the speed of one motor.
2. Run script.
Click Run button to run the close-loop control script. The execution time is 10 seconds defined in
EXE_TIME = 10
3. Observe the deviation with a closed-loop system.
Observe that, with the closed-loop feedback control system, the vehicle moves straighter than when it was using the open-loop control.
Task 4 - Other Things to Try
1. Change the initial speed setting and adjust the P parameter accordingly
SPEED = 20 P = 0.01
to make vehicle move straight.
2. Refine the control algorithm with integral and derivative parameters.
This example demonstrate the implementation of motor control for two-wheel EV3 vehicle. You learned that:
- Open-loop control does not ensure straight driving in a vehicle with independently-powered wheels.
- Closed-loop control uses the difference between two encoder outputs to synchronize the rotation speed of the two wheels.