Implementing PID control for a line following robot

The first requirement in implementing PID for a line follower is calculating the error of the robot. To calculate the error we will need to know the current position of the robot with respect to the line. There are a number of ways of knowing this.

A simple approach would be to place two IR sensors on either side of the line. The IR sensors should be tuned to give an output voltage that is promotional to the distance between the line and the sensor. The output can then be connected to the ADC pin of a microcontroller and the error can be calculated.

Though this method may seem simple and easy to implement it has a few drawbacks. Firstly, a robot using this method will have problems following a line whose width is varying. Secondly, the output of these sensors will be highly susceptible to interference from external light. And lastly, for this method to work, you have to ensure that the track is completely flat. Any change in the distance between the sensor and the surface will affect the sensor readings.

A better approach would be to use the traditional sensor array. Using an array of sensors, you can easily calculate the error by knowing which sensor is on the line. Consider you have an array of 10 sensors each placed 1cm apart. When the 7^th sensor from the left detects the line you can be sure that the centre of the robot is 2 cm to the right of the line. Using a sensor array the processor can calculate the error faster as there is no ADC required to be done and thus you can increase the sampling rate. You can also use the robot as a grid solver.

When building a sensor array, there are a few things to keep in mind. Firstly, the more sensors you have in an array, the better will be the performance of the robot, because more sensors translates to more range of error. For line following anything between 6 to 10 sensors (depending on your robot) will be sufficient. The distance between each adjacent sensor determines the resolution of the readings. It is always best to place the sensors as close to each other as possible because your resolution will increase as you place the sensors closer (a resolution of +/- 1cm is always better than a resolution of +/- 3 cm).

The next important thing in implementing PID after the sensors is the writing the code itself. The algorithm for a PID control for line followers would be something like this –

Error = target_pos – current_pos               //calculate error

P = Error * Kp                       //error times proportional constant gives P

I = I + Error                         //integral stores the accumulated error

I = I * Ki                             //calculates the integral value

D = Error – Previos_error       //stores change in error to derivate

Correction = P + I + D

The next step is to add this correction term to the left and right motor speed.

While calculating the derivative, the change in error can be divided by the time taken to get the rate of change of error, but I have skipped this step in my robot, because each loop takes almost the same time to execute.