MicroHTR (Part 2)

Part 1, 2, 3, 4, 5, 6, 7, 8, 9

PID is probably one of the most popular type of process controller. You may find many papers on the net describing the origins this system. I can read that PID controllers were developed in automatic ship steering. While the PID controller proved higher performances than helmsmen control, the Navy ultimately did not adopt the system, due to resistance by personnel! An other example of resistance to innovation…

I will not paraphrase the excellent explanations that you may find here and there. You may start reading this easy to understand application note from ATMEL in order to get the full picture from PID. I also recommend the excellent series of posts from Brett BEAUREGARD on PID. The aim of the MicroHTR project posts is to show the impact of the PID parameters on the proposed simple heater project:

Next plot relates to the most simple control: the on/off style! The blue line is the set point, the red line is the read temperature value and the green line is the power applied to the heating resistors.

This type of controller allows quite a large excursion of the read temperature around the set point as shown below.

It is simple and cheap, however, it may not fit applications which require a better temperature control.

Next plot relates to the use of the P term only in the PID equation. The code for the P controller may look like:

double PID(double processVariable)
/* Compute output signal */
	/* Compute error */
	double error = (_computedSetpoint - processVariable); 
	/* Compute output */
	double outputValue = (_Kp * error); 

The more the error (set_point – actual_value), the higher the correction. This type of controller is much more responsive than the previous one

However, the P style controller suffers from a major discripency: it fails to reach the target temperature! Its is precise, but not accurate.

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