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Intro to PID

Modified 2020-09-03 by Rara Ma

Open Vs Closed Loop Systems

Modified 2020-09-03 by Rara Ma

Open loop system, also known as a non-feedback system, is a continuous system where output does not affect the control action of the input (Electronics Tutorials). Ex: Toaster

A closed loop system, also known as a feedback system, is a system where the control action is based on the output (Electronics Tutorials). Ex: Body thermoregulation

PID Terms

Modified 2020-09-03 by Rara Ma

Process Variable, represented by y(t) : The parameter of the system that is being monitored and controlled.

Setpoint, represented by r(t) : The desired value of the process variable.

Control Variable, represented by u(t) : The output of the controller that serves as input to the system in order to minimize error between the setpoint and the process variable.

Steady-State Value: The final value of the process variable as time goes to infinity.

Steady-State Error: The difference between the setpoint and the steady-state value.

Rise Time: The time required for the process variable to rise from 10% to 90% of the steady-state value.

Settling Time: The time required for the process variable to settle within a certain percentage of the steady-state value.

Overshoot: The amount the process variable exceeds the setpoint, and it is expressed as a percentage.

Example relating to biological system:

Modified 2020-09-03 by Rara Ma

These terms are applicable to a biological system that we rely in. Thermoregulation relies on a negative feedback system.

The process variable: your body temperature.

Setpoint: Regular body temperature is 98.6 degrees F (or 37 degrees C).

Control Variable:

If you are cold, and you have a lower body temperature than usual, your body will generate more heat to restore back to regular body temperature.

If you are warm, and you have a higher body temperature than usual, your body will reduce heat in your body to restore back to regular body temperature.

The General Algorithm

Modified 2020-09-03 by Rara Ma

The error of the system e(t) , is calculated as the difference between setpoint and proces variable.

e(t) = r(t) - y(t)

The controller aims to minimize the rise time and settling time of the system, while eliminating steady-state error and maximizing stability (no unbounded oscillations in the process variable). It does so by changing the control variable u(t) based on the three control terms.