Module 4 Lecture Outline: Introduction to PID
- History of the first PID controller:
- It was used for automatic ship steering
- It was the first “governor design”
- The goal was stability not general control
- It was based on information on current error, past error and the rate of changes taking place.
- Examples of using data for past, present and future corrections
- Losing money
- Making money
- Heating a home
- PID Controllers
- Proportional-Integral-Derivative Controller
- Control loop feedback
- Used in Industrial Control Systems
- Calculate the error value of a desired set point
- Minimizes the error by adjusting the process control outputs
- PID Controller Algorithm
- 3 separate parameters
- Called 3 term control
- P= Proportional: Depends on the present error rate
- I= Integral: This is the data from the accumulation of past errors
- D=Derivative: Prediction of future errors based on the rate of change of the variable
- The weighted sum of these 3 actions are used to adjust the process via a control element
- Examples of a controlled element
- Control valve
- Damper
- Power supplied to a heating element
- An example of a closed loop element
- The Process
- A container with water needs to be a desired temperature
- Hot and cold water are mixed to create the desired temperature
- The temperature is adjusted by mixing hot and cold water until the temperature stabilizes
- Process variable: Sensed water temperature
- Manipulated variable: input of water and output of water, controlled by the PID controller
- Error: The difference in measured temperature and the set point. (Is the output water too hot or too cold?)
- Step by Step Process
- Measure the temperature of the water. (PV)
- Calculate the error between measured temperature and desired temperature
- The controller determines how much change is need for the taps to adding hot and cold water. (MV)
- Taps can be adjusted from really cold to really hot
- If the water is not heating up fast enough, the PID Controller may try to speed up the process. More hot water is added. This is called the Derivative action.
- As more and more hot water is added, and time goes by, this is an example of Integral Control.
- The controller measures how close the measurements are to the Set Point when the system is stable or instable.
- The controller may dampen or hasten future disturbances to control the facet temperature.
- Tuning
- Methods Used:
- Manual Tuning
- No math calculations required
- This is done online
- Requires experienced personal
- Ziegler-Nicholas Method
- Proven method
- Done online
- Uses some trial and error processes
- It is a very aggressive tuning process
- Software Tools
- Provides consistent tuning
- Online or offline
- Can use Automated Design System Controls
- May include valve and sensor analysis
- There is some cost and training involved
- Good process models available
- There is some math involved
- Limitations of PID Controllers
- Relies on feedback systems with constant parameters
- PID Controllers should not be used alone but with other instruments
- Examples of PID Controlled Environments
- Gravity Drained Tanks: Open Loop
- Gravity Drained Tanks: Closed Loop