Proportional and PID Responses in BAS Part 5 – Building Automation Systems Training
https://www.youtube.com/watch?v=ga3q4e-Zxx4
Welcome to building controls part 5 proportional and PID responses. This is the fifth course in the building control series if you’ve not already done so please participate in building controls one through four prior to taking this course. At the completion of this course you will be able to define the proportional control response, explain the addition of integral and derivative terms to form P I and PID responses and you will be able to explain the appropriate use of each control response. Proportional control is a simple and widely used method of control for many kinds of systems. Proportional integral derivative control also called PID control is somewhat more complex and may be suitable in certain situations. It is important to note that P , I and PID can be used in different applications and using each one has its advantages as we will see there are cases where using all P plus I plus D is not the best application.
Proportional control involves giving a response proportional to the stimulus to say that less technically the warmer it gets in a certain range of temperature the more cool air we put in so for instance looking at this sample proportional response table for a cooling application at twenty two point eight degrees Celsius we might have cool air flowing to a room at one thousand three hundred and sixty cubic meters per hour if the room happens to be in the US at 73 degrees Fahrenheit.
We would have eight hundred cubic feet per minute of cool air however if the temperature were to increase to twenty three point three degrees Celsius or 74 degrees Fahrenheit I might proportionally adjust my airflow to 1760 cubic meters per hour or one thousand thirty six cubic feet per minute each of these steps are at 200 cubic meters per hour or 118 cubic feet per minute so the higher the temperature gets the greater my system responds to it and the more airflow is produced drawing. This table as a chart shows you that the response is proportional to temperature you can think of proportional control as control that follows one simple rule for every degree of temperature difference from the set point increase the airflow by a certain amount. This equation represents the rule output equals degrees from set point x amount of air the controller will apply.
This rule repeatedly by checking the sensor changing the airflow if necessary waiting a fixed time and then repeating as necessary the control can be adapted by adding more terms you can think of this as adding more rules to follow when an integral component is added there will now be two parts in the control calculation the proportional part plus the integral part when a derivative component is added the control calculation will now have three parts simply put our proportional control term has more terms added to it to become this equation.
Let’s summarize some of the information that we’ve learned in this course. Proportional control is a simple control method that provides a response proportional to the stimulus. It is calculated from the difference between the actual and set point an integral term can be added which is driven by the amount of time that the measured variable has been offset from the set point on a graph of actual versus set point over time.
It is calculated from the area under the graph a derivative term can also be added which is driven by the rate of change of the measured variable this is useful to prevent overshoots on a graph of actual overtime.
It is calculated from the slope of the line side which response was appropriate to a slow responding system which response was appropriate for a moderate and fast responding system