Introduction to Control Loops in BAS Part 3


Introduction to Control Loops in BAS Part 3 – Building Automation Systems Training

Introduction to control loops this is the third course in the building control series if you have not already done so please participate in building controls 1 & 2 prior to taking this course. At the completion of this course you will be able to identify the five controller loop responses and you will be able to review the terms associated with controller loop responses. This class is one of a sequence that will provide you with an overview of basic control technology used in buildings so that you can see how it can contribute to energy efficiency if we can control an environmental system. We can tell equipment when to turn on and off, how slow or fast to run and how cool or warm the temperature of the air or water needs to be for an environmental control system or a building automation system.

To work effectively three things must take place data must be measured and provided as input to the system, measure data then can be compared to a set of desired outcomes or instructions and lastly an output is produced based on the measured data to change or maintain the environment in this section of the class.

We will examine the five controller loop responses and review the terms associated with building controls, let’s begin with an explanation of controllers. Controllers are the components of this system that take in the data from the input devices such as sensors or push buttons and process them using programmed logic.

Programmed logic can be thought of as the controller set of standards or instructions for what zone and equipment conditions such as temperature are supposed to be and the mathematical calculations to make the changes to keep zones and equipment conditions at those standards. Sensors get the measurements the controller takes those measurements and compares them with its on-board logic and then it sends a message to a control device.

The method with which the controller interacts with the control device is called the control loop response the control loop response refers to how the controller responds as we just discussed when the controller takes in a measurement from a sensor the controller internally compares that reading with a set of instructions or standards. For instance if the temperature in a room is 26 degrees Celsius or 79 degrees Fahrenheit and the controller has an on-board set of instructions or standards that say this room is supposed to be at 23 degrees Celsius or 74 degrees Fahrenheit the controller would set out to make a change the controller might be connected to a zone damper which is a type of device that looks a bit like a louver and is used to control the amount of air entering a space.

The method by which the controller controls that device is the control loop response there are five ways that a controller can make a response to the system a two-position response a floating response a proportional response also called by its initial P proportional plus integral or PI and proportional plus integral plus derivative also called PID response.

These five responses can be divided into two broader States digital and analog. Let’s look at each one of these responses in more depth beginning with the two position response to position always produces a digital response where the n device is either in an on state or off state the control device is also digital. Floating control is an analog control process it may be used to control either a tri-state device or an analog device.

Let’s look at the tri-state device first the tri-state device can accept one of three input signals clockwise rotation counterclockwise rotation or stationary. In effect the controller can tell a tri-state device to do one of three things increase the control output by driving in one of the two directions for example this might widen a vent opening decrease the control by driving in the other direction.

For example this might narrow a vent opening stay still and maintain the current output conditions this would keep the vent opening the same size.

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