Control Sensors Part 2 in BAS – Building Automation Systems Training
https://www.youtube.com/watch?v=D562a38-8uM
This is second in a series of building controls courses offered. It is recommended that you participate in building controls one an introduction to building controls before taking this course for best viewing results.
we recommend at the completion of this course you will be able to describe the categories and purpose of sensors commonly found in a building control system list types of devices within each sensor category for example within temperature. There are sensors for outside air room or zone temperature and equipment temperature and identify common problems with placement of sensors.
Building control systems are important fast it’s in any buildings energy management plan they help avoid waste and save money.
Sensors are vital components incorporated into building control systems. We must measure what we wish to control we need to have a way to make measurements accurately and repeatedly. Sensors measure the data that the controller uses to make decisions based on its set of programs standards and set points sensors are the first step of control. this course provides an overview of the various sensors integrated in a building control system and looks at the variety of designs and need for correct placement with closed-loop control sensors have two purposes first they need to measure current conditions to let the control system know what it needs to make a change.
Second they need to measure the effect of those changes to tell the control system when it no longer needs the change or perhaps reduce the intensity of that change.
sensors tell us how successful the control system is in making environmental changes for instance when the office is suddenly populated with people who are introducing heat into their environment and turning on lights computers copiers and making coffee the heat increases in that room a temperature sensor in that area would detect that the control system would need to increase cooling to that area once the temperature is back to the programmed set point.
The system depending on its setup could either stop cooling or decrease the amount of cooling going to that area the control system is always staying aware of that environment because of that one temperature sensor.
Sensors measure the conditions and provide a check to make sure changes take place because of this sensors can be a weak link in the chain of control if the control system is getting a signal from a faulty sensor.
The control system makes wrong decisions sensors must be installed in a manner that allows them to effectively measure the controlling variable they have to work and they have to be accurate. Sensors give their measurements to controllers that process that data how does the signal get from the sensor to the controller. The communication from the sensors to the controller is electrical it can be a contact closure a variable DC voltage a variable milliamp current or a variable resistance as such there is an electrical circuit between the sensor and the controller. The firmware in the controller is designed to interpret changes in the electrical inputs that it receives from the sensors since controllers get the data from their sensors from a small change in an electrical signal.
Too much distance between the controller and the sensor will cause errors in the measured variable some errors can be accommodated by making adjustments in software but there will always be specific guidelines on how far a sensor can be from the controller for accurate control for sensing temperature.
There are two primary types of sensor thermistors and temperature transmitters thermistors are constructed to produce a variable resistance as a function of temperature as the temperature changes the resistance changes in a nonlinear manner the controller will interpret these resistance changes. To determine the actual temperature there are many different types of thermistors some increase resistance when the temperature goes up while others decrease resistance when the temperature increases this is known as positive or negative temperature coefficient. The range of resistance around 25 degrees Celsius or 77 degrees Fahrenheit is important in the selection of the proper thermistor for a building control system.