Summary
Chapter 11 has been an introduction to the ideas behind controls. This is a vast field and we have only provided a glimpse of the subject. A more technical and detailed introduction to controls is available as a Self-Study Course in this series Fundamentals of Controls.
The chapter started off with some general discussion on control types: self- powered, electric controls, pneumatic controls, electronic (analogue electronic), and direct digital controls. Each of these types has a niche where it is a very good choice but there is a general trend towards DDC controls. We then considered a very simple electric control of a two-element hot water heater to show how controls can be considered in a logical way. Next we introduced the control loop and the difference between open loop control (no feedback) and closed loop control (with feedback). The parts of a control loop that you should be able to identify are: setpoint, sensor, controller, controlled device, and controlled variable.
To illustrate the main issues with modulating controls, we had you imagine pouring water into a glass. The ideas illustrated were
Proportional control is control in which the control action increases in proportion to the error from the setpoint
Offset is the change of apparent setpoint as the control action increases in a proportional controller
Gain the ability of the controller to make a large change in control signal
Overshoot is the result of applying too large a control signal and being unable to reduce it in time to prevent overshooting the control point Speed of reaction is the time it takes for the controller to initiate a significant
change
Having considered the standard control loops we went on to consider the four main types of DDC points:
Digital/Binary Input: a circuit such as a switch closing
Digital/Binary Output: providing power to switch a relay, motor starter, or two-position control valve.
Analogue Input: typically a signal from a temperature, pressure or electric current sensor.
Analogue Output: providing a variable signal to a valve, damper or motor speed controller, often via a transducer that changes the low power signal to a pneumatic or electric power source with the necessary power to drive the valve or damper.
Having introduced the four main point types, we introduced the concept of using a point identification scheme, then we considered a very simple example of a sequence of operations which are the logical instructions for the DDC controller to execute, to provide the required control. The required information to specify a DDC system control was then illustrated with a single air handler. The list of control points and schedule of operations is always required, but the schematic can be omitted, though doing so can lead to misunderstandings at the time of installation.
As well as accuracy, a major advantage of DDC is the ability to record data and either use it for more intelligent control or as information for the operator.
Finally we considered DDC architecture, introduced BACnet and interoperability and listed the pros and cons of DDC.
Bibliography
1. ASHRAE Self Directed Learning Course Fundamentals of Controls
2. ASHRAE Standard 135-2001 A Data Communication Protocol for Building Automation
and Control Networks
3. ASHRAE/IESNA Standard 90.1-2001 Energy Standard for Buildings Except Low-Rise
Residential Buildings
4. ASHRAE Guideline 13-2000 Specifying Direct Digital Controls Systems