Introduction to Direct Digital Control Systems
Purpose of this Guide:
The purpose of this guide is to describe, in generic terms, the various architectures, hardware components and software associated
with Direct Digital Control (DDC) systems. To accomplish this goal, a generic framework of the various components and configurations
used in current DDC systems has been defined. This framework is used as a yardstick for several DDC manufacturers so readers may
compare the relative features and benefits.
Due to the complexity and proprietary nature of DDC systems, it has become difficult to stay current with the designs, installations,
operation and maintenance of DDC systems. This guide was developed specifically to help building owners and consulting/specifying
engineers with these issues.
What is an Energy Management System?
For the purposes of this guide, an energy management system (EMS) is defined as a fully functional control system. This includes
controllers, various communications devices and the full complement of operational software necessary to have a fully functioning
control system. This guide addresses approximately twenty of the DDC vendors who serve the institutional and commercial marketplace
in the United States. Vendors who supply a complete line of all the necessary hardware and software are included. This guide does
not cover specialty markets (retail grocery, hotels), nor does it cover industrial or process controls.
What is Control?
The process of controlling an HVAC system involves three steps. These steps include first measuring data, then processing the data
with other information and finally causing a control action. These three functions make up what is known as a control loop. An example
of this process is depicted in Figure 1.
Basic Control Loop
The control loop shown in Figure 1 consists of three main components: a sensor, a controller and a controlled device. These three
components or functions interact to control a medium. In the example shown in Figure 1, air temperature is the controlled medium.
The sensor measures the data, the controller processes the data and the controlled device causes an action.
The Figure 1 could be an example of a pneumatic or electronic control system, where the controller is a separate and distinct piece
of hardware. In a DDC system, the controller function takes place in software as shown in Figure 2.
The sensor measures the controlled medium or other control input in an accurate and repeatable manner. Common HVAC sensors are used
to measure temperature, pressure, relative humidity, airflow stateand carbon dioxide. Other variables may also be measured that
impact the controller logic. Examples include other temperatures, time-of-day or the current demand condition. Additional input
information (sensed data) that influences the control logic may include the status of other parameters (airflow, water flow, current)
or safety (fire, smoke, high/low temperature limit or any number of other physical parameters). Sensors are an extremely important
part of the control system and can be the first, as well as a major, weak link in the chain of control.
The controller processes data that is input from the sensor, applies the logic of control and causes an output action to be generated.
This signal may be sent directly to the controlled device or to other logical control functions and ultimately to the controlled
device. The controllers function is to compare its input (from the sensor) with a set of instructions such as setpoint, throttling
range and action, then produce an appropriate output signal. This is the logic of control. It usually consists of a control response
along with other logical decisions that are unique to the specific control application. How the controller functions is referred
to as the control response. Control responses are typically one the following:
- Proportional (P only)
- Proportional plus Integral (PI)
- Proportional plus Integral plus Derivative (PID)
Controlled Device or Output
A controlled device is a device that responds to the signal from the controller, or the control logic, and changes the condition
of the controlled medium or the state of the end device. These devices include valve operators, damper operators, electric relays,
fans, pumps, compressors and variable speed drives for fan and pump applications.
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