DDC Control Loop | Building Automation - Direct Digital Controls are nearly everywhere in commercial and some industrial processes. We have several articles here at High Performance HVAC, which describe the basics of DDC and how DDC works. There is also a page here concerning DDC, where we describe the process of DDC, including the DDC control loop.

We wanted to make the process a little simpler for the layman audience and to make it more helpful for the new HVAC technicians and Control Technicians to learn the process in simpler terms. In the basic control loop, we have input, processing, and output.

DDC Control Loop | Building Automation

To understand Direct Digital Control (DDC) we must understand the basic control loop. Even the most adept HVAC control technicians benefit from their work from going back to the basic control loop to solve problems or break down and understand complex DDC control algorithms. For a basic DDC Controls or building automation control loop we need to understand three things:

DDC Control Loop | Building Automation - Input

The control loop begins with the input sensor or switch. A device that measures something such as temperature, humidity, carbon dioxide, carbon monoxide, or even a switch that closes or opens can be the input signal for DDC. An example of a switch input would be a fire detector alarm or dry contacts closing on a current transformer that senses a motor is energized.

The temperature sensor and other sensors are analog and are configured by the installation crew and start-up technician. These analog signals are scaled for the medium they are sensing, and they either send a 4 to 20 milli-amp signal or a DC signal of 0 to 10 or 2 to 10 volts.

Input comes from a sensor or device into the control. That can be analog or digital. In this step, we are Measuring temperature or C.F.M.’s (any variables) and collecting data. DDC building automation inputs measure a medium or monitor the HVAC systems such as smoke detectors and high/low limit switches. DDC inputs measure temperature, humidity, pressure, current, wattage, and air and water flow, among other things.

These input devices are connected directly to the controller for the next process in the DDC Control Loop.

DDC Control Loop | Building Automation - Processing

Actuator for Chilled Water - the actuator modulates from 0% to 100% based on the program and the input from the input device(s). This is floating control.

The reception of the input signal by the controller. The controller processes this signal based on the program. Depending on the program will depend on the control response to the output device. An output signal can be the output signal to a variable frequency drive (analog) or to dry contacts (open or close the contacts) or many other devices. The control response can be either

• Floating Control e.g. an actuator controlling an air damper or a chilled water valve where the CFM’s are controlled or the GPM is modulated with a VFD based on a pressure set point for the chilled water.
• Two-Position control e.g. a hot water valve that is either open or closed
• The proportional is typically abbreviated by a P and is a response sent out by the controller to the output device
• Proportional plus Integral is typically abbreviated by PI and is a response sent out by the controller to the output device
• Proportional plus Integral plus Derivative is typically abbreviated as PID and is a response sent out by the controller to the output device.

Explanations for PID

It is out of the scope of this article to explain P or PI or PID as it is somewhat complex however when enabled these control responses can smooth out motors and flow until a steady-state is achieved. The closest way to describe PID is to compare it to a thermostat anticipator where the anticipator eliminates or reduces temperature swing by optimizing how the system is controlled.

In the processing, we have a DDC or building automation controller to process information that holds the logic or programming. In this step, the DDC or building automation controller is processing the information from the input device(s) and based on the algorithm, possibly sending an output signal to a device to take appropriate action if necessary. The input device(s) does not need to be hard-wired to the local equipment controller, nor does an output response from the local equipment DDC building automation controller going to affect the DDC building automation controller’s local equipment.

Over a communication trunk, the DDC or building automation controller can receive input signals from distant automation controllers and issue output commands to those same or other distant building automation or DDC automation controllers. It depends on the program and set-up of the entire system in the algorithms of the building automation system as a whole.

DDC Control Loop | Building Automation - Output

We now have Input and Processing, and finally, we need Output to an Output device. That can be either an analog signal or a binary signal. That is where we turn something on or off or modulate something based on the input. The program in the processor of the controller. A good example of this is a VAV box where the tenant is too cold and adjusts the thermostat. The thermostat is the input device and is going to tell the controller it is too cold based on the current setpoint.

The controller processes the request and sends a signal to the VAV Box damper to close a little for reduced airflow while at the same time sending a signal to the hot water valve actuator to modulate open. Depending on the set up the actuator and valve may be floating or two-position, but either way, the valve opens, and hot water is introduced to the hot water coil. The tenant immediately feels the room getting warmer and is satisfied.

Some DDC thermostats are non-adjustable and only sense temperature. The temperature setpoint is controlled by the building manager or engineer in that case.

The actual device is controlled based on what the input is feeding to the DDC controls controller. The controlled device is taking action to maintain the program based on program variables. Output devices can be damper actuators, valve actuators, relays and contactors (electrical and or pneumatics (p-e or e-p), variable frequency or speed drives, compressors, blowers, and pumps.

Example of a DDC Control Loop:

A VAV box is reading 600 C.F.M.’s, and the zone temperature is 68 degrees F. The controller takes this input information and processes it based on pre-programmed set points. In this situation, the zone is too cold, so the DDC controller sends an output signal to the damper actuator to close it (some to maintain a minimum heating C.F.M. set point), energize a heating relay for heat, and (with a parallel box) turn the fan on for airflow.

(For more on VAV boxes see DDC/Building Automation. Because the output device is taking action it is reasonable to assume that our input sensor variables are going to change. The process starts all over again until all set points in the DDC building automation algorithm are satisfied.

In conclusion, we learned that the DDC Control Loop involves Input, Processing, and Output and involves sensors or dry contacts, a controller, and output devices.

DDC Control Loop | Building Automation

Resource: The Fundamentals of HVAC Direct Digital Control