by Roger Desrosiers
DDC control consists of microprocessor-based controllers with the control logic performed by software. Analog-to-Digital (A/D) converters transform analog values into digital signals that a microprocessor can use. Analog sensors can be resistance, voltage or current generators.
The microprocessor unit (MPU) in the controller provides the computation. Therefore, the term digital in DDC refers to digital processing of data and not that HVAC sensor inputs or control outputs from the controller are necessarily in digital format. Nearly all sensor inputs are analog and most output devices are also analog. In order to accept signals from these I/O devices, A/D and D/A converters are included in the microprocessor-based controller. The figure below shows several inputs and outputs. The microprocessor usually performs several control functions.
DDC provides more effective control of HVAC systems by providing the potential for more accurately sensed data. Electronic sensors for measuring the common HVAC parameters of temperature, humidity and pressure are inherently more accurate than their pneumatic predecessors. Since the logic of a control loop is now included in the software, this logic can be readily changed. In this sense, DDC is far more flexible in changing reset schedules, set points and the overall control logic. Users are apt to apply more complex strategies, implement energy saving features and optimize their system performance since there is less cost associated with these changes than there would be when the logic is distributed to individual components. This of course assumes the user possesses the knowledge to make the changes.
Elements of a Direct Digital Control System
8X DDC Controller: Figure 1
Figure 1 above is a typical state of the art DDC controller in use today.
Any point can be configured through software to be Analog In, Analog Out, Binary In, or Binary Out – no jumpers with Bright on-board LEDs assist in troubleshooting. You can directly connect a normal web browser for simple management. It's high speed communications allow the ultimate in flexibility and snappy response, 3 decimal rotary switches (0-9) allow simple addressing – no hex, no binary and all of the electronics are on one easily replaceable brain board for quick repairs.
All field devices and any logic or calculations associated with those devices are points. The word "points" is used to describe data storage locations within a DDC system. Data can come from sensors or from software calculations and logic. Data can also be sent to controlled devices or software calculations and logic. Each data storage location has a unique means of identification or addressing. A point can be an actuator, a temperature sensor, a control sequence or any other quantity or status that can be monitored or controlled. We recommend naming your points based on their function to make it easier for the operator. For example, if you have a temperature sensor that reads the outside air temperature, name the point “Outside Air Temp.” There are two categories of points: Hardware and Software
Hardware points are points that can be physically wired or connected through a wireless sensor to the terminal strip of a controller. They include field devices such as relays, actuators and sensors. Their function is to transmit data back to the controller or physically carry through a building automated control command.
There are four main types of hardware points. They are analog inputs, analog outputs, binary inputs, and binary outputs. Binary points have only two states such as ON/OFF, OPEN/CLOSE, or START/STOP. Analog points on the other hand, represent a range of measurement such as a temperature of 0°F to 110°F, a pressure of 1psi to 5psi, or a flow rate of 100 CFM to 200 CFM.
Whether a point is binary or analog, it must be either an input or an output. Points that monitor the status of a field device are inputs. Field devices send their condition or quantity to an input on the controller.
Points that control the status of a field device are outputs. The user can either control outputs manually, or allow for automatic control based on schedule, logic, PID, or other software outputs programmed in the building automated control.
Software points include calculations, points of reference, and logic statements. They are intelligent points that are not physically connected to the controller. Instead, they gather data and send commands to hardware points. An example of gathering data is the average supply temperature of all AHUs in the building. An example of sending commands is, "if Outside Air Temp is less than 50°F, then start VAV heat strips.
- The microprocessor
- A program memory
- A working memory
- A clock or timing devices
- A means of getting data in the basic elements of a microprocessor-based (or microprocessor) controller. (Fig. 3)
In addition, a communications port is not only a desirable feature but a requirement for program tuning or interfacing with a central computer or building management system.
Timing for microprocessor operation is provided by a battery-backed clock. The clock operates in the microsecond range controlling execution of program instructions.
Program memory holds the basic instruction set for controller operation as well as for the application programs. Memory size and type vary depending on the application and whether the controller is considered a dedicated purpose or general purpose device.
Dedicated purpose configurable controllers normally have standard programs and are furnished with read only memory (ROM) or programmable read only memory (PROM.)
General purpose controllers often accommodate a variety of individual custom programs and are supplied with field-alterable memories such as electrically erasable,programmable, read only memory (EEPROM) or flash memory. Memories used to hold the program for a controller must be nonvolatile, that is, they retain the program data during power outages.
Fig. 3. Microprocessor Controller Configuration for Automatic Control Applications.
All input signals, whether analog or digital, undergo conditioning (Fig. 3) to eliminate the adverse affects of contact bounce, induced voltage, or electrical transients. Time delay circuits, electronic filters, and optical coupling are commonly used for this purpose. Analog inputs must also be linear zed, scaled, and converted to digital values prior to entering the microprocessor unit. Resistance sensor inputs can also be compensated for lead wire resistance...
Performance and reliability of temperature control applications can be enhanced by using a single 12-bit A/D converter for all controller multiplexed inputs, and simple two-wire high resistance RTDs as inputs.
A/D converters for DDC applications normally range from 8 to 12 bits depending on the application. An 8-bit A/D converter provides a resolution of one count in 256. A 12-bit A/D converter provides a resolution of one count in 4096. If the A/D converter is set up to provide a binary coded decimal (BCD) output, a 12-bit converter can provide values from 0 to 999, 0 to 99.9, or 0 to 9.99 depending on the decimal placement. This range of outputs adequately covers normal control and display ranges for most HVAC control applications. D/A converters generally range from 6 to 10 bits.
The output multiplexer (Fig. 3) provides the reverse operation from the input multiplexer. It takes a serial string of output values from the D/A converter and routes them to the terminals connected to a transducer or a valve or damper actuator.
The communication port (Fig. 3) allows interconnection of controllers to each other, to a master controller, to a central computer, or to local or portable terminals.
Types of controllers:
DDC can be designed for system level or zone level control like that shown below.
Zone-level controllers can be applied to a variety of types of HVAC unitary equipment. Several control sequences can be resident in a single zone-level controller to meet various application requirements. The appropriate control sequence is selected and set up through either a PC for the system or through a portable operator's terminal. The following two examples discuss typical control sequences for one type of zone-level controller used specifically for VAV air terminal units.
VAV sequence of operation
In a pressure independent VAV cooling only air terminal unit application the zone-level controller controls the primary airflow independent of varying supply air pressures. The airflow set point of the controller is reset by the thermostat to vary airflow between field programmable minimum and maximum settings to satisfy space temperatures. On a call for less cooling, the damper modulates toward minimum. On a call for more cooling, the damper modulates toward maximum. The airflow control maintains the airflow at whatever level the thermostat demands, and holds the volume constant at that level until a new level is called for. The minimum airflow setting assures continuous ventilation during light loads. The maximum setting limits fan loading, excessive use of cool air, and/or noise during heavy loads.
System level controllers have more capacity and are more flexible then zone level controllers. System level controllers are used in central chiller and boiler plants, equipment rooms, and built up air handlers. Control sequences usually contain customized programs written to handle the specific application. The application of the controller must allow both the number and mix of inputs and outputs to be variable. The number of inputs and outputs required for the system level controller is usually not predictable.
Programming a DDC Loop
Most DDC systems use tables similar to the one shown below to set up a loop control for each part of the system. From this diagram you can see the loop is separated into several sections. The first section is the controller, which takes in a set point from an operator /programmer. It also takes in a feedback signal from a sensor and sends it to a control algorithm, which compares it to the set point. Any error found is the difference between the set point and the signal from the sensor also called the process variable. The output signal from this area goes through the digital/analog convertor. The corrective signal is sent as an analog signal from the controller to the final control element. (Chilled water/hot water valve etc.) For example, this loop could be controlling the temperature of a hot deck by modulating a hot water valve. The sensor could be an RTD thermostat, which would send back a feedback signal to the controller so the actual temperature in the hot deck can be compared to the set-point. If the set-point is 80* and the RTD says its 78* the controller would determine that the system needs to be warmer.
Simple Control Loop.
In Electronic # 6 we will talk about sequence of operations of various applications and web sites you can look up the gain more knowledge on DDC Controls. In the mean time: Be happy in your work and learn a lot!
Roger J. Desrosiers
About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.