The main purpose of the HVAC Heating, Ventilating and Air Conditioning system is to achieve clean indoor air quality and human comfort thermal comfort for the most desired comfortable and productive environment. There are many HVAC systems a designer or owner has the option to select based on the factors such as the type of the building, architectural layout, location, shape, surrounding climate, occupancy, envelop, level and frequency of activities, and the system operational function.
In addition to the above base factors that a HVAC system is expected to be selected upon, control accuracy and precision, energy management, system performance and efficiency, initial and operational cost, and finally, feasibility (short and long term rebound positive effect) are of the owner's and designer's critical concerns.
The HVAC control system is the key solution for the above concerns. Different types of control systems have been adopted for different applications since the invention of the HVAC control system. Conventional control systems (mechanical, electromechanical, hydraulic, and pneumatic based control systems) have played an important role in the development of the most powerful and accurate computer-based control system that is the Direct Digital Control (DDC) for HVAC control and energy management purposes.
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This paper will discuss the importance of the DDC application in the HVAC industry, the key elements, communications standards, operation, advantages, and finally the promising future of the DDC.
Control in machines is a very essential activity. For optimum results to be achieved there should be a good control setup put in place that will monitor the whole process of a specific system function. There are different types of control processes, they range from the manually operated systems, semi-automatic and fully automated systems. In ancient times most of the HVAC devices were operated manually and the operation personnel had to be within the vicinity of the systems to check that all the activities were progressing well. The environmental conditions were checked manually by use of thermometers and then the findings recorded. If the conditions were found to have gone beyond the acceptable limits, HVAC system equipments were turned on or off manually in order to rectify the problem. This process was tedious and most industrial processes went wrong due to its inefficiency. However, the trend has greatly changed due to the introduction of automated systems. The Direct Digital Control is one of the fully automated systems which require very little human intervention. The DDC system is very precise and accurate in its operation. The DDC is the result of many developments done on the HVAC control systems. It is the most challenging control system against the powerful pneumatic system that is still in use for many industrial control purposes. Pneumatic control technology is now becoming gradually outdated due to the rapid day-to-day advanced technology development devoted for DDC domain.. One of the major differences made by this developed technology is the introduction of the central controllers which are programmable and this makes it easy to customize the codes for a specific purpose. A DDC System not only uses digital components but there are analog parts which are incorporated into the systems and then the outputs fully digital.
As previously mentioned, the manual HVAC control systems have been slowly replaced with Direct Digital Control System. It took several decades from the time the first HVAC control system was developed to the time when a fully functional DDC system was implemented. Comparing with other conventional control systems, the DDC have different architectures and components which are used in its operation. The designs, installation procedure and the maintenance of DDC keep on changing as technology advances. This calls for more innovative minds and innovations in order to combat the changes.
This paper discusses the use of DDC and the developments that have accompanied it since its invention up to date and the improvements that its use has brought in the HVAC Industry. In the study of the DDC system this paper will cover the DDC elements, how the different parts communicate with each other as stand alone or distributed systems, and analyse the DDC system in details and consider the advantages it has over other conventional control systems. The DDC benefits in terms of energy saving and environment conservation will also be covered, and finally, the most current up to date technology and the future of the DDC applications will be discussed.
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1. History of HVAC Control Systems
The legacy of DDC can be traced back to the times when the inventions and discoveries were made by Nikolay Lvov, Michael Faraday, James Joule and many other scientists and pioneers in the fields of HVAC and control. These inventions were propagated by the industrial revolution. The idea of computation was started in 1940s when the first personal computers were invented. The computers were very big and could occupy up to a whole floor of a building. The computers used numerous vacuum tubes in their design. These computers generated a lot of heat. However, the vacuum tubes were soon replaced with transistors which meant that they no longer generated as much heat as before. In the early 1950s most of the operations were manual and the first electronic supervisory control system was made in 1954 by the Hughes Aircraft Company. In 1959 the first digital computer based control system called the Ramo-Woldridge was used. It had the capacity of 103 process measurements and 14 control outputs of which five were DDC outputs. With time, the idea of control was implemented in the HVAC industry. After the manual control, HVAC has gone through many control systems (electro-mechanical, hydraulic and pneumatic) at different ages of developments until the time when computer manufacturers started incorporating better technology with cheaper prices and computer control systems became more accessible. The invention of the DDC has been made possible with the improvements in the HVAC Control systems. As a result, better control has been achieved and thus improving the rate of productivity and precision hence boosting profits to the firms that are using the new technology. 
2. An Overview on DDC
The DDC is the heart of an efficient HVAC system , it has became the latest and the most recently used system for HVAC controls after the pneumatic and electromechanical control systems. It relies on microprocessor based controllers with the control logic in the system being performed by a software. These controllers are the core elements of the system, they handle extensive digital/analog input/output communication between sensors, probes, sub-controllers, and finally the controlled element which could be an actuator that adjusts the process variable (flow, temperature, level, or pressure), and allow for a feedback or feed forward signals to further adjust the desired process set-point. This whole process is reported in a real-time manner to the Central workstation for further coordination with the other building's controllable systems to achieve integration based on the pre-programmed parameters.
Based on the standardized network protocols, the DDC is able to be integrated with other sub-LANs, and can supervise, monitor, control, adjust and record the illumination, electric power control, HVAC, security and observation, access control, fire systems, lifts, and other engineering systems based on a specified, programmed event sequence.
The central diagnostic capabilities have made the use of DDC very effective and widely acceptable. The software updates make their efficiency improve day by day.
3. Control Loops
Controlling HVAC systems involve three steps which include: Measuring data, processing the data with other information and Causing a control action.
When controlling a system, we can either apply the open loop controller or the closed loop controller. The open loop controller will only use the current state and the model of the system to determine the output whereas the closed loop controller uses both the current output and the current input into the system to determine the next state. 
When the two controllers are used, we achieve a closed loop transfer function.
The control loop consists of three main components namely: Sensor, controller and controlled device. The three are represented in the diagrams below.
Open loop controller
Closed loop controller
Closed loop transfer function
A typical control loop
Controlled Medium Air flow
heating water returns
C - Controller
S - Sensor
CD - Controlled Device
HC - Heating Coil
The control loop shown in figure 4 consists of three main components
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The three components interact so as to control a medium. In this example, the air temperature is the controlled variable and the sensor measures the controlled variable whereas the controller compares the data fed from the sensor with the set point then corrects the drift by an algorithm process and sends an adjustment signal to the controlled device which causes an action for further adjustment on the controlled variable.
In a typical DDC system, the controller function is incorporated in a computer software as shown in figure 5 below.
A typical DDC control loop
Output of DDC
input into DDC
INPUT LOGIC OUTPUT
4. Direct Digital Control Elements
The DDC relies on three major functions:
a) A measurement element (Sensor, probe, Transmitter, Transducer)
b) An error detection element (Controller, PCU)
c) A final control element (Motor/Piston Actuator, VFD, VSD, Relay)
The sensor is used for measuring different types of controlled variable accurately. There are several HVAC sensors which are used for measuring different scenarios. Common HVAC sensors are used for measuring controlled variables such as: air flow, temperature, humidity, CO2 and pressure which usually affect the environmental condition of a given place. The measuring process is done repeatedly so that in case there is a change in the conditions, it can be easily detected. After the detection, the new signal is sent to the controller (via a transmitter which converts analog to digital or vice versa) to rectify the problem. There is additional input information which is the sensed data. This data influences the control logic and then the desired action is taken by the controller. 
As mentioned before, the DDC controllers are programmable and can handle extensive digital/analog data process from inputs (sensors, transducers and transmitters) that typically measure temperature, flow, humidity, pressure or level, and outputs to final controlled devices to adjust a process variable based on a preset parameters, the controllers also receive feedback signals from inputs again to further adjust signal command errors for best results based again on the set points.
The controller is responsible for processing data from the sensor then applies the logic of the control and later causes an output action to be generated. The generated signal can either be sent to an actuator for direct adjustment on the controlled variable or to a sub-controller for further signal adjustment before generating an output to the controlled device. The main function of the controller is to compare the input from the sensor with some set conditions which could include: throttling range and action or some set-points and then produce desired output signal.
The DDC controllers are programmable and can act as a stand-alone controller without interference from the central DDC software, this is an important feature as where a network failure occurs, the equipments can still be controlled by the programmable stand-alone controller and a peer-to-peer network will be established automatically as the stand-alone microprocessors have the ability to communicate with each others, however, the DDC LAN will not perform as feasible as at its initial designed purpose.
Control response is the way a controller functions and can take any of the following: Two-position, Floating, Proportional (P), Proportional plus Integral (PI), Proportional plus Integral plus Derivative (PID)
4.2.1 Two-Position Control
This method compares the value of a variable input with the instructions and then generates a digital output also known as (two-position output). These instructions could involve the definition of an upper and a lower limit. The value of the output will change progressively as the input crosses the limits. There are no laid standards for determining these limits as they are usually user defined. This type of control are used for simple control loops or in limit controllers.
4.2.2 Floating Control
This is a type of control that produces two possible digital outputs which depends on a fluctuation in a variable input. In this case, one output will increase the signal while the other
output will decrease the signal to the controlled device. It also involves an upper and a lower limit. This type of control also does not have standards for determining the limits. 
4.2.3 Proportional Control
This type of response produces an out put that has a variable change which is proportional to a varying input. The relationship between the input and the output is linear. In this response, there is just one unique value which corresponds to a full travel of the controlled device. There is also just one unique value corresponding to zero travel or no travel. In proportional Control the following terms are used.
Control point - Value of the controlled variable in a real-time fashion.
Offset - difference between the control point (value obtained) and the desired condition.
4.2.4 Proportional Plus Integral (PI) Control
This control involves the measurement of the offset (difference between the control point (value obtained) and the desired condition) over some given time. The offset is then integrated and a final adjustment made to the output signal.
4.2.5 Proportional plus Integral plus Derivative
This method adds a predictive element to the process. Apart from the proportional and integral component, this method requires that the slope is also computed. It is a precision control response. Its application is usually labor intensive. 
4.3 Controlled device
The controlled device is the part of the control systems that responds to the modified signal from the controller. It receives the controller signal and transforms it into a mechanical action, then applies the required adjustment on the controlled variable. The controlled device may include: valve, variable frequency drive, damper, electric relays, solenoid, fans and variable speed drives.
There are two controlled mediums:
A process which is maintained at predetermined values (controlled variable)
A process affected by the control system maintaining a controlled variable at a specified value or range (manipulated variable)
5. Data types and points
They describe the storage locations within a DDC system. The data can be extracted from the sensors, or the software. Each data storage location has a unique way of identification or address. 
5.2 Data type
The DDC mainly processes the following input-output data:
Digital: (discrete data or binary data which have values of either a 0 or a 1)
Analog: (numeric and have varying inputs) or accumulating data (also numeric and is where the resulting sum is stored). 
Digital inputs are dry contacts from a control device, analog inputs are voltage and current signals that measure variables such as humidity, pressure, level or flow form sensing devices and converted to percentage. Digital outputs are of 1 or 0 binary that either stop or start equipments via a relay, analog outputs are voltage or current signals that control a process variable controlled devices such as valves, motors or dampers.
5.3 Data Source
The data source represents the origin of data that is being used in the control process in relation to the location of the process.
For effective communication to take place between two different controlling equipments, the following are required: a common protocol for communication platforms, a common communication speed to avoid conflict and a known data formatting for compatibility issues.
This calls for an inclusion of a gateway or an interface because of the different proprietary protocols, communications speeds or the type of data formatting used. 
The protocols which are used in DDC include:
Open Systems Protocol - They are available to anyone and are not published/ defined by a standards organization
Standard Protocol - they are also available to anyone and they are created by a standards organization.
Closed protocol - these are only used by a specific equipment manufacturer.
6.1 DDC LAN-WAN- Internet Configuration
Different types of equipments (mechanical, electrical) are connected to microprocessors which exchange data between each others and all controlled by a central DDC software (LAN Configuration).
This software and microprocessors can communicate to other similar LAN configuration in other buildings (WAN configuration).
The WAN configuration can be connected to, and accessed from a web browser via compatible and standardized set of communication protocols such as: BACnet, ARCNET, TCP/IP (internet configuration).
The DDC system can be configured as independent (localized) closed-system or open-system based on accessibility and, monitoring and controlling options required by a group of buildings managed by a single company (centralized), or a single property to be monitored and controlled by its own (localized).
6.2 Open Systems
When using open systems, components from different manufacturers are used in the same network. They do not require a gateway for communication there is no specific standard that is used to visualize the data.
This is a non-proprietary open protocol proposed by a partnership of facilities management, network manufacturers and users which is published by ASHRAE (American Society of Heating, Refrigeration and Air-Conditioning Engineers) and adopted by ANSI (American National Standards Institute). This ANSI/ASHRAE 135-2008 (Data Communication Protocol for Building Automation and Control Networks)  is relatively complex and it standardizes the communication protocols between networks that control and monitor the HVAC systems for LAN, WAN and web-based access control. The standard has been updated severally so as to fit the rapid development of communication and automation technologies.
DDC BACnet based LAN or WAN can be accessed, controlled and monitored from remote locations via the Internet trough a centralized energy management system which is able to collect system information from different buildings. A gateway receives information in data form from the buildings networks connected to it via central servers for each building in a secured manner, this enables the DDC central operating station to monitor and control the use of energy according to the pre-determined parameters on a real-time basis from remote locations via the web browser.
BACnet works in conjunction with ARCNET, ARCNET (Attached Resource Computer Network) is an Ethernet-based set of protocols that govern the flow of data within the BACnet LAN (Local Area Network) and nodes.
BACnet and ARCNET are the most recent DDC and BMS (Building Management System) communication technologies and are being implemented for the DDC to be a web-accessed control system from remote workstations.
6.4 Overlay Systems
This is a high end workstation which is able to communicate with multiple manufacturers Proprietary systems. It displays data and allows for manual control and changes in set-points.
DDC controllers can share information through a data bus when they are networked together. When there are different DDC data networks which are linked together they can be controlled from a shared platform. 
7. The DDC vs. Pneumatic Control (Advantages)
DDC systems have several advantages over the convectional pneumatic control systems. The equipments are capable of performing control functions, energy management and system diagnostic functions. The energy management systems (which DDC belongs to) have analog and digital inputs from sensors and then process the data. These systems have more accurate control functions than any other convectional control systems. (Ruys, 1990).
The best method to illustrate the Advantages of a DDC system is to compare it with another HVAC control system, the best comparable control system is the conventional pneumatic control system as it is still being used widely in some industrial HVAC control facilities.
The following table can best illustrate the most advantages of the DDC system over the pneumatic system in different major categories.
Some of the advantages of DDC are as shown in the table below:
Category to be compared
Direct Digital Control
-High initial cost
-Cost decreases when additional loops required
-Feasibility due to energy saving and long term pay-back effect
-Lower initial cost
-Cost increases when additional loops required
-When complex control is required, cost increases significantly
Multi-loop controlling compatibility
-Ease to program complex sequencing
-Full PID control
-Control precision and accuracy
-Difficulty in accommodating complex controlling
-Proportional control limitation
-Single loop controlling
-Ease of adding new controls
-Ease of defining new control strategies at central
-Controllers are programmable
-Retrofit or addition of new loops require new or different controllers, recalibration, re-piping and rewiring
-Friendly user interface control software
-data storage and printing option
-Ease of retrieve process history
-Ease of changing set-points and operation parameters from a central operation machine
-Control operation is done at local control panels
-Control loops can be operated at a centralized control panel but frequent local inspection should be done for calibration and set-points error
-Ease of defining new functions by an operator
-New functions require more labor and equipments
Cost during life cycle
-Less cost of maintenance
-Ease of modification
-Less cost of control loops addition
-Retrofit and expansion requires additional costly controllers and equipments
-High energy saving due to accuracy in starting-up/shutting-down HVAC equipments based on programmable parameters and sequences
-Ability to perform control optimization
-Lack of prompt responding to programmable parameters and frequent devices failure
-Uncontrolled and continuous equipments run-time/excessive power consumption when communication failure occurs between local and centralized controllers,
8. DDC Benefits for the energy saving/management and its contribution to the environment
The use of DDC system has a great positive impact on the environment. This is attributed to the fact that it offers energy saving and manages techniques which in turn contribute greatly to environment conservation through limiting power consumption and the resulting emissions.
Some of the benefits of DDC in energy savings include:
The DDC has the ability to perform algorithm process to predict energy consumption trends and resets itself to meet the pre-set parameters based on the equipments capacity and the rate of occupancy.
The DDC system allows optimal start with pre-scheduled program. Equipments are started up in a specific pre-programmed event sequence to prepare a building's environment, this is done by a comparison process between the outside and inside temperature based on a set-point to start up the equipment. The space temperature is brought up further to another set-point based on either partial or full building occupation.
This process achieves the best possible thermal comfort and preserves energy at the same time as the reverse action is done when the building is unoccupied.
Less components are required to form a complete fully functional DDC system comparing to any other control system, also the coordination that exists within the system reduces the work involved in operating the HVAC system.. A decrease in the number of components means a decrease in cost and energy consumption.
A fully automated system as in the case with the DDC makes the control work more efficient. Whenever a fault is detected, it can be rectified in real time thus preventing energy loss in the system.
The DDC applies energy efficient routines and strategies like the demand monitoring and controlling based on pre-set parameters. Energy trends can be easily monitored and controlled. This reduces over-consumption of energy.
Once energy loss has been prevented in a system, the environment is also conserved in return. This is because the environment plays a major role in the provision of energy. When a system requires a lot of energy, the environment will have to be degraded in the process of trying to meet the ecological footprint requirements of a particular ecosystem.
Additional feature of the DDC is that it can provide healthy, safe and secured environment by measuring the level of CO2 concentration and shutting down the equipments where such a case reaches a pre-set point for safety measures, an alarm will be generated to the operator for further action.
The DDC can accommodate a fire alarm system and generates warning and alarm messages at different sub-LAN levels to the main workstation, also alarms can be set to be sent through pagers, automated telephone system or e-mails at different facilities management levels.
9. Future Applications of DDC and Expectations
The use of DDC has been on the rise in the recent past and the trend is likely to increase further. Currently, most of the control actions done in a manufacturing firm are automated. The use of DDC is still being explored further and in future it is likely to be used in more systems that require automation.
Elements of electronic, microcomputers and telecommunication devices are being incorporated into the DDC systems so as to improve their efficiency further. There is a new series of smaller and more intelligent control devices which are being manufactured. This means that in future we are likely to encounter the use of smaller devices which are more intelligent than the ones that we are seeing presently in the market.
In future we are also likely to see the whole problem of physical size limitations and costs of acquiring the gadgets overcome due to the increasing rate of technology advancement. Most of the convectional control systems that are available in the market and some DDC systems have a large physical size and their cost is also high. With the new developments, we are likely to encounter systems which are small in size and at the same time have a cheaper price.
We are likely to encounter the development of a protocol that will allow interoperability of platforms in real time without any form of hindrance. The standardized communication protocols devoted for building automation and control networks are in continuous development and frequent addendums to the AINSI/ASHRAE standard 135-2008 are in place .
The earliest HVAC systems were operated and controlled manually, consequently, significant energy loss, inaccuracy, frequent maintenance and human intervention were all affecting the projected performance and operation functions of those systems. As a result, extensive researches, inventions and developments were done to create a control system that reduces the above issues. Innovations led to the introduction of the early electro-mechanical control systems, further development done on the HVAC control system gave birth to the conventional pneumatic control system. Pneumatic control systems were very common in the early days and one could think that it was the end of innovation. The rapid development of the micro-processor industry made it possible to have a computerized and a fully automated system.
This brought rise to a new procedure of innovation which resulted into the invention of DDC. Since necessity is the mother of innovations, the DDC systems were developed as a result of this necessity of a fully automated control system.
The use of DDC results into less energy consumption, proper energy management, accuracy, precision and less labor cost. This eventually led to a proper energy usage and conservation.
The use of DDC has impacted greatly on the environment in terms of conservation and energy saving. The convectional control systems consumed a lot of energy and a lot of resources were used in order to ensure their effective operation. The use of DDC on the other hand incorporates circuit boards (micro-processors) which are much smaller and their energy consumption level is very low. This reduces their maintenance cost and many people switch to using them because of such advantages that they offer.
the DDC is able to analyze and adjust the trend of energy usage, which in turn, saves the energy, reduces its cost, and preserves the environment by limiting emissions and impacts caused by the required energy generation needed for the HVAC and other utilities.
The DDC has became the core of a good feasible high-efficient HVAC system, it is now the most recent High-Tech energy management system that manages a building performance to the maximum desirable pre-determined set of parameters which able to control, monitor, adjust, save and record mostly all of the building facilities and utilities when integrated with all of the compatible building's Sub-LANs.
The advantages which come along with the use of DDC has made it possible to gain popularity among other control processes which are in use. The DDC systems are automated and very accurate that makes them very effective and efficient as far as control purposes are concerned.
The use of DDC systems not only makes work more convenient to the operators but it ensures that the production process, human comfort and productivity, and energy saving factors are at their best level and properly monitored and maintained.
The use of DDC is being explored further and more intelligent systems are likely to be put in operation. In future the systems will function without human intervention at all. This will help to reduce the amount of labor used and at the same time preserve the environment as automated machines are known to be very efficient in terms of energy consumption.
The rapid development of the data exchange protocols and standards in the building automation systems field enhances the DDC system's reliability and compatibility to communicate with other communication protocols, and different types of network protocols will be integrated to form one standardized communication medium, which allows a DDC controlled facility to be accessed, monitored, modified and controlled at different network layers and gateways from different location on the earth by mean of the continuous development and addendums on the BACnet-based LAN, WAN and World Wide Web protocols.