Glossary of Industrial Automation

A

Terms	Applicable Categories
a constant read-in system	Counters
absolute	Rotary Encoders
absolute command	Servomotors / Servo Drivers
absolute maximum value	Push Buttons / Indicator Lamps
absolute-value alarm	Temperature Controllers
AC Relay	General Purpose Relays
acceleration time	Frequency Inverters
acceleration/deceleration time	Frequency Inverters
access right	Programmable Controllers
accuracy	Digital Panel Indicators,Signal Converters
accuracy of operating time	Timers
activation	General Purpose Relays
active power	Digital Panel Indicators,Signal Converters
adjuster setting	Photoelectric Sensors
adjustment level	Temperature Controllers
advanced function setting level	Temperature Controllers
Air Cleaning Systems	Air Cleaning / Static Electricity Components
Alarm History	Programmable Terminals
alarm hysteresis	Temperature Controllers
alarm latch	Temperature Controllers
alarm operation	Temperature Controllers
alarm with standby sequence	Temperature Controllers
Alarm/Event Display	Programmable Terminals
alarm/event function	Programmable Terminals
alarm/event settings	Programmable Terminals
Alarm/Event Summary & History	Programmable Terminals
alarm/event summary and history functional object	Programmable Terminals
allowable load resistance	Digital Panel Indicators,Signal Converters
allowable switching time for mode selector input	Safety Application Controllers
alternate	Push Buttons / Indicator Lamps
alternate action	Push Buttons / Indicator Lamps
alternative operation	Level Switches
ambient illuminance	Photoelectric Sensors
ambient operating humidity	Power Supplies
ambient operating temperature	Power Supplies
ambient operating temperature and humidity	Solid-state Relays
Amplifier Unit connector	Photoelectric Sensors
analog data allocation	Field Networks
analog frequency	Frequency Inverters
analog input	Temperature Controllers
analog line
Analog Meter	Programmable Terminals
analog reference	Frequency Inverters
analog signal	Temperature Controllers
annunciator	Temperature Controllers
annunciator	Digital Panel Indicators
anode arc	General Purpose Relays
apparent power	Digital Panel Indicators,Signal Converters
applicable load inertia	Servomotors / Servo Drivers
application condition	Basic Switches
AR area	Programmable Controllers
ASR	Frequency Inverters
AT	Temperature Controllers
AT calculated gain	Temperature Controllers
atmospheric pressure	Pressure Sensors
auto zero function	Counters
auto/manual switching	Temperature Controllers
automatic baud rate detection	Field Networks
Automatic Carrier Frequency Reduction Function	Frequency Inverters
automatic energy-saving operation	Frequency Inverters
Automatic Energy-saving Operation Function	Frequency Inverters
automatic reset	Counters
automatic torque boost	Frequency Inverters
Automation Systems	Code Readers,Field Networks,Programmable Controllers,Programmable Terminals,RFID Systems,Support Software,Support Software & Peripheral Devices,Wireless Components
auto-reset	Emergency Stop Switches,Enabling Components,Presence Detection Sensors,Safety Application Controllers,Safety Door Switches,Safety Limit Switches,Safety Relays,Safety Sensors
auto-tuning	Frequency Inverters
auto-tuning	Temperature Controllers
Auxiliary Area	Programmable Controllers
auxiliary output	Temperature Controllers
average processing	Temperature Controllers
average processing function	Digital Panel Indicators
AVR	Frequency Inverters
Axial Fans	Axial Fans

 

B

Terms	Applicable Categories
backlash	Servomotors / Servo Drivers
backlight	Programmable Terminals
backlight service life	Programmable Terminals
Backplane-free construction	Programmable Controllers
backup operation	Power Supplies
bank	Temperature Controllers
bank function	Digital Panel Indicators
bank switch	Digital Panel Indicators
base frequency	Frequency Inverters
Basic I/O Unit	Programmable Controllers
Basic Switches	Basic Switches
basic system	Programmable Controllers
batch counter	Counters
batch counting function	Counters
battery backup	Programmable Controllers
baud rate	Field Networks
BCD	Digital Panel Indicators
binary data	Wireless Components
bit	Digital Panel Indicators,Signal Converters
bit	Programmable Terminals
Bit Lamp	Programmable Terminals
Bit Slave	Field Networks
Bitmap	Programmable Terminals
bits per second	Field Networks
bits per second	Programmable Terminals
bit-strobe connection	Field Networks
black powder	General Purpose Relays
blanking	Safety Sensors
bleeder resistance	Solid-state Relays
brake	Frequency Inverters
Brake Control Function	Frequency Inverters
braking	Frequency Inverters
braking resistor	Frequency Inverters
braking torque	Frequency Inverters
break contacts	General Purpose Relays
brightness	Programmable Terminals
broadcast communications	Wireless Components
broken-line approximation	Temperature Controllers
Broken-line Graph	Programmable Terminals
brown powder	General Purpose Relays
BSMI Standard	Power Supplies
built-in amplifier	Photoelectric Sensors
built-in SSR output	Temperature Controllers
burnout (protection)	Digital Panel Indicators,Signal Converters
Busoff	Field Networks
Buttons	Programmable Terminals
byte	Digital Panel Indicators,Signal Converters

 

C

Terms	Applicable Categories
cable extension characteristic	Rotary Encoders
Cam Positioner	Counters
Cam Positioners	Cam Positioners
carrier frequency	Frequency Inverters
cascade control	Temperature Controllers
cathode arc	General Purpose Relays
Centronics interface	Field Networks
certification of conformance to technical standards	Wireless Components
chameleon lighting	Push Buttons / Indicator Lamps
chattering	General Purpose Relays
CIO Area	Programmable Controllers
city water	Level Switches
clearance distance	Limit Switches
clock pulse	Programmable Controllers
C-mode command	Programmable Controllers
Code Readers	Code Readers
coherency	Field Networks
coherer effect	General Purpose Relays
coil inductance	General Purpose Relays
cold junction	Temperature Controllers
cold junction compensation	Temperature Controllers
cold junction compensation	Digital Panel Indicators
Comm. Auto-return	Programmable Terminals
Command Button	Programmable Terminals
COMMAND INPUT mode	Counters
Commercial Switching	Frequency Inverters
common mode voltage	Digital Panel Indicators
Communication Setting	Programmable Terminals
communications error history monitor	Field Networks
communications error output setting	Field Networks
Communications Unit	Programmable Controllers
comparative output	Pressure Sensors
comparator	Field Networks
comparison output pattern setting	Digital Panel Indicators
compensating leadwire	Temperature Controllers
complementary output	Rotary Encoders
Complete Link Method	Programmable Controllers
Compo Com/DaikanYama	Programmable Controllers
CompoBus/S	Field Networks
Componet	Field Networks
compound contact	General Purpose Relays
CompoWay/F protocol	Temperature Controllers
condensate	Level Switches
conductance	Level Switches
conductivity	Level Switches
conduit size	Limit Switches
connection method	Digital Panel Indicators
Consecutive Line Drawing	Programmable Terminals
constant torque characteristics	Frequency Inverters
constant value control	Temperature Controllers
contact	Limit Switches
contact capacity of output	Level Switches
contact erosion	General Purpose Relays
contact film	General Purpose Relays
contact form	General Purpose Relays
contact gap	General Purpose Relays
contact operation monitor	Field Networks
contact protective circuit	Emergency Stop Switches,Enabling Components,Presence Detection Sensors,Safety Application Controllers,Safety Door Switches,Safety Limit Switches,Safety Relays,Safety Sensors
contact rating	Push Buttons / Indicator Lamps
contact relay output	Temperature Controllers
contact resistance	Basic Switches,Limit Switches,Push Buttons / Indicator Lamps
contact shape	Basic Switches
contact spring	General Purpose Relays
contact wear	General Purpose Relays
Contents Display	Programmable Terminals
continuous proportion	Temperature Controllers
Control Components	Cam Positioners,Counters,Digital Panel Indicators,Programmable Relays,Temperature Controllers,Timers
control cycle	Temperature Controllers
control mode	Temperature Controllers
control mode	Frequency Inverters
control output assignment	Temperature Controllers
Control Output Unit	Temperature Controllers
Controller Link	Programmable Controllers
Controller Link Unit	Programmable Controllers
convert	Programmable Terminals
cooling coefficient	Temperature Controllers
cooling fan	Frequency Inverters
cooling method	Power Supplies
copy function	Frequency Inverters
core	General Purpose Relays
COS connection	Field Networks
count value setting method	Counters
Counter with Backlight	Counters
Counters	Counters
CPS	Programmable Controllers
CPU Bus Unit	Programmable Controllers
CPU Bus Unit Area	Programmable Controllers
CPU Rack	Programmable Controllers
CPU Unit	Programmable Controllers
creepage distance	General Purpose Relays
creepage distance	Limit Switches
crossbar contact	Basic Switches
crosstalk characteristic	General Purpose Relays
CT	Temperature Controllers
CT input	Temperature Controllers
cumulative counter	Field Networks
cumulative operation time	Frequency Inverters
current	Power Supplies
current bank	Programmable Controllers
current output	Temperature Controllers
current overflow	Temperature Controllers
Cvsupport card	Programmable Controllers
Cvsupport pack	Programmable Controllers
CX-One	Programmable Controllers
CX-Programmer	Programmable Controllers
CX-Protocol	Programmable Controllers
cycle control	Temperature Controllers
cycle time	Programmable Controllers
cyclic connection	Field Networks
cyclic error	Timers
cyclic refresh	Programmable Controllers
cyclic task	Programmable Controllers

 

D

Terms	Applicable Categories
D action	Temperature Controllers
D constant	Temperature Controllers
damping control	Servomotors / Servo Drivers
Data Block Table	Programmable Terminals
Data Blocks	Programmable Terminals
Data Link Area	Programmable Controllers
data link table	Programmable Controllers
data log	Programmable Terminals
Data Log Graph	Programmable Terminals
Data Log Setting	Programmable Terminals
Data Loggers	Data Loggers
data register	Programmable Controllers
data tracing	Programmable Controllers
data type	Programmable Controllers
date object	Programmable Terminals
DC braking	Frequency Inverters
DC injection braking function	Frequency Inverters
DC Relay	General Purpose Relays
dead band	Temperature Controllers
dead band	Digital Panel Indicators
dead zone	Photoelectric Sensors
deceleration stop	Frequency Inverters
deceleration stop time	Frequency Inverters
derating curve	Power Supplies
derivative action	Temperature Controllers
derivative time	Temperature Controllers
deviation	Temperature Controllers
deviation alarm	Temperature Controllers
device monitor	Programmable Terminals
DeviceNet	Digital Panel Indicators
DeviceNet Area	Programmable Controllers
dielectric strength	Basic Switches,Limit Switches,Push Buttons / Indicator Lamps
dielectric strength	Temperature Controllers
dielectric strength	Timers
dielectric strength	Solid-state Relays
difference input	Digital Panel Indicators
different frequency	Proximity Sensors
Differential coil type	Proximity Sensors
differential time	Photoelectric Sensors
diffuse-reflective model	Photoelectric Sensors
diffusion alloy contacts	General Purpose Relays
digit drive system	Counters
digital	Field Networks
Digital Indicators	Digital Indicators
digital line
Digital Operator	Frequency Inverters
Digital Panel Indicators	Digital Panel Indicators
digital signal	Digital Panel Indicators
DIP SWITCH MONITOR mode	Counters
direct opening force	Safety Limit Switches
direct opening mechanism	Emergency Stop Switches,Enabling Components,Presence Detection Sensors,Safety Application Controllers,Safety Door Switches,Safety Limit Switches,Safety Relays,Safety Sensors
direct opening position	Safety Limit Switches
direct operation	Temperature Controllers
direct temperature controller connection	Programmable Terminals
directional angle	Photoelectric Sensors
disconnected line detection	Field Networks
disconnection detection function	Level Switches
Displacement Sensors	Displacement & Measurement Sensors
display color	Programmable Terminals
display device	Programmable Terminals
Display for the programming console function	Programmable Terminals
display range overflow	Temperature Controllers
display section	Programmable Terminals
distance-settable model	Photoelectric Sensors
distilled water	Level Switches
distribution reservoir	Level Switches
disturbance overshoot adjustment	Temperature Controllers
diversity	Wireless Components
diversity	Emergency Stop Switches,Enabling Components,Presence Detection Sensors,Safety Application Controllers,Safety Door Switches,Safety Limit Switches,Safety Relays,Safety Sensors
DM Area	Programmable Controllers
DOF	Safety Limit Switches
dog	Limit Switches
dog angle	Safety Limit Switches
DOP	Safety Limit Switches
Double Break Switch	Emergency Stop Switches,Enabling Components,Presence Detection Sensors,Safety Application Controllers,Safety Door Switches,Safety Limit Switches,Safety Relays,Safety Sensors
double snap action	Enabling Components
double throw	General Purpose Relays
double throw contacts	General Purpose Relays
double-winding latching	General Purpose Relays
down counter	Counters
DOWN mode	Counters
DPDT basic switch	Basic Switches
drain	Level Switches
Drip-proof Switch	Basic Switches
drive frequency	Push Buttons / Indicator Lamps
drive mode	Frequency Inverters
dual counter	Counters
durability shock	Timers
durability vibration	Timers
during regeneration	Frequency Inverters
DXF data	Programmable Terminals
dynamic characteristics	Temperature Controllers

 

 

Four steps on energy

Four steps on energy: "Industrial buildings are responsible for 31% of global energy consumption..."

SAMPLE PLC LADDER

Input comparison instruction

Input comparison instructions compare two values (constants and/or the contents of specified words) and create an ON execution condition when the comparison condition is true. Input comparison instructions are available to compare signed or unsigned data of one-word or double length data.

The input comparison instruction compares S1 and S2 as signed or unsigned values and creates an ON execution condition when the comparison condition is true. Unlike instructions such as CMP (Compare) and CMPL (Compare Long), the result of an input comparison instruction is reflected directly as an execution condition, so it is not necessary to access the result of the comparison through an Arithmetic Flag and the program is simpler and faster.

The input comparison instructions are treated just like the LD, AND, and OR instructions to control the execution of subsequent instructions.

 

long term timers

Example 1:
Long-term Timers
The following program examples show two ways to create long-term timers with standard TIM and CNT instructions.

Two TIM Instructions
In this example, two TIM instructions are combined to make a 30-minute timer.

 

TIM and CNT Instructions

In this example, a TIM instruction and a CNT instruction are combined to make a 500-second timer.TIM 0001 generates a pulse every 5 s and CNT 0002 counts these pulses. The set value for this combination is the timer interval × counter SV. In this case, the timer SV would be 5 s x 100 = 500 s. With this combination, thelong-term timer's PV is actually the PV of a counter, which is maintained through power interruptions.

                                                    long term timer        

 

PLC to servo driver

i have been asked about the connection from PLC to servo driver. <!--[if gte mso 9]> Normal 0 false false false MicrosoftInternetExplorer4 <![endif]--><!--[if gte mso 9]> <![endif]--> The connection is not as hard as you would have been imagined.
First thing that is important in doing the wiring is that we need to identify the power requirements of our PLC, our inputs and our outputs. Please take note that some PLCs are powered by AC power and transistor output PLCs are powered by DC power. As for the case of PLC that have pulse outputs, it is powered by DC power ( i dont know any of AC).

 

The basic wiring from PLC to servo driver is as the diagram below,

servo motor control

 

it is very easy to do a positioning control using a servo motor with the PLC. all you need to do is to configure the correct settings of the PLC, construct a correct wiring and write the program. if you buy a servo motor with the driver, make sure you hv the correct wiring. what im providing here is purely from the plc side.

i will sum it up into 3 steps,

1: configure the correct settings in your PLC.

make sure your plc has pulse outputs! you need two pulse outputs for a single axis. pulse can be output from the plc either by CW/CCW mode or pulse + direction mode. this mode must be of the same mode you setup in your servo motor amplifier. make sure also u set the built in inputs for encoder feedback. this is a must so that u know how many pulses has the motor moved.

2: correct wiring.

u also need to output the servo RUN signal to on the motor and Reset signal to reset any alarm. just for extra information, not all alarms can be reset from the reset signal, for some alarms, u need to reboot the servo driver.

for the input to the PLC from the servo driver, u need the feedback from the AC servo motor encoder. if you are using open collector, one feedback input is enough. it will count the Z phase from the encoder and pass it back to the plc to do the pulse count.

u also can tap into the PLC the 'pulse output complete' signal from the servo driver. this is important to make sure all pulse have been supplied and the motor is ready for next move.

3. write the program.

this is an example of how you can control a servo motor using ladder logic. this example shows how u can give a continuous supply of pulses in which the pulse will stop when the program instruct it to stop. as in the case of the example, the instruction SPED 0 will make the continuous pulse stop. in the example also it will touch on how u can supply independent pulse in which u instruct the PLC to supply a predetermine amount of pulse so that the motor make the linear system moves a certain distance. you can have a look at the sample program and better explanation HERE!!

Elements of a DDC System <4>

Multiple Dial LAN Support
In a system’s architecture, the local sites have the ability to call an alternate communication interface, if the primary is not available (Figure 17).

One-Way Dial System Architecture
One-way dial systems, Figure 18, are typically used to enable system owners to access their systems from a remote location, such as their home. It is used where auto-dial monitoring is not required. It can also be used by the installation and service company or by the commissioning authority to troubleshoot and program from remote locations. One-way dial can also be used to dial into remote site LANs or sub-networks.

Two modems are required, one located at the remote computer and one at the system site. Typically, the DDC operating software must be installed on the remote computer.

Communication
Communication between two different devices controlling equipment, requires a common protocol, a common communication speed and known data formatting. Vendors build their devices around these criteria, so communication between devices by the same manufacturer is routine.

Third Party Interfaces
In many installations, it is desirable for a proprietary building DDC system to communicate with other proprietary DDC systems controlling pieces of equipment. Examples would include a building DDC system and a chiller DDC system (Figure 19) or a fume hood DDC system. Communication between the two systems will require an interface or gateway, due to different proprietary protocols, communication speeds and data formatting.

The gateway or interface translates protocol between the two proprietary systems. The proper operation of the gateway is dependent on the continued use of the specific revised levels of software on both systems. It typically requires the support of the manufacturer at the corporate level to implement and cooperation between the manufacturers. In addition, the costs can vary widely.

Protocols
In the DDC world, there are the three classifications of protocols: closed protocol, open protocol and standard protocol.

A closed protocol is a proprietary protocol used by a specific equipment manufacturer. An open protocol system uses a protocol available to anyone, but not published by a standards organization. A standard protocol system uses a protocol available to anyone. It is created by a standards organization.

Open Systems

An open system is defined as a system that allows components from different manufacturers to co-exist on the same network. These components would not need a gateway to communicate with one another and would not require a manufacturer specific workstation to visualize data. This would allow more than one vendor’s product to meet a specific application requirement.

The sole use of an open or standard protocol does not guarantee that a DDC system will be an open system. A manufacturer has the ability to use open or standard protocols, yet create a closed system, thus continuing a building owner’s dependence on a single manufacturer. This can be accomplished by using unique communication speeds, unique data formatting and by not adopting the full range of an open protocol.

Note: A building owner/engineer should thoroughly research a manufacturer’s claim of an open system.

BACNET
BACNET is a standard protocol published by a standards organization (American Society of Heating, Refrigerating and Air-conditioning Engineers or ASHRAE). It is a specification for a protocol. DDC vendors create a communication protocol that complies with this specification.

BACNET is a relatively complex standard. The standard defines protocol implementation conformance statements (PICS) that define different levels of compliance. A given vendor may or may not support the level required for a given application. In other words, a vendor could meet a very low level of compliance and be BACNET-compatible. The key question is, “At what level?”

In Figure 20 the chiller control unit’s DDC will communicate with the building DDC system if each has a BACNET gateway and their PICS match.

Native BACNET
If a vendor states their product is native BACNET, they are using the BACNET protocol in lieu of a proprietary protocol on their LAN. In Figure 20, a native BACNET building system would be able to communicate to the chiller control DDC with one less gateway.

Overlay Systems

An overlay system is a high-end workstation that communicates with multiple manufacturers’ proprietary EMS systems. An overlay system supplier creates drivers to “talk” to the different systems. The vendors must have a cooperative relationship and revision control is important for continued successful use. The workstation typically displays data, allows manual control and setpoint changes, and handles alarms and messaging. Any detailed editing of the control sequence will still require knowledge of the underlying proprietary software.

LON
The Echelon Corporation has created an open protocol that uses a standard processor and a set of standard transceivers, which allows components from different manufacturers to co-exist on the same LAN. The protocol is available to anyone and is called LONTALK. A unique chip is required for any device that uses LON. Standard network variable formats have been established to allow the transfer of data from one device to another regardless of origin.

Presently, various vendors are competing to become the defacto standard for the network database structure. The network database is a map of the components and the relationship of the data moving between them. The operator workstation needs this structure to visualize the data.

Software suppliers providing the software for the operator workstation may be independent of those providing the software for the database structure and the EMS vendors. 

Elements of a DDC System <3>

Controller Classification
Controllers can be categorized by their capabilities and their methods of communicating (controller-to-controller). In general, there are two classifications of controller - primary control units and secondary control units

Primary controllers typically have the following features:
 

Real-time accurate clock function

Full software compliment

Larger total point capacity

Support for global strategies

Buffer for alarms/messages/trend & runtime data

Freeform programming

Downloadable database

Higher analog/digital converter resolution

Built-in communication interface for PC connection.

Secondary controllers typically have the following features:

Not necessarily 100% standalone

Limited software compliment

Smaller total point count

Freeform or application specific software

Typically lower analog-to-digital converter resolution

Trend data not typically stored at this level

Typical application is terminal equipment or small central station equipment.

Operator Interfaces
The next critical element in the system architecture is an operator interface. Operator interfaces are required to:

See data

Program the system

Exercise manual control

Store long term data

Provide a dynamic graphical interface.

There are five basic types of operator interfaces. They include:

Desktop computers which act as operator workstations

Notebook computers which act as portable operator workstations

Keypad type liquid crystal displays

Handheld consoles/ palmtops/ service tools

Smart thermostats

Desktop computers are centralized operator workstations where the main function is programming, building and visualizing system graphics; long term data collection; and alarm and message filtering.

Notebook computers may connect to the LAN through a communication interface that stands alone or is built into another device. The notebook computer connected to the LAN at a particular level may not have the same capability as a computer connected to the LAN at a higher level.

Keypad liquid crystal displays typically are limited to point monitoring and control. They may have some limited programming capability, such as changing a set point or time schedule.

Handheld consoles, palmtops and service tools are proprietary devices that connect to primary controllers or secondary controllers. Typically they allow point monitoring and control, controller configurations (addressing and communication set-up), and calibration of inputs and outputs.

Smart thermostats are sensors with additional capabilities. They connect to secondary controllers and have a service mode to allow for point monitoring, control and calibration. They also have a user mode that allows point information to be displayed, setpoint adjustment and an override mode.

PC/Network Interface
The communications interface shown in the Figure 11 is a communication interface device. It provides the path between devices that do not use the same communications protocol. This includes computers, modems and printers.

It may be a stand-alone component or it may be built into another device as shown in Figure 12. 

 

Each communications interface on Figure 12 may:

Translate protocol

Provide a communication buffer

Provide temporary memory storage for information being passed between the network and the external PC, modem or printer (mailbox function)

Larger System Architectures
When systems become larger than the capacity of a single sub-network, a higher level of architecture is added to allow the use of multiple sub-networks.

The site LAN wide area network or WAN is used to connect multiple sub-networks and site computers. Multiple sub-networks can be connected to a single site LAN/WAN that allows information sharing between devices on different sub-networks (Figure 13). There may be a limitation on the number of site computers. The site LAN/WAN may include routers if TCP/IP is used. If no routers are used, the protocol can be totally proprietary. If TCP/IP is used, the EMS site LAN/WAN can be the information system backbone within the facility or between facilities.

Multiple site computers can be added to the site LAN/WAN. They can connect the site LAN/WAN via a communications interface, which may be a router. Site LAN/WAN computers can send and receive information from the entire system. Information can be received by each of the site computers, but can not be subsequently shared from one computer to another. Sub-network computers may only be able to see their own sub-network.

Site LANs allow multiple computers to communicate with each other. They may use commercially available computer network software and hardware. Messages, alarms and other data can be re-routed to other computers on the primary site LAN. Information stored in other computers can be remotely accessed. This includes graphics, programming and stored trend and operational data.

Combined Components

  Some vendors combine multiple functions into a single device. In the following system architecture, Figure 14, the communication interface is built into the primary controller. A peer-to-peer LAN or sub-network is connected directly to the device.

In Figure 15, the key component in the system consists of a communication interface, a primary controller and an interface to the secondary polling network.

The addition of a site LAN allows a system to gain size in terms of the number of devices that are served, but in some applications, the location of the devices, rather than the number of devices, is the bigger challenge. In this situation, modem-based communication is used to expand the geography of the system.

Auto-Answer/Auto-Dial System Architecture
In auto-answer/auto dial systems, a specialized communication interface is substituted which introduces a modem and phone lines into the standard architecture. These communication interfaces are made with built-in modems or use external commercial modems. Auto-answer/auto-dial configurations are used to provide monitoring and access to remote buildings. They are used where traditional direct-wiring methods are impractical; and where central site monitoring is desired; or where remote access to controllers is desired.

In an auto-answer/auto-dial system, the central communications interface may call the individual sites or vice versa. Information and data can be passed to and from the layer above the central communications interface (Figure 16).

The auto-answer/auto-dial LAN architecture is typically used by installations with multiple facilities where control and monitoring needs to be centralized. Multiple LANs are used to maintain the groupings of devices, or to separate controllers into defined groups. 

Elements of a DDC System <2>

Controller

A control loop requires a sensor to measure the process variable, control logic to process data, as well as calculate an instruction, and a controlled device to execute the instruction. A controller is defined as a device that has inputs (sensors), outputs (controllable devices) and the ability to execute control logic (software) (Figure 7).



LAN Communication

Communications between devices on a network can be characterized as peer-to-peer or polling. On a peer-to-peer LAN, each device can share information with any other device on the LAN without going through a communications manager (Figure 8).

 

 

The controllers on the peer-to-peer LAN may be primary controllers, secondary controllers or they may be a mix of both types of controllers. The type of controllers that use the peer-to-peer LAN vary between manufacturers. These controller types are defined later in this section.

In a polling controller LAN, the individual controllers can not pass information directly to each other. Instead, data flows from one controller to the interface and then from the interface to the other controller (Figure 9).

The interface device manages communication between the polling LAN controllers and the higher levels in the system architecture. It may also supplement the capability of polling LAN controllers by providing the following functions: clock functions; buffer for trend data, alarms, messages; and higher order software support.

Many systems combine the communications of a peer-to-peer network with a polling network. In Figure 10, the interface communicates in a peer-to-peer fashion with the devices on the peer-to-peer LAN. The polling LAN-based devices can receive data from the peer-to-peer devices, but the data must flow through the interface.

Elements of a Direct Digital Control System

Handbook for DDC and application 

   
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.

Direct digital controls (DDC) data can be classified three different ways - by data type, data flow and data source.

Data Type
Data type is classified as digital, analog or accumulating. Digital data may also be called discrete data or binary data. The value of the data is either 0 or 1 and usually represents the state or status of a set of contacts. Analog data are numeric, decimal numbers and typically have varying electrical inputs that are a function of temperature, relative humidity, pressure or some other common HVAC sensed variable. Accumulating data are also numeric, decimal numbers, where the resulting sum is stored. This type of data is sometimes called pulse input.

Data Flow
Data flow refers to whether the data are going into or out of the DDC component/logic. Input points describe data used as input information and output points describe data that are output information.

Data Source
Points can be classified as external points if the data are received from an external device or sent to an external device. External points are sometimes referred to as hardware points. External points may be digital, analog or accumulating and they may be input or output points. Internal points represent data that are created by the logic of the control software. These points may be digital, analog or accumulating. Other terms used to describe these points are virtual points, numeric points, data points and software points.

Global or in-direct points are terms used to describe data that are transmitted on the network for use by other controllers. These points may also be digital, analog or accumulating.

Analog input points typically imply an external point and represent a value that varies over time. Typical analog inputs for HVAC applications are temperature, pressure, relative humidity, carbon dioxide and airflow measurements. Typical analog outputs include control signals for modulating valve positions, damper positions and variable frequency drive speed.

Typical digital inputs for HVAC applications represent the status (example: whether or not the motor is running) of fans, pumps, motors, lighting contactors, etc. A temperature high limit is considered a digital input because, although it is monitoring an analog value (temperature), the information that is transmitted to the controller is a digital condition (whether or not the temperature has exceeded a defined value). Digital outputs are typically motors or other devices that are commanded “on” or “off.” Digital outputs include fans, pumps, two-position (solenoid) valves, lighting contactors, etc.

A “true” analog output (voltage or current) is a varying DC voltage or milliamp signal that is used to drive a transducer or controlled device. Another type of analog output is pulse width modulation (PWM). PWM is accomplished by monitoring a timed closure of a set of contacts. The amount of time the contacts are closed is proportional to a level of performance for the controlled device.

Software Characteristics
There are basically three common approaches used to program the logic of DDC systems. They are line programming, template or menu-based programming and graphical or block programming.

Line programming-based systems use Basic or FORTRAN-like languages with HVAC subroutines. A familiarity with computer programming is helpful in understanding and writing logic for HVAC applications.

Menu-driven, database or template/tabular programming involves the use of templates for common HVAC logical functions. These templates contain the detailed parameters necessary for the functioning of each logical program block. Data flow (how one block is connected to another or where its data comes from) is programmed in each template.

Graphical or block programming is an extension of tabular programming in that graphical representations of the individual function blocks are depicted using graphical symbols connected by data flow lines. The process is depicted with symbols as on electrical schematics and pneumatic control diagrams. Graphical diagrams are created and the detailed data are entered in background menus or screens.

Architecture
System architecture is the term used to describe the overall local area network or LAN structure, where the operator interfaces connect to the system and how one may remotely communicate to the system. It is the map or layout of the system.

The network or LAN is the medium that connects multiple intelligent devices. It allows these devices to communicate, share information, display and print information, as well as store data. The most basic task of the system architecture is to connect the DDC controllers so that information can be shared between them.
 

Introduction to Direct Digital Control Systems (2)

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.

Sensor
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.

Controller
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 controller’s function is to compare it’s 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:

Two-Position
Floating
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.
 

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.

Intended Audience

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.