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Description
T100 MX2 PLC
Triangle Research International, Inc. |
An electronic controller is a piece of electronic equipment that receives incoming electric signals and uses preprogrammed logic to generate electronic output signals based on the incoming signals. While electronic controllers can be implemented for any application that involves inputs and outputs (for example, control of a piece of machinery in a factory), in a security application, these controllers essentially act as the system's "brain," and can respond to specific security-related inputs with preprogrammed output responses. These systems combine the control of electronic circuitry with a logic function such that circuits are opened and closed (and thus equipment is turned on and off) through some preprogrammed logic. The basic principle behind the operation of an electrical controller is that it receives electronic inputs from sensors or any device generating an electrical signal (for example, electrical signals from motion sensors), and then uses its preprogrammed logic to produce electrical outputs (for example, these outputs could turn on power to a surveillance camera or to an audible alarm). Thus, these systems automatically generate a preprogrammed, logical response to a preprogrammed input scenario. The three major types of electronic controllers are timers, electromechanical relays, and programmable logic controllers (PLCs), which are often called "digital relays." Each of these types of controllers is discussed in more detail below. Timers Timers use internal signals/inputs (in contrast to externally-generated inputs) and generate electronic output signals at certain times. More specifically, timers control electric current flow to any application to which they are connected, and can turn the current on or off on a schedule pre-specified by the user. Typical timer range (amount of time that can be programmed to elapse before the timer activates linked equipment) is from 0.2 seconds to 10 hours, although some of the more advanced timers have ranges of up to 60 hours. Timers are useful in fixed applications that don't require frequent schedule changes. For example, a timer can be used to turn on the lights in a room or building at a certain time every day. Timers are usually connected to their own power supply (usually 120-240 V). Electromechanical Relays and PLCs In contrast to timers, which have internal triggers based on a regular schedule, electromechanical relays and PLCs have both external inputs and external outputs. However, PLCs are more flexible and more powerful than are electromechanical relays, and thus this document will focus primarily on PLCs as the predominant technology for security-related electronic control applications. Information on electromechanical relays is included for completeness. Electromechanical Relays
10A Electromechanical
Cube Relays,
Automation Direct.com |
Electromechanical relays are simple devices that use a magnetic field to control a switch. Voltage applied to the relay's input coil creates a magnetic field, which attracts an internal metal switch. This causes the relay's contacts to touch, closing the switch and completing the electric circuit. This activates any linked equipment. These types of systems are often used for high voltage applications, such as in some automotive and other manufacturing processes. PLCs As noted above, PLCs are the technology of choice in most current electronic control applications. The PLC is a physical box holding an input/output unit and a central processing unit (CPU). The PLC works by analyzing inputs vs. its internal programmed logic. Depending on the state of the inputs, the PLC will respond with a specific output. PLCs can receive (input) and transmit (output) various types of electrical signals, and can control and monitor practically any kind of low-voltage mechanical or electrical systems. Several example applications are provided below:
- Example 1: PLCs can be used to activate security equipment based on an input signal. For example, PLC inputs can be wired to any kind of sensing device, such as motion sensors inside a building, glass break sensors placed on windows or doors, vibration sensors for very sensitive areas of a building, or any other sensors that user has deployed to secure the perimeter. An input from any one of these sensors (for example, a signal emitted from a glass break sensor when glass is broken) can generate a predetermined output signal from the PLC (for example, the input from a glass break sensor at an office window can trigger the PLC to send a signal to a surveillance camera near that window, activating the camera to begin recording activity near the window).
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Example 2: PLCs can also be used to control system processes. For example, an alarm signal input from a chlorine sensor could generate an automatic electric output signal that could activate a pump to add chlorine into a system. (While this scenario could occur, this example would more likely be controlled by a more complex SCADA system. For more information on SCADA systems, see the SCADA Product Guide).
PLC Inputs Inputs can be from devices that are hardwired directly to the PLC, or they can be from wireless devices (see discussion of data communications methods under Attributes and Features below). Any piece of equipment that can generate an electric signal may be input into a PLC. For security applications, typical inputs may be signals from intrusion sensors, motion detectors, process alarms, or card reader systems. PLC Outputs Outputs from these control devices can be connected to many different types of systems, such as alarms, lights, a video surveillance system, automatic door locks, etc. When a signal is sent from the controller, operating voltage is supplied to the output equipment, activating it. For example, an input signal from an intrusion sensor in a control room could generate an output signal that supplies power to a visual surveillance camera located in the control room, completing the electrical circuit, activating the camera, and allowing operators to view the interior of the control room. CPUs As with a standard desktop computer, all of the inputs to a PLC are processed in the CPU. The CPU reads the converted input signal, executes the user-specified logic program stored in the memory, and then generates appropriate output signals to be sent to the switching devices. The CPU consists of a digital processor, memory, and a power supply (24 VDC or 220 VAC). The digital processor executes the internal logic of the PLC. Increasing the processor speed increases the responsiveness of the PLC (i.e., higher processor speed equates to quicker response to a given input). Processor speed is measured in megahertz (MHz). Current CPUs typically run at 2000 MHz, although more advanced and expensive CPUs can run at more than 3000+ MHz. The CPU's memory, which is measured in bytes (typically gigabytes or megabytes) temporarily stores the operating instructions of whatever application is currently being used by the CPU (this is similar to a person focusing on one task and putting it "in the front of their mind," while retaining the ability to do other tasks by keeping them "in the back of their mind.") The larger the memory capacity, the more efficiently the CPU can execute its application, and thus adding more memory would increase the processor speed and increase the responsiveness of the PLC.
These components are prepackaged in the CPU and they do not need to be purchased separately; however, individual CPUs may have different processor speeds and memory capacity, and users can choose different CPU packages depending on their needs.
The CPU must have a power supply. The amount of power required depends on the size of the PLC. For smaller PLCs, the power supply is built into the unit. For larger and more complex PLCs, the power supply is external. It should be noted that the CPU power supply typically powers only the PLC and does not provide power to either the inputs or the outputs; therefore, inputs and outputs must have their own power sources to function. Attributes and Features As discussed above, PLCs are the predominant electronic control mechanisms in the marketplace, primarily because they are more flexible and more powerful than electromechanical relays. The two features that make PLCs the controller of choice for most current applications are their ability to be programmed to accept complex decision logic, and their ability to be integrated with wireless data communications. These two factors are discussed in more detail below. Programming Logic PLCs can be programmed using advanced programming language that allows different logical responses based on different inputs. Whereas electromechanical relays must be hardwired with simple yes/no "relay ladder logic" (a basic and easily usable control language) instructions, PLCs can be programmed to respond to more complex logic, such as C-programs. C-programs can run independently of ladder logic programs, and can handle complex calculations and manipulation of system data, and any dynamic control algorithms that may be too complex for ladder logic. Building both programs into one system allows information to be shared between components, increasing dynamic control of the system. This can be critical depending on system complexity and the needs of a specific application.
PLCs can be programmed by either computer software or via manual consoles. Many PLCs come with integrated software packages that allow the input/output logic to be programmed through a user-friendly graphical user interface. Some PLCs come standard with Windows-driven software, which may be programmed by anyone who has some working knowledge of PLC concepts, while other PLCs use custom-made software, which may require a more experienced user. Changing the PLC's operating parameters (i.e., changing its inputs and/or outputs) is relatively straightforward and can be done by modifying various parameters in the software package. In contrast, changing the inputs and outputs of an electromechanical relay system can require rewiring the entire system. Some PLC software may also include diagnostic functions, which may allow quicker and more efficient error detection. Data Transmission Data transmission between the input/output devices and the controller can be provided via two primary channels - hardwired and wireless. In hardwired systems, all of the components (input devices, output devices, and the controller) are physically connected to each other through wires or fiber optic cables. Wireless systems do not require physical connections between the input/output devices and the controller; instead, communications transmissions take place from the originator to the receiver over radio waves, satellite, or microwave frequencies. Both wireless and hardwire communications methods have advantages and disadvantages. For example, wireless data communications may be blocked by local terrain (trees, mountains, etc.), while it may be difficult to hardwire remote locations to a controller located at a central facility. The type of data transmission system appropriate for any given application will depend on the controller type and the user's requirements.
PLC-based systems can utilize both hardwired or wireless communications. In a PLC-based system, the input and output signals may be transmitted wirelessly between various input/output devices and a receiver co-located with the PLC, but the receiver must be hardwired to the PLC. In contrast, all components in an electromechanical relay system (input/output devices and the relay itself) are typically hardwired. Electromechanical relays can be modified to receive wireless inputs, but this can be a complicated process that is not usually cost-effective. Maintenance PLCs are efficient control systems. They require little power and little space, especially when compared to an equivalent system consisting of multiple electromechanical relays. PLCs are low maintenance and require few spare parts. In contrast, because electromechanical relays are mechanical, they wear down with repeated use. Thus they require a high amount of maintenance and need frequent replacement. Cost Timers Electric timers typically cost between $50 and $150, depending on the functionality required. Electromechanical Relays Electromechanical relays typically cost from $5-$40 apiece PLCs PLCs range from $30-$500. The primary factors influencing the cost of a PLC are the number of inputs and outputs and the power of the CPU. The higher the number of inputs and outputs for which the PLC is designed, the higher the cost. However, increasing the number of inputs and outputs also increases the number of external systems that can be controlled. Increasing the number of inputs and outputs also increases the CPU requirements. Higher-end CPUs have more memory and higher processor speeds than lower-end CPUs. The more powerful the CPU, the higher the cost of the PLC.
Finally, it should also be noted that PLCs are more cost efficient than are conventional relay systems, especially in cases where a large number of input/output instruments are needed or where operational functions are complex. Vendors
Disclaimer: The information provided in this guide does not constitute an endorsement by the Environmental Protection Agency of any non-Federal entity, its products or its services. In addition, EPA does not endorse the vendors and products listed on this site. EPA is publishing lists of vendors on this site in an effort to further public awareness of vendors identified as possible contacts for further information and possible purchase of the different types of security equipment. The Agency has selected the listed vendors on that basis. The list of vendors is not a complete list, and EPA does not endorse the products or services of these vendors. GE Supply
7602 Woodland Drive, Suite 200
Indianapolis, Indiana 46278
(800) 243-7313
www.ge.com
| Triangle Research International, Inc
1072 South De Anza Boulevard
Suite A-107-510
San Jose, California 95129
(877) 874-7527
www.tri-plc.com
| Delta Automation
2704 Charles City Road
Richmond, Virginia 23231
(804) 236-2800
www.deltaautomation.com
| Lutron Electronics Company, Inc.
7200 Suter Road
Coopersburg, Pennsylvania 18036
(888) 588-7661
www.lutron.com
| Siemens Energy & Automation
7883 Redpine Road
Richmond, Virginia 23237
(804) 743-5424
www.sea.siemens.com
| General Controls
2350 Brickvale Drive
Elk Grove, Illinois 60007
(847) 595-2152
www.generalcontrol.com
| Rockwell Automation
1201 South Second Street
Milwaukee, Wisconsin 53204
(414) 382-2000
www.rockwellautomation.com
| Honeywell, ADEMCO Group
165 Eileen Way
Syosset, New York 11791
(800) 645-7568
www.honeywell.com
| Maple Systems, Inc
808 134th Street SW, Suite 120
Everett, Washington 98204
(425) 745-3229
www.maple-systems.com
| AutomationDirect.com
3505 Hutchinson Road
Cumming, Georgia 30040
(800) 633-0405
www.automationdirect.com
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