Programmable Relays Solve Complex ApplicationsBy John Grocke, Published Aug 05, 2013, 12:00am EDT
Many times integrators are presented with complex applications, such as machine interface, mantraps, lighting control, or complex timing scenarios, that cannot be easily accomplished with standard access control functions or IP camera inputs and outputs. Often these problems are solved with great difficulty, using a box full of "ice cube" relays, timers and counters with a nest of wiring or an expensive and complex programmable logic controller (PLC). A simpler and more economical solution is to use a programmable relay, as pictured below:
What is a programmable relay?
A programmable relay is an economical, compact, expandable intelligent logic CPU that can replace many relays, timers and counters and is well-suited for small automation projects. Most programmable relay main CPU modules include:
- 8 inputs and 4 relay outputs
- Programming keys
- Built-in DIN-rail mount
- Port for USB or RS-232 cable and additional memory module.
- An optional LCD display for displaying status and programming.
- Connection port for additional I/O modules for increased capacity.
Examples of commonly available commercial off-the-shelf programmable relays are IDEC's SmartRelay, Siemens' LOGO! and Omron's Zen. Programmable relay starter kits including a CPU/base relay unit with LCD screen, programming software and USB cable can be purchased online for about $200, making them a cost-effective solution for complex problems. These programmable relays are quite compact and will fit inside most control cabinets or larger junction boxes, as shown below:
The relay's CPU can be programmed through control keys on the CPU (which can be cumbersome) or by an included Windows software package and RS-232 or USB cable. The programming software requires only a basic knowledge of electronic logic with no special programming skills necessary. The user simply drags logical blocks or functions into a ladder diagram or a free-from style layout and links the blocks as if making a physical connection. Once the design is complete, a simulation mode allows testing and modifying the program's functionality on-screen before downloading it to the relay's CPU. Trial versions of the programming software are available online from IDEC and Siemens, for experimentation before purchasing. (Note: They do not support Windows 8 yet)
Using IDEC's WindLGC programming software, here are a few examples of complex applications that were accomplished by using an economical programmable relay.
The first example is a simple 3-door Mantrap controller:
Here's a slightly more complicated design for interlocking cameras with directional tripwire analytics for verifying that two cameras detect motion within a given time before generating an alarm.
Here's another design to incorporate a traffic light at a one-way intersection at the entrance to a parking garage.
Recommendations & Limitations
Keep in mind:
- Relays come in different AC and DC voltages, be sure to order the proper voltage for your application. We recommend using 12VDC or 24VDC models.
- The inputs of the programmable relay are voltage sensitive, no voltage detected is "off" and higher than certain voltage detected is "on". A power supply will be required to power the relay and to provide the voltage to switch through the camera's onboard output or access control system relay. Verify that the output relay in the camera or access control panel is rated for the required "on" voltage.
- Some IP cameras do not have relay contacts, only 5V outputs. 5V is not enough to generate an "on" condition for a 12VDC programmable relay, a 5VDC to 12VDC converter is necessary in this case or a 5V relay board will work.
- Only a limited number of I/O expansion modules can be connected to the main relay CPU, maxing out at about 24 inputs and 20 outputs, depending on the manufacturer.
- IDEC and Siemens programming software programs do not support Windows 8 yet. It will need to be run on a Windows 7 machine or with a Win7 emulator. We contacted IDEC's product manager to inquire but have not received a response.
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