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Wireless Sensors Offer Single- and Multi-Channel Options

Dec. 7, 2009
Wireless Sensors Depend on Power and Node Needs: Big Bucks Can Be Saved in the Right Applications
By Dan Hebert, PE, Senior Technical Editor

Let's say your machine is really a system consisting of multiple modules. The main module contains most of the I/O, all hardwired, but wiring to separate and smaller modules is limited and always has been a pain. Wireless could be the answer.

Maybe your machine is huge with most of the wiring concentrated in one area, but there are a few areas far from the central nexus of control where you need to sense and monitor certain parameters such as presence, temperature or vibration. Wireless could save wiring time and expense.

Another application where wireless works well is a case in which it's impractical to hardwire a sensor. You might wish to monitor vibration on a motor shaft but realize it's impossible to hardwire a sensor because of rotation.

In these instances, there are two main ways to apply wireless monitoring and control. The first is with single-channel wireless, the only option for the rotating-shaft application. The second is via wireless multi-channel I/O nodes. Let's look at each option.
With the single-channel option, each wireless sensing device communicates directly to a centralized wireless gateway via its own radio. The gateway is located near and hardwired to the main controller. The gateway communicates with each wireless sensor individually, either one-way or bidirectionally.

The wireless communication radio can be built directly into the sensor or, more commonly, supplied via a separate wireless single-channel radio. "The cost to put radios on today's discrete sensors is prohibitively high," notes Cliff Whitehead, manager of strategic applications at Rockwell Automation (www.rockwellautomation.com). "Few discrete sensing applications could tolerate the price increase required to wirelessly enable the sensor instead of wiring it to a separate radio," adds Whitehead.

As there are few sensors available with built-in radios, most single-channel applications will use a radio hardwired to the sensor, with the radio communicating wirelessly to the gateway.

The main issue here is providing power to the sensor and to the radio. If the power is supplied via wires rather than batteries, then the advantages of wireless are greatly reduced. Fortunately, many battery-powered, single-channel radios can supply power to a sensor for years before a new battery is needed.

A much more widely used and much more practical approach is a wireless multi-channel I/O node. This approach works very well when there are number of sensors remote from the main machine controller. Rather than wiring each sensor back to the main controller individually, the sensors are instead hardwired to the local I/O node with both power and control connections.
"To enable the monitoring and control of a typical machine, we offer a simple wireless I/O node," says Bob Gardner, wireless product manager at Banner Engineering (www.bannerengineering.com). "The wireless I/O node provides six digital inputs and six digital outputs. The node is monitored and controlled wirelessly from a remote wireless-to-Modbus gateway. The gateway is hardwired to the PLC or HMI via a Modbus serial interface."

In addition to the digital node, Banner offers units with analog I/O for sensing parameters such as temperature, vibration and humidity. "Our battery-powered I/O nodes are popular because they need no power wiring and can operate for one to five years on a single battery," adds Gardner.

The wide disparity in battery life quoted by Gardner hints at one of the main application issues with wireless multi-channel I/O nodes, namely power management. For a simple system with low-power sensors and just a few discrete I/O points, a battery-powered node could be the right solution. But when the number of sensors grows or when the sensors need more power, it might be best to supply power to the I/O node via hardwiring, as the alternative will be frequent battery changes.

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