Switches and sensors verify that parts, doors, and shields are locked into their proper place prior to operating a machine. They are vital for assuring precision in production, and smooth and safe machine operation.

But all wired devices -- whether limit switches or proximity sensors -- have limitations in where they can be used and are vulnerable to wear that precipitates ongoing maintenance.

Proximity sensors have become widely used on industrial factory floors simply because they are usually a less expensive option compared to traditional electromechanical switches. Many manufacturers have found, however, that proximity sensors require ongoing, reactive maintenance, halting production while the sensor is repaired.

For example, multiple-axis machining, such as engine-casting machining, in which the fixture rotates the part for operation into position, is subject to movement and constant stress on the proximity sensor's conductor wires, which can cause the wires to fray and break at unpredictable times. When the wires break, the machine and employees are unexpectedly idle -- typically for 15 to 20 minutes each time -- while the wires are replaced. If these wires are replaced every three months, as is the case for some manufacturers, the cumulative lost productivity can be costly. If timing of the needed maintenance was predictable, preventive maintenance would eliminate this downtime.

Similarly, coolant carrying metal chips from the machining area are in continuous contact with the conductor wire, abrading the wire's sheathing, eventually causing shorts or continuity issues that necessitate cable replacement. Again, this results in lost productivity.

Of course, one of the biggest issues manufacturers face with sensors and their machines is the number of connection points. Each connection point is a potential source of failure. By eliminating connection points, manufacturers can help reduce interruptions to their production flow.

All of the above issues can be easily overcome using wireless limit switches. Wireless, specifically 802.15.4, 2.4GHz, has been proven to be reliable in a wide range of applications on the factory floor. A potential advantage of wireless is that the limit switches can control the power to the transmitters, which can extend battery life to two years or more under normal operating conditions. Diagnostics built into the RF electronics provide low-battery notifications, allowing for replacement of the battery outside of regular operations prior to causing a disruption in workflow. This is important because the power requirements of both the sensor and the RF is significant and requires regular battery replacement.

Wireless switches can be easily retrofitted into applications where standard wired switches or proximity sensors are used. Naturally, with any wireless device, an RF signal is transmitted which has to be translated back to a standard input signal -- PNP, NPN, or relay -- that can be used by the control system. This is accomplished by a wireless receiver mounted in the machine control panel. The outputs from this receiver are wired directly into the controller input cards just as is traditionally done. The controller cannot distinguish between standard inputs from a wireless device and a wired one. The only difference is that there are now additional diagnostic inputs for monitoring battery life and signal strength.

Wireless is changing factory automation by reducing switch installation costs and providing greater flexibility in applications. By eliminating wires, manufacturers can drastically decrease their maintenance costs and downtime concerns caused by connectors and wire integrity, and increase productivity. As new solutions are released and wireless adoption increases, the use of wireless devices to solve application problems will become even more mainstream.

About the Author:
Todd Hanson is Director of Wireless Solutions at Honeywell.