Sensor Sense: Wireless Sensor Connections

July 7, 2009
Wireless inductive systems provide power and signal connectivity so ordinary sensors can remain attached to removable or interchangeable pieces of equipment

Modern automation controls offer many new capabilities beyond systems of just a decade ago. But controls often need information on the position or condition of the object they manage. That usually means modern machines have more sensors than their predecessors in many positions throughout the machine, including some in locations where wiring is almost impossible. Barriers may block power and signal wires from the sensor; or, the sensors mount on parts of the machine that move, rotate, or get swapped with other parts while the machine runs.

For example, sensors on part carriers move with the part from one operation to the next. Sensor status confirms part placement and alignment at each station before an operation takes place. For another example, consider sensors attached to an interchangeable progressive die that monitors the operation of a stamping machine. Automatic die changers must make sure power and signal connections are solid for these sensors before stamping operations can begin.

Slip rings and high-density connectors have been used for such tasks, but the mechanical nature of these connections creates a contact failure point. Slip rings wear and can introduce electrical noise into the system, while connectors offer alignment problems and contact failures.

One method circumvents these problems by replacing the physical electrical connection with one that’s inductively coupled. A wireless inductive system transfers power and data signals from the primary system side across an air gap to a movable secondary side to eliminate wires between fixed and moving pieces.

Some inductively coupled systems offer only a single sensor connection across the air gap, while others can handle multiple sensors. Either version provides a virtual connection across the air gap. The wireless interface eliminates the need for batteries in the portable side, but the two transmission elements must remain within 20 mm. Power goes from primary to secondary side of the inductive coupling while information flows in the other direction from the secondary to primary as a pulse-coded data stream. For best results the primary and secondary elements should remain stationary at the time of signal transfer.

Pepperl+Fuchs (www.am.pepperl-fuchs.com) supplied information for this column.

Edited by Robert Repas

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