This question was posted on www.element-14.com:

We have a need to limit travel on a rope being retrieved onto a drum through a pulley. We need to mount a passive non powered sensor or tag of some type onto the rope and be able to detect it within a range of 1 mtr.

We have enquired about RFID but these are typically a computer controlled solution, all we want is a “yes the tag is in range” signal to a PLC to stop the motor on the winch.

Pinter Electronics Consultants’ answered:

Chris Pinter answers, “I think radios are a great way to solve problems when wires can affect results.  Wireless solutions typically cost more than wired ones and the quality of the communication link is dependent on many factors that a wired solution does not have to consider.

For your problem, I think maybe a different type of solution is better.    I would attach a magnet to the rope as a sensor.   The magnet will be more robust then a plastic box with an RFID or radio transmitter.   The magnet is also a passive device. You can then use a simple solenoid mounted on the pulley.  When the magnet passes the solenoid it turns to motor off causing the rope to stop.

Now, because I love radios so much I should mention that you can use a wireless system to transmit the signal from the pulley to the motor but a wire will be able to send the signal to the motor just as well.”

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