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Proximity Sensors in the Lower Die For Feed Detection



An effective way to verify feed progression is to install a proximity sensor in the lower die to detect a hole that should be present when the material is properly fed.   A sensor installed in this way can detect both shortfeed and overfeed.   A normally-closed sensor is used so that the sensor will actuate when the hole 'uncovers' the sensing field.


Proximity sensor detecting a hole to verify proper feed progression
Proximity sensor detecting a hole to verify proper feed progression

This arrangement is effective only if the strip stays at a constant height above the sensor. Unfortunately, in most dies the clearances necessary to allow the strip to feed are enough to make detection inconsistent, possibly resulting in several 'false' actuations per stroke.


Material lift causes false actuations
Material lift causes false actuations

Confining the strip to eliminate any vertical movement is one way to solve the problem, but this will likely result in feeding problems.   Fortunately, there's another solution.   The goal is to keep the sensor-to-strip distance constant; so rather than confine the strip, simply allow the sensor to track the movement.


Spring-loaded bushing keeps the sensor-to-strip distance constant
Spring-loaded bushing keeps the sensor-to-strip distance constant

One way to do this is to install the sensor in a bushing that is held against the bottom of the strip by a spring.   This arrangement allows the sensor to 'track' any incidental vertical movement of the strip, and helps to eliminate false actuations.

The photo below shows two proximity sensors that are being used in this manner.   Notice that the bushings feature a slight radius to make it easier to start the strip over the sensors, and that each sensor is slightly recessed inside the bushing to prevent the strip from wearing the sensor face.


Prox sensors installed in spring-loaded bushings keep the sensor-to-strip distance constant

Prox sensors installed in spring-loaded bushings keep the sensor-to-strip distance constant





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