In my last post (We Don’t Make Axes Out of Bronze Anymore) we discussed the evolution of technologies which brought up the question, can a prox always replace a limit switch? Not always. Note that most proxes cannot directly switch large values of current, for example enough to start a motor, operate a large relay, or power up a high-wattage incandescent light. Being electronic devices, most standard proxes cannot handle very high temperatures, although specialized hi-temp versions are available.
A prox is designed to be a non-contact device. That is, it should be installed so that the target does not slam into or rub across the sensing face. If the application is very rough and the spacing difficult to control, a prox with more sensing range should be selected. Alternately, the prox could be “bunkered” or flush-mounted inside a heavy, protective bracket. The target can pound on the bunker continuously, but the sensor remains safely out of harm’s way.
If direct contact with a sensor absolutely cannot be avoided, ruggedized metal-faced sensors are available that are specifically designed to handle impacts on the active surface.
Be sure to consider ambient conditions of the operating environment. High temperature was mentioned earlier, but other harsh conditions such as disruptive electrical welding fields or high-pressure wash-down can be overcome by selecting proxes specially designed to survive and thrive in these environments.
Operationally, another thing to consider is the target material. Common mild carbon steel is the ideal target for an inductive prox and will yield the longest sensing ranges with standard proxes. Other metals such as aluminum, brass, copper, and stainless steel have different material properties that reduce the sensing range of a standard prox. In these cases be sure to select a Factor 1 type proximity sensor, which can sense all metals at the same range.
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