Mechanical Limit Switch Sensing

Mechanical limit switch sensing is likely the first, and therefore the oldest method, in industry, of sensing the presence of some object.  Mechanical sensing is contact sensing.  It was intuitive to have either an arm or plunger be activated by a moving target and use a set of electrical contacts mechanically attached to that arm or plunger.  Mechanical limit switches are simple to install and understand for maintenance personnel.

Because of the target movement, single or multiple contacts could be activated, so normally open, normally closed, and combinations thereof could easily be integrated.  A spring is used to return the arm or plunger to the resting state once the target has left the mechanical limit switch.

Unlike other sensor technologies that are active devices requiring power just to be able to sense and provide a usable output, mechanical limit switches are passive devices, requiring or consuming no power of themselves.

The robustness of their electrical contacts can directly control reasonable loads in the 10-20 amp or more range, and handle either ac or dc circuitry.   Their electrical contacts are mechanically activated contacts, so mechanical limit switches are not solid-state devices, and their outputs are dry contacts.  Even though they do not have npn or pnp outputs, they can provide the equivalent if so wired, and if necessary.

Variations of mechanical limit switches can even tell which way a product passes, for example, whether from right to left, or from left to right, simply by the mechanical activation used.

The disadvantage of mechanical limit switches is that they are contact sensing and need initial relative movement between target and switch.  This intervention of the mechanical limit switch may slow or impede the target movement, or accidently obstruct it, e.g., a mechanical (paddle) limit switch as used in a feeder bowl.  It may even aesthetically deface the target.

In the harshest of environments, with lubricating oils being spray onto the targets, this accumulation of lubricants settling during periods of non-production, may cause the movements of mechanical limit switches to freeze up, so that maintenance personnel must intervene, and free-up the moving mechanical limit switch parts.

Because mechanical limit switches are contact sensing, their range is relatively short, compared to photoelectric, laser, or ultrasonic sensors.