Inductive sensing has been used in the marketplace for a long time. It was the natural upgrade from mechanical limit switches. Whereas mechanical limit switches confirmed a product present through physical contact, the inductive proximity sensor accomplished some of the same function, but in a non-contact device. The technology is that a varying electromagnetic field as created in the inductive sensor, can be reduced when in the near presence of a target with ferrous (iron) content. This reduction in the established electromagnetic field, can be detected by the inductive sensor’s circuitry, and used to change its output state.
Inductive sensors are only for the proximity mode. Typical variations are discrete output in normally open or normally closed, and analog. Electrical output configurations include 2-wire ac, 3 wire, and 4-wire.
Mounting considerations are flush mount in metal, requiring a shielded sensor, and non-flush mount in metal, which can use either an un-shielded (typical) or shielded sensor.
Generally, sensing range of an inductive sensor is related to face diameter.
The advantage of inductive sensing is that it is often the optimum technology for the harshest environments, surviving in water/oil direct spray, and even some metal chips from machining/cutting operations. Unlike a mechanical limit switch, it has no moving parts, and will not get stuck in some position due to the accumulation or residue of these lubricants over time. For these applications, the inductive proximity sensor can go where the devil fears to tread. Since it is non-contact sensing, it cannot scratch or damage the target product.
The disadvantages of the inductive sensing are derating curves for sensing ranges various types of metals with less ferrous content, e.g., aluminum, copper, brass, stainless. Some manufacturers offer enhanced designs with higher prices, to sense these metals with significant deratings.
While increasing the sensor diameter will increase its sensing range, it also increases the size of the minimum target it needs to be presented with for detection. It becomes a tradeoff of physical size, sensing range, and minimum target considerations. The relatively short range of inductive proximity sensors often results in the moving target hitting and destroying the sensor face.
Unshielded inductive proximity sensors have longer sensing ranges than shielded inductive sensors, but they cannot be flush mounted.
Inductive proximity sensing has relatively slower response times than what a photoelectric sensor can accommodate.