Inductive Proximity Sensors

by: Raphael Zollinger

The most commonly-used proximity sensor is the inductive type, which generates an electromagnetic field to sense metal objects passing close to its face. The sensor consists of an induction loop. Electric current generates a magnetic field, which collapses generating a current that falls asymptotically toward zero from its initial level when the input electricity ceases. The inductance of the loop changes according to the material inside it and since metals are much more effective inductors than other materials the presence of metal increases the current flowing through the loop. This change can be detected by sensing circuitry, which can signal to some other device whenever metal is detected. Common applications of inductive sensors include metal detectors, traffic lights, car washes, and a host of automated industrial processes. Because the sensor does not require physical contact it is particularly useful for applications where access presents challenges or where dirt is prevalent. The sensing range is rarely greater than 2 cm, however, and it has no directionality. This is usually the easiest sensing technology to apply in applications where the metal object to be detected is within an inch or two of the sensor face. These sensors are offered for a diverse spectrum of needs, from light-duty packaging applications to the environments on automotive welding equipment or washdowns in food processing plants. Thus, they also come in an array of shapes and sizes depending on their specs:

• Allen-Bradley

• ifm
• Stainless steel face and barrel for high strength
• Abrasion resistant and fire retardant
• 1200psi (8270kPa) washdown capability for contaminant and fluid ingress resistance
• Electrical protections against short circuits, overload, transient noise, false pulses and reverse polarity (DC models)
• Extended range models allow greater distance between sensor and target, reducing the likelihood of damage from collision
• The sensor can be placed farther away from the target which provides protection against mechanical damage
• The sensing face can rotate in one of five different directions. An integrated mounting bracket offers simple installation and the sensor’s Micro DC connector rotates 270 degrees for optimal mounting versatility. Two corner-mounted, highly visible LEDs indicate power and switching status.
• Proximity sensors are three-wire, PNP normally open units that are reverse polarity protected.
• They can run on 10 to 36 DC voltage with a supply current of 20 ma and a max load current of 250ma
• They have switch capabilities of between 60 to 100 HZ

• data sheets: prox and ifm
• price rages from $15 - $75 depending on the size and sensing distance
• can be found at Digikey and Mouser among many others