Proximity sensors shine on the shop floor

Sensing the presence or absence of objects, liquid levels in clear containers, or counting cans moving down a conveyor pose everyday tasks for discrete sensors on the factory floor. Noncontact proximity sensors form one branch of discrete sensors, with capacitive, inductive, photoelectric, and ultrasonic devices in common use—each differing in the sensing method.

By Frank J. Bartos, Control Engineering March 1, 2004

Sensing the presence or absence of objects, liquid levels in clear containers, or counting cans moving down a conveyor pose everyday tasks for discrete sensors on the factory floor. Noncontact proximity sensors form one branch of discrete sensors, with capacitive, inductive, photoelectric, and ultrasonic devices in common use—each differing in the sensing method.

Capacitive proximity sensors emulate a physical capacitor’s parallel-plates model. The sensor’s front face acts as one plate and the target surface as the other plate. Because capacitance varies inversely with the distance between capacitor “plates,” the sensor’s electronic circuits can be calibrated to trigger on a specific value to detect an object at set distance(s). Capacitive “prox” sensors can sense almost any metallic or non-metallic objects, including conductive as well as dielectric materials. They’re also adjustable for sensing objects inside or behind glass, plastic, or other clear materials.

Inductive proximity sensors detect virtually all metals, including non-ferrous metals. These sensors set up a high-frequency magnetic field in an inductive coil, which generates fluctuating current in the proximity of metallic objects. A detector monitors the current signals and actuates a switch or sends feedback for other control action. Rugged housings, often of barrel shape, make them common in manufacturing since they can withstand high temperature and pressure, shock/vibration, washdown, and even weld splatter.

Photoelectric prox sensors work with light beams sent from an emitter to a detector, and the variation of light intensity—including its absence—reaching the detector. These signals, when amplified, sense various nonmetallic and some metallic materials at short (or long) distances. Visible and infrared (IR) light beams are available, using LEDs or laser diodes as the emitter. While visible light simplifies sensor alignment, IR pulses, modulated up to the low-kHz range, are preferred for sensing accuracy, longer distances, and low power usage.

Photoelectrics also sense difficult-to-detect transparent materials and must accommodate the target object’s reflective characteristics like gloss and shine. Another concern in the factory is dust, dirt, oil, or other coating on optical surfaces that can alter sensing accuracy. In the latest sensors, signal conditioning and control circuits handle such limitations through sensitivity adjustment, background/foreground suppression, and self-teaching features.

Three operating modes further subdivide photoelectrics. In diffused sensing, reflected light from the target must reach the receiver directly to activate detection signals. Retroreflective or reflex sensing uses the target to interrupt the transmitted light beam, which actuates the detection signal, and is separately reflected to the receiver. In the above two modes, transmitter and receiver are located in the same module. As the name implies, through-beam sensing involves sending the light beam directly from transmitter to receiver located in separate, inline housings. The target interrupts this through-beam light to activate an output.

Sound waves also work

Ultrasonic proximity sensors provide another means of short- and long-range object detection—using sound waves rather than light waves. These sensors emit ultrasonic pulses, typically in the 40-250 kHz range, which the target surface reflects back to the detector. Usually one transducer module handles both transmit and receive functions. Ultrasonic sensors can detect more than presence of objects. By measuring the time for the echo to return to the receiver and using the speed of sound, the sensor system calculates distance to the target. Ultrasonics also have the ruggedness to handle harsh environments. Material types detected are similar to those for photoelectric sensors.

Microprocessors are extending the capabilities of discrete proximity sensors, while making them ever smaller (photo). They’re available from numerous manufacturers, with user options like 2- or 3-wire input (dc and ac), different output types (transistor, relay, etc.), sensing range adjustment, and a variety of housings.

fbartos@reedbusiness.com