Evolving applications mandate new proximity sensor designs

As automation equipment continues to decrease in size and increase in complexity, sensor manufacturers are striving to provide smaller sensors with greater capabilities that meet new user requirements. Many new designs can function in confined spaces where traditional, larger, barrel-shaped proximity sensors cannot.

To properly specify sensors, users must first consider their manufacturing process, application, and environmental conditions, and then select sensors that will provide the desired function with an acceptable level of maintenance. A variety of innovative new sensing options are available to meet these needs.

The need for range

A priority need these days among users of proximity sensor users is increased range. Proximity sensors most often fail when placed too close to their intended target. Under these conditions, they typically are damaged from repeated contact or sudden impact with the target itself. And, if the target is the product, it can be damaged as well.

The newest extended-range proximity sensors solve this problem by adding significant additional range. Today, a 30-mm barrel sensor has a range from 10 to 15 mm and a 12-mm barrel sensor, from 2 to 4 mm. Comparing alternate shapes, a 6-mm deep rectangular sensor has a 3-mm range vs the 2- to 4-mm range of a 12-mm barrel sensor that is 7-10 times the depth. A similar comparison can be made with an 8-mm deep rectangular sensor with a 7-mm range vs an 18-mm barrel sensor that has a 5- to 8-mm range, but 7-10 times the depth (Fig. 1).

Additional range allows sensors to be mounted farther away from the objects they detect, minimizing damage and replacement costs. Extended-range sensors are a vastly superior solution to spring-loaded brackets that try to cushion the impact of the target hitting the sensor, or metal-faced sensors that try to reinforce the sensing face to avoid damage. The best solution is a no-contact solution that eliminates impact and wear factors.

One extended range for all metals

Some manufacturers offer devices that sense mild steel, stainless steel, copper, aluminum, and brass at the same distance. This feature eliminates the need for sensor adjustment when target materials change, and allows users to inventory fewer sensors. The advanced design of many of these sensors eliminates the ferrite core of conventional sensors, giving them inherent immunity to weld fields and strong electromagnetic ac or dc fields, such as found in resistance welding operations. Some models provide significant sensing ranges even when fully embedded in steel.

The need for a precise fit

Proximity sensor shapes are evolving to better meet customers' processes. With automation equipment becoming increasingly smaller and space at a greater premium, proximity sensor manufacturers have had to rethink their sensor package designs. Dozens of new miniature rectangular- and cubical-shaped sensors are replacing traditional barrel sensors (Fig. 2). Models as thin as 6 mm (0.24 in.) can now replace barrel-style sensors that are up to ten times thicker, while still achieving longer ranges. Compact machinery previously impossible to automate can now be easily fitted with the new sensors. Other flat-style rectangular sensors achieve ranges up to 50 mm (2.0 inches). Tiny magnetic inductive cylinder sensors, designed specifically to fit in the groove of new-style pneumatic cylinders (Fig. 3), are now a mere 5 mm (0.20 in.) wide x 6 mm (0.24 in.) high.

Many of the newest sensors include advanced diagnostic features, such as integral 4-way LEDs and light bars with 3-sided visibility that provide status indication at a glance. Short-circuit, overload, and reverse-polarity protection are standard on most of the new devices. Intrinsically-safe models are also available.

Need for flexible mounting options

In addition, proximity sensors are available with a greater variety of mounting options than ever before—maximizing application compatibility. Some can be mounted either face up or face down (Fig. 4), while others can be mounted from two or more sides to capitalize on narrow depth, or to minimize width. Models with limit-switch mounting and a fully-rotatable sensing head offer significant mounting flexibility. Quick-disconnect fittings, available on most models, streamline installation and replacement.

Injection-molding: The shape of sensors to come

Although injection molding is not new to industry, it is a relatively new concept for proximity sensor manufacturing. Historically, sensors have been epoxy-potted to protect the circuitry from impact and moisture. Injection-molding (or overmolding) of the sensor around the circuit eliminates the need for potting, and streamlines design and manufacturing processes by reducing the required number of components and manufacturing steps.

Injection molding increases quality and throughput, while decreasing the overall cost of production. Advanced injection-molded housings seal better and are more resistant to the ingress of liquids, which increases their long-term reliability. Injection-molded sensors are also more rugged and more resistant to temperature changes, while providing greater aesthetic appeal. Temperature ranges as wide as -30° to 85°C are now available vs typical prox sensor ranges of -25° to 70°C.

Sensing technology evolution

Spurred by changing applications, the sensing evolution is expected to continue. Sensors that are small this year will probably be large by next year's standards. The variety of shapes, capabilities, and mounting options will continue to increase, keeping pace with the demand for high-tech, low-maintenance sensing solutions.

This article is written and provided by Brian Tarbox, Product Manager, Sensor Division, Turck, Inc. TURCK is a leading manufacturer of Inductive Proximity Sensors, Capacitive Proximity Sensors, Connectors, Cables, Cordsets, Fieldbuses and Automation Controls. For more information about Turck or their products, please visit their website at http://www.turck.com.