May 2009

Proximity sensing from factory to field

Proximity sensors have long seen use for metal detection in industrial automation. Whether to detect aluminum cans on a production line or a steel gate on a tank, these sensors were some of the first implemented in the industry and remain popular today. Often applications with different sensing ranges and special mounting requirements require different sensor types, which can cause a sizeable sensor inventory; sometimes thousands of sensors can be in place at a single plant. Today, a different kind of proximity sensor has made mounting hassles and excessive sensor inventories nearly a thing of the past.

Standard prox

Traditional inductive proximity sensors are for wear-free and non-contact detection of metal objects. The sensor consists of a coil wrapped around a ferrite core, an oscillator and detector circuit, and a solid-state output. They operate with a high-frequency electromagnetic field generated by a LC-resonance circuit with a ferrite core and a single coil. A metal object (target) entering the high-frequency field induces eddy currents on the surface of the target, resulting in a loss of energy in the oscillator. This generates a signal that turns the solid-state output to ON or OFF. When the metal object leaves the sensing area, the oscillator regenerates, which allows the sensor to return to its normal state.

In standard proximity sensors, the rated operating distance is a quantity that designates the nominal operating distance. Ferrous and nonferrous metals affect proximity sensors differently. Sensors sense them at different ranges depending on the metal they are detecting. A proximity sensors' standard operating range is based on its response to a 1-millimeter-thick piece of mild steel. When sensing metal other than mild steel, the sensor must correct to deliver the sensing distance suitable for the metal type-the more conductive the metal, the shorter the sensor's range. An 18-millimeter standard ferrite core proximity sensor has a range of 7 to 8 millimeters when sensing steel. If this same sensor is sensing aluminum, its range reduces by 20 to 30%, making the range about 2 millimeters.

To accommodate these correction factors, users traditionally had two choices; either purchase multiple sensors with varying sensing ranges (perhaps as many as one sensor per type of metal sensed), or use the same type of sensor for each application, simply remounting it each time they required a different sensing range.

No correction factors needed

Sensor technology has now evolved so we no longer need correction factors. The industry calls it factor 1 sensing. Factor 1 sensors detect all metals at the same range without adjustment, allowing them to remain mounted in their original location on the line while detecting a broad variety of metal objects. This design also means factor 1 sensors offer a longer overall sensing range than standard proximity sensors. Because users now only need to apply one type of sensor to detect different metals, factor 1 sensing can lead to reduced inventories and minimized maintenance and downtime.

Instead of operating using a single coil that eddy currents induce and affect on a target, as in standard proximity sensing, factor 1 sensors use separate, independent sender and receiver coils on a printed circuit board with no ferrite core. Because of this, ferrous and nonferrous metals have the same effect on factor 1 sensors and are rated for the same operating distance. The absence of the ferrite core also allows factor 1 sensors to operate at a higher switching frequency. They are inherently immune to magnetic field interference that commonly occurs in industrial environments. With this design, factor 1 sensors are particularly advantageous in assembly operations involving components comprised of various metals, such as copper, steel, and aluminum, as they can detect all components without readjustment.

The benefits factor 1 sensors provide can also prove useful in the field, measuring rotor position on wind turbines. To provide wind production even in low wind regions, turbine designs may use permanent magnets made from neodymium-iron-boron to excite the rotor of the turbine. This design makes the wind turbine more energy-efficient, since energy normally expended for this purpose may now be sent directly to the grid.

With this design, wind turbine manufacturers no longer need to use high-maintenance components prone to malfunction, such as transmissions, intermediate shaft, and couplings. Wind turbines using permanent magnets also no longer require excitation coils, slip-ring transmission, and direct-current generation.

In this application, factor 1 sensors can reliably measure the precise position of the rotors. During operation, their output can hold constant, despite changing wind conditions, by adjusting the working angle.

For each wind turbine, six factor 1 sensors (two on each blade) simultaneously determine the precise position of the rotors. To do so, each sensor records the end position of the rotor blades. A seventh sensor determines the position of the maintenance hatch of the turbine. With the data from all the sensors, the manufacturer's control system then ensures each rotor blade is in the correct position. The power is transmitted between the motor and the rotor blade via a lubricant-free and maintenance-free toothed belt. In this process, the power is distributed across several teeth, thus minimizing wear and increasing safety and reliability.