 A factor 1 sensor with coils on a PC board. |
By Bill Eaton, Eric Henefield, and Karen Keller
Standard proximity sensors have long been used on plant floors for metal detection. Whether to detect aluminum cans on a line or a steel gate on a tank, these sensors were one of the first implemented in the field and remain popular today. Though these sensors serve their purpose well, they are still far from perfect as they are unable to detect all metals at the same rated distance, and are particularly susceptible to physical and environmental damage. These sensors also cause many users headaches by forcing them to keep multiple different sensor types on hand for applications with different sensing ranges and unique mounting requirements.
This needn’t be a concern any longer, as sensor manufacturers have developed a different kind of proximity sensor that uses innovative technology to bring sensors in line with today’smodern industrial environment. Now, a single sensor can be used to sense aluminum,stainless steel, mild steel, copper, lead, and brass at the same rated distancewithout changing the position of the sensor.
Traditional inductive proximity sensors are designed for wear-free and non-contact detection of metal objects. The sensor consists of a coil and ferrite core arrangement, 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. When a metal object (target) enters the high-frequency field, eddy currents are induced on the surface of the target and the sensor is affected by these currents.
Ferrous and nonferrous metals effect proximity sensors differently and are sensed at different ranges depending upon the metal being detected. To sense different metals the sensing range must be adjusted to accommodate what is commonly referred to as a correction factor.
A proximity sensor’s standard operating tolerance is based on its response to a one-millimeter-square piece of mild steel. When sensing metal other than mild steel, the sensor has to be adjusted (or corrected) to the sensing distance per metal.
Proximity sensors are designated for embeddable or non-embeddable mounting, which indicates the location of the sensor’s face in reference to the mounting surface. Embeddable construction includes a metal band (shield) that surrounds the sensor body and helps direct the electromagnetic field to the front of the sensor. Embeddable sensors can be flush mounted in metal, making them better protected from mechanical damage, but causing a reduction in the sensing range. Non-embeddable sensors do not have a metal band surrounding the sensor (non-shielded) and have a longer sensing range, but they require a metal-free mounting area around the entire sensor. Metal parts near a non-embeddable sensor (in the metal-free zone) can influence the electromagnetic field and the operation of the sensor, i.e. increasing or decreasing the sensing range.
Standard proximity sensors can also be negatively affected if several sensors are mounted in a limited space. This can cause coupling between the oscillator coils, which in turn causes the sensors to trigger each other. Sensor manufacturers have different requirements for the mounting space between their sensors, but the space can be lessened by using sensors with different frequencies.
 Some factor 1 sensors allow for limited recessed mounting in embeddable designs without a reduction in sensing distance. |
Sensor technology has evolved so that correction factors need not be applied; this is widely referred to as factor 1 sensing. Factor 1 sensors detect all metals at the same range without adjustment, giving them a longer overall sensing range than standard proximity sensors.
Instead of a single coil inducing and being affected by eddy currents on a target as in standard proximity sensing, factor 1 sensors use separate, independent sender and receiver coils on a PC board and remove the ferrite core. Because of this, ferrous and non-ferrous metals have the same effect on factor 1 sensors and are rated for the same operating distance.
Not all manufacturers’ factor 1 sensors are the same: some operate using two coils, some three or four; some even incorporate a ferrite core into the design. Factor 1 sensors without ferrite cores are inherently immune to magnetic field interference that often occurs during electric welding operations, lifts, and electronic furnaces. The absence of the ferrite core also allows factor 1 sensors to operate at a higher switching frequency.
Since non-embeddable proximity sensors protrude from the mounting surface, they are more susceptible to physical damage from the target hitting the sensor or from accidental human contact. Other influences make both embeddable and non-embeddable sensors mechanically vulnerable, including temperature and voltage variations, and environmental factors such as exposure to washdown, high levels of noise, or weld fields.
Since factor 1 sensors are able to sense all metals at the same rated distance without observing correction factors, they are able to be mounted further away from the target and do not require alterations for different types of metal. The coil technology used for factor 1 sensors makes them able to observe smaller metal-free mounting zones and be mounted next to other sensors.
Damp or humid industrial environments, or those that require frequent washdowns by water, foam, or cleaning/disinfecting agents commonly found in food and beverage industries, produce adverse conditions for proximity sensors. It is increasingly important that sensors have the inherent ability to function in these applications.
Standard proximity sensors fail in these applications because residue penetrates through the front cap and connector insert due to ingress from temperature shock, or the housing materials cannot tolerate the acidity of the cleaning agent. To withstand the rigors imposed by these wet environments, sensor manufacturers have integrated design features into the front cap and connector insert and use different housing materials such as stainless steel to resist the ingress of water and vapors.
Factor 1 sensors are inherently weld field immune, so they are unaffected by strong electromagnetic ac or dc fields found in weld applications, such as resistance welding. Since factor 1 sensors use coil technology and lack ferrite cores, they can be implemented into multiple housing types allowing a factor 1 sensor to replace multiple different sensors.
Factor 1 sensors are suitable for industries that need to sense multiple different metals and have unique sensing environments. Whether the user needs a sensor with extended range, recessed mounting capabilities, a unique housing design or washdown suitable properties, factor 1 sensors can be used for almost any application. The flexibility and applicability of factor 1 sensors make them a viable alternative to standard proximity sensors: reducing inventory, maintenance, and the bottom line.
Bill Eaton is chief engineer with Turck Inc., 3000 Campus Dr., Minneapolis, MN 55441. He has nearly 40 years of industrial control experience, including 26 years in associated sensor technology.
ADVERTISEMENT
Eric Henefield is a product manager in Turck’s sensors division with 8 years of experience.
Karen Keller is a marketing coordinator with Turck, assisting with technical documentation. Turck Inc. can be reached at 800-544-7769.