When an application calls for detecting a metallic target that falls within an inch of the sensing surface, inductive proximity sensors are apt for the task. First introduced in the early 1960s, these durable components have proven their mettle in the sensing arena. In fact, they're the best-selling sensing technology in the world. Their immunity to dust and dirt buildup suits them well for harsh industrial environments. Additionally, the standardized physical and electrical characteristics of the general-purpose, cylindrical types of these sensors simplify their use.
Naturally, designers make some common mistakes when applying these devices. Knowledge in several key areas, though, can help careful users avoid these pitfalls. Successful object detection requires an understanding of the fundamentals of sensor design. The criteria for choosing between the various styles of inductive proximity sensors also must be kept in mind. And, the significance of key sensor specifications and the effect of mounting restrictions on sensor implementation should be recognized.
An inductive proximity sensor has four components: the coil, oscillator, detection circuit, and output circuit. The target material, environment, and mounting restrictions all have an influence on these items and on the senor's operation, magnetic nature, and shielding. The oscillator generates a fluctuating, doughnut-shaped magnetic field around the winding of the coil, which is located in the device's sensing face.
When a metal object moves into the sensor's field of detection, Eddy currents build up in the object, magnetically push back, and finally dampen the sensor's own oscillation field. The sensor's detection circuit monitors the amplitude of the oscillation and, when it becomes sufficiently damped, triggers the output circuitry (Fig. 1).
Inductive proximity sensors come in shielded and unshielded versions. Without any shielding, the doughnut-shaped magnetic field generated by the sensor's coil is unrestricted. As a result, the sensor will be triggered when any metal object comes from behind, along side, or in front of the device. In a shielded sensor, a ferrite core directs the coil's magnetic field to radiate only from the sensor's detection face. Even unshielded inductive proximity sensors have peeled-back ferrite-core shielding, which gives them a longer sensing distance than the shielded versions. At the same time, this feature prevents false readings caused by objects behind the detection face.
There are five categories of inductive proximity sensors: cylindrical, rectangular, miniature, harsh environment, and special purpose. Cylindrical threaded-barrel sensors account for 70% of all inductive proximity sensor purchases. Years ago, this style's behavior was standardized by the CENELEC organization, which determined characteristics such as body size, sensing distances, and output levels. It's easy to understand why a designer would automatically select this general-purpose sensor, since it would be the right choice 70% of the time.
Yet experience has shown that there are many proximity-sensing applications where one of the other, specialized sensors can provide a better solution. Designers who automatically specify a general-purpose sensor may encounter problems that would vanish if another style were selected. Target material, environment, and mounting restrictions should guide the choice of sensor style.
In the world of inductive proximity sensors, not all metals are created equally. The familiar specification in technical data sheets refers to a "standard detectable object" made of an iron (ferrous) material. Other metallic materials, such as stainless steel, brass, aluminum, and copper, have different influences over the inductive effect. They're usually less detectable than iron, too.
Designers should determine two things. First, is the target material made out of iron or another metal? Second, is it possible for the target material to change in the application's future runs? To calculate the sensing distance of nonferrous metals, multiply the standard sensing distance by a reduction factor. Typically, this value is 0.8 for stainless steel, 0.5 for brass, 0.4 for aluminum, and 0.3 for copper.
A full-line sensor supplier will have a sensor solution for the detection of troublesome metallic materials. These special inductive proximity sensors are known as "nonferrous sensing" or "all-metal sensing." Nonferrous sensors will detect metals such as aluminum better than they sense iron, while all-metal sensors will pick up on all kinds of metal at the same sensing distance.
What separates the nonferrous and all-metal sensors from general-purpose inductive proximity sensors is the number of separate inductive coils included in the proximity-sensor head. The nonferrous and all-metal types contain two or three separate coils in the sensor head, while the general-purpose sensor has only one. Consequently, the nonferrous and all-metal sensing styles tend to be larger and more expensive than their general-purpose counterparts.
Environmental conditions can significantly affect the sensor. Extreme temperatures will reduce its operating life, causing premature failure. Hot temperatures will make it more sensitive, while cold temperatures will lower its resistance to shock. Nevertheless, a full-line sensor supplier can offer solutions to specific environmental conditions.
In certain applications, metallic "chips" or filings accumulate on the sensor's side or face. To account for this, some modern inductive proximity sensors contain embedded microprocessors that detect the slow buildup of these chips over time and teach the sensor to ignore their effects. These sensors are "chip immune." The flat-pack proximity sensor also resists the effects of chip buildup. With its slim profile, it's virtually unaffected by chip buildup when its sensing face is vertically exposed.
Sensors may be exposed to cutting fluids or chemicals for prolonged periods of time as well. This can cause traditional inductive proximity sensors to become brittle and crack, shortening their lifetimes. In such cases, designers must again turn to a specialized model. Proximity sensors dipped, coated, or shot from Teflon suffer no ill effects from the material in terms of performance or reliability. Teflon's added cost can be justified by the material's stability in the presence of cutting oils and corrosive chemicals. It also prevents weld slag buildup.
High-temperature environments pose another challenge. Inductive proximity sensors generally are self-contained devices that include their silicon amplifiers and detection circuitry inside the sensor-head housing. Self-contained proximity sensors are practical for most applications until environmental conditions begin to exceed the standard operating parameters for a silicon-based circuit. Normally, silicon-based circuitry operates between −25°C and 70°C.
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