Selecting inductive proximity switches
Follow these guidelines to select the right inductive proximity switch for your application.
Proximity switches are the nerve endings for automation systems and equipment in a variety of industries. Without them, parts of many machines or processes would be stumbling in the dark. Therefore, careful selection of new or replacement switches is critical, and requires a clear understanding of the application environment, switch design, and installation methods.
This article explores the effects of environment and critical design options for specifying inductive proximity switches. It also reviews the importance of ingress protection (IP) ratings, housing materials, and configurations. Further, it offers selection and installation tips, and methods to reduce failures in both new and retrofit applications.
Low price, high performance
Inductive proximity switches have been used in automated equipment and processes for decades for position sensing, presence detection, part counting, and many other applications. They detect ferrous and nonferrous metals.
In its simplest form, a proximity switch is an inductive coil that creates a magnetic field when an oscillating signal is applied. This magnetic field is disturbed when a metal object enters the field. Switch electronics detect this disruption and energize an output circuit. This allows the sensor to detect targets within its range without contact, while rejecting the influence of many outside elements such as reflected light or stray materials.
A basic inductive proximity switch has virtually become a commodity. Over the last 15 years its price has dropped from about $100/device to less than $15 (see Figure 1). This cost reduction has been driven by the evolution of sensor technology and increased manufacturing efficiencies. Circuit technology has evolved from printed circuit board to flexible circuit film, and recently to using application-specific integrated circuits (ASICs).
ASICs allow programming of sensor characteristics after the unit is assembled. Previous technology required a trim resistor to be adjusted before final assembly to set the sensing range. This can now be done through the ASIC, resulting in significantly greater consistency and repeatability. Other programmed characteristics include normally open and normally closed switch functions. In addition to lowering production costs and assembly time, ASIC design is less complicated with fewer points of failure, and provides more durable and repeatable operation.
ASIC programming is performed by the manufacturer prior to shipping the sensor/switch to the user. As the name implies, ASIC technology allows suppliers to build one type of unit that can be programmed to accommodate many different applications. This has been a key factor in driving down the price of inductive proximity switches while maintaining a wide range of available features and functions.
Key selection considerations
When selecting an inductive proximity switch, users must determine optimum barrel size (diameter) and sensing distance first, and then consider other factors, which include:
- Types of metal to be sensed
- Housing material: plastic or metal
- Shielded or unshielded installation
- Prewired or quick disconnect
- Sourcing (PNP) or sinking (NPN)
- Normally open or normally closed output
- Switching frequency
- Temperature range
- Environmental requirements.
The two critical, interrelated selection points are the diameter of the proximity switch, and its sensing distance, which is defined as the distance from the sensor face to the target.
Inductive proximity sensors come in a variety of sizes, diameters, and even small rectangular housings for unique mounting applications (see Figure 2). Diameters from 3 to 30 mm and larger are available. The diameter has a significant effect on the sensing range because sensing distance increases with diameter. However, the size of the target to be detected should drive the switch diameter selection.
If the target size is about 12 mm, a 12-mm diameter switch is a better match than a 30-mm switch. A larger diameter switch is more expensive, occupies more space, and is more likely to sense objects outside the detection zone and generate false triggers.
Even when there is a large target size, this does not give the green light for a large-diameter switch because it must physically fit the application. Switch barrel length is also a consideration-the shorter the better in most cases.
Although barrel diameter affects sensing range, multiple sensing distances are commonly available for each barrel diameter. The sensing distance is typically specified as standard, extended, and triple range. A longer sensing distance can improve robustness as the target can be within a large range and still be detected. However, distance shouldn't be increased outside the normal range of the target because doing so can result in false detection of stray objects.
A shielded (flush-mount) or unshielded (non-flush) housing also affects the sensing distance. The sensing distance of a standard 12-mm shielded switch starts at 2 mm. The sensing distance of an unshielded switch starts at 4 mm. The sensing distances for extended range 12-mm shielded and unshielded switches start at 4 mm and 7 mm, respectively. For a little more money, a triple-range switch can increase the sensing distance starting point for a shielded 12-mm switch to 6 mm, and to 8 mm for an unshielded switch. An extra distance sensor costs more, but it increases sensing reliability because it protects the sensor face from impacts.
A shielded sensor generates a sensing field that emanates from the face of the sensor and can be identified by barrel threads running the full length to the sensor face. This allows the sensor to be mounted flush in a metal bracket or mounting surface.
An unshielded sensor can be identified by a protruding sensor face material extending past the barrel threads, with the sensing field beginning on the side of the sensor and extending toward the tip of the sensor with a shape similar to a candle flame. This improves sensing range, but the sensing area must be free from metal objects within three times the diameter to prevent false detection, which is a common problem. Costs are generally identical, so the decision is based solely on mounting requirements.
A common mistake when specifying a proximity switch is not reducing the sensing distance depending on the specific metal to be sensed. Highly magnetic ferrous materials, such as cold rolled steel, have a correction factor of 1. Stainless steel is ferrous but not as magnetic, so its sensing distance must be reduced by a typical correction factor of 0.7, while nonferrous and nonmagnetic materials, such as aluminum, have a typical correction factor of 0.4.