Selecting inductive proximity switches
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.
Common options and features
Switch design options include 2-, 3-, or 4-wire connection configurations; encapsulated wire (pre-wired) or quick disconnect; PNP or NPN; and normally open or normally closed contact arrangements.
Common switch installation issues include incorrectly specifying an NPN or PNP switch, or selecting an incorrect output contact configuration. In North America, a PNP switch with normally open contacts is typically used, with the controller input detecting 24 Vdc when a target is sensed. However, Asian-sourced equipment is often the opposite. Also, in many cases, end-of-travel switches for linear motion axis applications are configured as normally closed.
The configuration of 2-wire switches can be either NPN or PNP output, so the user doesn’t have to select output type at purchase. However, users must be mindful of leakage current required to maintain sensor power during an off state. This leakage current may cause a programmable controller input module to remain on regardless of the sensor’s output state. With 3-wire and 4-wire switches, leakage current is not a problem, but users must choose between NPN and PNP output when ordering.
Typically, 4-wire switches allow complementary normally open and normally closed contact configuration by the user. However, 2- and 3-wire normally open or normally closed contact configuration must be specified at purchase.
Another option that must be specified at purchase is pre-wired or quick-disconnect connection. If a pre-wired switch fails, wiring must be re-run between the switch and control panel. Switch replacement is much easier with a quick disconnect switch, but this type of switch is longer and requires the added cost of cables along with more upfront design effort.
Inductive proximity switches typically operate between 10 and 30 Vdc, with the 120 Vac switch mostly a relic of the past, especially considering that electrical safety requirements in Europe don’t allow its use.
Most switches have an LED to indicate the switch state. Some have LED lighting surrounding the barrel for easier viewing. This is a useful feature because it allows local detection of switch status.
Switching frequency must also be considered in some applications. An application counting teeth on a wheel may miss some counts if the switching frequency is not fast enough. Usually, a smaller housing diameter switch is quicker to sense and is more compact. However, the 12-mm switch is usually the least expensive. Below this size, the cost increases due to the need to fit all the components into a smaller space.
Environment and housing
A basic inductive proximity switch is nickel-plated, has a plastic face, and is IP65/67 rated to operate in environments with dirt, debris, and a little water (see Figure 3). Sensors for more demanding environments are made with a stainless steel body, and some are machined out of solid stainless steel with a metal face. These sensors have specialized circuits that sense through metal and provide a robust unit for the harshest conditions (food and beverage washdown). Switches with an IP69K rating have the highest protection, enabling close-range 1,400 psi high-pressure and high-temperature washdown.
Sensor label information has evolved over the years as well. A label tag hanging on the sensor cable used to include the electrical specs, schematic, and part number. Many times this label would tear or wear leaving no identification. Adhesive labels were then moved to the sensor body, and are still found on low-cost units today.
Many switches now have label information laser-marked on the metal housings. This mark is permanent. It won’t fall off or wash off when sprayed with a routine high-pressure caustic washdown, or simply due to the effects of temperature or time. This permanent mark can make a big difference when trying to determine the proper replacement part when a switch fails.
Some environments require a specialty switch. For example, areas with resistance welding equipment require weld-field immune switches to eliminate false triggering due to electrical noise. Extreme temperature environments might also require specialty sensors. For those sensors, typical temperature ranges are -13 to 176 F, but models are available to operate below -40 F and above 300 F.
A variety of issues can affect the reliability of inductive proximity switches in industrial applications. These issues include:
- Impact with the sensor face
- Coating of the sensor face
- Water ingress
Awareness of these problem areas can forestall problems when selecting new switches and replacing failed ones.
When a switch issue or failure occurs, it may not be wise to replace it with the same type of switch. The cause of the failure should be determined and that information should be compared to available switch functionality and options. Switch failure and replacement are not trivial events. Depending on the application, users could improve switch operation by considering upgrade options.
Sensor face impact is probably the biggest cause of switch failure. Although a sensor face should never be used as a hard stop for a cylinder or other moving item, occasional contact with a moving element can occur due to the many facets of machine operation. When this occurs, the sensor face can be worn away, or a piece of metal can be embedded in the face. Either of these problems can cause switch failure.
If the sensor face is plastic, replacing it with a metal-face version can prevent the problem from occurring, or at least lessen its frequency. Altering how the target travels in and out of the sensing field can help prevent over-travel or slack from causing sensor face impact.
Specifying a switch with an extended sensing distance reduces the chance for damage due to incidental contact with moving elements. With a limited sensing range, the sensor face may be too close to the target. When this occurs, the senor face can rub on the metal target, leaving a metal film on the face. This deposit can be detected, causing the switch to remain on or energized due to sensor hysteresis.
Another common switch failure is water ingress. Often the solution is simply tightening the quick disconnect cable. When replacing a switch in washdown or other wet applications, the environmental rating options for the switch should be examined to see if an upgrade to a model with a higher level of protection is warranted.
During equipment installation and startup, or due to failure of surrounding equipment, switch short circuit or overvoltage can be a problem. Excessive current to a device can melt it, so supplemental fast-blow fusing should be employed. Overvoltage can cause similar problems. Perhaps this issue can be resolved by installing a power filter upstream of the switch. Although most sensors have some level of short circuit and overload protection, adding these prevention measures is good practice.
When properly specified and installed, an inductive proximity switch will provide reliable operation for an extended period of time. If failures are occurring at an unacceptable rate, issues can often be rectified by changing to a different switch more suited to the particular application.
Andrew Waugh is the product manager for sensor and safety products at AutomationDirect. He has more than 14 years of experience with machine sensor and safety devices on packaging, assembly, material handling, and process control equipment.
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