Most operators and maintenance personnel don’t think about the sensors keeping a machine running, and about how the sensors reduce the risk of tooling crashes and damage due to malfunctions. Many proximity sensors are used in automated equipment, so it’s good to have a basic understanding of these critical devices.

Proximity sensors allow non-contact detection of machine tooling and objects in a variety of automation applications. This includes sensing the end-of-stroke, travel or index of tooling; as well as part counting, presence, verification and orientation functions.

Selection criteria

There are several kinds of proximity sensors including inductive, magnetic, capacitive and ultrasonic. Inductive proximity sensors detect the presence of metallic objects from a distance of a few millimeters up to about 40 mm, depending on sensor size and the type of metal. Magnetic sensors work like inductive sensors but use a magnet on the target to extend the sensing range.

Non-contact capacitive sensors work well for detecting both non-metallic and metallic materials including metal, plastic and wood. They can also detect liquids, powders and pellets through container walls up to about 40 mm thick. Ultrasonic sensors, using sound, can detect most materials as well, but at a much greater range.

Proximity sensors are commonly available for three sensing distances: standard, extended and triple distance—with the sensor body and related sensing area affecting sensing distance. The sensing distance depends on the application, and may influence how close a sensor can be placed to the sensed object. The sensor specifications must be carefully checked to confirm it meets installation requirements.

Not only may a longer sensing distance ensure a more robust installation, it may also help keep a sensor cooler by keeping it further from a heat source, extending its life. It may also eliminate the need for a more expensive high-temperature sensor.

Inductive and capacitive sensors are available in shielded and unshielded designs (see Figure). Shielded sensors can be installed with the sensor face flush mounted in a metal surface, but this reduces the sensing range. Unshielded sensors have greater sensing distance, but the sensor face must protrude from a metallic mounting surface. If mounting in a non-metallic material such as plastic, wood, etc., a shielded sensor is not required to flush mount the sensor.

Proximity sensors are available in a variety of sizes and physical configurations. Smooth cylindrical and threaded barrel proximity switches are available in diameters as small as 3 mm, up to 30 mm. They are also available in rectangular form factors. While most are single-piece designs, some ultrasonic proximity switches are a two-piece, thru-beam design, as with some photo-eye configurations.

Discrete sensors

Discrete sensors typically provide on/off, solid-state outputs at common industrial voltages to indicate the presence of an object. These outputs can be a sourcing PNP type or a sinking NPN type. Analog sensors provide a variable output, such as 0-10 Vdc, to produce an output proportional to the target distance.

Discrete sensors are typically available in 2-, 3- and 4-wire configurations. While often chosen based on personal preference, there are some important technical differences. A 2-wire sensor can connect to both sinking and sourcing inputs or devices. While there are only two wires to connect, these devices can have higher leakage current, often enough to keep an input on when it should be off.

A 3-wire sensor is probably the most popular, but must be specified as either PNP or NPN configuration. A 4-wire sensor provides the widest application range. It usually includes both PNP and NPN configurations in a single device, and it may be connected in a normally-open or normally-closed configuration.

With both discrete and analog sensor outputs, the rate that the sensor can detect a change in sensor state is the switching frequency. While not a concern in typical applications, the sensor switching frequency is a critical requirement if high-speed detection is needed, or for small spacing between targets.

Sensor connections

Proximity sensors are available with axial cables or quick disconnects (Q/D). Axial cables are typically 2m in length, are molded into the sensor body and exit axially. These types of sensors are the least expensive option, but the cable may not be long enough, and is not easily replaced if it fails. The axial cable exit may also not physically fit in all machine applications.

Quick-disconnect sensors require a separate cable to integrate the device. These cables are industry-standard designs such as M8, M12 or micro-AC, and are available in a variety of lengths with axial and 90-degree connections.

Proximity sensors can be installed in a wide variety of environments. Most can handle the everyday oil, dirt and dust of a factory, and some can also handle intensive cleaning and sanitary washdown. Many of these sensors can also handle temperatures up to 100°C.

However, it is important to meet or exceed the sensor’s specifications if replacing it. For example, sensors in food and beverage applications must be manufactured from materials capable of withstanding solutions used during equipment cleaning, and these materials must be approved by the FDA for use in food production environments.

Following these guidelines will help when choosing the right sensor for your machine automation application, ensuring many years of safe and trouble-free operation.