Making the right sensor choice

From proximity sensors to limit switches and from photoelectric sensors to machine vision sensors, an almost overwhelming array of sensors is available for a variety of plant applications.

By Mike Frey February 1, 2000

From proximity sensors to limit switches and from photoelectric sensors to machine vision sensors, an almost overwhelming array of sensors is available for a variety of plant applications. Although various sensors may be able to do the job, how can a plant engineer choose the one that will best meet his needs in the most reliable, efficient, and cost-effective way?

What factors should be considered when sensors are specified for specific applications? Here are some guidelines to follow for the four types of sensing and inspection solutions noted, including how they work and to what types of plant applications they are best suited.

Limit switches

Limit switches (Fig. 1), which require direct contact with the object being detected, provide a low-cost solution to presence/ absence applications. Less complex than many of their noncontact counterparts, limit switches are easy to install, use, and maintain. They are housed in rugged diecast or plastic enclosures, making them excellent choices for chemical and other harsh environments. Limit switches offer much higher current-carrying capacity than solid-state sensors and are therefore also effective for some control circuits.

Although limit switches use older technologies, their safety, ease-of-use, and flexibility features continue to evolve in response to user demand and worldwide safety requirements. More and more, for example, safety limit switches are used to prevent operators in manufacturing environments from opening machinery doors while the equipment is operating.

Safety limit switches that incorporate positive-action contacts are becoming more widely used. They ensure a positive break in contacts to increase worker safety and reduce liability. Positive-action contacts include the spring-actuated contact breaks found in all limit switches, as well as a mechanical backup system that forces the contacts open should they become tack welded together.

In some cases, safety is built into limit switches by including two sets of contacts in the switch: one normally closed (NC) safety circuit and one normally open (NO) control circuit. Some limit switches incorporate optically based, solid-state switching mechanisms that completely eliminate the disadvantages of electromechanical switching, such as short life span, contact bounce, and large profile.

Proximity sensors

Durable construction and noncontact sensing make proximity sensors ideal for most industrial applications. Because they are not susceptible to light, dust, color, or transparency of objects, they are easily used where photoelectric sensors cannot operate reliably. Two types are available: inductive and capacitive.

Inductive proximity sensors (Fig. 2) detect many types of metals. They consist of a coil and a ferrite core arranged to generate a high-frequency electromagnetic field at the sensing head. A metal object passing within the field causes the formation of eddy currents and a subsequent loss of energy. As more energy is lost, the amplitude of the oscillation is depressed. When the amplitude reaches the threshold level, the detector circuit sends a signal to switch the output. When the target object moves away and the amplitude increases, the detector circuit sends a signal to switch the output back to its normal state.

Inductive proximity sensors are popular because they are durable and low in cost. They incorporate solid-state technology instead of electromechanical switching and can operate at speeds of up to 3000 Hz. Their rugged construction makes their life span virtually endless.

Capacitive proximity sensors (Fig. 3) operate by detecting the electrostatic capacitance of materials. They can detect both metal and nonmetal objects, including liquids, powders, and solid materials. A capacitor consists of two metal plates separated by a dielectric material. A capacitive proximity sensor is basically an open capacitor comprised of two metal plates facing away from one another. As a target approaches, capacitance increases, raising oscillation amplitude. The sensor’s detector circuit sends a signal to switch the output state when oscillation reaches operating or releasing levels.

Capacitive sensors are commonly used to detect liquid levels in nonmetallic containers, but they can be used in a variety of applications. However, they are less stable than inductive sensors and are more likely to be affected by surrounding conditions.

Photoelectric sensors

Photoelectric sensors (see accompanying section on photoelectric sensing modes) are probably the most widely used type in manufacturing today. They offer flexibility, providing solutions for numerous and varied applications. Photoelectric sensors consist of a photoelectric light source, detector, (photodiode or phototransistor) and electronics that amplify a detected signal and enable the output circuit to open or close like a switch (transistor, FET, or relay). Photoelectric sensors operate by transmitting a beam of infrared or visible light that is either broken or reflected by a target. They come in four sensing modes: through-beam, retroreflective, diffuse reflective, and convergent beam. Mode is selected on the basis of application requirements. For example: Is a shiny object being detected? Or, how far from the sensor is the target object?

Photoelectric sensors are well suited for presence/absence applications. Examples include gate functions (presence of a label on a bottle), positioning (of a workpiece or cutting table), counting, and inspection (parts placement). Special-purpose sensors, among them luster, background suppression, and RGB color sensors, are available for more demanding applications such as detecting a clear label on a box or distinguishing between similar colors.

Machine vision sensors

Machine vision sensors range from what is basically a two-dimensional sensor to a pattern matching system. They can be set up in a matter of minutes by users who have no previous vision system experience. These solutions are designed to address basic presence/absence and orientation applications.

Traditionally, photoelectric sensors have been the most economical solution for these simple applications. Many users configure several photoelectric sensors for applications requiring two-dimensional inspections, sacrificing reliability and investing time and effort configuring and reconfiguring many devices. On the other hand, sophisticated vision systems often cost a great deal and have a functionality that far surpasses the requirements of the application. To solve basic presence/absence applications reliably, users need simple, low-end vision sensors with limited functionality.

Here is an example illustrating when a vision sensor might be specified and when a traditional photoelectric device might be used. In this situation, parts come from a feeder bowl to a “dead nest.” Parts are oriented according to a characteristic unique to one end of the part. All parts are identical. If high reliability is not crucial and if other parts are not likely to need to be detected, a photoelectric sensor is a good choice. However, if high precision is required and other applications may be added to this line, a vision sensor is preferable. Adjustment of many sensors is time consuming and costly and the level of precision achieved may not be sufficient.

Machine vision sensors are ideal when great precision is needed and a variety of parts may be inspected. Setup of the system is fast, requiring only a few minutes. Although the initial cost is higher, a vision sensor pays for itself quickly in adjustment and reconfiguration costs when the application warrants its features.

As sensor manufacturers respond to user demand for more specialized sensing solutions, plant engineers continue to face an array of sensing choices. Knowing the basics of how various sensors operate and the advantage that each type offers arms users with the knowledge they need to select the best sensing solution for the application.

— Edited by Jeanine Katzel, 630-320-7142,

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Photoelectric sensing modes

There are four photoelectric sensing modes: through-beam, retroreflective, diffuse reflective, and convergent beam. In a through-beam device, sender and receiver are mounted in separate housings opposite one another. The beam shines directly from the sender to the receiver. When a target interrupts the beam, the sensor is activated. This type operates well in dirty environments and is ideal for opaque objects. However, they cost more and must be precisely aligned.

In a retroreflective device, sender and receiver are mounted in the same housing and aligned opposite a reflector. The beam travels to the source, hits the reflector, and returns. When a target interrupts the beam, the sensor is activated. This type is easy to install and less expensive. However, sensing range is more limited and reflective targets can falsely trigger them.

In a diffuse reflective sensor, the sensor and receiver are mounted in the same housing, but the target acts as the reflector. The sensor is actuated when the reflected beam reaches the detector, rather than when the beam is broken. These sensors are easy to install and ideal for translucent or transparent objects. However, ranges are shorter and tolerance to attenuation is lower than with other modes.

In a convergent beam, or focal point, sensor the light source and receiver are set at the same angle from the vertical axis to capture the reflected light. These sensors have a narrow beam width and are designed to detect objects only at the point at which the two beams converge. This arrangement suppresses the background and lets the sensor focus entirely on the object in front of it. These devices are ideal for detecting small or dark objects.