Over the past few years, industry has experienced tremendous growth in the need for presence sensing in a variety of applications. This has been fueled by the increase in factory automation in Canada, and throughout the world, where the need for target detection is increasing.

In automated systems, objects can be detected either by electro-mechanical means (limit switches) or by solid state devices (photoelectric/proximity detectors). Solid state sensors do not require any contact with the target and therefore are suitable for a number of applications where the detection of objects is required, and in particular where physical contact will disturb a process. They do not have any moving parts that might jam, break or fail as is often found with limit switches under severe duty. Sensors have faster switching speeds than limit switches, with small sensors having the capability of switching (on/off) up to 5,000 times per second. This is important when production efficiency is required with targets moving at high speeds. There are no contacts in solid state devices, therefore no bouncing when the sensor state changes from open to close, or vice versa. This bouncing can cause undue disturbances and spikes on your control system.

Sensors are ideal for use in harsh environments such as cutting oil or submersible applications, and are available in all shapes and sizes to fit any machine or process. There are generally two types of solid state presence detection discrete and analog.

Discrete vs. analog

To put it simply, a discrete sensor is either on or off it merely detects whether an object is present or not. But if the process requires indication of where or how close an object is, an analog sensor will produce an output signal that's directly proportional to the distance between the object and the sensor. This is becoming more common with the increase of variable frequency drives in machinery, and other processes where speed control and accuracy can greatly impact manufacturing. Most applications today call for discrete detection, so that will be the focus of this article.

After the sensor is selected, the connections must be considered. Although simple for some, users can be confused by two, three or four-wire sensors. The focus here will be on two and three-wire versions, as they are the most common. One consideration for the sensor connection is the output type: solid state or relay. The main difference between these two is the switching capacity (in mA). A sensor with a relay output typically has a switching capacity about ten times that of a solid-state output. (Details of the exact capabilities are provided by the manufacturer.) Here are some considerations for two- and three-wire sensors.

Sourcing or Sinking?

Without spending much time with DC transistors, this often can be confusing. With DC sensors, the output switching circuitry uses a transistor, so polarity must be considered. The sourcing transistor is the compliment of the sinking transistor, therefore PNP sensors are connected to NPN loads.

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Current flows from the positve terminal, through the load, through the transistor, to the negative.

Current flows from the positive terminal, through the transistor, through the load, to the negative.

The easiest DC sensor to apply is one with universal options. This means that by changing wiring connections, a sensor can be connected to either NPN or PNP loads, and may even have N/O or N/C options. The sensing distance required will usually indicate the type of detection. Proximity sensors can detect up to 60mm, so beyond this range a photoelectric detector should be applied. Proximity sensors are available in different types for different applications.

Proximity Detection

The two main types of proximity devices are inductive and capacitive. Inductive sensors are designed to detect metal targets, using an oscillating field which acts as the interface between the target and the sensor. Once the electromagnetic field is induced into the target, it triggers the output, which sends the signal to the load. Capacitive sensors contain a high frequency oscillator, with a capacitor plate built into the end of the sensor. When the target is within the sensing area, a strong capacitive coupling develops and changes the output from one state to another. Capacitive sensors are ideal for sensing glass, plastic, liquids and other non-metallic objects.

Sensing Distances

Most sensors have standard capabilities with regards to the distance they will sense. The confusing part is how to judge a sensors performance for your application. Manufacturers list Sn and Su for sensing distance rates.

Sn - is the nominal sensing distance, which is the benchmark that most manufacturers follow. It is the sensing distance using a Òstandard target (for inductive proximity sensors, its made of steel and is the same diameter as the sensor) and applies to applications with a constant temperature and voltage supply. Su- is the useable sensing distance, a more practical measurement that allows for a 10 per cent fluctuation in voltage and temperature. This is usually the best range to follow because most applications do not have perfect conditions and use real targets, not standard ones.

Although many factors affect the performance of a sensor Ð like ambient
temperature, target size and sensor diameter Ð the target material may have the greatest influence. The simple rule for inductive proximity sensors is that the metal with the least conductivity will provide the greatest sensing distance for detection.

An 18mm proximity sensor will typically have an Sn of about 8mm. The Su for this sensor is about 6mm and using a copper target would reduce this to about 3mm. As technology has advanced, manufacturers have developed inductive proximity sensors that are not influenced by the target material. In other words, these sensors will detect copper, aluminum and steel at the same sensing distance. So from the point of view of an industrial user or distributor, this is an option to help reduce inventory.

Temperature can wreak havoc on a sensors performance, as extreme fluctuations may influence the electronics. Sensors can be supplied to meet either extremely high or low temperatures. Target thickness and size is also important. Simple rules of thumb are: the target should be the same size as the sensor face (or larger) to ensure detection; and the target with the greater thickness will be the easier to detect.

Photoelectric Selection

Photo sensors operate via the transmission of light to a receiver. Photoelectric sensors are equipped with light-emitting diodes that transform an electrical signal into a monochromatic luminous signal. Ambient light does not effect the sensor operation because the current flowing through the LED is modulated to produce a pulsing light transmission. Once this beam is broken, it triggers the output, which can be set for light or dark operating modes. The load will either switch when the receiver is dark (object blocks beam-dark on), or the load will switch when the light is on the receiver. To simplify the application, many photoelectric sensors now offer selectable light or dark switching modes.

Photoelectric Sensor Types

Summary

In the past, sensor technology was so complex that it kept traditional electromechanical users from converting to solid state devices. However,
today's universal sensors and software packages make choosing sensors a snap. Sensor selector software allows the user to simply fill in the criteria about a product or application and the programme makes the best selection given the data provided. The software will also provide drawings, specifications, mounting recommendations and options to simplify the application of sensors for any requirement.

As manufacturers face the need for controlled costs and increased production rates, more and more systems are automated. On these automated systems, sensors play a critical and growing role in the modernization of industry. Sensors are quite simple for most applications, and not only communicate with programmable controls, but can increase product quality and manufacturing speed, as well as reduce size and costs.