Thomas Stevic is a controls engineer at Star Manufacturing, an engineering and production company in Cincinnati. Contact him at [email protected].
When designing a control system that incorporates presence sensors, multiple options are available and each sensing methodology may provide the same detection results. If the sensing requirement is to detect a ferrous metal object within a range of 2 to 40 millimeters, a designer could be faced with a choice between using an inductive proximity sensor or a photoelectric sensor—two of the most common presence sensors.
Standard inductive proximity sensors are typically 8 mm to 30 mm in cylindrical diameter. Smaller- and larger-diameter units are also available, as are square and rectangular sensors. The proximity sensor typically detects an object at distances of 1.5 to 38 mm with the range greatly influenced by the sensor diameter.
This sensing range is a critical application requirement that must be carefully specified.
The two main types of inductive proximity sensors are shielded and unshielded. Shielded inductive proximity sensors have threads that extend to the sensing face of the sensor. Unshielded sensors have a projection, almost always a type of plastic, that extends past the end of the threaded barrel. Selection of a shield sensor is best if there is a concern of the effects of surrounding metals.
The inductive proximity sensor creates a magnetic flux. On a shielded sensor, the flux is oval-shaped, and the unshielded is more rounded. The net effect is that an unshielded sensor has a longer sensing distance than a shielded version. This magnetic flux induces electrical eddy currents in the metal object they are detecting. This, in turn, changes the oscillation amplitude that the sensor produces. The change in amplitude is detected by the sensor’s circuitry and causes the output to change state.
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When setting up an inductive proximity sensor, a good rule of thumb is to make the target of the sensor be at least equal to 50% of the area of the sensor face. This rule will almost always result in the sensor reliably operating at the published sensing distance when sensing ferrous material such as steel or iron.
Inductive proximity sensors are designed for the best performance when the target is a ferrous material. Inductive sensors will also work with nonferrous metal such as copper or aluminum, but the sensing distance will be diminished. The detection distance for stainless steel will be about 80% as compared to mild steel. For brass or aluminum the distance may be only 50%. Some manufacturers offer versions of inductive proximity sensors that are specifically designed to detect nonferrous metals.
Most general-purpose proximity sensors with a metal body use nickel-plated brass as the body material. Stainless steel is also often available when the environment requires it. Plastic body sensors are available with a variety of materials engineered to resist water, weld splatter and highly corrosive atmospheres. The shock and vibration resistance for a particular sensor is usually published.
Some lower-cost sensors may have unacceptable ratings or may not be fully potted to resist failure in an industrial application. For the most part, inductive proximity sensors are very tolerant of severe environments. They are mostly unaffected by dirt and contaminates in the air.
Photoelectric sensors have three general operational types, and manufacturers often use different terms for each. One type uses a light source aimed at a separate detector and called a through-beam sensor. When an object breaks the path of the light, the detector output changes state. These sensors increase the wiring requirements, but they are also the least likely to false-trigger.
The second type of photoelectric is called a retro-reflective sensor. The light source is aimed at a reflector, the reflector then returns the light back to the same device that generated the light. A retro-reflective sensor often uses a pulsed light output and polarization to ensure the light detected was generated by the same sensor. Two or more retro-reflective sensors may be placed side by side without cross interference. The reflector often twists the polarized light by 90° so the sensor can differentiate between the reflector and any other reflective surface.
The third type of photoelectric sensor is called a diffuse-reflective sensor. The diffused sensor outputs a light beam and detects the beam’s reflection from any object it hits. The diffuse type of photoelectric sensor is the closest equivalent to an inductive proximity sensor. Diffused photoelectric sensors will detect most any object placed within the sensor’s detection range.
Quite often the diffuse photoelectric sensor has a gain adjustment. This gain adjustment may be an analog potentiometer or a digital value entered into the sensor’s setup routine. By adjusting the gain, the sensing distance may be changed. The diffuse sensor’s performance will be affected by the color, texture and angle of incidence light is reflected from the target. The sensor will detect a smooth white surface that is at a right angle to the beam of light at a greater range than a dark uneven surface that is at a greater angle of specular reflectance.
Unless the light produced by a diffuse sensor is focused, the light pattern exiting the sensor is an inverse cone, meaning that the light will spread out in an ever-expanding beam as it moves further from the sensor’s lens. Focusing lenses are typically an added accessory, if they are available at all. The width of the light beam at several distances is usually included in the technical information. Because the beam of light expands, diffuse sensors have the greatest likelihood of false-triggering. By properly specifying and tuning the sensor, this problem can be reduced.
Most industrial photoelectric sensors are constructed with some type of plastic housing and lens. Sensors with a metal housing and glass lens cover are available. There are three main components to a photoelectric sensor—the electronics, including the light source; the lens; and the lens cover. Photo sensors are usually less tolerant of vibration and shock than inductive proximity sensors. If the sensor is deployed in a harsh environment, fiberoptics may be considered to remove the electronics from the harsh conditions.
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