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Photoelectrics Have the Light Stuff

Some sensors sense discrete objects, while others detect process variables such as pressure or temperature. Proximity switches, limit switches and photoelectric sensors constitute the bulk of discrete sensing components.

Of these, photoelectric sensors are known for their versatility.

Photoelectric sensors, unlike proximity and limit switches, do not always require installation close to the object to be sensed. This feature makes them a workhorse of discrete automation systems.

Guerrino Suffi, sensor marketing manager at Schaumburg, Ill.-based Omron Electronics LLC (www.omron.com/oei), describes a photoelectric sensor as “an industrial optical switch comprised of a light emitter source and a light detector sensor that is used to determine presence or absence of an object. The detection is based on the change of the condition of the received light.”

There exist three basic sensing modes in photoelectric sensors: through-beam, retro-reflective and diffuse. A through-beam sensor consists of a sender and a receiver in different housings aligned on either side of the object to be detected. A break in the light beam activates the output. A retro-reflective sensor contains light sender and receiver in the same housing. A separate reflector returns the light to the device, with a break in the light received triggering the output. The diffuse sensor also contains the sender and receiver in the same package, but it relies on light reflected from the object for detection.

Sensor characteristics

As shown in the chart, diffuse sensors must be placed close to the object to be sensed. Retro-reflective sensors allow more variation in installation. Through-beam sensors have much more power than the others, and can even be used to detect traffic across a driveway. The main drawback in through-beam sensors is installation. Aligning the sender and receiver, especially across large distances, can be challenging.

Tom Rosenberg, industry manager at Balluff (www.balluff.com), a Florence, Ky., sensor products company, notes a new product that contains the sender and receiver in a U-shaped housing that pre-aligns the sender and receiver, expediting installation.

Photoelectric sensors’ emitters are typically offered in two varieties, infrared light emitting diode (LED) (9400Å) source or visible beam LED (6600Å) source, states Omron’s Suffi. The infrared LED source has longer sensing distances and the possibility of smaller beam diameters. Naturally, the drawbacks are the invisible source itself being harder to set up with the naked eye. Infrared sensors are good for detection of opaque objects only, because they shine right through clear materials. The visible LED source has the advantages of alignment ease, color contrast detection, and the ability to detect translucent targets under the correct conditions. The main drawback is a reduced sensing distance when compared to the equivalent infrared sourced sensor.

Three critical technical parameters, according to Suffi, include excess gain, signal-to-noise ratios, and repeatability of the response time.

Excess gain is the actual light level detected compared against the minimum light amount required to change the output state of the sensor. The extra light energy can be used to overcome energy or signal loss due to dust, dirt, smoke, or moisture. An excess gain of “zero” means none of the light emitted is being detected.

Signal-to-noise (S/N) ratio is equal to the light level detected in the “light condition” divided by the light level detected in the “dark condition.” That is to say, detection ability from the object, divided by detection ability from the background. It is important to optimize an application’s S/N ratio for a higher sensing reliability.

Response time for photoelectric sensors is the amount of time required for the sensor to respond to a change in the input signal. It is the time required to transition from the ON state to the OFF state. This timing is critical for high-speed sensing and small parts detection.

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