A background check for photoelectric sensors

May 19, 2005
What's behind the target can make a tremendous difference in how well reflective sensors do their job.

Gary Frigyes
Ed Myers
Jeff Allison
Pepperl+Fuchs Inc.
Twinsburg, Ohio

The dramatic drop in sensing range for dark targets becomes evident from comparison graphs for ordinary diffused sensors. This example, which is typical, shows a 190-mm differential between sensing black and white targets. In contrast, use of triangulation techniques for background suppression can reduce black-white differentials. The difference is only 10 mm in the second example with an infrared light source, also a typical result.

Mechanical background-suppression techniques uses lenses to focus reflections from a target and from the background on two different light receivers. The sensor decides whether there is a target present based on which receiver is getting more light. Electronic background suppression uses a similar logic about reflections but employs a continuous array of detectors. The system senses a target when reflections hit a threshold level on the portion of the array corresponding to the reflection angle for the target.

Deadband refers to a region where the target is too close to the sensor to permit measurement. It begins at the point where reflections from the target significantly miss the receivers.

As a border between black and white areas on a target passes through the light beam of the proximity sensor, the sensor output can turn off momentarily. This Pepita effect can be fixed by either rotating the sensor 90° so the sensing axis aligns horizontally instead of vertically, or moving the sensor closer to the target.

The cross-eyed effect can arise if the target is smaller than the sensor light spot. Most light energy passes the target and hits the background surface, making the sensor believe there is no target present. The solution is to keep the diameter of the sensor light spot smaller than the target.

The blinding effect arises when too much light reflects back to the sensor. This overload can happen when a background-suppression sensor is aimed at a reflector, a mirror, or is perpendicular to a shiny object. The remedy is to angle the sensor slightly (about 5°) so it is not perpendicular to the target/background, thus reducing the light reflected back to the sensor.

Consider a photoelectric sensor operating in simple diffused mode (i.e., as a proximity sensor). The sensor uses the target to reflect light back and senses the reflection as an indication of the target's presence. A point to note is such applications work properly without a secondary device such as a reflector.

But there is a downside to working without a reflector: It may be tough to precisely control the sensing range even if the sensor has a sensitivity adjustment. This conundrum can cause significant problems when sensitivity is such that the sensor accidentally triggers on shiny objects well beyond its specified sensing range. In fact, it is not uncommon for a sensor with a specified range of 15 in. to falsely detect a piece of metal, Plexiglas, or other highly reflective object 6 ft away or more.

Targets can vary greatly in color and their color will directly affect the range of the sensor. This variability of sensing distance is known as black-white differential. The black-white differential is simply the difference in distance between where a diffused sensor detects a 90% reflective white card and where it detects a 6% reflective black card under the same conditions. Sensor manufacturers normally graph black-white differential data to give engineers a guideline for their applications.

There can be a dramatic drop in sensing range when dark objects are involved. The effect arises because diffused sensors recognize a target based on the light reflected back to them. The reflected light must be strong enough to overcome any ambient light or any electrical noise at the sensor receiver. So a black target absorbs large amounts of energy and therefore must sit closer to the sensor to be detected.

A special technique referred to as background suppression lets users precisely control the sensing range of sensors operating in diffused mode. Moreover, background suppression can make these sensors far less sensitive to target color.

Sensors with background suppression can use either visible-red or infrared (IR) light sources. The obvious benefit of visible red is that it simplifies the process of aligning the photoelectric sensor with the target. Of course, humans can't see IR, but these sources put out more power and are less sensitive to target color. Their higher optical power output arises because IR sources are more efficient than those putting out visible red. So sensors that use IR light can detect objects farther away. And photoelectric sensors with both background suppression and IR sources have smaller black-white differentials than similar sensors with visible-red sources.

Background suppression also provides a very small, bright and clearly defined light spot to give the user a high level of precision and repeatability. In addition, the small spot lends itself to detecting small objects in the presence of a background with the same reflective properties (i.e., verifying that a piston ring sits in the groove of a piston).

The basic principle behind background suppression is triangulation. An LED transmits light through a lens in a straight line toward the target. The target reflects light back to the receivers at some angle. The distance between the sensor and target determines the angle at which light reflects back. The closer the target is to the sensor, the greater the angle of reflection.

In addition, the sensor uses more than one receiver element to sense light. The elements sit at different points off the axis of light emitted. Thus the sensor differentiates between a target and the background by noting which element is receiving more reflected light. The reading is based on the distance from the axis of the outgoing light, not on the amount of received light.

Diffused mode sensors with background suppression can operate at either a fixed or variable range. And background suppression can take place either mechanically or electronically. Mechanical background suppression uses physical lenses to focus light reflected from the target and from the background onto light detectors. Electronic suppression generally replaces the receivers with a sensing element called a position-sensitive device (PSD) whose threshold sensing point is electronically programmed. There is a difference in cost between the two methods, as well as size and performance trade-offs.

Mechanical background suppression offers better optical performance and a sharper cutoff range. Mechanical background suppression is inherently more stable over temperature changes. But electronic BGS has a clear advantage in the presence of heavy vibration.

Mechanical background suppression also demands that the sensor have two receiving elements and an adjustable lens. The extra parts take up more real estate in the sensor housing. If the size of the housing is an issue, electronic background suppression is the better choice.

Background suppression brings with it a few challenges, most of which succumb to forethought and planning.

These sensors generally work at shorter ranges than standard diffused sensors. They also require some minimum distance between the target and the sensor. Targets too close to the lens reflect light at too wide an angle so none of it reaches the receiver. The minimum sensing range is typically under 10% of the sensor's full range.

Mounting and positioning systems are especially important for sensors that operate at a fixed sensing range. The reason is they have no sensitivity adjustments. This makes them tamperproof, but the mounting system may need to allow fine-tuning of the sensing distance and angle to the target.

Fixed background suppression is more difficult to install, but the lack of mechanical parts may make it a less expensive option. An alternative is a background-suppression sensor with an adjustable range. Here an external potentiometer serves as the adjustment mechanism.

There is a special case when a target contains two contrasting colors such as black and white. Suppose a sensor's light spot shines simultaneously on the two contrasting colors. If the target lies at the outer limits of the sensing range, the more reflective side of the target can reflect more light back to the far receiver than to the near receiver. As this border of the target passes through the light beam, the sensor output can turn off momentarily.

This action is referred to as the Pepita effect. It can happen if the sensor is improperly oriented with respect to the black-white border. It's also a possibility in targets with an extreme variation in reflectivity or their contours. The fix for the Pepita effect involves either rotating the sensor 90° so the sensing axis aligns horizontally instead of vertically, or moving the sensor closer to the target.

Another peculiar phenomenon is the cross-eyed effect. This can arise if the target is smaller than the sensor light spot. In this case, the target can't block the sensor light beam. Most light energy passes the target and hits the background surface, making the sensor believe there is no target present. The solution is to make sure the diameter of the sensor light spot is smaller than the target.

The blinding effect is a final challenge. It arises when too much light reflects back to the sensor. The light overload prevents the sensor from being properly adjusted between the near and far distances. This can happen when a background-suppression sensor is aimed at a reflector, a mirror, or is perpendicular to a shiny object.

The remedy for blinding is to angle the sensor slightly (about 5°) so it is not perpendicular to the target/background. This will reduce the light reflected back to the sensor so photo receivers don't get overwhelmed.

Dealing with darkness

The variable range feature makes the Series 28 sensors effective on a variety of applications. The ability to define the sensing range is particularly useful on rotary tower systems, which feature a carriage that rotates around the package. When the sensing range is adjusted to the width of the conveyor, the sensor, circled at left, ignores everything beyond the conveyor belt.

A packaging application needed a sensor able to not only detect all shades of black without error but also to work over an extended sensing range of at least 4 ft. Thus proximity sensors had to reliably detect flat black and glossy black materials. But a common problem with photoelectric sensors is that dark colors absorb more light than they reflect, which potentially shortens the sensing range.

A Pepperl+Fuchs' Series 28 photoelectric sensor with background suppression and an infrared light source handled the application. The background-suppression sensor is a diffused mode photoelectric that uses separate sensing elements to detect objects close together without detecting the background material. Its sensing range for black is just 5% less than its range for white targets, and it can operate at a variable distance. Developers installed the sensor on the yellow carriage of the top-loading machine. In operation it senses the contours of the package to let the machine automatically wrap any size package. Highly visible LEDs give operators a visual status indication from as far away as 50 ft.

 Edited by Leland Teschler



Pepperl+Fuchs Inc.
(330) 425-3555

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