By Jeff Allison, Pepperl+Fuchs GmbH
Here is a modern photoelectric sensor scheme to limit depth of perception.
Fig. 1. Triangulation of light from the emitter, E, to the target, to the receiver, R, for a background suppression (BGS) sensor.
As the old saying goes, use the right tool for the right job; the same goes for sensing. Using the right sensor makes the difference between seeing what is versus what is not. In a production environment the right photoelectric sensor might save you from envelopes not being counted correctly, machine parts not being error-checked, or even conveyor traffic that is not properly regulated.
Background suppression (BGS) sensing was developed in the late 1970s to address some of the limitations of the then conventional photoelectric sensing methods. Since its inception, BGS sensing has developed into a mature technology suitable for the demanding requirements of modern manufacturing. In the last decade, developers have increasingly selected BGS sensors due to improvements to the technology and a rising popularity in the sensor world.
In the past, options for photoelectric sensing were somewhat limited. As a result, sensors that seemed to work in production the first time around would have to be constantly prodded, tweaked, and cajoled to function as required. If a sensor falsely detected (false positive) or missed spotting an object (false negative), the sensor would have to be repositioned, readjusted, or replaced.
A conventional photoelectric sensor detects the intensity, or amount, of light directed to its receiver. In one condition, the sensor’s receiver sees a smaller amount of light, and in another condition, it sees a larger amount of light. The sensor’s output changes to indicate which condition is present.
BGS photoelectric sensors (Figure 1) build on the traditional integrated diffused approach by adding one more photodetector to the receiver so that the angle of the reflected light may be found from the difference between photodetectors, effectively determining distance through triangulation. In Figure 1 the light from the sensor emitter, E, is reflected by the target back to the receiver, R, at angles that depend on the target’s distance from the sensor. As the target approaches the sensor, the angle of the reflected light becomes larger which increases the signal at the receiver’s near element, N, compared to the signal at the receiver’s far element, F.
BGS depends more on where light is reflected from, as opposed to how much light is reflected. Three major advantages of BGS over traditional diffused methods are increased applicability, higher resolution, and background suppression.
BGS senses more surfaces
Variations in target surface texture, color, and reflectivity minimally impact performance. With this independence from the surface characteristics the BGS sensor detects a brown carton and a white carton, or a printed wrapper and an unprinted black wrapper, and a shiny foil wrapper with the same efficacy (figure 2).
Fig. 2. Sensing Consistency of BGS sensors. From the color extremes of white to black and everything in between, when a BGS sensor is programmed to a specific maximum distance, the color of the detected object has a negligible effect on the actual sensing distance.
Retroflective and through-beam sensors have similar performance with varied object surfaces. Diffused sensors depend entirely on the quantity of reflected light, which is directly related to surface absorbance.
BGS senses with higher resolution
With less than one-half the beam diameter of its diffuse counterparts, a typical BGS sensor has a small light spot. Much like the sampling rate in discrete systems, the light spot width is the limiting factor for resolution in photoelectric sensing. A target object should not be smaller than the sensor’s light spot. The larger the spot, the more often that both elements of the BGS receiver become saturated which makes for a less precise threshold distance. The smaller the spot; the greater the resolution.
As a result of the increased resolution, the object size that can be detected with BGS decreased. The intensity of reflected light required by diffused sensors is difficult to distinguish as the feature size diminishes.
BGS sensors are accurate and repeatable trigger sensors for detecting the leading edges of materials. Comparable retroreflective sensors have a beam size of 70 mm at a distance of 350 mm, whereas the BGS sensor at the same distance can use a 15 mm spot size.
BGS ignores the background
The last but perhaps most significant benefit of BGS sensors is its very namesake. Simply put, BGS suppresses the background information. It is a way to discriminate against objects that are behind the desired threshold, thus increasing the effective signal-to-noise ratio.
With an understanding of conventional and BGS photoelectric sensing, let’s take a took at some real world examples in which BGS sensors replaced traditional sensing technologies.
Example 1: Load detection on palletizer
The first example is a palletizer in the material handling industry that shrinkwraps varying sized loads automatically. The position and height of the load determine where and how the wrap is applied. The old solution employed magnetic sensors that required manual input of the load size.
Pallet contents could contain brown corrugated cardboard boxes, white boxes, and black boxes, which the legacy photoelectric sensors inconsistently detected. A reflector or thru-beam arrangement may have worked, but in this case it would have required mounting space that was not available on the equipment.
A long-range BGS sensor with a two-meter maximum sensing distance was selected to detect all types of loads consistently. The total number of sensors needed per palletizer was reduced, and the setup and changeover times were decreased.
Example 2: Leading edge trigger of textiles
The second application had used conventional diffuse sensors as a trigger of a textile to process. Sensing the leading edge signaled the machine’s controller to initiate an action such as cutting, sorting, folding, coating, stitching, or transferring the textile. The machine had been modified so that the sensor did not see machine components under the textile, but periodically the diffuse sensor would not see very dark textiles, such as black or navy blue. These were nuisance problems, but still resulted in lost productivity.
As a potential remedy, the engineering team evaluated fixed-range BGS sensors. The results were a precise trigger point and reliable color-independent detection.
In contrast to the prior system, which required push-buttons and potentiometers, the new implementation of a fixed-range BGS sensor was a “tamperproof” solution for its lack of external manual controls.
The reliable and precise triggering allowed the textiles to be detected and manipulated without the former nuisance misses.
Example 3: Error-proofing stamped parts
In the third case, an automotive parts manufacturer needed to confirm the presence of stamped holes and slots in metal parts. If the slots and holes were not present, the part would be rejected for rework to avoid a defective assembly. The size of the holes and slots could be less than one inch (< 25 mm).
Retroreflective and thru-beam sensors were not possible, as there was no space to mount a reflector or receiver housing opposite the part. Standard diffuse sensors, even laser-based ones with small light spots, would detect a portion of the machine or other components attached to the part.
A laser-based BGS sensor detected gaps and holes as small as 3/16 inch (4 mm) without sensing objects behind the hole. With the background out of the picture, a wider range of hole sizes could be detected.
While these examples offer a snapshot across several industries, machines, and applications, BGS is not limited to these industries or applications. They are suitable in a comprehensive list of other markets and applications, such as parts counting in assembly, presence verification in pharmaceutics, board positioning in solar cell handling, and low-level carton magazine detection in packaging.
Photoelectric Sensor Review
All photoelectric sensors have a light source and a light receiver. An object may obstruct or reflect light from the source to the receiver. Depending on the configuration of the sensor, the presence or the absence of the object may trigger the output. The three most common types of photoelectric sensing are diffused (proximity-sensing), retroflective, and through-beam (opposed).
Fig. 1. Typical photoelectric sensing modes: (a) diffuse and BGS, (b) retroreflective, and (c) thru-beam.
Diffused sensors (Figure 1a) detect the quantity of reflected light by an object. Some objects reflect a lot of light, while others reflect very little or even no light. When the reflectivity of the target object does not change, this diffused type of sensor may be adequate. In general, these sensors tend to be inefficient, imprecise, and impractical for applications that involve objects with varying reflectivity.
Retroflective sensors (Figure 1b) are similar to diffused, but instead of the object reflecting light it absorbs the light between the source and the receiver.
Through-beam sensors (Figure 1c) are similar to retroflective. Both detect a change in the light passed from source to receiver, but instead of a reflector there is a receiver separate form the source.
Filed Under: Sensors (position + other), Test + measurement • test equipment