How Do Light Convergent Reflective Sensors Work?

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Have You Ever Wondered How Sensors Detect Objects of All Colors and Textures?

Sensors are essential to enhancing productivity and safety in several industries, including factories, where automation has become increasingly commonplace. Photoelectric (optical) sensors are ubiquitous nowadays, safely controlling elevators, opening grocery store doors, and even detecting winners at racing events.

The continuous evolution of automated industrial equipment requires that sensors discover objects with exacting precision, while being immune to container and package design variations. Conventional diffuse reflective sensors function well with objects of certain material and color combinations, but can become unstable and inaccurate when specifications differ. In contrast, sensors based on convergent reflective technology are able to detect objects having a much wider variation in material, color, or reflectiveness.

This Tech Spotlight article focuses on light convergent reflective sensors, including how they work, their advantages and disadvantages, and example applications.

What Is the Difference Between Diffuse and Convergent Reflective Sensors?

A photoelectric or optical sensor works through the transmission and reception of light. A common method of target detection is by the use of diffuse reflective sensors. Another approach of target detection is with convergent reflective sensors. Let's learn how these two types of sensors work.

• How Do Light Diffuse Reflective Sensors Work?

In a diffuse reflective sensor, objects are detected based on the light that they reflect. A transmitter sends a beam of light in a specific direction. If there is an object present in the beam, it will reflect some or all of the light. A receiver, sharing the same housing as the transmitter, senses the reflected light, successfully detecting the object. Although it depends on the sensor size and power, diffuse reflective sensors typically have shorter detection ranges than other designs, generally less than one meter. Additionally, an object's color, finish, size, and shape directly impact the amount of light that is reflected back to the sensor.

• How Do Light Convergent Reflective Sensors Work?

A light convergent reflective sensor works based on a convergent beam principle. Unlike diffuse sensors, the convergent reflective sensor uses a lens system that focuses the emitter and receiver elements to a point in front of the sensor. The sensor is able to detect objects in the area (sensing area) where the emitting beam and the light-receiving area overlap, as shown in Figure 1. Because the light energy is focused on a smaller area, gain is increased, allowing the sensor to more easily detect narrow or low reflectivity targets, and making it color and texture agnostic (black, white, or shiny).

Figure 1: Light diffuse reflective type (left) and convergent reflective type (right) sensors

Image Source: Omron Light Convergent Reflective Type Sensor B5W-LB Series Manual.pdf

Let's Dig a Little Deeper Into the Light Convergent Reflective Sensor

A light convergent reflective sensor consists of an integrated light source (LED), a receiver (phototransistor), and an optical system (lens), as illustrated in Figure 2. Optical lens structures focus the beam from the light source, as well as the detection path of the receiver, limiting the detection range to the area in which the beam and detection path converge. If an object is present in the detection range, it will reflect light back to the receiver. Because both the LED and receiver are focused rather than spread out, sensitivity is much higher, increasing the range of textures and colors that can be detected. The sensor is unable to detect outside this sensing range.

Figure 2: Structure and Working of a Light Convergent Reflective Sensor

Let's Take a Look at The Advantages and Disadvantages of Light Convergent Reflective Sensors

The primary advantage of light convergent sensors is their more accurate detection of difficult materials, such as shiny textures or dark colors. Their other advantages over traditional diffuse reflective sensors include:

Advantages

• Greater Accuracy: Figure 3 shows one scenario where a convergent reflective sensor yields more accurate results when compared to a traditional diffuse reflective sensor. Because the beam of a diffuse reflective sensor might sense objects from farther away, it may confuse backgrounds for objects. A convergent reflective sensor limits detection range to a small area in front of the sensor, so objects just outside of this area are not detected. Additionally, a convergent reflective sensor can more accurately detect dark colors because of its increased sensitivity.

Figure 3: Background Effect with Traditional Reflective (left) and Convergent Reflective (right) Sensors

Image source: Omron

• More Stability: Convergent reflective sensors have more stable and accurate detection across the color spectrum, and for a larger variety of textures.

Figure 4: Object Shifting with Convergent Sensor

Image Source: Omron

• Ability to Detect Shifting Objects: A convergent reflective sensor is capable of detecting shifting objects, due to its increased sensitivity and wider sensing range. In the Omron B5W-LB, the enhanced sensing range is made possible by the use of four toroidal lenses, which help ensure a robust signal even at times when the distance of the object is not static.

• Ability to Detect Objects of Specific Shapes: Because the multiple lenses of a convergent sensor are focused on a small detection range, they have additional flexibility when used to detect objects of specific shapes. One example is a radiused object (cylinder or circle). A convergent sensor can focus on the nearest point (tangent point) of a radiused object, making it useful for applications like counting stacked cans or bottles.

Disadvantages

• Limited Detection Range: The primary disadvantage of a convergent sensor is its limited detection range. Objects that that are too close or too far will not be detected. Additionally, objects that are too far above, below, or to the left or right of the focal point of the sensor’s lens will not be detected.

• Object Detection at Predictable Distances: Convergent sensors are only suited to applications where detection targets appear at predictable distances from the sensor.

Omron's Light Convergent Reflective Sensors

Omron offers a range of light convergent sensors, including the B5W-LB series, which consists of a proprietary lens structure (with visible cutting filter), an infrared LED, a light receiver, and an analog circuit. The B5W-LB series is available in miniature and super-miniature form factors, with sensing range from 2-10 and 10-55mm, and digital and analog output models. Table 1 shows the product line of the B5W-LB series.

Table 1: The B5W-LB Series

Image Source: Omron Light Convergent Reflective Type Sensor B5W-LB Series Manual.pdf

What Can You Use Light Convergent Reflective Sensors For?

Light convergent sensors are ideal for detecting objects at predictable distances, as well as objects made from materials which are typically difficult to detect. They are used in a wide variety of equipment, including:

• Analytical Instruments
• Food Manufacturing Automation
• Sanitation
• Coffee Machines
• Vending Machines
• Dental Equipment
• Medical Packaging Machines
• Printers
• Robot Material Handling
• Automated Test Equipment

1. Cup Detection for a Coffee Machine: While a traditional diffuse reflective sensor can be used to detect the presence of cups in a coffee machine, the problems inherent in their design make them less than ideal. Coffee mugs come in a wide variety of colors and textures, many of which a traditional diffuse reflective sensor has trouble detecting. On the other hand, light convergent reflective sensors can successfully detect a much wider variety of colors and textures. A cup’s position is also important. The coffee maker should run only when the cup is in the correct position. Because of the small area created by the converging beams, light convergent sensors are much more accurate at detecting a cup’s position.

2. Touchless Sensors on a Vending Machine: Touchless buttons enable a user to "press" a button without physically pressing a button, providing better hygiene and decreasing the potential for mechanical failure. Touchless buttons can be created using light convergent reflective sensors. Because each “press” only occurs when the user’s finger comes in close proximity with the button, detection range is predictable. Light convergent reflective sensors will also not be confused by objects in the background, such as another person standing close by. They are able to detect a wide variety of colors and materials, making detection accurate whether the user is wearing gloves or using bare hands.

Figur-e 5: Vending Machine with Convergent Reflective Sensor

Image Source: Omron

Vending machines often have doors that open for users to retrieve products. Because vending machines have varying graphics and design, their doors may not always be the same color, making it difficult to use a traditional reflective sensor to detect if a door is open or closed. Light convergent sensors are better suited, because of their higher accuracy in detecting objects of different colors.

Multiple applications such as food manufacturing equipment automation, hand detection for sanitation equipment, and touchless sensing for dental equipment depend on light convergent sensors for object detection.

Let's Recap: Light Convergent Reflective Sensor Technology

Light convergent sensors have several advantages over conventional reflective sensors for many applications, the primary one being increased sensitivity to a wider range of colors and textures. Light convergent sensors have a smaller detecting area, making them ideal for applications where the target object is at a predictable distance from the sensor. Like traditional diffuse reflective sensors, they are self-contained, making them convenient for applications with space constraints.