A photoelectric system uses visible or infrared light to perform a variety of functions in both industrial and commercial applications, such as measuring distance or detecting the presence or absence of one or more objects.

Photoelectric sensing employs two optoelectronic devices: a transmitter, also called an emitter, and a photoelectric sensor, or receiver. The transmitter emits a visible or infrared (IR) light beam; the photoelectric sensor detects the light from the transmitter. More complex configurations may have multiple transmitters and receivers.

Transmissive and reflective sensing

There are two methods used to detect a moving object using the transmitter/photoelectric sensor combination (Figure 1).

Figure 1. Transmissive vs. reflective princples. Source: Light in Motion

In a transmissive sensing design, the transmitter and the sensor are physically separated. When no object is present, the photoelectric sensor receives the light beam from the transmitter. When an object passes between the transmitter and sensor, it interrupts the light beam and the lack of a signal constitutes a detection event.

A reflective sensing design uses the inverse principle. In this case, both the light emitting and light receiving elements are contained in a single housing. When no object is present, the photoelectric sensor does not receive the transmitted light signal. When an object crosses the detection area between the transmitter and receiver, it reflects the transmitted light back to the photoelectric sensor. In this case, the appearance of a signal is the detection event.

Figure 2: The H21Bx combination photoelectric device. Source: Light in Motion

Depending on the application, the transmitter and photoelectric sensor can be separate devices or combined in the same package. Figure 2 shows a combination device. The H21BX consists of an infrared light emitting diode (LED) coupled to an NPN silicon photodarlington transistor packaged in an injection-molded housing. The package is designed to optimize mechanical resolution, coupling efficiency, ambient light rejection and reliability. Inserting or removing an opaque material into the gap when the LED is operating switches the photodarlington transistor on or off.

Figure 3: Motor velocity sensing using an encoder and a photoelectric sensor. Source: Light in Motion

Figure 3 shows a typical application of a combination photoelectric device, as part of an optical encoder to measure the rotational speed of a motor.

Object detection with photoelectric sensing

In Figure 3 the rotating encoder is in a fixed position relative to the detector, but in many applications the position of the object relative to the transmitter and receiver is not known; it is random, located in an open delimited space. In other applications, the goal is to count the number of objects traveling through a constrained space (enclosure) called the sensing area.

The purpose may be convenience (an automatic door opener in a store); safety (a garage door opener interlock, or a light curtain around a factory work cell); or logistics (counting paper sheets, or parts moving down a conveyer belt).

The designer must consider several factors to implement a successful design. For object detection, these may include:

• Object shape and size
• Object optical properties such as reflectivity
• Sensing area shape and size
• Enclosure optical properties

Figure 4: Three possible configurations for random object detection. Source: Light in Motion

The designer has many options for locating the transmitter and receiver around the sensing area and Figure 4 illustrates some of the possibilities. The LIM158FS is a logic-output photodetector that includes a Schmitt trigger for improved noise immunity and an open-collector output to accommodate various logic-level interfaces. The small size of the LIM158FS allows the designer to create a fine mesh of detectors for small objects.

The configuration in Figure 4(a) is well suited for detecting and counting relatively large objects. The object must be larger in its smallest dimension than the distance “d.” The LED is positioned such that when the enclosure is empty, all detectors are “on” (high logic state); its emission pattern must be wide enough to illuminate the detectors located at each side.

The configuration in Figure 4(b) is preferred for applications where the enclosure or safety space is large, and the size of the object is variable. This is typically the case for a safety light curtain application. The LEDs must have a narrow emission pattern so that they illuminate only one detector.

The configuration in Figure 4(c) improves the capability of the system to detect small objects; the minimum size of the object is half that of Figure 4(b), and can be improved further by increasing the numbers of rows and reducing the alignment shift from row to row.

The designer can use other techniques to enhance the performance of the system or add functionality. For example, a design with multiple LEDs can energize them sequentially and record the response of the detectors. This can determine which LED triggers a detection to distinguish between two objects present simultaneously in the sensing area. More advanced techniques include the use of artificial intelligence (AI) and deep learning algorithms to recognize the signature of different objects and optimize the detection settings.

Design challenges with photoelectric sensors

In a traditional design, the photoelectric sensor and its housing are designed to fit in the sensing area. The design typically requires a printed circuit board (PCB) to hold the sensor, plus a harness that provides power and signal interfaces between the sensor and the rest of the detection system. As space is often very limited in the sensing area, finding room for the transmitter, receiver and the electronic interface can be a challenge.

In addition, locating the photoelectric sensor in the sensing area poses other design challenges. Certain components such as motors or switching power supplies, generate considerable amounts of electromagnetic (EM) noise that can interfere with the sensor electronics and cause incorrect operation, errors or even outright failure. Protecting against EM noise may require isolation barriers, additional shielding, components such as ferrite beads, filtered connectors or other measures; these techniques can increase the complexity, cost and size of the design

Additionally, the sensing area may be in an explosive environment. Inserting an electronic system may be prohibited or require a special casing to prevent sparks.

External light sources around the sensing area can also interfere with the operation of the sensor. Solutions may include additional filtering or improving the signal to noise ratio by increasing the transmitter output or the receiver sensitivity. Drawbacks of these solutions include increased cost or a longer design cycle.

Finally, the sensing area may be exposed to dust that accumulates on the openings of the sensor and degrades the response. Special sensor protections (sealed apertures) can protect against dust – but again at increased cost.

A distributed sensing design provides design flexibility

A distributed sensor configuration can build on the benefits of optical sensing and remove the pitfalls associated with the physical presence of the sensors in the sensing area.

The design adds optical fibers between the transmitter/receiver combination discussed earlier and the sensing area. Both transmitter and receiver are now located remotely; the optical fiber channels light from the sensor transmitter and then returns the light back to the photoelectric sensor. The result is increased flexibility and a simpler design.

Optical fibers can easily be integrated into both transmissive and reflective sensor designs, subject only to factors such as maximum fiber length and allowable bending radius. Figure 5 shows conceptual arrangements of each type.

Figure 5: Distributed transmissive and reflective sensor concepts with fiber optics. Source: Light in Motion

In a transmissive sensor, the functions performed by the throat depth and width of a traditional sensor are replaced by the relative position of the fibers in the sensing head. In a reflective design, the optical fiber increases flexibility in both the incident angle and the distance to the target. The resolution is defined by the fiber size.

Distributed design improvements vs the traditional approach

The distributed design solves many of the problems inherent in traditional designs. The optical fiber takes up very little space in the sensing area as there is no need for an in-situ PCB or wire harness. The design is easy to implement in explosive areas since there is no risk of a spark. And the optical fibers are impervious to electromagnetic noise from nearby components.

All those improvements are possible at the application design stage, rather than at the sensor design stage. A maker of robotic arms, for example, may rely on one or a few sensor-based designs for the electrical function that can be used in multiple arms by just designing a variety of sensing heads holding the fibers.

The flexibility afforded by a distributed system also allows for additional sensing methods such as:

• Distance measurements
• Object drop detection
• Media sensing
• Color sensing

About Light in Motion

Light in Motion offers a wide range of standard products that include products for both transmissive and reflective applications in a variety of options including wired, through-hole, and surface-mount packaging and hermetic packaging.

Transmitters include LEDs with a choice of wavelengths, output power and emission angles. On the receiver side, the company offers both phototransistors and high-gain photodarlington options.

Figure 6: A wide selection of devices is available to suit both transmissive and reflective applications. Source: Light in Motion

Figure 6 shows a few of the many standard photoelectric products available to provide the designer with maximum flexibility.

Light in Motion can also develop solutions for specific applications. Semi-custom products involve a modification to a standard product’s mechanical and optical design. Examples include the additional wires or connectors or additional product testing.

Full-custom products are ground-up designs that may include injection molding, custom PCBs, flex circuits or custom integrated circuits for either the detectors or the emitters. These products are typically modules that include features such as:

• Customer-specific housings
• Multiple sets of emitters and detectors
• Specific electrical circuitry
• Non-standard wavelengths

Light in Motion applications engineers are available to help customers select the correct product, configure a standard or distributed system for any application, or solve tough design challenges. Contact Light in Motion for more information.