Photo-Interrupters White paper


Photo-interrupters (also called Opto-interrupters, or Opto-switches) are part of the optosensors or photoelectric sensors family.
Photoelectric sensing employs two optoelectronic devices: a transmitter, also called an emitter, and a detector, or receiver. The transmitter emits a visible or infrared (IR) light beam; the detector detects the light from the transmitter. More complex
configurations may have multiple transmitters and detectors.

Historical background

Among all sensing techniques, one emerged in the 1980s due to the breakthroughs that were made at that time, and that lead to the industrial birth of the LED. In other words, engineers knew how to use light for sensing purpose. Silicon material was known to be a perfect material to convert photons into electron/hole pairs and electrical current, and a semiconductor packaging industry had already developed packages and techniques to harness this attribute.

Also, basic optics was available to optimize the handling of the light like focusing lenses and mirrors,

But the missing link was the device that would emit the light, without being bulky and in the right wavelength band as it had to be compatible with the silicon detection band.

The LED filled that gap and the Photoelectric sensor took its flight.

  • First patent for an IR LED in 1961 (Gary Pittman and Bob Biard, TI)
  • III-V material (GaAs)
  • Mass manufacturing at Monsanto in 1968
  • Fairchild introduced the Planar process in the 1970s
  • GaAlAs in the mid 1980s
  • Main factors for efficiency improvements:
    • LPE replacing VPE
    • Single Hetero Junction (SHJ)
    • Double Hetero Junction (DHJ)
    • Transparent substrate
    • Surface Etching

Since then, the capabilities of the Photoelectric sensors have taken many directions and have often become the preferred technique for many types of applications.

Transmissive and reflective sensing: operation

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


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

In a transmissive sensing design, the transmitter and the detector are physically separated. When no object is present, the detector receives the light beam from the transmitter. When an object passes between the transmitter and detector, 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 detector does not receive the transmitted light signal. When an object crosses the detection area between the transmitter and detector, it reflects the transmitted light back to the detector. In this case, the appearance of a signal is the detection event.

Benefits of photo-interrupters

The use of interrupter or reflector modules eliminates most of the optical calculations and geometric and conversion problems in mechanical position sensing applications. These modules are specified electrically at the input and output simultaneously — i.e., as a coupled pair — and have defined constraints on the mechanical input. All the designer need do is provide the input current and mechanical input (i.e., pass an infrared-opaque object through the interrupter gap) and monitor the electrical output. Other than normal tolerance, resolution, and power constraints, the only new knowledge required is the ability of the sensed object to block or reflect infrared light and an estimate of the effects of ambient light conditions providing false signals. This is true of both "off the shelf" commercial modules and limited volume custom modules, as the mechanical and optical parameters of any given module are fixed. Once the module is characterized for minimum and maximum characteristics, it is a defined electrical and mechanical component and does not require optical design work for each new application. This puts these sensor modules in the same design category as mechanical precision limit switches, except that the activating mechanism blocks or reflects light instead of applying a force. Thus mechanical wear and deformation effects are eliminated. Also, the switching signal is bounce-free.

Most commercially available interrupter modules are built around plastic packaged emitters and detectors. Reflective modules and other custom modules are built around both plastic and hermetic parts, depending on the required cost/performance trade-offs. It should be noted that due to the longer, angle critical , and generally less efficient light transmission path in a reflector module, lensed devices are dominant in these applications. This also explains the lack of standard reflective modules, because tight spacing between the module and the mechanical actuator must be maintained to provide adequate optical coupling, which leads to different mechanical mounting requirements for each mechanical system which is sensed.

Example of Transmissive photo-interrupter

3mm Gap with mounting tab1                             Darlington_interrupter_schematic.jpg
Figure 2. View and schematics example of transmissive photo-interrupter. Source: Light in Motion

Photo-interrupters are characterized by their mechanical outlines and their electrical output.

The most important mechanical characteristics are the slot width and the optical centerline  (which have to allow for the size and position of the object to be detected).

The electrical output is set by the type of detector: phototransistor, photodarlington or photologic.

Example of Application: Motor velocity sensing

Figure 3: Motor velocity sensing using an encoder and a photo-interrupter. Source: Light in Motion

Design challenges with photo-interrupters
In a traditional design, the photo-interrupter 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 photo-interrupter 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.

Most common applications are:
Instrument control

Food processors (motor control)

Motorized appliances (for example vacuum cleaners)

Fitness equipments

Counting objects (for example on a conveyor belt)

Door open/closed status

End of travel detection

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