Photoelectric sensor

A light barrier is a electronic- optical system which detects the interruption of a light beam. In this way, automatic devices can detect moving objects without contact. For example, barriers to self-closing doors can be recognized or invaders by alarm systems.

Operation

Light barriers consist of a light beam source ( the sender) and a sensor ( the receiver ) for this radiation.

As a light source are often light emitting diodes having a wavelength of 660 nm ( visible red light) or 880-940 nm ( infrared ) is used. Infrared light has the advantage to obtain a higher range of dark materials. Also, it is invisible. The advantage of red light is in the simpler setting of the sensor system by the visible light spot. For particularly precise applications ( detection of small parts, high repeatability ) typically is used red light from a laser diode. The receiver is usually a phototransistor, light -sensitive resistor or a photo diode.

The principle of operation of a light barrier is to register the object caused by a change of light intensity by the sensor and converting it into electrical signals, which are suitable for further processing.

To make a light barrier insensitive to extraneous light, the radiation, especially in far-reaching models, modulated, to distinguish them from the ambient light can. In addition, a human eye, appearing almost black infrared filter be placed before the receiver to such higher-frequency light shield the visible portion of the light.

To increase the range, the transmitter and receiver are usually with an optically converging system, such as a converging lens, is provided. This is formed by the LEDs at the front of the hemispherical cylindrical (about 2-7 mm in diameter, plus federal) encapsulation of transparent plastic, which embeds the semiconductor porous and thereby fixed the two connection wires. Alternatively, photo- transistors are in a side-light ausblendendes cylindrical metal housing mounted in the circular opening a small lens of plastic or glass is pressed defined. Often the case of small transmitters and receivers consist entirely of the black transparent for IR plastic.

To save an electrical lead to a second location, the transmitter and receiver are integrated often close to each other but visually separated in a case and be directed to the formation of the light barrier precisely to a retroreflector, which is mostly made ​​up of cube corners on the back of a plastic plate.

To adjust the direction of the transmitter beam can serve two screws. In darkness, a sheet of paper serve as a screen for visualizing the impact area of the beam when sufficiently visible red is included. Are helpful meter at the receiver output or indicator display, such as an LED or beep at the sensor. Are transmitter and receiver in one unit, the adjustment is simplified.

Early computer mice (similarly trackball ) worked with two friction wheels, recorded the horizontal and vertical movement of a heavy ball separated and transferred to two sector wheels with diameter of 2 cm. Each sector wheel sticking in the 3 mm wide gap of a fork-type light barrier, with two beams in order to detect not only the change of the shading around the spokes 30, but by a logic circuit and the rotational direction of the wheel.

The floor stop positions of building elevators are often controlled by fork sensors, which are mounted on the car and the position of pieces of sheet metal sample which slidably seated in the elevator shaft to adjust on a panned pair wires.

Types

It mainly distinguishes the types thru-beam sensor, reflex sensor and light grids.

Through-beam sensor

With Through-beam emitter and receiver are facing. This includes hybrid coupler and fork sensors, in which the transmitter and receiver are pre-assembled to each other at a distance of 3-120 mm. Transmitter and receiver are in separate housings, they must be aligned with each other is mounted in the assembly. Through-beam sensors have all types of the longest range (up to 80 m).

Retro-reflective sensor

Transmitter and receiver are here parallel to each other in a common housing. The light signal is reflected by a reflector.

Small reflective sensors are also known as reflex couplers, they often work with attached on moving parts reflective, self-adhesive film brands.

The versions with reflector still differ with respect to the use of a polarizing filter. Versions with polarization filters work only with a retro reflector or reflectors, but not with a smooth mirror surface. Such is recognized as an interruption; which creates additional security, since, for example including a bare metal object is detected reliably as an interruption. The use of retro-reflectors and reflective sensors simplifies the installation considerably, as less cabling is required and must be made no precise orientation of the reflector to the light barrier.

Diffuse sensor

The light signal is discarded at this through the object to be detected itself. The switching distance is therefore dependent on the reflection properties of the object surface. Transmitter and receiver are here parallel to each other in a common housing.

Apart from the purely energetic working reflection light scanners, especially the reflection light scanners with background suppression have great practical importance, since they are able to detect dark objects on a light background. Are they beyond for distance determination in a position they are called distance sensor - they then usually work on the principle of triangulation and include a position-sensitive photodiode ( PSD) instead of the phototransistor. With this reflection sensor can be constructed which various objects (eg on a conveyor belt ) can differ from each other. There are also distance sensors with switching function, which operate on the principle of transit time measurement. You can reach a greater range than reflex sensors (up to 75 m). see distance measurement (optical)

Light grids

In addition to the simple version with only one light beam, there are also so-called light curtains or light curtains that work with multiple parallel light rays. With these, a large area can be monitored, for example, access to a machine or an alarm secured room. With a light grid openings of buildings lift elevator cars are much better protected than with a single light barrier at the ankle as it was standard until 1970.

Fiber optic light barrier

Also, there are fiber optic sensors, where optics and electronics are disposed separately and connected by means of optical fibers. They are used for example in tight spaces. Disposable and reflective sensors are possible.

Areas of application

• Alarm systems ( light curtains)
• Exterior building security (photocells )
• Measurement of customer flows, customer frequency measurement
• Automation, packaging and food industry and production technology: position detection, object recognition, counting, position detection ( one-way and retroreflex )
• Encoders, such as computer mice, to servo motors and speed measurement ( fork couplers )
• Smoke detector ( not evaluate the interruption of the light beam, but registered scattered light from aerosols )
• Sanitary: automatic flushing urinals (light switch )
• Personal protection in hazardous machinery ( light curtains)
• Sports: Timing ( Through-beam sensor )
• Elevators: Door monitoring and position monitoring ( reflective sensor with retroreflector )
• Monitoring / control of garage entrances and doors as well as roll-up doors ( Through-beam, Retro -reflective photoelectric sensor with reflector)
• Position sensing in electronic devices (VCR, CD and DVD player ): Scanning the protection, cassettes, limit switches, inter alia, the CD tray and position detection and monitoring of a variety of other levers and motors ( mostly hybrid coupler, also reflex coupler ).
• Washes and washes for cars, trucks, trams, buses and trains
• Car tandem milking parlors ( parlor )