Photoelectric sensors open the door on new uses

A Panasonic Electric Works UK product story

Edited by the Engineeringtalk editorial team Sep 1, 2004

The sensing and detection of people is one of the many functions asked of industrial photoelectric sensors.

The sensing and detection of people is one of the many functions asked of industrial photoelectric sensors.

Some of their most common applications require one or more sensors to detect the presence of a person to allow the automated operation of opening or closing a door or barrier.

Furthermore, the movements and numbers of people who enter a building or plaza enables building management information systems to calculate the amount of "human traffic" entering or leaving a monitored area.

This information can be used to build up a picture of traffic for safety and security purposes, as well as a management tool.

The challenges presented to a photoelectric sensor in such an environment include the ability to reliably detect a target of any colour within its sensing range, a long sensing range for a self-contained diffuse type unit, and, in some cases, also the ability to ignore the background behind the target area to avoid triggering on false information.

For a typical self-contained diffuse type sensor this is difficult.

A diffuse type sensor's main principle of operation means that it measures the intensity of light that is reflected from the target.

The amount of reflected light is very much dependant on the colour and texture of target material.

In the case of detecting people, the colour and types of material worn by people can vary considerably.

This variation often results in unreliable operation.

This problem of unreliability with the basic diffuse type sensors has resulted in the redesign of reflective type sensors that use a principle of operation called fixed-focus sensing.

Rather than detecting the amount of returned light from a target object, a proportion of the returned light is focused onto a sensing element via a lens system.

The position of the focused light on the sensing element allows the sensor to determine the distance between the object and the sensor.

Using a mechanical adjustment system, normally a screw type system, the sensing element of the sensor can be moved in relation to the lens system to allow some fine tuning.

Typical reflective type sensors operate by sensing the variation in the amount of light in the incident beam.

However, the fixed-focus reflective sensing type incorporating the two-segment photodiode operates by sensing the variation in the incident beam angle.

The output is thus activated according to the distance of the object to the sensor.

This system helps this type of sensor to be unaffected by object colour or background and enables stable detection.

The sensor does not therefore rely on the amount of light returned by the target to determine the presence of it.

The result of this principle means that the sensor can reliably detect colours from white through to black at the same distance.

This is a major advantage when used in applications where people detection is required.

Moreover, the fixed focus technology used in this type of sensor provides the added benefit that even if there is a build-up of dust or dirt, or if there is a smoky atmosphere, the sensor is still able to reliably detect a target.

Demand for the automated operation of doors is set to grow, especially where commercial premises are concerned.

New legislation, due later this year, addresses the problem of access to businesses and shops that is experienced by disabled people on a regular basis.

Automating the opening and closing of doors can help meet the demands of the legislation that from October 2004, The Disability Discrimination Act Part III, places on all businesses to remove or alter physical features in their buildings that make access to services impossible or unreasonably difficult for a disabled person.

The most obvious step forward from photoelectric sensors is to use camera-based technology.

This technique will allow images from cameras to be processed in real time to detect the size, direction and speed of moving objects.

Offering higher functional stability than photoelectric sensors, even under difficult environmental and lighting conditions, there appear to be few practical limits to door opening behaviour that this technology can offer.

We live in an age of ever-increasing automation.

With much of our environment being power assisted, the trigger for a control system that opens a door, flushes a toilet or turns on a light depends on the reliable detection of human beings.

Photoelectric sensors and their future successors will continue to be key detection devices that can be depended on for these applications.