Machine Design

Light Sensors

Light sensors often use an infrared LED as a light source. Infrared LEDs have a greater intensity than LEDs that emit visible light. And when infrared photodiodes are used the sensors are relatively insensitive to ambient light.

Photoelectric light sources are often modulated at a given frequency to prevent interference from ambient light. But flashes or reflections can still fool light-activated sensors. This problem is solved by using a modulated receiver. Here the detector is synchronized to the light source frequency. For example, suppose emitted light has a frequency of 5 kHz, with a pulse duration of 10 ∝sec. The emitter LED and detector need only be on for 10 ∝sec. They can be off for the 190 ∝sec that light is not transmitted. Such third-generation devices have fewer false signals because the chance of ambient light activating the receiver is only 1 in 19.

One trend in photoelectric sensors is the development of modular devices. Sensor heads can be combined with a separate base and power supply, and with various logic options. The approach tailors a sensor for particular needs. Another advantage is that sense heads or logic options can be changed in the field without disturbing wiring or beam alignment. This minimizes downtime and allows unskilled personnel to repair equipment.

Fiber optics also are turning up in photoelectric sensors, both as optional sense heads, or integral to the sensor itself. The technology has a number of advantages over conventional methods such as through-beam or reflective sensors. Fiber-optic cables are small, some are only about +>1+>/->16-> in. in diameter and the fiber-optic head is usually just a little larger. As a result, it can fit into tight spaces. Fiber optics can also be used in explosive environments because all electrical signals are remote. Similarly, optic cables can reside next to high-voltage cables in plenums.

Reflective light sensors (RLSs) are frequently employed to sense an object's presence because they are easy to use. An RLS differs from other light sensors in that the target does not simply break a light beam but reflects light to a detector.

Through the years, RLSs have been refined so that they are less sensitive to ambient light and can detect minute or transparent objects. Some devices can even determine the distance between sensor and object, thereby offering an alternative to ultrasonic systems.

While in general, RLSs are becoming smaller, two divergent trends are emerging as well. One group of sensors is becoming simpler, basically containing only a light source and detector. Sensor output is a simple on/off signal, and users provide the power supply and signal processing circuitry.

The other group is becoming full featured. These devices contain a power supply and logic. Users program the device by setting switches or with a PC.

Simplicity has not come at the cost of technological innovation or accuracy. Sensor manufacturers are integrating advanced devices like solid-state lasers into their devices. Usually the power supply for the laser and an amplifier for the signal output are contained in a separate unit. An advantage of placing system functions in different packages is that the sensor is kept small.

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