Designers of automation systems are continuously on the lookout for sensing solutions. The sheer wealth of sensors on the market reflect the enormous variety of objects, materials and liquids that need to be detected. On top of this, there are the choices to be made on interfacing to other systems, as well as how the environment where the sensor is to be used will impact housing choice.

Many situations simply require the detection of an object, for example on a conveyor belt, table or transport trolley. However, it may not be possible to fit a reflector to use in conjunction with a retro-reflective sensor, or the receiving end of a through beam sensor. If the object to be detected lies in front of an interfering background, a diffuse sensor with background suppression could be the answer.

Such sensors use the principle of triangulation to measure the light intensity reflected by the desired target as well as the distance is it away. The minimum size of the object is then defined by the dimensions of the light beam being used. The distance to the object is configured by set screw, although some more advanced solutions have a “Teach-in” method that can be used.
Figure 1: For situations where a through beam sensor is impractical (L), the diffuse sensor with background suppression (R) might make suitable alternative
Featuring a red light emitter, this visible light source makes it simple to adjust, even in conjunction with small objects. Laser-based sensors are capable of detecting objects with a size of just 0.1 mm, while pinpoint diode-based sensors have a 2 mm diameter spot of light at their focus. Their design is such that the object being detected can vary in both colour and material without detriment to the detection capability, while objects moving in the background do not trigger unwanted detection.

A prime example of a robust, IP67 rated solution, is the FHDK 14P5104 Series of photoelectric sensors from Baumer. These are packaged inside a 14.8 mm x 43 mm housing, are available with a wired interface, M8 or M12 4-pin fitting, and feature a sensing distance Tw of 30 to 500 mm. They are also electrically robust, with both short circuit and reverse polarity protection, with a PNP output supporting up to 100mA output current for both light and dark operate function. The front optics are made from PMMA and the integrated electronics uses a flashing yellow LED to indicate both issues with alignment or a soiled lens.

Figure 2: The FHDK 14P5104 from Baumer uses a pulsed red LED light source in its diffuse sensor