Photoelectric sensors have solved numerous industrial applications over the years. There are basically three different sensing modes. The first is diffuse or reflex mode, next is retro reflective, which requires a reflector, and the third is through beam, transmitted or opposed. These field devices provide an excellent means of detecting target at a distance without contacting the object. All of the sensing modes are based on the sensor’s ability to detect a change in light reaching the sensor’s receiver. In this posting, we will review the diffuse or reflex photoelectric sensor.
Diffuse sensors operate on the principal that when a light source is shined on a surface, the light is scattered or diffused in many directions. A small portion of the light is reflected back to the sensor receiver. The receiver used in this style of sensor is designed to be sensitive to a weaker or smaller amount of light that is reflected back from the target surface.
One of the advantages of the diffuse or proximity style photoelectric sensors is the light emitter, or light source, and the receiver are in the same enclosure thus you only have a single device to mount and wire – making installation easier and faster. This is a huge benefit in space restrictive locations or in those applications where you cannot get to both sides of the target. Since there is only one device, the diffuse photoelectric sensor is typically cost effective compared to the retro-reflective or through beam sensors.
There are several variations of the diffuse based photoelectric sensors, which we will discuss in future blog postings. Some types of diffuse sensing include:
• Convergent or fixed focus
• Divergent or wide angle
• Foreground suppression
• Background suppression
• Color mark
• Registration mark
• Luminescence, refer to the previous posting Luminescence (UV) Sensing – Seeing the Unseen
When applying diffuse mode photoelectric sensors, there are a number of factors that need to be taken into account. Remember, this sensor works on the reflectivity of the target you are trying to sense. Typically, the sensing range charts you see in catalogs are based of the reflectivity of a Kodak test card that has a reflectivity index of at least 90%. Unless you have a clean room application and are detecting white paper, your sensing range will be less than the maximum shown.
Shiny objects or even worse shiny round objects can be very difficult or sometimes impossible to detect. When the light strikes the object surface it will be deflected away from the source and very little or usually no light will make it back to the receiver. An example of this would be trying to detect shiny aluminum wheels on a conveyor. If the shiny surface is flat then perhaps a solution to this problem may be to have the sensor parallel to the shiny surface.
If your application involves trying to detect a dark matte surface, such as a dark box or textured black matte plastic part traveling down a conveyor, the sensing range will be very short. In this case, once the sensor is adjusted to detect the object on the conveyor and a person in light colored clothing walks on the other side of the conveyor chances are you will have a false trigger with the sensor.
Other factors to take into account are dirty/dusty environments, target surface irregularities, backgrounds behind the target, target size, angle of the sensor to the target and distance between the target and the sensor.
Diffuse photoelectric sensors usually have a range of 30mm to 1000mm. The light source can be visible red, infrared or laser. Light sources will be a topic of a future blog.
Diffuse mode sensors have been used in a wide variety of applications in numerous industries providing many years of dependable service. These sensors are offered in a wide variety of housing options and with special functionalities.