Design News, February 2013

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s e n s o r s / m ac h i n e v i s i o n DESIGN APPLICATIONS Distance-Based Sensors Measure Up When some objects must be seen and others ignored, or when the object's presence and its position must both be known, SPT, MPT, or PRT distance-based sensors are the best options. by Jeff Allison, PePPerl+fUchs W When you visit someone's house, you probably use the doorbell at the front door to let him or her know of your arrival. But a single doorbell in the lobby of a 20-story apartment complex doesn't make sense. Because there are many tenants, you must first determine the apartment you want to visit and alert just the people in that apartment (by buzzer, phone, or doorbell) of your presence. Presence is helpful information in some cases, as in the first scenario. In that case, you push a button and someone knows you're present. But sometimes knowing presence alone isn't enough, and you must also know location information, as in the second scenario. In that case, the location of the apartment you want to visit and a signal of your presence are both required. Similarly in the world of sensing, presence is sufficient information for many industrial or commercial applications. And photoelectric presence sensing isn't just a staple of automation; it's a ubiquitous part of our daily lives. When our movement though an automatic sliding door is monitored to prevent the door from closing on us and when an intelligent faucet detects our hands and starts the flow of water, a photoelectric presence sensor is likely behind the scene. Photoelectric sensors serve their purpose of presence detection well. But simply detecting if something is present isn't always enough. Knowing that an object is within a certain range or identifying an object's precise location may yield more helpful information. Over time, distance-based photoelectric sensing has evolved as the go-to technology when presence sensing doesn't cut it. Today, three technologies have risen as the strongest methods of distance-based photoelectric sensing. They are Single-Pixel Technology (SPT), Multi-Pixel Technology (MPT), and Pulse Ranging Technology (PRT.) Single-Pixel Technology, alternately known as background suppression (BGS), is the simplest Single-Pixel Technology, alternately known as background suppression (bGS), is ideal for ignoring objects in the background and for consistent detection of objects in the near field, regardless of their color or reflectivity. [w w w. de s ig n n e w s . c o m ] f E b r u A ry 2013 T r e n d waTch : s e ns o r s / mach ine v is io n / a s up p le me nT To de s ign news T13

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