Getting a Better Sense

Best and Worst Practices When Spec Presence Sensing to Detect Unauthorized Personnel

By Control Design Staff

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We're integrating an ambitious manufacturing line project that has us a little out of our depth for one element of it. We use proximity and other presence sensors on our machines all the time, but we're being asked to spec presence sensing to detect unauthorized personnel in the areas where all the machines will operate. First glance shows us many pros and cons among several technologies: the more conventional photo cells that we know something about, but also laser-based sensors and infrared sensors. Anyone have some best/worst practices about these two?

—From May '12 Control Design


A Range of Sensing Options
There could be a number of reasons behind the need to detect unauthorized personnel in a specific plant floor area. One reason might be the security of intellectual property and the prevention of industrial espionage. The other reason might be personnel safety.

In the case of securing an area from prying eyes, a consideration is whether select personnel are authorized to be in the restricted area, or absolutely no one is permitted to enter. If only select personnel are permitted, some type of actively monitored access control scheme is called for. If no one is permitted to enter, an approach similar to an intrusion alarm system might make sense.

Typical intrusion detection systems use infrared motion sensors to detect a change in the static IR signature of the monitored area. Essentially, a human being moving across the field of view generates a dynamically changing IR signature, which triggers the alarm. Some IR motion sensors employ monitored, battery-operated wireless technology, which might solve some installation issues where hardwiring is not an option.

Another approach to personnel intrusion detection would be long-range infrared through-beam photoelectric sensors, which have the advantage of being invisible to the unaided eye. IR through-beams could be set up in a variety of heights and directions to fully cover the restricted area. Infrared through-beam sensors carry the advantage of the most robust sensing mode and enough power to burn through accumulated dirt and contaminants.

A lot of people have a misperception that lasers are the most powerful light source for sensors. In fact, most lasers used for sensing applications — especially visible, red-light lasers — are not extremely powerful for reasons of personnel eye safety. Also, laser through-beams are rather susceptible to false/nuisance triggering because of flecks of dust and water droplets interrupting the beam.

Another long-range presence-sensing technology to consider is ultrasonic. Many ultrasonic sensors have an adjustable set point, so you can tune it to ignore objects beyond the adjusted sensing range.

With respect to detection of personnel for safety reasons, the only approach is to follow government regulations and industry guidelines. Depending on the situation, safety light curtains, safety floor mats, safety area scanners, safety gate interlock switches, etc. may be needed to provide an appropriate level of personnel protection.

Henry Menke,
marketing manager, object detection, position sensing and measurement,

Safety Expert Recommended
In any application where personnel must be detected for safety purposes, the use of standard photo sensors is not appropriate. Without seeing your actual application, it is difficult to suggest the correct safety-rated presence sensing device.

However, there are basically three choices to consider for safety-rated devices. First are safety light curtains. These devices can be mounted either vertically or horizontally to detect the presence of a person. When used in a vertical position, the device will detect when a person crosses through the light curtain and will act to stop the machine. If the area the person is entering is such that they may cross through the light curtain and remain undetected in the hazardous area, then the use of a light curtain alone is probably not adequate. Light curtains could be used horizontally to detect a person in the area between the transmitter and receiver, but in those cases one of the other two technologies are more appropriate.

Second are safety mats. These devices detect the presence of a person when stepped on. These are often a cost-effective way to guard a specific area and prevent machine operation when a person is in the hazardous area. Safety mats come in a variety of sizes to allow custom configurations to ensure the entire area is protected.

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