How to Save Your Inductive Prox Sensor from a Hostile WorldFebruary 1, 2005 By: Joan Kassan, Marcel Ulrich Sensors
Will your inductive proximity sensor be working in a crash-bang manufacturing environment? Learn how to protect it from impact, abrasion, weld slag, and other shop floor assaults.
Inductive proximity sensors are inexpensive, durable, and resistant to industrial contaminants. But if your sensor will be exposed to target impacts, abrasion, corrosive cutting compounds, and other abuse, you need to pay very close attention to which sensor you choose and how you mount it.
Mounting. A case in point is Die-Matic Corp. in Brooklyn Heights, OH, a manufacturer of specialized tools and progressive dies and producer of precise metal stampings. With nearly 250 dies equipped with up to 40 sensors per tool, sensor failure is not an option. According to Die-Matic's sensor application specialist Steve Kerg, unplanned shutdowns can cost the company as much as $2000 an hour. The operating environment is severe. Many of the sensors are submerged in a cutting compound with a high chlorine content that makes PUR sensor cables brittle and easily cracked. A switch to PVC solved that problem.
Lack of proper protection in a welding operation can turn your shiny new inductive prox sensor into the sorry example.
Installing the sensors on the dies is a more challenging chore, though. Kerg estimates it takes upwards of eight hours per sensor to ensconce it in a protective die block (see Figure 1) and install the unit in its proper place. That might seem like a long time to spend on a single sensor, but, coupled with regularly scheduled maintenance, the practice has paid off by making sensor-related downtime nonexistent.
Figure 1. Taking the time to properly install sensors will, in the long term, save significant downtime and maintenance headaches. These proximity sensors are mounted in custom-made die blocks to protect them from harsh application elements.
You can also extend sensor life with cushioned mounting brackets (see Figure 2) incorporating an integral spring-loaded mechanism that protects against target overtravel. When the sensor gets hit by the target, the spring compresses and absorbs the impact; when the target retracts, the spring extends back to its original shape. Or you can outfit your sensor with inexpensive Teflon or Delrin caps that simply thread over the sensing face to protect it from abrasion.
Figure 2. Cushioned mounting brackets are a popular means of protecting sensors against target impacts.
Extended Range. Sensors are usually placed not at the maximum distance from a target but rather at 40%–80% of that distance to allow for various manufacturer and application tolerances. Standard cylindrical inductive sensing distances range from 0.8 to 15 mm, proportional to the diameter, so the actual sensor-target separation is typically <12 mm. As manufacturing equipment becomes more compact, the space available for special mounting brackets and assemblies shrinks. You might want to consider using "extended range" sensors (see Figure 3) to mitigate damage. .
Figure 3. Extended-range sensors offer 25%–200% more sensing range than conventional inductive sensors. The greater range allows them to be mounted farther from the target.
Increased sensing distance reduces target-sensor collisions by allowing the sensor to be mounted at a greater—and thus safer—distance from the objects they detect. Some of these extended-range sensors have as much as double the range of regular inductive proxes, and are available in virtually all industry-standard housing types
Some manufacturers have used special oscillator/coil arrangements to make inductive sensors with stainless steel sensing faces that "see through" the face metal and detect the opposing target metal. This type of sensor is particularly handy in the metal-forming industries where metal-on-plastic abrasion can quickly add conventional sensors to the scrap pile.
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