In quick review, an inductive sensor generates an electromagnetic wave at a specific tuned frequency. The tuned circuit inside the sensor has a high impedance at that frequency, creating a high ringing voltage. When any metal is brought near the output coil of the sensor, the coil becomes detuned. This shifts the oscillator frequency and, more importantly, reduces the ringing voltage in the sensor’s tuned oscillator circuit. The lower voltage triggers the output that indicates the presence of the metal target.
Most metal-detecting inductive sensors use plastic sensing faces. Plastic facings are inexpensive and do not interfere with the metal-sensing ability of the sensor. These products are an option when a sensing area is relatively benign. But plastic’s lack of durability may create problems in some situations.
Inductive sensors feature comparatively short sensing ranges, often under a half-inch. Such close proximity to the target means even slight variations in target clearances or vibration may subject the sensor to damaging physical abuse. The life span of a plastic-faced sensor is greatly reduced under these circumstances.
If an inductive sensor is to be used in abusive environments where impact and abrasion are commonplace, it’s now more common for the inductive sensor to have a metallic face — specifically, stainless steel.
The question arises, “How does the sensor detect the metal in the target, yet ignore the metal at the tip of its own housing?” To understand how requires a quick look into electrical physics. The trick hinges on the oscillator frequency and the wavelength of the signal it produces.
Metals such as copper, aluminum, and stainless steel tend to block or shield electromagnetic waves. The waves travel along and around the surface of the metal by a process known as skin effect. By reducing the frequency, the electromagnetic field no longer stops at the sensor’s metal surface, but extends beyond the surface into the surrounding space. Sensors with a stainless-steel face use such low-frequency alternating fields that their detection area extends well through the metal face of the sensor to detect metallic objects on the other side.
Laboratory tests compared the survivability of plastic and metal-face products to metal-brush abrasion and hammer impacts. The tests showed stainless versions withstood exposure levels more than 20× beyond their plastic-face counterparts. Considering stainless models cost only 25 to 30% more than plastic, they can be an economical replacement over an extended time with no trade-off in durability versus sensing distance.
Pepperl+Fuchs supplied information for this column.