Written by: Jeff Himes
I have led many inductive proximity sensor training classes where an “Ah ha” moment happens when discussing the effects of target size on an inductive proximity sensor. As more and more extended range sensing models arrive on the market, it’s even more critical to understand how target size affects sensing distance performance.
According to IEC 60947-5-2 the recommended target for an inductive proximity sensor has a defined material and size. The standard target material is carbon steel (Fe 360) with a rolled finish. The standard target is square in shape, is 1 mm thick (standard for test target), and will have a size equal to the greater of:
- Rule 1 – The size of the sensor’s active surface (ex. 8 mm, 12 mm)
– OR –
- Rule 2 – (3) times the rated operating distance (Sn) of the sensor
Of course the easiest rule to remember is the first rule indicating the target size should be at least equal to the size of the sensor’s active surface. For standard tubular sensors this would relate to square target sizes of 8 mm, 12 mm, 18 mm and 30 mm. The rule that tends to be forgotten and is the source for more errors is the second rule: (3) times the rated operating distance (Sn) of the sensor. Let’s look at a couple of examples to better understand the rules.
Example A – Target size calculation for a standard range M18 sensor with a rated sensing distance (Sn) of 5 mm:
- Rule 1 test – target should be at least the size of the sensors active surface. In this case that would be a target of at least 18 mm square.
- Rule 2 test – target should be at least (3) times Sn; therefore 3 x 5 mm = 15mm.
– Rule 1 wins in this case as it is the greater of the two methods. Therefore the target should be a minimum of 18 mm x 18 mm to achieve the sensor’s rated sensing distance.
Example B – Target size calculation for an extended range M18 sensor with a rated sensing distance (Sn) of 8mm.
- Rule 1 test – same result as above at 18 mm (sensor’s active surface size)
- Rule 2 test – target should be at least (3) times Sn: therefore 3 x 8 mm = 24 mm.
– Rule 2 wins in this case as it yields the greater result. With this extended sensing model the target should be at least 24 mm x 24 mm to achieve the sensor’s rated sensing distance.
* From these two examples it can be seen that for a standard sensing range sensor, Rule 1 will always win and for extended sensing range models (two, three or four times normal sensing range) Rule 2 will typically win.
Why does target size affect the sensor’s detection range? An inductive proximity sensor generates a magnetic field from its active surface that in turn generates eddy currents on its metal target. As the target gets closer, the eddy currents increase and lower the amplitude of the sensor’s oscillator signal until it eventually triggers the sensor. If the metal target is smaller than required, it will not generate as many eddy currents and the target must be closer to the sensor to have the same affect. Targets smaller than the recommended size will be detected in most cases – just at a shorter sensing distance.
Remember – extended sensing range models are the more popular versions being used today. That means Rule 2 will typically be the one to follow for the recommended target size. Targets smaller than this rule will normally be detected by the proximity sensor, but at a shorter sensing distance. If you need to achieve the full rated sensing range of the sensor, the recommended target size should be used.
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