When looking at a data sheet for an inductive proximity sensor, there are usually several different specifications listed with regard to the switching distance (or operating distance). Which of these various specifications really matter to someone trying to use a prox sensor in a real-world application? How can a specifier or user decide which sensor is going to work best in their situation?
Fortunately, there is an international standard that defines sensor switching distances and spells out test methods to assure that sensor specifications from product to product and even manufacturer to manufacturer can be directly compared “apples to apples.”
This standard is IEC 60947-5-2 Low voltage switchgear and controlgear – Part 5-2: Control circuit devices and switching elements – Proximity switches.
Operating (switching) distance s
In the diagram shown here, the letter “s” refers to a given sensor specimen’s actual switching distance when tested. It is defined as the distance (between the standard target and the sensing face of the proximity switch) at which a signal change is generated. For a normally open sensor, the target approaches the sensor axially, that is, the sensor approaches the active surface from the front (not the side). There are several subscripts used to describe different aspects of a sensor’s switching behavior.
Rated operating distance sn
… is the nominal switching distance of the sensor. It is simply used as a standard reference value. The rated operating distance is the best figure to use when comparing different sensor models to get an idea of their essential sensing distance capabilities.
Effective operating distance sr
…is the range of actual switching distances that any given proximity sensor will fall into when measured under specified conditions of mounting, temperature, and supply voltage. For well-designed and manufactured sensors, the sensor will be triggered between 90% and 110% of the rated operating distance. For example, various samples of a proximity sensor model with a rated operating distance (sn) of 8mm may deliver switch-on points anywhere between 7.2mm and 8.8mm.
Usable operating distance su
…takes into account the effects of the sensor’s full ambient temperature range (low to high) and variation of the supply voltage from 85% to 110% of the nominal voltage rating. The IEC standard requires the usable operating distance (su) to be between 90% and 110% of the effective operating distance (sr). For our example of a sensor with a rated operating distance (sn) of 8mm, the usable operating distance would fall between 6.5mm and 8.8mm. Pop quiz: why is the max of usable operating distance not 9.7mm (sr of 8.8mm * 110%)? Answer: the usable operating distance can always be less than but can never be greater than the maximum effective operating distance.
Assured operating distance sa
This is the distance of the target to the sensor where the sensor can be guaranteed to have turned on. If a target approaches within the assured operating distance, you can be confident that the sensor will detect it. It is 90% of sr which is in turn 90% of sn, which is in effect 81% of sn. Going back to our example of a sensor with a rated operating distance (sn) of 8mm, sa would be 81% * 8mm = 6.5mm. So in essence, sa = su(min).
Differential travel H
Now when the target recedes, at what distance will the sensor switch off? All good-quality sensors have a built-in property called hysteresis, which means that the sensor will turn off when the target is further away from the sensor than the point where it turns on. This is necessary to prevent chattering and instability when the target approaches the sensor. We want the sensor to turn on and stay on, even if the target might be vibrating as it crosses the threshold of detection. For most sensors, it is defined as ≤ 20% of the effective operating distance sr. The differential travel is added to the value of sr to define the switch-off point.
In practice, for any group of sensors, the minimum value of H would be zero and the maximum value would be sr(max) + 20% of sr(max). For our example of a sensor with a rated operating distance (sn) of 8mm, 7.2mm ≤ sr ≤ 8.8mm. So, the range of switch-off points would be 7.2mm ≤ sr+H ≤ 10.6mm. It might sound like a large range, but for any given sensor specimen the switch-off point is never greater than 20% of that particular sensor’s switch-on point.
The good news is that you don’t have to conduct sensor tests yourself or go through all of these calculations manually to determine a sensor’s performance envelope. The sensor manufacturer provides all of these useful figures pre-calculated for you in the sensor data sheet.
Learn more about the basics of the most popular automation sensor here.