How Inductive Proximity Sensors Work - Part 2
, by Jim Ryan, 3 min reading time
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Part 1 is here. It was a long post on the concepts. Today we take a look at one other thing about proxes: hysteresis.
Hysteresis in Inductive Proximity Sensors: A Good Thing
Inductive proximity sensors are designed to have hysteresis because in their context it is a good thing! Here we're speaking of sensors designed to measure proximity (distance) of a target from the sensing face - in which the output is affected actually by two preset distances:
- the ON POINT - the distance, on the target approach, where the sensor begins switching the output transistor. Referred to as the sensor's RANGE.
- and the OFF POINT - the distance, on target retreat, where the sensor stops switching the output transistor.
- The distance between ON and OFF is HYSTERESIS, indicated as a % of the sensor's range.
You might think of Hysteresis as a DISTANCE-STRETCH - since it stretches the range on the retreat of the target!
BTW, There are proximity sensors that have no hysteresis - as they are pure measurement devices providing an analog output, but we aren't concerned about them here.
WHAT IF THERE WERE NO HYSTERESIS?
That means just one single switch point for ON and OFF. Then in this case, if you set a part right barely inside that point, and left it there, a potential problem exists. SHOCK and VIBRATION! If a stamping machine nearby stamps - BANG! - the part moves away a few thou from the shock, and the sensor switches OFF. But what if a nearby jack hammer fires off, making the part vibrate in and out of the range a few thou many times a second? The PLC code likely is not going to like that "chattering" signal!
BUT THERE IS!
Because of this potential issue, proximity sensors have an ON POINT (the distance, on the target approach, where the sensor begins switching the output transistor). And an OFF POINT - the distance, on target retreat, where the sensor stops switching the output transistor). And this slight distance between ON and OFF (usually just 10 to 15% of the sensor published range) is a large enough distance to keep the sensor on during retreat, prevent the above situation from "chattering".
We can see from the spec sheet of a Weld Dynamix Z-PROX sensor:
The sensing range is 3mm, so with a Hysteresis at 15% (calculated as 3 x .15 = .45) that makes the OFF point (as the target retreats) at 3.45mm (calculated as 3 + .45 = 3.45).
UH OH! HYSTERESIS GROWS?
Given a stable temperature in the weld cell, the prox should provide reliable detection. But elevate the temperature in the environment and - POSSIBLE TROUBLE! Elevate the temperature, and the sensing range increases (happens usually over 150 deg F) and because Hysteresis is a percentage of the sensing range this also increases Hysteresis
Basically, if you heat it up real hot, when your part retreats your sensor might stay on (until it cools back down)!
So a sensor that normally works fine at 135 degree F, if it gains 40 more degrees, could actually sense the fixture or the mount - but only after the target retreats - it stays on until the core sensor temperature subsides. This is what happened in this post and it is extremely common in weld cells.
TRAINING
Bring us to your shop to give your team an hour of 100% hands-on training with proxes, cables, mounts, and cable protection. Your team will get to play with proxes and see their hysteresis with their own eyes. The price of the training? Low. Whatever you're guessing, it's lower than that. It only takes an hour, and it's 100% hands-on.