Written by: Jeff Himes
An inductive proximity sensor is meant to be a non-contact device. If contact is made with the face of the sensor by its metal target – it will typically fail. What if you want the reliability of an inductive proximity sensor – yet you want physical contact with the device too? Is this really possible? Yes – by using a mechanical device I call a Banking Screw.
A banking screw is a mechanical probe that has a spring-loaded plunger that can be “pressed” to trigger an inductive proximity sensor. The Banking Screw housing is typically threaded with a standard sensor thread such as M8 x 1, M12 x 1, M18 x 1, etc. and has two locknuts for mounting the banking screw in place. Photo 1 shows an M18 x 1 threaded banking screw. The tip of the unit (left end) is the spring-loaded plunger that moves approximately 4mm “in” when it is pressed. The rear of the banking screw (right end) is where the inductive proximity sensor is threaded into the unit.
Photo 2 shows the banking screw along with an M18 sensor. It is obvious from this photo that one requirement for this device is quite a bit of space. The banking screw is approximately 85mm long, the M18 sensor is 65mm long and the connecting cable head is approx. 40mm long. Therefore a total clearance of 165-170mm will be needed for the assembly. (The sensor recesses about 25 mm into the banking screw when fully threaded into the unit.)
Photo 2: Banking Screw and Sensor Prior to Assembly
To assemble the unit, the M18 inductive proximity sensor is simply threaded into the back of the banking screw until it triggers on the banking screw’s internal plunger. At that point the sensor will be activated with the banking screw in its “non-pressed” state. The sensor is then backed out (unscrewed) until it is de-activated. Now the sensor can be adjusted for a trigger point.
Trigger Point Adjustment – The trigger point of the sensor can be adjusted by threading it in/out of the banking screw. This will allow the sensor to trigger from 1-4 mm of plunger movement. The sensor must be threaded in/out and the plunger pushed in/out to observe the triggering relationship. Once set in place the sensor mounting nut should be used to lock the sensor in place. Photo 3 shows a fully assembled unit with banking screw plunger fully extended and without the inductive sensor triggered.
Photo 3: Banking Screw/Sensor Assembled but not Triggered
Note: Only a standard range inductive proximity sensor should be used. An extended range model may false trigger once mounted inside of the banking screw.
When a target comes in contact with the banking screw plunger, the plunger is pressed inward and in turn triggers the inductive proximity sensor. Photo 4 shows the banking screw plunger “pressed” and the inductive sensor in its activated state. Once the target moves away from the plunger, the plunger will return to its extended state due to its spring-loaded design.
Photo 4: Assembled Unit with Plunger pressed and Sensor Triggered
Advantages of using a Banking Screw:
1. Allows for positive mechanical contact to plunger without contact to sensor face
2. Provides mechanical barrier to heat if target has an elevated temperature
3. Isolates proximity sensor from mechanical contact
4. Provides a solid state output with no mechanical contact to sensor
If you have an application where mechanical contact is unavoidable or actually desired – don’t count out an inductive proximity sensor! By using a banking screw in combination with an inductive proximity sensor you can get the advantages of both.