Inductive Proximity Sensors: Mounting Tips & Tricks – Part 3
Inductive Proximity Sensor mounting requirements must be considered when implementing the sensor into your design, otherwise you may encounter reduced sensing distance, false triggering, or non-detection of the target.
Influence of surrounding metals
When an inductive proximity sensor is mounted into its sensing position, it is important to consider the effects of the mounting hardware and other metallic objects that may be present in the area of the sensor.
For shielded inductive proximity sensors, the device can be embedded into a metallic mounting fixture up to the point when the sensor’s face is at an equivalent height as the mounting surface. This embedded mount protects the inductive sensor from mechanical damage due to incidental contact with the target object. It is not recommended that a shielded inductive proximity sensor be recessed into a metal mounting surface. Objects, materials, or opposing surfaces that are not to be detection objects should remain clear of the Inductive sensor’s face by a factor of 3 times the sensor’s standard detection distance.
Unshielded inductive proximity sensors cannot be completely embedded into a metallic-mounting fixture. Due to its extended sensing distance, the unshielded inductive proximity sensor is susceptible to the influences of surrounding metals. One does not only need to consider that objects, materials, or opposing surfaces must remain clear of the sensor’s face by a factor of 3 times the sensor’s standard detection distance. In addition, one must consider that the inductive proximity sensor must be clear of surrounding metals by its size (diameter in the case of a cylindrical proximity sensor) in every direction with a depth clearance of 2 times its standard detection distance.
Failure to meet the inductive sensor’s clearance requirements can lead to false detection or reduced sensing distances.
Mutual Interference
When multiple inductive proximity sensors are mounted in close proximity to one another, either along side or in an opposing direction to another inductive proximity sensor, either inductive sensor can be subject to an effect called mutual interference.
Mutual interference is created when the field of a proximity sensor couples with the detection coil field of another closely mounted proximity sensor. The result can create an inductance that can result in the generation of a beat frequency in one or both of the sensors. This, in turn, causes the output of the proximity sensor to chatter (switch on and off erratically).
Mutual interference problems can be insidious due to their erratic nature. A sensing application where Inductive sensors are mounted side by side and closer then a manufacturer’s mutual interference distance specifications can actually perform seamlessly at one time and then suddenly display signs of chattering and false detection at another times. Specifications for separation distance of proximity sensors that are mounted side by side can vary from sensor body type and by manufacturer. Always examine and adhere to the manufacturer’s specification distances for mounting inductive proximity sensors to avoid potential mutual interference problems.
If your application and sensing requirements demand your inductive proximity sensors to be mounted closer together, consider the following tips. The selection of a shielded type of inductive sensor allows for closer mounting. Of course, one could also specify a miniature inductive sensor. The smaller size means smaller sensing distances and less probability for mutual interference. In addition, some manufacturers offer alternate frequency types.
Alternate frequency inductive sensors oscillate their magnetic coils at different cycle rates than their standard inductive proximity sensor counterparts. This prevents the inductive coupling that leads output chattering.
Lastly, if close sensor mounting cannot be avoided, the inductive proximity sensors can be multiplexed. Turning off and on alternate inductive proximity sensors and taking alternate reads can be a quick solution to a mutual interference problem provided that your application accounts for the response time hit.
Make sure you read the first two articles in my series about Inductive Proximity Sensors, Understanding Specifications and Design & Selection or download the entire article as a PDF by clicking here.
