Inductive proximity sensors (IPS) are non-contact sensing devices that are widely used in position detection due to their unique advantages such as resistance to fouling and abrasion, water tightness, long service life, low mechanical system maintenance cost, high mean time between failure (MTBF) value, and strong magnetic immunity. IPSs are particularly suitable for the industry field of position detection, especially in the aviation and aerospace fields.
Understanding the Fundamentals of Inductive Proximity Sensors
The primary transducer of IPS is a coil, and its inductance component correlates to the distance between IPS and the target. However, the resistance component of the coil changes significantly as the temperature changes due to the effects of the coil structure and materials. The resistance component also constrains the detection precision of IPS severely. The performance of IPS depends on the coil structure and the processing circuit. In certain conditions, the greater the inductance component is, the better the IPS sensitivity will be. However, the temperature drift will be severe as the resistance component increases simultaneously, and modeling calculation will be complicated by the simultaneous increase in distributed capacitance. The sensor signal processing circuit is important to the IPS performance. Many methods have been developed to reduce temperature drift by improving the processing circuit, including the analog and digital measurement methods.
Measuring Range and Sensitivity of Inductive Proximity Sensors
The measuring range of IPS is directly proportional to sensor diameter. Generally, the magnetic field generated by the sensor coil can be detected at a distance of about 2 times the coil diameter from the face of the sensor. This field extends not only to the front but also to the sides to some degree and in back of the coil. Its strength decreases exponentially with distance from the coil. The sensor range is typically 30-50% of the sensor coil diameter for all full-size targets.
In a well-controlled experiment, the sensitivity and accuracy of a developed metal detector using IPS were studied by finding limit distances at which the distance vs. spikes with ms count N was determined. The approximation of the distance vs. spikes with ms count N was obtained, which may be used for practical distance measurements. The range detection error mostly does not exceed up to distance D, which is 2.8 times higher than the accuracy of other inductive sensors at long ranges. At distances D, the approximation accuracy does not exceed 5%, which is sufficient for most industrial and aerospace applications.
Detection Distances for Different Metals and Sizes
Table 1 shows the detection distances for plates of different metals and sizes. Note that all the values are natural numbers for ease of interpretation.
Table 1: Detection Distances for Plates of Different Metals and Sizes (in cm)
| Plate Size | Steel | Brass | Aluminum |
|---|---|---|---|
| 25 × 25 | 22 | 28 | 25 |
| 12 × 12 | 10 | 15 | 14 |
| 5 × 5 | 1 | 7 | 7 |
For ease of generalization of the results, the target sizes were converted into relative values proportional to the coil diameter—see Table 2. Recall that all the coils in the experiments have a diameter of 10 cm.
Table 2: Detection Distances for Plates of Different Metals and Sizes (in terms of sensitive coils diameter)
| Plate Size | Steel | Brass | Aluminum |
|---|---|---|---|
| 2.5 × 2.5 | 2.2 | 2.8 | 2.5 |
| 1.2 × 1.2 | 1.0 | 1.5 | 1.4 |
| 0.5 × 0.5 | 0.1 | 0.7 | 0.7 |
Comparison with Other Inductive Proximity Sensors
Table 3 compares the proximity range of the developed sensor with sensors in referred works.
Table 3: Comparing the Proximity Range of the Developed Sensor with the Sensors in Referred Works
| Sensor | Max. Range/Coil Width |
|---|---|
| Cylinder coil and mixed analog-digital unit | 0.66 |
| Passive Wireless LC Sensor Based on LTCC | 0.75 |
| Multilayer PCB coil and ECS75 conditioner | 1.0 |
| Inductive Proximity Magnetoplated Wire Sensor | 1.0 |
| LTCC planar coil and TI LDC1000 chip | 1.26 |
| Single-coil chaotic metal detector | 1.4 |
| Current study | 2.8 |
As shown in Table 3, the developed sensor in the current study has a significantly higher proximity range compared to the other sensors, with a maximum range of 2.8 times the coil width. This is a remarkable achievement, making the sensor suitable for a wide range of industrial and aerospace applications that require long-range position detection.
Conclusion
In summary, IPSs are non-contact sensing devices that are widely used in position detection due to their unique advantages. The measuring range of IPS is directly proportional to sensor diameter, and the sensor range is typically 30-50% of the sensor coil diameter for all full-size targets. The performance of IPS depends on the coil structure and the processing circuit, and many methods have been developed to reduce temperature drift by improving the processing circuit, including the analog and digital measurement methods. The sensitivity and accuracy of IPS can be studied by finding limit distances at which the distance vs. spikes with ms count N is determined, and the approximation of the distance vs. spikes with ms count N may be used for practical distance measurements.
References:
– Kaman Sensors. (2012). INDUCTIVE TECHNOLOGY HANDBOOK. Retrieved from https://www.kamansensors.com/pdf_files/Kaman_Applications_Handbook_WEB.pdf
– NCBI. (2015, December 30). An Analog-Digital Mixed Measurement Method of Inductive Proximity Sensor. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4732063/
– NCBI. (2022, July 12). Sensitivity Optimization and Experimental Study of the Long-Range Inductive Metal Detector. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9319772/
– Festo. (n.d.). Inductive proximity switches. Retrieved from https://www.festo.com/gb/en/c/products/industrial-automation/sensors/inductive-proximity-switches-id_pim129/
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