This paper presents a simple and efficient seat occupancy detector. A seat occupancy detector is an integral part of the airbag safety system and, in its simplest form, provides the (occupied or vacant) status of the seat to the airbag control unit. Although the occupancy sensing methods based on a capacitive principle are efficient, they typically require electrodes to be placed in the surface layer of the sitting and backrest areas of the seat. The proposed sensor uses a simple electrode structure, and it is placed below the seat foam in the sitting area of the seat. These features promise a less-expensive sensor as it can be easily manufactured and installed in a seat. The new sensor combines inductive and capacitive proximity sensing principles. The sensor detects the presence of an occupant exploiting the shielding effect of the electric field while its inductive proximity feature senses the presence of conductive objects (e.g., laptop) that may be placed in the seat and helps to achieve reliable occupancy sensing. The measurement system uses a signal conditioning unit based on a carrier frequency principle. A prototype sensing system has been built, and its application as a seat occupancy sensing system in a vehicle has been verified. The developed system successfully senses human proximity and distinguishes it from other conductive objects.

A Combined Inductive–Capacitive Proximity Sensor for Seat Occupancy Detection

In this paper, a metal object detection (MOD) system, a kind of foreign object detection (FOD), which is based on mistuned resonant circuits and utilizes the variation of self-inductance of a sensing pattern, is newly proposed for wireless electric vehicle (EV) chargers. The sensing pattern that consists of multiple loop coil sets is mounted on the transmitting (Tx) pad of an EV charger, where a loop coil set has two coils connected in series with the opposite polarity to cancel out the induced voltage generated by the Tx coil. Variation of self-inductance of the loop coil set is detected by a parallel-resonant circuit, driven by a current source and operating at near 1 MHz, in order to enhance the resolution of the proposed MOD system. To increase the detection sensitivity of the proposed MOD system, instead of an exact resonant frequency, a mistuned operating frequency near the –3 dB point is utilized for the parallel-resonant circuit. In this way, the proposed MOD system can detect very small metal objects regardless of their position and orientation on the Tx coil without any blind zone. Through simulations and experiments, it is found that the proposed MOD system detects not only horizontal but also standing upright metal objects. A prototype MOD system, operating at 85 kHz to satisfy the standard J2954, was fabricated to verify its feasibility. The results showed that output voltage change of the proposed MOD system becomes 22.7% for a piece of the aluminum foil of 3 × 3 cm2 and 40.9% for 100 Korean Won coin.

Self-Inductance-Based Metal Object Detection With Mistuned Resonant Circuits and Nullifying Induced Voltage for Wireless EV Chargers

The presence of foreign metal bodies and fragments in foodstuff and pharmaceutical products is of major concern to producers. Further, hidden metal objects can pose threats to security. In particular, stainless steel is difficult to detect by conventional coil metal detectors due to its low conductivity. We have employed an HTS SQUID magnetometer for the detection of stainless steel particles which is based on the measurement of the remanent magnetization of the particle. Our aim was to determine the detection limits of HTS SQUID-based remote magnetometry, especially for food inspection purposes, and to make a comparison of this technique to commonly used eddy current coil and x-ray inspection systems. We show that the SQUID system's sensitivity to stainless steel fragments is significantly higher than that of coil systems if the samples are magnetized in a 100 mT magnetic field prior to detection. Further, it has a higher sensitivity than x-ray systems, depending on the density distribution of the product under inspection. A 0.6 mg piece of grade-316 stainless steel (a fragment of a hypodermic needle 0.5 mm long and 0.65 mm diameter) represents the detection limit of our system with a 150 × 150 mm2 inspection orifice.

https://iopscience.iop.org/article/10.1088/0953-2048/18/3/024/pdf

A SQUID-based metal detector—comparison to coil and x-ray systems

This paper presents a simple yet efficient proximity sensor obtained by combining inductive and capacitive sensing principles. Capacitive proximity sensors based on shielding of electric field are known for detecting presence of humans in its vicinity. Such systems may fail to distinguish the proximity of a human from grounded conductive objects. A novel, cost effective, sensing scheme that uses a single sensor, which performs both the inductive and capacitive proximity mode of operation is proposed in this paper. The measurement system uses a signal conditioning unit based on a carrier frequency principle. A prototype sensing system has been built and its application as a seat occupancy sensing system in a vehicle has been verified. The developed system successfully senses human proximity and distinguishes it from other conductive objects.

A combined inductive-capacitive proximity sensor and its application to seat occupancy sensing

This paper considers a dipole solution to the low-frequency electromagnetic responses of a typical in-line metal detector to metal contaminants. This solution is determined by the characteristics of metal targets and incident magnetic fields, which are treated separately as two independent factors. For the metal targets, the responses of sphere and wire samples are determined. The electromagnetic polarizability matrix of a metal sphere is directly computed from a spherical response function. The electromagnetic polarizability tensor of metal wire is derived from a measured eigenvalue matrix and a rotation matrix. The approximated responses of sphere and wire samples from the proposed solutions agree well with the measured responses from in-line metal detectors. In theory, the approximation method on metal wire samples is also applicable to the metal contaminants of other shapes.

On the Low-Frequency Electromagnetic Responses of In-Line Metal Detectors to Metal Contaminants

Metal detectors are extensively used to find undesirable metal objects in processed food. In such a typical metal detector, the coils are coaxially arranged with the transmitting coil in the center and two receiving coils on the sides. The receiving coils are connected to a differential amplifier. When the magnetic field generated in the transmitting coil is disturbed by metal objects, the amplitude and phase of the output voltage of the differential amplifier change, and, thus, the existence of foreign metal pieces is detected. The relationship between the amplitude and phase of the output and the electromagnetic properties of the metal objects, however, has only been discussed experimentally so far. The authors have already developed the SRPM method to simultaneously estimate the electrical and magnetic properties of a spherical sample by vectorially measuring the difference in the impedance of two circular solenoid coils, one with and the other without a sample. An attempt is made to theoretically analyze the properties, such as size, conductivity, and permeability of the metal objects from the output, i.e., amplitude and phase of the metal detector. Based on this method, an equation to estimate the vector voltage induced in the receiving coil by the metal object is derived by using a spherical sample to simplify the analysis.

Basic analysis of a metal detector

Metal detectors are extensively used to find undesirable metal objects in processed food. In such a typical metal detector, the coils are coaxially arranged with the transmitting coil in the center and two receiving coils on the sides. The receiving coils are connected to a differential amplifier. When the magnetic field generated in the transmitting coil is disturbed by metal objects, the amplitude and phase of the output voltage of the differential amplifier change and thus the existence of foreign metal pieces are detected. The relationship between the amplitude and phase of the output and the electromagnetic properties of the metal objects, however, has only been discussed experimentally so far. The authors have already developed the SRPM method to simultaneously estimate the electrical and magnetic properties of a spherical sample by vectorially measuring the difference in the impedance of two circular solenoid coils, one with and the other without a sample. An attempt is made to theoretically analyze the properties, such as size, conductivity and permeability of the metal objects from the output, i.e., amplitude and phase of the metal detector. Based on this method an equation to estimate the vector voltage induced in the receiving coil by the metal object is derived by using a spherical sample to simplify the analysis. The validity of the new formula was confirmed by experiments using iron, aluminum and SUS304 objects.

A. Tanaka

Papers

Basic analysis of a metal detector

Basic analysis of a metal detector