The principle of operation of a position and orientation inductive sensor is based on a detailed mapping of quasistatic magnetic fields induced in free space by a set of miniature coils (or loops) called “magnetic dipoles”. Depending on the task, one, two or a maximum of three coils in the mutually orthogonal planes can be used to produce the required magnetic pattern in free space. Each coil is energised by a sinusoidal current in the kHz range or a pulse sequence: independently or with a certain phase. The quasistatic condition means that the electromagnetic wavelength in free space is much larger than any dimensions in the system, including the coil diameters and the distance between the source of magnetic field and the sensor head. Therefore, the retardation effect caused by the finite speed of light can be completely neglected. The aim is to obtain a unique vector magnetic field (time-dependent) in each coordinate point. The magnetic field in an observation point can be measured by means of a set of miniature coils (or loops) comprising the sensor head which is attached to the tracking object. The sensor head may also include one, two, or a maximum of three miniature coils in the mutually orthogonal planes. To recognise the position and orientation of the sensor head, some algorithm has to be proposed that recalculates the magnetic field pattern into the coordinates and orientation angles.
In this project, Kalmus’s guidance method has been successfully adapted for robot navigation. Originally proposed by Henry Kalmus*, this method was designed to enable vehicles to follow one another. In Kalmus’s approach, two crossed excitation coils are powered by a low-frequency source (in the kHz range) in phase quadrature (a π/2-phase shift). The sensor head, mounted on a moving platform, consists of two crossed pickup coils and adjusts its azimuthal orientation relative to the line of sight between the excitation and sensor heads. Orientation is determined by measuring the phase difference between the pickup voltages. Leveraging this principle, we developed a navigation system that enables a slave mobile platform (“tank”) to follow a leading platform. The leading platform, controlled via remote control, carries the transmission coils that generate a quasi-static magnetic field. The slave platform, equipped with pickup coils, gathers information about its distance from the magnetic field source and its angular position relative to the source. This feedback allows the slave platform to autonomously follow the leading platform’s path. Based on our research, this represents the first documented application of inductive sensors for robot navigation. To adapt Kalmus’s guidance method for this purpose, we employed modern electronic components and digital signal processing techniques, ensuring a precise and reliable implementation.
*) H. P. Kalmus, “A New Guiding and Tracking System”, IRE Transactions on Aerospace and Navigational Electronics, Vol. 9, pp. 7–10, (1962).
Further references:
Motion tracking solutions from Polhemus (http://www.polhemus.com): http://polhemus.com/applications/electromagnetics and https://www.youtube.com/@Polhemus/videos
F. H. Raab et al., “Magnetic position and orientation and tracking system”, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-15, pp. 709-718, (1979).
Wireless power transfer: https://en.wikipedia.org/wiki/Wireless_power_transfer