Directly measuring inductance creates new position/motion sensing opportunities

September 16, 2013 // By Graham Prophet
Texas Instruments claims to have created an entirely new class of data converter, with its Inductance-to-digital converter, which the company believes will fundamentally change position and motion sensing, based on a coil stimulating a target, or detecting deformation of an inductor.

The inductance-to-digital converter (“LDC”) offers, in fact, two modes of detection, and that is detection of anything that possesses inductance: which can mean, anything that is conductive. In one mode, a sensor coil excites eddy-currents in a conductive target, which can be as simple as a piece of wire or a metal plate. Using a frequency range of 5 kHz to 5 MHz, the chip sets up resonance in the tank circuit of which the (electrically-unconnected) target forms a part. Any changes in the position or orientation or orientation of the target will alter the eddy-current losses; that change can be detected by the LDC to 16-bit resolution. In the second mode, the chip directly measures changes the inductance of a conductive element to which it is connected, by measuring frequency shifts; in this mode it achieves 24-bit resolution. Reference to inductance leads naturally to imagining the connected element as a coil; any distortion in the helix is detectable; so a spring can be its own load sensor. 24-bit sensitivity means that, to quote one example cited by TI, changes in the inductance of a bedspring can detect the breathing of the occupant of the bed.

This lies behind TI's claim that the LDC can use coils and springs as inductive sensors to deliver higher resolution, increased reliability, and greater flexibility than existing sensing solutions at a lower system cost. More generally, inductive sensing is a contactless sensing technology that can be used to measure the position, motion, or composition of a metal or conductive target, as well as detect the compression, extension or twist of a spring.

TI says that in many ways inductive sensing is complementary to capacitive sensing; capacitive sensing reacts “to everything” (with high sensitivity) whereas inductive sensing offers high selectivity – at short range. In the industrial arena, inductive sensing is well-know, but has generally required you to implement your own means of measuring inductance