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Directly measuring inductance creates new position/motion sensing opportunities

Directly measuring inductance creates new position/motion sensing opportunities

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By eeNews Europe



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 and inferring the quantity you want to measure. Lack of an integrated solution has limited applications in consumer and automotive products, TI adds.

Applications for inductive sensing, TI says, range from simple push buttons, knobs, and on/off switches to high-resolution heart rate monitors, turbine flow meters, and high-speed motor/gear controllers. Given their versatility, LDCs can be used in many different markets, including automotive, white goods, consumer electronics, mobile devices, computing, industrial, and medical.

LDC technology enables you to create sensors using low-cost and readily available PCB traces or metal springs. LDCs provide high-resolution sensing of any metal or conductor – including the human body.

You can achieve sub-micron resolution in position-sensing applications with 16-bit resonance impedance and 24-bit inductance values. Increased reliability comes from contactless sensing that is immune to nonconductive contaminants, such as oil, dirt and dust, which can shorten equipment life and interfere with optical detection. Flexibility comes with being able to locate the sensor remotely from the electronics, where PCBs cannot be placed. Use of low-cost sensors and targets, without magnets, cuts system costs. The LDC supports pressed foil or conductive ink targets, offering endless opportunities for creative and innovative system design. It consumes less than 8.5 mW during standard operation and less than 1.25 mW in standby mode. You can achieve lower cost and power than either a Hall-effect or optical system, TI claims.

Very high precision is available over short distances; short means, within one-half to one diameter of the sensing coil. The effect is always available in a similar ratio; so, a TI spokesman says, you can get 0.1-mm accuracy at 1 mm distance from a 1-cm-diameter coil; or 1-mm accuracy, 1 cm away from a 10-cm coil. In effect, the chip replaces the magnet in a traditional configuration, acting as a high-frequency electromagnet, and seeking disturbances in the field.

The illustration shows the output of a systems in which a steel plate is claimed in proximity to a PCB-coil – a resolution of around 5 µm is achieved at 8 mm distance. The graph is a statistical spread of the measurements, which may have some electrical noise component, and/or represent some minute mechanical perturbations of the system. Regard it, TI says, as the system-level noise floor of the setup. One of the characteristics of the technique is that as the distance to the target decreases, the available dynamic range also gets lower. You can measure the amount of metal that is over a coil, so a shaped metal target can directly yield lateral displacement – or velocity/acceleration. Where a Hall-effect detector might need a ferrous gear-tooth sensing wheel, with rectangular teeth, the LDC can resolve a non-metallic target with shaped teeth, and infer fractional-tooth position by interpreting the shaping.

Output data rate depends on the sensor frequency, and a settable parameter “response time” (which configures a digital filter). For example, according to the data sheet; if the sensor frequency is the maximum of 5 MHz and the response time is set at the minimum value, the data rate is maximised at 78.125 ksamples/sec.

To assist with design of the inductive sensors, TI has added a dedicated section to its Webench on-line tools, that calculates dimensions and simulates circuit behaviour for a variety of sensor configurations.

The LDC1000EVM evaluation module, which includes an MSP430F5528 microcontroller (MCU), is available for $29.00: an Inductive Sensing Forum has been set up in the TI E2E Community, where engineers can ask questions and get answers from TI experts.

The LDC1000, in a 16-pin, 4 by 5-mm SON package costs $2.95 (1,000) An automotive-qualified version will be available the first half of 2014.

TI; www.ti.com / www.ti.com/ldc1000-pr-eu

There is a video demonstration of the LDC1000 here; and you can download the “Guide to Inductive Sensing” infographic to learn how to build a sensor using an LDC.

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