Motor control

Inductive position sensor targets EV motor control applications

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There is a big gap between the theoretical concept of sensing a real world tangible physical variable and what it takes to do so in the harsh reality of the application. This is why there are so many approaches to sensors and related electronics to measure basic parameters such as distance, temperature, speed, pressure, mass, etc.

For example, sensing important factors that involve motor drive and position in the automotive environment is a multi-faceted mechanical, environmental, and electronic challenge. To remedy this situation, the LX34070 integrated circuit of Microchip technology targets electric vehicle (EV) motor control applications.

This high-speed inductive position sensor includes differential outputs, fast sample rates, and features that make it functional safety ready for ISO 26262 compliance in Automotive Safety Integrity Level (ASIL-C) classification. C) and AEC-Q100 certification for Level 0.

Inductive sensing isn’t new, of course, and uses an AC-excited primary coil – here fabricated as part of the PCB – to generate a magnetic field that couples to two secondary coils (also part of the board) . A small metallic target object disrupts this magnetic field, causing each secondary coil to receive a different voltage; the ratio between the voltages is used to calculate the absolute position (Fig.1).

By using board traces rather than transformer-based magnetic windings and coil structures, the LX34070 has negligible size and mass compared to alternative approaches. Accuracy is improved because the performance does not depend on the absolute strength of the magnet, but rather on their difference. The device further enhances ruggedness by actively rejecting stray magnetic fields, a major concern in electric vehicles.

These and other features provide designers with increased flexibility in where the LX34070 thin-and-light PCB-based solution can be placed in their EV motor control designs. The two output signals that represent absolute position can be used in a single-ended configuration to reduce pin count or as differential signals to maximize noise immunity if needed.

As expected, the IC is complex internally (Fig.2).

It includes an excitation circuit to drive the primary coil and two independent analog channels with input filtering and amplitude demodulation for the secondary side. In addition, differential/unbalanced output buffers with precise common-mode level and automatic gain control (AGC) maximize signal resolution over large gap ranges.

Of course, since faults are always a problem, there are circuitry for output short circuit protection, power and ground loss detection, and fault detection at the coil pins. excitation and receive coil pins.

The 36-page technical sheet explains the theory of operation, includes operating information, provides details on setting parameters in its EEPROM, and an application circuit with suggested component values (Fig.3).

Microchip also provides links to a wide range of Inductive Position Sensor Design Resourcesincluding evaluation kits and software tools, their take on “Advantages of inductive position sensors compared to Hall effect sensors“(Hall effect sensor vendors will no doubt have different views here), and a nine-minute video tutorial”Demonstration of the concept of inductive position sensing.”

The LX34070 operates from a 4.5 to 5.5 V supply with protection up to 18 V; it is housed in a 14-pin TSSOP package and is rated for -40 to +150°C operation.

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