Traditional Hall-effect sensors measure the magnetic field perpendicular to the surface of the semiconductor chip. By integrating a planar magnetic disk parallel to the integrated circuit's (IC's) surface, the resulting flux concentrator allows measurements parallel to the surface and provides several advantages. A few critical design factors for the sensor demonstrate its value.
In a typical application, the magnetic field on the shaft is perpendicular to the surface of the chip
The design of Melexis' MLX90316 Hall sensor starts with a pair of Hall-effect elements placed at opposite ends of both the X and Y axes of the CMOS-IC layout. The diameter of a soft magnetic disk is strategically positioned above the Hall elements so that its periphery translates the in-plane magnetic field to a perpendicular magnetic field for the sensor elements. By depositing the disk on the surface of the chip at the wafer level, Melexis minimizes alignment issues.
Because the disk is a soft ferromagnetic material, the magnetic field within and around the disk is fixed, but the field rotates synchronously with the external magnetic field. The addition of the ferromagnetic structure amplifies the external field for greater sensitivity and allows the measurement of both components of the flux density parallel to the chip's surface. Additional circuitry in the CMOS-designed sensor, including a microcontroller (MCU) with a digital-signal processor (DSP) core, amplifies, samples, and converts the two signals into the digital domain. Using a CORDIC (cordinate rotation digital computer) algorithm, the DSP determines the shaft angle. The computed angle is available as an analog output voltage from a ratiometric 12-bit DAC and a 12-bit pulse-width-modulated (PWM) signal with programmable frequency or a digital serial protocol.
The design provides increased accuracy because the measurements are less sensitive to flux-density variations from temperature, aging, or airgap. Among the initial applications that can benefit from this approach are several automotive measurements, such as steering-wheel position and steering torque in electrical power- steering systems, as well as accelerator, brake, and other noncontact position sensors.
Melexis programming unit PTC-04 connected to the supply, ground, and output accesses the internal microcontroller and DSP core to program the MLX90316 for the end application. For a larger image, click here
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