Udo Ausserlechner

Compensation of the Piezo-Hall Effect in Integrated Hall Sensors on (100)-Si

Silicon Hall sensors are known to suffer from a long-term drift in the magnetic sensitivity between 1% and 4%, depending on the degree of moisture in the mold compound of the package. This drift is mainly caused by changes of mechanical stress exerted by the plastic package onto the die. We present a system, which continuously measures the relevant stress components, estimates the sensitivity drift, and corrects for it digitally. An individual precalibration versus temperature is necessary to achieve the required level of accuracy. Results from laboratory characterization with pressure cells and lifetime drift during qualification runs show that this system can keep the drift of magnetic sensitivity well below 1%.

Ultra low-power monolithically integrated, capacitive pressure sensor for tire pressure monitoring

Tire pressure monitoring systems (TPMS) are gaining importance due to improved security standards in the automotive industry. In active systems, the power consumption of the system is of utmost importance for a required lifetime of more than 10 years. The presented integrated circuit comprises high resolution pressure, temperature and voltage monitoring, low-power oscillators, a specialized DSP with RAM/ROM and an EEPROM to store calibration data. The continuously running low-power oscillator for internal wake-up timing is operated with 30 nA. Due to the extremely low-power consumption of all circuit blocks, pressure measurements can be carried out in short intervals (0.5 s). This enables the implementation of a new intelligent dynamic wake-up algorithm to optimize the power-intensive HF-transmission intervals under different driving situations. Thus additional rotation detectors are not needed any more. The charge consumption without HF-transmission results in 48 mAh in 10 years.

An Integrated Hall Sensor Platform Design for Position, Angle and Current Sensing

A new platform design of a Hall sensor uses digital signal processing and digital 3rd order temperature compensation. The improved architecture of the front-end achieves low noise of <2 muT in a bandwidth of 400 Hz by using a 3rd-order multibit continuous-time DeltaSigma-ADC. By chopping the first stage of this ADC a low magnetic offset of <50 muT is realized in a typical automotive operating temperature range from -50 to 150degC. A novel circuit technique for a stress sensor is used to compensate stress and aging effects in the Hall sensor. All required digital calculations are done at low power and area with a new intelligent state machine, which also provides various digital interface protocols.

A miniature digital current sensor with differential Hall probes using enhanced chopping techniques and mechanical stress compensation

A 10kHz bandwidth 50Amax current sensor using a Hall effect gradiometer without magnetic core provides 80kHz update rate with a digital interface. Very low un-calibrated offset of 30mA (1σ) and after calibration typical 10mA over temperature is accomplished by a chopped multi-bit feedback continuous-time 3rd order ΔΣ-ADC. This also realizes low noise of 13mArms in 1kHz signal bandwidth. The ADC uses enhanced chopping techniques and additional digital feedback loops to avoid chopper ripple. New analog and digital stress-compensation circuits with lateral and vertical n-doped resistors achieve lifetime gain drifts below 1% and temperature compensation. Auto-zeroing ping-pong comparators offer a fast over-current detection of 1...2μs on a dedicated output pin. The monolithic integrated sensor chip and the 4kV galvanic isolated current rail fit into a very small 7×7×1mm3 package.

In-Situ Analysis of Deformation and Mechanical Stress of Packaged Silicon Dies With an Array of Hall Plates

A technique is presented that allows to simultaneously measure stress and deformation of packaged silicon dies. Required is a test chip with an array of Hall plates assembled in a package made of nonmagnetic materials. Exploiting the highly anisotropic magnetic sensitivity of the integrated Hall plates in an external magnetic field, makes it possible to measure the deformation. Moreover, the piezoresistive and piezo-Hall effect allow for measurements of several stress components without extra sensor elements. An optical measurement verifies the new technique, and a standard plastic package is exemplarily analyzed at different temperatures, under the influence of an external force, and at moisture load. The combined knowledge of stress and deformation allows for deeper insight into the mechanical impact of packaging on integrated circuits.

The Optimum Layout for Giant Magneto-Resistive Angle Sensors

360° angle sensors use small permanent magnets attached to the shaft. The magnet is polarized perpendicularly to the axis of rotation, and a magnetic field sensor is placed underneath on the axis. The sensor circuit consists of two orthogonal bridges having four giant magneto-resistive (GMR) elements. In a prior work it was shown that even though the magnetic field sensor may be calibrated perfectly, still significant angle errors may result from assembly tolerances of magnet and sensor. This work investigates how shape and size of the GMRs affect this error. Optimum layouts for GMRs fulfill three constraints: (i) the centers of gravity of all GMRs of both bridges lie on the axis of rotation; (ii) the sum over the deviation moments of all GMRs of each bridge circuit vanishes; and (iii) the sums over the moments of inertia around two perpendicular axes in the die surface are equal. Examples of optimized layouts are given. Layout and assembly tolerances interact to give an overall angle error, which is expanded into a second-order Taylor series. Monte Carlo simulations show that optimum layout reduces typical angle errors significantly and worst case angle errors moderately. For Gaussian distributed assembly tolerances with standard deviations of 0.1 mm and 1° and cylindrical magnets with 5 mm diameter 99.7% of all systems have errors less than +/-0.41°. Magnets with 10 mm diameter have only +/-0.16° error. It is shown that magnetic angle sensors are more robust against eccentricities of the shaft than many optical encoder systems.

Drift of magnetic sensitivity of smart Hall sensors due to moisture absorbed by the IC-package [automotive applications]

For automotive applications like cam-shaft, crank-shaft, throttle valve or accelerator pedal position sensing, Hall probes are integrated together with biasing and complex signal processing circuits into tiny plastic packages. Instabilities of magnetic switching points and magnetic sensitivity have been observed for some time. We show by way of experiments that the main contribution to this sensitivity drift originates from moisture absorption by the mold compound of the plastic package which changes the mechanical stress on the die. The mechanical stress affects the Hall factor of the Hall probe (piezo-Hall effect) and the current through the Hall probe (piezo-resistance effect) which finally leads to a drift of the overall magnetic sensitivity.

INACCURACIES OF ANISOTROPIC MAGNETO- RESISTANCE ANGLE SENSORS DUE TO ASSEMBLY TOLERANCES

A large class of angle sensors uses a small permanent magnet attached to the rotor. The magnet is polarized perpendicularly to the axis of rotation, and a magnetic fleld sensor is placed ahead on the axis. The sensor circuit consists of two full bridges at 45 - , each having four anisotropic magneto-resistive (AMR) elements. Even though the electronic system may be calibrated to have nearly no errors like ofiset, nonlinearity, and mismatch, still signiflcant angle errors may result from assembly tolerances of the magnet and the sensor. This work gives an analytical description of the angle error caused by tilts and eccentricities of magnet and sensor elements against the axis of rotation. Particular emphasis is given to worst case combinations of all tolerances. One part of the angle error can be cancelled by an optimized layout of the AMR-resistors. The remaining part is identical to the case of giant magneto-resistive (GMR) angle sensors. Errors of both AMR and GMR angle sensors are efiectively reduced by identical optimization of the shape of magnets. One such optimized shape is disclosed.

Closed Analytical Formulae for Multi-Pole Magnetic Rings

Multi-pole permanent magnetic encoders are used for wheel speed sensing in automotive systems. This paper discusses rings and discs magnetized along axial direction. The vector fleld is calculated analytically in 3D as sums over all poles. For the case of discs with vanishing inner and inflnite outer diameter the summation is done in closed form with a new summation formula. The results are discussed and several plots of fleld patterns are given in normalized form: At very small air-gap the fleld shows an overshoot. At moderate and large air-gaps it is a sine-function with exponentially decaying amplitude. The amplitude versus air gap, reading radius, thickness of the magnetic layer, and number of poles is studied and excellent agreement with measurements is found. The efiect of a steel-back on the fleld is explained. It is also shown how to maximize the torque transmitted in axially magnetized couplings.

Macroscopic simulation of isotropic permanent magnets

Abstract Accurate simulations of isotropic permanent magnets require to take the magnetization process into account and consider the anisotropic, nonlinear, and hysteretic material behaviour near the saturation configuration. An efficient method for the solution of the magnetostatic Maxwell equations including the description of isotropic permanent magnets is presented. The algorithm can easily be implemented on top of existing finite element methods and does not require a full characterization of the hysteresis of the magnetic material. Strayfield measurements of an isotropic permanent magnet and simulation results are in good agreement and highlight the importance of a proper description of the isotropic material.