A Low-g Accelerometer for Inertial Measurement Units
Inertial Measurement Units (IMU) for upcoming automotive chassis control systems require sensor clusters with multi-axial sensors for acceleration and angular rate. In this paper, we present a low-g accelerometer that fits the IMU requirements for advance
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A Low-g Accelerometer for Inertial Measurement Units
K. Kapser, M. Aikele, R. Gottinger, B. Hartmann, ContiTemic R. Burghardt, Continental Teves H. Seidel, Saarland University Abstract Inertial Measurement Units (IMU) for upcoming automotive chassis control systems require sensor clusters with multi-axial sensors for acceleration and angular rate. In this paper, we present a low-g accelerometer that fits the IMU requirements for advanced automotive applications and is capable of monolithic multiple-axes integration. For the target application a high offset stability and an overcritical damping of the sensing element are required. With simple design changes the squeezed-film damping of the sensor can be adjusted within a wide range (5Hz to 400Hz) using atmospheric pressure inside the cavity. Due to its symmetrical design and the differential readout principle, an offset drift of less than 50mg over the full temperature range (-40°C…120°C) was achieved. The sensor principle presented in this paper can be utilized to realize a sensitivity axis parallel to the wafer surface as well as perpendicular to the surface. Furthermore, monolithic integration into a two-axis (x/z) or a three-axis (x/y/z) sensing element is possible. The tri-axial element exhibits a four-mass design providing redundancy and thus an ongoing self-test capability.
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Introduction
In future advanced automotive systems an IMU (Inertial Measurement Unit) is used to provide comprehensive vehicle inertial data consisting of angular rates and acceleration values. All six degrees of freedom are covered (i.e. yaw, roll, and pitch for angular rate as well as lateral, longitudinal and vertical acceleration) with some safety relevant functions (e.g. yaw rate) potentially being redundant (figure 1). Various vehicle systems can take advantage of this information for applications such as dynamic drive control or roll over detection for the airbag. The compact spatial setup within one relatively small IMU allowing improved alignment of the individual sensor elements helps to greatly reduce cross-axis sensitivities. Nearly optimal alignment can be achieved by
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Comfort and HMI
the monolithic integration of several axes on one chip, as demonstrated in the three axial low-g sensor concept presented in this paper. Furthermore, a mathematical modeling concept of the known individual cross-axis sensitivities can be applied for further error reduction. The orientation of the IMU within the vehicle is not critical as the local coordinate system provided by the IMU can be transformed mathematically by signal processing (internal or external) to fit the desired vehicle coordinate system. Further integration with other sensors (e.g. wheel speed, GPS, etc.) is possible, thus enabling a dynamic reduction of the bias error, further increasing the precision of such systems.
Fig. 1.
Principle of an inertial measurement unit (IMU) containing different sensors and signal conditioning electronics.
One key component of such an IMU is a low-g acceleration sensor [1]. The r
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