Advanced and Intelligent Interface Circuits for Capacitive MEMS Sensors - The E and S in MEMS

微型机电传感器通过使用微小的硅质块来测量诸如加速度、转速、压力或力等量，其位置随着这些量的变化而改变。这转化为电容的变化，而电容又可以通过电子电路来测量。通过开发新的微制造工艺和优化机械传感元件的设计，人们投入了大量精力来改进这些传感器。这项研究提出了一种完全不同的方法来改进这些传感器的性能，即在这些传感器的电子接口和控制系统方面进行工作。这是通过在所谓的闭环力反馈系统中结合微机械传感元件来实现的，在该系统中产生的静电反馈力抵消了样品质量的运动。最有前途的控制系统是基于高阶Sigma-Delta调制器的。Sigma-Delta调制器(SDM)是电子模数转换的标准方法。本文提出将类似的拓扑结构应用于微机械电容传感器，以改善其带宽、动态范围、线性度和噪声底限等性能。另一个吸引人的特性是SDM具有可与标准数字信号处理器接口的直接数字输出信号。一种名为噪声整形的特性至关重要，它是衡量这种传感器系统工作情况的一个指标。高阶SDM与MEMS传感器结合使用，比目前已被研究的低阶SDM具有更好的噪声整形能力。为了实现这一目标，需要开发新的体系结构和控制系统拓扑，因为不可能将电子A/D SDM系统体系结构一对一地转移到MEMS传感器。这是一项具有挑战性的任务，需要进行理论分析，包括非线性控制理论、广泛的系统级仿真、硬件实现和传感器的严格测试。在这方面的研究很少，但有巨大的潜力产生真正的影响。通过这种方法，应该可以开发一种非常通用的接口芯片，该芯片可以与一系列微机械电容传感器一起使用，因为它对制造变化非常容忍，这是许多微机械传感器的问题。我们希望使用我们的工业合作伙伴Melexis提供的微机械加速度计来演示这种方法的有效性和潜力。这反映了这项研究计划背后的总体理念，使用现有的微机械传感器，并使用先进的电子和控制工程，使其更好、更多用途和更容易在系统层面上集成。

Micro-electro-mechanical sensors measure quantities such as acceleration, rate of turn, pressure or force by using a tiny silicon proof mass whose position is changed as a response to these quantities. This translates into a change in capacitance, which, in turn, can be measured by electronic circuits. Much effort has been put into improving these sensors by developing new micro-fabrication processes and optimising the design of the mechanical sensing element. This research suggests a radically different approach to improve the performance of these sensors, namely to work on the electronic interface and control systems aspects of these sensors. This is achieved by incorporating the micromachined sensing element in so-called closed loop, force-feedback systems where an electrostatic feedback force is generated that counteracts the motion of the proof mass. The most promising type of control system is based on higher order sigma-delta modulators. Sigma-delta modulators (SDM) are the standard approach for electronic analogue-to-digital conversion. It is proposed here to use similar topologies & architectures and apply them to micromachined capacitive sensors to improve their performance properties such as bandwidth, dynamic range, linearity and noise-floor. A further attractive property is that a SDM has a direct digital output signal that can interface with a standard digital signal processor. A property called noise-shaping is of crucial importance and a measure of how well such a sensor system works. Higher-order SDM, as proposed to be used here in conjunction with MEMS sensors, have much superior noise shaping than lower order ones, which have been researched to date. To achieve this goal it will be necessary to develop new architectures and control system topologies, as a one to one transfer of electronic A/D SDM system architectures to MEMS sensors is not possible. This is a challenging task that will require theoretical analysis involving nonlinear control theory, extensive system level simulation, implementation in hardware and rigorous testing of the sensor.Little research has been done in this direction, yet there is huge potential to make a real impact. With this approach it should be possible to develop a very versatile interface chip that can be used with a range of micromachined capacitive sensors, as it is very tolerant to fabrication variations, which are a problem for many micromachined sensors. We want to demonstrate the validity and potential of this approach using a micromachined accelerometer supplied by our industrial partner Melexis. This reflects the general philosophy behind this research programme, use an existing micromachined sensor, and use advanced electronics and control engineering to make it better, more versatile and easier to integrate at the system level.