A combined modulated feedback and temperature compensation approach to improve bias drift of a closed-loop MEMS capacitive accelerometer

目的:根据Allan方差定义,分析引起闭环MEMS电容式加速度计中长期漂移的因素,研究相应的消除和补偿方法,实现降低漂移、提高闭环MEMS加速度计稳定性的目的。创新点:首先设计利用载波调制反馈电压信号的方法,避开并滤除反馈通道上的低频噪声,降低Allan方差的偏置不稳定性;然后在反馈通道上加入参考信号,利用该参考信号经过模拟部分后被解调的相位来表征温度,进行温度补偿,进一步降低Allan方差的长期漂移。方法:首先,根据Allan方差的偏置不稳定性定义,其影响因素来自于闭环系统的低频噪声。根据文献资料和作者之前的实验测试情况,大部分低频噪声来自于反馈通道,因反馈信号是低频电压信号。为降低反馈通道低频噪声,设计载波调制反馈信号及通过高通滤波器滤除低频噪声的方案MFA(图4)。经测试,采用MFA的输出噪声和1小时的稳定性明显优于反馈信号直接反馈的方案DFA(图10-12)。在MFA基础上,继续增加一个参考信号,该信号通过整个系统的模拟部分后被数字系统解调,其相位信息携带了外界温度变化信息,从而可利用该相位信息进行实时温度补偿(图5、6)。对温度补偿方案进行静态温度范围测试(图16),补偿后的度系数是补偿前的1/46;在快速动态温度变化下(图17),实时补偿结果是未补偿的1/8,显示了较好的补偿特性。温度补偿后的Allan方差长期漂移获得明显降低,而偏置不稳定性略微升高(图18、19)。总体上看,利用MFA和温度补偿,闭环MEMS电容式加速度计的整体漂移特性得到较好抑制。结论:针对闭环MEMS电容式加速度计漂移问题,提出载波调制反馈和新型温度补偿方案。两种方案能够在同一闭环系统中同时使用,分别降低了偏置不稳定性和长期温度漂移,1 h的Allan方差显示,偏置不稳定性约为13μg,100 s积分时间的漂移为17μg,温度系数降到0.1 mg/°C,显示了较好的稳定性。

The bias drift of a micro-electro-mechanical systems （MEMS） accelerometer suffers from the l/f noise and the tem- perature effect. For massive applications, the bias drift urgently needs to be improved. Conventional methods often cannot ad- dress the l/f noise and temperature effect in one architecture. In this paper, a combined approach on closed-loop architecture modification is proposed to minimize the bias drift. The modulated feedback approach is used to isolate the l/f noise that exists in the conventional direct feedback approach. Then a common mode signal is created and added into the closed loop on the basis of modulated feedback architecture, to compensate for the temperature drift. With the combined approach, the bias instability is improved to less than 13 μg, and the drift of the Allan variance result is reduced to 17 μg at 100 s of the integration time. The temperature coefficient is reduced from 4.68 to 0.1 mg/℃. The combined approach could be useful for many other closed-loop accelerometers.