Patent Application
20080238537
In MicroElectro-Mechanical Systems (MEMS) gyros, electrical noise mixed in a non-linear component with the second harmonic of the motor drive signal that gets into the rate channel (herein called “Penguin” noise) is one of the biggest yield problems. The motor drive capacitors act like a summing circuit that sums the two drive signals, allowing them to cancel (almost) completely, leaving the noise superimposed on a DC level at the MID potential. One solution is to sum the same two drive signals external to the sensor by using two 20K resistors and then running the sum through a 1 pF capacitor into the unused input of the charge amp. The signal on the unused input is then subtracted from the signal on the main charge amp input by treating it as a common mode signal. This fix would work perfectly if the two signals were exactly the same size. Unfortunately, this is difficult to arrange because the external components vary in value, and the correct values to begin with are not known. Trimming one of the resistor values to get better cancellation is possible, but is discouraged because this adds to the production cost of the gyro
Therefore, there exists a need for efficiently and effectively removing noise and/or the second harmonic from the rate input of the gyro.
This invention solves the “Penguin” noise problem in tuning fork gyros by placing copies of the motor drive capacitors elsewhere on the sensor die to create a “dummy proof mass.” The capacitor copies are a non-movable but electrical equivalent of the true proof mass. Then, if the stationary capacitors of the “dummy proof mass” are activated using the same drive signals as the main motor drive capacitors and the output of the “dummy proof mass” is run into an unused input of the charge amplifier, a near-perfect replica of the noise signal on the main proof mass is generated. The stationary capacitors of the “dummy proof mass” have the same dimensions as the motor drive capacitors of the true proof mass.
When this invention is used, it is not necessary to use components outside the sensor, (the gyro board), to do the “Penguin” noise cancellation functions. This saves board area and reduces gyro costs. It also allows using driver components that have intrinsic noise on their outputs without screening the devices to eliminate such intrinsic noise.
Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
Each of the motor drive components 32 and 34 receives a square wave signal from a square wave generator 28. Before the second motor drive component 34 receives the signal from generator 28, the signal is inverted by an inverter 30, creating what is referred to as complementary motor drive to the gyro. This insures that the lateral (X-axis) motions of the proof masses 24, 26 resonate equal and opposite (out of phase) of each other. Connected between the drive amplifiers 56 and 58 and their respective motor drive capacitors 40 and 42 are first ends of stationary capacitors 68 and 70, respectively. The value of the stationary capacitors 68 and 70 are preferably equal in capacitance to their corresponding capacitors 40, 42.
The system 20 also includes rate sense electrodes 80, 82 that are located on a substrate of the system 20 (or below the proof masses 24, 26, respectively). The rate sense electrodes 80, 82 are biased to a predefined voltage. The proof masses 24 and 26 are electrically conductive (doped) to form a capacitor with the rate sense electrodes 80, 82. The proof masses 24, 26 are electrically connected to a differential amplifier or subtractor 74, which may alternatively consist of two charge amplifiers followed by a differential amplifier. The subtractor 74 also is connected with the stationary capacitors 68, 70. The subtractor 74 subtracts the combined signal of the stationary capacitors 68, 70 from the combined signal from the proof masses 24, 26. Because the stationary capacitors 68, 70 are comparable in capacitance to their respective motor capacitors 40, 42, they produce a similar noise signal. Therefore, because the noise signal that is experienced by the motor drive capacitors 40, 42 couples into the sense signal (the signal outputted by the proof masses 24, 26), then the subtractor 74 removes the noise signal from the generated sense signal.
While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.
