First off is a design for an accelerometer based tachometer. Basically, the prototype will have to measure accelerations and compute Revolution per Second (RPM). We thought of a couple of ways to do this. First involves measuring the centripetal acceleration. This was actually suggested by a colleague at work. If you remember from physics class, the centripetal acceleration a can be computed given perpendicular velocity and distance of point from the axis of rotation. There are several problems with this method, the limitations of the mems sensor, analog to digital conversion, and varying axis of rotation and these are probably not the least of the problems.
Problems with the first method: if you have already tried calculating for the centripetal acceleration 5 cm from the center of a 600 RPM electric fan, you would have already notice that the G exceeds standard maximum acceleration of commercial accelerometers. Also, even if the sensor is capable of high Gs, converting this to an ADC value will include quantization errors that will dictate your resolution. This is usually in tens of RPM and higher at lower RPM. Also, it should be noted that gravity may make computations a little more difficult depending on the orientation or axis of rotation.
Another thought occurred from looking at vibration measurement devices. Regardless of orientation, it is safe to say that the waveform will have to repeat itself every rotation. Fast Fourier Transforms may calculate for the component frequencies of this waveform. Suffice to say, this involves a little more complicated algorithm and digital signal processing.
Not one of these method had been implemented yet. Probably next week, we can know how FFT method would work. Not that I'm giving up on the first. We'll just have to be more creative.
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