02-15-2007, 03:34 AM
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Member
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Join Date: Feb 2007
Location: NYC
Posts: 17
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Quote:
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Originally Posted by JCL
Don't mean to give you a lesson in automotive mechanics and engineering, but since you asked for it....If you want to measure the speed of a rotating shaft with an accelerometer you will have to mount the accelerometer on the rotating component. How will you then take the signal off of it? Yes, you can get a wireless accelerometer, but that would be silly. The speed of a rotational shaft will best be measured with an optical or magnetic pickup that counts revolutions (revolutions/time is proportional to velocity). The neat thing about this method is that the sensor is mounted in a fixed position, and wiring isn't complicated by the rotating shaft.
This isn't about physics, or design. It is about automotive diagnosis and repair. The best and most suitable diagnostic aid you have available is a tire pressure gauge.
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What is this have to do with it. In my post I said there are accelerometers in the transmission among other places that always know vehicle velocity, pitch, etc... I never made mention of a rotating shaft. The transmission knows actual vehicle speed at all times. Now when the tires are rotating faster then what is programmed at given velocity due to slightly smaller tire diameter the X-Drive unit goes into default, Meaning all wheel lock. This is a soft Trouble code and does not let itself be known without a detailed scan. No dash warning lights will go off.
The point you were trying to make is that the car has no way of knowing how fast it is going other than wheel speed sensors. Thus when I stated that is has accelerometers to tell is exact speed you stated that accelerometers measure acceleration not speed. Back to my point Velocity (speed) = acceleration/time. Meaning as soon as you start moving the accelerometers report to the ECU and the ECU then computes Vehicle velocity based on acceleration/time formula. I'm not sure how intimately familiar you are to a modern automatic transmission but I can tell you with great degree of certainty that the transmission always knows exact vehicle speed, independent of wheel speed.
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