Accelerometer : only acceleration, or constant speed question...

Steel

I am working on a new product design, and need to determine if·an accelerometer isthe right part to put into it.

I need to measure speed.· I know that accelerometers measure acceleration force, but do they also measure constant force?

As an example...If I have an accelerometer on a car, and accelerate the car from 0mph to 60mph, and then drive 60mph for 30 minutes, does the accelerometer read force during the 0 to 60, and then settle back to "0" (as the 'acceleration' is no longer present), or does it continue showing the force during the maintained speed for 30 minutes?

If anybody could help me, I'd really appreciate it.· I need to create a speed sensor for acceration and constant speeds.

Shaun

Mike Green

An accelerometer measures acceleration. That's it!

It doesn't measure speed. It doesn't measure force. It doesn't measure distance.

If you want to measure speed, you need a speedometer.

In most cars, this is done by knowing the circumference of a tire and displaying the number of revolutions per hour times the circumference of the tire in miles or kilometers. The calculation is either done by the car's computer or mechanically in the speedometer gauge.

If you want specific suggestions, you'll have to provide more information.

Craig Eid

Mike is correct that you can not directly measure speed with an accelerometer. You can calculate speed by integrating the area under the curve of an acceleration versus time plot but this requires a very accurate measurement of acceleration and time. There are several inexpensive performance meters available in the automotive aftermarket - the GTech-Pro (http://www.gtechpro.com/) is an example - that use a high speed accelerometer in their speed and distance calculations. These products use a 32-bit microprocessor, combined with a very sophisticated algorithm, to perform the single and double integration and remove the calibration errors that occur during the integration.

The typical aftermarket data acquisition system calculates speed by accessing the OBD-II data or using a Hall effect sensor and a magnet or another type of proximity sensor to measure the number of revolutions occurring a defined period of time and doing the conversion that Mike described.

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Craig Eid

www.TriadRD.com