http://www.techbricks.nl made a NXT tachometer which measures revolutions per minute. I made one version for the RCX Mindstorms legacy rotation sensor and one for the NXT Mindstorms motor (the built-in tachometer). The speed (RPM) is displayed on the NXT screen in large digits. The upper number displays the revolutions per minute over the last second. The lower number displays the rpm over the last minute. It is the average of 60 samples of one second made in the last minute.
This way of measuring is much more accurate then the LEGO speed computer. The legacy RCX rotation sensor measures 16 ticks per rotation. The NXT motor measures 360 degree per rotation. The software is written in NXC. Go to my web site for more info: http://www.techbricks.nl/My-NXT-projects/nxttachometerspeedcomputer.html
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