tachometer accuracy
tachometer accuracy
(OP)
How accurate can an tachometer be? in %
In addition, I need to measure the linear velocity of a linear movement. The maximum velocity is 0.1 m/s.
What is the best way to get a accurate measurement of the linear velocity as a function of time? (and not too expensive).
Is it possible to get an accuracy within 2%?
Thanks.
In addition, I need to measure the linear velocity of a linear movement. The maximum velocity is 0.1 m/s.
What is the best way to get a accurate measurement of the linear velocity as a function of time? (and not too expensive).
Is it possible to get an accuracy within 2%?
Thanks.





RE: tachometer accuracy
1a. You also need to decide what sampling interval, or resolution in time, is appropriate for you. E.g., you can count revolutions for a minute, and get an accurate number, but it won't tell you the instantaneous angular velocity of the crank between firing events.
2. You can measure displacement and time with extreme accuracy these days. So if you could measure displacement every second, or the time it takes for your subject to travel 0.1m, you could have excellent accuracy.
2b. Same as 1a. If you want instantaneous velocity, it costs more.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: tachometer accuracy
RE: tachometer accuracy
There is no problem with "width of the needle" and the "acuity of my eye" because the result it not read by a human but rather a computer. A computer uses this information to control a system.
I realize there are issues with sampling intervals, cost. ect...
But I just want to know if I could get a tachometer so that the measured rpm at any instant is no more than 2% off than the real actual rpm, reasonably cheap, so that it could be used on a car.
RE: tachometer accuracy
If you try a bit harder 1Rpm resolution and accuracy in say 9,000Rpm would be feasible. Digital electronics and microprocessors are wonderful things for counting, and if you add or divide two numbers, or count events, the accuracy will be absolute. Digital readouts are very good too for exactly the same reasons.
The only possible source of error or uncertainty is the time measurement. Even the cheapest quartz clocks and watches are accurate within a few seconds per day these days. Percentage time error in that case being quite low.
You can make it as accurate as you want. Revs per minute is easy. How about revs per hour or revs per month ?
RE: tachometer accuracy
RE: tachometer accuracy
The trick is to use a microcontroller with the input capture feature. It logs the arrival time of each incoming pulse edge in hardware. The processor can then read this sixteen bit counter at any time afterwards, (provided it is read before the next pulse arrives).
From there you can log the instantaneous speed, average, calculate acceleration, or do whatever else you want.
RE: tachometer accuracy
When you decide how to decide what the 'real' rpm is, yes, you can measure it very accurately for not a lot of money. You just have to decide what you want.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
RE: tachometer accuracy
In fact, I use the (113) teeth of large Diesel truck engines to measure not only RPM, but also flywheel angular acceleration as a measure of the impulse that produces driveline torsional vibrations.
And of course, position, speed, and acceleration can be measured measured by radar.
RE: tachometer accuracy
You should also bear in mind that your 'accuracy' is limited by the laws of signal processing - specifically the shorter your sample time, the less accurate your resolution.
max Resolution = 1/sample time, so if you sample for 1 second the best you can manage is to within 60 rpm, anything less has no meaning.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: tachometer accuracy
The problem is always in picking up a clean signal from the rotating part to begin with. Optical and magnetic pickups can have a lot of jitter for all sorts of reasons, and that is what will limit you in the end.
Interesting about the misfires Greg. What does the ECU do about a detected misfire ?
RE: tachometer accuracy
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.