Re: RPM calculator
 

To calculate the delta time in Java do you use System.currentTimeMillis() before or after getting latest count or is there a better timestamp that is associated with the time the encoder count value was actually taken?

Re: RPM calculator
 

If you are using a 360 count per rev encoder on a wheel spinning at, say, 4000 rpm, and you are sampling at roughly 20ms, then the delta count per sample would be 480. One count out of 480 is 0.2% so you probably don't need to worry about that.

I suppose you could put the code to grab the encoder count and the system timer in a critical section to prevent getting swapped out between the two operations.

Re: RPM calculator
 

I meant to ask if System.currentTimeMillis() is the best time source -or- is there some other timeStamp to use?

Re: RPM calculator
 

Don't know about Java, but in C++ there appears to be a nanosecond timer in WPILib:

Re: RPM calculator
 

Excellent! In the WPILib I just found:

Timer.java

/**
* Return the system clock time in seconds. Return the time from the
* FPGA hardware clock in seconds since the FPGA started.
*
* @return Robot running time in seconds.
*/
public static double getFPGATimestamp() {
return Utility.getFPGATime() / 1000000.0;
}

Utility.java

/**
* Read the microsecond timer from the FPGA.
*
* @return The current time in microseconds according to the FPGA.
*/
public static long getFPGATime() {
return tGlobal.readLocalTime();
}

Thanks!

Re: RPM calculator
 

I generally use System.nanoTime() for anything in Java. It's overkill, but I've never found a reason NOT to use it.
You could measure the dT that it takes to get the latest count and see how negligible it is. (or isn't)