Kevin Watson's encoder code with RPM output

I’m the programmer on Team 473. I’ve been trying to use Kevin Watson’s encoder code to measure the RPMs of our transmission in order to write a shift scheduler. I’ve been trying to do this by getting the number of encoder counts which occur over 25 cycles of Process_Data_From_Master_uP. Since Process_Data_From_Master_uP runs every 26.2 ms, I can convert from rpms into encoder cycles per Process_Data_From_Master_uP runs. However, this is not giving me anything near an accurate RPM output (I have a digital tachometer to check the RPMs with).

Here’s my code from Process_Data_From_Master_uP:

	if (++counter >= (24))
		Left_Encoder_Count = Get_Left_Encoder_Count();
		printf("RPM Left: %5d
",(int)((Left_Encoder_Count - previous_count_left)*0.698666667));
		previous_count_left = Left_Encoder_Count;
		counter = 0;

0.698666667 is the conversion factor counts per 25 Process_Data_From_Master_uP cycle to RPMs. (1 rotation/1 min1 min/60 sec1 sec/1000ms655 ms (2526.2ms)/1 program cycle*65 counts/1 rotation)

Either my conversion factor is totally wrong or my method is totally wrong (or my tachometer is wrong, but since it’s digital I don’t think it is). Can anyone help?


Two things…

  1. The cycle time of 26.2 ms is approximate. To get accurate timing, write a timer interrupt routine.

  2. Integer arithemetic! You are multiplying by 1… Try doing the math outside of the printf. Multiply by 6987 and store into a long int variable. Now divide by 10000 and store in an int variable. Now printf your int.


Or multiply by 45788 (2^16 * 0.6986666…) and then right shift sixteen bits or just grab the high order sixteen bits of the thirty-two bit long. Man, I dig this stuff <grin>.