how to measure RPM and SPEED at the same time

[teeeeee]

i need to built meter that can display RPM and SPEED at the same time by use PIC16f877a. Now i can measure RPM on T0CKI by use timer1 and interrupt. I don't know how to measure SPEED at the same time. Please advice me.

[HenrikOlsson]

What do you mean by SPEED? Speed of what?
If, for example, you're measuring the RPM of a roller on a conveyor then speed of the conveyor belt is a function of RPM and the circumference of that roller, so by measuring the RPM you are already measuring speed. But I suspect you're doing something else and I think you need to elaborate a bit on what exactly that is and what you're trying to achieve.

/Henrik.

[comwarrior]

as henrik said above...
RPM is Rotations Per Min...
If you mean speed as in distanced travelled per second then you take the 'wheel' radius (half it's diameter) and times it by PI (3.14 2dp) which gives you the outer circumference. Then times the outer circumference by the RPM divided by 60 (watch the decimal places) and you have distance per second...
If you measured the radius / diameter in inches then the output will be inches per second (or can be converted to feet per second).
If you measured the radius / diameter in CM then the output will be CM per second (or can be converted to meters per second).

[HenrikOlsson]

Actually, the circumference is diameter * PI.

/Henrik.