Term - Wheel Speed
From VEX Wiki
The first concept to understand is figuring out how fast the robot moves across the floor based on how fast the wheel is spinning. For each time the wheel makes a full revolution, it rolls forward a distance equal to its circumference. So if we calculate the circumference of the wheel, we know exactly how far the robot goes per revolution.
The circumference of a wheel is equal to its diameter multiplied by Pi (about 3.14).
Once we know the circumference of the wheel, we can calculate how fast it is rolling based on its rotational speed. In the example below, we assume the wheel diameter is 4 inches, and the wheel is spinning at 50 RPM. Let's calculate how fast the wheel is rolling in inches per second. First, we calculate the circumference:
After we calculate the circumference, we can determine the ground speed based on the wheel RPM. Most of the following is just unit analysis and cancellation:
Now we know the way to calculate ground speed from wheel RPM. If we know the VEX Motor has an output of approximately 100 RPM, and we know what reduction we want our wheel to spin at, we can calculate the approximate gear reduction needed.
Let’s say we have a 5 inch diameter wheel, we want the robot to travel at about 4 feet per second, and we know the VEX motor spins at about 100 RPM. The first step is to calculate the diameter of the wheel, and also convert our target speed to inches per second.
So now we know that we need to move 48 inches in one second, and that each revolution is 15.7 inches; knowing this we can calculate how many revolutions per second the wheel needs to turn to achieve 4 feet per second.
Now we know that we need the wheel to spin at 183.42 revolutions per minute, and we know the VEX motor spins at 100 RPM, we can calculate the Gear Ratio needed to achieve our top speed.
