Calculating speed

From Combat Robot

Calculating the theoretical top speed of a robot is fairly simple, but requires several pieces of information:

• Find the no-load Revolutions Per Minute (RPM) of the motor, either by measurement or from the specifications. Note that no-load speed for a permanent magnet DC motor increases in proportion to the voltage -- the motor will spin about twice as fast at 24 volts compared to 12 volts.

• Determine the gear ratio used in the drive train.

• Determine wheel circumference in feet. The distance the robot will travel with each rotation of the wheel is equal to the circumference of the wheel -- about 3.1416 times the wheel diameter. A wheel 0.5 feet in diameter has a circumference of about 1.62 feet.

• Theoretical Top Speed = (No-load RPM/Gear Reduction Factor) * Wheel Circumference. The calculation gives speed in feet per minute. To convert to miles per hour, divide feet per minute by 88. Example: a motor with a no-load speed of 6000 RPM with a 12:1 gear reduction and a 2.8 foot wheel circumference yields (6000/12)*2.8 = 1400 feet/minute = 1400/88 MPH = 15.9 MPH.

Actual speed will be less than theoretical speed. The load placed on the motor to overcome frictional losses needed to move the robot results in a drop in RPM. Actual top speed for a typical robot with optimum gearing will be about 80% of the theoretical top speed.

The Team Tentacle Torque Calculator offers an automated calculation of the effects of gear ratios on speed, battery selection, and amperage draw for specific motors. Note that this calculator makes assumptions on robot design (E.G. all-wheel drive is assumed).