1. VEX Motors: Presented by: Mr. Harder & Thuan Doan Robotics Training Week 2010

2. General Topics Torque Speed Example problems Torque curves and specification sheets Gear ratio

3. What is a Motor/Servo? Converts electrical energy into mechanical energy Provides motion for robot Has specifications that must be designed for VEX Motor spins 360 degrees VEX Servo can only turn 180 degrees

4. Torque Force at a distance Rotational strength of a motor Measured in Newton-meters or foot-pounds  Be aware of unit conversions Speed Rotations per minute (RPM)  May be measured as radians/second

5. Power Power = angular velocity * torque P rot =  τ*ω Measures how strong and how fast a motor is spinning  Measured in Watts (radians -Newton- meters/second) This relationship shows that speed and torque are inversely related  Less torque = more speed  More torque = less speed

6. Gears, Chains & Sprockets Gearing can be used to change the output torque/speed Sprockets and chains can be used for same purpose  Slightly less efficiency loss See drive presentation for details

7. Power Loss in the Mechanism Some power from the motor is lost due to friction in the mechanism Gears, belts, cables Bearings, guides Tires, balls, or other deformable item Power loss is heat Heat and friction are usually not your friends

8. transmission efficiency is important Spur gears: 90% per pair Worm and gear: 10%-60% Chain: 85%-95% Rack and pinion 50%-80%

9. Motor Parameters Applied voltage ( V ) Voltage of your power source (7.5V for VEX) Stall torque (  stall ) Torque at zero speed Highest torque that motor can output Stall current ( i stall ) Amount of current drawn by motor when stalling Free speed (  free ) Speed of a motor under no load Fastest speed of a motor

10.  stall = 6.5 in-lbs i stall = 1.6 A  free = 100 rotations/minute VEX Motor Specifications Stall torque Stall current Free speed Reference voltage V = 7.5 Volts

11. Current vs. Torque Higher torque = higher current More torque you put on your motor, the more current it draws -> Stall current is reached at stall torque (remember?)

12. Speed vs. Torque Inversely related Review: (important concept) Stall torque -> speed = 0 Stall speed -> torque = 0 More torque -> Less speed

13. Max Power Max power = max output of the motor This is the power you should design for Torque at max power is half of stall torque Speed at max power is half of free speed

14. Peak Efficiency  Peak efficiency is about at about 1/3 – 1/6 of stall torque  Occurs at about 60% of Max Power  After max efficiency -> efficiency crashes

15. After this exhausting presentation … Definitions of all terms discussed How to calculate power, max power, torque, and speed (including unit conversions) Relationship between torque and speed Relationship between torque and current What causes power loss in a system Basic understanding of gear ratios