Introduction to Robot Design:

2. Introduction to Robot Design: Motors and Actuation GuiCavalcanti 5/19/2011

3. Overview • A little bit of physics first! • Roles of actuators • Types of actuators • Actuator sizing • Electric motors

4. Physics • All motion requires a force or torque • Work: • Force or torque exerted over a distance • Measured in Joules (J) of energy • Power: • Amount of work done in a given time • Measured in Watts (W) of power

5. Force • All motion is generated by forces acting on a mass • Newton’s Three Laws of Motion • Every body remains in a state of constant velocity unless acted upon by an outside unbalanced force. • A body of mass M subject to a net force F undergoes an acceleration A that has the same direction of the force and a magnitude that is proportional to the force and inversely proportional to the mass; • For every action there is an equal and opposite reaction.

6. Torque • Torque is a force acting rotationally through a radius • Torque is only produced by force perpendicular to the radius of force applied • Angular acceleration is proportional to torque applied and inversely proportional to rotational inertia;

7. Work Example • Lifting a weight involves pushing against gravity over a certain distance. No matter how fast you lift it, you’re expending the same amount of energy to lift it up. 100 lb 100 lb

8. Work Example • Moving an already-lifted weight sideways requires no additional physical work. What’s wrong with this statement? 100 lb 100 lb

9. Power Example • Lifting the block in 1 second takes a certain amount of power. Lifting the same block the same distance in 10 seconds takes 1/10th the power. 100 lb 100 lb

10. Power Take-Home Message • Any sustained source of force can do almost any task given enough time. 100 lb 100 lb

11. Physics • Some Forms of Energy: • Kinetic: • M is mass, V is velocity • Gravitational Potential: • G is gravitational acceleration (9.8 m/s/s), H is height from reference • Linear Spring: • K is spring rate, X is displacement

12. Example Problems • If you drop a 1 kg mass from 1 meter, how fast is it going when it hits the ground? • If you draw a 1 kg mass back 1 meter on a spring with a K value of 1000 N/m, how fast is it going when the spring is fully restored to its normal length?

13. Example Problems • If you drop a 1 kg mass from 1 meter, how fast is it going when it hits the ground? • KE = PE • 4.43 m/s • If you draw a 1 kg mass back 1 meter on a spring with a K value of 1000 N/m, how fast is it going when the spring is fully restored to its normal length? • KE = SE • 31.6 m/s

14. Example Problem • Let’s characterize your knee joint in a squat! • Figure out your weight in kilograms • Measure how long your leg is from your hip to your knee • Figure out how fast you can stand up from a squat by timing it • Use this data to compute • Maximum knee torque in a squat • Average rotational velocity during standing • Extension: What do you think would make good ‘envelope’ values if you had to replace your muscles with an actuator? Why?

15. Roles of Actuators • Actuator: • A mechanical system that combines a source of motion, a power transmission system and a feedback device to create desired, controlled motions

16. Types of Actuators • Pneumatic • Use pneumatic (air) pressure to generate motion in (generally) a linear fashion • Hydraulic • Use hydraulic pressure to generate motion in a (generally) linear fashion • Electric • Use electromagnetism to generate motion in a rotational or linear fashion

17. Pneumatic Overview • Common Actuator Forms: • Pistons • Vane motors • Power Source: • Compressors • Gas-engine powered • Electric motor powered • Typical Use: • High-force, high-speed equipment • Jackhammers, impact wrenches • Two-position, “Bang-Bang” equipment • Factory Automation

18. Pneumatic System • Required Pieces: • Compressor • Automatic Cut-off • Relief Valve • High-Pressure Storage Tank • Regulator • Valves • Pneumatic Actuators

19. Pneumatic System Compressor Power Source Regulator Storage Tank Relief Valve

20. Pneumatic Pros and Cons Pros Cons Very difficult to control incremental motion Very power inefficient for mobile systems Compressors are always loud, as a general rule Compressed air tanks can easily become bombs Very few hobby-level resources available • Easy to order custom, cheap actuators • Easy to create a functional system with the right pieces • Can create very high forces and speeds • Fairly inexpensive

21. Hydraulic Overview • Common Actuator Forms: • Pistons • Vane Motors/Pumps • Piston Motors/Pumps • Gear Motors/Pumps • Power Sources • Pumps • Gas-engine powered • Electric motor powered • Common Uses • High-force, low-speed equipment • Bobcats, Earthmovers, Diggers

22. Hydraulic System • Required Pieces: • Pumps • Variable displacement • Fixed displacement • Accumulators (Optional) • Return Fluid Tank (Optional) • Valves • Hydraulic Actuators

23. Hydraulic Pros and Cons Pros Cons EXPENSIVE Difficult to assemble, bleed, and work with Very power inefficient Very dangerous to work around leaking hydraulics Almost no hobby-level resources available • Easy to order custom actuators • Incredibly high force density • Easy to create a functional system with the right pieces • Can create very high forces and speeds

24. Electric Motor Overview • Common Actuator Forms: • Straight rotational motor • AC • DC • Gearmotor • Motor + Gearbox • Servomotor • Motor + Gearbox + Feedback device • Linear motor • “Unrolled” linear motor • Linear actuator • Power Source: • Batteries • AC Line Voltage • Alternators on Engines

25. AC Motors • Design: • Stator windings are fed alternating current • Iron rotor “squirrel cage” has electric fields induced into it • Constantly lags slightly behind the changing field, causing torque • Features: • Tend to have one fixed speed • Generally 3600, 1800, 1200, or 900 rpm • Asking for too much torque at speed causes motors to stall, not slow down

26. DC Brushed Motors • Design: • Many different magnetic coils exist on the rotor, get independently energized by brushes touching a commutator • Energized coils are attracted to nearest magnet • As motor turns, brushes suddenly touch a different set of coils • Features • Most common type of motor. Can be found everywhere, in everything • Incredibly easy to use and design around • Incredibly inexpensive • Two wires

27. Hobby Servos • Design: • Small brushed or brushless motor attached to a 150:1 to 200:1 gear train • Output is on a potentiometer or encoder • Signal sent to hobby servo is a position command • Motor controller inside servo reads feedback device and positions motor appropriately • Features: • Out of the box position control • Motors for every budget • Incredible ease of control • Wide range of hobby accessories and development

28. DC Brushless Motors • Design: • Many magnetic coils exist on the stator, while the rotor is made of individual magnets • Stator can be inside or outside the rotor • Electricity is routed to the stator in a well-controlled pattern to create motion • As motor turns, sensors detect position of motor and feed it back to the motor controller • Features • Highest power density of any electric motor • Fastest and longest-lived type of electric motor • Three wires

29. Stepper Motors • Design: • Four coils get individually energized in the stator and attract an iron gear-shaped rotor to line up as closely as possible • Coils are actively switched by controller • Can be used with or without sensors • Features • Easiest motor to command position control with – can rely on counting ‘steps’ to figure out where motor is if unloaded • Second-most common type of motor, found in office appliances everywhere

30. Electric System • Required Pieces: • Power Source • Battery • Line Voltage/Inverter • Gas-powered Generator • Specific Motor Controller • Gearboxes/Gear Reduction • Motors

31. DC Motor Curves