introduction to robot design:
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Introduction to Robot Design:
Gui Cavalcanti5/19/2011
Motors and Actuation
Overview
• A little bit of physics first!• Roles of actuators• Types of actuators• Actuator sizing• Electric motors
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
Force• All motion is generated by forces acting on a
mass
• Newton’s Three Laws of Motion1. Every body remains in a state of constant velocity
unless acted upon by an outside unbalanced force.2. 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;
3. For every action there is an equal and opposite reaction.
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;
Work Example
100 lb
• 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
Work Example
• Moving an already-lifted weight sideways requires no additional physical work. What’s wrong with this statement?
100 lb 100 lb
Power Example
100 lb
• 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
Power Take-Home Message
100 lb
• Any sustained source of force can do almost any task given enough time.
100 lb
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
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?
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
Example Problem• Let’s characterize your knee joint in a squat!
1. Figure out your weight in kilograms2. Measure how long your leg is from your hip to your
knee3. Figure out how fast you can stand up from a squat by
timing it4. Use this data to compute
1. Maximum knee torque in a squat2. 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?
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
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
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
Pneumatic System• Required Pieces:
– Compressor• Automatic Cut-off
– Relief Valve– High-Pressure Storage Tank– Regulator– Valves– Pneumatic Actuators
Pneumatic System
Power Source
Compressor
Relief Valve
Regulator
Storage Tank
Pneumatic Pros and Cons
Pros
• 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
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
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
Hydraulic System• Required Pieces:
– Pumps• Variable displacement• Fixed displacement
– Accumulators (Optional)– Return Fluid Tank (Optional)– Valves– Hydraulic Actuators
Hydraulic Pros and Cons
Pros• 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
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
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
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
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
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
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
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
Electric System• Required Pieces:
– Power Source• Battery• Line Voltage/Inverter• Gas-powered Generator
– Specific Motor Controller– Gearboxes/Gear Reduction– Motors
DC Motor Curves
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