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