Design Realization - lecture 20 docx

 Motors come in several flavors:  DC motors  Stepper motors  AC induction motors  AC Single-phase motors  AC Synchronous motors  The first two are highly controllable, and usually

Design Realization

lecture 20

John Canny10/30/03

Last time

 Real-time programming

This time

 Mechanics – Physics and Motors

 I is a 3x3 matrix, not necessarily diagonal

 If T = 0, then I  = -  x I  which is usually non-zero So  is non-zero,  changes with

time, and the object wobbles

 So even though an object wobbles when there is

no external force, the angular momentum is

conserved: q = I 

Physics of rotation

 Kinetic energy of rotation is ½ T I 

 In the absence of external torque, kinetic energy

of rotation is conserved

 But angular momentum conservation does not imply energy conservation

 Work done by a force = F x (Joules) where x is

the distance (m) through which the force acts

 Work done by a torque = T  (Joules)

 Power is rate of doing work

 Power of a force = F v (Watts).

 Power of a torque = T  (Watts).

 Power often expressed in horsepower = 746 Watts

 Motors come in several flavors:

 DC motors

 Stepper motors

 (AC) induction motors

 (AC) Single-phase motors

 (AC) Synchronous motors

 The first two are highly controllable, and usually what you would use in an application But we quickly review the others

 Induction motors are brushless (no contacts

between moving and fixed parts) Hi reliability

 Efficiency high: 50-95 %

DC Brush Motors

 A “commutator” brings current to the moving

element (the rotor)

 As the rotor moves, the polarity changes, which keeps the magnets pulling the right way DEMO

 Highly controllable, most common DC motor

DC Brush Motors

 At fixed load, speed of rotation is proportional to

applied voltage

 Changing polarity reverses rotation

 To first order, torque is proportional to current.

DC Brushless Motors

 Really an AC motor with electronic commutation

 Permanent magnet rotor, stator coils are

controlled by electronic switching DEMO

 Speed can be controlled accurately by the

electronics

 Torque is often constant over the speed range

Stepper Motors

 Sequence of (3 or more) poles is activated in turn, moving the stator in small “steps”

 Very low speed / high angular precision is

possible without reduction gearing by using many rotor teeth

 Can also

“micro-step” by activating

both coils at once

Driving Stepper Motors

 Note: signals to the stepper motor are binary, on-off values (not PWM)

 In principle easy: activate poles as A B C D A…

or A D C B A…Steps are fixed size, so no need

to sense the angle! (open loop control)

Driving Stepper Motors

 But in practice, acceleration and possibly jerk

must be bounded, otherwise motor will not keep

up and will start missing steps (causing position errors)

 i.e driver electronics must simulate inertia of the motor

Stepper Motor example

 From Sherline CNC milling machine:

 Forward or reverse (brushed)

 Many DC motors of all sizes available new and surplus for < $10

DC Motors – micro sizes

DC Motors – gearing

 Gearing allows you to trade off speed vs torque

 An n:1 reduction gearing decreases speed by n, but increases torque by n

 Ratios from 10:1 to many 1000s :1 are available

in compact “gearheads” that attach to motors

DC Motors – gearing

 But gears cost efficiency (20% - 50%)

 Gears decrease precision (due to backlash)

 Reduction gear train is normally not

backdriveable (can’t use for “force control”)

DC torque motors

 Some high-end motors are available for direct

drive servo or force applications (no gears)

 They have low speed (a few rpm), high precision (with servo-ing), and moderate torque

 Typically have large diameter vs length, and

use rare-earth magnetic material

 Cost $100’s (but maybe

less as surplus)

 Shaft encoders can be fitted to almost any DC motor They provide position sensing

 Many motor families offer integrated encoders

 Strain gauges can be used to sense force

directly Or DC brush motor current can be used

to estimate force

Linear movement

 Ball screws: low linear speed, good precision

 Motor drives shaft, stages move (must be

attached to linear bearing to stop from rotating)

Linear movement

 Belt drive: attach moving stage to a toothed belt:

 Used in inkjet printers and some large XY

robots

True Linear movement

 There are some true linear magnetic drives

 BEI-Kimco voice coils:

 AC motors are good for inexpensive high-power applications where fine control isnt needed

 DC motors provide a range of performance:

 DC brush: versatile, “servo” motor, high speed, torque

 DC brushless: speed/toque depend on electronics

 Stepper: simple control signals, variable

speed/accuracy without gearing, lower power

 Direct-drive (torque) motors, expensive, lower torque

 Linear actuation via drives, or voice coils