Nippon Pulse Basic Mechanics Presentation

Basic Mechanics
– Physics and Motors

Sold & Serviced By:

ELECTROMATE
Toll Free Phone (877) SERVO98
Toll Free Fax (877) SERV099
www.electromate.com
sales@electromate.com

Motion Control Requirements
Motion Control Applications must be:
1. Predictable
2. Verifiable
3. Controllable

Design Considerations:
• Accuracy
– Stiffness
– Environment Sold & Serviced By:

ELECTROMATE
www.electromate.com

Design Considerations

• Accuracy

Sold & Serviced By:

ELECTROMATE
www.electromate.com

Accuracy
Accuracy vs. Precision
• Accuracy doesn’t mean Precision

• Accuracy represents the ability to get
closest to a defined point.

• Precision is the ability to execute a
move and return to the same point
(regardless of accuracy). Sold & Serviced By:

ELECTROMATE
www.electromate.com

4

Accuracy
Accuracy vs. Precision

Sold & Serviced By:

ELECTROMATE
www.electromate.com

5

Accuracy
Resolution
While Resolution is not Accuracy…

It is part of what makes up
Accuracy

Two parts of Resolution
1. Electrical Resolution
2. Mechanical Resolution Sold & Serviced By:

ELECTROMATE
www.electromate.com


• Accuracy
– Stiffness

Sold & Serviced By:

ELECTROMATE
www.electromate.com

What Exactly is Stiffness?

M
O

OT
RV

OR
SE

STIFFNESS

Sold & Serviced By:

ELECTROMATE

MECHANICAL Toll Free Phone (877) SERVO98
www.electromate.com

Stiffness

M
O

OT
RV

OR
SE

STIFFNESS

Sold & Serviced By:

ELECTROMATE

MECHANICAL Toll Free Phone (877) SERVO98
www.electromate.com

Stiffness
Motor

• Motor Stiffness
– Mechanical stiffness

– Ability to respond to commanded motion

– Flux pattern (Magnetic Servo)

Sold & Serviced By:

ELECTROMATE
www.electromate.com

Stiffness
Mounting / Drive location

Sold & Serviced By:

ELECTROMATE
www.electromate.com


• Accuracy
–Stiffness
–Environment

Sold & Serviced By:

ELECTROMATE
www.electromate.com

Environment
Position Feedback Control Schemes

Sold & Serviced By:

ELECTROMATE
www.electromate.com

Environment
External forces

• Side Loading

• Motor resonance

• System harmonics Sold & Serviced By:

ELECTROMATE
www.electromate.com

Vibration – Nearby Conditions

• Other machinery

• Road traffic

• Building sway
Sold & Serviced By:

ELECTROMATE
www.electromate.com

Review of physics
• Newton’s law for translation:
F=ma
F in Newtons, m in kg, a in m/s2.

• Acceleration a = dv / dt

• Kinetic energy E = ½ m v2
E in Joules, m in kg, v in m/s. Sold & Serviced By:

ELECTROMATE
www.electromate.com

Physics of translation
• Momentum p = m v and so F = dp /
dt

• In the absence of force, momentum is
conserved.

• Momentum conservation implies energy
conservation. Sold & Serviced By:

ELECTROMATE
www.electromate.com

Physics of rotation
• Rotation is more complex; Euler’s equation:
T=Iα + ωxIω
T (torque) in N-m, ω in radians/sec, α in
radians/sec2, I in kg-m2, α = dω / dt

• I is a 3x3 matrix, not necessarily diagonal.

• If T = 0, then I α = - ω x I ω which is
usually non-zero. So α is non-zero, ω changes
with time, and the object wobbles.
Sold & Serviced By:

ELECTROMATE
www.electromate.com

Physics of rotation
• Angular momentum is q = I ω

• The rotation equation simplifies to T = dq / dt
because
dq/dt = I dω/dt + dI/dt ω = I α + ω x I ω

• So even though an object wobbles when there is no
external force, the angular momentum is conserved:
q=Iω

Sold & Serviced By:

ELECTROMATE
www.electromate.com

Physics of rotation
• Kinetic energy of rotation is ½ ωT I ω

• In the absence of external torque,
kinetic energy of rotation is conserved.

• But angular momentum conservation
does not imply energy conservation. Sold & Serviced By:

ELECTROMATE
www.electromate.com

Work
• 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 θ
(Joules) Sold & Serviced By:

ELECTROMATE
www.electromate.com

Power
• Power is rate of doing work.

• Power of a force = F v (Watts).

• Power of a torque = T ω (Watts).

• Power often expressed in horsepower =
746 Watts Sold & Serviced By:

ELECTROMATE
www.electromate.com

Motors
• 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.
Sold & Serviced By:

ELECTROMATE
www.electromate.com

3-phase AC
• Three or four wires that carry the same voltage at 3
equally-spaced phases:

• Single phase AC requires two wires (only 1/3 the
current or power of 3-phase).
Sold & Serviced By:

ELECTROMATE
www.electromate.com

AC induction Motors
• Induction motors – simple, cheap, high-power, high
torque, simplest are 3-phase.

• Speed up to 7200 rpm: speed ~ 7200 / # “poles” of
the motor.

• Induction motors are brushless (no contacts between
moving and fixed parts). Hi reliability.

• Efficiency high: 50-95 %
Sold & Serviced By:

ELECTROMATE
www.electromate.com

Single-phase AC Motors
• Single-phase (induction) motors – operate from
normal AC current (one phase). Household
appliances.

• Single-phase motors use a variety of tricks to start,
then transition to induction motor behavior.

• Efficiency lower: 25-60%

• Often very low starting torque.
Sold & Serviced By:

ELECTROMATE
www.electromate.com

Synchronous AC Motors
• Designed to turn in synchronization
with the AC frequency. E.g.
turntable motors.

• Low to very high power.

• Efficiency ?? Sold & Serviced By:

ELECTROMATE
www.electromate.com

DC Motors
• DC motor types:
– DC Brush motor
– “DC” Brushless motor
– Stepper motor

Sold & Serviced By:

ELECTROMATE
www.electromate.com

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. Sold & Serviced By:

ELECTROMATE
www.electromate.com

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.
• Load curve:
• Motors which
approximate this
ideal well are
called DC servo
motors. Sold & Serviced By:

ELECTROMATE
www.electromate.com

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.

Sold & Serviced By:

ELECTROMATE
www.electromate.com

Stepper Motors
• Sequence of (2 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. Sold & Serviced By:

ELECTROMATE
www.electromate.com

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). Sold & Serviced By:

ELECTROMATE
www.electromate.com

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. Sold & Serviced By:

ELECTROMATE
www.electromate.com

Stepper Motor example
• Step angle: 1.8°
• Voltage: 3.2 V
• Holding torque: 0.97 N-m
• Rotor inertia: 250 g-cm2
• Weight: 1.32 lb (0.6 Kg.)
• Length: 2.13" (54 mm)
• Power output = 3W

• Precision stepper motor: 0.02° /step, 1 rpm,
3W Sold & Serviced By:

ELECTROMATE
www.electromate.com

DC Motor example
• V = 12 volts
• Max Current = 4 A
• Max Power Out = 25 W
• Max efficiency = 74%
• Max speed = 3500 rpm
• Max torque = 1.4 N-m
• Weight = 1.4 lbs
• Forward or reverse (brushed) Sold & Serviced By:

ELECTROMATE
www.electromate.com

DC Motors – micro sizes
• Conventional (brush)
DC motor: 6mm x 15mm
• 13,000 rpm
• 0.11 m Nm
• Power 0.15 W
• V from 1.5 to 4.5 V
Sold & Serviced By:

ELECTROMATE
www.electromate.com

Brushless DC Motors
• Brushless DC motor:
16mm x 28mm
• 65,000 rpm
• 50 m Nm
• Power 11 W
• V = 12 V
Sold & Serviced By:

ELECTROMATE
www.electromate.com

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 3:1 to many 1000s :1 are
available in compact “gearheads” that
attach to motors. Sold & Serviced By:

ELECTROMATE
www.electromate.com

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”). Sold & Serviced By:

ELECTROMATE
www.electromate.com

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
Sold & Serviced By:

ELECTROMATE
www.electromate.com

Feedback
• 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. Sold & Serviced By:

ELECTROMATE
www.electromate.com

Linear movement
• There are several ways to produce linear
movement from rotation:
• Rotary to linear gearing:

Sold & Serviced By:

ELECTROMATE
www.electromate.com

Linear movement
• Ball screws: low linear speed, good
precision
• Motor drives shaft, stages move (must be
attached to linear bearing to stop from
rotating).

Sold & Serviced By:

ELECTROMATE
www.electromate.com

Linear movement
• Belt drive: attach moving stage to a
toothed belt:

• Used in inkjet printers and some large
XY robots.

Sold & Serviced By:

ELECTROMATE
www.electromate.com

True Linear movement
• There are some true linear magnetic drives.
• BEI-Kimco voice coils:
• Up to 1” travel
• 100 lbf
• > 10 g acceleration
• 6 lbs weight
• 500 Hz corner
frequency.

• Used for precision vibration control. Sold & Serviced By:

ELECTROMATE
www.electromate.com

Summary
• AC motors are good for inexpensive high-
power applications where fine control isn’t
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. Sold & Serviced By:

ELECTROMATE
www.electromate.com

Nippon Pulse Basic Mechanics Presentation

Recommended

Recommended

More Related Content

Viewers also liked

Viewers also liked (10)

More from Electromate

More from Electromate (20)

Recently uploaded

Recently uploaded (20)

Nippon Pulse Basic Mechanics Presentation