Actuators - cadara

Actuators



5.1
Mechanical movements


Actuators are an essential part of robotics, they are the
plant that drives
the robots; grating the robot the ability
to control and move its
mechanical parts.


5.2
Types of Actuators



Generally, actuators can be classified into two category of movement;
Linear and Rotary.


5.2.1
Linear Actuators



These actuators generate linear displacements, normally
in sliding
motion.


Fig 5.1 Linear

Actuator





Illustrated in Fig

5.1 is
the movement of the linear

actuator


5.2.2
Rotary Actuators


These actuators generate rotary displacements, normally
in spinning
motion.




Fig 5.2 Rotary Actuator










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5.2.3
Pneumatic &

Hydraulic Actuators



These are fluid powered actuators that are commonly

used in industrial robotic applications to handle varies

tasks from material handling to precision product

manufacturing.

This section will provide an introduction of the actuators'

working principles, highlight some important areas of

their application and do a comparison between the

actuators.


5.2.4
Pneumatic Actuators



Pneumatic Actuators uses compressed air to create movement.

Compressed air stored in storage cylinders or air
compressors are
pumped into the Pneumatic Actuator thus creating movements.

The working property of the linear p
neumatic actuator is

illustrated in
Fig 5.3 Pneumatic Actuator (Extended)
Fig

5.3

and Fig

5.4







Fig 5.3 Pneumatic Actuator


(Extended)






1. When compressed air is enters the pneumatic actuator
from valve A.

2. It pushes the piston and hence extending the piston

rod.

3. Air at valve B is vented into the atmosphere.








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Fig 5.4 Pneumatic Actuator


(Retracted)







1. When compressed air enters the pneumatic actuator
from valve B.

2. It retracts the piston rod.

3. Air at valve A is vented into the atmosphere.


These actuators are used for varies application such as

•

Pneumatic drills, as they are lighter, faster, and

simpler than an electric drill of the same power rating.

•

Replacement of electric actuators where electric sparks

(mines) and EMI (MRI scanners) can a safety hazards.






Fig 5.5
Dentist Drill















Fig 5.6 Jackhammer









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Shown above is a dentist drill employing a rotary
pneumatic

actuator and in Fig 5.6 is a jackhammer with a linear pneumatic actuator.


5.2.5
Hydraulic Actuators


Hydraulic actuators operates with incompressible liquid
such as oil and
water, they are normally used when large
amount of force is required in
operation.

The most common hydraulic actuators design is the piston type (Linear)
actuators.

The working property of a typical piston type hydraulic
actuator is
illustrated in Fig 5.7.





Fig 5.7 Hydraulic

Actuator







1.

Initially, when there is no hydraulic fluid pressure,
the spring holds
the piston fully extended

2.
As fluid enters the actuator, pressure in the
actuator increases.
When the hydraulic force is greater
than the spring force, the piston
retracts

3.

When the fluid is drawn out of the actuator,
hydraulic force
release and hence the piston is extended
by the spring.

4.

The fluid drawn from the actuator is returned back

to the hydraulic fluid reser
voir.

Hydraulic actuators are commonly used on heavy
machinery like
Airplanes, Space Shuttles, Cranes,
Bulldozers, Forklift, Vehicle jacks
etc.








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Fig 5.8 Vehicle jack









Fig 5.9 Plane landing gear








Shown in above is a hydraulic vehicle jack that is capable
of lifting up to
10 tons of load and in Fig 5.9 is a hydraulic landing gear of an airplane.


5.2.6
Electrical Actuators



These electromagnetic driven actuators are the most
commonly used
actuators used in robotics application.
Their application ranges from
industrial robotics all the
way to hobbyist robotics.

The advantage of using electrical actuators over fluid powered one are

• ease of interfacing to electronic circuitry (not signal


conversion is required)

• available in very small form factors

• electricity drive both the control circuitry and


actuators (no extra power source needed)

• easily and che
aply available

• The working mechanism of the electrical actuators


will be covered in the next section when


electromagnetism is introduced.






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5.2.7
Guideline to
Actuator selection



Here are some brief guidelines to the selection of
actuators.

• Amount of power required?

• Difference actuators type delivers difference among


of power

• Ease of power generation and driving actuators?

• Hydraulic actuator requires fluid tank

• Pneumatic actuator requires air compressor and


high pressure storage tanks

• Electrical actuator requires batteries/electricity


supply

• Size constraint?

• Is there a limit to the same

of the actuator?

• Availability?

• How easy is it to obtain the actuator?

• Price?

• How much does the actuator cost?

5.3
Electromagnetism

Electromagnetism is simply the physics of the
electromagnetic
field which is produced due to a changing electric field traveling in a
conductor.









Fig 5.10 Right Hand Rule







Illustrated in Fig

5.10

is the relationship between electric

current and magnetic field following the Right hand
rule.


These fields can be converted into forces that can drive
actuators.






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The reverse is also true when a changing magnetic field

cuttings across a conductor it actually generates

electricity!!!


5.3.1
Electromagnet


The simplest form of electromagnetism at work is the
electromagnet.
It is simply made up of wire coils, and
when a changing electric current
(AC) is passed through
it, it turn into a magnet.



Fig 5.11

Electromagnetic Coil







Shown in Fig 5.11 is a plain electromagnetic coil.

To produce a much stronger magnetic force, a ferromagnetic
material (i.e. soft iron) can be use to as a
core which can concentrates the
magnetic field that is
stronger than that of the coil itself.








Fig 5.12 Simple

Electromagnet









Shown in Fig 5.12 is a simple electromagnet that can be

easily constructed with a battery, long wire and an iron

nail.




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5.3.2
Applications of Electromagnets



They are widely used in common household, the tiny ones
come in the
form read/write head in the computer hard
disk, cassette tape recorder,
VCR etc. and earphones, speakers in entertainment systems.







Fig 5.13

Speaker Cross
-

Sectional View







Illustrated in Fig

5.13 is the cross
-
sectional view of a
speaker, the
electromagnetic coil can be clearly seen at
the back of the speaker.






Fig 5.14 Magnetic

Tape Head







Illustrated in Fig 5.14 is a simplified magnetic tape head
commonly found
in magnetic tape recorders.

The larger ones can be found in heavy industry, such as

metal junk yards where cranes fixed with huge

electromagnets are use to transport scraped metal. High
-



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speed bullet trains also made use of electromagnetism to
levitate on the
tracks to overcome the speed barrel due to fractions on the track.




Fig 5.15 Shinkansen


Bullet Trains








Shown in Fig 5.15 is

the bullet trains that run on Japan’s
Shinkansen
route.





Fig 5.16 Crane

Mounted With

Electromagnet





Shown in Fig 5.16 is a crane used in the metal junk yard, mounted with a
gigantic electromagnet that pick up large
pieces of scraped metal.


5.3.3 Electromagnetic Actuators


After discovering the basic working principal of

electromagnetism, let’s look at how it can be applied to

actuators.

There are mainly 2 types of Electromagnetic Actuators;
Solenoids
(Linear) and Electrical Motors (Rotary)








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5.4
Solenoids


Solenoids produce linear motion when electricity is
applied to its
coil the ferromagnetic core is pull or push
depending on the direction of
the current flow, this is
illustrated in Fig 5.17








Fig 5.17 Solenoid















5.5
Electrical Motors



DC motors are the most commonly used electrical motors
used in robots,
various type of DC motors may difference
in constructions but the basic
working principal behind
them are generally the same.

Similar to the solenoid electric current is applied to the
coils but instance
of interacting with a ferromagnetic core, DC motors interact with magnets.




5.6
Brushed Direct Current (DC) Motor



Brushed Direct Current (DC) Motor are rotary motor that

turns upon applying a significant voltage. The applied



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voltage determines the rotation speed where higher voltage would
relate to higher rotation speed.


Fig 5.18 DC motor








As the voltage increases, the current also increases

resulting in the motor to heat up. The heat if exceed the

permissible range the motor’s material can tolerant, the

motor becomes overheated and eventually burn out.

As such, all motors typically come with a voltage rating or

an operating voltage range to prevent such overheating.

Voltage rating on a motor indicates the typical permissible

voltage that can be applied that the motor would operates

continuously and n
ormally without overheating.

In addition, the typical speed in which the motor can
produce with
the voltage is also given. The speed
indicates the number of turns
the motor
(revolution) would rotate per minute (rpm) without load.

The working property of a typical brushed DC motor is
illustrated in Fig
5.19



Fig 5.19 How Brushed Dc


Motor Works







1. A simple rotary
-
brushed DC motor consists of a coil

as the rotor
3

and the permanent magnet (N
-

S) as the

stator
4
.


3

“Rotor”, Rotor,
http://en.wikipedia.org/wiki/Rotor_%28electric%29



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2. An electric direct current passes through and energized the
coil (rotor). The coil is connected to the via
contact points commonly
known as brushes.

3. As the current flows through the wires, magnetic
fields are
produced by these

current
-
carrying wires using
principle of the right
-
hand
rule.

4. The interacting alignment configuration of the magnetic fields
created by the wires and permanent
magnet, result in an upward
force on the positive pole
side and downwa
rd force on negative pole
side. These
opposing forces created within result in a clockwise
motion of the rotor as shown in fig 5.19

5. As the rotor rotates, the direction of the current
through the
coil is reversed; the magnetic field
subsequently produced by coil is
also reversed resulting in a repeat of the sequences.


Brushed DC motor has it advantages as it is commercially
and easily
available with various voltages

and speed
rating and typically less
costly.


However, brushed DC motor are approximately 70
-
80%

efficient only and are subjected to the problem of contact

irregularities of the brushes at high speed. As such,

brushed motor have typically lower maximum speed limit.

Friction produced by the brushes also constitute to the

problem of wear and tear which need replacement and

maintenance. In addition, DC motor is also subject to

electrical noise.


Overall, brus
hed motor are still widely use in robotics due
to its easy of
use and cost.



5.7
Brushless Direct Current (DC) Motor



A brushless DC motor electric motor is an actuator similar
to that of a
brushed DC motor however with a totally
different physical
configuration.






4

“Stator”, Stator,
http://en.wikipedia.org/wiki/Stator



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Fig 5.20 A Typical Brushless Dc


Motor








In a brushless DC motor, the coils are the stators and the

permanent magnet is the rotor. In such a configuration,

there is not a need to transfer a direct current to a

rotating armature and hence there are no brushes in a

brus
hless motor.






Fig 5.21 Configuration Of A


Brushless Dc Motor








Fig 5.21 show a typical conventional configuration of a

brushless DC motor where 3 coils or stators surround the

rotor. By use of an electronic controller or logic circuits, a

rotating magnetic fields created by the 3 coils such that

the rotor can be directed in a particular direction. To

facilitate the directing, information on the rotor position is

required and this is commonly achieved using Hall Effect
5

sensors
or rotary encoders
6
.






5

“Hall Effect”, Hall Effect,
http://hyperphysics.phy
-
astr.gsu.edu/hbase/magnetic/hall.html



6

“Rotary Encoders”, Rotary Encoders,
http://en.wikipedia.org/wiki/Rotary_encoder




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Brushless DC motor has it advantages in term of its
efficiency,
reliability, reduced noise and longer lifetime (no brush degradation).

However, brushless DC motors typically more costly and
are more
complex in its implementation. Additional logical
circuits and control
are required for ge
nerating the
rotating magnetic field.

Overall, brushless DC motors provide high performance
but are generally
more costly and complex to implement
which deter many robotics users.



5.8
Stepper Motor



A stepper motor is a brushless electric motor that can

rotate precisely to a particular angle in which the full

rotation is divided into a number of steps. T
he angle

resolution is depende
nt on the number of steps. Typically,

since stepp
er motors are used for positioning purpose,

stepper motors are rated using torque
7

(holding force)

and voltage.


Diff from the DC motor, stepper motor does no spin
continuously
when potential is applied to the motor.
Instead, stepper motor is used
to precisely hold the rotor
at a particular direction.







Fig 5.22 A Typical Stepper Motor













7

“Torque”, Torque,
http://en.wikipedia.org/wiki/Torque



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In a stepper motor, the rotor is a gear with magnetic
teeth as shown in
Fig5.23




Fig 5.23 A Stepper Motor


Configuration








A simple explanation of how a stepper motor works is as

follows

1. Each coil is magnetized one at a time attracting the
nearest few
magnetic teeth to it.

2. The next coil to be magnetized is mounted at an
offset such that
when it is magnetized, the gear rotates
slightly to align itself.

3. When one coi
l turns off and the other on, the gear
rotates towards the
required direction.

4. Using the same principle, the rotor is hold by aligning
particular teeth
of the gear to the coils

Stepper motor in other words can be deemed as a
brushless DC
mo
tor coupled with a controller for position control. As such, stepper
motor exhibit most of the
advantages to that of the brushless DC
motor and can
provide precise position control.

Its main disadvantages come as stepper motors are
subjecte
d to
slippages and are typically less power
efficient and more bulky. Cost
of stepper motors relate to
the precision of position control as the more
precise the
control, the more costly the motor is.



5.9
RC Servo Motor



RC Servo

motors are DC motors coupled with logical

controllers that provide velocity or position control

through the use of feedback information. Typically, RC

servo can provide only a limited degree of rotation


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control. (140
-

270 degrees). Similar to stepper motor,

RC servo are rated accordingly to their torque and

voltage.

By use of Pulse
-
Width Modulation
8

(PWM), the position of

the rotor is set accordingly. In recent years, new

generation servos uses serial data or daisy chain
9

method

as it reduces the need for multiple connecting wires.










Fig 5.24 Parts Of The RC

Servo












A typical RC servo consists of the following parts; DC

motor
(1), potentiometer(2), reduction gears(3)
,

actuator arm(4) and a digital controller as shown in

Figure 5.24


Fig

5.25 Circuitry Of

A Typical RC

Servo








8

“Pulse Width Modulation”, PWM,
http://en.wikipedia.org/wiki/

Pulse
-
width_modulation


9

“Daisy Chain”, Rotary Encoders,

http://searchnetwo
rking.techtarget.com/sDefinition/0
,,sid7_gci1115470,00.html



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RC servo works as follows:


A PWM signal is send to the digital controller.

Based on the PWM and reading from the

potentiometer (indicates position of actuator arm),

the appropriate drive required is sent to the DC

motor.

The DC motor rotates the actuator arms through

the reduction gear to provide more torque

(reduction gear and torque would be discussed in

the section 3.3).

The potentiometer updates the digital controller on
the new
position of the actuator arm.

Visit the following URL to find out more detail

information in the use and controlling of servo

motors.

RC servo has it advantage over stepper motors in position
precision control
in terms of torque and response time. In addition, it is easy to implement.

However new generation servos used

in robotics are
better
performance and more information feedback but
are subjected to higher
cost and more complexity.

5.10
Application of Electrical Rotary Motor in Robotics


Electrical rotary motor are vastly employed as actuators
in robotics in

various aspects. Below are examples of these rotary motor discussed
used in robots.


Brushed DC Motors
















Fig 5.26 Biomorph (Left) And Soccer Robot (Right) Employ The


Use Of Brushed Dc Motors


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Brushless DC Motor



Fig 5.27 Omni Directional


Soccer Robot From

Cornell University Employ

The Use Of Brushless Dc


Motors For Robocup

Compe
t
ition







Stepper Motor





Fig 5.28 The M6 Robot Built


For Locomotion Test On
Unstructured Environment Employs
The Use Of Stepper

Motors In Each Of The


Wheels



















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RC Servo







Fig 5.29 the humanoid robot

manus
-
i uses rc

servos as

actuator for each of its joints.












5.11
Guideline to Electrical Motor selection



Here are some brief guidelines to the selection of electrical rotary
motors.




Type of electrical motor required?



For rotation drive
-

brushed/ brushless DC motor,
stepper motor
or hacked
10

RC servos



For position control
-

stepper motor or RC servos



Speed (RPM) and voltage rating required?



Typically, higher speed motors required higher voltage rating.



Other considerations?

Cost



Reliability

Efficiency



Noise Immunity

Lifetime



Implementation
Availability






10

“Hacked RC Servo”, Servos that are hacked/ modified to provide full rotation control.



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5.12
Gears


A gear is a wheel with teeth around its circumference, the

purpose of the teeth being to mesh with similar teeth on

another mechanical device
--

possibly another gear wheel

--

so that force can be transmitted between the

two devices in a
direction tangential to their surfaces
11
.






Fig 5.30 Gear








Typically, gears are employed to increase or decrease

torque and speed. Speed and torque are inversely related

as increase

in speed would decrease torque and vice
-

versa. As such, gear is a very useful property especially

in robotics where mechanical advantage is needed.

There are many type of gears used for mechanical
advantages.
Different type of
gear provides different efficiency, stepping
up/down and also translates into
different mechanical direction.
















11

“Gear”, Definition adopted from
http://en.wikipedia.org



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5.12.1
Spur Gears

(~90% efficiency)






Fig 5.31 Spur Gears








Spur gears are the most common type of gears. They
have straight teeth,
and are mounted on parallel shafts. Sometimes, many spur gears are used
at once to create
very large gear reductions
12
.


5.12.2

Helical Gears

(~80% efficiency)




Fig 5.32 Helical Gears





Helical gears are an improvised version of the spur gears

where the edges of the teeth are not parallel to the axis

of rotation but set at an angle. In this configuration, the

teeth engage more often in compare to the spur gear

resulting in a smoother
and quieter run. Helical gear also

has it advantage to provide cross coupling which changes

the mechanical axis of rotation as shown in Figure 5.32







12

“Spur Gear”, Definition adopted from
http://auto.howstuffworks.com/gear2.htm




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5.12.3)
Bevel Gears

(~70% efficiency)












Fig 5.33 Bevel Gears






Bevel gears are another form of gear which is used to

provide the same cross coupling discussed earlier only.

Bevel gear changes the angle of operation and can be

designed to operate at different cross coupling angle.


5.12.4

Worm gears

(70% efficiency)





Fig 5.34 Worm Gears







A worm gear is another type of helical gear that looks like

a screw which can be coupled with a spur gear. Similarly,

this coupling provides the change in operating angle of

the rotation axis. The main feature of such a gear is that

it is not back
-
drivable. Back drivable implies that worm

(screw
-
looking) can dr
ive the gear but not necessary the

vice
-
versa direction. This special property makes it ideal



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for

driving large load without the need of holding torque.
However, it
efficiency is a disadvantage in compare to
other type of gears.




5.12.5




Rack and Pinion Gears
(~90% efficiency)




Fig 5.35 Rack And Pinion


Gears









A rack and pinion gear translates rotational motion into
translation
motion or vice
-
versa. This type of gear is typically employed in
steering in automobiles.



5.12.6

Gear Ratios

When two gears of different number of teeth are coupled,
the speed and
torque that the gears provide change.

When a gear with fewer teeth (pinion) drives another with
more teeth
(wheel), the speed is reduce. Speed is reduced as when the pinion
completes 1 revolution, the
wheel does not due to the teeth difference.

Similarly, in the same case, since there is a speed

reduction, there is an increase in torque as the same

amount of force in the pinion moves a much lesser angle

of rotation in the wheel. In othe
r word, the wheel move

with more torque.

As such, a unit of indication is used to calculate the change and this
unit is known as the gear ratio.





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Gear ratio is a unit to indicate the resultant speed and
torque when gears
are employed and usually, the number in the front is the gear where the
power is applied.

For an instance, when a gear is determined to be 3:1, this
indicates that the
number of te
eth of the gear in which
the power is applied to is three time
the number of teeth
of the resultant gear.



FIG 5.36 GEARS RATIO

10 Teeth Power applied to


this gear







To calculate the resultant speed and torque, the following
formulas can be
used.

Gear ratio given A:B

Resultant Speed

= Applied Speed * A/B
Resultant Torque

= Applied Torque * B/A



5.12.7

Type of Coupling


Type of coupling indicates the type of method used in to
provide the
gearing ratio. There are three methods:


Gear to gear, Belt driven, Rotary to Linear


5.12.8

Gear to gear



Fig 5.37

Gears To


Gear

Coupling





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Gear to

gear is the most common type of coupling

employed due to its simplicity. In areas of speed

reduction and torque increase, gear to gear is often

employed. A point to note is that gear
-
to
-
gear gives a

counter
-
rotation when even number of gears is employed.


5.12.9

Belt Driven



Fig 5 .38 Belt Driven


Coupling




Belt driven coupling uses a belt to drive the gears. In

instances, this belt can be seen as a chain that hooks the

two gears up. In
such a configuration, the rotation

direction is the same. An advantage of such a system is

that the gears can be positioned apart with the need of

more mechanism.


5.12.10

Rotary to Linear


Rotary to linear is one of the most common methods used
when rotary
actuators are used to provide linear motion.
Rack and pinion and worm
gear are typically employed to
provide such conversion.





Fig 5.39 Rotary To Linear


















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5.12.11

Guideline to Gear Selection

Here are some brief guidelines to the selection of gear.





Type of gear?



Cross coupling or non
-
cross coupling required Efficiency Size
and space



The required gear ratio?



Using the formula determine the required gear
ratio to obtained
the necessary speed and torque Multiple gears can be employed



Speed and torque are trade
-
off of each other. High
speed
-
low torque
and vice versa



Type of coupling?



Gear to gear
-

Suitable for compact gear box



Higher efficiency and less prone to slippage



Belt
-
driven
-

Suitable for long distance coupling



Subject to

slippage depending on the belt used



Rotary to linear
-

Suitable for steering and rotation
to linear
conversion



Material of gears Metal
gears



Strong



Need to be accurately coupled to reduce friction Longer lifetime



Less prone to slippage

Heavy



Plastic gears

Weaker



Wore out more easily

Light



More prone to slippage