classification of robots - parkway schools / homepage · force is left after we subtract the weight...
Post on 29-Jul-2018
214 Views
Preview:
TRANSCRIPT
Classification of
Robots
Chapter 4
Overview
• WHAT YOU WILL LEARN– How to classify robots by their power source
– How to classify robots by their work envelope and the kind of reach they have
– How to classify robots by their drive system
– How belt systems work, and the math that goes with them
– How chain systems differ from belt systems
– The different types of gears used in gear drives
– The math that goes with gear systems
– How the ISO classifies robots
How are Robots Classified?
• There are many ways to classify robots
such as:– By power source
– By the shape of the work envelope
– By the size of the robot
– By the weight it can move
– By the type of jobs it is optimized for
– By the type of drive system used to move the
robot
– Or any other method useful for comparison
Power Source
• A common first grouping of robots is by
the power source they use for
movement with the major division being
as follows:– Electrical
– Hydraulic
– Pneumatic
– Nuclear
– Green
Electric
• The two main subdivisions of this
category are AC or DC systems
• DC systems often provide greater
torque, but may require more
maintenance for the motors– Brushed motors generate sparks and dust, both
creating hazards to the process
– DC is often the choice for the hobby robotics world
as many of those systems are mobile, battery
powered robots
These LEGO Mindstorms creations run off battery or DC
power
Electric cont.
• AC is a common choice for industry and
for these systems the servo motor is
often used – Stepper motors are another choice and these
motors move a set portion of the rotation each
time power is applied
– Servomotors have encoders, which are devices
that sent back information about direction of
rotation, speed, and specific position in some
cases
An encoder used to provide
positional information to the robot
A FANUC robot run by servo motors. (the
black parts with red tops)
Hydraulic
• Hydraulic power is known for generating
large amounts of force and is still used
in robotics for heavy loads– With the improvements in servo motors, the
hydraulic robot is losing ground to comparative
electric models
– On a side note, this system uses some other form
of energy to generate the hydraulic pressure, but
the robot will move via hydraulics
Hydraulic cont.
• There are some down sides to hydraulic
robots:– Hydraulic leaks
– Cost of oil
– Fire hazard (mostly as a mist)
– Increased maintenance
– Increased noise
Pneumatic
• The largest problem with pneumatic robots
is the difficulty maintaining position– Because gas is compressible, stopping mid-stroke or
mid motion leads to drifting
– The only sure way to hold position is to use some kind
of hard stop and constant pressure
– Noise and leaks are another problem to contend with
– on the plus side these systems are very fast and most
industries have a ready supply of cheap pneumatic
pressure
Nuclear
• Nuclear powered robots carry their own nuclear reactor, though smaller than those found in nuclear power plants or subs– These robots are typically used by NASA or
similar agencies for deep space exploration
– These systems can run for years or even decades without human interaction, thus making them a perfect fit for space missions
– If used on earth, the nuclear material will need to be disposed of properly once spent
Green
• Green power is a term used to cover a
wide variety of power sources that
share the common characteristic of
power that is easy to replenish with little
or no ecological impact– Solar
– Wind
– Organic sources
– Natural heat sources
Geometry of the Work
Envelope• Another common way of grouping
robots is by the area they can reach or
the geometry of the work envelope– Cartesian
– Cylindrical
– Spherical
– Articulated
– SCARA
– Horizontally Base-Jointed Arm
– Delta
Cartesian Geometry
• These systems have a cubic or
rectangular work envelope– Many gantry-type robots fall into this group
• These robots often have two or three
major axes to move in: – X is front to back
– Y side to side
– Z up or down
– When there are only two major axes, X is often
the one omitted
•
Here you can see a gantry robot showing off the impressive
coverage of the system. To the right you can see an outline
of the different axes of movement
Cylindrical Geometry
• The work envelope of these robots
resembles a cylinder– These robots commonly have a rotary axis on the
base to spin the robot, two linear axes to move
the tooling into the general work area, and then
two or three minor axes for tooling orientation.
– These systems are good for reaching deep into
machines, save on floor space, and tend to have
the rigid structure needed for large payloads
Here is an illustration
showing the various
axes of the cylindrical
robot
Spherical Geometry
• This robots work envelop is a ball, cut
off by where the robot mounts
• Spherical, or polar, geometry, gives the
user a wide range of options for robot
positioning
• The primary difference between
cylindrical and spherical robots is that
the spherical units have a long reach
with a smaller size
Remember with
the spherical robot
you will not have
the full ball, part of
it will be cut off by
what the robot
mounts to and the
surfaces around
that
Articulated Geometry
• Articulated robots have a spherical-type
envelope that is constrained by the
construction of the robot– The articulated robot leaves linear motion behind
for rotational motion at the various axes
– This robot is also known as jointed arm, revolute,
and even anthropomorphic, because in many
cases, its motions look very organic and lifelike
– It has a chunked-up portion of the spherical
envelope due to the robot design and limitations of
the system
To the right is an articulated
geometry robot, a favorite of
industry. Above is an illustration
showing the work envelope of this
type of robot
SCARA
• Selective Compliance Articulated Robot Arm (SCARA) is unique in that it combines Cartesian linear motion with the rotation of an articulated system, creating a new motion type.
• SCARA has a cylindrical geometry with axes 1 and 2 moving in a rotational manner and axis 3 moving in a linear vertical way to manipulate the tooling into position while applying force.
SCARA cont.
• The orientation of axes 1 and 2 provides
horizontal rotation versus the vertical rotation
of the other systems that we have discussed,
in a similar fashion to axis 1 of the articulated
geometry.
• Another difference is that the wrist (or minor
axes) usually only has one, rotational axis.
• SCARA robots are popular in the electronics
field, where their motion and strengths seem
to be a good fit for the tasks required.
Above is a SCARA robot and to
the left is a diagram showing
the motions of the different
axes
Horizontally Base-Jointed Arm
• This is an adaptation of the SCARA
system, with axis 2 as the linear axis
instead of 3– Instead of the tooling rising up and down, as with
the SCARA, this system moves the whole arm up
and down
– These robots also tend to have a normal minor
axes complement of two or three versus the single
rotational of the traditional SCARA types
This configuration provides the power of the SCARA robot in
the vertical direction with flexibility in tooling orientation that
rivals any of the other systems we have looked at
Delta
• Delta robots have become popular in
industry and 3D printing over the past
few years due to their speed and unique
design, and with that unique design
there is a unique geometry
• the system is made up of three vertical
arms coming to a pyramid-type point at
the tooling below
Here are a couple of examples of Delta robots you might find
hard at work in industry
Delta cont.
• The result of this arrangement, due to
the sweeping motion of the three major
axes, is a cone similar to an acorn or
the nose cone of a rocket
• The majority of the work envelope is
closer to the base of the robot; the
envelope narrows as the tooling is
moved farther away from the overhead
unit
Here is an illustration
showing the work
envelope of the Delta
robot.
Notice how the work
area is larger near
the robot base and it
tapers the farther
away the tooling
moves
Drive Systems: Classification
and Operation• Another way we classify robots is how
the motors connect to the robot to move
it
• There are two broad categories in this
field:– Direct drive
– Indirect drive
Direct Drive
• Direct-drive systems have the rotating
shaft of the motor connected directly to
the part of the robot they move
• The torque (rotational force) of this
system is the same as the motor as
there is nothing to modify the system
between the motor and robot
Payload
• Payload refers to how much the robot
can move– With direct drive systems, the payload will be
based on the torque of the motors and how much
force is left after we subtract the weight of the
portion of the robot it moves and the tooling
– The payload of the robot is determined by the
smallest amount of force left, regardless of which
axis that might be
Reduction-Drive
• Reduction-drive systems alter the
output of the motor shaft via mechanical
means– As a rule, these systems slow the speed of the
rotational shaft in order to increase the torque or
force of the system
– They can also change the direction of rotation or
turn rotational motion into linear motion
– These systems often require more maintenance,
as they have additional moving parts
Belt-Drive
• In this reduction-drive system, both the
motor and the portion of the system we
wish to move with that motor have a
pulley attached and are connected by a
belt– V-belt, which is shaped like a V
– flat belt, which is a flat band of material
– synchronous belt, which has teeth at set
intervals along its length
The left section of belt is
V belt while the right is a
chunk of Synchronous
belt
Belt-Drive cont.
• Flat and V-belts rely on friction to
prevent slippage, with the teeth of the
synchronous belt maintaining positional
integrity– Slippage is when some of the rotation of the
pulley attached to the motor, known as the drive
pulley, is not transmitted to the pulley attached to
the system, known as the driven pulley
– Flat belts have the highest chance of slippage,
followed by V-belts and synchronous belts
To the right is an example of a
V belt system complete with
drive and driven pulleys.
Above is a Synchronous belt
used to move the fingers of a
robotic gripper.
Torque in Belt-Driven Systems
• The base formula for torque is T = F x d– T is torque, F is force, and d is distance
• We can adapt the torque formula for rotation as follows:
• T = F x R, where – T = torque
– F = force (note: the force at the drive pulley will be the same as the force at the driven pulley)
– R = radius of the pulley (radius is half the diameter of a circle)
• diameter is the distance from one side of the circle to the other, with the line passing through the center of the circle.
Pulley Ratio, and Speed in
Belt-Driven Systems
• Since the drive pulley attaches to the
motor, the revolutions per minute
(RPM) of the drive pulley is determined
by the motor
• The RPM of the driven pulley depends
on the size ratio between the drive and
driven pulleys
Pulley Ratio, and Speed in
Belt-Driven Systems cont.
• Rs = D2 / D1, where – Rs = speed ratio
– D1 = diameter of drive pulley
– D2 = diameter of driven pulley
• Also, Rs = RPM1 / RPM2, where – Rs = speed ratio
– RPM1 = revolutions per minute of the drive pulley
– RPM2 = revolutions per minute of the driven pulley
Velocity in Belt-Driven
Systems• Velocity is a measure of how fast
something (in this case, the belt) is moving– For this, we will use the following equation:
– 𝐵𝐷𝑉𝑓𝑡
𝑚𝑖𝑛= 𝐷1 𝑥 𝜋 𝑥 𝑅𝑃𝑀 𝑥
1
12
𝑓𝑡
𝑖𝑛
– BDV = belt-drive velocity in ft./min.
– D1 = drive pulley diameter
– π = pi, the constant 3.14
– RPM = revolutions per minute of the drive gear
– 1/12 = conversion factor for ft./min. If you want the belt-drive velocity in inches per minute or the pulley diameter is already in feet, leave this out of the equation.
Power of Belt-Driven
Systems• Power is a measurement of work, and
we can use the formulas for power to
find information we are missing
• There are several formulas for power as
it relates to the belt-driven system
• 𝑃 = 𝐹 𝑥 𝑣– which is power (P) equals force (F) times linear
velocity (𝑣)
Power of Belt-Driven
Systems cont.• 𝑃 = 2𝜋 𝑥 𝑇 𝑥 𝑅𝑃𝑀
– where power (P) equals two times pi (𝜋) times torque (T) times revolutions per minute (RPM)
• 𝐻𝑃 =𝑇 𝑙𝑏∙𝑓𝑡 𝑥 𝑅𝑃𝑀
5252– HP = horsepower
– RPM = revolutions per minute
– T = torque in ft. lb.
– 5252 = conversion constant for 𝑙𝑏 ∙ 𝑓𝑡 of torque
– 63025 = conversion constant for 𝑙𝑏 ∙ 𝑖𝑛 of torque (substitute this for the 5252 if using 𝑙𝑏 ∙ 𝑖𝑛. of torque instead of 𝑙𝑏 ∙ 𝑓𝑡)
Chain-Drive
• For the most part, chain-driven systems
work in the same way as belt-driven
systems, with a few exceptions:– These systems use sprockets, which have teeth
designed to fit into the links of the chain instead of
pulleys
– The a chain, usually made of metal, connects the
drive sprocket to the driven sprocket
– Like the synchronous belt, chains do not slip, but
they do wear out
Here you can see
several different
sprockets and the
chain that they
would run with
Gear Drive
• Gears come in many shapes, sizes, and
varieties, but they all have cogs, or
teeth, which are projections that match
up or mesh with similar projections on
other gears to transmit force
• Drive Gear – the one supplying power
• Driven Gear – one tied to the output
• Gear train or transmission – two or
more gears connected together
Gear Drive cont.
• Idler gears are extra gears added to a
system to change the direction of
rotation on a dedicated shaft, not an
output shaft– If there is an even number of total gears in a drive
system, the last gear will rotate opposite the input
gear.
– If there are an odd number of total gears in a
system, the last gear will turn in the same direction
as the input gear
An even
number of gears
means the
output will turn
opposite the
input
An odd number of gears is required to have the output, or
driven gear, turn in the same direction as the input, or
drive gear.
Gear Drive cont.
• Compound gears - two or more gears
on the same shaft, often made from one
solid piece of material– One of the gears in a compound gear will be a
driven gear, while the other will be a drive gear
– This is because one or more of the gears in the
compound arrangement serves as the power
source for a completely new gear train
– multiple gears on the same shaft must rotate in the
same direction
Here you can
see a
compound gear
with three
different gears
of varying size
on the same
shaft
Spur Gears
• Spur gears are made by taking a round
or cylindrical object and cutting teeth
into the edge– The teeth are not square, but tapered and
rounded at the end to reduce friction and other
stresses that occur as the teeth mesh with other
gears
– The teeth are parallel to the shaft running through
the gear
– Spur gears only mesh in parallel with one another
Inside this
tooling you can
see several
sets of spur
gears
Helical Gears
• Helical gears are similar to spur gears,
but their teeth are not parallel to the
shaft of the gear– they are set at an angle along the edge making
them part of a helix or smooth space curve
– Because of this unique shape, two gears can
mesh with their shafts in parallel or at a 90
degree-angle from each other
– They are also known as Skew gears
Here you can see a
couple of Helical gears
as well as how they
mesh together
Bevel Gears
• Bevel gears have their teeth cut along
a tapered edge that would make a
pointed cone if not flattened on the end– They are capable of any angle between 0 and 180
degrees with proper construction
– A bevel gears that has an equal number of teeth
and a shaft at a 90-degree angle is known as a
miter gear
– To reduce noise and smooth out operation, we
curve the tooth creating the spiral bevel gear
Bevel Gears cont.
• Zerol bevel gears use the same curved
tooth without the angled sides– These gears have a flat face instead of the
tapered, coned shape of the other bevel gears.
• Hypoid bevel gears are similar to the
spiral bevel gear, but if you draw a line
from the shaft set at an angle, it will not
meet the shaft of the other or mating
gear.
There is a
spiral bevel
gear and a
Zerol bevel
gear with its
trademark flat
face shown in
this cutaway
image
Worm Gears
• Worm gears are made up of a cylinder
that has one tooth cut around it known
as the worm and a spur or helical gear
of the desired design– These systems turn the force 90 degrees like the
bevel systems but are simpler in construction and
can generate torques up to 500:1
– The worm can always be the drive gear however,
in some configurations, the helical or spur gear
cannot
Here you can see a worm gear along with a matting spur gear
Rack and Pinion
• Rack and pinion systems consist of a
spur gear and a rod or bar that has
teeth cut along the length– This system converts rotational force into linear
force and is favored for moving systems over long
distances
Here is a rack and pinion system at work inside a tooling
setup to extend the movement range of the bases
Harmonic Drive
• Harmonic drive - a specialized gear system that uses an elliptical wave generator to mesh a flex spline with a circular spline that has gear teeth fixed along the interior– The circular spline is typically the driven portion of the
system
– the flex spline only contacts the circular spine at two points that are 180 degrees apart
– The wave generator is inside the flex spine, but is usually separated from the flex spine by ball bearings
This system can generate torques up to 320:1 and has no
backlash
The Math of the Gear
• We will first look at the pitch diameter,
which is the diameter of the imaginary
circle used to design the gear– This circle cuts through the middle of the gear
teeth, where the smooth side starts to taper at the
top
– It is designed to contact the pitch circle of another
gear when they mesh
Pitch Diameter
• 𝐷 =𝑁
𝑃, where
• D = pitch diameter
• N = number of teeth on the gear
• P = Diametral pitch or gear size– Diametral pitch is a ratio of the number of teeth
per pitch diameter and describes the size of the
gear teeth with a smaller ratio, denoting larger
teeth with more space in between then
Meshing Gears
• Once we know the pitch diameter, we can use this in a calculation to determine how far apart to place two gears for proper meshing– too close will cause binding or make it impossible
to mount the gears
– too far apart, can lead to gear damage, loss of power, and excessive wear
– When you are meshing two gears, the Diametral pitch and pressure angle must match for proper operation
Pressure Angle
• Pressure angle refers to how the
forces interact between two gear and at
what angles and determines how a gear
tooth is rounded or shaped
• There are two choices:– 14.5 degrees, which was the standard for many
years and is still readily available
– 20 degrees, which is able to transmit greater loads
and is thus the new favorite
Center-to-Center
• To find the center-to-center distance of
two gears, we use the following formula:
• 𝐶𝐶𝐷 =𝐷1+𝐷2
2, where
• CCD = center-to-center distance
• D1 = pitch diameter of gear 1
• D2 = pitch diameter of gear 2
Gear Ratio
• Just as we use pulleys of different sizes
in belt-driven systems to increase
torque or speed, we can do the same
with gears by increasing or decreasing
the number of teeth on a gear– We use the gear ratio to determine what happens
with the driven gear in reference to the drive gear
and thus the torque and speed changes as well
Gear Ration Formula
• 𝐺𝑅 =𝑁2
𝑁1, where
• GR = gear ratio
• N1 = total number of drive gear teeth
• N2 = total number of driven gear teeth
Gear Velocity Formula
• We use velocity to figure out how fast a
system moves via the following
equation:
• 𝑉 = 𝑝𝑖𝑡𝑐ℎ 𝑐𝑖𝑟𝑐𝑙𝑒 𝑐𝑖𝑟𝑐𝑢𝑚𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑥 𝑅𝑃𝑀– V = velocity
– Pitch circle circumference = D ∙ π and D = pitch
diameter
– RPM = revolutions per minute of the gear
Ball Screw
• Ball screws -a large shaft with a
continuous tooth carved along the outer
edge and a nut or block that moves up
and down the length of the shaft– The prime mover connects to the shaft either
directly via a coupler or through a belt, chain, or
gear-drive system, with all the options that creates
– The block or nut that moves along the ball screw
usually rides along the tooth via ball bearings and
attaches to whatever is being moved
ISO
• International Standards Organization
(ISO) - an organization that develops,
updates, and maintains sets of
standards for use by the industries of
the world– An ISO certification guarantees that a company is
making its products according to a defined set of
specifications for quality, safety, and reliability,
giving customers peace of mind
ISO Classification
• ISO groups robots into three broad
classifications: – Industrial (the one that ISO has worked with the
longest)
– Service
– Medical
ISO Industrial
• ISO defines an industrial robot as an
“actuated mechanism programmable in
two or more axes with a degree of
autonomy, moving within its
environment, to perform intended tasks”
(Harper 2012).
ISO Industrial cont.
• The ISO categories for industrial robots
are as follows:– Linear robots
– Articulated robots
– Parallel robots
– Cylindrical robots
– Others
• Notice how similar these are to the
geometry sorting we covered earlier?
ISO Service Robot
• The ISO defines a service robot as a
“robot that performs useful tasks for
humans or equipment excluding
industrial automation applications” (Virk
2003)
ISO Service Robot cont.
• This group of ISO robots has one
category with three subcategories:
• Personal care robots - in this instance are
those that can come into contact with people
to help with or perform action to improve the
quality of their life
– Mobile servant robot
– Physical assistant robot
– Person carrier robot
ISO Medical Robots
• ISO defines medical robots as “a robot
or a robotic device intended to be used
as medical electrical equipment” (Virk
2013)
• This is a new classification for ISO and
still under development
Review
• How are robots classified? This
section discussed why we classify
robots and some possible broad
categories.
• Power source. We examined this
group of robots and looked at some of
the strengths and weaknesses of each
category within.
Review cont.
• Geometry of work envelope. This
section showed how we can group
robots according to their work envelope
and discussed axes of movement.
• Drive systems: Classification and
operation. This section was one part
classification, one part drive systems.
We also explored some of the math
involved with drive systems..
Review cont.
• ISO classification. We explored how
ISO groups robots; we discovered that
industrial robots are classified by how
they work mechanically, giving us nearly
the same groupings as we present in
the geometry section of the chapter
top related