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