Chap02 robot anatomy

Kỹ thuật robot (robotics) là kỹ thuật liên ngành bao gồm kỹ thuật cơ khí, kỹ thuật điện điện tử, kỹ thuật máy tính, nội dung của kỹ thuật robot bao gồm nghiên cứu, thiết kế, chế tạo, vận hành và áp dụng robot. Kể từ giữa thế kỷ 20 cho đến nay, robot ngày càng đóng vai trò quan trọng trong mọi hoạt động của con người như sản xuất, dịch vụ, y tế, nghiên cứu khoa học, quốc phòng…Chương này trình bày về lịch sử robot, cấu tạo và phân loại robot, các lĩnh vực áp dụng của robot, cung cấp các khái niệm cơ bản sẽ được nghiên cứu chi tiết trong các chương sau

 C.B Pham 2-1

Ch 2: Robot anatomy

2.1 Robot anatomy

Manipulator is constructed of a series of Joints & Links AJoint provides relative motion between the input link andthe output link Each joint provides the robot with onedegree of freedom

Revolute joint (R) Prismatic joint (P)

2.1 Robot anatomy

 C.B Pham 2-5

Degree of freedom

Degree Of Freedom (DOF): are the set of independentdisplacements and/or rotations that specify completely thedisplaced or deformed position and orientation of the body

or system

Six degrees (P,R) of freedom needed to fully place theobject in space and orientate it

Point location in space specified by three coordinates (P)

Object location in space specified by location of a selectedpoint on it (P) and orientation of the object (R)

The number of DOF for mechanism:

ci: number of constraints imposed by joint i,

fi: number of DOF permitted by joint i,

j: number of joints in the mechanism,

n: number of links in the mechanism (mechanism has one link fixed),

 : number of DOF of the space in which the mechanism lives

We have:

Degree of freedom

 C.B Pham 2-7

1 1 )

1 4 4 ( 3

1 2 3 ( 3

1 9 8 ( 3

Degree of freedom

 C.B Pham 2-9

2.2 Drive systems

The drive system determines the speed of the armmovement, the strength of the robot, dynamic performance,and, to some extent, the kinds of application

Robot actuators quality: have enough power to acc/dec thelinks, carry the loads, light, economical, accurate,responsive, reliable and easy to maintain

Hydraulic system vs Pneumatic system

Hydraulic system

Pneumatic system

DC motor

 C.B Pham 2-13

The magnitude of the

force on the wire:

F: force on the conductor (in Newton) I: current through the conductor (in amperes) B: magnetic flux density (in gauss)

L: length of the wire (in meters)

 : angle between the magnetic field and current

DC motor

Brushless DC motor

Brushless DC motors (BLDC) operate without brushes by taking advantage of modern electronic switching techniques.

 C.B Pham 2-15 Advantages: extremely reliable, very efficient, and easily controlled.

Brushless DC motor

A schematic of a generalized three-phase BLDC

AC motor

AC motors work by rotating the stator field It makes use of the natural alternating nature of the AC wave to turn the field coils on and off sequentially.

 C.B Pham 2-17

As the stator field rotates past an

individual bar, the field strength in the

bar rises and falls This changing

magnetic field induces a voltage in the

bar, and the voltage causes a current to

flow The current flows through the bar,

through the end rings, and back

through other bars This current causes

the bar to have a magnetic field, and it

is this field, interacting with the rotating

stator field, that produces the

Ss: synchronous (stator) speed (in rpm)

 C.B Pham 2-19

Servo motor = motor + optical encoder

• Optical encoder

Two types of encoder: incremental encoder and absolute encoder.

Signal from the photocell

Absolute optical encoders use a glass disk marked off with a pattern of concentric tracks A separate light beam is sent through each track to individual photo sensors Each photo sensor contributes 1 bit to the output digital word.

Absolute optical encoder

Resolution = 1 / 2 N

 C.B Pham 2-21

Incremental optical encoders have only one track of equally spaced slots Position is determined by counting the number of slots that pass

by a photo sensor, where each slot represents a known angle.

Incremental optical encoder

Incremental optical encoder

 C.B Pham 2-23

Stepper motor

A stepper motor is a unique type of DC motor that

rotates in fixed steps of a certain number of

degrees Step size can range from 0.9 to 90°.

Stepper motor

Excitation modes for PM stepper motors

 C.B Pham 2-25

Smart motor

Characteristics of actuating systems

 Weight, Power-to-weight Ratio

 Operating Pressure

 Stiffness vs Compliance

 Use of reduction gears

Applications: electric motors are the most commonly used

 C.B Pham 2-27

Stiffness is the resistance of a material againstdeformation Hydraulic systems are very stiff andnoncompliant Pneumatic systems are compliant

* The stiffer the system, the larger load that is needed todeform it

* The more compliant the system, the easier it deformsunder the load

* Stiffness is directly related to the modulus of elasticity ofthe material

* Stiff systems have a more rapid response to changingloads and pressures and are more accurate

* A working balance is needed between these twocompeting characteristics

Stiffness vs Compliance

Reduction gears

Gears used to increase the torque and reduce the speed

Electric motors normally used in conjunction with reduction gears to increase their torques and to decrease their

Hydraulic actuators can be directly attached to the links

This Simplifies the design, Reduces the weight,Reduces the cost, Reduces rotating inertia of joints,Reduces backslash, Reduces noise and Increasesthe reliability of the system

 C.B Pham 2-29

Reduction gears

Planetary gears

 C.B Pham 2-31

Harmonic drive

A Harmonic Drive (also known as

"Strain Wave Gearing") is a special

type of mechanical gear system

that has no backlash, compactness

and light weight, high gear ratios (a

ratio of 100:1 is possible in the

same space in which planetary

gears typically only produce a 10:1

ratio)

Harmonic drive

• Wave Generator

The wave generator is a

component having small ball

bearings built into the outer

circumference of the elliptical

cam The wave generator is

usually attached to the input

shaft.

• Flexspline

The flexspline is a thin cup-shaped metal rim component with external teeth The bottom of the flexspline (cup bottom) is called the

 C.B Pham 2-33

Harmonic drive

The circular spline (ncs) has two teeth more than the

flexspline (nfs) and is usually fixed to a casing

Ratio =

fs

cs f

n n

n s 

Characteristics of harmonic drives

1 High-speed reduction ratio

2 Free of backlash (lost motion)

3 High precision

4 Small numbers of components and ease of assembly

5 Small-sized and lightweight

6 High torque capacity

 C.B Pham 2-35

Ball-screw drive

A ball screw is a mechanical linear actuator that translatesrotational motion to linear motion with little friction The ballassembly acts as the nut while the threaded shaft is thescrew Ball screws tend to be rather bulky, due to the need

to have a mechanism to re-circulate the balls

Synchronous / timing

belts having grooves

which mate with teeth on

the pulleys

Any slip of the belt

relative to the pulleys is

minor in degree and is

due to belt stretch, or

erosion of the grooves

Timing-belt drive

 C.B Pham 2-37

Comparison between drive systems

+ Good for large robots and

heavy payload

+ Highest power / weight

ratio

+ Stiff system, high

accuracy, better response

+ No reduction gear needed

+ Can work in wide range

of speeds without difficulty

+ Can be left in position

without any damage

+ Good for all sizes of robots

+ Better control, good for high precision robots

+ Higher compliance than hydraulics

+ Reduction gears used reduce inertia on the motor + Does not leak, good for clean room.

+ Reliable, low maintenance + Can be spark-free Good for explosive environments

+ many components are usually off-the-shelf + Reliable components + No leaks and sparks + Inexpensive and simple + Low pressure compared

to hydraulics + Good for on-off applications and for pick and place

+ Compliant systems

Hydraulic Electric Pneumatic

- May leak Not fit for

clean room applications

- Require pump, reservoir,

motor, hoses, etc.

- Can be expensive and

- Motor needs braking device when not powered

Otherwise, the arm will fall

- Lowest power to weight

Comparison between drive systems