THE HONG KONG POLYTECHNIC UNIVERSITY

INDUSTRIAL CENTER

INTEGRATED MANUFACTURING PROJECT

 

MU01: DESIGN AND MANUFACTURE OF A

STAIR-CLIMBER

FINAL REPORT

 

3 JULY 2012


 

ABSTRACT

The final report is dedicated to the Integrated Manufacturing Project established by the Industrial Center of the Hong Kong Polytechnic University. The objectives of the project are to design and manufacture a stair-climbing robot and the corresponding system of control within a budget of HKD2000.

 

The project statement was established and the project deliverables and technical considerations were clearly defined. The requirements of the projects were identified such that a list of Product Design Specifications was generated based on the stairs in the Industrial Center.

 

A series of solutions were generated with brainstorming through a mind map. Different conceptual solutions were deeply analyzed and the decision matrix was utilized to figure out the best design, which involved the use of both rack and wheels. The detailed design was then illustrated with Solidworks.

 

Although a lot of unexpected problems were encountered throughout the development stage, we could resolve most of them with the sincere help from Ir. KAREN CHENG and many other staff members. Our stair-climber succeeded in climbing upstairs and downstairs with the dexterous control of LEUNG TSZ KIT, one of our team members, and a group presentation has been completed. However, the climbing mechanism was not hundred-percent successful.

 

The project was well-managed such that we can complete the project in schedule and under limited budget. If more time and budget could be provided, we would like to improve some faulty features such as the controller and the position of the rack.


 

ACKNOWLEDGEMENT

Sincerely appreciate Ir. KAREN CHENG, who guided us onto the correct track throughout the design, manufacturing and problem-shooting stage. She has delivered valuable comments and recommendations to lead our team to consider deeply and thoroughly on the development of the stair-climber. For example, she discussed the selection of the correct motor and suggested different possible remedy actions to some important problems.

 

All the staffs and technicians in the Industrial Center were worth our acknowledgement for being helpful and assisting us to complete most of the proposed modifications. They demonstrated great skills and experience.

 

Although the final design was not a perfect one, it was much improved by the painstaking efforts from our team and the supervisors. Without them, the stair-climbing robot would not have been capable of climbing upstairs and downstairs successfully.

 

LAU KA HEI

Member from MU 01 – the Stair-climbing Robot


 

LIST OF FIGURES

1.1           The CAD drawing of the stair-climber

1.2           The CAD drawing of the controller

1.3           Project deliverables

2.1           The Flow of the design stage

2.2           The Flow of the manufacturing stage

3.1           The mind map constructed with the use of eDrawMindMap

3.2           The hand sketch of the “Tri-Star Wheel” Car

3.3           Simulation of the movement of the “Tri-Star Wheel” Car when moving upstairs

3.4           Simulation of the movement of the “Tri-Star Wheel” Car when moving downstairs

3.5           Simulation of the movement of the Tank

3.6           Simulation of the movement of the “Robotic Human”

3.7           The hand sketch of the Multiple-rack-and-wheel Car

3.8           Simulation of the movement of the multiple-rack-and-wheel car when climbing upstairs

3.9           Simulation of the movement of the multiple-rack-and-wheel car when climbing downstairs

3.10        The hand sketch of the Climbing Dog

3.11        Simulation of the movement of the climbing dog when climbing upstairs

3.12        Simulation of the movement of the climbing dog when climbing downstairs

4.1           The target setup of the stair-climber

4.2           The exploded view of the stair-climber

4.3           The exploded view of the wheel assembly

4.4           The exploded view of the rack assembly

4.5           The exploded view of the L-shaped-work-piece assembly

4.6           The electric circuit of the controller

4.7           Climbing upstairs – 1

4.8           Climbing upstairs – 2

4.9           Climbing upstairs – 3

4.10        Climbing upstairs – 4

4.11        Climbing upstairs – 5

4.12        Climbing upstairs – 6

4.13        Climbing downstairs – 1

4.14        Climbing downstairs – 2

4.15        Climbing downstairs – 3

4.16        Climbing downstairs – 4

5.1           The CAD drawing of the base plate

5.2           The CAD drawing of the side plate

5.3           The CAD drawing of the rack plate

5.4           The aluminum 6061 T-6 metal sheet

5.5           The CAD drawing of the 32-teeth gear

5.6           The CAD drawing of the 20-teeth gear

5.7           The assembly of L-shaped work-piece

5.8           The non-fabricated L-shaped work-pieces

5.9           The CAD drawing of the bush

5.10        The bush

5.11        The CAD drawing of the casing

5.12        The casing

5.13        The motor and motor stands

5.14        The wheels assembled with the shaft and bushes

5.15        The CAD drawing of the controller

5.16        The half-assembly of the controller

5.17        The manufacturing of silica gel

5.18        The silica gel

5.19        The circular components

5.20        The lathe

5.21        The regular components

5.22        The milling machine

5.23        The wire-cutting

5.24        The programming for wire-cutting

5.25        The CNC Turret Press

5.26        The CNC Press Brake

5.27        The drilling machine

5.28        The controller with electric circuit

5.29        The silica gel mixed with catalyst

5.30        The vacuum chamber

5.31        The silica gel poured into the container

5.32        The oven

5.33        The silica gel gasket

5.34        The incompletely cut rack

5.35        The hand-sketch situation of the supporting mechanism to the shafts

5.36        The real-life situation of the supporting mechanism to the shafts

5.37        The CAD drawing of the original bush

5.38        The wheel assembly with the original bush

5.39        The CAD drawing of the new bush

5.40        The new bush mounted onto the plate

5.41        The CNC Turret Press

5.42        The stair-climber failed to climb upstairs

5.43        The friction at the bottom of the rack was not enough

5.44        The gasket

5.45        The new rack-wheels

5.46        The counterweight at the front of the stair-climber

6.1           The Gantt Chart of our project

6.2           One of the meetings that were held after training for review of progress

6.3           The FACEBOOK group used to share ideas amongst team-mates

6.4           The DROPBOX used to share engineering drawings and documents

7.1           Use of the Controller

7.2           Friction Analysis

 

 


 

LIST OF TABLES

2.1           Requirement established by the project supervisor

2.2           Interpretation of the customer requirements

2.3           Product Design Specifications of the stair-climber

3.1           The advantages and disadvantages of different conceptual designs

3.2           The decision matrix

4.1           The bill of materials

4.2           Color of notations from Figure 4.2 to 4.11

6.1           Table of schedule

6.2           Budget of the project

 

 


 

CONTENTS

1         Introduction

1.1     Background

1.2     Objective

1.3     Project Statement

 

2         Preliminary Design

2.1     Introduction

2.2     Customer Requirements

2.3     Product Design Specifications

2.4     Project Flow

2.5     Summary

 

3         Conceptual Design

3.1     Introduction

3.2     Mind Map

3.3     Conceptual Designs

3.4     Decision Matrix

3.5     Summary

 

4         Detailed Design

4.1     Introduction

4.2     Structure of the Design

4.3     Working Principle

4.4     Motor Selection

4.5     Control System

4.6     Summary

 

5         Manufacturing Phase

5.1     Introduction

5.2     Material Selection

5.3     Process Selection

5.4     Problems Encountered and Remedy Actions

5.5     Summary

 

 

 

6         Project Management

6.1     Introduction

6.2     Schedule and Gantt Chart

6.3     Communications

6.4     Budget

6.5     Summary

 

7         Future Development

7.1     Controller

7.2     Casing

7.3     Wheels

7.4     Position of the Rack

7.5     Easier Control

 

8         Results and Conclusion

8.1     Results

8.2     Conclusion

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1        INTRODUCTION

1.1     BACKGROUND                                                                                                   (LAU KA HEI)

Nowadays, mechanical artifacts are commonly found in our daily life. They are currently used in many fields of applications such as office, hospital operation, industrial automation, military tasks and security systems. It is not difficult to observe that mechanical designs play an important role in assisting human tasks.

 

Stairs are one of the most commonly faced mobility challenges for robotic applications. With the inspiration from the Industrial Center of the Hong Kong Polytechnic University, our group has been involved in a project to design and develop a mechanical STAIR-CLIMBER that can climb up and down the stairs in a stepwise and safe manner.

 

1.2     OBJECTIVES                                                                                                       (LAU KA HEI)

After discussing amongst our group and our supervisor, Ir. KAREN CHENG, the following objectives were established at the beginning of the project.

I.         To design and manufacture a stair-climber which can climb up and down the stairs of particular specifications,

II.       To simplify the complex driving mechanisms into a simple electric circuit and develop a controller to control the movement of the artifact, and

III.     To maintain simplicity of our design throughout the project.

 

Figure 1.1 and Figure 1.2: CAD drawings of the stair-climber and controller

1.3     PROJECT STATEMENT                                                                                     (LAU KA HEI)

1.3.1      Project Deliverables

The following items are delivered at the end of the project.

I.         A mechanical artifact which can demonstrate the design of the stair-climber with an appropriately developed controller consisting of a simple electric circuit,

II.       Technical data such as data, sketches and diagrams of initial design, CAD and technical drawings, the Bill of Materials and the Gantt Chart.

III.     A simple webpage to illustrate the content of our project, and

IV.     Different forms of multimedia demonstration such as photos, videos, simulations and a power-point presentation.

Fig 1.3: Project deliverables

 

1.3.2      Milestones

I.         First compromised design: 10 April 2012

II.       Start of manufacturing: 11 June 2012

III.     Start of assembly: 26 June 2012

IV.     First successful testing: 4 July 2012


 

1.3.3      Technical Requirements and Considerations

I.         The size of the robot must fit those of the stairs; it will not be too large to be accommodated by each step of the stairs.

II.       The width of the robot must be well-defined within a suitable range such that it is zygomorphic for body balance on both sides. This can be achieved by a symmetric design and positioning of components.

III.     Since the stair-climber will be lifted up to climb the stairs, its weight to be supported must not be too large such that it will not burden or exhaust the weight supporter, which, most probably, is the motor. Components must be concisely designed and manufactured with proper materials.

IV.     The center of mass is arranged at the front side of the climber (the side ahead when going upstairs) such that it can facilitate the actions of climbing upstairs and prevent the artifact from toppling and flipping over when going downstairs.

V.       The method of controlling the robot must be well-considered. If a manual approach is employed, the user must be trained to familiarize with the robot; On the other hand, an automatic robot will involve the use of a digital computer such as the Programmable Logic Controller (PLC) for the purpose.

 

1.3.4      Limits and Exclusions

I.         The stair-climber can only climb the stairs with specifications similar to those in Industrial Center.

II.       Dry cells are used to deliver DC currents and they will be easily exhausted by the two heavily-loaded motors.

III.     It is confessed that the disassembly of the artifact can cause extreme inconvenience in case of maintenance.

2    PRELIMINARY DESIGN

2.1     INTRODUCTION                                                                                         (LAU KA HEI)

In this chapter, the customer requirements are identified at the beginning. Their needs are consequently interpreted in engineering terms and a list of preliminary design specifications is established. Simultaneously, the project flow for the design and manufacturing phase will be discussed.

 

2.2     CUSTOMER REQUIREMENTS                                                                        (LAU KA HEI)
After a detailed discussion with our supervisor, several requirements were understood to be accomplished during the development of the stair-climber and they are laid down in the following table.

Table 2.1: Requirement established by the project supervisor

Mission statement

Design and manufacture a stair-climbing robot

Design and manufacturing of the stair-climber

1.        Construction of a solid base to ensure the robot works properly when climbing the stairs

2.        Strength and toughness of the robot

3.        Use of battery power

4.        A maximum number of 4 motors to drive all the motions

5.        A switch to control power on and off

Functionality testing

Commission tests to ensure the robot is properly designed

Monitoring of project development

1.        Scheduled manufacturing process

2.        Budget estimation

3.        Development of the bill of materials

4.        A group presentation

 


 

Focusing on design, manufacturing and testing, we interpret the above statements into corresponding engineering requirements and they are described in the following.

Table 2.2: Interpretation of the customer requirements

Customer Requirements

Interpreted Needs

Design and manufacture a stair-climbing robot

1.        Mechanisms that provides forces for horizontal and vertical displacements

2.        Light-weighted design

Construction of a solid base to ensure the robot works properly when climbing the stairs

3.        A sheet-metal base onto which important components can mount

4.        A casing that protects important components of the robot

Strength and toughness of the robot

1.        Appropriate materials must be selected as the raw materials for manufacturing the components.

2.        Important components should be designed with strength.

Use of battery power

1.        The number of batteries to generate adequate electricity for driving the mechanical power producers

2.        The space for storing the dry cells

A maximum number of 4 motors to drive all the motions

Limitation to the variety of motions that can be performed by the stair-climber

A switch to control the power on and off

The route of the electrical power transmission must involve a single on/off switch.

Commission tests to ensure the robot is properly designed

1.        Since there are no specified stairs the stair-climber must be able to climb, the specifications of the stairs must be decided to verify that the robot can properly climb the stairs with specified range of dimensions.

2.        Construction of an artifact that imitates the stairs with suitable specifications to assist the robot in demonstrating its proper functions

 


 

2.3     PRODUCT DESIGN SPECIFICATIONS                                                          (LAU KA HEI)

The engineering requirements are further comprehended to generate a list of product design specifications which are concluded in Table 2.3.

Table 2.3: Product Design Specifications of the stair-climber

1.        Functional Performance

I.         Movement:

A.       The vertical displacement of the robot will be more than 0.2 m for climbing up and down each step of stairs.

B.       The horizontal displacement of the robot will be more than 0.2 m for climbing up and down each step of stairs.

C.       The movement of the robot will be stable and consistent.

D.       The safety factor is assigned to be 1.2.

 

II.       Power source:

A.       DC motors with working voltage of 6 V – 12 V will be selected.

B.       4 – 8 1.5 V dry cells will be used for each motor to deliver adequate amount of electricity.

C.       A regular space of width: 6 cm; length: 5 cm; height: 2 cm – 3 cm must be allocated for dry cell storage.

 

III.     Material selection:

A.       The solid base will be manufactured with alloy steels such as stainless steels.

B.       The casing will be manufactured with aluminum 6061-T6.

2.        Physical Requirements

I.         Size:

A.       The outermost dimensions of the stair-climber will be less than width: 2 m; length: 0.25 m.

B.       The size of the stair-climber will not affect its movements.

 

II.       Weight:

A.       As suggested by our supervisor, the total weight of the robot must not exist 2 kg such that the mechanical motion drivers will not be overloaded easily.

B.       The center of mass of the stair-climber will be designed to locate in the front region, probably 10 mm from the center position of the robot, so that the robot can stay close to the higher steps to facilitate proper and safe movement.


 

2.4     PROJECT FLOW                                                                                                (LAU KA HEI)

The flow of the project will be split into the two for the design phase and the manufacturing phase respectively.

 

2.4.1      The Design Stage

Figure 2.1: The flow of the design stage

 

The flow of the design stage will begin with a well-planned schedule. After that, a brain-storming process will be employed to generate a list of conceptual solutions with relevant sketches and descriptions. With the help of some decision tools, the final design of the stair-climbing robot can be developed and CAD software such as SolidWorks will be employed for simulating the design. The material for each component will be decided such that the Bill of Materials (BOM) can be built for making corresponding purchase from retailers. The manufacturing stage can then begin


 

2.4.2      The Manufacturing Stage

Figure 2.2: The flow of the manufacturing stage

 

The manufacturing phase will be divided into two groups at the beginning, namely the marking out of different components and the design and testing of the circuit. Once they have been finished, the stair-climbing robot can be assembled and consequently tested for functionality. If it cannot climb upstairs and downstairs successfully, its design has to be modified for further testing until successful performance can be observed.

 


 


2.5     SUMMARY                                                                                                           (LAU KA HEI)

Some fundamental engineering specifications are identified and all the conceptual designs in the next chapter and, certainly, the detailed final design in chapter 4, will satisfy the above requirements such that the stair-climber can climb up and down the stairs properly. A clearer picture has been drawn to facilitate the development of the final design. Furthermore, our team can work according to the flow of the project as established.


 

3    CONCEPTUAL DESIGNS

3.1     INTRODUCTION                                                                         (LEE HIN CHEONG)

At the beginning of the design stage, our group searched for existing methods for climbing stairs through the Internet. Several of them were deliberately observed and analyzed and some conceptual designs were generated through constructing a simple mind-map. Functions and criteria will be defined and put into the decision matrix to select viable designs while the Pugh’s Method will be used to weigh and combine viable features for the final design.

 

3.2     THE MIND MAP                                                                                  (LEE HIN CHEONG)

As a mechanical design project, some important aspects must be deliberately considered such as the motion drivers and transmitters, actions to climb the stairs and the methods of manufacturing and control the robotic motions. eDrawMindMap®, a software that is designed to develop detailed and easy-to-read mind map, is utilized for this purpose.

Figure 3.1: The Mind Map constructed with the use of eDrawMindMap

3.3     CONCEPTUAL DESIGNS                                                                  (LEE HIN CHEONG)

3.3.1      The “Tri-Star wheel” Car

A stair-climbing system can be achieved by the continuous rotation of a “Tri-Star” arrangement of the wheels. Three wheels are arranged in an upright triangle. The Tri-Star wheels are free to rotate and belt is used to bring motions to the wheels. Such a system can overcome rough surfaces in outdoor area.

 

The working mechanism of this proposed involves much in the rotation of a three-legged wheel holder, with a motor-driven wheel at each end. When it climbs up a step of the stair, the holder will touches the edge of the stair and rotate. The motioning wheels can grip on the surface of the next step of the stair and climb upward. The working mechanism for climbing downstairs is the reverse of that for upstairs.

Figure 3.2: Hand sketch of the “Tri-Star Wheel” Car


 

 

Figure 3.3 and Figure 3.4: Simulation of the movement of the “Tri-Star Wheel” Car when moving upstairs (3.3) and downstairs (3.4)

 


 

3.3.2      The Tank
The design is inspired by the motion of the tanks. A continuous track is used to drive the machine up and down the stairs. Several roller-and-sprockets are involved to drive the continuous track into motion. The track will only contact with the edge of the steps.

Figure 3.5: Simulation of the movement of the Tank


 

3.3.3      The “Robotic Human”

Similar to human structures, the design of the robot makes use of two “legs” which can step on the stairs. It consists of two separate linkages; each consists of two parts, namely the “thigh” and the “shank”, and they imitate human motions to step on the stairs when travelling up and down the stairs.

Figure 3.6: Simulation of the movement of the “Robotic Human”


 

3.3.4      The Multiple-rack-and-wheel Car

In this design, three racks are involved in the mechanism of the robot. Two wheels are mounted at one end of the each rack. One motor with a set of gears are used for the motions of all the wheels. The wheels control the movement of the robot on the floor, while each rack is controlled by one motor for elevation.

The working mechanism is that the front rack elevates and the robot will move near the stair until the middle rack touches the step. The elevated front rack will then lower to land on the next step. Such a procedure would then be repeated until all the racks can advance to the next step and the robot can then move forward for another elevation. The downstairs motion can be regarded as the complete reverse of that of travelling upstairs.

Figure 3.7: Hand sketch of the Multiple-rack-and-wheel Car


 

 

     

Figure 3.8 and 3.9: Simulation of the movement of the multiple-rack-and-wheel car when climbing upstairs (3.8) and downstairs (3.9)

 


 

3.3.5      The Climbing Dog

The design imitates the motions of dogs. It consists of four pairs of “legs” and each of them is driven by an independent motor so that they can “poke” on the ground. Furthermore, there are two additional racks to lift the machine upwards, and it can climb to the next step together with the “poking” motion of the legs. To climb the stairs, it will make use of the reverse of exactly the same set of actions.

Figure 3.10: Hand sketch of the Climbing Dog


 

 


 

 

Figure 3.11 and 3.12: Simulation of the movement of the climbing dog when climbing upstairs (3.11) and downstairs (3.12)

 


 

The advantages and disadvantages of different conceptual designs can be summarized in the following table.

Table 3.1: The advantage and disadvantage of different conceptual designs

 

Advantage

Disadvantage

1.        Simple mechanical design

2.        Stable

Wheel size increases for higher steps

1.        Stable

2.        Balanced

1.        4 motors are required

2.        Complicated gear arrangement

1.        Not easy to flip over

2.        Can turn to side

Cannot be driven by 4 motors

Great speed

Complex mechanical design

 

 

1.        Unstable

2.        Difficult to balance


 

3.4     THE DECISION MATRIX                                                                  (LEE HIN CHEONG)

After generate 5 different conceptual designs, a simple decision matrix is utilized to screen, and select viable design features for the final design.

Table 3.2: The decision matrix

 

 

Mechanical complexity

0

-

+

-

+

Chance of success

-

+

-

+

-

Endurance

 

0

0

+

+

-

Stability

 

+

+

+

+

-

Ease of control

+

0

-

+

-

Cost

 

+

0

0

-

0

Difficulty in Manufacture

+

+

+

-

-

Maintenance

 

0

0

+

-

0

Safety

 

0

+

0

+

-

Sum of +

 

4

4

5

5

1

Sum of –

 

1

1

2

4

6

Net Score

 

3

3

3

1

-5

Ranking

 

1

1

1

4

5


 

Observing and analyzing the results from the decision matrix, it can be found that the concept of the “Tri-Star” wheels, the multiple-rack-and-wheel car and the climbing dog rank the highest. Their features can be combined for the final design, which involves the use of both wheels and rack for motions, moves in a more stable manner and has a higher chance of being successful.

 

The idea of imitating the tank is not employed for its difficulty in manufacturing and mechanical complexity, while those of the robotic human are its instability and safety.

 

3.5     SUMMARY                                                                                           (LEE HIN CHEONG)

After this chapter, a brief description to the detailed design has been established as a result of the use of the decision matrix, combining different conceptual solutions. The computer-aided design (CAD) will be developed next chapter according to the frame set by the description.


 

4        DETAILED DESIGN

4.1     INTROCUTION                                                                                                    (LAU KA HEI)

In this chapter, the final design of the stair-climber will be discussed. The working principle of the stair-climber is illustrated with the breakdown of the design structure. The required torque driven by the motor is computed, while experiments are performed to test for the functionality of the motor. A simple electric circuit is also constructed to control the motions of the stair-climber. Figure 4.1 shows the target setup.

Figure 4.1: Target setup of the stair-climber

 

Our group believes that a simple design similar to this stair-climber can be capable of satisfying all the requirements as mentioned in the project statements.


 

4.2     STRUCTURE OF THE DESIGN                                        (LAU CHUN WAI, CHU KA HO)

The stair-climber consists of numerous components. All of them have their own specific use. Only the proper assembly can result in their smooth motions. The exploded view of the stair-climber is shown in Figure 4.2 below.

Figure 4.2: The exploded view of the stair-climber

 

The final design of the robotic climber installs one rack and two motor, the front motor is responsible for the linear propulsion and the rear one is responsible for the moving up or down motion of the rack. The reason of using two motors rather than a motor is to ensure the independence of the horizontal motion and moving up or down motion and thus obtain the stability of the robot.


 

The robot can be commonly divided into two major parts. One is the base part and another one is rack part. For the former part, most of the components are mounted on the base plate. It is because the base plate is made of the cast stainless, which can provide a support and even withstand the impact when the climber moves up or down. In the final design, the forward or backward motions are mainly depended on the wheels. There are two different types of wheels, front wheels and rear wheels. The front wheels with a greater size is responsible for driving the robot in horizontal direction, the rear wheels is responsible for supporting the robot when it lifts up. The wheel connectors are acted as the fixtures to the front wheels for fixing them onto the 5mm shaft. Due to the manufacturing difficultly, there is no any wheel connector installed on the rear wheels. The plates installed on the two sides of base are to maintain the wheels alignment correctly.

front wheel

Figure 4.3: The exploded view of the wheel assembly


 

For the rack part, the elevation of the robot is mainly relied on the rack; the rotational force generated by the motor is transmitted by the 20 teeth gear to the rack. With the assistance of the rack wheels, the robot can transform its motion from standing to sliding. There are two blue gaskets mounted on the rack. Their function is to increase the friction between the rack and the ground. Once the robot without the gaskets, it cannot stand on the ground firmly during climbing motion with a great extent of vibration. The instability of the robot would be caused and further it would flip at a certain vibrating level.

rack

Figure 4.4: The exploded view of the rack assembly

 

Two pairs of L-shaped work pieces are separately installed on the upper casing and under the base plate. Their main function is to restrict the rack motion other than vertical, such as the side sliding motion. A 3mm shaft with two bushes can smooth the rack motion and reduce the friction respectively, which connects each pair of work piece.

L-shaped

Figure 4.5: The exploded view of the L-shaped-work-piece assembly


 

4.2.1      BILL OF MATERIALS                                                    (LAU CHUN WAI, CHU KA HO)

The bill of materials was established to clearly state all the components involved in the assembly and the quantity required for each item. The list can facilitate the purchase of materials for the next stage of our project – the manufacturing stage.

 

Table 4.1: The bill of materials

BOM 2BOM

 


 

4.3     WORKING PRINCIPLE                                                                                     (LAU KA HEI)
Our team makes use of a simple mechanical design to fulfill the requirements of climbing up and down the stairs. Simplifying the working mechanisms, there must be an upward and forward displacement for the robot when it is climbing upstairs, while a downward and backward displacement when downstairs.

Table 4.2: Color of notations from Figure 4.2 to 4.11

Blue arrows

forces acting on the body of the stair-climber

Red arrows

forces acting on the rack

Green arrows

weight of the stair-climber

Yellow arrows

manner of rotation of the motors

Orange arrows

moment acting on the body of the stair-climber

 

4.3.1      Climbing upstairs

Figure 4.6: Climbing upstairs – 1

 

The stair-climber can travel in a straight line and it will manually stop when the front wheel touches the first step of the stairs.


 

Normal reaction on the wheels

 

Normal reaction on the rack

 

Downward force on the rack

 

Figure 4.7: Climbing upstairs – 2

 

The front wheels keep on rotating counterclockwise, while the rack at the back side will continuously exert a force onto the floor when it extends against the latter as driven by the motor. The upward reaction forces acting on both the rack and the wheels will lift the whole body upwards.

 


 

Moment out of paper

 

Downward force on the rack

 

Normal reaction on the rack

 

Normal reaction on the wheels

 

Figure 4.8: Climbing upstairs – 3

Figure 4.9: Climbing upstairs – 4

 

In Figure 4.4, the front wheels reach the edge of the first step of the stair. The reaction acting on the wheels by the stair at this position would drag the stair-climber forward; while the rack will continue its downward extension for a certain length until the front wheel completely travel forward on the top of the first step. The small wheels at the end of the rack are to facilitate such a forward motion.


 

Weight of the robot

 

Moment into paper

 

Upward force on the rack

 

Figure 4.10: Climbing upstairs – 5

Figure 4.11: Climbing upstairs – 6

 

The stair-climber as shown in Figure 4.6 starts to withdraw the rack from the floor. Without any support, the body will returns to its original plane of travel for its own weight, as shown in Figure 4.7. The front wheels will continually travel forward to prevent the body from toppling downstairs for the weight of the rack.


 

4.3.2     

Moment out of paper

 
Climbing downstairs

Normal reaction on the rack

 

Downward force on the rack

 

Figure 4.12: Climbing downstairs – 1

Figure 4.13: Climbing downstairs – 2

 

The mechanism travelling downstairs is similar to the reverse of that of upstairs. In Figure 4.8, the car moves to a position that the rack can extend vertically to the next lower step and, when it reaches, the normal force acting on the rack will generate a counterclockwise moment such that the stair-climber will be positioned as shown in Figure 4.9. The body can then travel backwards.


 

Normal reaction on the wheels

 

Upward force on the rack

 

Weight of the robot

 

Moment into paper

 

Figure 4.14: Climbing downstairs – 3

Upward force on the rack

 

Weight of the robot

 

Normal reaction on the wheels

 

Figure 4.15: Climbing downstairs – 4

 

When the front wheels reach the edge of the upper step, as shown in Figure 4.10, they will keep on rotating clockwise, while the rack starts to be withdrawn from the floor, which creates a clockwise moment again for the weight of the robot. The downward reaction acting on the wheels by the stairs due to the continuous rotation of the wheels will keep on balancing this moment until all the wheels reach the ground to receive support from the floor.

4.4     MOTOR SELECTION                                                         (LAU CHUN WAI, CHU KA HO)

4.4.1      Requirements

Before selecting an adequate motor for the robot, some assumptions are made and listed as follow:

I.         The total weight of the robotic climber is about 2 kg.

II.       The safety factor is 1.2.

III.     The efficiency of selected motor nearly attains 100%.

IV.     The friction between the gears is negligible.

 

Then, the distance between the motor and the rack should be obtained to calculate the required torque.

 

Distance between motor and rack
= ½ x (number of teeth of gear x module number)
= ½ x (20) x (1)
= 10 mm

 

Minimum torque provided by motor

= (2 kg) x (0.01 m) x (9.81 m s^-2)
= 0.1962 N m

 


 

In order to ensure the torque provide by the motors are high enough for performing required movement, safety factor 1.2 should be applied in this case.

 

Required torque
= (0.1962 N m) x 1.2
= 0.2354 N m

 

Specifications of the motor
= 20 kg cm
= (20 kg cm) x (9.81 m s^-2) / (100 cm / m) N m
= 1.962 N m

 

4.4.2      Specifications of the Selected Motor

Motor dimension: diameter 35.6 mm with length 56 mm

Gearbox dimension: diameter 37 mm with length 26 mm

Voltage range: 3-9 V

No loaded current: 700 mA

Weighted current: 2 A

Rotation Speed: 90 RPM

Torque: 20 kg/cm

Output shaft diameter: 6 mm

Shaft length: 15 mm

Weight: 290 g


 

4.5     CONTROL SYSTEM                                                           (LAU CHUN WAI, CHU KA HO)

In the design of our product, we decided that we use wired controller without sensor. Therefore, we have to provide instruction to the robotic climber to move up-down and forward-backward through the controller. Moreover, since we learned to use electric circuit to control a system, we decided to use the knowledge got from the lessons to complete the project. The followings are the required specification of our controller.

I.         Wired with the robotic climber and the controller

II.       Able to move on two planes

III.     With electric circuit acting as control system

IV.     With two switches to control directions

V.       With one main switch to manage the power supply

 

From specification 1, the wires connecting controller and the robot should be long enough. From specification 2 and 4, since there might be large power consumption for batteries, two specific batteries for power supply were needed in the circuit. Moreover, one switch must fully control a linear direction. From specification 5, without the one main switch turned to be “on”, there is no power supply no main which status of the linear switches are. In conclude, we designed an electric circuit as below:

 

Figure 4.16: The electric circuit of the controller

 

4.6     SUMMARY                                                                           (LAU CHUN WAI, CHU KA HO)

The design of the stair-climbing robot has been explained with the structural breakdown and step-by-step working principle. The course of the selection of the motor and the development of the electric circuit as the control system were reviewed.


 

5        MANUFACTURING PHASE

5.1     INTROCUTION                                                                                            (CHU KIN WING)

After the complete development of the detailed design, the manufacturing stage will begin. In this chapter, we shall discuss the selection of materials for manufacturing different components. The reasons for choosing a particular manufacturing process will also be explained. More importantly, the problems that were encountered during this stage and their corresponding remedy actions and the make-and-buy decision will be discussed.

 

5.2     MATERIAL SELECTION                                                                           (CHU KIN WING)

It was anticipated that our stair-climbing robot would be suffering vigorous impact during testing and demonstrations so careful selection for materials used in production was very crucial. We wanted to ensure the robot’s materials were strong and durable enough to endure the hard collision during travel, and, at the same time, light in weight to lessen the burden taken by the robot so that the motors’ torques required could be reduced. Thus the safety factors of the motors’ performances were increased.

 

After comprehensive discussion on the robot design and its components’ material requirement, the selection process was completed and the justification would be explained.


 

Figure 5.1 – Figure 5.4:

(From left to right) Base plate, side plate, rack plate and aluminum 6061 T-6 metal sheet

 

Base plate, side plate and rack-wheel plate were the main structure of the robot and needed to withstand great impact force during climbing up and down the stairs. Therefore these components were made of stainless steel.

 

One 32-teeth gear and two 20-teeth gears would be fabricated by wire-cut. It would be easier and faster to make them by using brass rod in wire-cut. On the other hand, the brass gears were able to withstand the shear stress while operating.

Figure 5.5: 32-teeth gear

Figure 5.6: 20-teeth gear


 

Figure 5.7: Assembly of L-shaped work-piece