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Structural Design Report of Robotic Arm

February 28 2003



Lunar Consulting have been assigned the task of creating a robotic arm that moves parts from a conveyor belt system to the recycle bin.  The arm has been built using a simple truss design and has been successfully statically tested.  This report outlines the methods and procedures used in manufacturing, and explaining the design process of the arm in order to optimize the given criteria.  This report includes a discussion on the initial research conducted, an outline of the criteria, a design development section discussing all the alternate design possibilities, an explanation of the manufacturing process and a calculations section determining the factor of safety of the arm and the stresses experienced in each truss member.

It was determined that a triangular design would be the most effective design.  Furthermore finite element analysis showed that smaller triangles (1.5”x1.5”x1.5”) would be very effective for this application.  Using the smallest available diameter rod of 1/16” would reduce the weight of the arm significantly reducing stress on the servomotor. 

The critical calculated factor of safety was found to be 2.5.  Thus the arm is safe and ready for manufacturing and static testing.

Manufacturing involved creating three faces of the structure and putting together the final triangular prism shaped arm.  Joints were soldered together carefully and consistently.   

The results of the static testing showed that the arm deflected approximately 1 mm when vertically loaded using a 1 kg mass and it had a total weight of 111 grams.  It would also support a 2 kg load in the horizontal direction, which takes care of any forces due to momentum as it moves. 

It is recommended by Lunar Consulting that routine maintenance and inspection procedures are performed in order to keep the arm functional.

1.0 Introduction

Racing Pistons Inc. has been manufacturing pistons for racing applications for 40 years and have achieved the highest quality levels in their field. Currently on their manufacturing cell of their racing piston production line, there is a sub-cell which performs the functions of a quality control system.

The quality control system has two aspects to it, firstly a visual inspection system, this is designed to test each part and ensure every required specification is within tolerance. A piston not within the desired tolerances would be defined as a defective part and is removed from the conveyor belt before the next stage of the manufacturing process. 

The second part of the quality control system is the removal of the piston from the conveyer belt.  Lunar Consulting has been awarded the contract to design and construct a prototype system to complete this task.

Therefore the concept of a single degree of freedom robot arm was recommended by Lunar Consulting. This arm would allow the defective part to be removed from the conveyor belt to a recycling bin. Control of the arm is achieved by the use of a control system which is coupled together with a servomotor and planetary gear reduction to drive the system with the assistance of an encoder.

The first stage of the system design process is the design of the conceptualized single degree of freedom robotic arm. Various constraints and criteria have to be applied to ensure a viable design is produced. These constraints and criteria were developed in the production of a proposal report for submission for project tender and are highlighted in Appendix A.

The objective of this report is to highlight the procedures used in the development of a solution to the aforementioned criteria and constraints. These procedures include the following aspects: research, design development, design selection, design verification, manufacturing, and static testing.

A detailed analysis of the project procedure was created for the proposal report. These procedures are highlighted in Appendix B.

2.0 Research

Initially background research was carried out to provide more information on the nature of the problem in terms of an engineering perspective.  This research takes many different forms and is listed below.

Constraints and Criteria:     Analysing the constraints and criteria that were developed for the proposal report, (refer to Appendix A) It is clear what role the design should fulfil. There are a couple of points to note, which are not obvious from reading the section. They are:

·         The deflection of the structure at the free end must be no more than 1/8th of an inch

·         The arm tolerance must be 20 plus or minus Ό inch.

Current Truss Designs:        The solution to this type of problem has been completed many times previously, whilst not in the form of the design problem highlighted in this report (i.e. the specifics are not identical but the general design ideas are uniform over design problems of this nature). Therefore preliminary research was carried out to identify current standardized designs of trusses and robotic arms.  A diagrammatic representation of the truss structures investigated is shown in Appendix E. 

Concept of Failure:   Structural members may fail to perform their intended function if excessive elastic deformation, yielding (plastic deformation) or fracture (break) occurs. For a fail safe design it must be determined what modes of failure can occur and then establish suitable criteria that accurately predicts the modes of failure. The mode of failure depends on the type of material used and the manner of loading e.g. static, dynamic, fatigue. There are two types of elastic deformation which result in structural failure. They are:

 ·         Deformation under the conditions of equilibrium, such as deflection of a beam or angle of twist of a shaft under gradually applied loads. The ability to resist this form of deformation is called the stiffness of the member.

·         Buckling of a large displacement under conditions of unstable equilibrium.  This may occur when the axial load exceeds the Euler critical load.

Two methods are used to determine the possibility of failure in the system; they are the Von Mises theory of failure criterion and the Euler buckling theory of failure. Von Mises theory of failure states that failure occurs when the energy of distortion reaches the same energy for yield/failure in uniaxial tension. The following equation (Von Mises) will be used to predict the safety factor of the design. [1]:

Where is the principle stresses on the structure and is the yield stress. The critical buckling load (elastic stability limit) is given by Euler’s equation.

Where Pcr is the critical load for buckling, E is the modulus of elasticity, I is the 2nd moment of area of strut about the neutral axis, and L is the length of the member. This equation is useful in determining the length of the struts to prevent buckling.

Welding and Soldering:        As the manufacturing process involves the use of soldering, background research was performed to identify the best ways of joining and soldering. This process is important as to minimise the structural discontinuities at the joints.  The structural discontinuities could cause stress concentration areas and ultimately lead to the failure of the robotic arm.

Material and Preliminary Structure Testing:         A description of the material properties used for the construction of the robotic arm is included in Appendix C. The values are the result of material tests performed on sections of the same type of brass rod to be used in the manufacture of the robotic arm. The tests performed are mainly to gain accurate information on the density, (weight analysis) and yield strength (failure analysis). By determining accurate values for these figures the design verification should have more evidence to support the conclusions.

3.0 Design Development

3.1 Constraints

The constraints determined by Lunar Consulting that are relevant to the design of the arm are as follows.

  1. Withstand a vertical load of 1 kg in the vertical direction.
  2. Withstand a 2 kg load in each of the horizontal directions to simulate dynamic loading.
  3. The arm must be within 23” to avoid interference with the Lexan shielding.
  4. The capture distance between the disk and the electromagnet must be 1/8”.
  5. The deflection of the arm under the loading of the electromagnet and the disk should be kept at a minimal such that the final distance from the table surface to the mounted disk be greater than 0.25”.

 3.2 Criteria

Lunar Industries strives for customer satisfaction by engineering better designs with no additional costs.  The following criteria set by Lunar Consulting will ensure a better arm design.

  1. The deflection of the structure at the free end must be no more than
  2. The weight should be under 130 grams
  3. A safety factor greater than 2 must be applied to the structure

3.3 Conceptual Design

The constraint and criteria form the basis of the idea for the design.  These generated ideas will help shape the conception of the design.  To facilitate the design conception the design of the arm was broken down into the following sections.

  1. Main structure
  2. Truss structure
  3. End support
  4. Load support
  5. Material choice
  6. Joining

 3.3.1 Main Structure

The main structure is the foundation of the arm, as it will define the supporting elements, and how the load force is distributed.  Figure 1 shows a few possible designs for the main structure, and Table 1 describes the advantage and disadvantage for each design. 

Figure  1 : Main structure design concepts


Design Shape



Rectangular Design

  • Easy to manufacture

  • Easy to mount and load weights

  • Can implement a simple  truss structure

  • Not an efficient weight to load ratio

  • Uncommon design

  • Inefficient load distribution

Triangular Design

  • Easy to manufacture

  • Easy to mount and load weights

  • Good weight to load ratio

  • Can implement a simple  truss structure

  • Efficient load distribution


Decreasing Triangular Design

  • Very good weight to load ratio

  • Easy to mount and load weights

  • Can implement a simple  truss

  • structure

  • More efficient load distribution

  • Difficult to manufacture

Cylindrical design

  • Good weight to load ratio

  • Efficient load distribution

  • Difficult to manufacture

  • Uncommon design

  • Requires complicated truss structure

  • Difficult to analyze

Table  1 :  Advantage and disadvantage of design concepts

 It has been decided that the triangular design is the best choice because of the manufacturing ease, the weight to load ratio, and the fact that a simple truss structure can be implemented.  Furthermore the triangular design is a proven structure as it is prevalent in bridges and cranes.

3.3.2  Truss Structure

The purpose of the truss structure is to distribute the loadings across the entire structure so it can carry the loads.  Appendix E show various truss structures that were considered.  The choices were then reduced to the Warren and the Pratt truss structures as seen in Figure 2.  The reason for the choice is that both truss structures are based on simple geometries which make manufacturing easier.  Furthermore the two truss structures appear to have fewer elements in the structure, which minimizes the weight.  

Figure  2:  Truss Designs

In a Warren truss structure some diagonals carry compressive forces while others carry tensile forces.  In a Pratt truss the diagonal struts are in tension, the vertical struts are in compression

By choosing the amount of trusses, the angle of the joints, and the materials, both truss structures can be engineered to support the load, and be optimized to minimize the weight and deflection.  In terms of weight to load ratio the Warren truss structure will be more efficient, however the deflection with a Pratt truss structure will be less.   

3.3.3  End Support

The bending moment experienced by the arm due to the loading will cause huge forces towards the pivoted end of the arm.  Thus the end must be stable and be able to withstand the bending moment and the forces that it will be subjected to.  In order to minimize the loads on the latter members, the structures should be supported at as many places as possible, to minimize the reaction forces exerted by each member.

Figure 3 shows the first concept where attachments are soldered on to the structure and four bolts are used to fix the structure in place.  The only concern with this design is that the joints are under heavy forces and might fail when torque is applied to the bolts

Figure  3 :  Fixed outer mountings

Figure 4 shows the second design concept where two support clips are positioned across the bottom rods.  This design distributes the forces across a greater area, and the probability of failure is lower than in the first design since the clips are not fixed structures and can deflect and move when torque is applied to the bolts.

Figure  4 : Mounting using support clips

3.3.4  Load Support

This section deals with the concept of the load support at the free end of the arm.  The electromagnet must be fastened to the arm using a screw.  Figure 5 shows a simple method of accomplishing this task by laying a couple of rods across two trusses, and passing the screw between it.

Figure  5 :  Load support

3.3.5  Material Choice

The choice of material was constrained to brass rod.  This rod is available in diameters of 1/16”, 1/32”, and 1/8”.  The choice of diameter can be made once the stress analysis and the finite element analysis (FEA) of the model is completed.  One of the major concerns is strength.  Failure of the material can occur by buckling or fracture.  It is apparent that failure from tensile fracture is unlikely because of the high yield strength of brass.  The most likely failure is that through buckling.  Buckling can be minimized by choosing a larger rod diameter, or by minimizing the length of the elements.  The latter option is more desirable since it will help reduce the weight of the structure.

3.3.6  Joining

All the members are connected to each other by solder.  The solder joint is extremely important to the structural integrity of the arm.  Although the soldered joints are strong, a poor soldered joint can easily fail.  Failure of one joint can cause a dynamic redistribution of forces on to other elements and can propagate failure.   This can eventually lead to the catastrophic failure of the entire arm.  Thus it is important to have good solder joints, and also to have a truss structure that can allow failure in a few joints while maintaining the structural integrity of the entire arm.  This can be accomplished by having a truss design with more elements so that the stress can be evenly distributed. 

3.4 Final Design Concept

Combining the ideas from the conceptual design the features of the final design are summarized in Table 2.   Furthermore engineering drawings are presented in Appendix F.



Reason and Comments

Main Structure

Triangular structure

  • Easy to manufacture, light weight, and strong

Truss Structure

Warren Design

  • The Warren design will be modified for   a 3D structure.

  • The structure is strong,  light and will have  minimal deflection.

  • The number of elements will help distribute forces and lessen the chance of catastrophic failure when one joint fails.


End Support

Support Clip

  • Less likely to fail since forces are distributed

Load Support

Two rod support

  • Simple and strong

Material Choice

All beams and truss are to be made using 1/16” Brass rod

  • Minimize weight

  • Thus the truss structure has to be have elements with short lengths to prevent buckling


Solder joining

  • -Joining influenced the truss structure

  • To  prevent total failure from single joint a truss with many elements was chosen

Table  2:  Summary of Final Design

Figure  6:   Incremental section of robotic arm

The design is seven and a half of the incremental sections shown in Figure 6, joined together in a long section. A detailed design including manufacturing drawing of the truss structure is shown in Appendix F.

Design Statistics:        The diameter of the rod used is 1/16”, the weight of this rod is 2.48 grams per inch and therefore the mass of the arm can be estimated. It is as follows:

Number of Warren trusses


Mass of Warren truss

11.4 grams

Estimated mass of solder

14 grams

End attachments

11 grams

Total mass of structure

111 grams

Manufacturing...continued >>>>