Multi-Disciplinary Engineering Design ConferenceKate Gleason College of EngineeringRochester Institute of TechnologyRochester, New York 14623

Project Number: P09203

RP1 MOTOR MODULE SECOND GENERATION

ABSTRACT

The primary goal of the Robotics Platform 1 kilogram (RP1) is to provide an off-the-shelf robotics platform designed for a nominal payload of 1 kilogram and follows previous RP lines for 10 and 100 kilogram payloads. The objective of this project was to develop an independently controlled servo-motor driven wheel for the RP1 project family in conjunction with team P09204 who are developing the electrical systems for the RP1 project family. The final project family will support several applications including: educational programs, research & development projects, and outreach programs within and beyond the RIT KGCOE community. As such care needed to be taken to ensure few limitations were placed frame design, which will be developed sometime in the coming year.

BACKGROUND/INTRODUCTION

The Robotics Platform family was started with the intention of introducing students to the concept of designing a product within the context of a family of closely related products, and to develop an open-sources, open-architecture, modular, and scalable robotic ground vehicle platform. Initially started in AY 2006-2007 with the 10 kilogram and 100 kilogram nominal payload variants, each successive team is expected to build on the technologies developed by previous projects. This is done in an attempt to mimic industry, specifically similar to the automotive industries “next model year” approach to design.

In the winter of AY 2007-2008 two senior design teams were formed with the task of designing the first 1 kilogram nominal payload variant (RP1). Largely based on the previous RP10 Gen 2, the RP1 was meant to be a scaled down and improved version of the RP10. Unfortunately the mechanical design team placed too much emphasis on the family of designs and delivered a product only slightly cheaper and lighter than the RP10. This lack of significant design changes prompted the creation of two senior design teams, P09203 and P09204, tasked with the creation of the RP1 Gen 2.

The focus of this paper is to demonstrate the design, analysis, and evaluation methods used in developing the second generation 1 kilogram nominal payload robotic platform motor module (RP1 Gen 2 MM) by team P09203.

Figure 1: RP1 Motor Module

Copyright © 2009 Rochester Institute of Technology

Lauran Farnsworth / Project Manager Michael Egan / Chief Engineer

Matthew Missel / Design Lead Greg Leaper / System Analyst

Servo

Upper Housing

Encoder

DC Motor

Lower Housing

Servo

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NOMENCLATURE

RP1 1kg nominal payload capacity robotics platform

Gen GenerationRP10 10kg nominal payload capacity robotics

platformRP100 100kg nominal payload capacity robotics

platformMM Motor Module – independently controlled

servo-motor driven wheel.IM Idler Module – non-driven motor module

variantMSD Multi-disciplinary senior design course I, II

sequence.CAD Computer Aided DesignDC Direct CurrentFEA Finite Element AnalysisCNC Computer Numerical ControlCOTS Commercial, Off-The Shelf

CUSTOMER REQUIREMENTS

Developing concepts require a critical set of customer requirements and engineering specifications. In order to identify customer requirements the project history was researched in order to form a ground work understanding. The customer needs were refined based on feedback from the customer, Dr. Hensel, and the project guide, Dr. Walter. Gathered needs were ranked on importance and used to develop customer specifications. As a second generation product, specific care was taken to ensure mistakes learned in the previous generation were not repeated and to determine what the customer expected from the second generation robotic platform. These high-level customer needs include:

1. Nominal payload of 1 kilogram2. Infinite effective motion3. An improved system efficiency over RP1

Gen 14. Professional look and feel5. Open-source & open-architecture6. Must be in compliance of existing regulatory

and RP family structure7. Design must be robust8. Design should be intended for mass

production9. Utilize readily-available COTS components

CONCEPT DEVELOPMENT

As a second generation product, the first action take by P09203 was to benchmark the previous generation. It was quickly noted that it was difficult to tell the RP10 MM and RP1 MM apart, and there were a number of problems with the drive system. The RP1 Gen 1 utilized a delrin steering turntable which added considerable friction to the steering assembly. The steering and drive assemblies both utilized a complex drive train that allowed the drive wheel to spin infinitely in place. However it was determined that this complex drive system significantly reduced system efficiency, and increased the MM’s overall size and weight. It was also determined that motor selection was driven primarily by what DC motors were available that had a range of gear boxes large enough to be suitable for both the drive and steering systems.

Figure 2: RP1 Gen 1 MM

Based on customer specifications and a more expansive knowledge of the family history a number of concepts were explored. Using a Pugh diagram two concepts were explored in greater detail an omni-wheel design and a traditional drive and steer design. The omni-wheel design utilized an omnidirectional wheel, which would allow for infinite effective motion without the need of a second motor to change the orientation of the wheel. The traditional drive and steering design would effectively be an improved first generation MM, relying on a single steerable drive wheel. These concepts were presented to team P09204 and it was determined that the omni-wheel design

Project P09203

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would require a major redesign of the electrical and programming components; therefore, a traditional drive and steering design was selected.

DESIGN METHODOLOGY

Once a concept was selected it was possible begin designing the MM. Early research primarily revolved around the drive motor, drive wheel, gear/timing belt, and steering motor availability since those components would constrain possible designs more than any other factor. With knowledge of available components the design was broken up into three subsystems drive, steering, and structure (See system architecture).

Figure 3: System Architecture

Drive subsystemTeam P09204 began work one quarter ahead of

team P09203, which limited mechanical design chooses since they needed to make a number of assumptions in order to precede through MSD I. One such limitation was the need to use a DC motor for the drive power and an encoder for distance measurement. This knowledge along with customer specifications generated previously allowed for the generation of simple force diagrams. Since there is a linear relationship between the angular velocity of a DC motor and the motor’s torque output it was possible to build a system model describing the straight line performance of the completed robotics platform. This system model was coded into Simulink© and allowed for the simple evaluation of all easily available COTS DC motors.

Figure 4: Simulink Code

Approximately 50 motors were under consideration based on the simple force diagram and expected losses due to friction. This list was condensed based on simulations to a list of approximately 5 motors. Based on available COTS gears/timing belts and wheels the motor which most closely met all customer specifications was chosen (Copal SH 50:1). At this point a 1 inch radius wheel and 1:1 gear drive was chosen for the design as the best compromise to minimize size, reduce complexity, and provide an appropriate gearing for the drive motor.

Figure 5: Velocity Profile

To reduce the overall size of the MM a gear drive of 1:1 was selected. A gear drive was selected over a timing belt drive because of availability in the size range we were looking into and it was determined from previous RP projects that timing belts need to be tensioned, which would add to the level of complexity of the project. The available COTS gears drove the selection of ¼” axle stock, which in turn drove the drive encoder selection, and ultimately the bearing selection.

Steering subsystemThe steering subsystem was driven primarily by

two factors. First, team P09204 had already begun design on the electrical components for a servo to provide steering for the MM; this limited our options as to steering mechanisms. Second, it was determined that the COTS turntables traditionally used in the RP family are inadequate for robotic applications and that a solid delrin turntable introduces too much friction into the system. Standard servo motors are effectively DC motors geared to a potentiometer and an output shaft. This design allows the output shaft to be positioned without an encoder, allowing for simple control of steering without additional components but limits the MM to a 180 degrees steering sweep.

Servo motor selection was handled in a similar manner to the DC motor selection. A force diagram

Copyright © 2009 Rochester Institute of Technology

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was first generated and then based on customer specifications a Simulink© simulation was constructed. Since this model depends primarily on the interaction between the drive wheel and the ground, dampening coefficient were largely based on experimental data from RP1 Gen 1 with appropriate loading. Requirements from P09204 limited servo selection to servos manufactured by Futaba® and an appropriate servo was located (S3010). It should be noted that the experimental data was considered extremely conservative due to the excessive friction in the RP1 Gen 1 turntable.

The output shaft of Futaba® servos is a 25 point 6 millimeter diameter spline that needed to be mated to. A COTS product was found but considered excessively large. A custom broaching method was investigated but it was eventually determined to be infeasible.

A turn table consists of three main components an axial thrust bearing, a method of centering, and a connection method. A number of bearing layouts were considered including solid bearings and roller bearings; however, it was determined that the best solution to reduce friction, and excessive slop while maintaining a low profile was to use a ball bearing radial bearing and a ball bearing thrust bearing. Different connection methods were discussed before a c-clip design was chosen, since it has an incredibly small profile while having few downfalls.

Structure subsystemThe structure subsystem offered to most design

freedom and as such we were able to develop two basic designs in parallel. The first variant revolved around the idea of multiple structural components (ME1) making up the upper and lower housing. This type of design could be easily manufactured, could provide room for electrical components and wires were sent up through the turntable which would reduce the possibility of tangling. The second design centered on a solid upper and lower housing (MM1) each made out of a solid piece of aluminum. This design looked more professional, utilized space very efficiently, and was very light in comparison to the multiple component design; however, this design would require a CNC to manufacture and provided little protection of the wires. The benefits of each design were evaluated in a method similar to that used in concept selection and it was decided that the structure would be two solid pieces (MM1). Size restrictions with this design left no room for electrical components on board of the MM, which required P09204 to make a minor design change.

Figure 6: Structure Design Options (left: ME1 right: MM1)

Different techniques for mounting the MM to the frame were also explored. The vast array of applications the MM is intended to be used in prompted the development of a mounting method that would allow for quick attachment of a MM the frame while maintaining minimum restrictions on frame design. A slot system was conceived to achieve this goal. This system consists of a plate (1.75”x~2”x0.125”) which is mounted to the frame and a slot located in the upper housing that receives this plate. The mounting plate can be customized in any manner by the frame designers as long as the height and thickness are controlled, and nothing interferes with the mounting surfaces.

FEASIBILITY/REVISION

The initial designs were all developed in SolidWorks© 2007, which allowed for simple importation into the FEA package COSMOS works©. The force diagrams developed for motor analysis allowed us to easily develop the loading conditions of the MM. Simple loading conditions were used to generate force distributions so that more complex analyses could be performed. The unusual shape of the upper and lower housings limited the ability of hand calculations and required minor simplifications to facilitate mesh creation. The lowest recorded factor of safety in all tests was 7.4 and the maximum deflection was recorded at the furthest point from the attachment point to be 0.0001559 inches.

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Figure 7: FEA results deformation

Despite FEA numbers all well within expectable tolerances a number of concerns were brought up and addresses. Concerns were raised about the tabs used for securing the servo to the upper housing, difficult and unnecessary processes used to manufacture the lower housing, misalignment of the steering axle could damage the servo if excessive, and the possibility that the upper housing could be an extruded product.

The lower housing was simplified in some areas to reduce the number of machining operations. Different bearings were selected which further simplified machining operations but required the addition of a retaining ring. The thrust bearing groove was deepened in order to remove the corresponding bearing groove on the upper housing. To reduce the amount of material needed and to simplify the machining of the lower housing the steering axle was changed to a pressed pin.

The upper housing was simplified in a similar manner to lower housing. This simplification was primarily accomplished by redesigning the upper housing so that it could be easily extruded. As stated before the thrust bearing groove was shifted to the lower housing which saves considerable amount of time per each upper housing. The need for a COTS servo coupling meant that it wasn’t feasible to place a flexible coupling on the steering shaft while maintaining the profile and cost of the MM. Each standard servo comes with four rubber mounting blocks, which would allow the servo to move slightly in order to accept axle misalignment. Unfortunately the placement of the mounting holes was measured incorrectly so the mounting blocks were not implemented; however, the possible misalignment should be far below levels that would damage the servo.

After the design changes were made all FEA analysis were rerun, with results slightly better than previous tests.

MANUFACTURING

Rapid PrototypeDuring MSD I a rapid prototype was

commissioned to better illustrate the initial design concept. The models created in SolidWorks© were simplified to speed up creation time and then a rapid prototype was printed in ABS plastic. The prototype served both to illustrate our design to the customer and to evaluate the use of space within the MM. When placed next to RP1 Gen 1 it is clear to see that RP1 Gen 2 is approximately 1/3rd the overall height of Gen 1 and slightly approximately 1/9th the total volume of Gen 1.

CNC MachiningThe complex design of lower housing required the

use of CNC machine to manufacture. At this time we were able to solicit the help of RPO (Rochester Precision Optics) who manufactured the lower housings for free as part of a class demonstrating complex CNC operations. Materials, SolidWorks models, and technical drawings were given to RPO and the parts were manufactured on a 4-axis CNC mill. All CNC machining was done to technical drawing specifications and all machining techniques were documented so they could be reproduced at a later date.

Manual MachiningSimper parts such as the upper housing and drive

axles were manufactured manually using standard machining equipment. Care was also taken to modify the COTS drive wheel, which needed to be modified in order to fit onto and secured to the drive axle. All manual machining was done to technical drawing specifications and all machining techniques were documented so they could be reproduced at a later date.

ToleranceThe large number of mating surfaces required an

extensive use of geometric tolerances to ensure proper assembly and operation of the MM. Tolerances varied depending on application of the feature. Fine tolerances were kept to a minimum wherever possible and were primarily used only when a surface needed to mate with a bearing.

AssemblySince this project is intended for use by a large

number of customers it was important to clearly illustrate the process of assembly. Pictures were taken at every step during the assembly process and used to generate an assembly plan. Two non-engineering students were given the assembly plan and the needed

Copyright © 2009 Rochester Institute of Technology

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components, and asked to assemble the MM. Feedback from these students was then used to clarify the assembly such that someone with little knowledge on the MM would be able to successfully assemble one.

EVALUATION

To evaluate whether or not the design would meet customer needs an array of tests were developed. These tests include verification of: velocity, acceleration, robustness, component lifetime, module attachment, encoder operation, motor module assembly, module turning, efficiency verification, battery discharge time, and motor torque. Tests were divided into four categories: platform level testing, steering module testing, drive module testing, and electronics testing.

PlatformPlatform level testing was to be performed on a

completed robotics platform which includes a test frame, a MM, and the electrical components developed by team P09204.

Velocity Test: failed to meet specification

The motor specifications given by the manufacturer were found to be grossly inaccurate; this meant that the final speed of the robotic platform was

Acceleration Test Drop Test Component Lifetime Module Attachment Motor Encoder Test Motor Module Assembly

Steer ModuleSteering tests were performed on a completed

MM and verified the articulation and efficiency of the steering system. Module Turning Test Steering Module Efficiency

Drive ModuleThese tests are performed on a simplified motor

assembly and a completed MM.

Motor Torque: Pass

Based on the recorder experimental data the stall torque of each DC motor is within the expected tolerances and the motors will provide enough torque to successfully operate each module.

Motor Angular Velocity: Failed to meet manufacture specifications

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Drive Module Efficiency Motor Axle Bending

ElectronicsThis test verifies the completed MM and

electronics packages to verify it will operate for the desired amount of time. Battery Life

HIGHLIGHTS OF FINAL DESIGN

FUTURE WORK

Some observations were made through the course of manufacturing and assembling the MM but could not be implemented into the final design. The current module is manufactured out of solid pieces of aluminum 6061; however, this is impractical for large volume manufacturing. Ideally the lower housing would be a cast or molded plastic component and the upper housing would be an extruded component. This change would require extensive research and was simply out of the scope for this generation. The upper housing also contains more material then what is needed and should be optimized for both material selection and more efficient geometry.

In order to reduce the overall height of the MM the steel washers that we intended to be used with the thrust bearing in the turn table were not used. This means that the steel ball bearings of the thrust bearings are in direct contact with the upper and lower housings, which are constructed from aluminum. After minor testing the MM was disassembled and there was a visible ring left on the upper and lower housings from the ball bearings. In the future a harder material should be used to prevent the ball bearings from damaging softer and more expensive components such as the upper and lower housings. The easiest solution would most likely be to design a recess that can accommodate the steel washers that come with the thrust bearings.

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CONCLUSION

*********To be completed at a later date******

ACKNOWLEDGMENTS

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The RP1 Motor Module team P09203 is very grateful to everyone that contributed to making the project a success. We would like to thank the Gleason Foundation, our principal sponsor, for their financial support, Dr. Edward Hensel, our primary customer, for his vision and direction, Dr. Wayne Walter, our faculty advisor, for his guidance, and Todd Fernandez, our teaching assistant, for his support. We would also like to thank John Bonzo, Professor Wellen and Dr. Kempski for their technical support and insight.

Finally we would like offer our gratitude to the people at RPO including Daniel Missel and Larry Brugger for their expertise, time, and materials.

REFERENCES

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Copyright © 2009 Rochester Institute of Technology