Proceedingsedge.rit.edu/edge/P11213/public/P11213 - Final Technical... · Web viewThe Mechanical Engineering department wants the MSA to serve as a learning tool for incoming students

Multi-Disciplinary Senior Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: 11213

LAND VEHICLE FOR EDUCATIONMODULAR STUDENT ATTACHEMENT

Dylan Rider/Team LeadMechanical Engineer

Andrew Komendat/Interface ManagerMechanical Engineer

Oyetunde Daniel Jolaoye/EngineerElectrical Engineer

Jared Wolff/EngineerComputer Engineer

ABSTRACT

The Modular Student Attachment (MSA) is a four bar linkage with an end effecter mounted on the Land Vehicle. The MSA is used to pick up things of very little weight. The linkages extend to the desired distance and the end effecter grips the object. The primary purpose of the Land Vehicle for Education’s Modular Student Attachment is to provide a learning platform for mechanical engineering students and possibly in other curricula in the future as well. As the name suggests, this platform is modular in nature so many attachments could be developed for use by students in all curricula. The following document will summarize the reasoning behind the development of the Modular Student Attachment. It will also detail the development, assembly, test, and any conclusions drawn from the design of the MSA. A major problem with the MSA was over-weight. The servo could not move the linkages, and this makes the servo stall most times. The linkages had to be shortened because the weight of the MSA was too much for the servo to work with. This was a major challenge faced and was solved, making the MSA a working machine.

NOMENCLATURE

MSA – Modular Student AttachmentLVE – Land Vehicle for Education

GPIO – General Purpose Input/OutputWOCCS – Wireless Open-Source/Open-Architecture Command & Control SystemI/O – Input/outputFEA – Finite Element AnalysisPCB- Printed Circuit BoardUSART-Universal Asynchronous Receiver/transmitterSPB- Serial Peripheral Interface BusMATLAB- Matrix LaboratoryRIT- Rochester Institute of TechnologyCAD- Computer-aided Design

INTRODUCTION

The Mechanical Engineering department wants the MSA to serve as a learning tool for incoming students. The MSA is designed to show freshmen the various basic tools that they will be introduced to and teach them how to use it by using those tools first on the MSA. For the MSA to serve as a learning tool, it must be designed and fabricated using the tools that the freshmen would be exposed to in their first year at RIT. Incoming students are targeted because the M.E. department wants to introduce them to the basic tools at the very beginning of their program. The idea behind the MSA is that as the students go about the design and fabrication, they will learn the various techniques and tools used in the department and upon completion, they will be familiar with those tools.

Copyright © 2011 Rochester Institute of Technology

Proceedings of the Multi-Disciplinary Senior Design Conference Page 2

Coming out of high school, most incoming students into college are confused about what to expect in. The MSA is intended to help them have a little fun and also teach them about software tools like CAD and functional calculations. This would provide them with hands-on approach to mechanical engineering right out of high school.

PROJECT GOALS

The MSA was intended to meet a wide range of educational goals while conforming to numerous constraints. The primary goals incorporate the implementation of CAD modeling software as part of the design process. This involves creating models of parts the students can build in space before investing money and time, an important step in engineering. This includes parts, assemblies, drawings, and functional models.

Another important mechanical engineering experience that is intended to be incorporated in this project is a form of analysis. Provided that freshmen engineers will have little to no engineering education when taking the course, an application of mathematical background should show students the importance of careful calculations before stepping into manufacturing phases.

Cost was certainly a driving factor in the design. The mechanical engineering department had a set budget for the freshman projects, and was very strict with regard to prototype, mass production, and running costs. While considering the budget heavily, the design must also be flexible such that other MSA projects and assemblies can be attached, as well as leaving open potential for other majors to utilize the platform.

Finally, the project is intended to further introduce students to manufacturing in the RIT facilities. With the manufacturing phase, students should begin to gain confidence in producing their own custom-made parts, following the safety guidelines and being cautious of safe machining practice.

PROCESS (OR METHODOLOGY)

Specification DevelopmentThe customer needs and engineering specifications were developed through several meetings with the customer, as well as the input from a variety of professors. Though the scope of the MSA was ambiguous at first, continues refinement came about

through the initial phase of concept generation and formulation of needs and specifications.

The most important need of the MSA is to teach first year RIT Mechanical Engineering students design principles. For this to happen, the MSA must require the use of CAD modeling skills to build. The MSA must also utilize in-house facilities for the manufacturing of MSA components. Since the MSA is targeted to freshmen, it must involve simple calculations a first year student is capable of handling. Cost is a big issue when undergoing this project. The MSA must be of a low cost so that more would be purchased. It must have low manufacturing costs as well as use off-the-shelf parts when possible. The MSA must be easy to store in the allocated storage and it must also be safe to use. Another big need is that the MSA must be impressive such that other schools and faculty would want to emulate it.

Feasibility Analysis / Conceptual DesignThe use of controls in this design may be trivial but it was important to design a system where modularity was the main focus. There were several alternatives including combining boards with the LVE control team, using the WOCCS board as a control board and using a commercial PCB solution.

The current design was chosen over all alternatives because it provided a modular platform to expand upon in the future and an efficient means of converting voltage for use by the servos. Some notable features of its modularity include the ability to remove/upgrade the microcontroller daughter card, a robust power supply for use in many applications, 10 general purpose input/output pins brought out to the outside of the LVE.

The control module design also incorporated an ATMega ISP header, USART header, SPI bus header, and I2C header. All these interfaces were included to provide flexibility for future projects should they desire to use the design.

The MSA servos get their power from the switching buck circuitry. The switching buck circuitry, in this case, provides a steady 5V to the Chassis I/O header. The 5V is used for any devices including servos that may connect to the chassis I/O header. The buck steps down the battery voltage, which varies in range from 8.5V down to 6.9V. Anything below 6.9V will cause the buck circuitry to shut down.

Initial calculations for power supply requirements are summarized as follows:Assumptions:

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Using a servo with 100 oz in stall torque to drive the linksAssuming max torque from setup 100 oz in typical Servo Current:285 mA no load @ 5V~1000 mA max holding @ 5V

MaxI=~1000mA

Thus choosing the TPS54331, which has a max current limit of 3A, provided a factor of safety of 3 when it came to max current load. From the calculations it was determined if the MSA was constantly in use, drawing 0.25 C from a 4Ah battery, the MSA could operate up to 4 hours continuously.

Thus, initial power consumption calculations proved that such a design would be within the capability of the LVE power supply. A number of comparison studies were also performed with regards to different design setups. This included rough geometry and weight selections for components, generally conservative, and maximum torque calculations.

A major driving factor of the concept included budget restrictions. The cost of designs was incorporated to the concept selection based on parts and components required, as well as a few assertions regarding the end condition of the design.

Layout / PCB Design

Figure 1: Design and Pin Layout

The sizing constraints put forth by the chassis team drove the PCB design. The final constraints were 2 inches by 3 inches with #4-40 mounting holes at each corner of the board.

Some reference materials used for layout included the TPS54331 data sheet, which had the pertinent information about the buck circuitry layout.

All other layout procedures followed PCB layout best practices set forth by industry experts. Figure 1 shows the schematic diagram for the control of the MSA. Figures 2 and 3 shows the layout for the top and bottom of the MSA control board. Figure 4 shows the layout for the Chassis IO Board.

Figure 2: Top Control Board Layout

Figure 3: Bottom Control Board Layout

Figure 4: Chassis IO Board Layout

Module Testing and DebuggingGenerally, after fabrication, PCBs are tested and validated to ensure proper operation when assembled as an end product. The testing included ensuring the proper operation of each piece of circuitry on the MSA control board. Some testing occurred before stuffing



the board to ensure there were no PCB defects. This process entailed using a multi-meter to measure continuity of traces throughout the board. The remaining tests involved a populated board. The aforementioned tests include:

Ensuring proper buck operation by measuring:

o Voltageo Max currento Low Voltage Dropouto Over current Dropout

Ensuring the daughter card was operational by:

o Programming a simple program that toggles a built in LED

o Program development code on the device.

Ensure all traces and related serial headers are connected by:

o Running development code and connecting servos to the outputs through the Chassis I/O header.

Software DevelopmentSoftware for operating the MSA and LVE is completely open-source and available on the EDGE website for the project. The software allows selection between a controller (handheld) or a GUI on a personal computer. The software code itself contains the capability to manipulate the rotational speed of the drive and gripper motors, as well as the potential to set limitations on servo range to prevent mechanical interferences.

After all hardware testing is completed, true software development can begin. Software development was performed to ensure compatibility with the LVE controller design. This included ensuring the serial protocol was correctly set to receive commands from the LVE controller. All data was transferred at 115200 BAUD with no parity and one stop bit. The exact data scheme can be seen in Figure 14 below.

Byte 2

8- REV

8- FWD

7 - REV

7- FWD

6 - REV

6- FWD

5 - REV

5- FWD

Byte 1

4 - REV

4- FWD

3 - REV

3- FWD

2 - REV

2- FWD

1 - REV

1- FWD

Figure 14: MSA Data Scheme

The protocol was simply to send two bytes of information to the MSA controller whenever an input

from the controlling PC changed. i.e. The end user presses and releases the MSA buttons on the provided game controller. One servo is controlled with two bits. One bit controls the forward action and the other bit controls the reverse action of the targeted servo. This scheme provided the ability to control up to eight operating servos at once. To ensure the simplicity of the design the speed of the servo was controlled by a constant set in the program. In this case, the constant was set for optimal performance of the MSA 4-bar linkage and gripper.

Mechanical DesignFor the mechanical portion of the design, there were some several concepts for the MSA before the final selection was made.

Figure 5: First Concept of the MSA

In one design shown in Figure 5, the 4-bar linkage assembly incorporated two motors to operate a gripper. The design requires an interesting geometric analysis for the student while minimizing required motors and power.

The second concept shown in Figure 6 contains a simple robotic two-hinged arm design. The design allows a wider range of capable motions, but requires the use of three motors and has significantly higher torque requirements.

Figure 6: Second Concept of the MSA

The third concept shown in Figure 6 contains a similar design to the first concept, but contains a slotted 3-bar linkage design rather than 4-bar. The design again utilizes just two motors and can have a very versatile travel motion.

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Figure 7: Third Concept of the MSA

Settling on the final concept was driven by various factors, cost being the main driver especially it comes to the servos. The drive servo chosen was at the limit of the MSA functionality. The links of the MSA are designed in such a way that holes are drilled into them to give room for different applications.

The first concept was settled upon because it gives the maximum amount of torque with less power consumption as shown in the analysis below. The modularity of the design is such that the system can be easily interchanged with other projects because of the simply removed plate, and standard bolting pattern.

Figure 8: MSA Prototype CAD Assembly

Drive Motor SelectionThe selection of a drive motor was primarily driven by budgetary restrictions. The larger the torque output by the motor and the more robust the motor design, the wider the range of potential solutions which will be available to the students in their design, hence less failure. However, the most powerful of motors exceed the budget allowed for the project.

Early in the design phase torque calculations were performed for a number of different design projects to simulate the motor output necessary to produce an effective project. A number of project concepts were hampered or rejected based on the results of torque

analysis with a variety of inputs and loads to the system.

From component research servo motors were selected because of their low cost, availability, and simplicity to implement with the power and controllers in the system architecture. The servo comes with a variety of output gears which give a number of potential attachments for student parts and project actuation. The limitations, however, include the robustness and resistance to impact loads on the servo, including linear and rotational forces. Considering these possibilities, the servo was chosen as a reasonable tradeoff between power output and robust capabilities.

To ensure that the motor selected contained the power required to operate the MSA, a torque calculator was produced to determine the requirements of the system. Using the 4-bar linkage setup, calculations were performed to find the torque through the entire drive range of the system. A MATLAB calculator runs iterations at each degree through a full range of motion. The results feedback a torque required at each position, and shows the limitations of the system. A simple square, equal-sided setup requires the torque as seen in the Figure 8.

Figure 9: MSA Torque Curve: Square Geometry

As seen the maximum torque is required when the arm is at full extension, and when in the fully retracted position, the geometry is such that no torque is required to maintain position. For different geometric setups, two scenarios were discovered as limitations to the system. Such analysis was done by varying the lengths of each link from 4 to 8 inches, maintaining the other links at 6 inches. Analysis could be continued with infinite range and refinement of linkage setups. One such scenario is seen below.



Figure 10: MSA Torque Curve: Passive Inflection, Rear Drive

In the torque curve shown in Figure 9, an inflection point is seen in its trajectory. However, the system is capable of driving through this point. This is where the orientation of two of the links is linear. This inflection point is such that it can be operated through the full range of motion. Another scenario is seen in Figure 10.

Figure 11: MSA Torque Curve: Active Inflection, Rear Drive

The above situation is a fatal flaw in the 4-bar design and is a limitation on the system which must be accounted for by the designer. In this scenario, an inflection point is found away from the drive motor link. This causes the system to reach a point where it can become stuck, and its efforts will likely go to waste. This can result in the system being stuck in its range, or gravity may cause the opposite joint from the drive motor to become stuck inside its travel trajectory. Fortunately, the overhanging weight of the gripper should always prevent the joint from reaching this limitation, and should not be encountered.

Based on the maximum torque required from all these cases, a drive motor was selected that provided a reasonable range of link geometry to the students while keeping cost relatively low.

Gripper SelectionThe selection of a gripper was chosen based on available grippers in the market. The gripper was a simple selection that was intended to be run by a servo for simplicity of programming. The gripper was not manufactured in house because of the interface to the servo, which could not be accurately modeled without parts being purchased. With a better representation of

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the drive servo now available, a gripper could likely be produced using RIT manufacturing capabilities of the rapid prototyping 3D printer in the Brinkman Lab. This could bring about a cost reduction of nearly $15 per gripper.

The gripper selected for the current MSA assembly is seen in Figure 11.

Figure 12: MSA Gripper Assembly

Mounting DesignThe MSA was required to be mountable and removable from the LVE platform with ease. The driving motivation behind this required flexibility revolves around the required assembly of the MSA, as well as a number of sections using the same LVE platform. The platform must be robust to dropping and impact loads, and not break down from the wear and tear of common removal (threading, etc.). As such, a 0.25” thick acrylic plastic mounting plate was designed. A standard 1” spaced bolt pattern allows for flexibility with future MSA attachments and projects. The platform will be light-weight and remain durable.

To confirm durability and robustness to impact loads, Finite Element Analysis (FEA) was performed simulating a 5G loading for the maximum payload of the MSA. The load was placed at the center mounting location, a worst-case scenario for structural integrity. The results indicated no yielding or failures at the impact load, and minimal deflection. This scenario will exceed any foreseeable case in operation.

Figure 13: MSA Mounting Plate FEA

The student designed links themselves deemed disposable did not require FEA analysis as disposable parts were excluded from the drop test requirement. However FEA analysis on the link, bracket, and pin assembly was performed to validate the robustness of the mounting and hinging points. Again a 5G load test was preformed as with the base plate. The results indicate unnoticeable deformations and stresses in the pin and brackets. It can be noted, the analysis also concluded the student link parts would greatly buckle and bear most of the stress, as this is acceptable for the link components; the links can be observed to be a mechanical fuse in the event of a drop and will deform and bear the brunt of the damage.

Figure 14: FEA modeling of link, pin, and bracket

To allow simple attachment and removal of the MSA, a press fit-pin and cotter pin connection design was incorporated. Custom machined press-fit pins made of soft steel will be pressed into 0.25” holes in the LVE studs. The holes in the pins shall allow cotter pins to secure the mounting plate to the LVE. The design allows attachment without the use of threaded connectors which are often counter-threaded after repeated use.



Figure 15: MSA Mounting Assembly

Module ConstructionThe assembly of the MSA consists of a number of simple components, and required only screw drivers and adjustable wrenches. The assembly and manufacturing plan for the system includes detailed information regarding the construction of the design.

Application to Multiple DisciplinesThe MSA presents the ability for programs outside Mechanical Engineering to incorporate the LVE for educational purposes. This gives the LVE and MSA a much more attractive marking strategy and can potentially become a profitable tool in the classroom.

Industrial Engineering students will likely take the same freshmen-level course Mechanical Engineering student will be in, already being introduced to the LVE. However, additional coursework might involve process improvement for manufacturing of the LVE or MSA, budget reduction, or automation.

Electrical Engineers might incorporate a circuit to the MSA control system internal to the LVE. Such systems might include sensory feedback on the MSA platform back to the transmitting board and display, or design of voltage regulators or other components integral to the MSA control board or I/O board.

Computer Engineers or Computer Scientists might be involved with programming of the LVE controls. Processes might include creation of a GUI interface for the MSA and LVE, coding for protocol between the LVE and MSA control boards, or programming an automated operation for the entire assembly.

RESULTS AND DISCUSSION

The specifications for the MSA subsystem to the LVE were verified with a series of 18 tests, including endurance testing, educational goals, customer surveys, temperature tests, and numerous others. The test plan also details the analysis behind all the engineering specs related to the MSA that do not require analysis.

From endurance testing performed, the MSA has proven to be an effective attachment and design project for the students over 2 hours of operation. Though the drive servo will become a limitation to student designs, particularly over the lifetime of the LVE system, the prototype model is fully operational and can receive commands via the handheld controller or the GUI available from the downloadable program.

A primary concern is the analysis behind the drop test specification. Though the specification is likely a bit extreme, as not many robots could survive a 3 ft. fall. This is a difficult design limitation, to support an impact load of an object at least twice its own weight. Provided the student-made components (links) are considered disposable parts and not subject to the drop test, the MSA should be able to survive the drop test by analysis.

Timing specifications for work required for individuals and teams is likely to be a parameter tuned through trial and error. Fortunately, the design project contains a number of flexible opportunities to tweak project difficulty. This can include the number of parts to machine, design objectives, parts to assemble, and analysis required. With this flexibility in the project scope, the MSA can be fitted to meet all the instructor’s education goals for freshmen engineers. From a simple trial project with a current freshman mechanical engineer, a 10/3 scaling ratio for timing was used to validate all timing requirements.

Other notable results include an 80% overall approval rate from ME professors on the impressiveness survey, 0.4 oz. of waste per student design, and sub 100 degree Fahrenheit temperatures on all parts.

CONCLUSIONS AND RECOMMENDATIONSA number of improvements could be made in future revisions of the MSA and LVE. In particular, budget restrictions are of primary concern. Further steps towards budget savings such as gripper selection or material selection should be investigated. Lighter weight materials could allow for a reduction in required power and drive torque output. Savings could possibly be made in computing power with consolidation of control boards. Also, a closer examination of mechanical tolerances and stack ups might result in a design that is more aesthetically pleasing and versatile.

REFERENCES

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ACKNOWLEDGMENTS

The MSA team would like to express its deepest gratitude to all the individuals that have contributed their knowledge and experience into this project. Thanks specifically to Dr. Hensel and Dr. DeBartalo for their insight and assistance with the design. Additionally we would like to thank our faculty advisors: Philip Bryan, Leo Farnand and Vincent Burolla.



Project P11213

Documents

Proceedingsedge.rit.edu/edge/P11213/public/P11213 - Final Technical... · Web viewThe Mechanical Engineering department wants the MSA to serve as a learning tool for incoming students