Multi-Disciplinary Senior Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: P11211

LAND VEHICLE FOR EDUCATION: CHASSIS, MOTOR, POWER

Jonathan Fabian - EE Ryan Sutton - ME

Jesse Keyser - ME Matthew O’Neill – ME

ABSTRACT

A robotic platform was designed to be used in a freshman level engineering class. The robot is designed to accept a modular attachment designed and built by the students that will be mounted in order to accomplish some task defined within the class. Some examples of this include, picking up, moving and rearranging small objects. This project was a new iteration of older families of projects, of varying sizes and budget ranges.

This platform is capable of supporting the desired attachment and operating successfully, in addition to being manufactured primarily in-house. Although the current design fell short when it came to the customer’s mass production budget (overall, the system was over budget by ~10%, not including shipping costs). This led to redesign proposals that focused on driving down overall cost. If all the proposed recommendations are implemented, a total savings of $448.75 would be recognized, leading to a per unit price of $125.46 for 10 chassis.

NOMENCLATURE

LVE – Land Vehicle for Education, the robotic platform designed during the course of this projectMSA – Modular Student Attachment, A student designed device meant to extend the functionality of the LVE for educational competitionsWOCCS – Wireless Open-source/Open-architecture Command and Control System, the RF Module used to wirelessly control the robot

RF – Radio Frequency, the spectrum of electromagnetic energy that surrounds and penetrates everything, can be harnessed for communicationsH-Bridge – Motor control solution for Permanent Magnet DC motors, can utilize PWM for speed control.PWM – pulse width modulation, method of controlling the voltage across a circuit by rapidly toggling on and off a switching deviceFEA – Finite Element Analysis, a numerical technique for finding approximate solutions of partial differential equations (PDE) as well as of integral equations. This is utilized here to analyze stress in complex 3d situations.

INTRODUCTION

The Land Vehicle for Education (LVE) is a robotic platform designed for use with a future mechanical engineering class. It is designed so that freshmen engineering students can create a modular attachment utilized to perform a set task. It is based upon prior senior design robotic platforms, such as RP1, 10, 100, as well as the LV1. For communication, it pulls from the WOCCS family of projects to provide a low cost, open source/open architecture wireless solution. This iteration represents a further refinement of the platform, focused specifically on cutting the production costs associated with the device in order to make it economically feasible for production.

PROCESS

Before starting the design, it was necessary to communicate with our customer, the head of the

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mechanical engineering department Dr. Edward Hensel, to understand the project goals and his requirements for the design. Following our meeting with Dr. Hensel, we synthesized a list of 16 critical customer criteria, as seen in table 1, which related directly to our subsystem of the LVE. The needs were rated on their importance to the customer, and served as a guideline for development activities.

Table 1. Table of Critical Customer NeedsCustomer Need # Importance Description

CN1 4 MSA Interfaces are Multipurpose and Easy to use

CN2 9 Cheap and Easy to Maintain

CN3 8 Inexpensive and Easy to Manufacture

CN4 7 Easy to Operate

CN5 13 Easy to Store

CN6 6 Be able to withstand multiple years of use/abuse

CN7 3 Be able to handle the weight of MSA and payload

CN8 11 Safe for use by students and faculty

CN9 12 Sustainable Design

CN10 2 Stable platform for the MSA

CN11 5 Standardized attachments for the MSA

CN12 16Standardized geometric tolerances and dimensions

CN13 10Employ manufacturing processes available in RIT Labs

CN14 15Logical and easy to follow manufacturing Instructions

CN15 1 Stable source of power for Controls and MSA

CN16 14 Aesthetically Pleasing

This information, in conjunction with the system level specifications, was used to develop preliminary engineering specifications, shown in table 2a and 2b, that guided our decision making process during the design selection procedure. The systems level specifications were developed by the systems level engineers in response to the overall customer needs.

Table 2a. Table of Engineering SpecificationsEngr.

Spec. #System Spec

Chassis CN Description

1 1.2 3/13 Chassis Cost per mass production

2 1.3 3/13 Chassis Cost per Prototype

3 2.1 5/7 Unloaded LVE Weight

4 2.2 7/10 Able to support specific payload weight

5 2.3 4/7 Move LVE at adequate speed

6 2.4 4 Turning radius

7 2.5 5/10 Height of Chassis

8 2.6 5/10 Base Area of LVE Platform

9 2.7 4/7 Ability to travel up incline

10 2.8 6 Able to Withstand being dropped from minimal height

11 3.1 2/4/15 Run Time

12 3.2 2/4 Recharge Time

13 4.1 8 Surface Temperature

14 4.2 8 Number of tissue layers torn through when run against edge

15 4.3 9 Minimize Material Waste

16 4.4 3 Max. Lead Time for parts

17 4.5 12 Use limited number of screw types

18 4.5/4.7 9 Minimize machined parts per LVE

19 5.4 16 Limit exposed wires

Table 2b. Continuation of Engineering SpecificationsEngr.

Spec. # Metric Value

Importance

1($/

Unit)<15

0 1

2($/

Unit)<25

0 2

3 (lbs) <10 6

4 (lbs) <5 5

5 (mi/h) >0.5 10

6 (in) <12 8

7 (in) <8 16

8 (in^2)<14

4 7

9 deg 15 17

10 (ft) 3 4

11 (min) >90 3

12 (hr) <4 9

13 (°F)<13

0 12

14 (Count) <3 13

15 (lbs) <1 19

16(Week

s) <2 18

17(Count

) 4 14

18(Count

) <20 15

19(Count

) <5 11

The assumptions made during this design included that the LVE will be run on industrial carpet, hardwood flooring, concrete and rubber track. It was also assumed that the only obstacle that would be encountered would be a 15 degree incline. It would be battery operated, wirelessly controlled, with additional functionality of autonomous control in the future. Lastly, we assumed that we would use permanent magnet DC motors.

Several concepts were generated and rated using various grading criteria. Concepts produced were a Rectangular shape, a Circular shape, a Triangular shape, a Car shape with Flat Roof, a “Pick-up”shape with MSA Mounted in the Bed, and a Trapezoid shaped vehicle when viewed from the side. Some of the concepts that we modeled are shown in Figure 1a-c. The selection criteria was “Ease of Manufacturing”, “Aesthetically Pleasing”, “Material Waste”, “Storage Ability”, “Internal Space Utilization”, “Platform Surface Area”, and “Cost”. These were each weighted and then graded for each design. We also followed

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this process for deciding on materials, MSA attachment method, wheel mounting position, battery type and drivetrain/steering method.

Figure 1a. Circular Shape

Figure 1b. Rectangular Shape

Figure 1c. Pickup Shape

The concept chosen was the “pick up” with two wheel differential steering and castors. The “pick up” went through several iterations to meet ours as well as the other LVE family groups’ needs. In the end the final design was a “reverse pickup” with the front wheels being the differential drive wheels and the rear wheels being castors. The MSA platform or the “pickup bed” is mounted at the front to allow for easier placement of the MSA when the vehicle is driven. The “cab” of the pickup is where the control boards and RF board are mounted. The battery is located within the chassis under the MSA platform. The motors are mounted under the chassis attached using motor mounts. The finalized concept can be seen in Figures 2a-b.

Figure 2a. Final Prototype Exterior

Figure 2b. Final Prototype Interior

During the concept selection process, there were a number of physical system needs that had to be

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determined, such as the torque and speed requirements for the motors, battery capacity, clearances, layout of components and structural integrity.

To determine the motor torque requirements the total weight of the LVE needed to be estimated. This was accomplished by using the maximum allowable weight of 10 pounds for the LVE Chassis and 5 pounds for the MSA. Using a factor of safety of four, the torque required to move this weight was calculated to be 25.5 in-oz. The next step was to understand the terrain the LVE would traverse. The most demanding task would be to climb an incline of fifteen degrees. The following calculations show how we came to the required torque of 25.5 in-oz for flat ground with a factor of safety of four and 112 in-oz for the fifteen degree incline with a factor of safety of two because it was not as critical of a costumer need.

The factor of safety required was determined using table 3, as recommended by the faculty advisor.

Table 3. Table used to determine requisite factor of safety

ConsequencesLow High

Uncertain 1.2 2Certain 2 4

Rolling Torque:

1) T=C r N f∗r

Static Torque:

2)

T=msystem ar+ Iα=msystem ar+12

mwheel r2( a

r )Incline Torque:

Figure 3. Incline Free Body Diagram

3)T=[msystem a+msystem gsin (θ ) ] r=msystem r [a+gsin (θ ) ]

Speed Requirement

The speed required in the technical specifications was stated as 0.5 mph. When choosing motors appropriate for the system, both torque calculated and max speed were taken into account. This was determined through the use of the torque-speed characteristic equation of the motor, with the Ka and ra values determined from the manufacturers motor specifications. Ka is the motor constant supplied by the manufacturer and ra is the resistance of armature. Figure 4 shows the graph of torque versus speed for the given motor, which ensured that it would meet our speed requirements at the system voltage level.

4) wr=UaKa

− TKa2 ra

Figure 4. Plot of Motor Torque Versus Speed

Battery Capacity

Battery capacity was driven by the customer requirement that the LVE must operate for a set length of time. To ensure that this was met, the estimated current draws from the various components and subsystems was determined, with a factor of safety of 2 included. With this information the necessary battery capacity could be determined by simple multiplication, giving the minimum required battery capacity to meet the customer requirement.

5) Itotal=Imotors+ Imsa+ Icontrols+ Irf6) BattLife=BattCapacity /Itotal

Battery Type Selection

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Battery type was driven by the need to support the necessary power requirements, cost constraints, and ease of use. Lithium-Ion, Ni-Cad, Ni-MH, and Lead-Acid batteries were examined. Ni-MH was chosen because it offered the best combination of cost, ease of use, and power supplied. The Lithium-Ion batteries were too expensive when factoring in the necessary monitoring circuits, and lead-acid batteries would have necessitated an increase in weight and driven up other costs in the design.

Wheels: Interface

The selection of the drive wheels for the LVE was an involved process that included several driving factors. The most critical being the ability to meet the max torque requirement to traverse an incline of 15 degrees. This calculation would rely on the overall mass of the LVE and the radius of the wheels. Figure 5 shows the max torque vs. radius graph that assisted in the proper wheel selection. The yellow data point shows that the 45mm radius wheel selection would require an 84 in-oz. motor. Next, the drive wheels would have to meet the 0.5 mph top speed requirement for the LVE. An analysis was done comparing the RPM of several motors vs. several wheel radii. Lastly, the cost was a large driving factor in the wheel selection. After completing and reviewing our analysis for these factors, the 90x10mm Pololu wheels were selected for the LVE.

Figure 5. Torque Required for 15 degree Incline per Motor

Figure 6. Wheel with Motor and Hubs

Material Selection

The materials considered for the construction of the LVE were aluminum, steel, carbon fiber, plastic and titanium. The selection criteria were weight, cost appearance, durability and machinability, with an emphasis placed on cost and durability. A concept matrix was generated and lead to the decision to use aluminum and steel for the LVE base platform and uprights, and plastic for the LVE body panels. The reason aluminum was not used exclusively for the structural components were due to a concern expressed by the customer of the durability of the threads in the aluminum components. Plastic was used for the body panels in place of aluminum due to reduced weight, cost and machinability. The ability to use the laser cutter to cut out the RIT logo in the back body panel was a very attractive option that would help satisfy the customer needs.

Figure 7. Lasercut Back Panel with RIT Logo

Machining

Four primary machining processes were used in the construction of the LVE. The base platform and uprights were machined using the band saw and mill. The body panels were machined using the laser cutter and break. The machining time was approximately 20 hours in total, with the steel and aluminum components consuming approximately 16 hours and the body panels consuming the remaining 4 hours. The parts were produced using the mechanical drawings generated from the LVE CAD model.

Clearances/Layout of Components

The size and shape of the LVE presented challenges when designing the layout of the various internal components of the device. The electronics were concentrated in the tower section of the LVE, which resulted in a very confined space needing to accommodate three boards, and their associated wiring and connectors. In addition to the physical volume constraints, the location of the casters on the bottom presented challenges for the proposed mounting locations of the boards, with the desired layout

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interfering with the caster plates. The casters were eventually moved, which allowed the boards’ supports to clear the plates. The front of the LVE (the area below the MSA attachment) was dedicated to the battery compartment to avoid damaging components with the battery. Additionally, the low height of the front section limits what could be placed in it, even without the constraints posed by the battery. A partial internal layout can be seen in figure 8, which shows the internal location of the main components that needed to be accommodated.

Figure 8. Internal Electrical Layout of LVE

MSA Interface

The MSA Interface was the major mechanical interface the needed to be managed across team boundaries. It was a very fluid interface, with many changes over the course of the project in response to customer requirements and manufacturing concerns. Initially, the chassis was to include the plate for the MSA to mount to, with the interface itself being the holes in the plate. This was adjusted when it became apparent that the customer would require more MSA units then chassis units so that the MSA now included the mounting plate. This allowed the plate to be used independently of the chassis for construction, and then easily attached to the chassis when the students were ready to test. Following this change, there was still discussion back and forth between the teams as to where the mounting holes for the plate would be and what type of connectors would be used.

Stress Analysis (FEA Analysis)

Stress analysis was performed on both the base plate of the chassis as well as the upright supports used in the final design. The uprights were loaded with 5 lbs. normal to the top face (Figure 9) and resulted in a maximum deflection of 9.7 x 10-7 in and a peak stress of 8.5 psi.

Figure 9. Stress of loaded Vertical Support

The base plate was loaded with 5 lbs equally distributed over the msa support attachment points and 0.33 lbs over the “cab” attachment points, with the wheel mount locations held constant (Figure 10). This resulted in a max deflection of 1.4 x 10-4 in and a peak stress of 102 psi. All forces were calculated from the expected weight at those point times a factor of safety of 4.

Figure 10. Stress of loaded Base Plate

Stress Analysis of Unibody Design

A unibody design was proposed by the customer to reduce cost. The following is an analysis of the feasibility of the design. A normal force of 10 lb. was applied uniformly on 4 faces, with the same wheel mounting locations as on the base plate analysis held constant (Figure 11). This produced a max Von Mises Stress of 3554 psi, and a max deflection of6.9205 x10-4 inch.

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Figure 11. Stress of loaded Base Plate

Additional Considerations for the Unibody Design:Problems:

1. Difficult to maintain internal components2. Cannot replace individual structural

components – if chassis breaks, an entirely new chassis is needed

3. Difficult to bend multiple sides in metal bender

4. According to Baja-SAE Project Manager, ability to bend on campus is non-existent

5. Lack of internal clearance for tools6. Need to modify MSA attachment (pins are

not possible)7. Steel will require protection to prevent

rusting8. Difficult to keep hole tolerances on bent plate9. Difficult to machine on RIT equipment due to

size limitations10. Unibody part is more expensive then parts it

would replace (~$92 oppose to ~$40)

Advantages:1. Less Parts2. Similar weight characteristics to current

design3. Will support MSA weight

RESULTS AND DISCUSSION

In order to validate our project and make sure it meets the needs of our customer we devised a list of testing criteria to put the LVE Chassis through. All of the tests were derived from our Engineering Specifications. Each tests’ purpose was to ensure that we developed what the customer wanted. The testing was completed with the Chassis as a complete subassembly. This means that the testing was done without the controls or MSA. The complete system assembly testing with controls and MSA is to be carried out by the Systems group. The following tables show the tests and the results. In the end we successfully passed all of our tests except for two.

There was a failure during the drop test as well as not meeting the mass production cost. We were over the mass production cost by $20.33.

Table 4a. Verification TestingTest # Test Name Related Engineering

Specification Pass Criteria

1 Drop TestLVE will be able to

withstand being dropped from 3 feet on the wheels

LVE shows no signs of fatigue and passes functional test after

being dropped from 3 ft on the wheels

2 Payload Weight

LVE will support a specific payload weight of

5 lbs

LVE platform will support 5 lbs with no obvious deformation while loaded or upon

removal of load

3 Unloaded LVE weight

The unloaded LVE will weigh < 10 lbs LVE weighs < 10 lbs

4Base Area

of LVE Platform

The LVE Platform will be less than 144 in2

LVE Platform is > 144 in2

5 Tissue Test < 3 Tissues torn through

< 3 Tissues torn through when tissue is

ran along each exposed edge

6Minimize Material Waste

< 2 lbs of reusable material waste at the end

of machining

< 2 lbs of reusable material waste at the

end of machining

7Limited

number of screw types

Use < 4 screw types for LVE Chassis

< 4 screw types used for LVE

8Battery

Recharge Time

It will take < 4 hrs to recharge LVE Battery

< 4 hrs to recharge battery

9 LVE Run Time

The LVE will run for > 90 minutes

It will take > 90 minutes to drop battery below 7V

10 Unloaded LVE speed

Move LVE at speeds > 0.5 mph

LVE maintains a top speed > 0.5 mph

11 Turning Radius < 12 Inch Turning Radius

The LVE will be able to turn within a 2 ft

diameter circle

12 Surface Temperature

All exposed surfaces will be < 130 degrees F

Exposed surfaces < 130 degrees F

13Chassis

prototype cost

The LVE chassis prototype cost will be <

$250

LVE prototype cost < $250

14

Chassis Per Unit Cost

(Mass Production)

The LVE per unit mass production cost will be <

$150

LVE mass production cost < $150

15Maximum

lead time for parts

Maximum lead time for parts will be 2 weeks

Lead time for parts < 2 weeks

Copyright © 2011 RIT Senior Design Team P11211

Table 4b. Testing ResultsTest

# Test Name Results Pass/Fail

1 Drop Test

LVE ran for one ten foot straight-line pass and then the gearbox

stripped a gear

Conditional Failure

2 Payload WeightLVE platform

supports 5 lbs with no deformation

Pass

3 Unloaded LVE weight 8.102 lbs Pass

4 Base Area of LVE Platform 96 in2 Pass

5 Tissue Test< 3 Tissues torn through for each exposed edge

Pass

6 Minimize Material Waste 1.06 lbs Pass

7 Limited number of screw types 2 Screw types Pass

8 Battery Recharge Time 1 hr 10 min Pass

9 LVE Run Time Observed 90 minutes Pass

10 Unloaded LVE speed 1.104 mph Pass

11 Turning Radius LVE stays within the 2 ft diameter circle Pass

12 Surface Temperature

111 degrees F at motor Pass

13 Chassis prototype cost $204.21 Pass

14Chassis Per Unit

Cost (Mass Production)

$170.33 Fail

15 Maximum lead time for parts

All parts < 2 week lead time Pass

CONCLUSIONS AND RECOMMENDATIONS

Overall, the design met the majority of the customer needs. It is capable of supporting the desired attachment and operating successfully, in addition to being manufactured primarily in-house. It did not, however meet the customer’s mass production budget requirement (overall, the system was over budget by ~10%, not including shipping costs). This led to redesign proposals that focus on driving down overall cost.

Some of the drawbacks of our design are that it weighs more than other alternatives considered; this leads to the requirement of larger motors and battery. This weight also puts a larger amount of stress on the motor mounts, and the shaft protruding out of the gearbox on the end of the motor. This design had a larger number of parts and overall more complex machining.

We believe the motor failures were due to the weak gearbox. We assumed the attached gearbox would be robust enough to support the torque output of the motors being that these came together as a unit.

Major changes were instituted to drop the mass production price below the customers’ requirements, starting with changing the major structure to a simple box. This change dropped the overall weight of the chassis dramatically (8.1 lbs down to 4.8 lbs), which allowed for additional changes. This includes a reduction in the size of the motors, which proved to be

a major cost savings. If all the proposed recommendations are implemented, a total savings of $448.75 would be realized, leading to a per unit price of $125.46 for 10 chassis.

These changes aren’t without their tradeoffs however, such as the need for modifications at the MSA attachment points and a less aesthetically pleasing robot. The customer must weigh these considerations and make a final decision about which path they wish to pursue. Improving upon the existing design with better direction from the customer will be a project for a future MSD team.

ACKNOWLEDGEMENTS

We would like to thank the RIT Mechanical Engineering Department, Dr. Edward Hensel, Phil Bryan, Leo Farnand, Vince Burolla as well as Robert Kraynik, and Steven Kosciol of the ME machine shop.

REFERENCES

1) Hibbeler, R. C., and P. Schiavone. Engineering Mechanics Dynamics. 11th ed. Singapore: Pearson/Prentice-Hall, 2007. Print.

2) "Pololu - 154:1 Metal Gearmotor 20Dx44L Mm." Pololu Robotics and Electronics. Web. 29Apr.2011.<http://www.pololu.com/catalog/product/1109>.

3) "All-Battery.com: Tenergy 7.2V 3000mAh High Power Flat NiMH Battery Packs W/ Tamiya Connectors for RC Cars." All-Battery.com - Rechargeable Batteries & Chargers. Web. 29 Apr. 2011. <http://www.all-battery.com/72v300mahflatnimhhighpower38adrainratebatterypackswithtamiyaconnectors11120.aspx>.

4) McMaster-Carr. Web. 29 Apr. 2011. <http://www.mcmaster.com/>.

5) Jones, Joseph L., and Anita M. Flynn. Mobile Robots: Inspiration to Implementation. Wellesley, MA: A.K. Peters, 1993. Print.

Copyright © 2011 RIT Senior Design Team P11211