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Multi-Disciplinary Senior Design Conference

Kate Gleason College of Engineering Rochester Institute of Technology

Rochester, New York 14623

Copyright © 2011 RIT Senior Design Team P11211

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 project

MSA – Modular Student Attachment, A student

designed device meant to extend the functionality of

the LVE for educational competitions

WOCCS – 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 communications

H-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 device

FEA – 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


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 Needs

Customer 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 16 Standardized geometric tolerances and dimensions

CN13 10 Employ manufacturing processes available in RIT Labs

CN14 15 Logical 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 Specifications Engr.

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 Specifications Engr.

Spec. # Metric Value Importance

1 ($/Unit) <150 1

2 ($/Unit) <250 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 2̂) <144 7

9 deg 15 17

10 (ft) 3 4

11 (min) >90 3

12 (hr) <4 9

13 (°F) <130 12

14 (Count) <3 13

15 (lbs) <1 19

16 (Weeks) <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

this process for deciding on materials, MSA

Proceedings of the Multi-Disciplinary Senior Design Conference


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

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

Consequences

Low High

Uncertain 1.2 2

Certain 2 4

Rolling Torque:

1)

Static Torque:

2)

(

)

Incline Torque:

Figure 3. Incline Free Body Diagram

3) [ ( ) ]

[ ( )]

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)

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)

6)

Battery Type Selection

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

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 of

6.9205 x10-4

inch.



Figure 11. Stress of loaded Base Plate

Additional Considerations for the Unibody Design:

Problems:

1. Difficult to maintain internal components

2. 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 tools

6. Need to modify MSA attachment (pins are

not possible)

7. Steel will require protection to prevent

rusting

8. Difficult to keep hole tolerances on bent plate

9. Difficult to machine on RIT equipment due to

size limitations

10. Unibody part is more expensive then parts it

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

Advantages:

1. Less Parts

2. Similar weight characteristics to current

design

3. 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 Testing

Test # Test Name Related Engineering

Specification Pass Criteria

1 Drop Test LVE 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

4 Base Area

of LVE Platform

The LVE Platform will be less than 144 in

2

LVE Platform is > 144 in

2

5 Tissue Test < 3 Tissues torn through

< 3 Tissues torn through when tissue is

ran along each exposed edge

6 Minimize Material

Waste

< 2 lbs of reusable material waste at the end

of machining

< 2 lbs of reusable material waste at the

end of machining

7 Limited

number of screw types

Use < 4 screw types for LVE Chassis

< 4 screw types used for LVE

8 Battery

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

13 Chassis 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

15 Maximum

lead time for parts

Maximum lead time for parts will be 2 weeks

Lead time for parts < 2 weeks


Table 4b. Testing Results Test

# 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 Weight LVE platform

supports 5 lbs with no deformation

Pass

3 Unloaded LVE

weight 8.102 lbs Pass

4 Base Area of LVE

Platform 96 in

2 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

14 Chassis 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/72v300mahflatnimhhighpower38

adrainratebatterypackswithtamiyaconnectors1

1120.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.

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