Proceedings of the Multi-Disciplinary Senior Design Conference Page 7
Project Number: P11211
Copyright © 2011 RIT Senior Design Team P11211
Proceedings of the Multi-Disciplinary Senior Design Conference Page 7
Land Vehicle for education: chassis, Motor, Power
Jonathan Fabian - EE / Ryan Sutton - MEJesse Keyser - ME / Matthew O’Neill – ME
Copyright © 2011 RIT Senior Design Team P11211
Proceedings of the Multi-Disciplinary Senior Design Conference Page 7
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 / DescriptionCN1 / 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 / Description1 / 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 / Importance1 / ($/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^2) / <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 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
ConsequencesLow / High
Uncertain / 1.2 / 2
Certain / 2 / 4
Rolling Torque:
1) T=CrNf*r
Static Torque:
2) T=msystemar+Iα=msystemar+12mwheelr2ar
Incline Torque:
Figure 3. Incline Free Body Diagram
3) T=msystema+msystemgsinθr=msystemra+gsinθ
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-TKa2ra
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+Irf
6) BattLife=BattCapacity/Itotal
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