Multi-Disciplinary Engineering Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: P10201

LAND VEHICLE INTEGRATION PLATFORM

Christopher Wakeley / Project Manager, Andrew Coulter / Payload design, Albert Kuramshin / Power Source Design, Daniel Vick / Interface Manager, Raymond Casper / Strength Analysis

ABSTRACT

The mechanical engineering department at RIT has been trying to develop a robotic platform to help engineering freshmen learn different engineering concepts such as stability. The robotics project has been going on for a couple years and each year the project is given to senior engineering students to create a robust, working robot. The goal of this team is to design a well built chassis for all the components to fit on the chassis and then integrate all of those components to create a working robot. The team interfaces with the motor module team that is responsible for providing the wheels and motors to move the robot, the motor controller team which is responsible for the circuitry that powers the motors, and the wireless command and control team which is responsible for providing a wireless system to control the vehicle.

INTRODUCTION

Introduction: Land Vehicle 1 (LV1) project is a second generation of the family of 1kg land vehicle platforms. The overall goal of the LV1 project is to create an easy-to-use, 1kg, modular robotics platform to be used by freshman engineering students as a learning tool. LV1 project is comprised of several teams: Motor Module (P10202), Motor Controller (P10203), Wireless Command and Control System (P10205) and the Platform Integration team (P10201).

The goal of the LV1 Platform integration team is to integrate current and future robotic platform motor modules and motor controllers while improving previous robotics platform design in order to meet all required customer needs.

NOMENCLATURE

LV1: Land Vehicle 1kgRIT: Rochester Institute of Technology

OVERVIEW

Customer Needs

As the initial phase in the design, the customer needs had to be identified.

(1) The design must reuse as many parts from previous designs as possible.(2) The design must look professional and well organized.(3) The design must be same size or smaller than the previous generation.(4) The Vehicle must be stable on flat surface throughout its operation range.(5) The platform must be able to secure and carry a payload of 1 kg.(6) The design must be open source and open architecture.(7) Power source of the design must be portable and shall be able to power the system for a useful period of time.(8) The design must have configurable ground clearance.(9) Vehicle should be simple enough for freshman RIT students to assemble and operate.(10) There should be a variety of ways in which the platform be able to be assembled.

Engineering Specifications

After the customer needs have been identified they have been translated into the engineering specifications so the design process could be started. Engineering Specification document has been created. The full engineering specifications can be found at http://edge.rit.edu/content/P10201/public/Home under the detailed design header. Meetings with the Motor Module Team (P10202), Motor Controller Team

Copyright © 2010 Rochester Institute of Technology

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(P10203), Wireless Command and Control System (P10205) were held to identify the interface specifications, the interface specification document was created.

Interface Specifications

For the robot to function and be put together there were several interfaces that the team needed to deal with. The interfaces are separated into two types: electrical and mechanical. The electrical interface is to deliver power to all the subsystems of the robot. The subsystems include the motor controller and wireless link. Mechanical interfaces are to allow the subsystems to be secured to the chassis to make the robot. The mechanical interfaces were the mounting and placement of the motor controller, motor modules, and the wireless link. The document found at http://edge.rit.edu/content/P10201/public/Home under the design detail header. The document was constructed to allow all the team involved to develop their project and know that their projects could be integrated together to build the robot.

Concepts

Figure 6. Picture of the finished chassis concept.

On the next stage of the design process was concept selection. At the beginning of the process team has examined the materials left by previous teams. Seeing an example has facilitated concept generation process. To approach the concept selection more structurally the concepts were generated individually for each sub-component of the design: power source, frame, payload adapter and interfaces.

At the concept selection process most of the generated concepts were discarded for being unreasonable. Pugh chart, was used to select the most optimal solution out of the remaining. Factors that were considered in concept selection were customer needs, engineering specifications and interface specifications as well as overall functionality.

Power Supply Analysis

One of the customer needs was a portable power source. Multiple concepts were selected to meet that criterion such as battery, fuel cells, portable generator many others. After ranking the concepts using the Pugh chart it was clear that using battery was the most optimal solution. The battery concept was then broken down into a wide range of sub-concepts based on battery chemistry. Concept selection was then reapplied for the sub-concepts and it became clear that Li-ion chemistry would be the best fit for the application. Most benefits of the Li-ion chemistry were the ability to be recharged and had the highest energy density. Next it was important to select the supply voltage. Originally power supply voltages of 11.1V and 14.8V were considered in order to provide backwards compatibility with an older generation. However once the motor team selected their motors to run at typical 6V, the power supply voltage was reconsidered to avoid unnecessary power loss at conversion as well as extra cost of regulators and the cost associated with having a more powerful battery. For those reasons it was decided to supply 7.4V. Design backward compatibility would be ensured by using older generation power supply when using chassis on the older generation vehicle. To select the final concept various 7.4V Li-ion batteries from different vendors were compared on the basis of monetary cost, mass, volume and shape. The final selected concept was then double checked against system’s power requirements and maximum current draw. It was also made sure that the battery was under the allocated monetary budget weight budget and space. The final concept seemed to satisfy the requirements in the most optimal way.

Gantry Strength Analysis

Using Solid Works the appropriate 3D model was generated and used as the model imported into ANSYS Workbench. From here the appropriate material properties were applied as per matweb.com for 6063 T5 Aluminum. To satisfy the drop test requirement we came to the assumption that a 0.08s deceleration rate, less than one tenth of a second was going to be used. This resulted in a 10g deceleration rate. Once this was known the 1kg payload could be converted to a pressure distribution across the top surface. This ended up being 98N over the 0.0045m2

area. As seen in the results below, the maximum Von Misses Stress seen is 19.1MPa, a factor of safety of 7.6 from the 145MPa yield strength. Also the maximum deflection is about 0.1mm, which is negligible.

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Figure 1. Simulated Strength analysis of the Gantry and frame.

Base Plate Analysis

In order to model the base plate, symmetry was taken advantage of with respect to all three planes of the plate, greatly simplifying the work to be done by the simulation. To accomplish this, a line was extruded into a plane that was assigned shell elements with a thickness of 1/32”, half of that of the full plate, and a cross section of the rail was extruded into a volume. These were split into a series of areas and volumes that all contained four (or 6 for volumes) sides for even meshing, which increases the accuracy of the result.

To determine the strength of the base plate after being subjected to a 3ft drop test, an acceptable impact time was first determined to be under one tenth of a second, namely 0.08s. From this deceleration time, a deceleration rate of approximately 10g’s was calculated, and this rate was used to determine the loading under several circumstances. Three situations were considered, the effect of the battery on the base plate by itself, the force exerted on a rail by a single block clamp, and the impact of a single block pressing down on the rail during a drop.

For the force the battery exerts on the plate, the rate of acceleration was combined with the mass of the battery (0.2kg) to get a total force of 19.92N. This force was divided by 4 due to symmetry and converted to pounds, and a force of 1.1027lbf was applied to an area of 1.03125in2. The resulting load applied was a pressure of 1.069psi to the area occupied by the battery. The edges of the plate were secured in all degrees of freedom to represent the interface with the frame, and symmetry boundary conditions were applied to the edges of the plate. While the Deflection at the center of the plate exceeds 1/16”, the maximum stress is 828psi, compared to a yield strength of the material of 1200psi. This shows that the flexibility of the Nylon and the ability to withstand a shock load. This model is also a worst case scenario because it ignores any rigidity added by the rails and the components bolted to the plate.

In order to model the loading applied to the rail by the blocks, two different loading scenarios were considered. First the possibility of a block pressing down on the rail from above was considered, and then

the possibility of a block being suspended from the rail was considered. To calculate the load applied to the rail, symmetry was used to place the block in the worst possible location, directly centered in the rail. For this reason an area of 3/8” x 3/16” was used to apply the load down onto the base of the rail, and the largest mass component (controller = 0.4kg) was used, divided among four locations. Using these assumptions, a pressure of 15.68psi was applied to the area to simulate a 10g load pressing down on the rail. The maximum deflection in this case is minimal, only 0.00002”, and the stress is only 94psi, showing that when combined with the battery loading in the center of the plate, the stress will still be within the elastic range. This is a result of the rails spreading the load out to the sides of the plate.

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YZ

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STEP=1SUB =1TIME=1DMX =.287E-04

Figure 2. Stress analysis of the rails when a stress is applied downward on the rail.

For the block pulling out of the rail the same force

was used, one corner of the controller under a 10g load, but the area was found differently. The area used was the same as that used for the block pushing down, with the open area at the top of the rail subtracted, resulting in a pressure of 43.81psi.The highest risk case, the block pulling out of the rail was considered in a worst case sense by ignoring the friction generated by the clamping force placed on the rail by the block and spacer. The result was a maximum stress of 166.7psi, meaning the rail would not be permanently deformed. The magnitude of the deflection was 0.000665” in either direction, which amounts to a remaining contact width of 0.1237”, assuming an initial contact width of 0.125”.

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55.5674.08

92.599111.119

129.639148.159

166.679

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STEP=1SUB =1TIME=1SEQV (AVG)DMX =.755E-03SMX =166.679

Figure 3. Stress analysis of the rail when pulling out on the rail.

Payload Analysis

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For simplicity and ease of design, the payload interface plate was designed to be identical to the base plate. However, instead of having a set of rails on the top and bottom, the payload plate only has one set of rails on the top. The plate was then screwed onto the top of the frame with four holes drilled through it.

In regards to the strength analysis, the payload adapter plate is considered to be identical to the base plate. There was no need to analyze it with the added components such as the battery. It was found that the plate could easily support a 1 KG load.

Interface Analysis

In order for the robot to work several interfaces had to be discussed as far as how they would work. The interfaces that are of major concern are the chassis to the motor controller, to the motor module, the wireless link and the payload.

Figure 4. Base plate shows the rails that the controller boards and wireless link will mount to.

Figure 5. Base plate that shows the interface to attach the battery to the chassis.

Motor module interface

Figure 6. Motor mounting plates that the motor modules will mount to.

The purpose of the motor module interface is to provide the robot with a way to move. The interface was designed keeping last years design in mind. The interface used the spacing of the mounting plates hole distance the same. This way the chassis would be able to be integrated with both this years and last years design. The interface that was design to be used with last years model and future models is a steel plate that is 2in x 4. The plate will have holes that will allow the plate to be mounted to the side of the chassis frame and then allow the motor module to mount to the plate. This will ensure that the motor module will interface with the new and old modules. The plates are designed also to allow the robot to change its ground clearance depending on the mounting position.

Motor controller interface

There are two interfaces that the chassis had to come up with for integration. The two are providing electrical power to the electrical components of the controller boards and providing a place for the motor controller to mount to.

The mechanical interface was designed so that its modular in nature for any future motor controller design. The new motor controller will be mounted on a plate that the motor controller sits on, that will be screwed down by 8-32 flat head screws that will be secure the plate to the rails. For the older generation the interface will consist of drilling holes out in the base plate and then using screws and spacers to mount the controller boards to.

The electrical interface was design so that the chassis would provide the power via a battery, to the controller boards. The interface consists of switch which will allow the system to be shut off to save battery life, wires which will provide the path for current to flow to the controller boards.

Payload interface

The payload interface was design to be purely mechanical in nature. The payload may require electricity to work but the power source being the battery does not accommodate for this. The battery was designed to supply enough power to the motor controller boards, motors via the controller boards, and the wireless.

The payload was initially designed to be a metal plate the size of the base plate with a lip all the way the edge of the plate to prevent anything from slipping off. The lips or rails are designed to have slots cut out to tie the payload down to the chassis. However, the other design that is to be implemented is, a duplicate of the base plate and drill holes in the top of the gantry and base plates to screw down.

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Wireless link interface

There are two interfaces that the chassis had to come up with for integration with the wireless link. The two are providing electrical power to the electrical components of the wireless and providing a place for the wireless link to mount to.

The mechanical interface was designed so that its modular in nature for any future wireless link designs. The new wireless link unit will be mounted on a plate that the motor controller sits on, that will be screwed down by 8-32 flat head screws that will be secure the plate to the rails. The previous generation will be zip-tied to one of the motor module plates or to one of the legs on the gantry. This will ensure that the wireless link does not move or slip off of the chassis.

The electrical interface was design so that the chassis would provide the power via a battery, to the wireless link. The interface consists of switch which will allow the system to be shut off to save battery life, wires which will provide the path for current to flow to the wireless link unit.

RESULTS

Major specifications of the battery were: output voltage range, energy stored, maximum discharge current, size and mass. To ensure that battery would function well as a part of the whole system, it had to satisfy specifications listed in the engineering specification document and the interface document. To test for that a component level test plan was created for the battery. It consisted of five tests to ensure that the battery is under allocated mass, supplies voltage within the allowable range, can discharge specified amount of energy, is under allocated size and can supply specified peak current. After the battery was received from shipment the testing has begun.

First test was to ensure that the battery is under

the allocated weight budget of 0.8 kg. The test was performed by measuring mass of the battery on the scale. The mass was measured to be 0.2 kg, which has satisfied the pass criteria.

The second test was to ensure that the battery output voltage does not go above the input range of the components of the LV it powers. To pass the test the fully charged battery could not output more than 9V. The battery was charged according to the instructions. After the charging was complete, the output voltage was measured to be 8.5V which has fallen under the pass criterion.

Third test was to ensure that the battery can power the LV for one hour. To pass the test, the fully charged

battery had to provide 25Wh energy before its output voltage would drop below 6V. Fully charged battery was loaded with 2 Ohm resistor and the voltage at the output was measured and recorded every 5 minutes. The experiment has stopped after the battery voltage has dropped under 6V. Recorded data was used to compute the total energy supplied to be 31Wh.

Fourth Test was to ensure that the battery could fit into the designed battery strap. To pass the test the battery had to fit into the 9cm by 4cm by 4cm box. The battery was placed on the table and its dimensions were measured to be 7 cm by 4 cm by 3.7cm. The test was passed.

Fifth test was to ensure that the battery can supply the peak current drawn by the system. To pass the test the battery had to be able to supply 7.5A of current. The battery was loaded with a variable resistor and the output current was monitored. The resistance was slowly decreased to until the battery shut off. The maximum supplied current was recorded to be 5A. The test was failed with the first battery. A second battery was tested that was made to meet the desired specifications. After testing this new battery the peak current was 9 A which passes the test.

Consequences of the failure of the last test were that the battery would shut off when the discharge current would exceed 5A. This would turn off the robot. To deal with problem, battery manufacturer was contacted. The technical support has tested several equivalent batteries and claimed that they can discharge 9A. It was decided that those batteries to be ordered. When they would be received they would be tested. If they are to satisfy the tests, the old batteries were to be returned for a refund as defective. If the new batteries would also fail the last test, two batteries were to be used per vehicle.

The results from the mechanical tests show that the chassis can be built using documentation and built under the given time allowed. The chassis also is smaller than the previous version which meets our specifications for size. After weighing the chassis the mass was determined to be well below the specifications, the mass was 760 grams before the controller boards and wireless were added. The chassis can be assembled multiple times and not fail. The chassis operated in the given temperature range. The 2nd generation components will fit on the base plates after screw holes are drilled out. The analysis showed that all the components will fit on one side. However components can be mounted to both sides with the use of space to ensure clearance of the components on the boards to the base plate. The newer generation components can be mounted to the chassis without any major problems. The one drawback is the amount of space that everything has to

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fit in. The payload adapter plate can hold a load of 1kg with no deformities in the plate or the structure of the robot. The ground clearance range and the minimum ground clearance met the desired specifications after running the test to find the range of ground clearance and the minimum ground clearance. The wheelbase range met the desired specification after the test was conducted. When the chassis was dropped the first chassis sustained several breaks in the weld joints. The second chassis was dropped from three feet and it survived without any damage to the chassis frame.

CONCLUSIONS

Based on the results the chassis designed met all the required customer needs. While customer never specified that the chassis had to survive a drop test, it was implied that the chassis should survive a fall off of a table top which is roughly 3 feet off the ground. The chassis could be made stronger to survive the fall be reinforcing the joints using metal braces. This would allow the chassis to be more durable more rigid. Another way to strength the chassis would be to use a different material that is easy to weld together like steel. Using steel may make the structure more sturdy and durable but the weight may increase too much to the point where the performance of the robot suffers. The same type of aluminum could be us could be used but choose a thicker cut of aluminum. In addition the chassis frame and gantry should be sent to a manufacturer to be professionally welded to increase the strength and appearance of the chassis.

When choosing a battery the manufacturer should be contacted to ensure that the battery meets the specifications that are desired. The manufactures specifications that were listed were inaccurate. This mistake was corrected by contacting the manufacturer and requesting them to send out a new battery t hat has the desired specifications.

Figure 7. Picture of the finished chassis concept.

ACKNOWLEDGMENTS

The team would like to thank our customer for the financial contributions to our project. In addition we would like to thank our guide(Phil Bryan) and our Teaching assistant (Leo Farnand).

Project P10201