DESIGN AND DEVELOPMENT OF A SUSPENSION SYSTEM USED IN ROUGH- TERRAIN VEHICLE CONTROL FOR VIBRATION SUPPRESSION IN PLANETARY EXPLORATION Arvin Niro College of Engineering University of Hawaiʽi at Mānoa Honolulu, HI 96822 ABSTRACT Interplanetary Exploration has been a big part of NASA s curiosity into exploring what is beyond our atmosphere and NASA s rovers have allowed us to do just that. They are equipped with state of the art technology that allow us to see and measure if life can exist elsewhere. In this project, we aim to design and build a suspension system that can be incorporated onto a rover that was constructed last semester at Kapi olani Community College by Eric Caldwell and Lee Do. This rover features wireless technology and a mecanum drive system that allow it to be extremely maneuverable. However, due to its maneuverability, this generates uncontrollable vibration in the rover. In an attempt to suppress the vibration, a double wishbone suspension design was used with a spring and damper system to reduce and dampen the vibration generated from the mecanum wheels. The result of this allowed a suspension system to be designed and optimized for this specific rover by utilizing SolidWorks and finite element analysis to decide the overall design of the system. INTRODUCTION Suspension systems are commonly seen in automotive vehicles where it is used to dampen and reduce the amount of vibration generated from the ground to the vehicle. This allows passengers to enjoy the car ride without having to worry about getting hurt going over potholes or speed bumps. By using a combination of springs and dampers, the vehicle is allowed to control how much force the chassis sees. The application of a suspension system on a rover that is exploring other planets is a possibility. This is because the amount of sensors and equipment that are onboard the rover may be sensitive to any type of vibration and may case imperfections in the data recorded. This was similar to the case on our rover that I helped build at Kapi olani Community College. In the fall of 2013, I was part of a group that designed and built a rover (Figure 1) at KCC. The rover featured an onboard router that allowed for wireless control and video feed through the onboard webcam, wired through the use of a CAN bus system. The rover also featured the use of mecanum wheels, which are individually driven wheels that have individual rotors mounted at a 45 around the wheel. These wheels allow the rover to maneuver in three directions: forward & back, rotation, as well as side to side. When programed, the wheels move in a certain direction to allow the rover to achieve what is called holonomic motion, which is when the rover is capable of controlling all degrees of freedom, in this case three. One of the problems that the group mentioned when they tested the rover was how shaky the video feed was while they drove the rover, as well as how much the vibration interfered with the sensors that 47
were mounted onboard. This was due to the design of the mecanum wheels. Because it featured individual rollers mounted around the entire wheel, it caused the rover to vibrate each time a roller hit the ground, thus causing the vibration. For this reason, I was tasked to design a suspension system for the rover to reduce the amount of vibration generated from the wheels. Figure 1 Rover built with mecanum wheels (Webcam not shown) METHOD Before designing the suspension, research needed to be conducted to learn about the different types of suspension designs that are currently used. Due to the design of the mecanum wheels, we decided to go with a double wishbone suspension design (Figure 2), most commonly seen in Formula One and off-road vehicles. The beauty of this design is that it is allows each wheel to be independent of other wheels on the vehicle, thus allowing for fine tuning and design of an individual wheel without affecting others. Because our rover features a single gearbox and motor for each wheel, a double wishbone setup provided a perfect solution. Figure 2 Double Wishbone Design Photo provided by carbibles.com The gearboxes on the rover are from AndyMark, a company specializing in robotic components. Using the downloadable CAD files of the AndyMark Toughbox, we were able to determine the exact dimensions of the front plate as shown in Figure 3. Using SolidWorks, a 3D Computer Aided Design Software, allowed us to design the double wishbone suspension as shown in Figure 4. The first design featured equal upper and lower control arms to allow for even vertical movement. An upright connects the two control arms on the outside which will house the ball bearing of the output shaft that connects to the mecannum wheel. This all connects to the redesigned faceplate that will be replacing the current plate on the Toughbox. Universal joints were designed to allow the drive shaft to reach from the gearbox to the wheel, as well as allow for vertical movement while still transmitting the power from the gearbox at various angles. 48
Figure 3 SolidWorks rendering of Andy Mark Toughbox Figure 4 SolidWorks rendering of the first wishbone design for the rover Our designs went through finite element analysis (FEA) by using the built in tool provided by SolidWorks. The part I was most interested in was the new faceplate that would replace the current faceplate on the gearbox. A force of 100 lbs. is applied at a 45 angle to the shaft, which will simulate the force of the shock when it is mounted. Using the Von Mises and Deflection graphs as shown in Figures 5 and 6, we were able to redesign the parts to ensure that they could withstand the force. Once completed, parts were 3D printed (Figure 7) at KCC, to allow for us visually see our design and get a feel for how it will actually behave when mounted onto the rover. When the 3D parts were tested to fit on the rover, there were problems with clearance and design issues about the current design. Therefore a second design was made. Figure 7 ABS 3D Printed parts of the first wishbone design RESULTS The second designs still utilized the equal length control arms, but were extended to allow for clearance as well as mounting of the shocks and dampers. The first design did not account for the mounting points for a shock and damper system. This is due to the fact that these types of components are not common among rovers and therefore had to be searched. When it came to the shock, we found mountain bike shocks that would fit within the design of our suspension system. They are approximately 110mm (4.33 in) long, with a 550 lb. steel spring as 49
shown in Figure 8. These were the stock springs, but there are plans to reduce the stiffness. As for the damper, they were purchased from McMaster-Carr that had dampers small enough to fit (Figure 9 Left). They are approximately 5.28 when extended, and 4.3 when compressed, and can handle up to 112 lbs. of force. Also purchased from McMaster-Carr were new connectors for the dampers, since the original connectors featured a ball-joint end which was not ideal for our setup (Figure 9 Right). Figure 8 Mountain bike shock Figure 9 Damper with ball joint connection (left) and easy adapt clevis connection (right) One of the components that were not accounted in the original design that changed was the manufacturing of universal joints. Originally they were designed specifically for the first design, but due to the high cost to manufacture the custom universal joints, we decided to use off-the-shelf universal joints. The problem with them was that they were too long (Figure 10 Left), and in order to fit them in the design, the length of the control arms had to be extended, and the universal joints had to be machined down (Figure 10 Right) to fit within the design. 1/2 inch diameter rods with 1/8 inch keyways connected the universal joints to the output shaft of the gearbox and upright of the design (Figure 11). The keyway allowed for the power to be transmitted through the universal joints to provide the rotational movement of the wheels, but still allowed for vertical movement during rotation. Figure 10 Original universal joint (Left) and machined universal joint (Right) Figure 11 1/2 diameter shafts with 1/8 keyway and key 50
Once all the new designs were drawn, and analysis was performed (Figure 12-15), the parts were taken to be 3D printed. All of the components were then test fitted into the printed design as shown in Figure 16. Figure 16 ABS 3D Printed parts of the second wishbone design with all components installed From here, the designs will be taken to be machined out of 6061 Aluminum. 6061 was chosen because it is a common material widely used in automotive and aerospace applications, and it also demonstrated during FEA tests that is can withstand the force applied. CONCLUSION As a result, I was able to design and prototype a suspension system for the rover. As mentioned before, the last step will be to machine the parts out of 6061 aluminum and install them onto the rover. Once that is complete, tests can be conducted to see how the system behaves under different loading conditions to allow us to see what can be changed. One of the things that will probably change is the stiffness of the spring. Because it is such a stiff spring, the system will still behave the same way without a suspension system since the amount of force to compress the spring is too high. For that reason, softer springs will be replaced. Although I was not able to successfully build a working suspension system to be used on the rover, I was able to go through the engineering process of designing and prototyping my designs for the rover. This allowed me to see that there are pros and cons to every decision that was made, as well as understand the importance of utilizing software to conduct analysis to save on time and money spent to design a prototype. This project also allowed me to further enhance my fabrication and SolidWorks skills as I had some experience before, but wanted to improve on them. Understanding how FEA works and learning how I can change my designs based on the results was one of the new skills I developed. Also the importance measuring twice, cut once, to allow for perfect fits of the components when I assembled them was key. 51
ACKNOWLEDGEMENTS I would like to thank my mentor, Dr. Aaron Hanai, for all his help, knowledge and assistance in everything that I did on this project, whether it is designing parts in SolidWorks, or understanding the physics or math behind the concepts. Also thanks for helping with the purchase of some parts that I needed for this project. I would also like to thank the Kapi olani Community College STEM program for allowing me to use the facility to conduct my research. Thank you to Lee Do and Eric Caldwell, who were past fellows of the Hawai i Space Grant, and were also part of the project in the fall of 2013, for helping me with additional help with the rover. Thank you to everyone else who gave input on my designs as well as help on where to find parts (William Kaeo). Lastly, thank you to the Hawai i Space Grant Consortium for all your monetary support in funding this project. REFERENCES Longhurst, Chris. Coil Spring Type 1. Digital image. The Suspension Bible. N.p., 13 Apr. 2014. Web. 1 May 2014. <http://www.carbibles.com/suspension_bible.html>. 52
FIGURES Figure 5 Von Mises graph of the first design Figure 6 Displacement graph of the first design 53
Figure 12 Von Mises graph of the second design Figure 13 Displacement graph of the second design 54
Figure 14 Von Mises graph of the second design lower control arm Figure 15 Displacement graph of the second design lower control arm 55