Lunar / Mars Rover Suspension

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1 Lunar / Mars Rover Suspension HW1: A Plan for a Simulation-Based Study ME 6105 Modeling and Simulation in Design January 30, 2007 Nathan Young Stephanie Thompson Robert Thiets

2 1. The Decision Situation NASA s Vision for Space Exploration includes a goal to return to the moon by the year 2020 as a launching pad to manned exploration of Mars and other planets in future decades[1]. To meet this goal, the recommendations of the Exploration Systems Architecture Study (ESAS) include a strategy in which the establishment of a lunar outpost is used also as a test bed for martian technology, known as Mars-forward testing[2]. One important task for both lunar and martian exploration is the design and development of unpressurized and pressurized rovers for use in both environments. In this project, we will concentrate on the design of the suspension for a manned lunar rover with extensibility to the Martian landscape. Specifically, we seek to provide a recommendation for a passive or semi-active suspension and the target values for effective spring rates and dampening coefficients. For the purposes of this course, we assume that we are part of a larger rover design team, and that our group has been tasked with the design of the suspension system. Our group has the authority to determine the relevant specifications of the suspension system, including, for example, spring rates and damping ratios, as well as the layout of the system. Interfaces with other rover systems will be captured in given parameters and constraints relating to the payload capacity and variations, allowable levels of shocks and vibrations and other important factors. 2. Objectives Hierarchy The fundamental objectives for the design problem are organized into the hierarchy shown in Figure 1. A suspension which maximizes the research value of a lunar or mars rover has been determined to be the most fundamental objective. In order to maximize research value, the suspension must maximize reliability, safety, and launchability. Safety is maximized by maximizing the stability of the rover and response to user input while at the same time minimizing the unwanted acceleration of payload, vibrations caused by terrain, and the vertical travel of payload. The maximum launchability of the rover in achieved by minimizing the size, cost, and weight. The means objectives of the design problem are organized into the network shown in Figure 2. The key objectives which will be used in the design study are: maintain correct vehicle ride height, dampen vibrations, dissipate impact energy, maximize reliability and maximize response to user input. As shown in the means objective network, this combination of key objects should be sufficient to reach the fundamental objective of maximizing research value. These keyobjectives have attributes which can be measured and will be discussed in subsequent sections. Page 2 of 10

3 Figure 1. Fundamental Objections Hierarchy for Rover Suspension Figure 2. Means Objective Network for Rover Suspension Page 3 of 10

4 3. Identification of Design Variables After developing a means objective network and functional objective hierarchy, we have identified primary design variables which represent the objectives within our design problem formulation. Before we can explain the design variables which comprise our decision alternatives, a definition of decision must be posed. A decision is a present action to achieve a future outcome and an irrevocable allocation of resources that would take additional resources, perhaps prohibitive in amount, to change the allocation. The design variables include the suspension control strategy (active, semi-active, or passive), effective spring rate, damping considerations, and the suspension geometry. 3.1 Suspension Control Strategy For this design variable, we will determine the means of control with respect to the suspension system. Current methods to control overall system response include a completely passive system, the use of a controller board, or actuators. Passive suspension systems consist of conventional springs and shock absorbers such as those used in most cars. The springs are assumed to have almost linear characteristics while most of the shock absorbers exhibit nonlinear relationship between force and velocity. In passive systems, these elements have fixed characteristics and hence, have no mechanism for feedback control [3]. Semi-active suspensions provide controlled real-time dissipation of energy [4]. For an automotive suspension this is achieved through a mechanical device called an active damper which is used in parallel to a conventional spring. The main feature of this system is the ability to adjust the damping of the suspension system without any use of actuators. This type of system requires some form of measurement with a controller board in order to properly tune the damping [5]. Active suspensions employ pneumatic or hydraulic actuators which create the desired force in the suspension system [6],[7].The actuator is secured in parallel with a spring and shock absorber. Active suspension requires sensors to be located at different points of the vehicle to measure the motions of the body suspension system and or the unsprung mass. This information is used in the online controller to command the actuator in order to provide the exact amount of force required. Active suspensions may consume large amounts of energy in providing the control force; and therefore, in the design procedure for the active suspension the power limitations of actuators should also be considered as an important factor [5]. 3.2 Effective Spring Rate The effective spring rate represents a lumped spring rate and is denoted by k (lb/in or N/m). This lumped parameter includes contributions from a spring, deflection of suspension structure, and the tire. One effective spring rate will be determined for each wheel of the rover. Page 4 of 10

5 3.3 Damping Damping refers to the energy dissipation by a specified system to control the recoil of the spring upon loading. As mentioned above in the suspension control strategy discussion, the type of damping is often related to the type of system control chosen to regulate damping. The various types of dampers relating to these controls are the hydraulic/piston assembly, actively controlled hydraulic/piston assembly, and hydraulic or pneumatically actuated system. 3.4 Suspension Geometry There are two types of suspension geometry that we are considering: kinematic geometry and component geometry. By kinematic geometry, we refer to the structural configuration and relative orientation of struts, shock absorbers, springs, and other system components. Component geometry refers to specific geometric parameters such as cross sectional area and length. The component geometry is usually dependent upon the kinematic geometry and overall system architecture. 4. Identification of Design Problem Structure The design problem structure is formulated in terms of an influence diagram. This structure, shown in Figure 3, represents the chance events and intermediate computation outcomes that influence the decisions that we make to meet our objective of maximizing safety. 4.1 Chance Events Chance events are events which affect design decisions but cannot be controlled by the designer. This requires the designer to determine which events are in fact measurable and which are required to be lumped. In the context of the rover suspension design, chance events include payload, the operating environment, terrain, and human factors Payload The rover suspension is a general purpose suspension for a variety of lunar and martian applications. Due to its generalized purpose, the suspension system must be robust to various loading conditions. Specifically, the suspension must meet the design objectives despite variations in the mass of astronauts, cargo, and scientific payload aboard the rover Environment To comply with the Mars-forward testing strategy recommended by the ESAS report, the suspension system must be useful in both a lunar and martian environment. Variations in gravity and atmosphere between the two environments must be considered Terrain Both the lunar and martian terrain are unpredictable with respect to the actual topography. This topographical variation represents a chance event with respect to the suspension operation. Page 5 of 10

6 4.1.4 Human Factor Finally, the human factor is a lumped chance event which represents the variation of the human operator including driving habits (braking and acceleration), steering and possible errors. 4.2 Intermediate Computation Outcomes After the determination of the design variables, measures of effectiveness were specified to quantify the attainment of objectives. In this context, the measures of effectiveness for our rover suspension system include the settling time, rise time, variance of ride height, percent overshoot, and Roll Couple Percentage Settling Time Settling time is the time required for a response to reach and stay within 2% of its final value [8]. In terms of our suspension design, this represents the required time to damp vibration energy to an acceptable value. In the context of the rover suspension, this metric should be at a minimum, meaning that vibrations in the suspension quickly dissipate Rise Time Rise time is defined as the time for the waveform to go from 0.1 to 0.9 of its final value [8]. This measure of effectiveness represents the time for the suspension system s spring to reach 90 percent of its final value. For our purposes, we try to maximize this value in an effort to reduce the acceleration of suspension system to reduce impact to the undercarriage of the car Ride Height Displacement In our context, the displacement of the ride height is defined as the displacement of the rover s center of mass relative to an expected value for the ride height. By monitoring the displacement of the ride height, we can evaluate the ground clearance of the vehicle and evaluate the deflection of the suspension system Percent Overshoot Percent overshoot is defined as the amount that the underdamped response overshoots the steady-state, or final, value at peak time, expressed as a percentage of the steady-state value [8]. This value represents the deflection of the suspension past its typical steady state value, which would be represented by a completely static system Roll Couple Percentage Roll Couple Percentage is defined as the ratio of sprung weight which will be transferred between wheels during acceleration, braking or cornering. Roll Couple Percentage represents the suspensions ability to absorb body roll and therefore maintain traction. Roll Couple Percentage is calculated by dividing the roll rate (spring rate leveraged by suspension geometry) of the front suspension by the summation of all roll rates. Page 6 of 10

7 Figure 3: Influence diagram of decision elements in the design of a manned lunar/mars rover. 5. Identify the Simulation Scenario for an Energy-Based System Model In this section, the energy-based phenomena and corresponding assumptions are discussed. The physical phenomenon includes a mechanical model with a focus on the vibration response of the system and corresponding assumptions listed in section Energy-Based Models for Design Objectives Maximize Launchability and Safety of Passengers/Equipment: For these objectives, the suspension system will require a mechanical model to minimize weight, maximize stability, reliability, energy dissipation, and vibration. The need for other models such as electrical and thermal models will be determined based upon the evaluation of decision alternatives. Page 7 of 10

8 5.2 Energy-Based Models for HW2 For HW2, we intend to address the mechanical model of our system as it is the core of the suspension and will provide a sound foundation for subsequent phases in the design process. 5.3 Physical Phenomena and Energy Domains Within this mechanical model several physical phenomena and energy domains are evident at this level of abstraction. The physical phenomena evident in our current evaluation are system dynamics, elastic deformation, heat transfer, and basic Newtonian mechanics. The energy domains relate to the aforementioned mechanical model of our system. This model will require a vibrational response of the system and mechanics. 5.4 Assumptions The following list represents current core assumptions that are integral to the development of a simulation model. Load is applied through center of mass. No drag on rover body due to air friction. Constant heat transfer and mechanical properties. Manned rover. Tires are designed based upon suspension and required payload. 6. Plan Assessment In this paper we present our plan for an energy-based simulation study. There are several steps in this plan for which the research team lacks specific knowledge or expertise at this time. The ability of this plan to be executed within the scope of this class is therefore uncertain. The most uncertain steps of the plan are listed below along with contingency plans. 6.1 Assessing the needs and requirements of rover suspension Since no team member has any aerospace expertise, there may be additional aerospace related requirements or constraints which have not been uncovered by our research at this point. If it is discovered that a key issue as not been addressed, the fundamental objectives will be revaluated. If it is determined that the project will still be relevant without the missing requirement, a simplifying assumption will be made and the current plan will still be valid. If it is determined that the issue is critical, the fundamental objectives and means objective network will be updated to reflect the addition. It will then be necessary to make a simplifying assumption to remove the key objective which will have the least impact on the relevance of the model. 6.2 Modeling suspension contact with terrain No team member has any expertise on the interaction between tires and the lunar/martian surface. It is currently assumed that tires will be designed based on the specifications generated by this project and will therefore meet the assumed performance characteristics. If this assumption turns out to be inadequate, a model of an existing lunar rover tire will be integrated into this project. Equations for this model will be obtained from literature. Page 8 of 10

9 6.3 Identifying suspension geometry The suspension geometry will be chosen based on research of previous rovers and will not be the main focus of this simulation-based design study. It is possible that a specific suspension geometry would require alterations to key-objectives or design variables. If these alterations conflict with the existing plan, the feasibility of other suspension geometries will be considered. If no viable configurations are found, the fundamental and means objective will be altered to match a chosen suspension geometry. New key-objectives and design variable would then be created to match the new fundamental and means objectives. 6.4 Accurately representing mars/lunar surface The simulation of the lunar and martian surface requires that the terrain be expressed in mathematical terms. At this time, it is unknown if any mathematical models or terrain data can be found for the lunar and martian surfaces. If no data can be found, a mathematical model of a random surface which would approximate descriptions of typical lunar and martian surfaces would be used. 7. Learning Objectives Nathan Young: In ME6105 Modeling and Simulation in Design, I intend to develop a theoretical understanding of the principles of modeling and simulation as they pertain to the development of both analysis and decision models (while having fun). In developing this understanding, I will learn how to frame design decisions, discriminate between design alternatives using evaluation techniques in the course, develop analysis models, critically evaluate analysis models, better operate in a group setting, and effectively present the results of a simulation model. Stephanie Thompson: In this course this semester, I would like to emphasize the following learning goals: Learn how to model design decisions Learn how to use simulations to support design decisions Learn about modeling methods and how to apply them to design problems By learning how to model design decision and use modeling and simulation to support design decisions, I believe that I will have a better foundation upon which to expand current methods in decision-based design in my PhD research. In addition to these learning goals, I would like to learn how to write a scholarly paper on modeling and simulation in design. To achieve this goal, our group intends to prepare and submit a paper to the 2007 AIAA Modeling and Simulation Technologies Conference and Exhibit. Robert Thiets: My learning objectives for ME6105 include developing an understanding of the structure of design problems and the role of modeling and simulation in design. I would like to learn how to appropriately choose the level of abstraction of a design problem or model. I would also like to Page 9 of 10

10 have a firmer grasp on how uncertainty in a model effects decisions. I would like to become proficient at using the modeling and simulation software to create and solve energy-based models for systems. 8. References 1. The Vision for Space Exploration. 2004, National Aeronautics and Space Administration. 2. Exploration Systems Architecture Study Final Report. 2005, National Aeronautics and Space Administration. 3. Miller, L.R. Tuning passive, semi-active, and fully active suspension system. in IEEE Proceeding of the 27th Annual Conference on Decision and Control Austin, Texas, U.S.A. 4. Crosby, M.J., Karnopp, D.C., The active damper. Shock and Vibration Bulletin, (2): p Taghirad, H.D., Esmailzadeh, E., Automobile Passenger Comfort Assured Through LQG/LQR Active Suspension. Vibration and Control, : p Esmailzadeh, E., Servo-valve controlled pneumatic suspensions. Mechanical Engineering Science, (1): p Wright, P.G., Williams, D.A., The application of active suspension to high performance road vehicles. ImechE, C239: p Nise, N.S., Control Systems Engineering. 4th ed. 2003, Hoboken, New Jersey, U.S.A.: Wiley Page 10 of 10

A Simplified Vehicle and Driver Model for Vehicle Systems Development

A Simplified Vehicle and Driver Model for Vehicle Systems Development Martin Bayliss Cranfield University School of Engineering Bedfordshire MK43 0AL UK Abstract For the purposes of vehicle systems controller