Modeling a Formula SAE Racecar in ADAMS/Car Valid for R3 and 2010 Versions
Part 1: Suspension Joseph Little Mississippi State University
Table of Contents 1. Intro
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A. ADAMS and ADAMS/Car B. Getting Up and Running C. This tutorial 2. Running an Analysis 3. Viewing Results 4. Model Construction 5. Conclusion
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Introduction A. ADAMS and ADAMS/Car ADAMS stands for Automatic Dynamics Analysis of Mechanical Systems. The heart of ADAMS is ADAMS/Solver, a program that solves high order differential equations of motion. Other programs in the ADAMS product line allow you to create a mathematic model your dynamics system thr ough a graphical interface. The process is analogous to FEA, where in FEA, material is defined boundary , conditions are specified, and then stress/strain plots are typically output. In ADAMS, multiple bodies are defined and constrained, initial conditions (such as initial velocities or forces) are specified, the graphical and numerical data are output describing the motion of the bodies in the system. ADAMS/Car is an interface fo r ADAMS/Solver designed specifically for Automotive Engineering. It makes use of predefined templates to streamline the process modeling a vehicle. B. Getting Started MSC offers a University Bundle of software to several academic organizations, one of which is Formula SAE. If your school does not already have access, visit the following address and follow the instructions to get the ball rolling. http://www.mscsoftware.com/Contents/Academia/Learn/Competition.aspx On this page, click Contact Us and use the MSC University Program link to send an email; a reply will be sent letting you know what y ou will need to do. Included in the bundle is ADAMS/View, which is the most general interface for ADAMS/Solver. I highly recommend doing the tutorial for this program a nd becoming familiar with this program to get a feel for how thing work in ADAMS. It is easy to get into ADAMS/Car and not understand the mechanics that are making everything work. The tutorials for any ADAMS product can be found by going to Help >> Getting Started tutorial. Doing the ADAMS/View tutorials and then the ADAMS/Car tutorials will provide a good foundation for beginning this tutorial. ADAMS/Car by default arrives with several templates, but one more specific to FSAE (along with an instructional PDF on how to get it loaded into ADAMS) can be found at http://support.mscsoftware.com/kb/results_kb.cfm?S_ID=1-80028801 This is one of many helpful articles that can be found in the MSC Knowledgebase. To access the Knowledge Base an account/log-in is required. There is a link to do this near the log in area. Loading this database is a perquisite for beginning this tutorial. Also, with this log-in, you can access the MSC user forums, which are a great source of help with issues running simulations. Make sure to read the forum rules. C. This Tutorial This tutorial is intended the present the user with a minimal amount of knowledge need to model their suspension in ADAMS/Car. This tutorial requires that you have the FSAE database loaded, a tutorial for this can be found at a link above. This tutorial is not intended to teach the principles of suspension dynamics, and ideally the user will have a good idea of what their suspension design goals are. While it is feasible to begin using ADAMS with a “clean slate” , most users will begin with a fairly well defined preliminary suspension setup. After a short preview to the capabilities of the program, this guide will
show how to create a fully defined suspension in ADAMS/Car with easy-to-follow instructions and graphics. ADAMS Help provides many helpful tutorials, namely in the Getting Started section. Completing these tutorials is the best way of getting a good grasp of the program. These tutorials use demo subsystems and assemblies; thought it may be best go through the tutorial word for word at first, they can usually be applied directly to the FSAE models with little to no modification.
2. Running an Analysis A good place to start is to run some analysis on the baseline FSAE model. This is mainly to get a feel for what the program is capable of. The best approach for ADAMS is the Crawl, Walk, Run strategy. Start with the working FSAE model, them run some simulations with i t. Then go back and begin modifying the subsystems to represent you car. Making mistakes will be less discouraging after seeing what the baseline model is capable of, then following a simple, incremental process to modify the model to represent your vehicle. The FSAE Template tutorial ends with a Full Vehicle Simulation. Your results in the post processor will not look like the ones shown in the pdf, that would require some plot creation and animation loading, which will be discussed later. For now we will back-track a little and run a Quasi-Static analysis on a front suspension subsystem. To run a Suspension Analysis, we need a Suspension Assembly instead of a Full Vehicle Assembly. Point to File >> Open >> Suspension Assembly . Right click in the Open Suspension Assembly section box click. This will bring a menu with several options. Point to Search >>
/assemblies.tbl . Then double-click fsae_front_with_steer.asy.
This is important : Right clicking in text boxes and using the Search, Guess, Pick, etc. submenus is a unique and fundamental part of using ADAMS. There a re several examples in this tutorial where an entry box will
be filled with a long string of abbreviations separated by underscores. Most often, it got there using the right click menu in this method This is also important: In my slides, I have used an image editor to show several steps on the screen at once. For example, in the above image, the File >> Open >> Assembly will close after selecting Assembly, before the Open Assembly box appears, and obviously the opened assembly will not be in the background before you open it, but I have merged several shots to streamline the tutorial. Now we will run our first suspension analysis. Point to Simulate >> Opposite Wheel Travel and fill out the box as shown.
The ‘a’ is just a prefix for the output name of the simulation. This could alternatively be “first _run” or something similar. The idea is to be a ble to identify your data sets when running multiple simulations.
The output step tells how often you want the solver to output positions, velocities, forces, etc. for the graphics and results data. Setting this to 50 will output each piece of data and an image 50 times throughout the duration of the test. In tests that run f or a specified amount of time, a good value is usually 10 times the number of seconds, which will output every 0.1 seconds.. Bump is the distance above the modeled position where the simulation will begin and Rebound is the distance below the modeled position where the simulation will end. I have my units set to Inches in this particular shot, but the default is metric. This can be changed in Settings >> Units ; however ADAMS will revert to mm whenever the program is restated. In general, the analysis windows will ask for information familiar to people with a basic knowledge of vehicle dynamics. However, pressing F1 with the window active will bring a help window describing each input. This is a helpful technique in just a bout any window of dialogue box.
After the box is filled out, click OK to run the simulation.
3. Viewing Results Now that simulation has been run, an animation of can be played by pointing to Review >> Animation Controls and clicking the play icon. This particular animation shows the specified articulation of the suspension. This can be viewed frame by f rame to insure that the simulation ran a s intended.
Now is a good time to talk about graphics area manipulation. Right clicking on an empty area gives the view control menu (when right clicking over a non-empty area, view control will be a sub-menu). It is highly recommended to memorized the keyboard shortcuts associated with translate, rotate, windowed zoom, and zoom. Next, we will look at the data collected from the simulation. Switch to Adams/Postprocessor by pressing F8. The postprocessor loads with your result set listed in the simulation box near the bottom of the screen. We will be loading a preconfigured plot file containing 9 pre-made plots. Do this by pointing to Plot >> Create Plots the right click in the Plot Configuration File Dia logue Box and select Search >> /plot_configs.tbl. In the file explorer choose mdi_suspension_short.
The plot configuration file has 9 plots, each comparing a different suspension characteristic to wheel travel.
The 9 plots in choose mdi_suspension_short all use wheel travel for an independent axis, however a plot can be created using any two data sets, or a data set vs. time. To make a plot, select the analysis from the simulation box, then select to data for the dependant axis from the request box. After selecting the data from the request box, several choices will a ppear in the component box, one of which must be s elected.
The above example shows the selection of caster angle (left wheel) vs. roll angle (measured from contact patches). Selecting the Data option under Independent Axis brings up the I ndependent Axis Browser, otherwise time can be selected for the independent axis. Note that +testrig has to be expanded to show several of the most relevant data sets. After this selection process, select OK in the Independent Axis Browser (if you selected Data, not Time, for the Independent Axis) then click Add Curves. In this example, the following plot is generated:
Also, an animation can be loaded by pointing to View>>Load Animation then clicking + next to the name of the simulation and choosing the appropriate animation. The name if this analysis was “example”.
4. Model Construction Now that your familiar with the Suspension Simulations, you can begin to model you own suspension. The model from the previous exercise was the front suspension assembly. This included the arb, steering, and suspension subsystems. We will begin with the suspension subsystem. The construction (or in this case, modification) requires information that can be generally grouped into two categories, Kinematic and Dynamic. Kinematics This assembly consists of rigid bodies, joints, a nd forces. Kinematics concerns the first two. Kinematic analysis gives information like Camber Change vs. Roll Angle, Spring Travel Linearity, and Roll Center Migration. Though ADAMS main benefit is the ability to go far beyond Kinematic Analysis, understanding the model at this stage sense is critical at this stage in or to advance to more complex simulations. You will model your kinematic suspension using hardpoints. You probably will get your hardpoint information from some other program in which you’ve already modeled your suspension, such as So lidworks…
or a kinematics program…
Or even a 3 view sketch on graph paper, something of the sort will be needed for modeling in ADAMS. You could begin with the template model and build from there, just keep in mind that many rules have been created since this template was made that need to be taken into account, and that the template may need heavy modification to perform well in your application. Before you begin, you may want to create a new database. Do this by navigating to Tools >> Database Management >> Create Database . Use Save As to save modified subsystems to this database. Many of the modified subsystems will still reference files in the FSAE template database, so it needs to a n active database with the same name on any machine you want to run your new model on. Alternatively there are export options that are described in the help section. Now that you have you own database, we will create a new front suspension subsystem. Point to New >> Subsystem. Enter a name for your front suspension subsystem. It is a good idea to stick to the FSAE template naming conventions, as they will help you differentiate subsystems, assemblies, templates, etc. only replacing FSAE with your acronym. For example, if your team’s name was Mississi ppi State Motorsports, your front suspension subsystem might be called msm_frontsusp, where as the template front suspension subsystem is called fsae_frontsusp. Select front for the minor role, this lets ADAMS know that it is creating a front suspension from the template, not a rear one. For the template, navigate to the FSAE template directory and select _fsae_frontsusp.tpl. Note the leading underscore that indicates this is a template file. This isn’t a requirement, just a helpful convention. Select OK to create your subsystem.
Now that your subsystem is created, you could begin be modifying it to represent your suspension immediately, but instead we will insert it into a front assembly and edit it from there. Both methods are acceptable, but editing from the assembly tends to be more convenient as the subsystem won’t need to be opened individually.
The first step will be modifying the default hardpoint data in the template. This is done b y pointing to Adjust > Hardpoint > Table . This will bring the Hardpoint Modification T able, where you can enter the 21 points (actually more, but points with left and right counterparts are mirrored) that k inematically define a SLA suspension setup and ARB setup.
Note: The rigid bodies have geometry associated with them, which is what you see in the graphics area. One thing that is important to know is that geometries are “children” of rigid bodies. The locations of parts in your model are controlled by Hardpoints. The geometries are also linked to the hardpoints, but they are “just for looks”, i.e. the end of a strut could appear to be floating in space, but could still function perfectly in simulation. CG and inertias can be calculated from geometry, but can also be entered manually. The image below shows all of the hardpoints labeled. The numbers correspond to the tables below, which further describe each hardpoint that needs to be entered. Note that some ARB hardpoint are included in the Front Suspension Subsystem and additional point need to be defined in the ARB subsystem menu. If you haven’t designed ARBs yet or don’t plan to, ignore the points in the Front Suspension subsystem (setting pt 2 equal pt 10 may make the bellcrank geometry look correct) and disable the ARB subsystem. Do this in File >> Manage Assembly >> Toggle Subsystem Activity, select the ARB subsystems in the dropdown menu and select Inactive.
Suspension Hard Point Chart 1
arb_bushing_mount
2
arblink_to_bellcrank
Location where Anti_Roll bar mounts to the chassis. Only important for determining forces applied to the chassis by the mount.
Arb / Bellcrank Pivot, the geometry of the bellcrank also depends on this point Note: Other arb points can be modified in the arb subsystem table. Switch to this using the drop menu at the top of the hardpoint modification table window.
3
bellcrank_pivot
4
bellcrank_pivot_orient
5
lca_front
Point 5,7,12 & 13 are the contol arm/chassis pivots lca is lower control arm and uca is upper control arm
6
lca_outer
Points 6 & 13 are the upright/control arm pivots. They also define the steer axis.
7
lca_rear
See 5
8
prod_outer
PRod/upright pivot. Outer end of push/pullrod.
9
prod_to_bellcrank
PRod/Bellcrank pivot. Inner end of push/pullrod.
10
shock_to_bellcrank
Shock/Bellcrank pivot center. This point does not affect the geometry of the bellcrank.
The location of the center of the pivot of the bellcrank
Along with 3, defines the axis about which the bellcrank rotates. Can be any other point on this axis
Note: Since shock_to_bellcrank does not update the geometry of the bellcrank, it may appear to float in space, but remember, geometry is only for mass calculations (which can be overwritten) and appearance, the point will still rotate properly with the bellcrank .
11
shock_to_chassis
Shock/Chassis pivot center Note: ADAMS considers the spring/damper a single part, the shock. For information on what to do with separate spring/damper locations, see below.
12
uca_front
See 5
13
uca_outer
See 6
14
uca_rear
See 5
15
tierod_inner
Either the tierod/steering actuation pivot or tierod/chassis pivot, depending on weather wheels are steered.
16
tierod_outer
Tierod/upright pivot.
17
wheel_center
Geometric center of wheel.
Anti-Roll Bar Hard Point Chart 18
arb_bend
19
arb_bushing
Location of the cylindrical joint/bushing that constrains the arb’s rotation.
20
droplink_to_arb
The droplink is the member that pushes/pulls on the lever arm. This point is the lever arm end. The other end is arblink_to_bellcrank, 2 in the suspension subsystem graph.
21
arb_middle
The center of the arb. For a straight, latitudinal bar, this point will likely be the same as 18 except with 0 for the y coordinate.
Anti-Roll Bars are typically activated by a lever arm. This is the location at which that arm connects to the anti-roll bar. Also can be thought of as the end point of the anti-roll bar, or the “bend” location.
A point not listed Is the Global Center. This will be used in the future when assembling a Full Car Assembly, and will be 0,0,0 unless you have used a different origin for your front suspension hardpoint than for your entire car. Though the spring and shock can have separate hardpoints assigned to them in some programs, for ADAMS, model them as a single unit, connected to the chassis at on end and the bellcrank at the o ther. Setting the static preload in the spring property menu will assure it functions correctly. After all of the hardpoints are entered, set static toe and camber.
Next, we will enter some mechanical property descriptions. First define some basic info the system like weight, CG height, and sprung mass. You could either enter the tire information manually or select a tire property file of for the tire. Tire property files are available from the FSAE tire test consortium (TTC).
For the springs, click Modify Property File, the middle of the three button in the bottom right corner of the modify spring box.
In the curve manager there are several options for setting your spring rate. Most likely, you will choose Linear Spring and enter a rate.
Now simulations can be ran as described before. Note that tutorials for other simulations are in the Getting Stated section of ADAMS Help. One e ssential technique will be analysis of a design change by overlaying graphs. Simply run an analysis, make a change, then run the analysis with a new prefix. Now when you go to the post processor, there will be 2 analyses. Following the plot creation method shown above can be repeated to show as many overlapped curves as desired (as long as the surf box is unchecked). Simply plot the same data from both analysis and compare the graphs.
Dynamics Definition of dynamics characteristics will mostly be for Full Car simulation, however, dynamics simulation can be useful at the subsystem level for looking at damping characteristics and member loading. The additional suspension subsystem information required for dynamic simulation are: da mping curves, part masses, and part moment inertias. The da mper information is entered much the same way the spring information is, except this time the curve tool will have to be used. Part mass, cg location, and inertias can be entered manually by right clicking on a part and pointing to [part name] >> Modify.
Alternatively, masses, cg location, and inertias can be calculated from geometry, of source the accuracy of this will depend on that of the geometry.
Now a dynamic simulation can be run . Point to Simulate >> Suspension Analysis >> Dynamic… and fill out the box as shown.
This will oscillate the wheels at a magnitude of 10 (model units) at a frequency of 1Hz. Changing one function to COS would offset the oscillations. Also, several other loads can be applied. The results of these animations will generate force data for components and suspension damping rates.
5. Conclusion Although this tutorial focuses on suspension, much more can be done with full vehicle simulation. Ever wondered what the trajectory of a Formula car doing 120 MPH up a 45 degree ramp would be? Easy, the program is a full dynamics engine, so there are no bo undaries on what you can and can’t simulate. Overlaying a dozen skidpad simulations and animation with successive camber iterations: Also easy, once you’ve built your model. But the suspension is your starting point, and in most applications, the majority of your work. The next tutorial will go into other subsystems and then fu ll vehicle assemblies.
