“Effectively comparing vehicle concepts is crucial to design winning racecars. For that, we* have set-up a lap time simulation based on tire test data and a two track model in Simulink. This allows us […] to model different vehicle concepts on various tracks and evaluate the performance.”
I probably can’t say it any better than Fabrice Oehler in his keynote talk at the MATLAB Expo user conference in June 2016 in Munich (link to the recorded presentation). Successful Formula Student teams use lap time simulation in the early stages of their design. Below I am presenting 3 scenarios for you to explore. This article does by no means claim to be complete, it is summarizing what winning teams have shared with us and shall be a motivation for you to get started.
[* editor’s note: AMZ Racing Team of ETH Zurich]
Three scenarios how succesful teams perform lap time simulation
#1 Minimal effort – decent outcome
Probably one of the simplest approaches to model a vehicle is assuming it to be a point mass. Thus, lap time simulation is nothing but balancing the time dependent forces acting on the mass such as lateral or aerodynamic forces. Using Google and the search terms “lap time simulation MATLAB” will reveal a bunch of tools readily available to you. It is great to see the open source spirit within automotive student teams! What these tools all have in common is a limited number of parameters that one can adjust such as engine power, gearbox characteristics, aerodynamics, tires and mass.
Following that path may be ideal to get started with lap time simulation, especially if you are new to that field. But be aware that you might hit limitations pretty soon.
#2 Full control – ideal for holistic approach and know-how expansion
The most common approach based on my observations is certainly to use Simulink® and Simscape™ to set up a lap time simulator. Teams from TU Munich and ETH Zurich (see intro) went that road and certainly are happy and successful with it. I suggest you check out their contributions to the MATLAB and Simulink Racing Lounge and form your opinion.
[VIDEO] MATLAB and Simulink Racing Lounge “Lap Time Simulation; Essential Part of Concept Development“
[VIDEO] MATLAB and Simulink Racing Lounge “Steady-State Lap Time Simulation“
What both approaches have in common is that they rely on single track or two track models. Both teams used tire models and relied on test data, e.g. from the Formula SAE Tire Test Consortium, to obtain realistic assessments on the road-vehicle interface.
An aspect where both approaches differ is the driver model. For AMZ Racing, the driver is considered as controller. TUfast completely neglects driver influence by evaluating lap time only based on the ggV-diagram, often also denoted as the performance envelope (car at its limit all the time). Make sure you watch the key takeaway section of both videos about the pros and cons. That will give you some insight which strategy you may want to apply.
TUfast and AMZ Racing have not (yet) used Simscape™ when the videos were published but I certainly know that they do know. Using Simscape™, your models may be more detailed using less blocks. The situation for you (automotive student teams) is really comfortable because MathWorks offers free-of-charge Simscape online training to get you ramped up quickly.
Simscape™ enables you to rapidly create models of physical systems within the Simulink® environment. With Simscape, you build physical component models based on physical connections that directly integrate with block diagrams and other modeling paradigms. You model systems such as electric motors, bridge rectifiers, hydraulic actuators, and refrigeration systems, by assembling fundamental components into a schematic.
#3 Stand-alone vehicle modeling platforms
Student teams, especially Formula Student team have access to a variety of vehicle modeling platforms, e.g. IPG CarMaker is pretty common in the community. Not only the vehicle modeling capabilities make it a good choice, also its interface to Simulink for developing control strategies and programming ECUs are strong assets. Also here, I don’t want to evangelize too much but rather hand over to specialists. I suggest you have a look at the joint video, that Prasanna Kannan of IPG and myself recorded.
I hope that you found at least one hook or starting point for your own approach to lap time simulation. Make sure you use the feedback options that this blog provides. I am happy to contribute to a lively discussion!
Certainly, this article cannot replace a lecture and thorough studies about vehicle dynamics. The variety of materials is huge. One of my favorite books is “Race Car Vehicle Dynamics” written by William and Douglas Milliken and published by SAE, Inc.