Motion Controller Accelerates NASCAR
Automotive test rig manufacturer sets out to improve NASCAR’s standard chassis test stand with improved dynamic capabilities and reduced maintenance.
Programmable multi-axis motion controllers are revolutionizing the performance tuning of mechanical systems because they can simulate real-world stresses on those systems. They can "play back" a sequence of realistic motion stimuli in a laboratory environment and enable measurements to be taken that would be difficult and time consuming to get in the real world.
Consider the task of trying to measure how a race car chassis flexes in response to road and aerodynamic forces on the track. There are many different wheel positions that occur as a vehicle goes into corners and its weight transfers between the tires. Front end camber can change, and tires may "bump steer" at different places on the track. Not only is it extremely hard to take measurements of the forces being applied to the chassis while the vehicle is moving, available track time is limited. If a racing team could test and tune its car’s chassis and suspension responsiveness off the track, it could optimize the car’s performance more precisely and gain a competitive advantage.
For years, teams have sought ways to test their chassis adjustments with the car on a test stand. The first such system to be developed, called the "K-Rig" (K for "kinematics"), entered use some four years ago, and has become the standard for National Association for Stock Car Auto Racing (NASCAR) teams.
The K-Rig uses linear hydraulic actuators to apply forces independently to the wheels. It proved effective, but hard to tune precisely, and it was hard to use for dynamically testing car-body torsional characteristics.
With these early-system limitations in mind, Accelerating Developments International Inc. (ADI) of Concord, N.C., one of the original K-Rig suppliers, set out to design an upgrade to the system which it called the KD-Rig (D for "dynamics"). This upgrade would improve on the K-Rig’s dynamic testing capability, achieve tighter control, and reduce system maintenance costs.
Tighter control and lower maintenance cost requirements led ADI to replace the K-Rig’s hydraulic actuators with an electromechanical solution, using electric servos driving ballscrews to convert rotary into linear motion. ADI engineers prototyped this version and it worked fairly well, but the system still didn’t have the ability to program forces with sufficient accuracy to meet their goals, and the system was still hard to control; it would sometimes either overshoot or undershoot its position and force targets.
ADI engineers spent about 3 months trying to make the existing controls follow the force and position profile before deciding that a new motion controller was needed. At the recommendation of ADI general manager Jay Drake, who had used motion controllers by Delta Computer Systems Inc, of Vancouver, WA. in the past, they added one of Delta’s RMC150 eight-axis motion controllers to the system. They were then able to program and tune the system to work flawlessly within a couple of weeks.
The controller was able to reach a high level of precision using the same system of electric servos and ballscrews as the original KD-Rig, so there was no need to consider going back to hydraulics on any but the lowest cost applications. The precise control provided by the controller enables the new KD-Rig system to repeatedly hit a target axis position precisely to within seven decimal places.
Navigating the curves
Unlike other motion controllers, which can require a lot of up-front programming, the Delta controllers can run individual motion instructions that produce very sophisticated motion profiles. For example, ADI was recently able to create a pre-processor that would generate a spline function instruction that causes a continuous curving motion to be applied to the system.
The RMC motion controller creates curves by interpolating data points provided by the user or a host controller. It provides many options for creating and following curves to satisfy a wide range of applications. In the case of KD-Rig, the data can come from recordings of motion stimuli made on the track. The curves can be used for both time-based motion, where the curve defines the position of the axis at certain times, and master-based motion, where the curve defines the position of the axis based on a master, such as the position of another axis.
In the KD-Rig, the resulting flowing motion profiles have no discontinuities, just like the real motion of a race car on the track. The previous system sometimes exhibited interrupted motion, which reduced test realism and data accuracy.
Motion control programs are also easy to expand, giving ADI the ability to extend and evolve the test sequences that the machine supports. The latest KD-Rig uses eight motion axes. In addition to one linear motion system for each of the four wheels, optionally are three aerodynamic load actuators ("aeroloaders") to pull the body into an attitude simulating the effects of aerodynamic down forces, and an eighth axis can be used to simulate steering inputs.
Sensor inputs provided to the Delta Computer Systems motion controller come from a magnetostrictive linear displacement transducer (MLDT) mounted on each actuator to provide position feedback, and a load cell on each actuator to provide information on the force being applied. The controller provides direct interfaces to these devices, as well as direct outputs to drive the servo motors.
Ethernet built in
To provide overall control and a user interface for the system, ADI chose to use a PC in a control cabinet running VLC by Steeplechase as the control program and an HMI from Indusoft. The data acquisition is handled by Pi Research, a program that many motorsports teams use.
In addition to supporting programming of the motion controller, Delta’s RMCTools software is used on the PC as a diagnostic/tuning tool. All communication between the control PC and the motion controller takes place over the controller’s built-in Ethernet interface.
The vertical wheel actuators are bolted into position where the wheels normally attach. A test can be preprogrammed to put all the wheels through a certain range of motion, or motion commands can be sent to each wheel independently. The chassis’ torsional characteristics can be tested by keeping three wheels on a plane and then exerting a force on the fourth wheel.
The system also accepts a "drive file" which generates motion based on the same data that a car obtains from driving on a real track. A problem with using drive files is that the files can be very large when they include a full lap of data, so the flexibility to use test sequences of different lengths gives KD-Rig an advantage.
A properly tuned suspension helps the car follow its desired path, and can help the car go faster when it’s not going straight ahead. Aerodynamically, a car may even go faster straight ahead if it is set up correctly.
According to the company, ADI test rigs are being used by about 90% of the NASCAR teams and are beginning to be used in the Indy Racing League (IRL). ADI has also been approached by several automotive OEMs for information on how the systems could help car manufacturers do long-term life testing.
The KD-Rig received the Testing Technology of the Year Award for 2007 at the Professional Motorsport Expo in Cologne, Germany, in December.
ADI continues to extend the capabilities of the KD-Rig system. For example, the company is developing an enhanced system with 16 axes of motion control that enables the car to run under power on the stand, permitting testing of braking, acceleration, and deceleration scenarios. As a clear sign of ADI’s satisfaction with the performance that they have obtained from the Delta Computer Systems, the new system will incorporate two Delta RMC150 multi-axis motion controllers.
Peter Nachtwey is president of Delta Computer Systems. Contact him by email at peter@delta motion.com