Automotive Chassis Testing

Automotive Chassis Testing Bill Greco, Mechanical Engineer, Warrington, Pa.

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There's a science to testing metal structures for rigidity and performance under stress, The development of new testing methodologies is continuing at a fast pace, aided by new technologies. For example, motion controllers have been developed to support special capabilities for exerting forces on structures that can deliver, in a very short time, the loads and movement that assemblies would otherwise require an enormous amount of time. Motion control electrical devices connected to hydraulic and electromechanical actuators induce stresses and measure responses that would be difficult to accomplish under actual real world conditions. Consider the issues associated with testing an automotive chassis for strength and reliability to road conditions. In the past, measuring instruments were typically attached to a car's chassis, the car was run on the road, and data was assessed. Between test sessions, the data was analyzed and the chassis would be tweaked and tuned to change its driving characteristics. Following this, the car was sent back out and the data was taken again. It was difficult to optimize, because so many variables were changing at the same time. It can also be time consuming and expensive to schedule "track time" to try out differnet versions of the design. A good way to test a chassis would be to do so in a controlled environment where one can isolate the various factors causing stress and see how changes to the chassis under test modify the system's response to different variables. Some comapnies have been building automotive chassis testers for years. Among the company's customers are the leading NASCAR and Indy Racing League (IRL) teams. Test rigs on which race car chassis are mounted and subjected to stresses to show how they perform, Each rig has multiple actuators. The actuators can be moved individually, or they can be synchronized to operate together. To operate the actuators, a programmable motion controller is used. Motion controllers have built-in support for: Curve following, Position controling. Synchronization, and/or force limit of multiple motion. Because motion controllers can output information on the conditions of the motion that they produce, they make ideal data acquisition instruments as well as controllers. A closed loop motion controller is used, it makes decisions and adjusts con trol of the motion based on feedback from sensors attached to the chasis. The test rig system uses eight motion control axes for position control and force monitoring. The controller can get its position inputs from transducers mounted on the actuators at the wheels, or it can close the control loop using a voltage from potentiometers attached to the shock absorbers. Both modes employ force limiting using load cells to make sure that the motion doesn't bend the frame. Information from the load cells can also be used by the motion controller to allow the simulation of forces that are measured on the road and can enable testing to see if there's any problems between components. Jogging allows small adjustments in actuator motion conrol. Use of a hydraulic cylinder at each wheel that can move the wheels independently is standard operation. The cylinders can double as jacks to facilitate under-car suspension adjustments. Discrete moves between specific points are produced relative to a current location. A PC only needs to send the points and PLC does all the motion control work. Plus, because of its programmability, a motion controller can provide the capability to add new test features. To produce a motion "target" for the motion controller to follow, a controller typically works with a program and a recording of the car's motion It’s possible that up to 200,000 data points that a motion controller can sequences through.

Automotive Chassis Testing Page 2 of 2 Bill Greco, Warrington, Pa. [email protected]

To produce a realistic simulation of the stresses encountered in a typical trip. The motion can be sped up or slowed down to change the speed of the simulated drive. A motion program is composed of instructions that generate smooth accelerations and decelerations according to mathematical equations. A motion controller connects to a human-machine interface (HMI) which is implemented with a PC running control and visualization programs from software. Chasis tester’s and their customers are able to vary the speed of the motion and reverse the motion. The motion can be frozen at any point to capture data that would otherwise be impossible to capture with the chassis moving using the old way of road testing. It is possible to produce more realistic data on how a chassis responds to road stress if the motion is slowed down. Customer’s are able to run test files from varrying raceway’s and roads. ln the age of toss away products new automotive components can be engineered to operate for millions of cycles and offer better reliability than has ever been obtained before.