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 innovations in supporting technologies. For example, motion controllers have evolved to support special capabilities for exerting real-world forces on structures that can deliver, in a matter of hours or days, the loads and movement that assemblies would otherwise encounter in a whole lifetime of use. And motion control electronics connected to hydraulic or electromechanical actuators can submit assemblies to stresses and measure responses in a “clean” environment that would be difficult or expensive to accomplish in the field.
For example, consider the issues associated with testing an automotive chassis for strength and responsiveness 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 collected. Between road sessions, the data was analyzed and the chassis would be tweaked and tuned to modify its driving characteristics. Following this, the car was sent back out and the data was taken again. It was difficult to “tune” the system optimally, 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 new configurations on the road.
A better way to test a chassis would be to do so in a laboratory environment where one can isolate the various factors causing stress and see how tweaks to the chassis under test change the system’s response to those factors. Such research is one key activity at Accelerating Developments International, Inc. (ADI) of Concord, North Carolina.
ADI has been building automotive chassis testers for years. Among the company’s customers are the leading NASCAR and Indy Racing League (IRL) teams. ADI’s product is a line of “test rigs” on which race car chassis are mounted and then subjected to stresses to show how they perform. Each rig has four actuators, one located at each wheel. The actuators can be moved individually, or they can be moved together in synchronized motion (see Fig. 1).
For operating the actuators, using a programmable motion controller is the key. State-of-the-art motion controllers have built-in support for:
- Curve following
- Position control and/or force limiting/ control
- Synchronization of multiple motion axes
- Interface with PLC’s, HMI or PC instrumentation software
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.
The motion controller that ADI selected is the RMC150 provided by Delta Computer Systems, Inc. of Vancouver, Washington. The Delta RMC150 is a closed-loop motion controller, meaning that it makes decisions and adjusts control of the motion based on feedback from sensors attached to the system. The ADI test rig system uses all eight motion control axes provided by the Delta controller 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 provided by 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 track and can enable testing to see if there’s any binding between components.
The Delta Computer Systems motion controller also enables jogging (i.e., making small adjustments in actuator position), discrete moves between specific points, and moves relative to a current location. The attached PC only needs to send the points and the Delta RMC does all the motion control work. Plus, because of its programmability, the motion controller provides the capability to add new test features.
ADI’s engineer, Mike Messick, programmed the motion controller to apply real-world forces to a race car’s suspension at each of the four corners, to simulate the action of the car as it rounds the track. To produce a motion “target” for the motion controller to follow, the controller typically works with a “drive file,” a recording of the car’s motion as it raced on a real track.
Drive files can contain up to 200,000 data points that the motion controller sequences through, in effect “connecting the dots” to produce a realistic simulation of the stresses encountered in a typical trip around the track. The motion can be sped up or slowed down to change the speed of the simulated lap.
To connect the data points with smooth motion, the Delta controller runs motion programs composed of instructions that generate smooth accelerations and decelerations according to special mathematical equations called spline functions.
The motion controller connects to a human-machine interface (HMI) which is implemented with a PC running control and visualization programs from software companies Steeplechase and Indusoft. Using the HMI, ADI’s engineers and their customers are able to vary the speed of the motion and even reverse the motion around the track. They can also “freeze” the motion at any point to capture data that would otherwise be difficult to capture with the chassis moving.
One thing that the ADI team learned is that it is possible to actually produce more realistic data on how a chassis responds to road stress if the motion is slowed down. “Last week, we had just installed a new test rig for a top NASCAR team,” explained Jay Drake, ADI’s General Manager. “Our customer was able to run test files from the Phoenix raceway that produced data that appeared to be identical to what they had obtained previously from the track.”