Figure 1. HIL test applications use real-time simulations to allow system-level software testing with incomplete systems by electrically emulating the interaction between the ECU and the simulated system.
Mechanical testing applications such as dynamometer- and servo-hydraulic-based test systems may use models to simulate the mechanical interaction between real and simulated components of the system. With real-time simulation, you can test control systems without having all system parts in place. This practice reduces test cost by finding control systems errors earlier and preventing potential equipment damage. It broadens test coverage to help ensure higher quality products by enabling you to test conditions that would not otherwise be possible in pure physical testing. It also reduces test time because you can perform many tests from within a test cell instead of requiring field testing.
Automotive Model-Based Test Cells
Tecnalia, a European research organization, offers an example of how mechanical testing is benefiting from the use of real-time simulations. It created a parameterized modeling environment, Dynacar, so users can easily create models of their specific power train systems by virtually assembling the components used in their designs and filling in the parameters for each component. To implement a system-level test with only one of the components physically present, the test engineer can replace missing components with a real-time simulation. The physical component is coupled to the real-time simulation via dynamometer and/or hydraulic actuation systems. With this technology, you can cost-effectively evaluate physical systems across a variety of configurations. For example, the manufacturer of driveline systems can test the operation of the component with multiple combinations of wheels, transmissions, and motors by simply switching the models in the real-time simulation.
Figure 2. A real-time simulation provides the operating points for actuator control loops that implement the mechanical simulation for the device under test.
Structural Test Using Real-Time Simulation
You also can use real-time simulations for mechanical testing to develop safer civil structures. Due to the sheer size of the structures, finding ways to test new designs can be extremely expensive. For mechanical testing, the test rig generally needs to be much larger than the device under test (DUT), so the size required to test new building designs is very large. Research facilities such as Lehigh University; the University of Nevada, Reno; and the University of Colorado Boulder use real-time simulations to complement the testing of critical structural members of a simulated building frame. A stimulus applied to the software-modeled mechanical structure controls the forces that are applied to the member being tested. Likewise, sensors on the member being tested feed information back into the model using instrumentation such as SC Express hardware from National Instruments. You can test a portion of a building rather than the entire structure at a much smaller scale. In addition, you can isolate a structural member so as to not cause unnecessary fatigue damage to other parts of the structure that are outside the area of concern during testing.
Figure 3. This example uses sensor feedback to provide closed-loop control of mechanical actuators to couple the real-time simulation to the physical device under test.
The Benefit of Real-Time Simulation in Mechanical Test Cells
Unlike HIL test applications, for which the simulation is used to provide an accurate representation of electrical interactions, real-time mechanical simulations deliver a representation of the physical interactions between a mechanical component being tested and a modeled mechanical system (displacement, velocity, load). Instead of sending the device under test an electrical signal representative of the system state in the simulation, a physical force setpoint is provided by the simulation and used by a closed-loop control algorithm to mechanically apply this force to the device under test.
Real-time simulations have proven to be a valuable and often necessary tool to keep pace with the growing complexity of mechanical control systems. The key to achieving these advantages is implementing a real-time testing platform that can execute models efficiently and connect to the real world using hardware I/O with the appropriate accuracy and performance. National Instruments offers the software and hardware tools necessary to implement real-time tests using simulations that reduce development and test costs while increasing quality.
As control systems continue to evolve, the functionality of mechanical test systems must continue to expand so test engineers can meet strict quality and time to market requirements. Integrating real-time simulation into these mechanical test systems allows for broader test system coverage while reducing test time. NI VeriStand is a software tool that you can use to easily integrate simulation models into your mechanical test system. To view demonstrations of NI VeriStand, visit ni.com/veristand/getting started. To learn more about National Instruments signal conditioning modules, visit ni.com/sc-express.