Real-time test (RTT) technology refers to implementing test applications in a real-time environment, primarily to help test systems achieve higher reliability and/or determinism. Therefore, RTT technology plays a crucial role in the development of many products and systems. For example, test systems with requirements for durability, lifespan, and others that need to operate continuously for extended periods or in unattended environments require a real-time execution platform that provides superior reliability. Examples of systems requiring RTT technology include environmental test cells, dynamometers, hardware-in-the-loop (HIL) simulators, and other similar test systems that perform closed-loop control, requiring a real-time execution platform with low jitter determinism. By examining several RTT applications, we can see how they have evolved to address the various challenges faced by test engineers today.
Real-time testing technology
A common real-time testing technique utilizes closed-loop control to automatically manipulate physical variables within a test system, such as temperature, position, torque, or acceleration. For example, in implementing an environmental test system, such as a pressure chamber, the test chamber, in addition to providing stimulation to the unit under test (UUT) and observing its response, must also perform these functions when specific conditions are reached. Because the pressure within the chamber is affected by numerous factors, such as chamber leakage or changes in UUT characteristics, test engineers must use a closed-loop control algorithm to monitor pressure sensor readings and automatically adjust the control signals of the compressor and safety valves to ensure that the real-time pressure curve matches the requirements of the test plan. To achieve such automatic control, the closed-loop control system must detect the system status and adjust control commands at defined time intervals.
Figure 1. Real-time testing systems such as pressure chambers use closed-loop control systems to automatically achieve the pressure conditions required for the test plan.
Another example is hardware-in-the-loop testing. This real-time testing application can more effectively test electronic control systems. An electronic control system consists of an electronic control unit (ECU) and the system or environment it controls.
When testing an electronic control system, considerations such as safety, system feasibility, or cost may prevent all necessary tests from being performed on a complete system. However, due to the closed-loop coupling between the ECU and other parts of the system, we cannot fully test the performance of the electronic control unit without using the complete system.
Hardware-in-the-loop simulation uses software models representing other parts of the system to simulate the interactions of sensors and actuators between the control unit under test and the rest of the system. It creates a virtual environment for the ECU, maintaining closed-loop coupling within the system. To accurately simulate sensor and actuator interactions, the test system must perform highly deterministic calculations of the execution model over continuous or deterministic time intervals.
Figure 2 Hardware-in-the-loop (HIL) testing is a real-time testing technique that uses software to simulate missing system components to test electronically controlled devices.
Evolution of Real-Time Test (RTT) Systems
As products and systems become increasingly complex, the implementation of testing systems faces greater challenges. To address these challenges, real-time testing systems need to integrate multiple functions, resulting in systems that can simultaneously meet various requirements that were previously fulfilled by separate real-time testing applications.
This trend is well illustrated by the emergence of model-based dynamometers. Typically, a dynamometer testing system includes a real-time test application using a proportional-integral-derivative (PID) control algorithm to generate different load and speed conditions for the unit under test (UUT). This testing system applies static excitation curves to the PID controller and the UUT to execute and verify the device. Model-based dynamometer systems evolved from traditional dynamometers, using models to execute advanced control algorithms and generate dynamic excitation curves.
Engineers at Wineman Technologies used NI's RTT platform to produce a six-wheeled, stand-alone chassis dynamometer system. To fully test their vehicles, the dynamometer needed to generate diverse test conditions to simulate driving conditions on different terrains.
For example, a model-based dynamometer must be able to simulate the following states: two wheels traveling on snow, one wheel gliding in mud, two wheels traveling on loose gravel, and the other wheel off the ground. Furthermore, the system needs to simulate the terrain changes from wheel to wheel as the vehicle moves.
To implement this testing system, engineers needed to combine their experience in implementing dynamometers and HIL simulators to create a system with more features than traditional dynamometer testing systems, features more commonly found in HIL testing systems. Specifically, they had to deterministically execute complex models to provide dynamic excitation to generate six relevant speed/torque curves and implement advanced control to accomplish this task.
The integration of real-time testing requirements can also be seen in the application of the European research institution Robotiker Tecnalia. In their research and development of hybrid electric vehicle (HEV) powertrain systems, engineers used NI's real-time test platform to build a dedicated HIL test system.
Instead of performing a complete electrical simulation of the interaction between the vehicle's sensors and actuators and the ECU, they replaced the powertrain traction drive software model with actual electromechanical components. These components were then linked in a closed loop with software models simulating other parts of the vehicle, resulting in a more accurate and flexible testing system (see Figure 3).
Figure 3. Mechanical components are added to the HIL simulation to help develop and validate HEV powertrain systems more efficiently.
Because physical components are added to the simulation, a load-loading mechanism needs to be added to the traction drive so that the simulation can control its load-loading conditions. The HIL simulator provides simulated load values, which are applied to the traction drive via mechanical coupling.
When implementing this dedicated HIL testing system, Tecnalia's engineers needed to create both an HIL simulator and a dynamometer-based load-loading system. Working together, the two could provide an electromechanical simulation of an HEV powertrain.
Consumer expectations, regulatory bodies, and competitive pressures are driving products to achieve increasingly complex functionality at an ever-faster pace. As companies struggle to balance surging complexity demands with shorter development cycles, higher reliability requirements, and unchanged or even reduced budgets, the crucial role of real-time testing technologies in the development process is becoming increasingly apparent.
