HIL is a new frontier in test
14 May 2008
HIL simulation has become an essential tool in the embedded system designer’s toolbox. It now plays a major role in development programs across a broad range of industries
An increasing number of system developers now use HIL (hardware-inthe- loop) simulation in the development and test of real-time embedded systems.
Embedded systems are designed to control systems such as land vehicles, satellites, spacecraft, unmanned aerial vehicles (UAVs), aircraft, weapon systems, marine vehicles and jet engines. Typically, the complexity of the control system increases with the complexity of the system being controlled. They tend to be expensive and individual and difficult to test.
It is a given that when interfacing control software to the target hardware for the first time, not everything will go smoothly. The time to identify and fix problems is often limited; HIL simulation can substantially lower the cost of resolving conflicts before the final hardware integration begins.
The first generation of HIL test systems were designed for single applications, and in automotive systems engineers are frequently used to test engine control units. The question is 'Why buy a system that is limited to only one job, when that piece of equipment could do several?’ Today, HIL simulation is used to test a variety of embedded systems and the trend is away from single application test system architectures, towards a more cost effective multiple use paradigm, typical of other more general purpose testinapplications. Designers can further reduce costs by leveraging existing COTS (commercial-off theshelf) hardware and flexible software to reduce design time.
What is HIL simulation?
Pure simulation is often used to understand the behaviour of a system, or to predict an outcome under different internal and external influences. However, if the simulation is being used as a basis for proving control feasibility, the risk of investment can be further reduced using an HIL simulation approach. For most real systems, there are characteristics that are unknown or too complex to model by pure simulation. To develop, for example, controls for a head positioner in a disk drive system, it would be foolish to build all the hardware at the start without considering the system as a whole.
HIL simulation is a dynamic test technique that simulates the I/O behaviour of a physical system that interfaces to an embedded system in real-time. It is dynamic because the values of stimulus signals generated by a simulator are a function of the system’s response from the previous cycle. Other variables, such as test profiles and in-line analysis results, may also influence the calculation of stimulus values. However, it is the dependence on the response from a unitunder-test that differentiates HIL simulation from other test techniques.
In a closed-loop control system, the current state of the system being controlled is fed back to the controller through sensor measurements. The controller uses these measurements to help determine the appropriate actuator values in order to attain a desired operating condition (see figure 1).
To control wheel slippage while braking, for example, an ABS (anti-lock braking system) uses an ECU (electronic control unit) to provide closed-loop control of the vehicle’s brakes. The ECU receives information regarding individual wheel speed, vehicle speed, brake position and other conditions necessary to determine the appropriate brake actuator command for each wheel to maintain maximum traction while stopping in adverse conditions. Physical testing of the ABS ECU ultimately requires a vehicle and test track; however, engineers can thoroughly test the ECU without a vehicle or even a brake system using HIL simulation.
The value of HIL simulation
Why use HIL simulation? Why not connect the embedded system under test to the ‘real plant’, that is the dynamic system being controlled, to perform development and testing?
In some cases, this is the more effective way to develop an embedded system. In other cases, HIL simulation is more efficient. According to Applied Dynamics International, the metric of development and test efficiency is typically a formula that includes cost, duration and safety.
The cost of the approach will be a measure of the cost of all tools and effort. The duration of development and test affects the time-to-market for a planned product. The safety factor and duration are typically equated to a cost measure.
In avionics, HIL simulation is used in the development of aircraft flight controls. For example, a flight simulator would include simulations of aerodynamics, engine thrust, environmental conditions, flight control dynamics and more. Mechanical hardware prototypes can be connected to the simulator, allowing test pilots to evaluate flight performance given various parameters.
The alternative to HIL simulation would be to place prototype flight controls in early aircraft prototypes and test for usability during flight test. This approach fails when measuring the three conditions.
Cost: A flight test is extremely costly and therefore the goal is to minimise any development occurring with flight test.
Duration: Developing flight controls with flight test will extend the duration of an aircraft development program. Using HIL simulation, the flight controls may be developed well before a real aircraft is available.
Safety: Using flight test for the development of critical components such as flight controls has a major safety implication. Should errors be present in the design of the prototype flight controls, the result could be a crash landing.
Examining these criteria, it is clear that HIL simulation would be the better option for integrating this avionic system.
The automotive industry
The automotive industry has adopted HIL simulation as standard practice in test. Driven by governmental regulations and demand for better fuel economy and greater performance, the design of modern automobiles and the process of developing them has radically changed. The automobile has evolved from a mostly mechanicalhydraulic system to include complex electronics and software algorithms implemented on ECUs.
For example, BMW used their existing HIL test systems for the test and development of the BMW Hydrogen 7, the first commercially available hydrogen-powered luxury saloon car worldwide. The BMW CleanEnergy hydrogen initiative aims at avoiding carbon dioxide emissions by changing from carbon dioxide based fuel, such as petrol and diesel, to hydrogen. With the concept of efficient dynamics for all engines, the BMW group is working at solving the fundamental conflict between higher driving performance and concurrent lower fuel consumption. The development of the controller, the functions and the application of the BMW Hydrogen 7 have run through the tried and tested BMW development process.
The HIL systems for the Hydrogen 7 motor control are based on the NovaSim HIL simulators from MicroNova. MicroNova is a manufacturer and integrator of HIL test systems with experience as an engineering partner for CleanEnergy functions development and software development.
The core of these simulators is based on a National Instruments hardware platform in which a real-time computer is connected to various I/O boards via a PXI bus. The simulation models are coupled with the hardware via National Instruments LabVIEW software, which also enables the simulator operation. The goal of the simulator group at BMW is to use as many standard, commercially available components as possible to reduce cost and engineering effort.
From power generation to automotive, to aerospace, HIL simulation is helping to cut costs and improve quality and reliability. The key to successful HIL system development is to use standard tools which can be supported long term. By enabling tests earlier in the development cycle, and removing the limitations of using physical tests alone, HIL simulation is helping to reduce development costs and increase the quality and reliability of embedded designs in every industry.
SARAH BRADY is technical marketing engineer, National Instruments UK & Ireland
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