HIL Testing Concept Explanation

HIL Testing

Hardware-in-the-loop (HIL) testing is an efficient black-box method for testing and verifying control system software.

Instead of being connected to the actual equipment on the vessel, the control system is connected to a HIL simulator with sophisticated models of the vessel and its equipment. This enables systematic and comprehensive testing of control system functionality and failure handling without risk to people, equipment or environment.

The concept of HIL testing

A control system interacts with vessel systems through a set of input/output (I/O) signals. Input is provided by sensors that measure dynamic states and parameters, as well as by operator stations and other control systems. Based on the input and internal models in the control system, the control system calculates control signal output to the actuators.

HIL testing is accomplished by isolating the control system and its operator stations from its surroundings, and replacing all actual I/O with simulated I/O from a HIL simulator in real time. The HIL simulator imitates the vessel, vessel systems and environment, responds to the commands from the control system in a realistic manner, and provides realistic and consistent measurements. The control system cannot sense any difference between the real world and the virtual world in the HIL simulator. The HIL simulator thereby facilitates systematic testing of control system design philosophy, functionality, performance, and failure handling capability, both in normal and off-design operating conditions.

HIL simulator visualization


  • A HIL simulator includes sophisticated mathematical models of actuators, dynamics and sensors
  • A HIL simulator constitutes a “virtual world” for the control system
  • HIL testing is performed in a safe test bed, without risk to people, equipment or environment
  • HIL testing is black-box testing
  • No source code is revealed or exposed through HIL testing
  • HIL testing is the de facto industry standard for control system software in the automotive and aerospace industries
  • Every vessel is unique regarding equipment and configuration, increasing the potential for errors

Black box

In HIL testing, the control system is viewed as a black box. The system vendors do not disclose any of their source code, since no first-hand knowledge of the inner workings of the control system is necessary. Test cases and acceptance criteria are based on functional descriptions, user manuals, class rules and regulations, and industry standards.

Best practice

In the automotive, avionics and aerospace industries, HIL testing has been established as best practice to meet the requirements for performance and reliability. Comprehensive HIL testing of electronic control units (ECUs) is done both by automobile manufacturers and their suppliers. It is considered essential to reach the necessary safety levels and avoid vehicle recall campaigns. ECU examples include ESP, ABS, cruise control, automatic four-wheel drive, fuel injection, ignition, and turbocharger control. In the space industry, NASA has utilized HIL testing extensively at its Independent Verification & Validation (IV&V) Facility for testing mission-critical software components on spacecraft. In the aerospace industry, HIL testing has been used in programmes such as Future Combat Systems and Joint Strike Fighter. Example of our PMS-HIL simulator and some failure modes:

PMS-HIL simulator

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Simon Mockler

Simon Mockler

Senior Risk Engineer

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