Hardware-in-the-loop (HIL) simulation, HWIL, or HITL, is a technique that is used in the development and testing of complex real-time embedded systems. HIL simulation provides an effective testing platform by adding the complexity of the process-actuator system, known as a plant, to the test platform. The complexity of the plant under control is included in testing and development by adding a mathematical representation of all related dynamic systems. These mathematical representations are referred to as the "plant simulation". The embedded system to be tested interacts with this plant simulation.
How HIL works
HIL simulation must include electrical emulation of sensors and actuators. These electrical emulations act as the interface between the plant simulation and the embedded system under test. The value of each electrically emulated sensor is controlled by the plant simulation and is read by the embedded system under test (feedback). Likewise, the embedded system under test implements its control algorithms by outputting actuator control signals. Changes in the control signals result in changes to variable values in the plant simulation.
For example, a HIL simulation platform for the development of automotive anti-lock braking systems may have mathematical representations for each of the following subsystems in the plant simulation: Vehicle dynamics, such as suspension, wheels, tires, roll, pitch and yaw; Dynamics of the brake system's hydraulic components; Road characteristics.
Uses
In many cases, the most effective way to develop an embedded system is to connect the embedded system to the real plant. In other cases, HIL simulation is more efficient. The metric of development and testing efficiency is typically a formula that includes the following factors: 1. Cost 2. Duration 3. Safety 4. Feasibility
The cost of the approach should be a measure of the cost of all tools and effort. The duration of development and testing affects the time-to-market for a planned product. Safety factor and development duration are typically equated to a cost measure. Specific conditions that warrant the use of HIL simulation include the following: Enhancing the quality of testing Tight development schedules High-burden-rate plant Early process human factor development
Enhancing the quality of testing
Usage of HILs enhances the quality of the testing by increasing the scope of the testing. Ideally, an embedded system would be tested against the real plant, but most of the time the real plant itself imposes limitations in terms of the scope of the testing. For example, testing an engine control unit as a real plant can create the following dangerous conditions for the test engineer: Testing at or beyond the range of the certain ECU parameters (e.g. Engine parameters etc.) Testing and verification of the system at failure conditions In the above-mentioned test scenarios, HIL provides the efficient control and safe environment where test or application engineer can focus on the functionality of the controller.
Tight development schedules
The tight development schedules associated with most new automotive, aerospace and defense programs do not allow embedded system testing to wait for a prototype to be available. In fact, most new development schedules assume that HIL simulation will be used in parallel with the development of the plant. For example, by the time a new automobile engine prototype is made available for control system testing, 95% of the engine controller testing will have been completed using HIL simulation.
The aerospace and defense industries are even more likely to impose a tight development schedule. Aircraft and land vehicle development programs are using desktop and HIL simulation to perform design, test, and integration in parallel.
High-burden-rate plant
In many cases, the plant is more expensive than a high fidelity, real-time simulator and therefore has a higher-burden rate. Therefore, it is more economical to develop and test while connected to a HIL simulator than the real plant. For jet engine manufacturers, HIL simulation is a fundamental part of engine development. The development of Full Authority Digital Engine Controllers (FADEC) for aircraft jet engines is an extreme example of a high-burden-rate plant. Each jet engine can cost millions of dollars. In contrast, a HIL simulator designed to test a jet engine manufacturer's complete line of engines may demand merely a tenth of the cost of a single engine.