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.
Early process human factors development HIL simulation is a key step in the process of developing human factors, a method of ensuring usability and system consistency using software ergonomics, human-factors research and design. For real-time technology, human-factors development is the task of collecting usability data from man-in-the-loop testing for components that will have a human interface. An example of
usability testing is the development of
fly-by-wire flight controls. Fly-by-wire flight controls eliminate the mechanical linkages between the flight controls and the aircraft control surfaces. Sensors communicate the demanded flight response and then apply realistic force feedback to the fly-by-wire controls using motors. The behavior of fly-by-wire flight controls is defined by control algorithms. Changes in algorithm parameters can translate into more or less flight response from a given flight control input. Likewise, changes in the algorithm parameters can also translate into more or less force feedback for a given flight control input. The “correct” parameter values are a subjective measure. Therefore, it is important to get input from numerous man-in-the-loop tests to obtain optimal parameter values. In the case of fly-by-wire flight controls development, HIL simulation is used to simulate human factors. The flight simulator includes plant simulations of aerodynamics, engine thrust, environmental conditions, flight control dynamics and more. Prototype fly-by-wire flight controls are connected to the simulator and test pilots evaluate flight performance given various algorithm parameters. The alternative to HIL simulation for human factors and usability development is to place prototype flight controls in early aircraft prototypes and test for usability during
flight test. This approach fails when measuring the four conditions listed above.
Cost: A flight test is extremely costly and therefore the goal is to minimize 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.
Feasibility: It may not be possible to explore certain critical timings (e.g. sequences of user actions with millisecond precision) with real users operating a plant. Likewise for problematical points in parameter space that may not be easily reachable with a real plant but must be tested against the hardware in question. == Use in various disciplines ==