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Why use Hardware-in-the-Loop Simulation?
This question is an important part of understanding real-time technology.
To restate the question using a control systems term: 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 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 test efficiency is typically
a formula that includes the following factors:
- Cost
- Duration
- Safety
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.
Specific conditions that warrant the use of HIL simulation include the following:
- Tight development schedules
- High-burden-rate plant
- Early process human factors development
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. Companies
such as Gulfstream®
and United Defense®
have executed highly efficient
and successful programs
using the Simulation-Centric
process.
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 an 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, an 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 three 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.
Given all three criteria listed at the beginning of page, it seems that HIL
simulation is, once again, the best option for developing usability and human
factors.
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