Because it is there…(When to use your H.I.L.)

There is a famous quote from George Mallory; when asked why he was trying to climb Mount Everest he said “Because it is there.”(1) Unfortunately this is sometimes the approach that is used for when to use a Hardware In the Loop (H.I.L.) system; it is used “because we have it.”

Integrating a model into a H.I.L. system requires additional work which is not used on the final product. As a result, use of the system should be restricted to when it is needed. In this post I will review the primary reasons for using a H.I.L. system.

Validation of a 3rd party controller

Some “Third Parties”

The simplest reason for using a H.I.L. system is when the unit under test (UUT) or part of the overall system is a third party controller where you do not have access to the source code for simulation based tests. Common tasks in this example are fault detection and boundary condition testing.

Timing studies

Connecting a controller to the H.I.L. system enables complex timing studies of the controller, e.g. determining the mean, max, and statistical variation in timing for the algorithm.

Fault injection / Noise

While fault injection(2) and noise simulation can be performed in a simulated environment it generally requires modifications to the control algorithm to support this; as a result it is preferable to perform these tasks on a H.I.L. system.

Difficulty in modeling physical systems

In some cases, the creation of a physical model has a high cost in development time (3) whereas the actual physical component(4) can be hooked into the H.I.L. system easily. In this case having the physical component enables development that would not be possible in simulation and would be difficult or impossible in the actual environment.


  1. George Mallory died in his attempt to climb Everest, with Sir Edmund Hillary being the first to reach the top of the mountain.
  2. On my first job we designed an automated Break Out Box which was marketed as Auto B.O.B (this was for the automotive market). The running joke when there were any problems on the project was “It is Bob’s fault.”
  3. High cost in this case means the total engineering development time.
  4. When a physical component is hooked into the system it is called a “buck.” The first time I encountered this was with an early A.B.S. system. The brake dynamics were too fast to model and the tire/brakes system are easy to install.

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