Hardware-in-the-loop (HIL)
A testing method in which the real controller runs against a real-time simulation of the machine it is meant to control.
Testing a brake controller by crashing into things is expensive and dangerous. In a HIL rig the actual control unit runs its actual firmware, but its sensors and actuators are replaced by a computer simulating the vehicle, engine or aircraft in real time. The simulator generates wheel speeds and pressure signals, the controller responds, and the simulator works out what would have happened.
The payoff is being able to try black ice, sensor dropouts and short circuits thousands of times a day, unattended. HIL is the last step in a chain that starts with model-in-the-loop and software-in-the-loop, where the same tests run against models and compiled code with no hardware involved. It is standard practice in automotive and aerospace, where safety standards effectively demand it.