HIL testing in one paragraph
In hardware-in-the-loop testing the device under test (DUT) is the real controller hardware running its real software: an engine control unit, a battery management system, an inverter controller or a drone flight controller. Everything it would normally sense and drive, such as the vehicle, the machine or the environment, is replaced by a real-time simulation that reads the controller’s outputs and returns sensor signals. The loop is closed, and ideally the controller cannot tell that it is not in the real system.
That makes HIL the stage where software meets real timing, real interfaces and real electronics, before the complete system exists, or when testing the complete system would be unsafe or too expensive.
MIL, SIL, PIL and HIL: where HIL sits
HIL is one step on a ladder of increasingly real test environments, usually drawn along the right side of the V-model. Each step finds errors the previous one cannot:
| Stage | What is real | What is simulated | Typical findings |
|---|---|---|---|
| Model-in-the-loop (MIL) | Nothing: controller and plant are models | Everything | Control-law design and logic errors |
| Software-in-the-loop (SIL) | Production code, compiled for a PC | Plant, sensors and timing | Coding and integration errors, regressions |
| Processor-in-the-loop (PIL) | Code running on the target processor | Plant and I/O | Numerical and execution-time issues |
| Hardware-in-the-loop (HIL) | The complete controller with its I/O | Plant and environment, in real time | Interface, timing, fault-handling and integration errors |
| Field or flight test | Everything | Nothing | Effects that no model captured |
The building blocks of a HIL test bench
Whatever the industry, a HIL test bench, also called a HIL test rig or HIL simulator, is built from the same parts:
- Real-time simulator. A computer that runs the plant model with a fixed, deterministic time step, fast enough for the controller’s loop rates.
- Plant and environment model. The physics the controller acts on: vehicle dynamics, motors, batteries, aerodynamics and sensors. Its fidelity sets what the test can prove.
- I/O and signal conditioning. Analog and digital channels, PWM, CAN, serial buses and Ethernet, at the voltage levels and timing the controller expects.
- Device under test. The real controller, ideally with production firmware, wiring and connectors.
- Fault insertion. The ability to open, short or corrupt signals to test failure detection and safe states.
- Test automation and logging. Scripted test cases, pass/fail criteria and synchronised recording for regression testing.
- Physical stimulus where needed. Some sensors cannot be convincingly simulated as electrical signals. Then the sensor itself is moved, loaded or heated on a physical rig.
How to set up a HIL test bench, step by step
A HIL bench is a measuring instrument for software. Building one follows the same order regardless of the product:
- Define the questions. Write down which requirements the bench must verify: control performance, fault reactions, timing or regression after every software release.
- List the controller’s interfaces. Every input and output with its signal type, range, update rate and latency budget. This list sizes the I/O hardware.
- Choose the model fidelity. Use the simplest model that still exercises each requirement, and add detail only where a test depends on it.
- Close the loop and measure timing. Measure the end-to-end latency from controller output to returned sensor signal and compare it with the controller’s loop period.
- Validate the bench itself. Compare bench results with known data, earlier tests or flight logs before trusting it with new software.
- Automate and version everything. Script the test cases, version models and scripts together with the software, and archive every run.
When physical motion belongs in the loop
Classic HIL is electrical: the simulator replaces the sensors by generating their signals. For a flight controller this means feeding it simulated IMU, barometer and GNSS data. That tests the software path, but not the real sensors, their mounting, or their vibration and alignment errors.
A motion platform closes that gap. With the flight controller on a 3-axis test rig, its own gyroscopes and accelerometers sense real roll, pitch and yaw while the vehicle model and the rest of the environment stay simulated. Encoders on each axis provide the ground truth for the attitude the controller estimates.
This is the approach behind INFINITE-HIL X1, a 3-axis platform with continuous 360° rotation and 21-bit absolute encoders that combines physical attitude motion with model-based test inputs. The drone HIL testing page walks through that test loop step by step.
Where HIL testing is used
- Automotive: engine, transmission, braking and driver-assistance ECUs, the origin of most commercial HIL systems.
- Aerospace and UAVs: flight controllers, autopilots, avionics and actuator controllers.
- Power electronics and energy: inverters, motor drives and battery management systems, often with fast multiphase models.
- Robotics and industrial automation: motion controllers and PLC logic tested against simulated machines.
What HIL testing cannot tell you
A HIL bench is only as good as its models and interfaces. It will not reveal effects that nobody modelled, such as unexpected aerodynamics or electromagnetic interference, and it can hide timing problems if the simulation runs differently from reality. That is why HIL complements field and flight testing instead of replacing it: it makes those tests rarer, safer and more focused. For UAVs, the flight controller testing guide shows how the stages fit together.
Planning a HIL test bench for a UAV, a flight controller or another electromechanical product? Tell us about your system. INFINITE-HIL designs custom HIL test and simulation systems around the product and the budget.