Case Studies

Hardware in the Loop (HWIL)

Hardware in the Loop (HWIL)

Customer: Fortune 100 Aerospace Company
Project: Missile Avionics Subsystem Testing
Challenges: Short Turnaround, Mission Critical


If a rocket exhibits a design problem once it’s flying, you cannot recall it to the factory for repairs. When it comes to avionics hardware and software, especially for rockets and spacecraft, exhaustive testing must occur while everything is still on the ground.

The Challenges of HWIL

“Hardware In the Loop” (HWIL) provides the ultimate testing for flight avionics subsystems using a real-time simulation. The HWIL testing approach involves connecting exact hardware interfaces to every input/output connection of the avionics device under test (called the “Unit Under Test” or “UUT”). Each hardware interface is driven by a real-time simulation in order to present the UUT with high fidelity, real-world physics and interactions/responses. During the testing, the UUT actually “thinks” it’s flying in the vehicle while every interaction and special condition is exercised, logged, and observed.

When our customer needed an HWIL system “yesterday” to qualify a critical – and overdue – missile avionics chassis, they knew they had a significant problem: HWIL systems are notorious for being time-consuming to design and build, and they can also be technically difficult and expensive. One reason for this is that HWIL equipment needs to connect to every input/output interface of the UUT. The I/O interfaces often span a wide range of signal types (serial, analog, digital, high voltage/power, Mil-Std-1553, SpaceWire, etc.) and typically have high channel counts. In addition, the HWIL equipment needs to rapidly respond to interactions with the UUT to mimic the rest of the vehicle. Response time often needs to be less than 1 millisecond (guaranteed) and may also involve running a real-time vehicle/physics simulation to develop an appropriate response.

Coincidentally NetAcquire had provided several hundred HWIL systems of various types (for more than a decade) to another group at the same company just down the road. As the missile avionics team searched for solutions, one engineer told a colleague “you should give NetAcquire a call”.

Project Set-Up

With the short schedule, there was no time to waste. The customer provided a bulk download of vehicle and avionics interface specifications. The NetAcquire applications engineering team quickly analyzed the input/output needs and simulation requirements necessary for an HWIL test system. NetAcquire systems have an inherent benefit: they already support more than 50 different hardware-signaling interfaces as COTS ordering options. With mix-and-match provisioning of interfaces into a single chassis, the hardware design came together quickly. Of equal importance was the software. The customer already had a software simulation of the vehicle dynamics but, in order for this to work correctly, the NetAcquire system needed to guarantee real-time operation and low-latency responses to the UUT. Again, COTS product capabilities saved the day. NetAcquire systems were already designed from the ground-up with a deterministic real-time operating system and associated application software that provided fractional millisecond response time guarantees.

Field Programmable Gate Arrays (FPGAs) were a valuable asset to the success of this project. An FPGA is a logic device that can be reprogrammed by software in the field to “wire together” logic blocks to allow implementation of specialized interfaces such as custom serial protocols. NetAcquire has developed several high-density PCI Express FPGA boards that, in conjunction with downloadable firmware, can quickly implement vehicle-proprietary interfaces. In the case of this customer’s UUT, eight specialized serial interfaces were developed as downloadable FPGA firmware without incurring the cost or lead time of developing a custom hardware interface board.
To further expedite the integration and test schedule, the customer asked NetAcquire to design and integrate the HWIL equipment into two full-height 19” racks. These racks included a meticulous power distribution/grounding design, uninterruptible power supplies (UPS), GPS time synchronization, NetAcquire-developed signal conditioning/isolation circuitry, and multiple high power electrical loads.


NetAcquire efforts and the resulting successful HWIL delivery made a dramatic difference in getting the customer’s critical flight hardware into production. This caught the attention of the division Vice President who personally presented NetAcquire with a special vendor commendation that included a thank you for “a superb job”. The DVP said the NetAcquire team and solution was “exactly what we look for in a partner”, and praised our “willingness to go the extra mile” and “dedication to first pass success”.

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