How to choose between digital twin and HIL?

How to choose between digital twin and HIL?

Recently, there has been a lot of discussion about the use of “digital twin” technology to accelerate product development, software debugging and solving electromechanical interaction problems. Digital twin technology has been popular in recent years, just like the “hardware-in-the-loop testing” of similar popular tools a few years ago. The same (usually called HITL or HIL).

Recently, there has been a lot of discussion about the use of “digital twin” technology to accelerate product development, software debugging and solving electromechanical interaction problems. Digital twin technology has been popular in recent years, just like the “hardware-in-the-loop testing” of similar popular tools a few years ago. The same (usually called HITL or HIL).

What are digital twins and HITL? Generally speaking, for digital twins, you will create a software model to control the system, then provide it with the inputs and outputs of the controller under test, and see how your controller performs in terms of its expected functions.

How to choose between digital twin and HIL?
Figure 1: In principle, a digital twin is a virtual model of the entire application and process, allowing designers to visualize and test designs through a single software entity.

Conversely, for HITL, you can build to interact with the core and directly use the actual hardware (circuits and machinery) to evaluate the performance of the controller (Figure 2). In other words, DT is almost all software and models, while HITL, as the name suggests, has some actual circuits and even electromechanical components.

How to choose between digital twin and HIL?
Figure 2: This top-level view shows the key components of the HITL test system, which uses representative real-time responses, stimuli, and functional examples to connect all I/Os of the HITL test system. The unit under test (here is the Electronic control system of the car).

The example of using a car engine and its ECU (Electronic Control Unit) can make this clearer. For the digital twin scenario, you model the engine completely as a software structure, and this model “talks” to the software of the controller being developed. In contrast, with HITL, you are modeling the engine, but now the modeling software connects the actual circuit I/O to the controller under development, and then the controller will actually communicate with the interface. HITL usually requires rack-mounted equipment, which means a lot of circuits are required (Figure 3). One of the attractions of digital twins is that they eliminate the need for most, if not all, hardware.

How to choose between digital twin and HIL?
Figure 3 As the name suggests, HITL integrates hardware, and in the meaning of two words: electronic and electromechanical components.

HITL systems can even be used as standard products, such as high-precision and high-dynamic three-axis and five-axis flight motion simulator (FMS) systems for the development and production testing of missile guidance and seeker kits (Figure 4).

How to choose between digital twin and HIL?
Figure 4 The HITL system can be provided as a standard product application, such as a flight motion simulator system for testing missile guidance and guidance components.

So, which of these two is better? As with engineering questions almost always, the answer is simple: “it depends on the situation.” It depends on factors including the time to create the respective model, the confidence in the model, and the complexity of the simulated I/O. Some supporters of digital twins say that HITL is “past tense” and is no longer needed. Some supporters of HITL claim that digital twins have been overhyped and that HITL is more faithful to the model. Others believe that the best solution is a combination of the two and should be applied with caution.

Not surprisingly, the problem is mainly about the model rather than the method. We know that it is difficult to develop a good digital model in the real analog world, and the model with the highest accuracy is the last 10% of the model. There are many subtle unknowns, extreme situations, anomalies, nonlinearities, inflection points, and the creator of the model simply does not understand or quantify more of them. Relying too much on model accuracy is just the latest manifestation of the classic but still valid adage “garbage in, garbage out”.

There is no doubt that it is absolutely necessary to use various models, whether they are digital twins or HITL, Spice, RF software packages or simulation and analysis tools such as COMSOL Multiphysics, Mathworks MATLAB and Simulink, and ANSYS HFSS. But to be realistic about the sophistication of these models, always keep in mind that the model can show the accuracy of three, four or more significant figures, but the actual accuracy is usually much lower if there are situations in the real world that the model cannot capture.

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