The next level of virtual verification?
This first blog post will be a brief survey about the use of virtual verification within the development of mechatronic systems. There will also be some considerations about future concepts within the field, which will give you some clues about possible topics for the upcoming posts.
This time, we will consider the development of an arbitrary mechatronic system, a system consisting of both hardware and software. Into the hardware, we count all physical components that range all the way from processors and integrated circuits through actuators and sensor to engines and ventilation circuits. While with the term software, we refer to the embedded code uploaded on processors and integrated circuits.
Back in the days, which is not that many years ago, the graph in the figure above could have been a good schematic view of a development process. Here, we have time along the x-axis, a start point to the left and a delivery point to the right. At this period, it was more or less necessary to have a sequential process, where the actual hardware had to be available before the development of the software could be started.
In this graph, it can be noticed that,
- The knowledge about the system, the green curve, is increasing with time. The knowledge is obtained by testing different solutions and the more tests that can be performed, the more knowledge the developers will obtain about the system.
- The possibility to make changes, the red curve, is instead decreasing with time. The closer one gets to the point of delivery the more limited is the possibility to make changes. The short period left to the delivery makes it hard to get large changes in place and even small changes can rock the foundation of the rigid structure that the system has become close to the delivery point.
- The yellow marked area between the two curves and x-axis, within the considered developing time, is a measure of the effective work that can be performed during the process.
Clearly, the goal should be to maximize the efficient work during the process. The more useful work, the more tests can be performed and more bugs and faults can be found. Fewer bugs and errors result in a better quality of the system and a better final product.
One important variable that we have disregarded in this graph is the cost. We do all know that there always exist alignments within companies whose main responsibility is to keep the costs as low and the income as high as possible. One efficient way to obtain this is to reduce the development time, to shorten the time-to-market, which is exactly what is visualized in the figure below.
Directly, two significant consequences can be noticed. First, the amount of productive work is more limited. Secondly, the sequential procedure, where the software development starts first after the hardware is in place, does not longer fit within the time for development.
The introduction of Model-Based Design, MBD, has made it possible to separate the software into different components. Some of these are in direct contact with the hardware and are interacting with it. While others, like the controller software components C-SWC, which have the purpose to control the behavior of some physical quantities, have several layers of software between themself and the hardware. To test the C-SWC, it might not be necessary to have the actual device in an early stage of the project. Instead, virtual models describing the dynamics of the physical quantities, and how they are perturbed by other quantities, can be the object to test the C-SWC code against, a so-called plant model.
The entry of plant models and separable software components made it possible to start the development of the software earlier and test it on desktop computers instead of on physical test benches. The effect of the virtual testing is visualized by the graph in the figure above. Before virtual testing was introduced, one had to wait to have an available test bench before testing the software, described by the blue curve in the graph. With the introduction of MBD, “only” plant models were needed to verify part of the software for bugs and faults in a virtual environment. The bugs and errors are found at an earlier stage of the process, which is what the red curve is telling us. Still, there is a need for physical testing, but the amount has now been reduced.
The look of the schematic view of the development process changes with the introduction of virtual testing, see the figure above. The curve for obtaining knowledge about the system has a steeper behaviour at the beginning of the process. This corresponds to the possibility to perform virtual tests at an earlier stage. Faster obtained knowledge increases the effective work performed during the process, and the question now is, how can one get even more knowledge at an early stage?
One way is to increase the number of tests that are performed and a virtual environment is an ideal location for performing tests in large numbers, see the figure below. A physical test bench is usually designed for a specific type of test, if one wants to do something outside its specification one has to rearrange the setup or build a new test bench. This can be both expensive and cost a lot of time. With virtual testing, a new test bench can just be some lines in a script away, which makes it simple to switch between test configurations and set up automated processes.
A growing field within virtual testing is Model-Based Testing, MBT, where software algorithms are used to design the test cases, run the test procedures and analyze the result. These algorithms can automatically produce a substantial number of test cases and do even feedback information from the results back to the process in order to create new and better test cases. An example is the TestWeaver algorithm that is described to play chess with the system under test.
Testing a system under test (SUT) is like playing chess against the SUT and trying to
drive it into a state where it violates its specification.
Most, if not all, applications presented so far have been introduced to benefit software development. Plant models and separable software gave the developers access to the virtual test benches. Will it also be possible to use virtual hardware models to actually improve the development of the physical hardware, as well as the software?
If the virtual hardware can be in place early in the process, it would be possible to test combinations of different components in virtual test benches and obtain early knowledge for both hardware and software developers. This will, of course, require much more detailed models of the hardware that exist today, including non-trivial behaviours and limitations that could be triggered from the virtual test environment.
Hardware models of fine granularity will benefit the development of both the hardware and the software. With a structure of common virtual test benches, into which both hardware and software teams are delivering models, it will be possible to test the robustness of the systems in new ways. For example, how the software will react to signals coming from hardware components that are old and not functioning perfectly anymore? Or, how the hardware components should be designed in order to hold for the large forces which can appear with rapid actions from the control algorithms? To be able to test this kind of scenarios at an early stage will not only generate knowledge within the hardware and software teams themself but also put the teams closer together to make it possible for them to find solutions together.
Model-Based Testing and virtual hardware are both two examples of concepts that will increase the knowledge about the system at an early stage and decrease the need for expensive physical test environments.