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Oscilloscope Reinvented The convergence of power stage modeling and signal processing within the HIL gives its oscilloscope function a high-bandwidth probeless access to all signals within the test setup. This convergence opens up unheard-of possibilities for automatic testing and reporting.
By Subhashish Bhattacharya NC State University, Toni Gualtieri, Predrag Nikolic and Dusan Majstorovic Typhoon HIL Inc. Introduction It is estimated that the cost of quality in power electronics (PE) industry amounts to 2-4% of its overall revenue. The lion's share of those resources is spent on control systems quality assurance, quality control and mitigation of the controller-related field problems. Now, the question is if there is a way to test power electronics control systems more efficiently than with the current methodologies? Before we can answer this question let us look into the state of the art PE laboratory first.
Figure 1. Testing power electronics controls; a) the power electronics lab setup, b) the HIL setup: 1. a deep memory oscilloscope with a large number of channels, 2. test setup control, supervision and protection, 3. a grid side transformer, power supplies, etc., 4. a power electronics converter, 5. An energy source/load, 6. controller hardware/software under test and 7. a HIL with HIL Connect. Unique Challenges of PE Testing Figure 1a illustrates a typical high power test stand with the PE controller under test and the oscilloscope highlighted. The oscilloscope in a typical PE test stand has to process a large number of input channels, with a large dynamic range and a multitude of probes with, sometimes, extreme insulation requirements. The large dynamic range means that a very deep memory oscilloscope is needed and the high number of probes is both costly and time consuming to deal with.
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To make things more "interesting", the testing involves elaborate safety procedures while the interpretation of the results is often done manually, i.e. skilled technicians interpret the downloaded measurements, store them, and write reports about their findings. Figure 1b proposes an alternative approach which all but removes the need for testing the PE controller in a high power laboratory. The "trick" is to package the 5 elements from Figure 1 into a compact, desktop device that emulates the power stage with high fidelity and time resolution, and at the same time includes a powerful oscilloscope, as well as test control, supervision, analysis and automation functionalities.
Figure 2: PE Testing the HIL way 1. a deep memory oscilloscope with a large number of channels, 2. Python script test control and supervision , 3-5. digitized PE hardware, 6. controller hardware/software under test and 8. a HIL Connect custom physical interface between the physical controller and the HIL system. PE Testing the HIL way Once the power stage is digitized inside the HIL device, there is no need to worry about physical properties of the signals (all that remains is the low voltage custom interface between the PE controller and the HIL, element 8 in Figure 2). Suddenly, with high ener-
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gy components out of the picture, PE testing becomes a signal-processing problem, which is much easier and less costly to solve.
Figure 3: HIL scope key parameters
The HIL oscilloscope The convergence of the power stage model and signal processing within the HIL gives its oscilloscope a high bandwidth, probeless access to all HIL internal signals. Figure 6: Performance advantage of HIL test setup. The oscilloscope's 32 Mpts of storage are directly connected to the HIL processor data bus which enables probeless data access of the HIL scope. The signal processing and display functions are implemented in SW and have found a place on the PC. The cycle time, steps 1-4, from Figure 4 depend on the size of the acquired data sample size (400 Bytes to 128 Mbytes), and is in the 20ms-10s range.
Figure 4. HIL Scope time diagram
Fully control the test procedure on both, the HIL and the controller • under test • collect and analyze the test data • automatically generate test reports
Figure 6 compares variable cost per test points between the "average" high power laboratory and the HIL test setup. Although exact numbers may vary, the speed and efficiency of unsupervised 24h/365 day automatic test system with the test analysis and reporting functionality of the HIL provides orders of magnitude lower cost per test point than the power laboratory. Additionally, to further increase the capacity of the HIL test setup, all that is needed is additional HILs, while in case of the power laboratory, it is additional space, additional personnel, and additional high power equipment. Conclusion By digitizing the power laboratory by means of HIL technology i.e. by "removing" high energy physics from PE controller development and testing, we have reasons to expect a measurable reduction of quality cost, improved quality and shorter time to market.
Figure 5: Feature-rich HIL Scope GUI. In addition to featuring the rich oscilloscope mode from Figure 5, the HIL Scope can also operate in capture mode, in which the data is stored in the PC for post-processing. In this mode, the scope can also be controlled via the Python script for easy integration into the automated PE controller testing procedure.
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Testing made simple and cost effective The control system test architecture from Figure 2 all but removes the personnel and material risk of the high power laboratory, and replaces it with the comfortable, risk-free development of Python scripts that:
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