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Posted on 29 June 2019

New Hardware-in-the-Loop Platform for Rapid Development of High-Reliability EV and HEV Propulsion Drives

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Main propulsion drives are one of EVs (electric vehicles) and HEVs (hybrid electric vehicles) safety-critical systems. A configuration with only a DC-link current transducer instead of two phase-current transducers promises improved sensor reliability when matched with equally reliable software implementation of the phase-current reconstruction algorithm. With a new ultra-low-latency (ULL) hardware-in-the loop (HIL) emulator, it is now possible to develop, debug, optimize and type-test such safety-critical real-time software functionality within one integrated environment.

An affordable HIL environment promises to revolutionize the power electronics design tool-chain

By Evgenije Adzic and Nikola Celanovic, Typhoon HIL GmbH

 

Typhoon HIL development environment

State of the art HIL systems with their often massive computational power, excel at modelling large, relatively slow systems but typically have too long a latency to provide sufficient time-resolution needed to model the fast-switching dynamics of power electronics (PE) systems. A highfidelity ULL HIL400 system dedicated to HIL emulation of power electronics circuits offers a “time microscope” that expands the time resolution of the existing (digital) HIL solutions. It comprises a programmable, application-specific processing architecture, fast IO subsystem and a custom tool-chain which, all together, make a 1ìs simulation time-step and latency possible. Typhoon HIL also offers a range of interface boards that simplify the hardware interface between HIL400 and some of the standard DSP evaluation boards and rapid control prototyping platforms such as the ezDSP TMS320F2812 used in this application example.

Figure 1 illustrates the HIL development environment comprising the HIL400 and an ezDSP TMS320F2812 evaluation board with the hardware shown to the right. The ezDSP TMS320F2812 controller is set on a HIL400 interface board from Typhoon HIL. On the left-hand side of Figure 1 can be seen the PC-based tool-chain comprising HIL400 and DSP tools. HIL400 tools include an intuitive schematic editor with a circuit compiler and the HIL400 control panel which provides a convenient way to assign signals to IO interface pins, program their gains and offsets, operate contactors, change grid voltage (where applicable), start/stop the HIL400, etc. The DSP tool-chain in this example, is TI’s Code Composer Studio.

Typhoon HIL400 development environment

The HIL400 signals are viewed on the oscilloscope, as would be done in a high power laboratory, only in this case, there is no need to worry about safety requirements or deal with current and voltage probes etc. Moreover, because all the internal signals of the modelled circuit are available at the HIL400 analog outputs, it is now possible to also view the signals that are otherwise difficult (or impossible) to measure in the lab, such as machine flux.

The most reliable way to measure inverter phase-currents is by means of a single DC-link current-transducer and a reliable phase-current reconstruction algorithm implemented in software. Figure 2b shows that within every sampling cycle, there are two time-intervals where the DC-link current equals one of the phase currents (or the negative value of the phase currents), which is enough to reconstruct all three phase currents if we assume a negligible common-mode current, i.e. ia+ib+ic=0.

Inverter DC-link current relationship to: a) PWM signals (i.e. voltage vectors) and b) motor line-currents

It is important to emphasize that the illustrations of Figures 2 to 4 are actual real-time oscilloscope measurements of the TMS320F2812 gatedrive signals controlling the motor drive within the HIL400. In other words the configuration of Figure 1 is a real-time simulator and a laboratory test-bench all-in-one: a true “power electronics laboratory in a box”.

The current reconstruction technique requires that the DC-link current-sampling instants be synchronized with the PWM pulses. High fidelity requires that dead-times, gate drive delays and signal processing delays (which depend on the actual control platform and algorithm implementation) be taken into account. Only by means of a ULL emulator it is possible to take into account those delays that are of the order of a microsecond and may also be time-varying. As one can see from Figures 2 to 4, the switching frequency in this example, is set to 4kHz. Figure 3a illustrates that sampling the DC-link current twice per PWM period is not enough because it results in a current measurement error. Instead, in order to accurately reconstruct the average value of the phase current located at the middle of PWM period, it is necessary to sample the DC-link current four times per PWM period (twice for each phase) as shown in Figures 3b and 4b and to calculate their average values.

DC-link current sampling scheme: a) two samples in the lagging side of the PWM b) four samples in the same PWM period

a) Modified PWM pattern b) Advanced DC-link current sampling scheme

However, as the results from Figure 5 show, this is still not enough to correctly reconstruct the phase currents. The cause of this, as the setup from Figure 1 clearly reveals, is the short vectors that are always present at the 60 degree sector-crossing and for low modulation indices. One way to solve this problem is illustrated in Figure 4a where the PWM signals are modified (shifted) in order to provide long enough DC-link current sampling windows.

Reconstructed line-current waveform and current dq-components a) Reconstructed line-current waveform b) Reconstructed current dq-components

The complete control system for the virtual drive was assembled. Rotor speed was estimated using a Model Reference Adaptive System (MRAS) and phase currents estimated by means of the current-estimation algorithm from Figure 3a and Figure 4a. By using the arrangement of Figure 1, it is easy to observe the influence of microsecond-order changes in the PWM and sampling pattern on the drive-current waveforms in the millisecond range.

Summary and outlook

This article showed some of the ways in which the rather unique, 1µs time-resolution of the HIL400 is helpful when developing high reliability EV and HEV propulsion-drive control algorithms. Clearly, there is much more to the HIL400 which is, in fact, a universal PE development platform, enabling PE specialists to study with great ease and confidence, all the fine details of the interaction between the controller and the PE hardware in any and all operating conditions, as they occur in real-time. It can be said that this is the beginning of a new approach to PE system development and a paradigm shift in fast prototyping and type testing.

 

 

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