LEROY AUTOMATION is a leading European manufacturer of automation systems for on-board train and rail signalling control-command systems. For several decades, LEROY AUTOMATION has been accompanying major rolling stock manufacturers, locomotive subsystem integrators and original equipment suppliers for new built rolling stock vehicles as well as for many train fleet overhaul projects. 

As part of on-board train architectures, energy management is an important topic where efficient power converters are key subsystems. Thanks to the auxiliary power converters or inverters, the current-voltage characteristics can be adjusted to match the perfect requirements of subsystems installed into subway cars and railway vehicles. They also fulfil a significant role in safety by protecting the entire system, as they are resistant to over-voltages and short-circuits. The intelligence of the auxiliary inverters is implemented onto powerful embedded controllers, where high-speed digital signal processing tasks are common. Today, LEROY AUTOMATION discusses how to implement some key features of auxiliary inverters’ management onto proven commercial-off-the-shelf (COTS) technologies: the BRIO hardware platform.

Figure 1: BRIO R107 – CANopen-based Programmable Logic Controller (PLC)

In parallel, LEROY AUTOMATION has been asked to think about an innovative solution for the BRIO to be able to detect in real time a ripple voltage on a high-voltage DC line around 900Vdc. The technical specification was restrictive: the AC ripple signal can range from 0.35Vrms to 70Vrms, and can be on any 2 frequencies between 0 and 1 KHz.  In addition, the system had to be fully configurable and monitored from the CANopen interface (ripple frequencies to be detected, detection temporal window, Vrms threshold to declare the ripple present or not, measured energy for both frequencies), and had to activate specific digital outputs on frequency detection.

The final system solution could then be presented as the following block diagram:

In Digital Signaling Processing (DSP), the DFT (Discrete Fourier Transform) is an efficient tool to find a specific frequency in a signal. Simulations have shown that a DFT linked to Hann windows, gave good results with an acceptable accuracy on signals recorded on train.

The DFT equation by itself is not so complex:

It just says that “X(k)” is the level of the frequency “k” in the complete signal represented by a set of “N” samples “x(t)” of the analyzed signal.



The Hann window consists in deforming the analyze signal before applying the DFT algorithm.  Indeed, the DFT implies a temporal windowing of the signal over N samples, which affects the signal spectrum. Applying a pre-deformation such as Hann windows prior to the DFT improve the frequency response of the DFT.

The Hann window is also not very complex in terms of digital signal processing function. For a set of N samples, it gives a coefficient to apply on the sample “t” by the following formula:

In order to illustrate the efficiency of the Hann window, let’s take a pure sinus signal. 

We can see that in frequency domain, the spectrum representation gives much better results with Hann window than without. Indeed, the energy peaks that characterize the harmonics of the original signal are much clearer in red, which allows a frequency detection with a better accuracy. 

Thus, in order to detect 2 specific frequencies “K1” and “K2” in the analyzed signal, the algorithm to implement was:

It is one thing to make an offline simulation on a Windows-based PC, with mathematical tools. But it is another challenge to make the same in real-time, in an on-board PLC, with the very same accuracy.

As mentioned earlier, the BRIO is based on a STM32 micro-controller and an FPGA device.  It could have been easy to implement the algorithm in the micro-controller. A few ANSI-C programming lines would have been enough.  Nevertheless, the micro-controller was already busy to manage the other functionalities of the BRIO itself, including CANopen messages, and the real-time criteria would have been difficult to meet.  It was then decided to implement the algorithm in the FPGA device.  At this point, the difficulty remains in implementing calculus in the hardware chip.  Indeed FPGAs, and specifically small matrices, are not adapted well for floating-point computing with cosine and sine functions. 

The idea was to take benefit of each device strength with a modular design approach: microcontroller for complex floating-point and cosine/sine off-line calculus, FPGA for real-time fixed-point computing. 

The real-time part of the algorithm implemented in the FPGA can be represented as follows:

Each time a parameter is modified (window size, K1, K2, sampling frequency), the micro-controller computes only once the 5 tables that contains cosine and sine coefficients needed by the Hann window and the DFT. Furthermore, the FPGA only needs to perform simple operations such as multiplications and additions for which it is very well suited.

The digital part of the algorithm worked fine but a challenge remains.  The signal to analyze is composed of a DC part that can be up to 1,000Vdc.  On the other hand, the ripple can be as low as 0.5Vac.  It was necessary to remove the DC part of the signal in order to keep most of the ADC resolution for the AC part.  It was also necessary to keep the low-pass filter required before any analogue-to-digital conversion.  The resulting band-pass filter has then required a lot of calculus and simulations to be flat enough over the required frequency range, and for the whole temperature range (from -40°C to +70°C).  It had also to fit the small footprint available on the BRIO printed-circuit board (PCB).  

At the end, we were able to modify one analogue input of the BRIO to implement the following band-pass filter:

At this time, interfacing the frequency detection system with the CANopen stack was the easiest part of the project. Some specific PDO messages have been implemented to configure dynamically the detection parameters and to report the detection results in a very efficient way.



As a conclusion, this project has been a great engineering success.  Operational tests on-board subway cars gave excellent results, totally in line with theory and simulations.  BRIO was already known as a highly available and reliable I/O module with PLC-programming capabilities.  But this particular auxiliary inverter application has revealed all the potential and benefits of the BRIO hardware with the partitioning of digital signaling processing algorithms and techniques onto a mixed FPGA and micro-controller platform.

About LEROY AUTOMATION

LEROY AUTOMATION is a company headquartered in Toulouse with a North American subsidiary located in Montreal. For 40+ years, LEROY AUTOMATION has been designing, manufacturing and marketing automation products and embedded electronic equipment for on-board rolling stock vehicles, automation solutions for electrification networks, and railway control systems. Especially designed for harsh conditions, the company’s products are suited for demanding electromagnetic and extreme thermal, as well as for high vibration operating environments. From feasibility studies, detailed engineering and design, to maintenance and repair services, LEROY AUTOMATION collaborates with its worldwide customers during the complete life cycle of their products and systems. For several decades, LEROY AUTOMATION has cooperated with ABB, ALSTOM, CRRC, SIEMENS, THALES, and others in international project and product developments along with railway and mass transit authorities, industrial and military customers, system integrators and OEMs worldwide.

All trade marks and registered marks are acknowledged.

For more information, please contact:

LEROY AUTOMATION SAS

250 rue Max Planck

31670 Labege (France)

T. +33.562.240.550

sales@leroy-autom.com

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