RAILWAY TECHNOLOGY UPDATE
The purpose of this page is to provide a source of information regarding the impact of railway technology changes on the manufacture and functionality of modern rolling stock.
DEADTIME OUT
While working on a customer's traction project a frequent control question was encountered - how to effectively counteract inherent deadtime present in a control system. The system in question was a DC motor drive and in this instance it was feasible to change the hardware to reduce the deadtime problem. However, in many cases this is not possible, so what are the alternative control solutions?
Deadtime arises from a delay in propagation of a control demand to the controlled output, and during this time there is no response at the output. Common traction sources of deadtime are fluid transmission in hydraulic and pneumatic controls, backlash in a gearbox or coupling, and firing delay in a phase controlled rectifier. A measure of deadtime significance is calculated from the product of deadtime (in seconds) and required bandwidth (radians / second). If this figure is greater than 0.5 then additional compensation is often necessary to achieve acceptable stability and response.
It is a common misunderstanding that deadtime can be tuned out by appropriate derivative term within a PID controller. Derivative correction can compensate for phase lag in the controlled plant, but it is always ineffective against pure time delay. Instead the classical deadtime compensation method was conceived in 1957 and is known after its originator as the Smith Predictor. Its control structure is shown in the block diagram below. It requires a reasonably accurate model of the plant without deadtime and a separate series function representing the cumulative deadtime delay estimate of the plant.
By feeding back the undelayed estimated model output, the PID control response is driven by a fresh signal. Only a small error signal, which incorporates latency, is added to finely adjust the PID response to eliminate closed loop differences between the model reference and the actual plant. There are many variations of the Smith Predictor scheme to alleviate sensitivity to model errors, but increasing complexity can lead to diminishing returns, and often the simplest implementations are the most effective.
SIMULATIONS TOOLS UP
Some colleagues have enquired what is happening in the world of proprietary simulation packages for electronic / electrical / mechanical / system modelling.
Such tools should have a rightful place in the traction development cycle along side dedicated CAD/CAE mechanical engineering packages.
The answer is similar to events in many engineering sectors - "consolidation".
Those familiar with rail traction modelling a decade ago, or if you read Paper 3, will know there were two distinct generic families of simulator.
- Electronic circuit design tools
- System modelling tools
It would have been a reasonable guess to expect these families to merge, but actually the opposite has occurred, driven by the dominating influence of the large scale micro-chip manufacturers.
As chip life cycles have shortened, fabrication scales reduced, and transistor component counts multiplied, so development simulation has become paramount.
A lucrative simulation industry has been built around silicon based MEMS (Micro-Electro-Mechanical Systems), and a set of high-end vendors are competing for the market.
The players include ARM, Cadence/CoWare, Coventor, Dolphin, Mentor Graphics and Synopsis.
All of these vendors have a family of products to support the main MEMS electrical and physical modelling centerpiece.
The chip simulation packages utilize the public domain SPICE engine in conjunction with other physical modelling tools such as ANSYS and Hardware Description Language (HDL).
For example, Synopsys produce software tools for the design, synthesis, and validation of FPGA and ASIC integrated technology. Also in the same portfolio are packages for CAD, mixed-signal analysis and analogue simulation.
The following Synopsys tools are amongst those available:
- Analogue Modeling and Simulation(combined HSPICE and Saber bundle including Cosmos)
- Front-end Design and Verification (including 'Physical Compiler' and Leda HDL)
- TCAD
- System Development
The irony that HSPICE (a h.f. SPICE variant) is teamed up with Saber, a long term rival.
All this means that there is no sensible alternative to SPICE for circuit development - it's the industry standard. A full featured version called LTSPICE is available via the recommended links. The LTSPICE engine allows full numerical integration control and the plotting facilities are versatile with very good FFT spectral analysis options. The schematic capture is less intuitive than most, however, with experience is quite acceptable. See LTSPICE download package details for licence terms.
With SPICE Windows versions starting from free downloads to top end UNIX licences at over £100,000, an entry level for every organisation exists.
Importantly, migration from one product to another is possible, although more difficult at the top end.
Over the years SPICE behavioural modelling (block diagram representation - included in LTSPICE) has developed to permit circuits to be part of larger systems.
However, it is yet to be a contender for large scale system modelling. There is little incentive for such a development, given the profitability of the micro-chip market.
The system modelling market is already dominated by MATLABTM. MATLAB, from the Mathworks, solves system problems represented in matrix form.
Originally it solved continuous time system equations, but has consistently expanded with comprehensive tool-boxes covering areas from digital (z-plane) design to robust controller optimisation.
Tool-boxes for power electronics and circuit components exist to extend MATLAB capabilities, but complex, mixed-mode, circuit only simulations, are not handled so well.
Like SPICE. MATLAB is the industry standard in its field. However, there are alternatives, most of which offer MATLAB compatibility or at least similarity.
Matrix-X was a strong brand in the 1990's, but now VISSIM, LABVIEW and SCILAB are the main alternatives to MATLAB.
VISSIM has good MATLAB compatibility and code generation facilities, but does not offer an extensive range of control tool-boxes like those provided by Mathworks.
LABVIEW has gained popularity in recent times, and is particularly well geared to process control and data analysis problems.
SCILAB offers a terrific free product capable of solving many small or medium sized problems. It has strong UNIX (or rather GNU) origins, and is now also ported to Windows OS's.
In conclusion, the simulation tools sector is a maturing market. Two industry leading standards in SPICE and MATLAB have emerged.
These products deserve their support for good and differing reasons.
For major engineering organizations to significantly resource and fund projects based on other products not committed to either of these standards could result in future maintenance difficulties.
BEARINGS DOWN
The existence of shaft induced voltage (SIV) has been known for nearly a century.
Conduction of induced bearing current through paths to ground has been minimised by careful motor design, frame earthing strategy, shaft brushes or insulated bearings.
The effect has been to negate conducted currents, otherwise particularly evident at low speed, when a good electrical contact between rolling elements exist.
This results in a non-arcing bearing current, which has no adverse effect on life expectancy.
However, publicity in the Drives and Controls journal has raised the profile of long term bearing damage resulting from the use of IGBT inverters.
Just when designers thought these issues were mastered, along came IGBT inverters in the 1990's, with higher switching frequencies up to 10kHz, designed to reduce harmonics and noise.
A high dV/dt at switching instants is inevitable, which discharges current through parasitic capacitances, firstly between the stator winding and motor frame, and secondly from stator to rotor bars and rotor core.
The discharge current spikes can be positive or negative according to the direction of the switching common-mode voltage which creates them.
The resulting bearing current caused by repeated discharges in IGBT inverters have the capacity to break down the bearing grease dielectric impedance, particularly posing a risk as motor speed increases.
When the grease breaks down the high energy discharge currents cause electro-erosion of the bearing, evidenced as pitting or fluting of the raceway and often associated with a grey discoloration.
The problem becomes more acute with increased motor size because of the corresponding increase in parasitic capacitance.
Consideration to switching frequency reduction should be given in this case.
Additionally ceramic coated insulated bearings can prevent discharge currents, but is expensive and not always 100% reliable.
The risk to rolling stock operators considering modernising old thyristor or even GTO equipment with high frequency IGBT inverters and choppers is self-evident. The higher the switching frequency the more rapid the failure mode once arcing conditions are established. Motors not originally designed to work in a high switching frequency environment could have significantly reduced life expectancy.
There are also electrical interference and longitudinal voltage compatibility issues creating by discharge currents, and the capacity for premature bearing wear to adversely affect the harmonic footprint of a vehicle.
These problems can be overcome by use of careful simulation supported design and extensive testing, but are not recommended to be ignored when re-engineering or refurbishing equipments.
|
