In large industrial systems, optimizing automation can lead to enormous increases in efficiency. However, this must not lead to overloading the equipment since it could result in millions of dollars worth of damage. Applying the real-time-capable Soft-PLC SmartPLC and using only two industrial PCs, infoteam succeeded in optimizing an altitude test stand for aviation turbines. With the new automation solution, the same system can now perform test scenarios that were previously not possible.
Aviation turbines must withstand enormous loads. Naturally, the units must be put to the litmus test before their use. The Institut für Luftfahrtantriebe (Institute for Aviation Propulsion) of the University of Stuttgart, short lLA, operates an altitude test stand for this purpose that is unique in Germany. This test stand consists of a control room from which the system is controlled and the tests for the sample test turbines are conducted.
The system part consists of a closed pipework in which a total of five compressors create positive or negative pressure. thereby simulating of different flight altitudes. If the system is in full for accepting the test turbines operation, the compressors gave a power consumption of 35 M1V. For this reason, the test stand is exclusively operated in the evening and during the night at more favorable energy costs. Even if these machines consist of solid parts such as cast-iron housings of several centimeters in thickness and arm-size screws, there is a physical phenomenon that can destroy them within seconds and create multi-million dollar damages.
This phenomenon is called the pump surge. In case of an overload, the air flow breaks off abruptly and the powered compressor blades run the risk of being completely destroyed. Thus, a pump surge must never occur. The compressors are protected against such a failure by using two redundant control systems: the pump prevention for safe operation up to the power range limit and the pump protection to prevent exceeding the power range limit. Both protective measures work independently of one another on every compressor with several pressure and temperature sensors.
The actuator is a hydraulic bypass vane that controls the air flow. In case of an overload, it quickly opens to prevent any damages. While this cancels the test, it does save the compressor. A new customer project by a European consortium of propulsion unit manufacturers caused ILA to think about an improvement to the existing system. infoteam’s work order consisted of implementing an automation solution that would allow for operating the system closer to the power range limit with the existing equipment.
For this purpose, the pump protection and pump prevention control systems would have to be reimplemented and the sensor technology be replaced. infoteam appeared as system integrator and, as general contractor, was responsible for planning, goal-compliant and on-schedule implementation as well as commissioning.
The solution
To meet these requirements, infoteam deve-loped an automation solution with two industrial PCs. The heart of the control system is the real-time-capable Soft-PLC SmartPLC. This is a software that converts a conventional industrial PC into a programmable logic controller. The Soft-PLC works with the corresponding Open-PCS programming system from infoteam. This allows for planning, implementing, testing and commissioning the automation programs for the Soft-PLC.
The programming system is IEC61131-3-compliant and supports the programming languages function block diagram language, ladder diagram, operating language, structured text and instruction list. By using the PCs as the platform, two important requirements were implemented directly on the computers: logging of all incoming and outgoing signals and visualizations.
Logging & visualization
The ability to subsequently and effectively analyze the tests the values of all incoming and outgoing signals are recorded as records in a logging file for each control system separately in the control cycle. The size of these files after a test series can easily reach the megabyte range. This shows already one decisive advantage of the industrial PCs compared to the implementation by means of a traditional programmable logic controller: the implementation of the logging and the analysis by means of Analyze tools or even with MS Excel is child’s play. To be able to view the status of the compressors on a screen, the most important signals must be directly displayed on a screen. A programmable logic controller is usually not capable of this type of visualization.
The industrial PCs can perform this task without additional hardware. Just as decisive is the fact that the systems are expandable with respect to the software without changing the hardware by using the powerful PCs from Siemens. While infoteam considers Siemens to ensure a long-term provision, high quality and excellent service, the extensive standardization of the PC architecture does, however, provide the advantage of greater independence from suppliers in comparison to a PLC.
Real-time capability
In selecting the operating systems, infoteam chose Windows XP, expanded by the RTX real-time add-on from Venturcom. In this case, RTX is situated between the hardware abstraction layer (HAL) and the operating system and controls all resource accesses by Windows. This is the only way, for example, to activate and process a timer by RTX with high precision even though another thread is currently writing to the hard disk, i.e., performing a very slow and indeterministic access.
This is where the Soft-PLC shows its strength, because it is a multi-threaded application in which several RTX threads are executed in synchronization to each other and prioritized. RTX allows for the creation of Soft-PLC as an application in which high-priority deterministic threads run mixed with low-priority indeterministic threads. As a result, for example, the thread that is intensively using the hard disk for logging and running with low priority can be interrupted if the high-precision timer thread is called for reading the pressure sensors.
Features of the automation solution
The control of the system is carried out by a redundant system with two functional diversity control loop circuits that each feature their own periphery and their own PC. The primary control is responsible for keeping the compressor pressure ratio on the setpoint value, even if the mass flow changes. For this purpose, it has control of the bypass vane. If opened, the mass flow increases and the pressure ratio drops. To avoid the pump limit from being exceeded, the pump pressure ratio of the compressors, as a function of the corrected speed and the fixed blade position, are recorded in a table.
This ratio plus a safety margin is the limit for the setpoint value. However, two aspects must be taken into consideration: On the one hand, the control system cannot completely prevent an overshooting of the actual value in case of fast changes. On the other hand, the pump pressure ration is influenced by numerous parameters, such as contamination, thermal state (vane gap), air humidity, and Reynolds number, and is, therefore, inherently unpredictable. However, including all possible influencing variables in the safety standard would severely restrict the operating range of the system. For this reason, the recognition of a pump surge is indispensable.
Diversity is a proven means for managing systematic errors. For the most part, software in automation is structurally simple and therefore easily testable. This allows for largely eliminating errors in the source code. Diversity on the level of source code and machine code is, therefore, frequently overestimated in ist effectiveness. In practice, the important errors are predominantly specification errors.
For diversity to be effective here, it must also be applied on the specification level. For these two reasons, a decision was made to maintain the existing concept of a functional diversity of pump prevention and pump protection.
Pump protection
The pump protection is responsible for detecting the occurrence of a pump surge, independent of the operating point of the compressor and to interrupt by quickly opening the bypass vane. If a compressor is restricted, the pressure ratio increases with falling throughput. With the axial compressor, this is accompanied by an increase of the angle of incidence of the rotor and stator blading. If the maximum angle of incidence is exceeded, a stalling – the hydrostatic lift -- similar to an airplane occurs, and the pressure ratio collapses instantaneously. This deviation between mass flow and pressure ratio – the characteristic curve family of the compressor – becomes obvious by comparing the current operating point with a valid operating point from the characteristic curve family.
For this purpose, the characteristic curve family is recorded in a three-dimensional table. However, it cannot be directly interpolated based on the characteristic curve family representations; rather, it is necessary to use an iterative equation solver. For a safety-relevant software in cyclical processing, ruggedness and determinacy are of special importance. For this reason, a bisectional procedure with a defined number of iterations is used. While it is slow in comparison to many more intelligent procedures, it can ensure a defined accuracy in a defined computing time.
Pump control
The control of the pressure ratio is a single-variable fixed setpoint control and, therefore, a classic application for a PID controller. The gain factor of the controlled system, i.e., the change of the pressure ratio for a certain vane opening, is not constant, but dependent upon the operating point. For this reason, the controller gain is determined by the pressure ratio and the position of the inlet guide series.
An additional challenge was the fact that the controller parameters of the existing system where unknown. Because the new system features a delay time due to the new metering installations and the quantification. For this reason, the transfer function for commissioning was determined using jump response and vibration experiments and the controller parameters were set according tom the Ziegler-Nichols method.
The periphery
The pressure sensors report the current pressure values using a total of three RS485 paths of the pump prevention. For this purpose, they use a proprietary protocol that was implemented by infoteam in a separate driver for interfacing with the Soft-PLC. The rest of the sensor technology of the pump prevention is connected via Profibus DP and consists of digital and analog IO terminals from Wago. The actuators – the four hydraulic vanes – are connected to the industrial PC of the pump prevention via CAN to read the actual position of the vane and to specify the set position of the vane. The pump protection with its second industrial PC receives all the other IO signals via its Profibus.
The pressure sensors used are standard sensors with Profibus-DP connection. While they are no way near as precise as those of the pump prevention, they are much faster – and speed is of the essence in pump protection. The access of pump prevention and pump protection to the same vane appears to be problematic. The solution to this conflict consists of connecting the pump protection via direct IO to a second input of the vane controller that operates only digitally. If a signal is present, the vane opens; if no signal is present, the vane receives the set position via CAN.
CAN and Profibus driver are interfaced using the standard drivers of the CAN and Profibus card manufacturers and integrated in the Soft-PLC.
Summary
The decision on a solution with industrial PCs paid off. After a project time of only three months, the complete system was put into operation successfully and on schedule. The convincing part of this solution compared to a competitive offer that envisioned implementing the automation solution with four individual programmable logic controllers, is the better price-performance ratio and the greater flexibility with respect to continued expansion of the test stand. To this effect, scientists are working on a physical real-time simulation to be able to replicate the physical characteristics of the test stand using simulation software.
The findings about the pump limits of the individual compressors will allow for an even more precise control of the system in the future.For more information on infoteam, please visit their web site at www.infoteam.de .
