Development of Control System of Icing Wind Tunnel

A 3m×2m Icing Wind Tunnel is a continuous transonic wind tunnel where icing test and conventional aerodynamic test can be conducted. In the wind tunnel, several systems are arranged and control system is complicated, and the total pressure, air speed and temperature control systems are featured by long-time delay, non-linearity and strong coupling properties. To meet the performance requirements of control system, controlled system of wind tunnel are analyzed. According to the analysis results, PROFINET industrial real-time network technology is applied toward wind tunnel control system and corresponding design scheme is worked out, and the characteristics are analyzed. In addition, appropriate control strategy is put forward for flow field parameters multi-variable control system. According to the commissioning process and effects, it is proved that the wind tunnel control system can satisfy the performance and specification design requirements of the wind tunnel.


Introduction
Icing Wind Tunnel is important test equipment via which aircraft icing mechanism and relevant theoretical research can be conducted, influence of different icing patterns and ice accumulation process upon the aerodynamic characteristics of aircrafts is researched, so as to ensure flight safety of the aircrafts under icing weather condition.Meanwhile, in the Icing Wind Tunnel, aircraft low Reynolds number test can be conducted.Therefore, it can meet the subsonic aerodynamic test requirements for the high-altitude aircrafts (airfoil, semi-scale model and full-scale model ) [1][2][3].
Compared with conventional wind tunnels, the Icing Wind Tunnel is equipped with several and large-scale systems (some of them have thousands of control I/O nodes) and internal control of subsystems is complicated.There exists coupling between the flow field pressure, wind speed and temperature control system in the Icing Wind Tunnel, therefore, such System is deemed as a typical MIMO system with non-linearity, large time delay and strong coupling property.A spraying system may exert strong interference upon the total pressure, temperature and wind speed control system.Furthermore, according to the requirements, the Icing Wind Tunnel can be used to conduct low-temperature high Reynolds number test by corresponding operation control modes, in addition to icing test.
Through development of the Icing Wind Tunnel Control System, the following key technologies are addressed: -Due to large quantity and large size of systems arranged in the Icing Wind Tunnel, a distributed control scheme is used for it.However, concentrated real-time control function shall be provided to meet the requirements of flow field multivariable control to hardware platform.-The"concentrated control + distributed control" scheme is use to achieve remote closed-loop control of network.The network system shall be of real-time, punctuality and reliability, so as to facilitate implementation of flow field multivariable control strategy.-The control strategy for Non-linearity, large time delay, and strong coupling MIMO system.This Paper, in light of the features and requirements of the control system of the Icing Wind Tunnel, provides the overall design scheme applying PROFINET technology toward the control system, and introduces the design idea concerning the wind tunnel flow field parameter control strategy.In addition, this Paper sets forth the actual effect which verifies the feasibility of design scheme of control system.

Controlled Systems and Requirements of Icing Wind Tunnel
The layout of Icing Wind Tunnel is as shown in Figure 1 [1,2].It is mainly composed of fan system, altitude simulation system, cooling system, spraying system, anti/ de-icing system, humidification system, model support system, etc.

The technical specifications and requirements
The technical specifications and the requirements of wind tunnel are as follows: -Max.wind speed: main test section: 210m/s, second test section: 78m/s; highspeed test section: 256m/s.Wind speed control accuracy: better than 0.3%.-Altitude simulation: maximum altitude: 20000m.Altitude control precision: control accuracy better than ±100Pa,in the range between local elevation to 7000 meters.Accuracy better than ±50Pa,in the range between 7000 meters to 20000 meters.-Temperature range: ambient temperature ~-40°C.Temperature control accuracy: better than ±0.5°C.-Liquid water content: 0.2~3g/m 3 .Median Volumetric Diameter(MVD): 10~300μm.

Controlled systems description
Wind tunnel operation control refers to control upon the following systems [2,3]:

Fan system
The Fan system is composed of fan and its drive system, with the power of motor output axis being 6000kW, rated rotation speed of motor being 475rpm, maximum rotation speed 600rpm.The fan blade pith angle is adjustable when it is in shutdown, with adjustment range between -20° to +20°, adjustment positioning accuracy better than ±6′.
The Fan system is used to control air speed.The system objects include: inverter system, variable blade pitch angle mechanism, cooling fan of electric motor, motor stator internal anti-dew electrical heater and automatic grease-filling system.The system can monitor the axis temperature, vibration, motor temperature and other status.Siemens-ROBINCON perfect harmonic-free series of HV frequency converters are used.Control of air speed is implemented via adjustment of fan speed.Both altitude simulation system and spraying system can impose influence upon the air speed control system.

Altitude simulation system
Altitude simulation system is mainly used to control pressure of air in the Icing Wind Tunnel, thus simulating different altitudes.The System is composed of vacuum pump set (water ring pump + roots pump), piping, valves and relevant control units.The water ring pump set adopts inverter speed control and coordinates with the air make-up regulating valves on the main inlet piping.It is mainly used to simulate vacuum pressure control when the altitude<7000 meters (39kPa).The vacuum pump set, consisting of roots pumps and water ring pumps, is used to vacuum pressure control when the altitude is between 7000 meters to 20000 meters <7000m (39kPa~5kPa).
Two 2BEC72A and one 2BEA203 are used as water ring vacuum pump and four LQ2500 and one LQ600 air-cooling roots pump are used as roots pump.
To match up various working conditions in the system, air make-up regulating valves with different nominal diameters are arranged on the air inlet main pipes of the vacuum pump set and are used to control the vacuum pressure in the wind tunnel by coordination with the vacuum pump set.
The controlled objects mainly cover: variable-frequency speed regulating control of vacuum pump set, flow rate control of air make-up regulating valves, cooling of vacuum pump set and monitoring of softened water.The spraying system, cooling system and fan system will impose influence upon the altitude control system.

Cooling system
The cooling system is mainly used to control total temperature at stable stage in the wind tunnel.This system involves startup, operation energy level control of refrigerating compressor unit; control of lubricating oil station (used to lubricating the refrigerating compressor unit) and of auxiliary systems like cooling water circulation system; control of ammonia liquor circulation pump set; control of liquid supply electromagnetic valves, return-gas electromagnetic valves; electric regulating valve system subject to low-pressure circulation air suction pressure closed-loop control; in the way, operation safety interlocking of the system is implemented.
The maximum refrigerating capacity of the cooling system is 11000kW (temperature at the evaporator outlet: -30°C), the minimum refrigerating capacity: 330kW.Such system is composed of 12 compressor drives and auxiliary systems.The temperature in the wind tunnel is regulated by controlling the compressor units and low-pressure circulation cylinder air suction pressure.Fine control of temperture is implemented by controlling the low-pressure circulation cylinder suction pressure (to adjust the energy levels of compressor units).

Spraying bar system
The Spraying Bar System (SBS) is used to simulate the cloud and mist environment aircrafts pass through under which the cloud layer contains super-cooled water drops.The SBS is able to generate water droplets with diameters (MVD) and Liquid Water Content (LWC).Furthermore, the system is capable to generate Super-Cooled Large droplets (SLD),with the range of freezing drizzle conditions.
The SBS consists of spraying bars, water supply subsystem, air supply subsystem and control subsystem.The SBS has 20 bars.Each bar is equipped with 50 spraying nozzles, totaling 1000 nozzles.Each nozzle water supply line is equipped with a solenoid valve that can be remotely switched on and off.Nozzles can be controlled independently, and by controling the proportion of air supply pressure to the water supply pressure,can meet requirements for any combination and implement different cloud & mist parameters.
Two types of nozzles can be replaced mutually to form MVD covering 10~300μm.The system's controlled objects include: water and gas heater system, more than 50 valves of water and gas lines, 1000 solenoid valves.

Anti/de-icing system
Anti/de-icing system falls into: hot air de-icing system and electrical deicing system.The former is used to Anti/de-icing the wind tunnel parts and aircraft, so as to ensure test is conducted successful.The later is used to conduct aircraft anti/de-icing test.
The aircraft anti/de-icing system currently provided mainly includes: hot gas anti-icing system, aerodynamic de-icing boot system and aerodynamic pulse de-icing boot system.Different systems put forward different requirements for mass flow, pressure and temperature of output hot gas, and all require precise control.The system is typical multi-variable control system.

Humidification system
The system controls the humidity in the wind tunnel through controlling the temperature of water and gas heaters and regulating the flow rate and pressure.Humidity control range: 70%~100% (-15°C~-20°C), 100% (-20°C~-40°C).

Model support mechanism system
The model support mechanism has been designed in order to support the model weight and the aerodynamic loads generated by model aircraft.It includes upper and lower turntable mechanism, yaw angle mechanism and pitch angle mechanism equipped for primary test section.Lower turntable mechanism, yaw angle mechanism and pitch angle mechanism equipped for secondary test section.Left/right turntable mechanism and sting support mechanism for high-speed test section.
The controlled mechanisms vary from the test sections.Motion controllers will be used to achieve the synchronous control strictly of the related angle mechanisms.

Wind tunnel auxiliary system
Readiness of wind tunnel auxiliary system is the essential interlocking condition precedent for startup of wind tunnel.This system is composed of variable diffuser widened angle system, plenum chamber lifting platform and its hydraulic system, aerated sealing gas control system (gate and wicket of plenum chamber, gate of stilling chamber, variable diffuser front/rear flange and variable diffuser sealing ring) and gas distribution system.

Overall Design of Control System of Icing Wind Tunnel
For the coupling between the flow field control system, such scheme integrating the characteristics of distributed control system and centralized control system shall be applied toward the control system architecture.
In light of the features, the control system employs the overall architecture scheme of PROFINET-based control system.By this scheme, the control system can combine the advantages of the distributed control system and centralized control system in terms of performance.

PROFINET performance [4-12]
PROFINET (Process Field Net) is updated automatic bus criteria based on industrial Ethernet technology put forward by PROFIBUS International (PI).Such technology, by combining the automation technology and Ethernet network technology, can integrate other profibus system on a seamless basis.It is a complete network resolution available in the automation field, and covers real-time Ethernet, motion control, distributed automation, fault safety and cyber safety technologies, and can be completely compatible with the industrial Ethernet and existing profibus technology as cross-vendor technology. PROFINET

b) RT communication
PROFINET provides an optimized real-time communication channel based on Ethernet Layer 2, as shown in Figure 2. By using such real time channel, data processing time in communication stack is shortened greatly, thus improving the automated data refresh rate.The typical response time of real-time communication is between 5 ~10ms, satisfying the requirements for data exchange among sensors, executors and controllers.sufficient time accuracy while a large quantity of data is transmitted; meanwhile, the communication task of the processor on PROFINET equipment can be eased.In case of 100 nodes, the response time is less than 1ms, jitter error less than 1μs, in this way, punctual and defined response can be supported.

Overall design of control system
Trunk network of the control system of Icing Wind Tunnel is of distributed control system network structure built with PROFINET industrial Ethernet.The trunk network is of optical fiber transmission media and ring-network structure, as shown in Figure 3 [13].The entire control system architecture is divided into three layers: wind tunnel operation management layer, network layer and subsystem execution layer.The wind tunnel core control PLC and local controllers of subsystems are connected to the trunk network of the control system via switch with synchronous real-time communication function.The subsystem controllers are used to implement operation control and safety interlocking of local subsystems according to the instructions from the wind tunnel core control PLC.Configuration and function design of the system is set forth below [13].

Design of wind tunnel operation management layer
Design contents include: wind tunnel core control PLC, wind tunnel test run supervisory computer, engineer station, operation monitor, database server, data analysis station, measurement system host, projection & display system, etc. Wind tunnel test run supervisory computer is mainly used to prepare and publish wind tunnel control missions and monitor important wind tunnel parameters and status.Such supervisory computer publishes the test missions (working conditions) to the "wind tunnel core control PLC" which implements dispatching control of subsystems (as shown in Figure 3).In addition, it forwards the wind tunnel operation data from the wind tunnel "core control PLC" to the database server for storing (as shown in Figure 4).Both the supervisory computer and operation monitor acquire wind tunnel operation data from the wind tunnel "core control PLC" via TCP/IP communication.
S7-400PLC is used as wind tunnel core control PLC.It, as the Controller of the PROFINET I/O Device/Controller communication mode, communicates with subsystems in RT communication, thus implementing operation control and safety interlocking of the wind tunnel.
The engineer station is used for programming of subsystem, program debugging, maintenance or monitoring upon several systems during the test via PROFINET network.
Database server is used to enable network server and database functions; namely, storing and inquiring of test control data, parameters, measured data and parameters, status information parameters and fault information.In addition, the database server can output various data curve, time sequence and reports as per specified requirements.
Data analysis station provides the aerodynamic staff or user with test data for analysis and processing.

Design of network layer
Network layer is composed of main switch and sub-switches via which subsystems are connected to the trunk network.
Main switch is provided with IRT webmaster function, and supports PROFINET, integrate with redundancy management function and supports layer 3 exchange technology, VLAN technology, SNMP, web-based management and PROFINET diagnosis functions.Thanks to supporting VLAN technology, the main switch can define different VLANs to prevent data network broadcast storm and improve cyber reliability.The sub-switch is equipped with IRT switch module, with two 10/100Mbit/s RJ45 ports, two 100Mbit/s multi-modulus BFOC interfaces, and it is provided with fault signal indicating function, connection setting buttons, redundant power supply input, PROFINET IO design and network management function.Therefore, ring network functions can be provided.
Also, such network supports TCP/IP, RT and IRT communication, satisfying the system requirements for different communication modes.

Design of execution layer of subsystems
Internal network of subsystems can be configured independently based upon actual requirements, and is controlled on a concentrated basis by local controller of subsystems.Local controller communicates with the wind tunnel core control PLC via RT communication (PROFINET I/O Device/Controller communication mode) for concentrated control and management.
Internal expansion of subsystems mainly includes: Ethernet network, Profibus, Profibus-DP Drive and Modbu.Profibus-DP Drive bus is used to support motion control system (TCPU controller is used).
See Figure 4 and Figure 5 for schematic diagram of instruction and data transmission among systems and composition diagram of measurement & control software.
According to above, the design scheme is provided with the following characteristics: -To support several communication protocols and satisfy the different requirements at different layers.-Be of ring network structure, optical fiber communication, to improve reliability of network control.
-To combine the advantages of concentrated control & management and distribute control, with functions superior to conventional distributed control system.

Control Strategies of Wind Tunnel Flow Field Parameters
Wind tunnel test run control is currently divided into three operating models: icing (anti/de-icing) test run, ambient temperature test run; high-altitude low Reynolds number test run.
Under icing (or anti/de-icing) test run operating model, control system is made to control the wind speed, altitude (total pressure) and total temperature(at stilling chamber); cloud & mist parameters are controlled by combination of large-size and small-size nozzles and regulating of water and gas pressure and temperature.If model anti/de-icing test is conducted, the model anti/de-icing control system needs to be put into tuning.Both the altitude control system and temperature control system are featured by long-time delay, the wind speed, altitude and temperature control system is a MIMO coupling system.In addition, when the spraying system starts to work, it may impose different influence upon the wind speed, altitude simulation and temperature control system.However, flow field parameters as required by tests shall be get stable rapidly.
Ambient temperature test run and high-altitude low Reynolds number test run operating model are mainly used to conduct force test.For offset heat generated by fan system, the cooling system shall work.High-altitude low Reynolds number test requires the altitude simulation system to participate into pressure control, and the following working conditions shall be implemented: constant pressure (constant altitude), stepped variable wind speed for force test; constant pressure (constant altitude), constant wind speed and continuous or stepped variable model attitude for force test, etc.For coupling between wind speed control and pressure control, speediness and stability of wind speed and pressure shall be addressed during the stepped variable control of wind speed.During the regulation of wind speed and total pressure, temperature control shall meet specified control precision requirements, so as to ensure the repeatability of the test.
In relation to control strategy, the control algorithm of conventional control theory for the long-time delay system and multi-variable system generally relies upon transfer function [15].However, transfer functions focus on the objects described by linear ordinary differential equations.For the controlled system with severe nonlinearity and time-varying properties, it can be expected that sound control effect can not obtained.According to the characteristics of the Icing Wind Tunnel, we, based on references [14,[16][17][18][19][20][21][22][23][24][25][26], utilizes the "open-loop stability" feature of the flow field for the design of control strategy, and divide the flow field control process into "startup establishment process" and "fine tuning process".In this way, the complicated multivariable decoupling algorithm is decomposed into single-variable control algorithm, and the coupling is deemed as disturbance, so as to improve the system's robustness and resolve the coupling issues, thus implementing precise control upon flow field parameters.
For control on "start establishment process" of flow field, the required target value obtained through test is corresponding to "preset" control target value of relevant systems.System coupling will not be considered for control at this stage.In light of establishment time of system corresponding parameters and through time sequence scheduling, the systems can be controlled according to the "preset" control target value, then switched to the "fine tuning process" after establishment of the flow field.
Intelligent PID or self-adaptive control algorithm that is of increment cascade control structure is used for the "fine tuning process", so as to improve the system's robustness and implement precise control upon flow field parameters.
With further commissioning, if the required target value obtained through test is corresponding to the "preset" control target value of relevant systems, expert knowledge-based can be built, thus building a intelligent control system based on expert knowledge.
Control block diagrams of total pressure (simulation altitude), wind speed and temperature are as shown in Figure 6, 7 and 8.The intelligent controller as shown in the figures below is used to enable the "knowledge-based" to calculate the "preset" control target value of relevant systems based on the given target value, and automatically complete switch between "start establishment process" and "fine tuning process" and intelligent PID algorithm according the parameters change.Its software functions are implemented jointly by relevant subsystem controllers and "wind tunnel core control PLC", therefore, the cyber communication will be subject to the higher requirements of punctuality and reliability.

Control Commissioning
Ventilation test for the Icing Wind Tunnel was deemed successful in July 2013, and the first icing test was conducted in October of same year.From July 2013 to date, we have been conducted commissioning of control system.During the commissioning, the control network system has been operated stably and reliably, without any fault.Control software is developed in convenient and rapid manner.Control strategy of flow field control can meet all specification requirements.Icing test effect of aerofoil front-edge is as shown in Figure 9, and the flow field parameter control response curve (wind speed: 35m/s; total pressure: 39kPa, total temperature: -20°C) during control commissioning is as shown in Figure 10 [13].After the air speed, total pressure and the total temperature stability, the air speed changed, the flow field parameter response curves is as shown in Figure 11.The Control commissioning results shows the precision of total pressure, wind speed and total temperature is superior to design requirements.At present, we have completed the primary commission of wind tunnel and are conducting further and optimization commissioning.

Conclusion
Based on the actual conditions and effect of commissioning of icing tunnel control system, it can be concluded that: -Distributed control system based on IRT network is superior to conventional distributed control system and can integrate distributed control and concentrated control.Thanks to application of ring network structure and optical fiber transmission, the cyber reliability is improved, thus satisfying the requirements of large-size systems and correlative multi-variable control systems.-PROFINET, by supporting various communication protocols, can achieve consistent communication from management layer to equipment layer.It has advantages including succinct, reliable structure and sound openness.-Control process is divided into "star establishment process" and "fine tuning process".By using smart control strategy based on expert knowledge, the multi-variable coupling control of flow field parameter can be implemented successfully.With further commissioning, the "knowledge-based" can be improved and optimized continuously, thus optimizing the flow field parameters control performance specifications.

Figure 1 .
Figure 1.The layout of Icing Wind Tunnel supports three communication modes: TCP/IP protocol communication, real-time (RT) communication; IRT communication.a) TCP/IP communication TCP/IP and IT protocol is applied based on Ethernet technology.It can meet wind tunnel management-level (such as wind tunnel operation management level or wind tunnel information-based system) communication requirements, with response time about 100ms.

Figure 3 .
Figure 3. Block diagram of control system network of icing wind tunnel