Development of a Wireless Application for Remote Control of Space Test Facilities S. Scaranzin, E. Bonelli, F. Saito, F. Scortecci AEROSPAZIO Tecnologie s.r.l. Rapolano Terme, Siena, Italy
[email protected]
Abstract—A wireless application based on a PDA and a mobile phone has been developed at AEROSPAZIO Tecnologie to allow remote control of space test facilities and increase the staff mobility. The application developed in house has been compared with available software over different networks. The test activities and results are described. Wireless; space test facilities, remote control; PDA; mobility.
To date more than 2500 h of thruster endurance test have been performed in few months of continuous operations. The flight qualification of Electric Propulsion systems often requires several years of uninterrupted lifetime test. In this regard, the test facilities must guarantee all the time reliable continuous operations in any environmental condition.
I. INTRODUCTION Modern times are certainly the age of mobile communication. Having access to the office or to the Internet from everywhere in the world (home, airport, hotel, etc.) facilitates mobile personnel allowing the companies to save time and money.
Therefore, for the value of the test activities and the criticality of the processes involved in the experiments a specific facility monitoring and control system is required. For these reasons, an advanced wireless remote control application was developed at AEROSPAZIO so as to allow operators to perform a reliable monitoring of the state of the facilities and of the test article and to react readily from everywhere in case an alarm is notified.
In this regard, a wireless communication system has been developed at AEROSPAZIO Tecnologie to allow remote control of test facilities, including a Large Vacuum Test Facility (LVTF) used to perform tests on satellite Electric Propulsion systems and space simulation.
II. T HE CONTROL SYSTEM The LVTF consists of a 120 m3 stainless steel vacuum chamber equipped with several stages of pumping system, vacuum gauges, test equipment and diagnostics [3].
Presently, the LVTF (Fig. 1) is used for the endurance test of the RIT-22 thruster [1, 2] developed by EADS Space Transportation GmbH and proposed for ESA scientific missions (like BepiColombo to Mercury foreseen for the year 2012) as well as for commercial satellites (like the new European platform Alphabus).
A test article, for example an electric thruster, is placed inside the chamber and switched on for testing, when the right pressure is achieved. A Data Acquisition System (DAS) and a Facility Management System (FMS) have been developed to automate LVTF operations and control the test behaviour from computer. A. The Data Acquisition System All the signals coming from sensors in the test are managed by the DAS. The DAS hardware is based on a PC architecture consisting of a 12 bit National Instruments®1 DAQ card multiplexed and conditioned by an extensible SCXI™ system. To date a 32 differential channels conditioning module is installed with 200 kS/s max sampling rate, selectable gain and low-pass filter.
Figure 1. The Large Vacuum Test Facility.
1 Product and company names listed are trademarks or trade names of their respective companies.
The DAS software, developed by AEROSPAZIO in collaboration with the Dept. of Information Eng. of the University of Siena [4], allows the user to configure the DAS hardware and stores the data converted in the appropriate units of measurement.
B. The Facility Management System The FMS has been developed by AEROSPAZIO [5] in order to operate and control the test facility from computer. The FMS software, running on a PC located in the test laboratory (Facility Control PC - FCPC), manages several digital I/O cards and control electronics in order to: •
monitor the state of the pumping system
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allow the operation of valves and pumps
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execute predefined procedures to reach different vacuum levels
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check and show data acquired by the DAS
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show the test article operations, managing webcams
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detect major equipment failures and automatically react starting safety operations and sending a specific voice message to a list of phone numbers
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store all the events in a log file.
In order to operate the facility continuously for long duration tests (up to several years without interruption for lifetime tests) a number of automatic procedures have been implemented for normal and safety operations. However, although the FMS sends a voice message to notify an equipment failure, the warned person can check the LVTF conditions and eventually restart the test only from FCPC.
B. Remote Control from a PC Equipped with a Connection to the Telephone Network through Duplex Cable The next purpose consisted in providing remote control of the FCPC located inside AEROSPAZIO from another computer equipped with a connection to the telephone network through duplex cable. As first step, a commercial software (Laplink® Gold) allowing point to point modem connections was installed on both computers. After the connection is established, Laplink authorizes FCPC desktop control by the host PC. Unfortunately, desktop control of a remote PC requires many data to be exchanged, so the modem transfer rate (56 Kbps) reduces the monitor refresh frequency. Moreover, the call cost could increase too much depending on computers distance and call duration. Then, a Virtual Private Network (VPN) was developed between FCPC and other computers outside the company used for remote control. The advantages of using VPN are the following: •
it is possible to access the A-LAN over the Internet at the cost of the Internet Service Provider (ISP)
•
data exchanged are encrypted and so hidden from Internet users. However, the data can be accessed securely by authorized users through the VPN
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VPN supports the most common network protocols (TCP/IP, IPX and NetBEUI). Therefore, it is possible to remotely run any application using these network protocols.
Therefore, some remote control systems have been investigated, in order to allow staff mobility in the company during working hours and everywhere outside the company during holidays.
There is no need for the host PC to have direct access to the Internet in order to create a VPN connection.
The requirements of the systems dealt on are: safety, confidence of data exchanged, low cost and an appropriate transfer rate.
In fact, there are two ways to establish a VPN connection from a host PC: by dialing an ISP, or by connecting directly to the Internet.
III.
T HE REMOTE CONTROL SYSTEM FROM PERSONAL COMPUTERS
The first goal is to allow remote control from computers in the AEROSPAZIO offices and from a PC equipped with a connection to the telephone network through duplex cable as, for example, in the personnel houses. A. Remote Control on LAN The facility remote control from AEROSPAZIO computers was attained using RealVNC™ by RealVNC Ltd (VNC) over the AEROSPAZIO Local Area Network (A-LAN). VNC is an open-source software, which allows remote desktop control of one computer (the server) using a simple program (the viewer) running on another computer connected to the server by TCP/IP protocol. Running the VNC server component on the FCPC and the viewer component on computers in offices, A-LAN satisfied all the requirements listed before, using the standard IEEE 802.3 protocol with a speed of 100 Mbps.
In the first case, considering the AEROSPAZIO situation, the VPN connection first dials to an ISP, then another call is routed to the remote access server that establishes the communication protocol tunnel (PPTP). In the second case, the user needs a computer where IP connectivity is established during the operating system startingup. For example, using a PC equipped with an ADSL service or a PC member of a local area network linked to the Internet, the user creates a VPN tunnel dialing directly the IP address of the remote access server. Anyway, after authentication, the user can access FCPC in the A-LAN, and use VNC software to remote control AEROSPAZIO facility. IV. T HE REMOTE CONTROL SYSTEM FROM PDA In order to improve staff mobility outside the company where a connection to the telephone network through duplex cable is not available, a notebook or, better, a less bulky PDA, equipped with a wireless Internet connection can be used to remote control the AEROSPAZIO facility.
An Internet connection can be established by a Wi-Fi card if the PC is near to an access point (for example in hotels, airports, …) or by a mobile phone via GSM/GPRS network. In the first case there are no expense, but access points are not available everywhere. On the other hand, in the second case the connection costs can be very high especially using a remote desktop control application (for example VNC for PDA) due to the large amount of data exchanged. In fact, many telephone company tariffs depend on the amount of data exchanged and not on the call duration. Therefore, AEROSPAZIO decided to develop a Wireless Remote Control System (WRCS) to fully control LVTF facility from everywhere using a PDA and a mobile phone minimizing connection costs. A. The Wireless Remote Control System Architecture The WRCS architecture developed by AEROSPAZIO includes the following devices: •
a desktop PC (FCPC) equipped with an Athlon™ 2600 CPU and Microsoft® Windows® 2000 Professional operating system, IEEE 802.11g connection at 56 Mbps, Ethernet card at 100 Mbps, digital and analogic I/O for AEROSPAZIO facility control
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ADSL Switch/Router NetGear® with static IP address connected to ADSL line at 1.6 Mbps for Internet connection and equipped with IEEE 802.11g+ connection at 108 Mbps
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Pocket PC ASUS® A730W equipped with Microsoft® Windows® CE 2003 operating system, IEEE 802.11b connection at 11 Mbps and Bluetooth® integrated
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GSM/GPRS mobile phone Motorola® A835 with Bluetooth connection integrated.
A scheme of the system layout is shown in Fig. 2.
Figure 2. The WRCS architecture.
The PDA can connect directly to the AEROSPAZIO router through the wireless LAN if it is near to the device. Otherwise, if the user is outside the company, the PDA can link the AEROSPAZIO router following the subsequent steps: •
the PDA links the mobile phone using Bluetooth
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the mobile phone works as a modem connecting the PDA to the Internet through GSM or GPRS network
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PDA creates a VPN tunnel up to the AEROSPAZIO router.
The router is wireless connected with the FCPC, which controls the facility. B. The Wireless Remote Control System Software The WRCS software is a Client-Server application implemented to fully control LVTF equipment using a PDA. The client component of the WRCS software, developed using Microsoft® eMbedded Visual Basic 3.0®, runs on the PDA. It can create a VPN or wireless LAN connection with FCPC (Fig. 3). Then, the user can send commands, ask for information on the plant and see them on the control windows. Each window shows data (state and measurements) of related devices and sensors. For example, the control window describing first two stages of the pumping system is shown in Fig. 4. The server component, implemented using Microsoft® Visual Basic 6.0, runs on FCPC embedded in the FMS software. It formats and sends to the client the required information, evaluates commands forwarded by the user and if possible executes them sending back the updated state of the plant; otherwise an error message is generated. After the connection is established, the two components communicate using Ethernet packets.
Figure 3. Log-In window of WRCS software.
•
readings from sensors (alphanumeric string): each reading is represented in scientific format with three numerals for the mantissa and two for the exponent and is separated from the others by an underscore. For example, if the second number is 2.03E-4, the pressure gauge 1 is measuring 2.03 x 10-4 mbar inside the vacuum chamber. If the sensor is not working, the string Fault replaces the reading.
Every coded message is few bytes long (for example about 30 bytes describe the state of the facility), so the connection’s cost is minimized and refresh of PDA screen is very fast over a low bandwidth line too. Moreover, the WRCS software exchanges data only on demand to reduce further the traffic over the network. However, the LVTF safety is not compromised, because FMS updates sensor readings and devices state at regular intervals (typically every second) and, in case of equipment failure, executes safety procedures. Figure 4. Pumps & Valves window of WRCS software.
Every packet contains a coded message consisting of few ASCII characters in the form shown in Fig. 5. The software recognizes the part of the Ethernet packet useful to control the LVTF, because the Start Transmission Delimiter and the End Transmission Delimiter mark the boundaries of the message body. If one of the delimiters is not received or if the message body contains invalid data, an error is caused. The body of the message is different depending on the transmission direction. If the message comes from the PDA, the body can contain a string to request readings from sensors or to change the state of a device (a command). Examples of commands are: av1_on to open the angle valve 1, rot_on to switch on the rotary pump, tmp_autostart to begin the automatic procedure that waits the correct conditions to run up the turbomolecolar pump, etc. If the message comes from the FCPC, the body contains two section: the first, consisting of two ASCII characters, identifies the type of message, the second contains a coded string that can represent:
V. COMPARISON BETWEEN REMOTE CONTROL SYSTEMS Considering that the main aim of this work is to allow AEROSPAZIO staff to remote control the facilities with the minimum mobility constraints, using a PDA and a mobile phone becomes necessary. Therefore, it is interesting to compare the performance (cost and speed) of different networks and software. In particular, some tests have been performed using VNC and WRCS software to control FCPC through GSM and GPRS networks. Costs of the tests have been calculated using tariffs of the Italian society Tre Mobile Video Company. Note that while GPRS costs depend on the amount of data exchanged, GSM expense increases with the call duration. The control session, considered as benchmark for comparison, comprehends the following operations: •
connection to FCPC
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check of measurements and devices state
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sending of a command
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check of measurements and devices state.
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the state of the facility (numeric string): each numeral corresponds to a specific device of the facility and represents its state. For example, if the fifth numeral of the string is 1, the gate valve is open
Unfortunately, remote control with VNC on GSM does not satisfied the AEROSPAZIO speed requirements due to the large amount of data exchanged on the low bandwidth network (9.6 Kbps).
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an error message (alphanumeric string): for example, if the user send an unknown command, the server answers command_error
On the contrary, a VNC/GPRS control is satisfying, because the transfer rate (115 Kbps) is more than ten times faster than GSM. Therefore, the desktop refresh is fluid and the call duration is about 6 min. However, the GPRS coverage is less widespread than GSM.
Figure 5. Scheme of the coded message used by WRCS software.
Using WRCS software, the amount of data exchanged to achieve operations described before is much smaller, so it can be used both over GSM and GPRS networks. In both cases, the considered control session takes about 5 min. In fact, even if
GPRS is faster than GSM, most time elapses without exchanging data while the user checks the facility state. Tab. 1 summarizes results from benchmark tests. In particular, the cost ratio between VNC/GPRS and WRCS/GPRS in the considered example is about 2:1. However, if the user need to perform many operations and checks, the ratio between data exchanged increases rapidly, and so does the cost relation. TABLE I. Control Sw VNC
COMPARISON BEETWEEN REMOTE CONTROL S YSTEMS USING PDA AND MOBILE P HONE Network GPRS
Speed a (Kbps) 115
Duration (mm:ss) 5:23
Data (KB) 335
Costb (€) 0.6 + 1.34
WRCS
GSM
9.6
5:35
-
0.15 + 0.9
WRCS
GPRS
115
5:02
15
0.6 + 0.12
a. It is referred to nominal value. b. Calling unit + calling rate
VI. FUTURE IMPROVEMENTS Voice and video data from the LVTF and the test article will be included in the WRCS software in order to improve the sense of presence in the laboratory. Further tests will be performed to check the system performance on the UMTS network whose deployment is presently ongoing in our area. REFERENCES [1]
[2]
[3]
[4]
[5]
H. J. Leiter, R. Killinger, H. Bassner, J. Mueller, and R. Kukies, “Development of the radio frequency ion thruster RIT XT,” IEPC-01104, Proc. of the 27 th International Electric Propulsion Conference, Pasadena, CA, October 2001. H. Leiter et al., “A survey of the 'New grid systems for ion engines' technology project,” Proc. of the 4th International Spacecraft Propulsion Conference, Chia Laguna, Italy, June 2004. F. Scortecci, E. Bonelli, B. Michelozzi, F. Saito, S. Scaranzin, and A. Turco, “Performance of a large vacuum test facility for spacecraft propulsion testing,” Proc. of the 4th International Spacecraft Propulsion Conference, Chia Laguna, Italy, June 2004. E. Bonelli, Sistema di acquisizione dati per prove in camera a vuoto di propulsori elettrici satellitari. Master Thesis, University of Siena, June 2003. F. A. Saito, Gestione e controllo remoto di una camera a vuoto. Degree Thesis, University of Siena, June 2004.
