Aggregation of automated instrumentation via internet using remote VISA calls.

Sandu, Florin ; Nedelcu, Adrian Valentin ; Moraru, Sorin Aurel 等

1. INTRODUCTION

The problem addressed by the authors is the discovery, aggregation and invoking (in automated test sequences) of remote instrumentation resources (mainly equipment but also interfaces and software objects). In a larger context, such capabilities should be advertised, searched and probed (assessed) by controllers that build a distributed test & measurement (T&M) configurations based on "ontologies". The work aims to enlarge the access to existing high-tech laboratory infrastructure for distance-learning purpose. It is included in an E.U. "Leonardo da Vinci" Transnational Network that should integrate distributed experiments as educational SCO (Sharable Content Objects)

2. STATE OF THE ART

Most of the outstanding remote laboratories (with real and/or virtual experiments) are presented in published surveys (e.g. of Gomes and Garcia Zubia, 2007) that include also best-practice and case-studies.

One of the best approaches belongs to researchers from the University of Genova (Bagnasco et al, 2006) who manage instruments as web-services. This is a top-down approach of the OSI (Open System Interconnect) stack. Compared to this, we would rather approach the problem bottom-up, using the Virtual Instrument System Architecture ("VISA").

3. RESEARCH COURSE

3.1 Publishing the full-capability remote GPIB control

A high-tech automated "tower" of instruments can be (almost) fully controlled remotely by using the complete GPIB (General Purpose Interface Bus--IEEE488) drivers offered by the equipment manufacturers, in order to be used with National Instruments (NI) "LabVIEW" (LV). The integrative LV Virtual Instrument (VI) developed at "Transilvania" University of Brasov--Romania (Fig.1) groups the front panels of 4 instruments (3 with full controllability and one--the HM1007 oscilloscope--limited to waveform visualization). The VI-s offered by NI, HP (for HP34401A) and Hameg--for HM8142 and for HM8131 were integrated with authentication blocks, instruments' reset, stimuli annulling and outputs' disabling. For remote control (via Internet) of a LV VI panel, the built-in LV web-server must be activated and configured on the workbench-server by the administrator.

[FIGURE 1 OMITTED]

Users should address http:// <workbench-server name> : <port> / <application name>.htm. Out of security reasons only the web-server is directly visible on the Internet, but not the workbench-server (it was connected--together with the workbench-servers of other networked lab-s--in the Intranet of the web-server, through a switch connected to its second network card). For this reason port forwarding was used to route the remote users' requests directly to the workbench-server (addressed with its Intranet address, 192.168.200.x).

3.2 Controlling remote instruments with NI-VISA

VISA is the origin of the well-known plug-and-play concept in modern operating systems. Session-orientation of VISA (based on interrupts' servicing) encouraged us to use the most recent virtual instrumentation software--like NI LV--together with NI Measurement and Automation Explorer (MAX) in order to integrate remote and local resources and to build test sequences.

Using NI Remote VISA Servers, the authors implemented a method of sharing instruments and even interfaces to remote users that can integrate them in their experiments (if they are only didactical, even Units-Under-Test, UUT, can be far from the user, if not, at least UUT and intelligent transducers should be local and other equipment can be remotely).

The configuration implemented consisted of a instrumentation (workbench) PC, running NI Remote VISA Server, to which all the instruments are connected using different interfaces (such as GPIB, USB, serial and parallel). The workbench server is also connected in the Intranet of the web-server which is "visible" in the Internet.

Port forwarding has also been configured in order to route Remote VISA data (coming through the dedicated port 3537) from the web-server to the workbench server. Any security requirements can thus be fulfilled by configuring access lists for port 3537. A client who has access to the system will use only the IP address of the web-server for remote access to the "published" experimental resources. The resources' "offer" of a certain server can be viewed in a tree-like manner in the Remote Systems window of NI MAX (Fig. 2). The availability of these resources can also be tested in MAX, using a graphical user interface.

[FIGURE 2 OMITTED]

3.3 Implementing remote laboratories with NI-VISA

The authors have used the Remote VISA technology for the implementation of remote experiments. The workbench consists usually of the following typical hardware parts:

* Experiment board(s)--the UUT

* Stimuli generators (e.g. programmable power supplies and/or function generators)

* Measurement instruments (e.g. digital multimeters, DMMs)

* Switches (in relay matrix and/or multiplexer configuration); a robust implementation is suggested by the authors in Fig.3.

Mechanical relays were chosen because of their larger bandwidth, of their better separation between the control circuit and the controlled one; the relays are controlled via parallel port of the workbench. "Test Points" are connected to common relay terminals; in order to multiplex them, "normal opened" terminals are connected to DMM input by appropriate DIP switches. Because the parallel port, LPT (which has also the advantage to be latched) is a resource which is "visible" through VISA, the relays can be controlled using Remote VISA. In order to do that, pins 11 (Busy) and 12 (Error) had to be tied to GND. Otherwise, the hardware driver will think the peripheral (it is talking to) is busy or wrong and will not output any data.

The software (see also Cheij, 2002) is built in NI LabVIEW (LV). A typical remote experiment (e.g. with an electronic UUT) can have a diagram like in Fig. 4.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

It sweeps a given range of input stimuli (e.g. between a start and a stop voltage, with a user-defined number of steps) and performs certain measurements (e.g. of voltages or currents).

A measurement cycle involves opening a VISA session to each of the instruments used in the laboratory. The syntax for identifying a remote instrument is the following: visa://ip_adrress_of_the_webserver/instrument_ID.

After opening a VISA session, an instrument can be controlled using standard SCPI--Standard Commands for Programmable Instruments (Pieper, 1998) commands (e.g. the syntax for measuring a DC voltage is: MEAS:Voltage:DC:?). Such commands are sent to the instrument using the VISAWrite.vi LV function. The measured value can be viewed by calling the VISARead.vi function, whose output is converted to double format and placed in an array.

4. CONCLUSIONS

Successful integration of local and remote instrumentation into automatic test sequences was demonstrated. Experiments were centered not on UUT but on instruments. Full access to their capabilities implies some specific security issues that are challenging further research.

5. ACKNOWLEDGEMENT

The research was financed by the European Union, inside the "Leonardo da Vinci" Trans-national Network RO / 06 / B / F / NT175014 "VET-TREND" (Valorization of an Experiment-based Training System through a Transnational Educational Network Development).

6. REFERENCES

Bagnasco, A., et al (2006). Exposing Measurement Instruments as Grid Services in Distributed Cooperative Laboratories, Networking Instrumentation, and Measurements, Davoli, F., et al (Ed.), Springer Science and Business Media, Inc., ISBN 0387298118

Cheij, D. (2002). Software Architecture for Building Interchangeable Test Systems, IEEE Aerospace and Electronic Systems Magazine, Vol. 17, No1, pp 27-30, ISSN 0885-8985

Gomes, L. ; Garcia Zubia, J. (Ed.), (2007), Advances on Remote Laboratories and e-Learning Experiences, Published by University of Deusto, ISBN 978-84-9830-077-2

National Instruments Corp., U.S.A. (2003), Remote Instrument Control with NI-VISA, accessed on the following website: http://zone.ni.com/devzone/cda/tut/p/id/4789; date of access 19.02.2008

Pieper, J. M., (1998). Standard Commands for Programmable Instruments ,SCPI Consortium, ACEA, Wierden, 1998, Available on: http://www.scpiconsortium.org, Accessed on: 2007-09-17