Proceedings of the Fifth IEEE International Caracas Conference on Devices, Circuits and Systems, Dominican Republic, Nov.3-5, 2004
Labdileit : Electronic Instrumentation Laboratory Through Internet Raúl Turón, Rodrigo Picos, Miquel Roca, Eugeni Isern, Eugeni García-Moreno a
Grup de Tecnologia Electrònica, Universitat de les Illes Balears,
[email protected], +34-971-173-207
Abstract: In this work we present a remote laboratory (Labdileit) which allows the possibility to measure some experiments on electronic circuits through Internet. Labdileit has been implemented aiming at educational purposes, but an extension of the work could be considered for R+D activities. The system is based on a PC computer as a server. This sever contains a DAQ card and a GPIB card connecting different measurement equipment such as a digital multimeter, an oscilloscope, a waveform generator and a power supply. The server automates the measurements on the circuit. This laboratory is available at http://labdileit.uib.es Index Terms: remote laboratories, programmed measurements, e-learning.
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education, and that were not possible before. In the field of very specific measurements (research and development) where the need of very sophisticated, and also very expensive equipment is mandatory, a remote laboratory allowing to do measures from a terminal connected to Internet could be very useful. Experiences in this direction are already very common in fields like astronomy. All the aspects commented in this introduction motivates the appearance of different laboratories which are available now in Internet as can be seen in [1-7]. This work is a part of an ALFA project from the European Community where some researchers who have developed other laboratories as AIM lab or Lab-on-web, have participated. The paper is organised as follows: Section 2 details hardware (Subsection 2.1) and software aspects (Subsection 2.2) of the architecture we have considered. Section 3 shows the different experiments which are now available in our remote laboratory. Finally, in Section 4 the main conclusions of the paper are commented on.
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I. INTRODUCTION The use of Internet is very important in present society, where the number of Internet users is growing every day. Today it is very frequent the use of internet services as e-mail, electronic bank, e-business, chat, videoconference, etc. A new area where the use of Internet is becoming very important is e-learning, that is, remote education. A lot of universities and educational centres offers remote courses to the students through Internet. The students can follow several topics through a terminal connected to the Web. However, experimental areas are difficult to get through internet, because laboratory courses are one of the most important parts, and it could seem to be difficult to do this part of work in the framework of remote courses. In this way remote laboratories through Internet could play a very important role, allowing the student to perform experiments directly from his terminal. An important aspect to be considered is that this kind of experiences offers to centres with reduced budget the possibility to perform experimental measurements on electronic circuits with a simple PC computer connected to the net, with no need of electronic laboratories with expensive instrumentation equipment. Another advantage of remote labs is the possibility to increase the available experiences through the creation of remote labs networks. In this way the user could access to different laboratories and perform different experiments in different sites. An extensive use of these activities allows the consideration of new pedagogical uses, as experimental demonstrations included in traditional lectures, laboratory modules as homework exercises for the students, etc. aspects which will be very important in the new European space of
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II. LABDILEIT ARCHITECTURE The architecture considered is a typical client-server architecture as it is commonly used in Internet applications (Fig. 1). In this architectures the client open the session, sending a request action to the server. The server receives this request, process it, and sends the answer to the client. This kind of interaction allows the use of the server by multiple users. The part of the client needs only a Web navigator with some standard plug-ins, which allows the visualization of interface pages with the server. In this part the client customizes the experiments (parameters selection) through a form that will be sent to the server. The part of the server includes software elements to make possible the interaction between server and client and the instrumentation equipment needed to perform the measurements on the electronic circuits. This equipment includes GP-IB instruments, data acquisition cards, or any other instruments that can be connected to a PC (via serial or parallel ports or even through USB). The protocol used in this work to develop the server side is HTTP, which is included in the TCP/IP standard. The HTTP protocol allows the user the possibility to exchange data with the server side, which is a requirement for this work.
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This DAQ belongs to the set of low cost cards, the sampling frequency being low, then it could be used only for low frequency experiments. The consideration of high frequency experiments will imply the use of a more advanced DAQ, but from the point of view of measure automation and Internet customization only simple changes will be needed. Two experiments using the DAQ card are currently available. However, taking measurements with high accuracy and/or speed is better achieved by using dedicated instruments, instead of using a general tool like the DAQ card. In this sense, a PCI-GPIB card is also needed in the server. This card allows the connection of many different instruments, thus giving a huge flexibility to the system. In this work, the chosen card was a PCI-GPIB card from National Instruments. To achieve the educational purposes of this work, the instruments connected to the GPIB bus are: a digital multimeter 34401A from Agilent Technologies, a power supply with double output Tti-PL330TP from Thurlby Thandar Instruments, a TDS1002 digital oscilloscope from Tektronix and a AFG320 function generator from Tektronix as well. For the moment there is an experiment in the server using the digital multimeter and the power supply, but in the future other experiments using all the equipment will be included in the set of available experiments. Finally, the server is a PC computer (Pentium IV family), running Windows 2000. This PC performs communication operations with the client (web server tasks) and controls electronic measurements through both DAQ and GPIB.
Fig. 1: Client-Server Architecture
A. Hardware aspects of Labdileit architecture In order to make the system more flexible we considered the possibility to use more than one kind of instruments, as shown in Fig 2. where a schema with the instrumentation equipment connected to the server is depicted. In this way, the instrumentation equipment used in Labdileit can be classified in two different sets: -Data acquisition card (DAQ) -Measurement instruments controlled by GPIB bus.
B. Software aspects of Labdileit architecture This virtual laboratory is based on the HTTP protocol. This way, the software is composed by different layers (Fig 3). The first layer includes the Apache server and the measuring program POSTSERVER.vi. The Apache server makes the web site available to the internet and interfaces it with the users. The POSTSERVER.vi is an application dedicated to answer the measuring requests, and only supports the POST method from the HTTP protocol. This last application has been done using LabView™, from National Instruments™, which is a de facto standard for instrumentation control. LabView is a graphical environment that allows easy implementation of complex tasks and has vast libraries of instruments, thus freeing the designer from the task of programming each instrument. LabView also includes powerful tools for mathematical processing and easy creation of user interfaces. Moreover, LabView includes tools that allow the easy integration of internet functionalities in the programs. One of these tools is the LabView Internet Toolkit, thought it has not been used in this project, because it was not available in time. Instead, we used CGI programming, for which LabView has also some tools, thought they are a bit more difficult to implement.
Fig. 2: Instrumentation equipment configuration
Labdileit uses a 6024E DAQ from National Instruments Therefore, when designing new experiments for this card, the circuits should take into account the performance of the card. This way, there are a number of critical design parameters that will limit the possible experiments: the number of input/output ports (analog and digital), the voltage range of these ports and the maximum sampling frequency of the card.
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Finally, the Labdileit site has been created using HTML, including JavaScript and SVG. The first two technologies are well-known standards, while the SVG is a new W3C standard, derived from XML, that allows easy creation of vector graphics, among other functionalities.
III. IMPLEMENTED EXPERIMENTS In this section the currently available remote experiments will be described. They can be classified into two groups: those where the measurements are performed with the DAQ, and those that make use of the instruments connected through GPIB. The verification of Ohm’s Law in a resistor (Experiment 1) and measuring the transference function for a A/D converter (Experiment 2) belong to the first group. An experiment to plot the output voltage-current characteristic of a bipolar npn transistor (Experiment 3) is presented as an example of the second group. All the experiments have been physically implemented on an interconnection board, which is permanently connected to the DAQ and the GPIB. In all the experiments the user can select and adjust some parameters and/or input voltage sweeps from a predetermined list of values.
Fig. 3: Software elements in Labdileit.
The POSTSERVER application (an example is depicted in Fig 4) is dedicated only to control the instruments and make the measurements. To perform these tasks, it gets the requests from a queue maintained by the Apache server. Once the request is performed, it processes the answer. This answer is formatted in HTML and SVG and send to the remote user. Letting the control of the queue to the Apache server guarantees that more than only one user will be able to use the Labdileit at the same time, because all the users have to post their requests, which are queued and attended sequentially in time by the POSTSERVER.
A. Experiment 1: Ohm’s Law In this experiment the user can validate the Ohm’s Law on a resistor, and use it to estimate its resistance. As shown in Fig. 5, the circuit makes use of a voltage divider made of a fixed known resistor and a user-selected unknown resistor (chosen from a set of three single resistors in seven possible parallel combinations, by means of analog switches).
Fig. 5. Input form and schematic of the Ohm’s Law experiment.
The user selects the input voltage sweep: Vi max, Vi min, voltage step (‘Pasos Vi’, in the actual implementation), and the parallel resistor combination, and the server makes the measurements using two input ports of the DAQ. These measurements are used to plot an I-V characteristic for the unknown resistance. A linear regression is then used to fit a straight line to the measured data, its slope being the
Fig. 4: POSTSERVER.vi example
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estimation of the resistance value. The user can also choose if for each input voltage value, a single measurement or averaged multiple (5 or 10) measurements will be used to determine the I-V points for the linear regression. The outputs of this experiments are the I-V characteristic of the unknown resistor (SVG graphic) and its calculated resistance value (Fig. 6). Finally, the user has also the option to download the actual measurements as an ASCII file.
Fig. 8. Example of the results obtained in the A/D experiment.
C. Experiment 3: I-V characteristic of a BJT transistor This experiment differs from the two ones previously presented in that it makes use of the GPIB connected instruments, instead of using the DAQ card. In particular, this experiment uses a digital multimeter and a two-output digitally-programmable power supply, both of them controlled by the POSTSERVER through the GPIB bus.
Fig 6. Example of the results provided by the Ohm’s law experiment.
B. Experiment 2: A/D converter transfer characteristic In this experiment measurements are taken to plot the transfer characteristic of a ADC0804 A/D converter. One analog output port of the acquisition card is used to apply the voltage to be converted, while four input digital ports provide the four most significant bits of the ADC0804 output (Fig. 7). The user can select the input voltage sweep (Vi max, Vi min, and number of steps). The experiment output consists of a graphical representation of the A/D transfer characteristic, restricted to the four most significant bits (Fig. 8). Again, the client can download an ASCII file with the collected data, for an eventual post-processing. Fig. 8. Input form and schematics of the BJT experiment.
Now the user can measure the output I-V characteristic (IC vs. VCE) of a 2N2222A npn BJT transistor. The circuit configuration and the form used to select the measurement parameters are shown in Fig. 8. The user selects from a predefined list the VBB and VCC voltage sweeps (maximum value and number of steps for VCC, and maximum value and number of curves for VBB). Both voltage values are then applied by the power supply, whereas the collector current is measured by using the digital multimeter configured as an amperimeter. The experiment output consists of a plot (SVG graphic) with the I-V characteristic (Fig. 9), and optionally as before, the measured current values in an ASCII format.
Fig. 7. Input form and schematics of the A/D converter experiment.
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Fig. 9. Example of I-V characteristic obtained in the BJT experiment.
IV. CONCLUSIONS As the use of internet grows, more and more demands are placed on the services providers. One of the more active fields is that of the e-learning, including here from life-long learning to primary school. However, technical areas are difficult to get through the web, because of the need to perform experimental work, that needs in many cases the physical presence of the user. One way to overcome this drawback is using simulation tools (who has not used Spice or Matlab simulations in a course?), but they are sometimes too ideal. Another way is using remote laboratories like the one presented in this paper. These remote labs offer the possibility to make real measurements to people who has not the possibility to physically access a laboratory to perform them. This way, technical learning through the web can be improved. The laboratory presented in this paper is a cheap way to achieve a real implementation of a remote lab. It only includes a PC server connected to the internet, a DAQ card and a PCB, in its basic configuration. Additionally, a GPIB card can be connected, thus improving its functionality. This way, simple experiments can be performed, thus demonstrating the feasibility of these remote labs, even using quite cheap equipment. Currently, the laboratory enhancement is focused in two directions. On one hand, the design of an extensive set of experiments covering a typical undergraduate electronic course. On the other hand, we are working on the inclusion of simulation capabilities in Labdileit to offer the possibility to contrast the simulations with the measurements.
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[2]
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