Student Projects
Contact Information
University: Università degli Studi di Salerno (Italy)
Team Members: Franco Catino, Antonio De Caro, Mario Migliaro, Fiorenzo Sansone
Faculty: Electronic Engineering (Master's degree in Electronic Engineering)
Faculty Advisors: Prof. Antonio Pietrosanto, Dr. Alfredo Paolillo
Course: Instrumentation and Automatic Measurement Systems
Email Address:
Project Information
Title: Metrological verification of oscilloscope “measure” functions.
Description: The objective of this project is to develop an automatic software to evaluate the calibration status of an oscilloscope, via IEEE 488.2 interface, relaying on LabVIEW, on NI GPib-USB controller, on the scope “measure” functions and on a Fluke 5500A calibrator.
The software could be used to set an automatic testing station for oscilloscopes. The developed program sets an automatic measure process, and verifies every oscilloscope “measure” function, calculating error and standard deviation from the reference value set with the calibrator. The data obtained and its relative calibrator uncertainty is stored in a spreadsheet file, for easy elaboration and consultation. In addition it is possible to visualize the average error with the relative calibrator uncertainty in graphical form. Evaluating the data, the user can apply the needed correction for every measure and make the needed calibration adjustment to the device.
Products:
The software was built using National Instruments LabVIEW and a National Instruments GPib-USB controller. Using the Gpib interface, an oscilloscope of TeKTronics TDS series 200, 300, 400, 500, 600, 700 and 3000, and a calibrator Fluke 5500A were remotely controlled.
The Challenge:
The main challenge in this project is to find the perfect coordination between the different steps needed to achieve a meticulous metrological process. To obtain useful calibration data is necessary to execute measurements; changing waveform type; sweeping frequency, amplitude and offset for every oscilloscope “measure” function, in order to optimize measurements and cover the entire frequency and amplitude range of the device. Each measure must be accurate and linked with the relative calibrator uncertainty, so it is very important to set properly the time base, vertical scale and trigger of the scope, every time the wave generator of the calibrator switches. In order to create a software useful in a measurement lab, the program must be able to communicate with devices under test, provided with different Gpib primary addresses, via 488.2 interface.
The Solution:
Using the advantage given by LabVIEW modular approach to software engineering, and LabVIEW superior administration of communication interface as IEEE 488.2, our team has developed a software focused on a simply and clear user interface.
Fig.1: Software hierarchy and function order of execution.
A main.VI file is used as user interface and controls the execution of more subVi-s. Each one of them execute one task of the verification process: selecting the “measure” function to test; retrieving the instruments connected; selecting the instruments to verify; initializing the calibrator; and executing the measurements.
Fig.2 and Fig.3: User interface and dialog window for “measure” selection.
From the main front panel it is possible to start the measure process, stop the program, search for connected device, select the speed of the process (the speed is related to the number of iterations per measure), select the “measure” functions to test, save in spreadsheet format and visualize the data obtained from the verification in graphical form. While measuring, partial output file are saved for every single type of “measure” in a new folder. Date and time are stored in respectively in folder and file names.
The file can be easily opened with common spreadsheet application as Excel, OpenOffice Calc and Matlab. The graph displays the average error with the relative calibrator uncertainty range, related to the reference values generated by calibrator, for every “measure” function tested. Below there is a sample of an output file saved.
Fig.4: Output file storing a single type of “measure”.
In the output file calibrator reference values, scope measure, average error, standard deviation, and calibrator uncertainty of the different types of measure are stored.
Uncertainty is evaluated using the calibrator manual as reference, and for indirect measurement (as Duty Cycle measure), uncertainty propagation formula was applied.
The uncertainty given by our software can be completed with other external uncertainty sources (as cable noise or electromagnetic interference) by user.
The function FINDINSTRUMENTS.VI is useful to communicate with devices under test provided with different Gpib primary addresses, via 488.2 interface. Using a serial polling command and query for identification, this function scans all the possible primary address of Gpib bus, creating an array of clusters, filled with the address of the connected instruments and their respective identifier. A simple sub-vi consisting in a dialog window, displaying the device connected, is launched to select the instruments we will use in the process.
Fig.5: Dialog window for instruments selection.
The measurement sub-vi (named “MEASURE.VI”), sets the oscilloscope and launches the measure. The software is default set to test the channel 1 of an oscilloscope. Time base, vertical scale and trigger of the scope are automatically set every time the wave generator of the calibrator switches. A wait command for “autoset complete” is sent before measure starts.
The main.VI file controls also the execution of a sub-vi consisting of a progress bar. PROGRESS.VI graphically evaluates the state of execution of the metrological verification progress, communicating with a global variable, and also can be used to stop the procedure, clicking on an abort button. The global variable is needed to control the execution of the sub-vi-s from the main function, storing reference to progress index used in progress bar.
Fig.6: Progress bar.
The main Vi file (named “MENU.VI”) is set to recall calibrator initialization and measurement sub-vi-s in order to execute measures, changing waveform type, frequency, amplitude and offset for every oscilloscope “measure” function, optimizing measures and covering the entire frequency and amplitude range of the device.
Fig.7: Instruments used for testing.
Further improvements of software will be addressed to include more devices, communication protocols and manufacturer instructions sets. The main goal is obtain a software able to set an automatic process of metrological verification for the most diffused oscilloscopes and calibrators, and subsequently to create a total automatic software for oscilloscopes complete calibration.
