Student Projects
Contact Information
University: Università degli studi di Salerno
Team Member(s): Anna Rizzo, Paolo Sacco, Ivano Forrisi, Piero Castelluccio
Faculty Advisors: Ing. Alfredo Paolillo
Email Address: rizanna88@yahoo.it
Project Information
Title: Characterization of a triode valve
Description:
This project work consists of two different parts: “generation and acquisition” and “curve fitting”. In the first part the objective is to evaluate the current-voltage characteristics (Ib-Vb) as the control voltage changes. The second part “curve fitting” allows to make the fitting on the acquired curves, estimating the parameters of the Child’s model “µ” and “A”
Child’s Law:
Products:
Labview8.6
NI GPIB-USB-HS
The Challenge:
Create an user friendly software that manages the process of measurement for the characterization of a triode valve. The software will allow the management of acquired data and the estimation of the model best suited to the triode valve tested.
The Solution:
The objective is to evaluate the current-voltage characteristics (Ib-Vb) as the control voltage changes. The measurements are made considering the system in stationary conditions.
In the following, the hardware structure of the test system will be described and how the software, developed with LabVIEW, works. Two Fluke 45 multimeters have been used to make tension/current measurements; two Agilent power supplies used to bias the valve and a Pc with a “General Purpose Interface Bus”.
For a correct operation of the software, specifications of the connected devices and software parameters must be respected
Hardware Configuration
The devices used interface by bus IEEE488 (GPIB) to th e personal computer (driver), which handles its operation by monitoring the valve state. As following :
Figure 1
Since the stabilized power supply connected to the terminals “H-H” o “H-HT”(Heather ) is not driven by the Pc, it doesn’t require an interface 488.
The second power supply is connected to the output terminals(U-M) and to the input (IG-M) of the circuit allowing to change the operating point of the valve. In this way is possible to programmatically change the operating point (Vc,Vb, Ib ).
The multimeter (1) is connected to the output to the valve and measures voltage and current at its terminals (IP-IK) and (IK-M).
The multimeter (2) measures the grid voltage with reference to ground (IG-M).
Addressing of devices
GPIB communication standard requires the devices connected to the bus to be univocally addressed. It has been decided to assign the address “0”, generically associated with the system controller, to the pc, and the address “1 ” to the multimeter (1) and the address “2” to the multimeter (2) and the address “3” to the power supply.
The address association to the instruments must be done manually, by operating directly on the front panels of the instruments and by changing the preliminary properties of the interface connected to the driver.
The above mentioned addressing must be respected because any change could cause the missed or erroneous operation of the software.
In figure 2 the devices addresses and the relative links are illustrated in details.
Figure 2
Description of the software interface
The software is composed by two panels called “Generation and Acquisition loop” e “Curve Fitting” :
Generation and Acquisition loop
Figure 3
The front panel “Generation and Acquisitio n loop” allows to the user to manage the acquisition already present in memory into a text file or to make news ones:
- acquisition of the curves by means of instrumentation (generation and acquisition button)
Figure 4
With this button it is possible to make the acquisition achieved by means of the instrumentation.
The scanned range of the output voltage (U-M , fig. 1) and of grid voltage (IG-M , fig. 1) can be defined via software at less than the limits indicated on the panel.
Figure 5
By planning the step parameters the increase of the software during the acquisitions are fixed by changing the number of points for curve.
Figure 6
In order to give an indication to the user about the necessary time at the end measure procedure, on the front panel “Generation and Acquisition loop” there is a progress bar.
Figure 7
When the acquisition is finished the curves are displayed on the graphic and saved into a text file in a user’s directory choice.
Figure 8
- loading from file of the curves already acquired in a previous moment (load key)
Figure 9
By pressing the “load” button the file data are charged in the program and displayed for next elaborations.
“Uncertainty” button:
Figure 10
With this function it is possible to calculate and display the uncertainty related to the current Ib measurement.
Figure 11
Down on the right there is an indicator with the maximum value of uncertainty found in the measures:
Figure 12
Note: in the plots the uncertainty intervals of the data items have been increased of a 50 factor for an easier readability.
On the front panel “Generation and Acquisition loop” there is the button “Clear Graph” which allows to delete all the graphics on the “Acquisition” graphic.
Figure 13
Curve Fitting
Figu re 14
The second panel “curve fitting” allows to make the “fitting” on the acquired curves, estimating the parameters of the Child’s model “µ” and “A”
Child’s Law:
Start Fitting (From file
Figure 15
This button allows to load data items from a text file and to make the fitting, afterwards the measured curves and the respective fitting curves are put on graphics.
- Start Fitting (from last measurements):
Figure 16
This command made the fitting of the curves just acquired.
- Clear Graph:
Figure 17
On the second panel “curve fitting” there is the button “Clear Graph” again which allows to delete all plots on the graphic.
- Threshold:
Figure 18
The user has the possibility to ignore in the fitting acquired points whose plate voltage is lower than the specified threshold, for a more effective estimation of the parameters.
- estimated parameters:
Figure 19
This indicator shows the parameters estimated from the fitting function for input data items.
- Error Graph and f(minimum indicator):
Figure 20
This graphic shows the objective function to be minimized versus the variation of the threshold, in order to allow a proper choice of the threshold value.
There is also an indicator which displays the value of this function in the current step.
Termination of the program
The program execution simply ends by pressing the exit key Exit:
Figure 21
Model used for the assessment of uncertainty
The following expression has been used in assessing the uncertainty:
-
are the uncertainties of current and voltage measurements related to the instrument used. For Fluke45 it is calculated considering the terms related to accuracy and resolution.
expresses the dependence of valve current output on the output voltage. This relationship has been assumed linear and modeled by a straight line whose slope was calculated as ratio between finite differences on data acquired.
expresses the dependence of output current on the grid control voltage. This coefficient is evaluated using the model of child:
