Automatic data analysis

Transcription

1 NOVA technical note #1 1 Automatic data analysis Case study: automatic IV curve and power curve from fuel cell measurements Fuel cell characterization is usually performed by measuring the IV and power curve of the fuel cell. Two different strategies are possible: Constant potential mode: this method works in potentiostatic mode (using the Autolab or an electronic load in combination with the Autolab). Starting at the open-circuit potential (OCP), negative over-potential steps are applied and the discharge current is monitored. Constant current mode: this method works in galvanostatic mode (using the Autolab or an electronic load in combination with the Autolab). Starting a zero current (OCP), negative current steps are applied and the potential of the cell is monitored. This case study illustrates how to use the data analysis commands of NOVA in order to perform fully automatic data handling and analysis in fuel cell measurements. Experiment 1 The constant potential method is used in the experiment. The procedure and the data analysis tools used in this procedure can be easily adjusted for a constant current method. The experiment uses a repeat for each value loop to apply a sequence of overpotentials (relative to the open-circuit potential) on a fuel cell and measure the discharge current and cell potential for each over-potential. Each measurement takes 12 s, the interval time is 0.1 s. Figure 1 shows a simulation example on the Autolab dummy cell (a), for one of the steps in the repeat loop. 1 This note describes a simulation that can be executed on the Autolab dummy cell (a).

2 NOVA technical note #1 2 Figure 1 Current (blue) and Potential (red) measurements on the dummy cell (a) At the end of the experiment, the user wants to calculate the average value of current and potential over the last 30 data points for each step. He also wants to calculate the power using the averaged current and potential values. Finally, the user wants to plot the averaged potential values and the calculated power values versus the averaged current values. All this can be done completely automatically using the following tools: The Windower command to select the last 30 points for each overpotential The Calculate signal (twice) command to calculate the mean values of the Current and Potential values provided by the Windower The Calculate signal command to calculate the values of the Power using the average values of the current and potential The Build signal command to filter all the average values of current and potential and the power values for each over-potential The Custom plot (twice) command to build the V versus I plot and the P vs I plot.

3 NOVA technical note #1 3 The procedure Figure 2 provides an overview of the simulation procedure, to be used on the Dummy cell (a). Figure 2 Overview of the simulation procedure This procedure starts with a Timed procedure which is used to determine the open-circuit potential of the cell. The measured OCP is then used as reference potential. The OCP value is then set (Set potential 0 V relative to the OCP). The cell is switched on and there is a delay of five seconds. The next part of the experiment is located into a repeat for each value loop. The over-potential values (relative to the OCP) are predefined in the repeat loop (in figure 2, from 0 V to -1 V, 100 mv step, 11 values). The values of the repeat for each value loop are linked to the Set potential command. This command is followed by a Record signals (> 1 ms) command, which measures the potential and the current during 12 seconds, with an interval time of 0.1 s. Two predefined plots are used (E vs t and i vs t).

4 NOVA technical note #1 4 The final timed procedure switches the cell off. At the end of the experiment, the user has a total of 11 record signals items in the data presentation. Each Record signals item provides two plots: i vs t and E vs t. Figure 3 shows the data recorded using the procedure on dummy cell (a). Figure 3 overlay of all the data recorded using the procedure Modification of the procedure In this section, we will illustrate how the data analysis command in NOVA 1.5 can be used to perform complex data handling automatically. Step 1 data extraction Since the user wants to calculate the average of the current and the potential over the last 30 data points for each value, the first step is to extract the 30 last data points from each measurement. This can be done using the Windower tool. More information on the Windower can be found in the user manual (page 208). Switch to the procedure editor and open the commands browser. Locate the Windower command in the Data handling group and add a windower to the procedure, on the Record signals (> 1ms) command (see figure 4).

5 NOVA technical note #1 5 We have to do this because we want to window the data provided by the record signals command. Figure 4 Adding the windower to the Record signals (> 1 ms) command Note: the mouse pointer will display a small + sign when the Windower is dragged on top of the Record signals (> 1ms) command. Release the mouse button to drop the windower on the record signals command. Once the Windower has been added to the procedure, we need to specify the source of the windower. The source is the signal that is going to be used to extract the data points required for the analysis. This is done by linking the source parameter to the signal that is used to select the data. Since we want to select the 30 last data points, we are going to use the Index signal (see figure 5).

6 NOVA technical note #1 6 Figure 5 Linking the Index signal to the Windower source Once the source has been defined, it is possible to specify the boundaries for the selected source. Each Record signals command provides a total of 120 data points, indexed from 1 to 120. Since we want to extract the final 30 points from each measurement, the boundaries will be from index 91 to index 120. Click the button next to Boudaries parameter of the Windower (see figure 6).

7 NOVA technical note #1 7 Figure 6 Editing the boundaries of the Windower Click the OK button of the Boundaries Editor to validate the selection. The Windower command will be updated. The original signals provided by the Record signals command are now also located below the Windower (see figure 7). However, the signals below the windower will only contain the values of the last 30 data points.

8 NOVA technical note #1 8 Figure 7 The final Windower will now extract the values of the last 30 points Step 2 signal calculation Now we can calculate the average value of the current and the potential. Locate the Calculate signal command in the data handling group and add it to the procedure, dropping it on the Windower (see figure 8). Note: more information on the calculate signal tool can be found in the User manual.

9 NOVA technical note #1 9 Figure 8 Adding the Calculate signal command to the Windower Note: since we want to perform the calculation on the data provided by the Windower, the Calculate signal command must be added to the Windower command in the procedure editor. Once the Calculate signal command has been added to the procedure, it is possible to create a new signal using the available values provided by the Windower and an equation. In this case, we want to calculate a new signal called Average current. Open the Calculate signal window by clicking the button of the Calculate signal command (see figure 9). Define the Name of the signal (Average current), the units (A), the expression MEAN(current) 2. Click the Parameters frame to validate the expression. NOVA will identify the current parameter as the only parameter of the equation. Click the Single value checkbox to force the calculation to return a single value and click OK button to close the Calculate signal window. 2 It is possible to copy and paste this expression into the Expression frame of the Calculate signal window

10 NOVA technical note #1 10 Figure 9 Using the Calculate signal tool The Calculate signal command will be updated, displaying the current parameter in the procedure editor, below the Calculate signal command and the result of the calculation, the Average current signal will be added to the signals available in the Windower. Add a link between the WE(1).Current signal from the Windower and the current parameter of the Calculate signal command (see figure 10).

11 NOVA technical note #1 11 Figure 10 Linking the WE(1).Current values to the Calculate signal command Using this approach, the average value of the last 30 WE(1).Current values will be calculated and the result of the calculation, called Average current, will be added to the available signals. Add another Calculate signal command to the Windower in the procedure to calculate the Average potential signal, using the same approach (see figure 11).

12 NOVA technical note #1 12 Figure 11 Calculating the Average potential value The procedure will now also calculate the Average potential using the last 30 WE(1).Potential values. Add another Calculate signal to the Windower. This Calculate signal command will be used to calculate the Power, by multiplying the Average current value with the Average potential value (see figure 12).

13 NOVA technical note #1 13 Figure 12 Calculating Power Note: the names of the Parameters cannot contain a space. In figure 13, the _ was used instead. The links are used again to link the parameters of the Power signal calculation with the Average current and Average potential signals (see figure 13).

14 NOVA technical note #1 14 Figure 13 Calculating the Power values At this point, the procedure will extract the values of the last 30 points from each Record signals step in the experiment, calculate the average value of the current and the potential using these values and calculate the power using the average current and potential values. Step 3 gathering the data from all the measurements Now we need to extract all the values of Average current, Average potential and Power in order to build the required plots. Select the Build signal command from the data handling group and add it to the procedure, dropping it onto the top-most line in the procedure editor, the title (see figure 14). More information on the Build signal tool can be found in the User manual page. Note: since we want to gather the values for each individual windower, we need to add the Build signal to the top-most level of the procedure, e.g. on the first line of the procedure editor.

15 NOVA technical note #1 15 Figure 14 Adding the Build signal command to the procedure The Build signal command will be added the procedure, on the same level as the Timed procedures and the Repeat for each value commands (see figure 15). Figure 15 The Build signal command added to the procedure Click the button located next the Build signal command in the procedure to open Filter and select signals interface (see figure 16).

16 NOVA technical note #1 16 Figure 16 The Filter and Select signals interface Click the + button located next the Data handling group in the Filter and Select Signals interface and expand the Windower group. The calculated signals (Average current, Average potential and Power) will be displayed in the list. Double click the Average current signal to add it to the frame on the right-hand side (see figure 17). Figure 17 Adding the Average current value to the Build signal

17 NOVA technical note #1 17 Repeat this procedure for the Average potential and the Power signals (see figure 18). Figure 18 The final Build signal interface Click the OK button to close the interface. The three signals provided by the Build signal command will be added to the procedure. Step 4 plotting the data Add two custom plot command to the procedure to build the required plots (see figure 19). Links towards the signals provided by the Build signal command are used. Set the Measurement plot number of the two additional plots on plot #2 (see figure 19).

18 NOVA technical note #1 18 Figure 19 Adding two custom plots to the procedure The two plots can be customized (plotting style, axis location, etc ). When this procedure is performed on the dummy cell (a), the following results are obtained (see figure 20). Note: click the measurement view. button in the toolbar to display both plot in the

19 NOVA technical note #1 19 Figure 20 Final plots