5.2 DAQ

The Measurement wizard allows you to configure your online FRF measurement. Note that most settings can be changed during the measurement and also after completion when working off-line. Such changes include:

• Changing calibration values
• Re-mapping sensors in case cables had been swapped
• Changing the frequency limit
• Changing impact hammer window settings
• Changing shaker profiles

The following sections describe the steps in the DAQ wizard.

Connect hardware

In the first step, a connection can be established with a data acquisition system (DAQ). DAQ hardware requirements are listed in System requirements. The PAK MKII hardware can be connected either directly or through the PAK live.hub service. For typical applications, the direct method is recommended. The PAK live.hub is required when using multiple MKIIs.

Changing Ethernet adapter settings

To use the MKII you will need to connect it to the computer with an ethernet cable. You also need to change some adapter settings of your ethernet on your laptop. To do so, complete the next steps:

1. Open the Control Panel.
2. Select Network and Internet and then Network and Sharing Center.
3. Select Change adapter settings on the left side. A window will open.
4. Right-click on the Ethernet option and select Properties. A new window will open.
5. Select Internet Protocol Version 4 (TCP/IPv4).
6. Click Properties.
7. Select Use the following IP address and type the following addresses”.
   1. IP address: 192.168.100.10
   2. Subnet mask: 255.255.255.0
   3. Default gateway: 192.168.100.1
8. Press OK.

PAK live mode

DIRAC can only be used for measurements with the MKII running in PAK live mode. If the MKII is not in live mode, the firmware can be updated using PAK tunes. Find your device in the PAK MKII devices list on the right, and then click Update PAK MKII within the Firmware and Licenses section. This may take up to 20 minutes.

Connecting to an MKII directly

To connect directly, enter the DAQ IP address of the MKII (found on the MKII screen) within the Connect Hardware page of the Test Settings card and click Connect. A typical address would be 192.168.100.5. The DAQ status should turn green and show Running.

The following video shows how to connect to an MKII directly and gives some troubleshooting tips.

Connecting to an MKII through the live.hub

Using the cloud, one computer/server can host the PAK live.hub for several measurement devices. Using the host computer with the PAK live.hub installed, access the service by entering localhost:17671 in a browser. Here, you can add and connect to several measurement devices. It also allows for pulling the license (CS_IO_USE) for high data throughput, which is typically required in a multiple-DAQ setup.

Next, DIRAC will connect to the cloud server. In DIRAC, within the Connect Hardware page of the Test Settings card, enter the IP address of the computer hosting the cloud service and click Connect. If you are hosting the service on the same computer as you are running DIRAC, enter 127.0.0.1. The DAQ status should turn green and show Running.

In case you want to use multiple frontends, it is advised to connect them via a network switch. Make sure the frontends are in the same IP-address subnet, so, for instance, 192.168.100.xxx.

The following video explains how to connect to an MKII through the PAK live hub.

Enable PTP synchronization

If more than one system is connected through the cloud, you must enable PTP synchronization. Use the switch PTP synchronization to enable this and press PUSH. Beware: this will disconnect DIRAC from the cloud and will reboot the PAK systems for which the state has changed.

When the systems are online again, reconnect and observe that the PTP synchronization is indeed Synchronized. This might take about a minute to update.

Measurement settings

The second step is about the impact/shaker measurement settings. Here you can define one or more input sources (impact hammers and shakers), set the sampling rate, sampling time, frequency settings, and define filtering curves in case more than one input source is defined.

Input sources

You can add input sources by clicking on the + icon on the top right. This will open the sensor database, where you can find the impact hammers and shakers. For conventional FRF testing, it is commonplace to use a single impact hammer only. The pre-trigger settings of the source can be set using a pre-trigger percentage (a percentage of the time window) and a trigger level. In the single-input case, filtering shall always be set to none (more about this further on).

Sampling settings

Here the sampling settings are defined. The sampling rate can be set for a list of possible sampling rates in Hz supported by the DAQ. The sampling time establishes the length of the measurement window in seconds and can be set freely.

Note that sampling settings cannot be changed when measurement data already exists within the project. If needed, all data can be deleted by right-clicking within the Impact Locations card and selecting Delete All Measurement Data in This Project.

You can set a value in the Zero padding field to increase the frequency resolution of your FRFs. It corresponds to an interpolation of the measured FRF. Since DIRAC only saves the measured time data, this does not increase the file size of your project. The zero-padding option can be changed at any time.

The Nyquist factor offers three options: 2.0, 2.56, and Custom. This specifies the effective frequency bandwidth of the measurement. When set to 2.0 or 2.56, the frequency limit becomes the sampling rate divided by that factor. According to the Nyquist- Shannon sampling theorem, factor 2.0 is the maximum observable frequency, whereas 2.56 is the “conservative” factor commonly used in NVH engineering. For the case of sampling at 16384 Hz, this yields respectively 8192 or 6400 Hz bandwidth, which will be displayed in the field Frequency limit.

The Custom setting allows to manually specify a frequency limit to match the frequency range of interest. Often this is considerably lower than the sampling rate. Setting a lower frequency limit during testing has the added benefit that all other frequency-domain calculations are performed faster. The option is also non-destructive, meaning that the saved measurement data is unaffected, and one may always choose a higher frequency limit later on for analysis.

Filtering settings for multi-inputs merge

If you are using multiple inputs (hammers, shakers) for your measurements, you need to merge the data of each of them. In DIRAC, we implemented spectral filtering up-front in the calculation. This will ensure that each input source adds energy in the frequency range where it is most effective.

To filter the inputs from multiple sources, complete the next steps:

1. Click on the DAQ bottom on the top right corner of the Prepare or Measure modules.
2. Click on Measurement Settings.
3. Click on + to add an input source.
4. Select the source from the database.
5. Click on Use selected sensor.
6. Choose the correct filtering.
7. Set the correct cut-off frequency  (the frequency where the filter = 0.5).
8. Set the correct roll-off frequency (the frequency range of the filter).

Map sensors

Sensors and excitation sources are mapped by dragging and dropping them onto the desired channels on the DAQ system. Multiple channels can be selected and mapped simultaneously. DIRAC checks for compatible modules and channel types to ensure that ICP and Microphone channels cannot be mixed up.

Some general things to note:

• In the case of multiple DAQ systems, the drop-down box on the top right can be used to switch between the DAQs.
• The DAQ channels must be active to map channels. The DAQ channels are activated by right-clicking on the DAQ channels and selecting Activate All Channels.
• In case multiple excitation sources (impact hammers) are planned for, but not yet connected, it is important to already map these to the channel where they will be measured on. This ensures that the proper channels are recorded.
• Unmapped channels are not recorded. Therefore, changing sensor mapping can only be done for channels that were initially marked for measurement by having a sensor associated with them.

Channel settings

The sensor settings provide configuration options for the measurement channels. The calibration values, mode (ICP/voltage/microphone), voltage range, coupling (AC/DC), and grounding (differential/grounded) of the sensors can be set here. Other information about the sensors. e.g. name and serial number, were previously defined in the Prepare module and must be updated there. By clicking Show advanced, high-pass and low-pass filtering can be defined.

The table will show the values as they are configured in DIRAC, which is information that gets stored in the project. If an inconsistency is observed between the channel definitions in DIRAC and the actual values on the DAQ, this will be marked by a yellow background on the value field. Here, you can choose from two options:

• Corrected on the MKII side by pressing Push settings to DAQ. This will send the updated configuration.
• Assume the values as set on the DAQ by pressing Pull settings from DAQ. This will make changes to the values in DIRAC.

Remarks:

• Calibration settings can be pasted from a spreadsheet program such as Excel. The calibration unit is set in the definition of the sensor.

For TEDS-enabled sensors, the TEDS information can be read out by clicking the TEDS button next to a channel, or all TEDS information can be read out simultaneously using the button in the lower right.

TEDS is the memory within some sensors that is utilized for storing information about the sensor, such as manufacturer name, sensor type, model number, serial number, and calibration data.

If the TEDS button is pressed for a sensor that does not have TEDS, old sensor information from PAK may be retrieved that is not relevant for that sensor, and the sensitivity may be updated incorrectly. The TEDS readout button should only be used with sensors that definitely have TEDS and verified afterward.

Map shakers

If you use a shaker for your measurement, you have to map it to the output device and define some measurement settings.

• Output device: select from the drop-down the output device (the sound card where the shaker is connected).
• Sampling rate: type the value, or leave it empty to use the default one (same selected in step 2: Measurement Settings).
• Shaker mapping: choose the correct output channel from the drop-down. The stereo sound card has 2 channels, so select the correct one.

Windowing and Profiles

The last step is about the impact windows and shaker profiles that are used in the FRF calculation.

Windowing (Hammers)

In the Windowing tab of the hammer, you can define the force/exponential window parameters. They can either be entered manually on the right, or the orange bars within the plots can be dragged to the desired parameters. The most recent impact is shown in the plots.

Profiles (Shaker)

In this window, you create all the shaker profiles that you need for your measurement.

To create a new profile, click the + button, and a new profile is added. Set up the following:

• Profile name: type the name of the profile.
• Type: Choose the profile type from the dropdown.
• Number of blocks: type how many times the profile is repeated.
• Cross reference channel: channel used for the division. DIRAC, by default, picks the force channel.

After creating a profile, you can play it to check it:

1. In the Global Profile settings, select the profile from the dropdown.
2. Press the Play button.

Ready for measurement

Once the last step has been completed, a summary page shows the settings as set, alongside an encouraging image to get started measuring! Now would be a good time to save the DIRAC project, though, for instance, by pressing CTRL+S.