Shaker testing

In some cases, it is beneficial to use a shaker to excite the system instead of one or multiple impact hammers. With a shaker, you can: 

• Improve the repeatability and precision of the excitation.
• Input more energy into the system.
• Have control over the amplitude of the input energy.
• With a small shaker it can be easier to access areas that are difficult to reach with a hammer (below a car).

In DIRAC, you can perform measurement campaigns with shakers. To do so, you need the dir-shaker license. If you wish to use both shaker and hammers as input sources, you will also need the dir-impact license.

When performing a shaker measurement in DIRAC, you must follow the same procedure as for a measurement with impact hammers, with a few extra steps. In this article, you will find all the extra steps, explained in detail. 

Add the impedance head and shaker to the database

The measurement setup using a shaker excitation can be described by the following components:

• A signal source: generated from the DAQ system or an external signal generator.
• An amplifier.
• A shaker.
• A stinger: a rod with high stiffness in the axial direction to make sure the excitation is in the right direction
• An impedance head (or one or multiple sensors): the sensor (usually force type) placed at the end of the shaker and glued to the test candidate.

In DIRAC, you must add both the shaker and the impedance head to the project. To measure the FRF a shaker must always be linked to an impedance head. It is always recommended to add both a master and an instance of the shaker and the impedance head to the sensor database (Similar to how you would previously be able to add sensors). You must create two instances if you want to use the same shaker with multiple impedance heads.

Already created impedance heads and shakers can be found in the Hardware Database. You can open it from the Prepare module by clicking on the button or with the keyboard shortcut D.

Create a new impedance head

To create a new impedance head, you must add and save it in the Hardware database. To do so, complete the next steps:

1. Access the Hardware database ( button or keyboard shortcut D).
2. Go to the Impedance Head tab.
3. Click the button.
4. Fill in all the Sensor and Rendering information, including the Channels window.
5. Press Save.

Create a new shaker

If you want to use a new shaker for the first time, you need to add it and save it in the Hardware database. To do so, complete the next steps:

1. Access the Hardware database ( button or keyboard shortcut D).
2. Go to the Shaker tab.
3. Click the button.
4. Fill in all the Sensor information.
5. Link it to an impedance head, by clicking on and selecting the correct one.
6. Press Save.

Add the shaker to the project

After creating the new shaker and impedance heads, you must add them to the current project. To do so, you need to add the shaker only, since it already has the information of the impedance head. To add a shaker to the project, complete the next steps:

1. Access the sensor database.
2. Go to the Shaker tab.
3. Select the correct shaker from the list.
4. Press Use selected sensor.

DAQ

After preparing the project, it is time to set up the measurement and connect to the hardware. You can do it in the DAQ window. Here, in the Connect Hardware tab, you connect to the MKII directly or through the PAK live.hub. You can read how here.

In the Measurement Settings you define all your measurement settings and set up the input sources, if you have multiple of them. You can read more information here.

You then need to map all sensor channels (including the hammers and impedance heads) to the correct portholes of the MKII. You do this in the Map Sensors tab. In the Channel Settings tab, you push or pull the channel settings to or from the MKII. You can find more information here.

Map Shakers

After setting up and mapping all channels, it is time to connect the shaker and correctly map its channels, in the Map Shakers tab. To do so, complete the next steps:

1. Select the shaker output device (sound card it is connected to) from the dropdown.
2. Define a sampling rate. By default, DIRAC uses the one set in the Measurement Settings tab.
3. In the Shaker Mapping, choose the correct output channel from the dropdown. (the stereo sound card has 2 channels).

Windowing and profiles

In the last step, set the correct force and response windows for hammers and define the shaker profiles. Here, select the shaker source and add all the desired profiles. To do so, complete the next steps:

1. Click on the button to add a new profile.
2. Edit the profile settings. They are:
   • Profile name.
   • Type: choose the desired profile type from the dropdown.
   • Number of blocks: how many times the profile is repeated.
   • Cross reference channel: channel used for the calculations. DIRAC, by default, picks the force channel.
3. If you have periodic signals, edit their settings. They are:
   • Block pause (%).
   • Lower Frequency (Hz).
   • Upper Frequency (Hz).
4. In the global profile settings, set the signal fade in/out.
5. Test the created profile by selecting it from the dropdown and pressing the play button .

Measuring with a shaker

When all is set up, it is time to start measuring. To do so, complete the next steps:

1. Go to the Measure module.
2. Select the excitation location where the shaker is attached (from the Impact locations card or the 3D Viewer).
3. Open the Shaker Control card.
4. Select the desired profile.
5. Press the Play button .