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"Excessive DC level" message

Experimenter
Experimenter

Trying to do a MIMO Stepped Sine test with 30 accelerometers and one shaker, but cannot get past the System Identification step. 

 

First the software does some “Settling…” (right middle)

 

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And then fails with a red “Failed”. (right middle)

 

2.png

 

 

The message is “Excessive DC Level” on some channels.

 

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What is this all about?

 

 

2 REPLIES

Re: "Excessive DC level" message

Siemens Legend Siemens Legend
Siemens Legend

At Siemens, any time a closed loop vibration control test will be run, we want to make sure your test article and shaker system:

  1. Are well controlled, and get good and accurate results.
  2. Will not get damaged due to problems with malfunctioning accelerometers or other situations.

There are in fact two different DC level checks, each aimed at making sure the transducers are working properly before shaking the structure:

  • “Excessive DC level” check – How close the measurement AC coupled channels are to zero without the test shaker running. For AC coupled transducers, their value should be very close to zero.
  • “Excessive Delta DC level” – Check on both DC and AC coupled channels on how stable the channels levels are over time. Without the shaker running, the levels should not change over time.

If these DC level checks cannot be passed without the shaker running, there might be a grounding, icp power, high environmental vibration, or thermal issues causing the transducer to drift.

 

For example, ICP accelerometers are prone to drifting or offsets, especially immediately after the power is applied to the accelerometer. After the power is applied, it can take a minute for the accelerometer to settle to zero (see Figure 1).

 

icp_drift.pngFigure 1: After applying ICP power, it takes some time for an accelerometer to read zero.

 

Any changing or drifting signal makes the amplitude less predictable and should be addressed prior to running the test.

 

Excessive DC Level Check – For AC Coupled Transducers Only

 

The “Excessive DC level” checks evaluates the following for every AC coupled channel (ICP or Voltage AC mode in Channel Setup).  An AC coupled channel has a high pass filter with a very low cutoff frequency.  The filter removes any offsets from the data so no offsets are present in the data:

 

1. Without the shaker running, it calculates the mean over individual measurement blocks as shown in Figure 2. The mean should be close to zero (although never perfectly zero) without the test running and in AC coupled mode.

 

dc_level_check.pngFigure 2: The mean of the signal, without the test/shaker running, is calculated on successive blocks and evaluated for how close to zero it is compared to the channel input range.

 2. The mean is compared to the channel input range, and must be below a certain threshold as indicated by the “Maximum background noise to full range ratio” setting.

  

maximum_background.png

 

For example, if the channel range is 10 Volts, the mean value must be below (10 V) * (0.1%) = 0.01 Volts.  If the mean was 0.01 volts or less, the test would pass.

 

Where to change this setting is shown below.  It depends on the application that you are running.

 

Excessive DC Delta Level Check – For AC and DC Coupled Transducers

 

The closely related “Excessive Delta DC level” looks at the variation in the mean between successive time data blocks while the test is not running as shown in Figure 3. The variation should be close to zero if the signal is stable.

 

dc_delta_level_check.pngFigure 3: The “Excessive Delta DC level” check looks at the difference in successive time data blocks. Ideally, the difference should be zero from block to block if the transducer is stable.

This check is used for channels in both AC and DC coupled mode, where a DC offset on the data could be expected. Any DC offset on the data channel should be constant over time.  DC offset drifting and non-constant behavior is not expected without the shaker running.

 

The parameter “Max Background noise variation to channel range ratio” is used to evaluate the difference between successive blocks.

 

maximum_background_variation.png

 

Solutions: What to do?

 

The first thing to do when encountering this problem is to ensure all the transducers are working properly.  Check for the following:

  1. ICP Settling – Make sure ICP accelerometers have adequate time to settle.
  2. Electrical and Thermal Issues - Look for and eliminate any thermal drift or electrical grounding issues. Make sure all accelerometers and other transducers are properly mounted.
  3. External Vibration Sources - Reduce or eliminate any external sources of vibration. For example, nearby equipment that is running.

If trying the items above does not resolve the issue, change the settings used in the DC level checks:

  1. For AC coupled channels, increase the “Maximum background noise to full range ratio” percentage (for example, increase to 0.5 percent from 0.1 percent).
  2. For AC and DC coupled channels, increase the “Max Background noise variation to channel range ratio”.

 

Simcenter Testlab

 

Depending on the application, you can find the DC level check settings in different areas:

 

Vibration Control: Sine, Random, Shock

 

When using Simcenter Vibration Control (formerly called LMS Test.Lab) products, the DC level check settings can be found in the Selfcheck worksheet under the “Advanced..” button and Safety tab as shown in Figure 4.

 

vibration_control_settings.pngFigure 4: In Simcenter Vibration Control, DC Level check settings are under the “Advanced…” button in the Selfcheck worksheet.

 

MIMO Swept and Stepped Sine

 

In the System Identification worksheet, click on the “Settings” button in the upper left. The DC level check settings will be on the middle right as shown in Figure 5.

 

stepped_swept_sine_settings.pngFigure 5: In Simcenter MIMO Sine Sweep and Stepped Sine, DC Level check settings are under the “Advanced…” button in the System Identification worksheet.

Other Ideas

 

There is also a TestLabEnvironmental.ini file you can increase “MaxSettlingTime_Seconds=30.”  From 30 seconds.  Typical Path to find this file is C:\Program Files (x86)\LMS\LMS Test.Lab {revision}\central\Configuration or C:\Program Files (x86)\Simcenter\Simcenter Testlab {revision}\central\Configuration.

 

Another possible solution to get pass the Excessive DC issue is to increase the range of the input channel.  This will increase the allowable DC and delta DC value. However, changing the control channel range will also increase the value of the open loop threshold.  It is not highly recommended to use the channel range to address this issue.

Re: "Excessive DC level" message

Experimenter
Experimenter

Followed what you said, and got through the System Identification.

 

I do have some machinery running nearby that might be causing some additional vibration.  I can't shut it off.

 

So I increased the Max Background noise variation to 5% from 0.01%:

 

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And then got through the System Identification:

 

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And the test ran fine after that!  

 

-Thanks