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Overloads

Siemens Experimenter Siemens Experimenter
Siemens Experimenter

What is an Overload?

 

An overload occurs when the voltage of an incoming signal is higher than the voltage range the sensor or data acquisition system can handle. When an overload occurs, the data beyond the range limit is clipped (Figure 1).

 

Figure 1: The amplitude of the incoming sine wave is 1 volt but the range limit is 0.5 volts. Therefore, the signal gets clipped at 0.5 volts.Figure 1: The amplitude of the incoming sine wave is 1 volt but the range limit is 0.5 volts. Therefore, the signal gets clipped at 0.5 volts.Overloads distort the amplitude and frequency content of the data.

 

Overload Types:

 

There are three main types of overloads:

 

  1. Data Acquisition System overloads
  2. Transducer overloads
  3. Out of band overloads

 

1. Data Acquisition System Overloads 

 

This type of overload occurs when the range of the data acquisition system is lower than the range of the incoming signal. The incoming signal is clipped as shown in Figure 1 above.

 

Usually when this occurs, a red light appears on the acquisition system and an overload flag is set.

 

2. Transducer Overloads

 

This type of overload occurs when the physical input the sensor measures is greater than what the sensor can handle. This causes the incoming data to be clipped before it is digitized. This will happen even if the range of the data acquisition system is greater than the range of incoming data. See Figure 2 below.

 

Figure 2: The range limit of the data acquisition system is set at 1.0V. But the transducer can only handle signals up to 0.5V. Therefore the incoming signal is clipped despite being under the range limit of the data acquisition system.Figure 2: The range limit of the data acquisition system is set at 1.0V. But the transducer can only handle signals up to 0.5V. Therefore the incoming signal is clipped despite being under the range limit of the data acquisition system.

To fix this, try using a less sensitive transducer. For example, instead of using a 100mV/g accelerometer, switch to a 10mV/g accelerometer. For the same amount of vibration, less voltage will be generated.

 

Be sure to check transducer data sheet and find the max level the transducer is rated for. For example, an accelerometer may have a max level of 500g. If the data acquisition system is reading 500g the transducer may be overloading. 

 

3. Out-of-Band Overloads

 

An out-of-band overload occurs when frequency content beyond the specified bandwidth is of higher amplitude than the range limit of the system. This can be a very difficult overload to diagnose because the data displayed in the bandwidth region will be within range, yet the system will indicate an overload.

 

Figure 3: The frequency content at fover is above the specified bandwidth. Despite this, the frequency content is above the range limit of the data acquisition system and will cause an overload.Figure 3: The frequency content at fover is above the specified bandwidth. Despite this, the frequency content is above the range limit of the data acquisition system and will cause an overload.

To fix an out-of-band overload:

 

  • Use a less sensitive transducer: The less sensitive transducer will bring down the voltage amplitude. Beware: when using a less sensitive transducer the signal to noise ratio will be lower!
  • Use an external low pass filter: The external low pass filter will remove the higher frequency content that may be causing the overload. Be cautious and ensure the external filter is not also being overloaded: the external filter will need a larger range than the data acquisition system.

What Does an Overload do to the Data? 

 

Scenario: A transient signal was acquired. The range of the signal is 1V. The signal was acquired once with a 10V range and once with a 0.16V range. The signal that was acquired with a 0.16V range is clipped in the time domain (see Figure 4a below).

 

Figure 4: a) Incoming signals in the time domain. The two traces (red and green) are the same incoming to the DAQ system. The green signal was acquired with a range of 10V (no overload). The red signal was acquired with a range of 0.16V (overload). b) Incoming signals in the frequency domain. Green is the non-overloaded signal. Red is the overloaded signal. Notice the red spectral content smeared throughout the frequency range.Figure 4: a) Incoming signals in the time domain. The two traces (red and green) are the same incoming to the DAQ system. The green signal was acquired with a range of 10V (no overload). The red signal was acquired with a range of 0.16V (overload). b) Incoming signals in the frequency domain. Green is the non-overloaded signal. Red is the overloaded signal. Notice the red spectral content smeared throughout the frequency range.

An overload can affect the data across the entire measurement bandwidth. It can raise or lower amplitudes and generate additional noise on the measurement.  

 

Why is there so much spectral content introduced when the signal overloads?

 

In the time domain, the signal becomes clipped. Essentially turning the “sine” shaped signal into a “square wave” shaped signal.

 

The Fourier series of a square wave consists of all odd harmonics of the frequency of the wave, continuing to infinity. Because the clipping creates square waves at various periods and frequencies, the fundamental odd harmonics of these square waves are also created at many frequencies. The odd harmonics of these fundamentals are carried out across the entire frequency range creating lots of spectral content.

 

This phenomenon is called harmonic distortion (see Figure 5 below).

 

Figure 5: a) The spectrum of one square wave. b) The spectrum of three square waves at different frequencies. c) The spectrum of four square waves at different frequencies.Figure 5: a) The spectrum of one square wave. b) The spectrum of three square waves at different frequencies. c) The spectrum of four square waves at different frequencies.

You can see that even with just three or four square waves at different frequencies, harmonic distortion effects the spectrum greatly.

 

Overloads greatly affect the resultant spectrum!

 

WARNING: Even if the overload is out of range you will still get harmonic distortion across your entire frequency range. The overload will still effect your data even if it’s out of band!

 

LMS Test.Lab Tips!

 

1. Range 

 

In the LMS Test.Lab Channel Setup workbook, under the “Range” column, the dynamic range of the channel can be set. Try maximizing this range. Most SCADAS systems have a max range of 10V.

 

Figure 6: The Range column in Channel Setup defines the max dynamic range of the channel.Figure 6: The Range column in Channel Setup defines the max dynamic range of the channel.

2. Auto-Ranging

 

In LMS Test.Lab under the Acquisition Setup tab there is a level bar which gives an indication of whether or not the data acquisition system is in overload. A white bar indicates the range is too large. Green bars indicate the range is set appropriately. Orange indicates that the signal is within the upper limit / overhead region. A red bar indicates the range is set too low and the channel is overloading.

 

Figure 7: The level bar indicates whether the range is appropriate for the incoming signal.Figure 7: The level bar indicates whether the range is appropriate for the incoming signal.

To auto-set the ranges, click 1) start ranging, 2) hold level, then 3) set ranges.

 

Figure 8: The Autoranging sequence.Figure 8: The Autoranging sequence.

This will set the range to the appropriate level. Hopefully minimizing the risk of an overload.

 

Under the “More…” button in Figure 9, there are some additional helpful options.

 

Figure 9: The “Range Checking” window opens after selecting the “More…” button from the autoranging panel.Figure 9: The “Range Checking” window opens after selecting the “More…” button from the autoranging panel.

It is possible to set the dB level of overhead used for the ranging.

 

Overhead is essentially “headroom” for safety. It is additional room in the range to protect against spikes in the data which may cause overloads. A common overhead setting is 6 dB (or 50% of the input range). You can always increase or decrease this number to suit your needs.

 

It is also recommended to activate the “Use full range when autoranging” box. When activated, the range is set to the full range before the autoranging procedure starts.

 

NOTE: Use the autorange feature to set the ranges appropriately. It is not wise to always use the max range of the system to avoid overloading. See the explanation of why in this article

 

3. Monitoring 

 

Monitor the incoming data in the Measure workbook by dropping in data from the Data Explorer. It is important to monitor incoming data to ensure the data is as expected. This will make it easier to detect what type of overload occurs.

 

Figure 10: In the Measure workbook, it is possible to monitor data online. To do this, create a display and drop in data from the Data Explorer.Figure 10: In the Measure workbook, it is possible to monitor data online. To do this, create a display and drop in data from the Data Explorer.

4. Overload Logging 

 

In the Measure worksheet, after stopping a run, the “accept or reject run” dialog box will appear (unless the message is suppressed). If an overload occurs, the dialog box will warn the user that an overload occurred and an overload report will be generated.

 

Figure 11: The “Accept or reject run” window with an overload warning.Figure 11: The “Accept or reject run” window with an overload warning.

If no overload occurred, the normal dialog box will appear (as shown below).

 

Figure 12: The “Accept or reject run” window.Figure 12: The “Accept or reject run” window.

 

5. Overload Report 

 

To get the details of an overload report, browse to the run in the Navigator workbook. Two reports will be listed: the “OfflineOverloadLogging” and “OnlineOverloadLogging”. To view the contents of the report, right click the report and click view.

 

Figure 13: Two overload reports will be generated. To view the report, right click and click view.Figure 13: Two overload reports will be generated. To view the report, right click and click view.

This will open a “File view…” window. Double click on the .txt file within the window to open the detailed overload report. The report contains which channels overloaded and at what points in time.

 

Figure 14: Double click the file in the “File view…” window. This will bring up the overload report.Figure 14: Double click the file in the “File view…” window. This will bring up the overload report.

Related Digital Data Acquisition Links:

 

LMS Test.Lab acquisition tips:

 

Other Links:

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