Simcenter Testing Solutions Overloads

Direct YouTube link: https://youtu.be/rQXvpMKkIN8

 

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 (left) but the range limit is 0.5 volts (right).
 

In the figure above, the signal gets clipped at 0.5 volts which causes an overload. An overloads will distort the amplitude and frequency content of the data.

This article covers overloads, how they affect data collection, and how to avoid them:
1. Overload Types
   1.1 Data Acquisition Overloads
   1.2 Transducer Overloads
   1.3 Out of Band Overloads
2. What Does an Overload Do?
3. Spectral Content and Overloads
4. Simcenter Testlab Overload Tips
   4.1 Range
   4.2. Auto-Ranging
   4.3 Monitoring
   4.4 Overload Logging
   4.5. Overload Report

1. Overload Types

There are three main types of overloads:

• Data Acquisition System overloads
• Transducer overloads
• Out of band overloads

1.1 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 SCADAS data acquisition system and an overload flag is set.

1.2 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: In this example, two transducers with different sensitivities (10 mV/g versus 100 mV/g) measure the same transient event.  The output range of the 100 mV/g accelerometer is exceeded which causes it to read a smaller peak amplitude and creates a slow decaying offset after the transient event due to the accelerometer being saturated.

To fix this, try using a less sensitive transducer with a greater output range. 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. 

By default, there is no indication on the SCADAS hardware (i.e., red light) nor in the SimcenterTestlab software that an overload occurred when a transducer overloads.  If in a situation where the transducer range is less than the channel range, additional fields can be added in the Channel Setup to enter the transducer limits as shown in Figure 3.
 

Figure 3: Under "Tools -> Options -> Channel Setup Visibility" add the "Limit EU Auto", "Limit +EU" and "Limit -EU" fields in the channel setup. In this case, overloads will occur when the acceleration signal exceeds +/- 50 g's on the first channel.

Move the "Limit EU auto", "Limit+EU", and "Limit-EU" to the visible column.  Uncheck "Limit EU auto" to enter the limit values provided by the transducer manufacturer.  Exceeding these limits will cause an overload indication both online during data acquisition and in post processing.
 

1.3. 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 (Figure 4).

Figure 4: 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.

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.

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.

2. What Does an Overload Do?

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 5 below).

Figure 5: Incoming signals in the time domain (left). 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.

3.  Spectral Content and Overloads

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 6 below).

Figure 6: 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!

4. Simcenter Testlab Overload Tips

Simcenter Testlab has several ways of detecting and avoiding overload conditions during data acquisition.

4.1  Range

In the Simcenter Testlab Channel Setup workbook, under the “Range” column, the dynamic range of the channel can be set (Figure 7).

Figure 7: The Range column in Channel Setup defines the max dynamic range of the channel.

Try maximizing this range. Most SCADAS systems have a max range of 10V.

For more information on range settings, see the article: Gain, Range, and Quantization.

4.2 Auto-Ranging

In Simcenter Testlab 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 as shown in Figure 8.

Figure 8: 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 as shown in Figure 9.

 

Figure 9: 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 10, there are some additional helpful options.

Figure 10: 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.

4.3 Monitoring

Monitor the incoming data in the Measure workbook by dropping in data from the Data Explorer (Figure 11).

Figure 11: 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.

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.

4.4 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 12).

Figure 12: The “Accept or reject run” window with an overload warning.

If no overload occurred, the normal dialog box will appear (Figure 13).

Figure 13: The “Accept or reject run” window.

 

4.5 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 14).

Figure 14: 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 (Figure 15).

Figure 15: Double click the file in the “File view…” window. This will bring up the overload report.

The report contains which channels overloaded and at what points in time.


Questions? Email charles.rice@siemens.com or contact Siemens Support Center.

 

Also check out the Siemens SCADAS Data Acquisition Hardware Brochure.

 

Related Digital Data Acquisition Links:

• Index of Testing Knowledge Articles
• Simcenter SCADAS
• Siemens Simcenter SCADAS Mobile and Recorder Hardware
• All About Accelerometers
• Gain, Range, Quantization
• Anti-Aliasing Filters
• Averaging Types: What's the difference?
• What is Fourier Transform?
• Time-Frequency Analysis: Wavelets
• Spectrum versus Autopower
• Autopower Function...Demystified!
• Power Spectral Density
• Shock Response Spectrum (SRS)
• Windows and Leakage
• Window Types
• Window correction factors
• Exponential Window Correction Factors
• RMS Calculations
• The Gibbs Phenomenon
• Introduction to Filters: FIR and IIR
• Digital Data Acquisition and Signal Processing Seminar
• Digital Signal Processing YouTube Playlist

Simcenter Testlab acquisition tips:

• Simcenter Testlab Signature
• Hands-free acquisition (almost)
• Cool channel setup tricks for triaxial accelerometers
• GPS triggered data acquisition
• How to calculate a VECTOR SUM
• Tips and tricks for acquiring torsional orders
• No more tach! Use OBDII to get engine and vehicle speed...
• Record Raw CANBUS Data and Decode Offline!
• CAN bus Measurements
• Single Plane Balancing in Simcenter Testlab
• Measuring strain gauges in Simcenter Testlab
• Simcenter Testlab: Measuring a String Pot