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Simcenter Testlab SCADAS and Long Strain Cables

Siemens Valued Contributor Siemens Valued Contributor
Siemens Valued Contributor

Question:

 

Doing a test with relatively long cables for strain measurements (about 10 meters+ or so) using Simcenter SCADAS and VB8 strain cards. 

 

Any important things to keep in mind when using long cables to measure strain?

 

Answer:

 

There are probably two big concerns:

  1. Noise on the strain measurements caused by the long cables acting as antennas and attracting electromagnetic and electrostatic interference.
  2. Resistance in the cable affecting the strain gauge calibration factor which results in an inaccurate strain amplitude measurement.

 

Noise or Interference on Measurements

 

Strain gauges produce very low level voltage signals which are transmitted through the cables.  Because the levels are low, the signals are susceptible to even small amounts of electromagnetic and electrostatic interference.

 

Electromagnetic

 

Electromagnetic interference comes from from nearby electric power sources, including electric motors and electric transformers as shown in Figure 1.

 

EMI.pngFigure 1: Electromagnetic interference in a signal wire is created from the magnetic field generated by nearby electric sources.

 

Current flowing through an electrical device generates a magnetic field. Any wire carrying a measurement signal passing through this magnetic field picks up electromagnetic interference.

 

Electrostatic

 

Electrical charges can accumulate on a signal wire, when electrical charges are discharged from a source (Figure 2). Ever experienced a spark when shaking hands with someone during the winter?  This is an example of electrostatic discharge.  In fact, the spark can happen without actual contact between the two hands. The hands just have to be close enough that the spark jumps from one to the other.

 

In measurements, electrical sources like fluorescent lights can create electrostatic interference on an unshielded signal wire. 

 

ESI.pngFigure 2: Accumulated charges, created by electrical discharges from a nearby electrical source, cause electrostatic interference in a signal wire.

 

It is important to have good shielding (Figure 3) around the signal wire to prevent electrostatic interference.  Charges accumulate on the shielding, rather than the signal wire. 

 

shield.pngFigure 3: A braided shield, like the one used in the SCADAS VB8, gives the wiring some flexibility that a foil shield would not.

 

The shield takes these accumulated charges away from the signal wires via a low resistance path to ground. Shielding is a thin metal foil wrapped around the signal and supply wires.  A braided shield gives the cable flexibility so it can bend easily when routing the cables.

 

What can be done?

 

Some things that can be done to abate electromagnetic and electrostatic interference:

  1. Use higher excitation voltages to make the strain gauge signal levels higher. This will make the measurement signals have a higher amplitude relative to electrical interference.  There is a trade-off with thermal drift with is explained in the knowledge base article 'Selecting an excitation voltage for strain gauges'.
  2. Route signal cables far away from any electrical power sources.
  3. Do not inadvertently leave power cables near strain cables.  If necessary, make sure the power cables and signal wires cross each other orthogonally, rather than parallel.
  4. Use a full bridge with differential signal wires to take advantage of common mode rejection.
  5. Make sure the cables are well shielded electrically.  The shield should provide a low resistance path to ground.

 

Calibration Factor Sensitivity

 

A calibrated strain gauge is provided with a sensitivity or calibration factor (for example 1000 uE/mV).  The calibration factor, used to convert the voltage signal produced by the gauge to strain, depends on the voltage supply level used when calibrating. 

 

For example, suppose the gauge was calibrated with the supply at five volts, and the resulting calibration factor is 1000 uE/mV.  If the supply was changed to one volt, the calibration value would be less. For a one volt supply, it would be 200 uE/mV. The calibration factor changes with the voltage supply.

 

Typically, strain gauges are calibrated in a lab, and do not require long cables when calibrating.  With a short cable, the voltage supplied and the voltage that reaches the gauge, are the same.

 

However, if a long cable is used while measuring, the voltage supplied and the voltage that reaches the gauge may be different (Figure 4). The longer the cable, the higher the electrical resistance. This resistance reduces the voltage across the cable. 

 

For example, the voltage could be reduced across the supply wire by 10% (for example, supply of 5 volts reduced to 4.5 volts at the gauge).  This would result in a 10% amplitude error on the measurement.

 

long_and_short_cables.pngFigure 4: The length of the voltage supply wire creates a resistance and drop in the voltage carried by the wire. A shorter wire (top) has less of a voltage drop than a longer wire (bottom).

There are different methods to account for the drop in the supply voltage when using a long cable:

  1. Calibrate with Long Cable: Calibrate the gauge with the long cable to be used in the final application rather than a short cable.  When doing this, if a resistor is used in the calibration process, it should be applied at the gauge itself so the resistance of wire is accounted for in the calibration process.  The calibration resistor cannot be applied at the source.
  2. Lead Wire Resistance Estimation: By determining the resistance of the signal (or lead) wire, the voltage drop can be calculated and the calibration factor be adjusted accordingly.  This is based on an estimate of the resistance given the length and type of wire.
  3. Sense Lines: Sense lines are additional wires that are used to measure the voltage supply at the gauge. The wires run in parallel to the supply lines and carry no current. The voltage that reaches the gauge is measured and the voltage supply is adjusted accordingly to ensure the voltage supply at the gauge is as expected.

These three options can be handled with the Simcenter SCADAS VB8 card as described in the next section.

 

Simcenter Testlab and Simcenter SCADAS

 

This section describes how the previously mentioned methods ensure an accurate strain measurement using Simcenter Testlab and the Simcenter SCADAS VB8 card.

 

Calibrate with Long Cable

 

If using Simcenter Testlab to calibrate the strain gauges with long cables, the calibration resistor should be placed at the gauge.  Details about using Simcenter Testlab to calibrate a strain gauge are given in the knowledge base article ‘Using Simcenter Testlab to measure strain gauges’.

 

external_sense1.pngFigure 5: When calibrating with an external resistor, the External Sense field in Simcenter Testlab and SCADAS should be set to “External Shunt, No Sense Lines”.

In the knowledge base article, internal shunt resistors of the SCADAS hardware are used. This would not give the correct result when using long cables.  Instead, a shunt resistor placed at the gauge itself should be used.  The “ExternalSense” field should be set then to “External Shunt, No Sense Lines” as shown in Figure 5.

 

Lead Wire Resistance Estimation

 

The number of ohms of resistance of the cable can be entered in a field (for one wire only) in the Channel Setup workbook.  Using the ohms per meter or ohms per foot and multiplying by the corresponding signal wire length, and the total ohms of resistance can be calculated.

 

This value can be entered into the software:

  • In the Channel Setup workbook, choose “Tools -> Channel Setup Visibility” from the main Testlab menu.
  • Add “Bridge Lead Resistance” from the source field names to visible field names, and press “OK” as shown as Figure 6.

bridge_lead_resistance.pngFigure 6: The “Bridge Lead Resistance” field in Channel Setup visibility menu.

  •  Enter the calculated ohms of resistance in Channel Setup in the field. The default value of 0.7 ohms corresponds to the resistance of the LEMO to Open Wire pigtail adaptor cable provided with the SCADAS VB8 cards. If this cable is being utilized, it should be added to the calculated signal wire resistance.

 

Sense Lines

 

Sense lines are dedicated wires that measure the voltage being supplied at the gauge.  These sense wires are in addition to the signal wires and voltage supply wires (Figure 7). These wires have no current so they can measured the voltage at the gauge correctly.

 

many_cables.pngFigure 7: Sense lines are additional wires that are run to the gauge in addition to the voltage supply and signal wires.

Sense wires get connected to Pins 2 and 5 on the SCADAS VB8 card as shown in Figure 8.

 

pinout.pngFigure 8: The pinout for the LEMO connections of the Simcenter SCADAS DB8 and VB8 cards.

The voltage supply will be automatically adjusted by the SCADAS data acquisition hardware to maintain a certain supply level at the gauge.

 

To utilize sense lines:

  • In the Channel Setup workbook, choose “Tools -> Channel Setup Visibility”
  • Add “External Sense” from the source field names to visible field names, and press “OK”.
  • In the Channel Setup, change the “ExternalSense” field to “Internal Shunt, Sense Lines” (Figure 9).

 

external_sense2.pngFigure 9: Select “Internal Shunt, Sense Lines” to utilize the sense lines of the VB8 and DB8 cards.

Sense lines increase the amount of wiring that is required.  For example, a full gauge increases from 4 wires to 6 wires!

 

Questions?  Email john.hiatt@siemens.com or post a reply to this article.

 

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