Simcenter Testing Solutions Simcenter Testlab: Measuring a String Pot

Figure 1: String Potentiometer or “String Pot”
 

String pots consist of a measuring cable, a spool, a spring, and a rotational sensor. The end of the cable is attached to a moving object. As the object moves, the cable un-reels and reels. The rotating spool creates electrical signals proportional to the linear extension and velocity of the cable.

The cable is wound tightly around the spool, and the diameter of the spool is constant. The spring is in place to maintain the tension and retraction of the cable.

2. Simcenter SCADAS VB8-II and VB8-III Cards

To measure a string pot with Simcenter SCADAS hardware, a VB8-II or VB8-III (referred to as VB8 for remainder of article) card is required. The VB8 card has a voltage supply to power and measure a string pot directly.  

The VB8 card (Figure 2) has 8 channels with software selectable signal conditioning per channel.  Users can select from: ICP, Voltage, Bridges (Quarter, Half, Full), Potentiometer, and Active Sensors.
 

Figure 2: VB8-II card and with Open Wire (other end is LEMO) cable and BNC wire (other end is LEMO) cable,
 

The card comes with small “pigtail” wires.  To support string pots, there is an open wire pigtail which is used to connect to the string pot. 

A VB8 card also supports other types of transducers in addition to string pots, such as strain gauges and ICP based accelerometers and microphones. The card can be installed in any SCADAS Mobile frame. The card can be mixed in the SCADAS frame with other signal condition cards (like V-24, V8-E, etc).

3. Cable Wiring 

String pots come in a variety of models.  Most require three wires to be connected to work:

The Simcenter SCADAS has seven connections, three of which are used to measure a string pot as shown in Figure 3: 
 

The connections on the SCADAS are typically through the pigtail open wire which has specific colors that correspond to each pin.

The connections are as follows:

Why connect the ground of the string pot to the negative supply voltage of the SCADAS? This is a little trick to get the maximum voltage output from the string pot. The sense line connections of the SCADAS are not used with a string pot.

4. Simcenter Testlab: Channel Setup

In Simcenter Testlab Signature Channel Setup worksheet, set the following to use the strain gauge (Note: If some fields are not showing, make them visible under “Tools -> Channel Setup Visibility”):

Point

Enter the desired name for the string pot measurement channel.

Input Mode: Potentiometer

Set the input mode to Potentiometer as shown in Figure 4.
 

Figure 4: Set input mode to potentiometer to use a sting pot.
 

Measure Quantity: Displacement

Bridge supply: Set the voltage supply level. Lower voltages make the signal levels low and susceptible to noise. 

Electrical Unit: mV/V

A string pot is “ratiometric” sensor.  This means that its sensitivity depends on supply voltage. The higher the supply voltage, the more output signal from the string pot. This setting ensures that the sensitivity will be entered in mV of output for one volt of supply. This should be the default setting after selecting “Potentiometer” for the Input Mode.

Actual Sensitivity

The mV/V/EU (Engineering Unit) value. This value can be calculated during calibration (next section) or can be entered directly from the specification sheet if it is known.
Range:10 V

Best practice to start with a full range of 10 V. 

An example setup of a string pot is shown in Figure 5.
 

Figure 5: Simcenter Testlab Channel Setup settings.
 

To determine the calibration values and/or check that the string pot is reading the correct values, a DC calibration is performed as described in the next section.

Some helpful Channel Setup tips: Simcenter Testlab Data Acquisition Tips

5.  Simcenter Testlab Calibration

The sensitivity and offset value for the string pot can be calculated in the Simcenter Testlab software via the “Calibration” worksheet (Figure 6).
 

Click on the “Calibration” worksheet. Once the worksheet is opened, click in the upper right corner, and select “DC Calibration” (default is AC Calibration) as shown in Figure 7. For more information on the difference between AC and DC based measurements, see the knowledge article: "AC versus DC Coupling - What's the difference?".
 

Figure 7: Select "DC Calibration" in the upper right hand corner of the Calibration worksheet.
 

To proceed, a method of measuring the displacement at a few known lengths is needed.  An example is shown in Figure 8.
 

Figure 8: String pot calibration setup for 6.5 centimeters.
 

In the upper right, select the channel to be calibrated.  Enter at least two values of displacement that can be measured with the calibration setup (Figure 9).
 

Figure 9: After selecting the channel to calibrate at the top, enter more than two values to be measured in the “Expected value” column.
 

Then extend to the string pot to each of these known positions.  While holding the string pot at each individual length, press “Start”, wait a bit until the screen shows data, then press “Stop”. For example:

The screen should look something like Figure 10 below after the measurements are finished:
 

Figure 10: Once the calibration is finished, there should be a linear, upward sloping line in the lower left.  
 

After testing several discrete points, the software fits a line to the data.  This line is shown in the lower left.  If the line fits the data adequately, then press the “Accept” button in the lower right of the Calibration workbook.

6.  Simcenter Testlab Measurement

Click on the “Measure” workbook to start measuring (Figure 11).
 

Figure 11: Measure workbook.
 

The amount of time to view the live measurement can be set by pressing the “More…” button (Figure 12):
 

Figure 12: The monitoring duration can be set under the “More…” button.
 

In the “Monitoring” field the history size can be set in seconds.  For example, 20 seconds can be viewed rather than the default 2 seconds.

To see the 20 second time history, a display must be opened (Figure 13):
 

Figure 13: Press the “FrontBack” icon to open a display.  Right click on the Y-axis and change the format to “Real”.
 

In the upper left is a series of icons.  Press the first icon to open a “FrontBack” display.  Right click on the Y-axis and change the format to “Real”.

Now add the string pot data into the display. Choose “Data Explorer” in the upper right corner of the Measure worksheet. Drag the string pot trace the from “Online Data -> Fixed Sampling -> Monitor” folder (Figure 14) into the display.
 

Figure 14: Drag and drop the Stringpot time history data (blue icon) into the FrontBack display from the Data Explorer.
 

This will show the data live while the system is armed and during the acquisition.

Make sure that “Time Recording during Signature” is on under “Tools -> Add-ins”.  Press the Arm and Start button (the button with the arrow) to take the measurement (Figure 15).
 

Figure 15: Press the Arm and Start (with arrow) buttons to take data.
 

Each time a acquisition is performed, a new run is created.  For more information on using Simcenter Testlab see the knowledge article “Getting Started with Simcenter Testlab”.

Questions? Email peter.schaldenbrand@siemens.com

Related Durability and Data Acquisition Links

• Index of Testing Knowledge Articles
• Simcenter SCADAS
• AC versus DC Coupling - What's the difference?
• Single Ended vs Differential Inputs
• Cool Channel Setup tricks for Triaxial Accelerometers
• Data Acquisition: Anti-Aliasing Filters
• Simcenter Testlab Data Acquisition Tips
• History of Fatigue
• Stress and Strain
• Calculating Damage with Miner's Rule
• What is a SN-Curve?
• Strain Life Approach
• Neuber's Rule
• Rainflow Counting
• Mean Stress Corrections and Stress Ratios
• Difference between 'Range-Mean' and 'From-To' Counting
• Power Spectral Density
• Shock Response Spectrum (SRS)
• How to Calculate a Shock Response Spectrum with Testlab?
• Some Thoughts on Accelerated Durability Testing
• Goodman-Haigh Diagram for Infinite Life
• Measuring Strain Gauges in Simcenter Testlab
• Rosette Strain Gauges
• Simcenter Testlab Thermocouples
• Digital Image Correlation for Static Measurements
• Calculating Damage in Simcenter Tecware Process Builder
• Simcenter Testlab Neo: Strain Life Method
• Strain Gauges: Selecting an Excitation Voltage
• Simcenter Testlab SCADAS and Long Strain Cables
• Durability On-Demand Webinars