Contents 1. What is a String Pot? 2. Simcenter SCADAS VB8-II and VB8-III Cards 3. Cable Wiring 4. Simcenter Testlab Classic 4.1 Channel Setup 4.2 Calibration 4.3 Measurement 5. Simcenter Testlab Neo 6. Displacement versus Acceleration
1. What is a String Pot?
A string pot is a transducer used to measure displacement. The word “string pot” is a short form of string potentiometer. An example of string pot device is shown in Figure 1.
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).
String pots come in a variety of models. Most require three wires to be connected to work:
Supply voltage: Voltage required to power the string pot.
Ground: Ground or reference voltage.
Signal: The signal output voltage of the string pot proportional to displacement.
A Simcenter SCADAS Mobile/Recorder has seven connections, three of which are used to measure a string pot as shown in Figure 3:
Figure 3: Typical connections between a string potentiometer (generic model) and Simcenter SCADAS
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:
String pot voltage supply -> Simcenter SCADAS Pin 1 (positive voltage supply) corresponding to white wire
String pot ground -> Simcenter SCADAS Pin 6 (negative voltage supply) corresponding to brown wire
String pot signal -> Simcenter SCADAS Pin 3 (positive signal wire) corresponding to grey wire
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.
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.
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.
The sensitivity and offset value for the string pot can be calculated in the Simcenter Testlab software via the “Calibration” worksheet (Figure 6).
Figure 6: Calibration Workbook.
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:
With no extension of the string pot, press “Start” and “Stop” for the zero value.
Move the string pot to 5 centimeters and hold. Press “Start” and “Stop”.
Move the string pot to 10 centimeters and hold. Press “Start” and “Stop”.
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.
4.3 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”.
5. Simcenter Testlab Neo
The steps to setup, calibrate, and measure a string pot in Simcenter Testlab Neo Time Data Acquisition are similar to those for Simcenter Testlab Classic Signature as described in the previous section.
While the steps are similar, the user interface is different.
Go to the "Channels" tab of the "Instrumentation" task of Simcenter Testlab Neo Time Data Acquisition. In the "Home" ribbon, select the Universal view as shown in Figure 16.
Figure 16: While in the "Channels" task, choose "Universal" view In the Home ribbon.
Then in the setup grid for the channels, set the "Conditioning" field to "Potentiometer" for the channel(s) with a string pot (Figure 17):
Figure 17: Choose "Potentiometer" for the Conditioning field of the channel with a string pot.
This will automatically set the units to displacement. In the "Supply" field, enter the voltage supply for the string pot sensor. It must be a value other than zero for the sensor to work.
To calibrate the string pot with various displacement levels, go to the "Multi-point Calibration" tab in the "Calibration" task located at the bottom of the screen (Figure 18):
Figure 18: Move to the "Multi-point Calibration" tab in the "Calibration" to calibrate the string pot.
Select the string pot channel to be calibrated. Then enter the different displacement lengths to be used for calibration in the "Calibration Table" as shown in Figure 19.
Figure 19: On the left side, select the string pot channel to be calibrated. Enter the different lengths to be used for calibration on the right.
Measure the different lengths using the state control button in the lower left of the screen. A line appears in the lower display to show the fit to the different calibration lengths used.
If in doubt, click on the "Walkthrough" button in the lower right to be guided through the calibration process.
When the calibration process is finished, go to the Measure task to perform a measurement.
String pots measure displacement directly. Sometimes users attempt to calculate displacement by double integrating acceleration data. This can lead to problems as shown in Figure 20:
Figure 20: Displacement (red, left limits) derived from acceleration (green, right limits) has unrealistic values.
This is caused by numerical integration issues, especially at low frequencies close to zero Hertz. Some accelerometers, like ICP style, are not designed to measure below a few Hertz, which causes the problem. Often, using high pass filters (with a low frequency cutoff) or detrend operations can help reduce these numerical errors.
However, if in doubt, using a device like a string pot, which is designed to measure displacement, will yield reliable results.
