Collecting Parachute Riser Force Data With Load Links

This guide describes how to collect parachute opening shock data and other information from load cells or load links inserted in parachute risers. Since there are few commercially available systems designed specifically for parachute opening shock testing, the parachute researcher must usually design their own system, in particular, when the scale of the load and forces is outside the limitations of a commercially available system.


About load cells and load links

Load cells measure expansion (pulling) and compression forces and are made using strain gauges, which are small elements that are attached to the load cell and change electrical resistance when flexed. Many types of load cells are available commercially and some may be used for parachute riser applications if their size and shape permits. Load cells are very linear unless they are very lightly loaded or until they are loaded near the point of destruction. (Note that load cells may have one maximum load rating that describes the maximum safe load before destruction, and another that describes the maximum load for accurate measurement purposes.)

Custom load cells (load links) can be constructed specifically for parachute risers by fabricating a bar of metal to which riser material can be connected and which has an area that has been narrowed for the application of strain gauges. Strain gauges on load links are usually connected in an arrangement called a full "Wheatstone" bridge circuit, which is 4 strain gauges, and are amplified to a voltage suitable to drive a data acquisition system.

If you decide to use a commercially available load cell, you may have the option of ordering it with the signal conditioning circuit built in. If the correct power required for the signal conditioning circuit is available, and if the voltage output signal from the circuit is within the range of the data acquisition system input, this may simplify the construction of the riser load measurement system.


Building a riser force data measurement system with Industrologic products

A load cell or load link with a strain gauge bridge, an Industrologic SGAMP-2 strain gauge amplifier, and an Industrologic PDAS-3 data logger can be used to build a riser force measurement system.

(If you choose to have Industrologic assemble these components and do the wiring from a commercial load cell to the SGAMP-2 and from the SGAMP-2 to the PDAS-3, you may wish to continue to the section Setting up and calibrating the system at this point.)

If you are constructing your own load links you will already be familiar with the arangement of the strain gauges and their wiring. If you are using commercially available load cells, the manufacturer will provide the wiring information. The 4 wires from the load link or cell are connected to the SGAMP-2 which provides the "excitation voltage" to the bridge as well as the signal inputs for the outputs of the bridge.

The SGAMP-2 is usually powered by the battery voltage available at the PDAS-3 signal connector, so the power, ground, and the output signal from the SGAMP-3 are connected to a connecter that mates with the PDAS-3 signal connecter. (If your system uses more than one load link or has other voltage signlas that will be recorded by the PDAS-3 your wiring may be different.)

Refer to the SGAMP-2 Reference Manual and the PDAS-3 Reference Manual for detailed wiring information.


Setting up and calibrating the system

Two methods of calibrating and using a riser force system are described below, a quick approximation that can be done with known suspended weights in a "lab" environment, and a more precise method based on actual data during a parachute opening. If the actual data is expensive to obtain you may want to do the quick approximation first in order to set the gain of the amplifier so that data from the the more precise method used later is at the optimum resolution.

You will need to refer to:

  • The SGAMP-2 Reference Manual for information about how to make adjustments to the amplifier for Offset and Gain described below.
  • The Help function in one of the the PDAS3 Download/Graphing programs found on the PDAS-3 Software page for information about how to display the PDAS-3 input signals with that program.
  • The PDAS-3 Reference Manual for information about how to configure the unit to record and download data after calibration is complete.
1. Connect the PDAS-3 to a computer running one of the the PDAS3 Download/Graphing programs and start a continuous display of the input signal voltages.

2. Adjust the Gain control on the SGAMP-2 to minimum.

1. Quick and approximate calibration and use

This method of determining riser force data will allow you to quickly set a voltage that represents a particular force (weight), and is quick and intuitive.
  1. Adjust the Offset control on the SGAMP-2 to an offset of zero volts when the load links are in a "no-load" condition.
  2. Apply a known weight to the load link and note the voltage. Divide the weight in pounds by the voltage to obtain the "pounds-per-volt".
  3. During data collection the force in pounds will be the pounds-per-volt multiplied by the voltage at a particular sample time.
  4. If the maximum voltage observed is low for the expected maximum force, the gain can be increased. (The offset voltage during a no-load condition will need to be re-adjusted each time the gain is changed.)
Note: If the temperature changes from the time this calibration is done to the time of the data collection, the offset voltage may change.

2. More precise method

This method of determining riser forces allows considerable long term offset voltage drift due to temperature, because the offset voltage and suspended weight voltage are both recorded just seconds before and after the load condition during data collection.

Calibration

  1. Adjust the Offset control on the SGAMP-2 to a small voltage above zero when the load links are in a "no-load" condition, for example, 0.1 to 0.2 volts.
This "offset voltage" will insure that the no-load voltage will always be positive and within the range of the PDAS-3 inputs, since it is expected that temperature will change the long term output of the strain gauges and/or amplifier. (This offset voltage will be subtracted from all voltages read during data collection to return the reference voltage to zero.)

Converting the voltage data to force data

To obtain the force in pounds at a particular sample time the voltages recorded during data collection must be viewed or graphed to make note of two specific voltages.
  1. Note the average offset voltage during a no-load condition.
  2. Note the average voltage during parachute descent.
pounds-per-volt  =  suspended weight  /  (  average suspended weight voltage  -  offset )

The force in pounds for each sample time can be calculated by this formula:
force  =  ( sample voltage  -  offset )  *  pounds-per-volt

Note: Observing the no-load voltage both before deployment and after landing will provide the most accurate results. Before deployment the load links may be compressed due to the physical arrangement of the payload.

Most Industrologic PDAS-3 customers choose to do these calculations in a spreadsheet, since the data from a PDAS-3 file is in easy to use space-delimited format. Using a spreadsheets allow the data to be graphed in a number of ways as well as allowing other data to be included in the spreadsheet.


Example Calculations

The following simplified graphs show the steps used to calculate the peak force using the method and formulas described above.

This example is for a load with a suspended weight of 100 pounds.

Step 1
Step 2
Step 3
Step 4




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