The suspension system of parachutes is typically made from ropes (referred to as cordage). Measuring loads in the suspension system cordage has thus far proven very challenging because of the dynamic nature of the parachute. The suspension lines must be deployed along with the parachute, and experience rapid acceleration and dynamic motion as the parachute inflates. The addition of bulky load cells to the suspension lines would change the dynamics of the system and corrupt the data.
Measuring parachute suspension line loads is important for quantifying design loads that are used to size the structural members. Typically, these loads are determined analytically, and there is no data to support the analytical prediction. Measuring the actual loads would increase confidence in the design of the parachute, which is important considering it is a single-point failure in recovery and landing systems.
A small device was developed that is inserted inside a hollow-braided cord that gets squeezed as the rope is loaded, and can measure the tension in the rope by measuring the squeezing forces. This device can be inserted into the existing rope in the system without any alterations to the system, and with no significant reduction in strength. By making the device small and internal, the problem of massive and cumbersome load cells is nearly eliminated, and only the challenge of wiring remains. The proof-of-concept device weighs 2.5 grams and uses strain gauges to measure rope loads up to 10,000 pounds (≈44.5 kN).
The device is a pill-shaped object with a slot in the middle. It can be made from any metallic material, but aluminum alloy is preferred because of the low elastic modulus and density. Strain gauges are installed on the internal surfaces of the slot. The slot effectively creates small beams in the device. When tension is applied to the rope, and the device is squeezed, the beams bend and the bending strain is measured by the strain gauges. The output of the strain gauges is related to the tension of the rope by a calibration factor that is determined from laboratory testing of the device.
The wires of the strain gauge are routed out of the end of the device, down the inside of the rope, and can pass outside of the rope at any point through the braid (see figure). The device also has features to tie into the rope to prevent the device from shifting or migrating, which is especially important while the rope is unloaded.
By appropriately sizing the diameter of the device relative to the diameter of the rope, the strength of the rope isn’t reduced significantly by placing an object inside. Care is taken in the design of the device’s shape to minimize stress concentrations in the rope. Smooth and continuous shapes are preferred. The design also includes low-friction coating on the surface of the device to minimize the effects of friction between the device and the cord fibers.