Wireless health and usage monitoring systems (HUMS) are flying in four U.S. military rotorcraft on an experimental basis. If the system proves successful, it could one day be standard equipment on all military and civilian helicopters for a price that is an estimated 90 percent less than current wired systems, which cost up to $200,000 each.Past success with wired systems used to measure structural loading on Navy F-18 fighters suggest that a wireless HUMS could double an aircraft’s useful life and provide significant ongoing maintenance and operational cost savings. It also could be used as a training tool for pilots and to report an aircraft’s condition, including exceedances, in real time to an operations center.
The premise is simple, said Steve Arms, president of MicroStrain, the Williston, Vt. company that has developed the technology. “You can extend the life of rotating parts if you track them more accurately.”
Arms said the current system of using slip rings and wired strain gages on a few test helicopters, generally during certification of airframes and follow-on components, is a poor predictor of part health and life on individual helicopters. The process is called regime recognition. “You have the pilot do different maneuvers and fly different regimes and then record the worst-case scenarios to estimate usage,” said Arms, who called the process “a relatively coarse way of lifing parts. The same pilot can fly a similar regime on the same aircraft and get vastly different loads on rotating structures.”
The system can be used to measure strain and vibration on rotating parts and vibration on nonrotating parts.
Arms said a wireless system is the best way to track, and thereby extend the service life, of rotating parts on an entire fleet. “It has to be wireless. The parts are spinning and no one is going to fly sliprings on an entire fleet and string wires on everything.”
Simply outfitting test helicopters with slip rings, wires and strain gages can take six to nine months per helicopter, Arms said. The time required to install a MicroStrain wireless HUMS system? One day.
The system is installed on three Sikorskys: a Navy MH-60S at Air Station Patuxent River, Maryland; and two Army H-60s, one at Fort Eustis, Va., and the other at Moffett Field in Mountain View, Calif. It also is flying on a Marine Bell-Boeing V-22 Osprey tiltrotor based at Cherry Point, N.C.
The guts of the system are small and lightweight. Strain sensors are connected to a tiny wireless node about half the size of a credit card that is powered by an energy harvester, in this case, an Infinite Power Solutions Thinergy rechargeable micro energy cell that is tuned to the helicopter’s predominant frequencies. The cell generates power by tapping into the helicopter’s natural vibration. “When it undergoes cyclic loading, it generates energy,” Arms explained. A tuned energy harvester can generate approximately 3 milliwatts. “You not only get rids of wires, you get rid of battery maintenance,” he added.
Sensor data is then transmitted to a “WSDA box,” or wireless sensor data aggregator, a hand-sized device that is basically a single board computer running on the Linux operating system. The WSDA includes Ethernet, USB and Arinc 1553 connectivity as well as wireless IEEE802.15.4 in the 2.4 GHz radio frequency band. Commercial WSDAs use a GSM cell phone that pushes data to the cloud where it is hosted on a secure server.
The WSDA also features an embedded inertial sensing suite that provides vehicle pitch, roll and yaw data as well as triaxial accelerations and triaxial angular rates. It uses temperature-compensated, commercial micro-electromechanical (MEMS) inertial and magnetic sensors that are similar to those found on the stability systems of modern automobiles.
According to Arms, the measured parameters include pitch link loads, lead-lag damper loads, mast bending moments, altitude, airspeed, aircraft weight, power on the mast, rpm and types of torque. “We can get a really good idea of what kinds of forces the [rotor] blades are experiencing–flap, edgewise and torsional bending moments–without putting strain gages on the blades,” he said.
The system would eventually allow users to track aircraft parameters and performance from anywhere in the world in real time, he added. Exceedance levels can be programmed into the system, with automatic e-mails or text messages dispatched when levels are crossed. Pilots will be able to see quantitatively how their inputs influence aircraft loads and longevity.
Arms envisions customers being able to build their own smartphone apps for custom parameters and he aims to fly the system on more aircraft by year’s end. “There is so much potential,” he said.