This article focuses on that helicopters often display a rotor torque reading on their instrument panels to tell pilots how close they are flying to transmission design limits. With no measure of torque, a limit based on engine power is necessarily conservative. Torque monitors can increase a helicopter’s time between overhauls. Mechanical resonance, it seems, developed when the two engines were operated at matched torques. The torque meter lets the pilot split the torque to the two engines by 2 percent, diminishing any resonance. Permitted speculation, the Siemens researchers observed applications for their instrument beyond the traditional realm of torque measurement. In automotive applications, for instance, the researchers envision their torque measurement system one day being used in car engines to provide data in real time to the engine controls.
Hellcopters often display a rotor torque reading on their instrument panels to tell pilots how close they are flying to transmission design limits. According to Paul Darden, chief of instrumentation at Bell Helicopter in Fort Worth, Texas, the Bell-Boeing V-22 Osprey Tiltrotor aircraft goes a step further. It brings torque readings from both rotors into a control loop, relieving a pilot's concern about overtaxing the gears.
"A helicopter that has no system for measuring rotor torque takes on a bit of a penalty in performance," Darden said." If you fly to a limit based on engine power, that limit has to take unknown losses into account," he said. Power to the tail rotor, for example. Although the main rotor shaft does not transmit this power, an engine power reading includes it, he explained.
With no measure of torque, a limit based on engine power is necessarily conservative, Darden said. "With direct rotor torque measuring, you get a considerable improvement in performance because the pilot can fly right up to the drive system limits," he said.
Closed loop measurement makes the V-22 easier to fly, Darden said, because the flight control system helps monitor and regulate torque. The Osprey incorporates triple redundant fly-by-wire controls, he said. Flight computers control all hydraulic actuators for the rotor system and flaps electronically, he explained. To regulate torque, the three flight control computers accept inputs from three separate, independent torque measuring channels coming off each rotor.
Bell-Boeing engineers conceptualized the torque meters that are used aboard the V-22. Engineers at GKN Aerospace of East Cowes, Isle of Wight, designed them. GKN Aerospace furnishes Bell with kits containing the electronics and the strain gauges on flexible printed circuits, Darden said. Bell installs the kits on the V-22s.
Each channel of the torque system uses conventional strain gauges mounted on the inside of the hollow rotor shaft. "This is done partly for protection from the environment," Darden said. Mounting the strain gauges correctly turns out to be critical. "The performance of the torque meter is only as good as the quality of the bond line you get," he said.
The V-22 uses folding blades that are de-iced electrically. To provide for these abilities, a fairly stout slip- ring assembly accompanies each rotor, Darden explained. The Bell-Boeing engineers, in working up a scheme by which to power the shaft-mounted electronics and retrieve strain gauge signals from the spinning rotors, elected to use the slip-ring system that would be in place on the Osprey anyway.
Other helicopters, without slip-ring systems entrenched, would probably rely on telemetry to both power and interrogate shaft electronics, Darden said.
Besides the use of slip rings, another unusual feature of the Osprey torque measuring system is its accuracy, Darden said. "The torque displayed to the pilot or sensed by the control system is within 1 percent of what it actually is," he said. "That is over a broad temperature range. It is digitally corrected. The strain gauges are compensated for temperature and other effects."
Accuracy is one of the factors bracing the argument for torque meters on helicopters, Darden explained. "The benefit is improving the utilization of the aircraft. If you have a system with significant error, say, 5 percent, you don't really get full benefit because you can only fly up to 95 percent of the aircraft's indicated maximum torque," he said. "With a 1 percent system, you can fly up to 99 percent and you don't have to back off." The system is by no means inexpensive, he added, but the increase in the power that is safely available justifies the cost.
According to Brian Sharpe, head of technical development at GKN Aerospace, enhanced performance is one of the benefits his company promotes in selling its torque meter. " Since a helicopter requires around 70 percent torque simply to remain airborne, depending upon weight, airspeed, attitude, etc., only 30 percent of the sustainable power developed by the engines is available for airborne n1aneuvers," he said.
The high accuracy of the Tiltrotor system comes partly by way of a calibration and validation software suite that GKN Aerospace wrote for Bell-Boeing. "The non-linearity and drift and offset are all taken out using this software," Sharpe said. The measurement system is based on a custom ASIC, or application specific integrated circuit, he said. Calibration data is stored in electrically erasable, programmable ROM.
Torque monitors can increase a helicopter's time between overhauls, said Sharpe. In one application, a twin-engine helicopter was able to lengthen the life of a splined shaft simply by operating one engine at a slightly different torque than the other, and alternating lead engines daily. Mechanical resonance, it seems, developed when the two engines were operated at matched torques. The torque meter lets the pilot split the torque to the two engines by 2 percent, diminishing any resonance, Sharpe said.
Industrial Measurements, a consulting engineering firm in Derby, England, specializes in torque measurement. Its managing director, Terry Allen, said that just about everything from the smallest aerospace or automotive application (with torque in the range of a tenth of a Newton-meter) to the largest steel rolling mill or ship tailshaft (where torque measurements of several million Newton-meters are common) have fallen under the gaze of its instruments at one time. His company has even instrumented an occasional wind turbine, he added.
Allen made a distinction between a strain gauge system, which measures strain through the change in resistance of a metal foil or wire that is bonded to a stressed element, and a phase displacement system, which measures the windup of a rotating shaft. Phase displacement systems rely on magnetic pickups to detect the phase difference between notched or toothed flanges spaced apart on a common shaft.
Though pricier than their strain gauge counterparts, phase displacement systems are capable of handling higher speeds and temperatures with greater accuracy, Allen said.
They are typically used on turbines spinning at 70,000 to 80,000 rpm, he said. A strain gauge system, on the other hand, could go to 25,000 rpm if it was powered and interrogated telemetrically, or typically to 8,000 rpm if connected by slip rings, he said. He added that special high-speed slip-ring systems were available.
Phase displacement systems do not mount any electronics to the spinning shaft. This makes them especially useful in high- temperature conditions that could damage sensitive components, Alien said. The absence of shaft electroncs also enhances the reliability of such systems. But space requirements and the cost of these systems—brought about partly by the precision a stationary detector needs to sense the tiniest phase difference between rotating teeth- makes them practical mainly for turbo-machinery and test stands, he said.
Of the strain gauge systems, those using slip rings are less expensive than those that rely on telemetry for power and data transfer because slip ring systems use fewer electronics, he explained. Shaft mounted electronics can typically survive to about 150°C, after which a phase displacement system begins making better sense.
Telemetry or rotary transformer systems are better suited to retrofits, Alien said. His company is developing a new retrofit system to simplify installation, he added.
Torque on the Water
Torque measurement in marine applications, according to GKN Aerospace's Sharpe, is used for fuel management, power balancing, and overload protection. It provides a way to compare power with ship's speed to detect hull fouling." In the marine industry, torque measuring tends to be a means of limiting power input so that nothing is damaged," he said. The Royal Canadian Navy has ordered systems for its lroquois class.
"Marine systems are a mature technology because they've had to go through extensive testing and be qualified to withstand as much as 200 gs of shock," Sharpe said. "Most are NATO codified. The time will come when they have to be redesigned because of component obsolescence."
GKN Aerospace has provided both naval and merchant vessels with torque monitoring systems. Customers include the Royal Navy, the Royal Malaysian Navy, and the Indian Navy, as well as Shell, BP, P&O Bulk, P&O Ferries, and Lykes Containers.
Marine torque monitoring often includes data storage to allow ship operators to compare torque and fuel- use data as a means of gauging power plant efficiency.
The company has also instrumented race cars. Its torque meters have been used on the clutch shafts of Formula 1 cars both during their development, for balancing cylinders, and before they race, for setup. But that can be tough duty, Sharpe said. "The clutch shaft can get above 200°C, which is unkind to the electronics," he explained.
So such systems remain "very much a hand-built thing to do," he said.
"The whole torque-measurement world is looking for a viable replacement, one that combines the same accuracy and reliability of strain gauges with systems that can be mass produced," Sharpe said.
Lasers and Mirrors
Two engineers and a physicist have been testing what may be just such a system at the Siemens research lab in Erlangen, Germany. This system dispenses with strain gauges of any kind, as well as shaft-mounted electronics and the need for slip rings or telemetry. Instead, it relies on lasers and mirrors to directly measure the phase displacement between two points on a rotating shaft.
According to Guenter Lins, who developed the system with Reinhard Maier and Gerd Griepentrog, the optics turned out to be quite simple. Using a rate rising program, the researchers simulated the conditions for which optical focus is small. "We needed very high resolution in time," Lins said. "We were able to do it with a simple, off-the-shelf silicon mirror, which was not very expensive," he said.
The mirrors are key to the system because they are shaped to define a precise line on the shaft. Both concave mirrors-measuring about 1 cm across-mount to discs on either end of the motor. One disc attaches to the motor shaft just outside the fan housing. The other disc fastens to the motor shaft near the coupling. The mirrors point axially along the motor shaft.
The farther apart the mirrors are spaced on the shaft, the more they wind up for a given amount of torque, Lins explained. But as far as shaft-mounted hardware is concerned, the mirrors are basically it. Unlike strain gauge systems, no electronics ride the shaft. The other components of the system, which includes lasers, photodiodes, beam splitters, and circuitry, reside off the shaft, where they are easy to get at.
To measure torque, the system casts laser light through the beam splitter (two prisms glued together along their hypotenuses) into the rotational path of the shaft-mounted mirror. As the mirror comes under the laser, the beam bounces back to the splitter, which reflects it 90 degrees toward a photodiode.
The photodiode, Lins explained, uses two photo elements separated by a 50- micrometer-wide dead band. As the mirror comes under the laser beam, at some instant an equal amount of light is reflected back to the two active elements on the photo-diode. At the moment the light shines equally on both sides, each photo element generates an electric current of identical strength, thereby marking the mirror's center.
As the system is busy marking the passage of one mirror, an identical setup of laser, beam splitter, and twin-element photodiode is awaiting the passage of the second mirror, Lins said. A counter starts on the signal from the first photo diode and stops on the signal of the second, measuring the time between the events. Under no-load conditions, the time delay represents the phase angle between mirrors, a measure of the tolerance applied in mounting them. When the shaft is loaded, the phase angle increases, providing a direct measure of shaft windup from which the torque is calculated.
The Siemens researchers tested the system on a 15-kW asynchronous motor, said Griepentrog. During the four-week test, the researchers used an eddycurrent brake to load the shaft. They compared measured torque against predetermined load moments. The results were in close agreement with calculated data, he said.
"Since the distortion amounts to only a few hundredths of a degree, the measuring instrument has to be very sensitive," Maier said. "Using a zero-point detector, the resolution can be determined within nanoseconds," he added-about 0.0003 degree.
Although the instrument remains under development, Maier thought commercialization would not be an extraordinary step. "The optical system in one of the photoelectric barriers in our sensor is similar to that of a CD player in operation and cost," he said. Siemens has been asked by a few machine tool builders and chemical makers about the possibility of their buying systems.
Darden at Bell Helicopters said he thought optical systems held promise for use aboard helicopters. But they would need to be rugged enough to withstand the shock of flight, he cautioned. The oil mist in a helicopter gearbox could blind the optics, too, he said. Flying in icing conditions could frost the mirrors as well.
Terry Alien said that an optical system could work well in laboratory settings. Whether it could work as well beneath an automobile or inside a steel mill, he wasn't so sure. Dirt and grease are often concerns with optical systems, he said.
Still, one advantage of the Siemens approach to optical torque measurement, Maier said, was the immunity of the photodiodes to fluctuations in light intensity. "The accuracy of the measurements isn't affected by dirt on the optical system or the aging of the laser diodes," he said.
Permitted speculation, the Siemens researchers saw applications for their instrument beyond the traditional realm of torque measurement. In automotive applications, for instance, the researchers envision their torque measurement system one day being used in car engines to provide data in real time to the engine controls. "The torque of the combustion engine could be determined in order to tune engine controls and optimize the ignition point," Griepentrog said. Right now, automakers measure engine torque on test stands only, he said.