The Miniature, Rugged Alternative to LVDTs and Linear Potentiometers
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The Miniature, Rugged Alternative to LVDTs and Linear Potentiometers
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Application Note for Aircraft/Aerospace |
IntroductionThis Application Note was developed to assist engineers and technicians with the process of selecting, installing, and using SpaceAge Control position transducers for aircraft and aerospace applications. It details why, where, and how SpaceAge Control miniature and subminiature position transducers are used in aircraft/aerospace environments. Known also as string pots, yo yo pots, cable displacement transducers, and draw wire transducers, SpaceAge Control position transducers were first developed in 1968 to support NASA flight test programs. Since that time, these products have been used in an extensive number of aviation and space projects ranging from general aviation to commercial aircraft and from Formula 1 aircraft racing teams to environmental systems for space habitats. Contact us for a representative list of aircraft/aerospace customers and programs. Product Technology SummarySpaceAge Control position transducers are cable-actuated displacement-sensing devices. The majority of aircraft/aerospace applications use the analog electrical output provided by precision potentiometers, but synchros, resolvers, and encoders are also available. Figure 1.0 shows how a position transducer works. Operationally, the products work by mounting in a fixed position and attaching the displacement cable to a moving object such as an aileron or landing gear component. As movement occurs, the cable extracts and retracts. An internal power spring maintains cable tension. The threaded cable drum rotates a precision encoder, potentiometer, servo, or synchro that produces an electrical output proportional to the cable travel. Depending on the mounting arrangement and signal conditioning, the electrical output can indicate any number of lengths, motions, angles, or rates.
Figure 1.0: How a Position Transducer Works
BenefitsBefore discussing where and how to use SpaceAge Control position transducers, let's look at what benefits these products bring to position measurement challenges and why you might want to consider the products for your application. SafetyProbably the foremost benefit of using SpaceAge Control position transducers in aerospace applications is the additional safety factor that exists with these products relative to other choices available. The predominant alternatives to these types of transducers are:
The inherent flexibility of a cable-actuated position transducer eliminates any binding or bending problems. Should a mounting or product malfunction occur, the displacement cable connection assembly can be designed to have a minimum breaking strength of less than 10 lbs (44.48N). While non-contact transducers do not present a restriction of movement hazard, they do present difficulties in ensuring data integrity due to the dirty environments aircraft and aerospace vehicles are exposed to. Leaking hydraulic and lubricating fluids, smoke, mist, and other visual and atmospheric elements reduce or eliminate the effectiveness of non-contact technologies. The result is data that cannot be relied upon and possible safety consequences if a control or feedback loop is involved. Small Size and Light WeightThe realities of physics mandate that size and weight be considered in nearly every engineering design. This is especially true in the aircraft and aerospace world. Because the significant aspect of the cable-actuated position transducer is a stainless steel cable, SpaceAge Control products have an inherent size and weight advantage compared to other choices. Rod-and-cylinder products require room to store the rod and cylinder when the product is in its fully retracted state. For example, a typical rod-and-cylinder solution requires 42 inches (1.07 m) of operating space to measure 20 inches (0.51 m). A SpaceAge Control product can measure the same distance using less than 22 inches (0.56 m). When fully retracted, the rod-and-cylinder product takes up over 20 inches (0.51 m) of linear space. The SpaceAge Control product takes up less than 2 inches (0.05 m). See Figure 2.0. Correspondingly, a rod-and-cylinder design will weigh significantly more than the SpaceAge Control product. A representative comparison for products with a 20-inch (0.51-m) range show rod-and-cylinder designs weighing 22 to 32 oz. (0.62 to 0.91 kg) while the SpaceAge Control product weighs around 3 oz (0.09 kg).
Figure 2.0: Comparison — Rod-and-Cylinder vs Cable Position Transducer
While a non-contact sensor may seem to provide slight size and weight advantages over a cable position transducer, often that advantage is outweighed once the non-contact sensor electronics and signal conditioning are taken into account. Mounting FlexibilityThe flexible cable combined with innovative mounting bases and mechanisms enable these position transducers to be mounted in accordance with your application requirements. There is no need to re-design your project or product. This flexibility is particularly useful when measuring flight control surfaces that simultaneously rotate and extend or when monitoring landing gear components that are moving in three dimensions. It is also useful when tight spaces provide no room for the transducer. Cable pulleys can be used to re-route the displacement cable through conduit and other channels, letting the transducer be placed remotely from the measurement location. Ease of InstallationQuick-connect displacement cable connectors and varied mounting arrangements make the products easy to install and remove. This saves time in setup as well as reduces time spent at the design station and in the machine shop creating specialized installation fixtures.AccuracyMost SpaceAge Control products use threaded cable drums and precision potentiometers (providing infinite resolution), high-resolution encoders, servos, or synchros. SpaceAge Control's threaded cable drum design ensures the cable is laid in the same place each cycle. Non-threaded drums allow the cable to wind over itself in a haphazard fashion and reduce repeatability. These SpaceAge Control design features combined with precision-machined aluminum parts and assembly by skilled technicians result in unparalleled repeatability and resolution. Accuracy for a 30-inch (0.76 m) Series 161 unit has been shown to be better than ±.025% of full-scale output. DurabilityUsing high-quality precision sensing elements and anodized machined aluminum with ball-bearing-mounted shafts, SpaceAge Control position transducers will often outlast the aircraft or aerospace vehicle itself. Many products have been environmentally qualified to DO-160D / ED-14D requirements. UsesDue to the sensitive nature of many aircraft/aerospace projects, we frequently never know how our products are used. Nevertheless, for over 40 years, we have come to recognize the most frequent uses of our products:
Representative ApplicationsBelow are a few recent applications of SpaceAge Control position transducers with notes on product usage. Contact us if you would like more details on these applications. NASA: High-Temperature Component Load Test and Bell Crank Angle
By locating the position transducers on both the flight control surfaces and cockpit controls, actuation delay, non-proportional control, and pilot-induced oscillations may be monitored. Military Fighter Speed BrakeA Series 173 product with a 3-inch (76.2-mm) range was used to measure the angle of an aircraft speed brake for a military fighter aircraft. The speed brake operated at relatively high velocity and required that the position transducer be provided with high cable tension. Business Jet Landing GearSeries 160 position transducers were outfitted on the landing gear of a large business jet to obtain displacement data during takeoff, landing, and gear stowage. Data was used to verify the landing gear performance. Space Station Environment ControlA modified Series 162 position transducer is used as an ammonia tank level sensor for a Space Station environmental control system. The position transducer is in a nitrogen atmosphere and attached to a collapsible metal bellows. A low-profile sensor was critical for this application and a custom cable-redirection pulley is used to change the cable direction 90°. Simulator Data AcquisitionOver twenty Series 150, 173, 160, 161, and 162 products were used in series of data acquisition flight sessions to acquire flight control and related information. The information was then used to develop FAA Class C and D flight simulators for pilot training and certification. UAV Actuator FeedbackA high-altitude UAV (unmanned air vehicle) was outfitted with two Series 173 units to provide position feed-back to wing-tip-mounted actuators. The actuators were used to torque the wing such that the entire outer wing section acted as a flight control surface. Series 150 products were also used to provide feedback on the V-tail flight control surfaces. Product Usage NotesWhich Series To UseIn general, the product size, displacement, and resolution will determine which SpaceAge Control position transducer is best suited to your appli-cation. An exception to this guideline is when increased cable tension is required for very-high-frequency-response applications. Series 150, 17X, and 180 products are not avail-able with ultra-high cable tension and therefore ultra-high cable tension requirements must be met with Series 16X products. Position Transducer MountsMounting of SpaceAge Control products is simple using a choice of mounting bases and designed-in mounting flexibility. Rotation ability of most bases allows precise alignment of the product after it has been mounted. Series 6, 160, 161, 162, 161H, and 161H
Series 150 through 180
Other Mounting Methods
Accessories
Displacement Cable Attachment
NOTE: If you need to cut uncoated displacement cable, you should anneal the cable with a flame at the cutting point to ensure the cable does not fray after cutting. Other Considerations
More InformationTo learn more about products and accessories not mentioned in this Application Note, visit the Literature Room or the data sheet for mounting bases and accessories ConclusionIf you have any questions not answered by this Application Note or would like to discuss your application, please contact us by phone, fax, or e-mail. Note that for flight test applications, SpaceAge Control also produces pitot-static probes, air data booms, trailing cones, and trailing bombs for measuring angle-of-attack, sideslip, static pressure, total pressure, and temperature. Let us know if you would like more information on these products. A representative list of aircraft and aerospace vehicles where SpaceAge Control position transducers have been used:
Additional Resources
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