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Application Note for Position Transducers for Flight and Pilot Controls
This Application Note is intended as an aid to engineers and technicians in selecting, installing and using position transducers for aircraft flight and pilot controls. Such controls include those found on simulators, FADECs (full authority digital engine controls), and autopilots.
SpaceAge Control first produced position transducers in 1968 when the company developed draw-wire transducers in support of NASA flight testing activities. Since then, an expanding customer base has used these position transducers for an increasing variety of applications.
Initial applications for the transducers were confined largely to flight test and ground test instrumentation, for example, sensors for data acquisition; and on-board sensors for aircraft and spacecraft indicators. They are now flying as flight control input sensors for flight data recorders and health and usage management systems (HUMS), and as controls on environmental control systems and accumulators (bellows).
SpaceAge Control position transducers have been used in space, on supersonic test aircraft, helicopters, UAVs and even on the replica of the Wright Flyer. In aerospace ground applications, they have been used in flight simulators and as sensors in ground test instrumentation. In addition to the aerospace industry, the broad user base also includes robotics, industrial control, crash dummy and automotive manufacturers; research and development labs; and autoracing teams.
This Application Note summarizes the uses and suitability of position transducers for flight and pilot controls, and will address the following topics and issues:
The majority of SpaceAge Control's position transducers are cable-actuated displacement measurement sensors, which comprise two subassemblies: a mechanical cable drum assembly, and an electromechanical sensor, which is fitted to one end of the cable drum shaft.
Use Fig 1.0 from "Application Note for Ground Vehicles/Transportation", henceforth referred to as 5004A(B), which is the control number located on the last page of that document.
The cable assembly consists of a multi-strand stainless steel wire cable that winds onto a spring-loaded threaded drum. When the cable is drawn from the threaded drum, the drum shaft rotates the attached sensor, which in turn creates an output signal that is proportional to the cable draw distance.
The most common installation method is to mount the position transducer on a fixed structure and to attach the free cable end to a component, the movement of which is to be measured. As the component moves, the cable extends or retracts to follow movement, with tension being maintained by the position transducer's internal spring.
This section provides a brief summary of the physical characteristics and measuring ranges of SpaceAge Control's position transducers. Complete descriptions can be found at www.spaceagecontrol.com, where data sheets for each product are available
Size, Mass and Measuring Range
SpaceAge Control offers an extensive range of product configurations. Table 3.3-1 compares two products from that range, ones that are frequently used in the aerospace industry and which represent both ends of size and mass range - the Series 150 and the Series 160. In turn, the Series 16x units have numerous optional configurations for a variety of measurement lengths and cable tensions, as detailed on the product data sheet.
Table 3.1-1 Position Transducer Size and Mass Comparison
Sensor Output Choices
Our off-the-shelf products feature either a potentiometer or an optical encoder. Also offered are position transducers that incorporate the following types of rotary sensors:
Signal Interface Options
Depending on the product, SpaceAge Control offers a range in signal interface options for its off-the-shelf configurations, as follows:
The Series R rotary transducer provides high life-cycle and low cost rotational sensing for harsh environments. The unit is environmentally sealed (IP67) and features FlexSignal™ integrated signal conditioning that allows a variety of electrical outputs including but not limited to 0-5 VDC, 0-10 VDC, ±5 VDC, ±10 VDC and 4-20mA. Additional detail is available at www.spaceagecontrol.com.
Representative Applications and Uses
SpaceAge Control's aerospace customers have used our position transducers for flight and ground tests and, more recently, in permanent installations as input sensors for flight data recorders, health and usage monitoring systems or cockpit displays. Applications include the measurement of a variety of aircraft components such as:
Figure 4-1 Position Transducer Uses
Each application will have its unique installation constraints, largely defined by the availability of space and electrical power. In some situations, the position transducer can be used to measure the movement of a bell crank attached the control surface itself as illustrated in Figure 5-1. If this isn't possible, the engineer can find a location at which a position transducer can measure the movement of control cable, push rod, hydraulic actuator or rotating shaft that moves the control surface, as illustrated in Figures 5-2, 5-3, 5-4 and 5-5 respectively.
Use 5004A(B) Fig 3.1
The second photo at this link shows PTs hooked to control cables. Suggest a much simplified illustration showing one PT hooked to a control cable and a 2-headed arrow to represent bi-directional movement. http://opl.ecn.uiowa.edu/gallery/view_album.php?set_albumName=CARP&page=2
Need a new illustration. Suggest using the photo in pmc0602.htm titled "Series 173 installed to monitor aileron movement" as a guide and create a new illustration along the same lines as requested for Fig 5-2.
Need a new illustration. I ran across an illustration of a PT hooked up to a hydraulic actuator on the web site. It was dynamic and was included with two or three other such illustrations that together composed a single illustration. I can't find it again to provide as an example, but I think this should be fairly straight forward.
Suggest a modification of AGARD Vol 8 drawings 7.3-11 and 7.3-14 to show a PT hooked up to the partial pulley gizmo that is attached to the tube in 7.3-11 and to the lever-type attachment attached to the tube in 7.3-14. This would show two options, with the second showing a small off-axis movement.
In many cases, however, physical constraints preclude these relatively straightforward installations and require the instrumentation designer to mount the transducer remotely from the measurement location. Pulleys or Bowden cables allow the measuring cable, or an accessory to the measuring cable, to be routed in two or three axes around obstacles, as illustrated in Figure 5-6 and 5-7, respectively.
(My typo incorrectly shows this as a second Fig -1) Use the s054b.htm Fig 3 that shows the use of pulleys to get around corners. One suggestion--the addition of a third pulley might be useful in showing routing of the cable in a second plane perpendicular to the one in the current illustration.
(My typo incorrectly shows this as a second Fig -2) Use AGARD Vol 8 Fig 7.3-15 as a guide, and show a PT attached to the end of the Bowden cable.
This installation flexibility makes displacement cable transducers particularly adaptable for different measurement uses within an instrumentation design. Secondarily, this adaptability may well reduce the number of unique signal interfaces that the engineer must accommodate, and reduce the number of unique part numbers that must be ordered and stocked.
As with the rest of SpaceAge Control products, the design of our mounts has been driven by the desire to satisfy customer requirements. The simplest of mounts orient the position transducer either vertically or horizontally and permit up to 360° in one axis. Representative configurations are shown in Figures 5.1-1 and 5.1-2.
through 5.1-3 Use the illustrations (not to include the dimensioned drawings) shown in s021r.htm for part numbers 160040-1, 160015-1 and 160030, respectively.
Figure 5.1-2 Horizontal Mount
More versatility is available with our universal mounts, which permit up to 360° in two axes, as illustrated in Figure 5.1-3.
Figure 5.1-3 Universal Mount
In addition to the widely used screw-down mounts, SpaceAge Control offers a mount that incorporates an adjustable steel strap, as illustrated in Figure 5.1-4.
Figure 5.1-3 Strap-down Mount
Cable Termination Hardware
Figure 5.2-1 depicts some of the cable terminations that are available and which can accommodate a wide variety of connection requirements.
(Another cut and paste typo here. Title incorrectly refers to Bowden cable, when it should be about terminations) Use the illustration from s021r.htm titled "Displacement Cable Terminations".
Mounting Accessories for Off-axis Measurements
One of the limiting characteristics of early configurations of cable-actuated transducers was that the transducer body had to be carefully aligned with the cable extension and retraction axis. If the movement of measured object was not along this axis, it was possible, in some instances, for the cable to rub on the cable exit. SpaceAge Control developed three different cable exit accessories that eliminate interference between the cable and cable exit.
In many of these difficult measuring situations, the position transducer can be mounted so that measurement axis is confined to small angle in a single plane. An idler, illustrated in Figure 5.3-1, is a well-suited accessory.
Use the illustration of the idler arm that is on page 6 and 7 of s004a.htm.
Other situations are more complex and involve movement in multiple planes, requiring the cable to extend and retract along various axes in a conical area. If the angle is small (lesss than 20?), a cable guide shown in Figure 5-3-2 prevents the cable from departing the threaded cable drum and proves to be an effective solution.
Use the illustration of the cable guide that is on page 7 of s004a.htm.
For larger off-axis measurement angles, SpaceAge Control has introduced the RoundAbout™ cable exit, an accessory that won one of Design News magazine's 2005 Product of the Year award. It permits measurements to any point around the position transducer's location, except for the conical area that is blocked by the transducer body. The RoundAbout™ cable exit is available as an attachment that is fitted on the position transducer, or fitted with an integral mounting base for locations between the position transducer and the moving object. These configurations are shown in Figures 5-3-3 and 5.3-4
Use RoundAbout illustration shown in Fig A1 from s050y.htm
Use the illustration (not to include the dimensioned drawing) shown in s021r.htm for part number 301225
Accuracy and Calibration Considerations
A comprehensive discussion of accuracy and calibration is beyond the scope of this document, but we have addressed the subject in a separate paper titled "Application Note for Draw Wire Transducer Accuracy", which can be found on our web site at this link: www.spaceagecontrol.com/s054j.htm.
Some brief comments are appropriate here, however, to address some likely concerns that the reader may have regarding repeatability and effects due to temperature, vibration and cable sag.
SpaceAge Control's AccuTrak™ feature is based on the use a grooved, threaded drum to assure the high repeatability that is characteristic of our sensor outputs. During retraction, the grooves guide any segment of cable to same location on the drum that it occupied before extension. This prevents cable spread or overlap that cause variations from the desired drum rotation/cable extension relationship.
General repeatability for our analog transducers is ± 0.025% of full-scale extension and ± 1 count for transducers with encoders.
The SpaceAge Control web site provides a formula for calculating the error due to thermal expansion of the wire cable. Generally, mechanical thermal error can be disregarded due to its minor effect. The effect is particularly immaterial if the item being monitored has the same coefficient of thermal expansion as the draw wire transducer.
SpaceAge Control transducers are used as voltage dividers (using voltage variance) instead of rheostats (using resistance variance). Any changes in resistance occur uniformly throughout the resistive material, thereby preventing any voltage change due to temperature effects and protecting the transducer from electrical error caused by temperature change.
A cable draw transducer is uniquely capable of measurements in some high temperature conditions. NASA used one of our position transducers to measure movement of an object in such an environment. Engineers mounted the transducer outside of the high temperature compartment and routed the displacement cable through a hole in the compartment wall, as illustrated in Figure 6.2-1.
Use 5004A(B) Fig 3.0.
The spring-loaded cable drum maintains tension on the displacement cable when it is extended. SpaceAge Control offers a number of tension options to adapt the transducer configuration to various vibration environments.
One of our aerospace customers recently conducted tests to compare a SpaceAge Control position transducer and an LVDT in a missile vibration environment. The position transducer proved to be superior, because vibration caused binding between the LVDT rod and cylinder that hampered free movement.
(Tom, should there be something here about frequency response? If so, I'll need a roughly worded input or some bullet points from you.)
This effect, which is generally insignificant, is a function of factors such as the length of displacement cable extension, cable weight, cable tension and acceleration (g force) perpendicular to the cable. The SpaceAge Control web site features a calculator for determining this effect, which is expressed in the magnitude of cable sag and the effect, in percent, on the measurement.
Comparison with Rotary, Rod-in-Cylinder, Non-contact and Magnet/Rod Sensors
SpaceAge Control position transducers compete with a number of other displacement measurement devices such as LVDTs, RVDTs and non-contact sensors (Hall effect, laser and ultrasonic.) Figure 7-1 graphically displays a comparison of cost, accuracy and measurement range of these competing sensors.
The National Instruments paper on "How to choose among LVDT, RVDT, Potentiometer...." located at http://zone.ni.com/devzone/conceptd.nsf/webmain/5E45F01A9456E5C386256A9B0060230F contains an illustration that is attributed to SpaceAge Control. I don't know if you provided the raw data for the illustration or the illustration itself.
Not displayed in that figure are other factors that the instrumentation engineer must consider. For example, the high accuracy expected of an LVDT comes with some drawbacks. The footprint of cable displacement transducer is small compared to the space occupied by an LVDT, even when the rod is fully retracted within the LVDT cylinder. Figure 7-2 illustrates this relationship.
Use 5004A(B) Fig 2.0
Some non-contact sensors offer lifetime levels that a cable displacement transducer cannot achieve, but this lifetime benefit may be significantly and adversely affected by vibration and temperature changes or extremes.
(Tom, I need some help here.)
The preceding text describes many of the product features that have been introduced in compliance of SpaceAge Control design factors, which emphasize a cost-conscious introduction of improvements that increase the versatility, reliability, durability, accuracy and producibility of our products. We emphasize our customers' needs when we make improvements, but do not compromise the quality and precision that have gained our customers' respect.
The following features also contribute to the overall quality of our products:
SpaceAge Control uses this patented method to attach the sensor to the cable drum. It eliminates the use of torsion springs, clutches, gears and other devices that cause backlash and its consequent measurement error.
This feature (Tom, I couldn’t find a lot about this. Do you have any info?)
As noted in the discussion of sensor options, SpaceAge Control's design team will modify our existing products to accommodate unique customer requirements. Most frequently, this involves the installation of an optional sensor, such as a synchro or resolver, but we have also modified mounts to meet specific physical interface requirements, replaced the wire cable with another material, such as nylon filament, or changed rewind tension, to name a few such adaptations. We are prepared to satisfy our customers' needs, and view adaptations as an opportunity to expand the versatility of our product line.
SpaceAge Control has completed DO-160D/ED-14D testing on its position transducers. The qualification levels verified in those tests are summarized in Table 10-1.
Figure 10-1 Environmental Testing Summary
(Tom, need info)
(Tom, need info)