Providing the Ultimate Solutions in Precision Displacement Sensors
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Providing the Ultimate Solutions in Precision Displacement Sensors
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LVDTs, Vehicle Roll Testing, Pulleys and Cable Life |
Position Measurement & Control - Issue 44
PRODUCT FOCUSThe New Gold Standard? Raytheon Chooses Firstmark Controls Position Transducers Over LVDTs For Vital Tests It might not have the flash and appeal of a Pepsi versus Coke taste test, but for a Senior Principal Mechanical Engineer at Raytheon Corporation, this comparison was much more important. Instead of the fizz and flavor of soda, the engineer was looking for measurement results in a harsh vibration environment -- more specifically, the comparison of measurements by a Firstmark Controls position transducer with those of a linear variable differential transformer (LVDT) in that environment. Placed side-by-side on a shaker table at one of Raytheon's labs, the two units were subjected to Raytheon's system vibration test. The winner would move on to become an integral part of a missile system funtional test -- where accuracy and reliability is of utmost importance. This wasn't a test to be taken lightly.
Figure A1 - Without exact alignment and extensive mounting hardware, LVDT rods can bend or LVDT bushings can bind. LVDTs have had a reputation among some circles as the "gold standard" for displacement sensing. Some Raytheon personnel wanted to go with the well-known LVDTs. In addition, some personnel were skeptical about the use of cable-based position transducers. From what they had seen from other "string pot" manufacturers, cable-based position transducer reliability and accuracy was spotty. However, the engineer's research indicated Firstmark Controls designs resulted in robust products suitable for highly dynamic environments such as monitoring the gun actuators on missile defense installations. As the shaker table commenced its lateral movement vibration test, the results were substantially different between the two units. The LVDT experienced problems with its ball joints, providing imprecise results and binding. The LVDT's shaft proved to be weaker than expected and was subject to bending when large loads were placed on it. On the other hand, the Firstmark Controls position transducer repeatedly provided accurate results. Even the technicians, initially skeptical of the cable-based position transducers' capabilities, firmly believed that it was the better sensor for the application. Firstmark Controls's position transducer had performed so well that Larry was ready to try the unit on a test involving real missile systems at Raytheon's Virginia location. Don't be surprised if the position transducer provides the same accurate results in real life as they did on a severe vibration test. APPLICATION FOCUSVehicle Roll Testing Firstmark Controls Position Transducers Help Engineers Solve The Problem Of Repeatable Rollover Testing Safety should never be compromised when it comes to testing a car. Unfortunately, when it comes to testing roof strength during rollover, every test method in practice has had trouble creating repeatable results. Methods such as ramp rollover and dolly rollover failed to exactly duplicate the situation, making it difficult to compare test results and provide definitive conclusions. Xprts LLC sought to fill this void by designing a new dynamic rollover test called the Jordan Rollover System (JRS). The JRS features a test fixture with a simulated roof buck (complete with properties similar to production roofs), vertical, lateral, and roll-impact conditions to simulate the impact of severe loads on a vehicle roof, along with a roadway velocity simulator underneath. The JRS holds a replaceable car frame on the test fixture, then lifts, rotates, and releases to simulate the impact of rollover. Firstmark Controls position transducers are mounted on either side of the cab and used in conjunction with accelerometers for feedback and data acquisition.
Figure B1 - Jordan Rollover System with vehicle mounted and ready for testing.
Figure B2 - Test report graphic showing position transducer output versus time. The JRS used identical parameters for the three tests in terms of mechanical positioning. The tests used velocities of 13.5 mph, 13.1 mph, and 14.3 mph on the JRS' repeatable roof buck design. Data from the transducers illustrated the displacement in the car frame upon impact occurred roughly at the same time (approximately 1.5 seconds) for all three test speeds. The accuracy of Firstmark Controls units helps Xprts LLC engineers fix problems that have plagued modern car design for years. The consistency of the position transducer's pre-impact displacement readings, as well as the fixed mechanical parameters indicated that this test is repeatable and should prove to be useful for automakers at various levels of design. TECHNICAL FOCUSSmall-Diameter Pulleys May Not Be Groovy Pulley Groove Diameter Greatly Affect Draw Wire Transducer Life Position transducer displacement cables may not go over the river and through the woods, but they do often need to bend around corners and turn at angles. Many position transducer users use pulleys to act as guiding mechanisms as the displacement cable gets to its eventual destination. However, did you know that when improperly used, a pulley can significantly reduce the lifetime of the displacement cable? Several different aspects of a pulley can either maintain or greatly decrease a cable's life expectancy. Industry tests have shown that a pulley's design can increase or decrease the bending life of a cable. Poorly designed pulley grooves lower the bending life by about 10% below the expected value. Overall cable life can be diminished by a poorly designed groove simply by the fact that a groove may rub and wear on the cable as it runs through the pulley. On the other hand, increasing a pulley's diameter does wonders for cable bending life. Doubling the pulley's diameter can increase a cable's bending life tenfold and beyond. A pulley's groove radius is directly proportional to the strength efficiency of a cable. Cable life expectancy is generally calculated for cables running over a flat surface. A large enough groove radius provides support for the cable similar (but never totally equal to) a flat surface.
Figure C1 - A larger pulley diameter reduces displacement cable strain, reduces fatigue, and increase lifetime. To calculate the efficiency of cable strength, we need to determine the ratio (R) of the pulley diameter (Dp) to the cable diameter (Dc) with the following equation:
where:
For R equal to 40 or greater (e.g. 0.8-inch pulley diameter and 0.018-inch cable diameter), cable life reaches its maximum potential of about 95% of the estimated life relative to no pulley. The following values show cable strength efficiency as a function of ratio:
This data illustrates that cable strength is not linear based on the ratio. Your safest bet is to try to use a pulley-cable diameter ratio of at least 20. ISSN 1527-5108 • Document Number S050AD(051213DRAFT) |
