Position Measurement & Control - Issue 43
PRODUCT FOCUS
Simulators and Position Feedback
SpaceAge Control Position Transducers Are An Integral Part Of Making Motion Simulators Work
It could be the speed and intensity of driving a race car at 200 mph around the bend. It could be the sensation of flying a jet through mountain canyons. It can be the twists, turns, and dips of a roller coaster, or it could be the out-of-this-world sensation of traveling on a space ship. Whether you're riding Star Tours at Disneyland or helping NASA pilots learn the nuances of flying the Space Shuttle, motion simulators can be found in the worlds of training, education, and entertainment. And, where you find motion simulators, there's a good chance you'll find SpaceAge Control position transducers integrated into the technology that brings you virtual thrills and spills.
Figure A1 - The Atlas Lite motion simulator technology demonstrator uses Series L and 6 position transducers.
Motion simulators are a complex intermingling of software, hardware, and controls. Position transducers act as a bridge between all three components, translating information about the hardware's location into data that can be interpreted by the software and fed back to the user via control systems. One group currently using SpaceAge Control position transducers in this application is Carleton University's Mechanical/Aerospace Engineering department in Ottawa, Canada. Carleton engineers are working on motion simulators as an ongoing project and are putting SpaceAge Control Series 6 and L position transducers to good use as an integral part of the feedback control system.
For example, the Carleton students use position transducers with a maximum range of 60 inches (1524 mm) and 5 kΩ resistance. The potentiometer is mounted on an L bracket underneath the simulator's frame. To measure distance, the transducer's displacement cable is attached to the right side of the simulator's motion platform. The potentiometer feeds data to the simulator's control system through a connection via the motion control board's fourth axis. As the motion platform moves, the displacement cable retracts or extracts and provides corresponding voltage change that is converted to displacement data by the control board. The engineers chose to replace their original mouse sensor with SpaceAge Control position transducers because of their absolute readings, simple setup, and "set it and forget it" operation.
For more information on motion simulator technology, Carleton University's project page is filled with information on the subject.
APPLICATION FOCUS
Crates And Barrels (And Position Transducers)
Cal Poly Engineers Use SpaceAge Control Transducers To Help Wine Industry
When an earthquake strikes facilities in wine country, the winery managers' first thoughts are about getting to a safe place. However, their next thoughts are, "What's going to happen to my wine barrels?" For California's wine industry, this is a serious matter - nearly 90% of California's $45 billion wine industry is dangerously close to active earthquake faults. To make matters worse, all of those wine barrels are placed on steel racks with little regard for potential earthquake damage. That means that when gallons of wine are violently rocking in oak barrels, there are few preventative measures to compensate for the dynamic forces at play. Enter Cal Poly Civil Engineering - with a little help from SpaceAge Control.
Figure B1 - A four-high, two-barrel wine barrel stack ready for testing
Cal Poly's Civil Engineering team knew they couldn't address the problem without fully understanding it. To do this, they simulated an earthquake for barrels filled with water (since no one wanted to waste good wine) using a giant steel table with an area of 225 square feet at The Parsons Earthquake and Geotechnical Engineering Laboratory. Steel racks holding the barrels were placed on the shake table, tied down with 3/8-inch (9.525 mm) braided steel wire for safety. The following SpaceAge Control products gathered data:
Four position transducers were mounted from the table surface to the barrel's center of mass to aid in determining rocking-sliding behavior.
Lateral transducers were placed on the equipment frame to measure the relative uplift of the barrel on each end.
Two transducers were mounted on the shake table to monitor relative sliding and rocking behavior.
In addition, each tested barrel carried an accelerometer at its center of mass to capture acceleration data.
The Cal Poly team put the shake table to the test and began gathering data. As the results soon showed, the chaotic activity caused by the fluid within the barrel meant that typical mathematical models could not be applied. The experiments did prove, however, that ground motion pulses with lower frequencies (2 Hz versus 4, 6, and 8 Hz) caused the barrels to slide significantly more. In fact, ground motion was nearly five times greater at low frequencies. The table, rack, and barrels all had similar time periods of slide movement. However, the barrel's displacement was far greater than the table or rack, supporting the theory that barrel slide is the cause of most earthquake-related damage. In addition, system damping from 1% to 7% failed to have any effect on the table/rack/barrels system. Testing with various coefficients of friction (with increases ranging from 5% to 25%) indicated an increased displacement with a decreased transmission of force between the barrel and the racks (slide). Increasing friction accomplished what damping could not do.
Figure B2 - Laboratory instrumentation including Series 6 position transducers used as rack, rocking, and laterial displacement sensors.
While the experiment proved fruitful in many ways, there is further research to be done. For example, this particular set of experiments failed to provide the amplitude of any rocking motion (though a rocking period was defined). The Cal Poly team proposed recommendations for further tests to ensure the safety of the wine industry's investments - and you can be sure that SpaceAge Control position transducers will be there to help engineers gather data in the future.
TECHNICAL FOCUS
Why Do Potentiometric Position Transducers Get A Bad Rap?
Negative Comments About Potentiometers Are Often Just Myth
Experienced sensor users may have heard whispers and myths regarding the limited capabilities of potentiometers. Is there any truth to these accusations? Like any sensor, potentiometers are not perfect. There are some obvious faults with potentiometers. As with any device using a contacting mechanism, there can be very modest mechanical noise due to the wiper moving across the potentiometric element. This mechanical contact also wears the potentiometric element's surface over time resulting in a limited life cycle. However, these are often insignificant effects for a technology that can be highly effective when used properly.
Figure C1 - The potentiometer: simple, elegant, and picked on.
Sometimes problems occur when specific potentiometer technology is used in the wrong application. For example, control potentiometers that use wirewound or cermet elements are designed for circuit feedback and low-dynamic performance. When this type of potentiometer technology is used for a dynamic position measurement or any other type of instrumentation it's not designed for, noise, short lifetimes, high non-linearity, and modest resolution can be the result.
Negative comments about potentiometers are often based on experience from years ago when long lifetime, infinite resolution, and low noise potentiometer technology was not available. Similarly, the context of a potentiometer's use affects its abilities. When subjected to vibration, temperature, or other elemental factors outside of design specifications, potentiometers are adversely affected. Of course, all sensors behave similarly when placed in conditions outside of their operating range. So why are potentiometers singled out?
Jealousy?
Ignorance?
Urban myth?
It's not a coincidence that negative comments about potentiometers originate from manufacturers of optical encoders, LVDTs, magnetoresistive encoders, synchros, and other competing products. Our advice on the subject: check out the specifications carefully. Evaluate via lab and/or field tests. Educate yourself on the facts to find out if potentiometers are right for your application. If you have questions about how potentiometers can be used to their maximum potential, contact us for more information. Or, request an evaluation position transducer to see if you can become a mythbuster!
ISSN 1527-5108 • Document Number S050AC(051116)
