Position Measurement & Control - September 2002 (S050F)

Application Focus	Application Corner
In Action	News You Can Use
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APPLICATION FOCUS

Bellows Displacement Monitoring

Tight Spots and Cable-Type Displacement Sensors Are A Natural Fit

Cable-type displacement transducers are the sensor of choice for many bellows displacment monitoring applications. Their small size, flexible mounting, and short length relative to maximum displacement are all reasons these products are specified repeatedly for this measurement challenge.

Bellows position measurement and draw wire displacement transducers are often a natural fit.

Bellows position measurement can be made easy with cable-type displacement sensors.

What is a bellows? Generally, a bellows is a ribbed structure that can be displaced. An accordian is an example of an item that incorporates a bellows. More specfically, a bellows is an apparatus for controlling a pressurized environment. It is a chamber that is contracted and expanded by pumping to force a gas through a nozzle.

Bellows are used for many purposes due to their small size, reliable operation, and lightweight. In many applications it is important to monitor the displacement of the bellows to act as a control mechanism. Key criteria for any sensor to considered for a bellows measurement task is compact size, low weight, and easy installation.

Firstmark Controls cable-type displacement sensors are ideal for this type of application due to the use of a flexible cable that gets wound around a threaded drum. This design reduces the "footprint" of the sensor and the mass. The flexible cable also is easily mounted using threaded studs, swaged balls, pins, or other secure techniques.

Please contact us if you would like to learn more about simple and reliable bellows displacement measurement.

APPLICATION CORNER

The Application Corner answers your questions about using position transducers in specific applications. If you have an application question you would like answered, please contact us by phone, fax, e-mail, or Web form.

Repeatability Benefit of Threaded Drums

Q. What is the repeatability benefit of the threaded drum?

A. The repeatability benefit of the threaded drum is that it ensures the displacement cable wraps onto the drum in the same location each cycle. Non-threaded drums can allow the cable to wrap up on itself, may have gap variability between each revolution of the cable, and have cable placement inconsistency from cycle to cycle.

Quantitatively, let's take the example of the displacement cable wrapping over itself on a non-threaded drum for only one revolution and then not wrapping up on itself. Using a 2-inch (50.8 mm) diameter drum as an example and a 0.027" (0.6858 mm) diameter displacement cable, the threaded drum will provide an output of 6.326" (160.67 mm). If, however, a non-threaded drum is used and the cable wraps up on itself for only one revolution, the circuference on that revolution will be 6.368" (161.747 mm). This cable overlap adds a repeatability error of ± 0.335% for the non-threaded drum solution.

Note that all standard Firstmark Controls position transducers use threaded or grooved drums for enhanced repeatability.

Non-threaded drums can introduce repeatability errors on draw wire-type displacement sensors.
In the figure above, the overlapped cable (red section) will change the output and affect accuracy.

Displacement Cable Connection Via Magnet

Q. Do you have any sources for magnets for connecting the displacement cable to my application?

A. There are a number of sources for magnets with hooks, threaded studs, and other connection features. A good search engine will likely turn up a lot of possibilities. Sources you may want to consider include:

Quadrature Output

Q. What is quadrature output?

A. Quadrature output refers to the 90-electrical-degree phase relationship between the A and B channels of incremental encoder output. For more information on quadrature output terminology, visit US Digital's Glossary or Gurley's Glossary.

IN ACTION

Every month we see numerous fascinating applications of our position transducers. Here are a few recent ones:

Actuator Final Assembly Fixture Monitoring High-resolution Series D60 position transducers are used to monitor flight control surface actuator output during final assembly and test. The Series D60 was selected due to its high repeatability, high resolution, and easy installation.

Firstmark Controls D60 digital output dispacement sensor monitors actuator final assembly fixture.

Window Acceptance Testing Spadix Technologies, Inc. uses Series 160 products to test its automated robotic window manufacturing equipment.

Korea Aerospace Industries (KAI) T-50 On August 20, 2002, the Korea Aerospace Industries (KAI) T-50 advanced trainer took to the skies on its first flight. Future flights will be conducted with a suite of Firstmark Controls miniature position transducers. The T-50 advanced supersonic trainer will have the maneuverability, endurance, and modern systems to prepare future pilots to fly current and next-generation fighters such as the F-16, F-15, F-22 and the F-35 Joint Strike Fighter.

ISSN 1527-5108