Position Measurement & Control - Winter 1997 (WI97)

Contents

• Application Note for Ground Vehicles/Transportation
  
• Firstmark Controls Position Transducers: Small Size, Light Weight
  
• Applied Safety Technology Corporation Knee Slider Advances Crash Research
  
• Application Corner
  

Firstmark Controls position transducers have been used in a broad range of ground vehicle applications including vehicle development, performance validation, lifecycle testing, crash test, racing, and OEM use. We have recently developed an Application Note that focuses on the use of Firstmark Controls position transducers in the broad ground vehicle/transportation area. Six pages in length, the Application Note covers typical uses, actual application descriptions, mounting and installation tips, and other technical information. The document is targeted at instrumentation engineers and technicians in the auto, heavy vehicle, rail, recreation vehicle, and racing fields.

To request a copy of the Application Note for Aircraft/Aerospace, please complete and submit the Request More Information form or view it online.

One reason to use Firstmark Controls position transducer products is their small size and light weight. The realities of physics mandate that size and weight be considered in nearly every engineering design. This is especially true in the sensor, test, and instrumentation world. Because the significant aspect of cable-actuated position transducers is a stainless steel cable, Firstmark Controls products have an inherent size and weight advantage compared to other choices.

Rod-and-cylinder products such as LVDTs, linear potentiometers, and magnetorestrictive transducers 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 Firstmark Controls 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 Firstmark Controls product takes up less than 2 inches (0.05 m).

Correspondingly, a rod-and-cylinder design will weigh significantly more than the Firstmark Controls 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 Firstmark Controls product weighs around 3 oz (0.09 kg).

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.

You may have seen slow-motion video of crash test dummies being thrown around vehicles, limbs flying, and heads jerking forward. To fully document the dynamics of vehicle crashes and the vehicles’ occupants, these dummies have transducers and instrumentation throughout their bodies.

Applied Safety Technology Corporation (ASTC, formerly Vector Research, Inc.) of Milan, Ohio specializes in the development and manufacturing of anthropomorphic dummy components and instrumentation. The company was recently tasked by the Vehicle Research & Test Center (VRTC) to develop an alternate method of measuring dummy knee shear. Dummy knee shear occurs when the tibia (shin bone) moves relative to the femur (upper leg bone).

VRTC had developed a specialized Hybrid III dummy lower leg assembly used in obtaining leg-related crash test data. This lower leg, known as ALEX (Advanced Lower EXtremities), used a linear potentiometer to measure the knee shear that occurs during a given test. Use of ALEX by auto companies and VRTC indicated that the linear potentiometer used to measure knee shear in the Hybrid III dummy tended to bind up and could fracture under high-load, off-axis leg movements. Three years ago, VRTC contracted with ASTC to develop an improved knee shear measurement technique for an advanced dummy known as ALEX II.

ASTC began investigating the requirements of knee shear measurement and after careful analysis and testing, developed a unique product named the EX-593 ball bearing knee slider. The product incorporates a modified Firstmark Controls Model 150-0121 position transducer that converts the mechanical leg movement into an electrical signal. This modified position transducer has the electrical cable exiting the side of the transducer instead of the corner. The Model 150-0121 position transducer is attached to the knee slider mechanism via a mounting plate. The displacement cable is attached using a counter-bored hole and the cable stop. The ball bearing knee slider makes use of 0.9 inches of the transducer's 1.5-inch range.

Summarizing ASTC's choice of the Model 150-0121 position transducer, Paul Depinet, Chief Development Engineer for ASTC stated, "Off-axis and high-friction problems were eliminated, improving the reliability of the measurement and reducing the cost of testing."

Performance of the new design was verified by doing a standard knee slider calibration as well as a VRTC comparison of other knee shear measurement designs.

ASTC was started in 1991 and has representatives in 12 countries around the world. For more information on the EX-593 ball bearing knee slider or ASTC, please contact the company at:

The Application Corner is dedicated to answering your questions about using position transducers in specific applications. If you have an application question you would like answered, please let us know by phone, fax, or mail.

Q. I need some accessories to complete my displacement measurement application. Do you provide items like power supplies, digital meters, and data loggers?

A. In general, we prefer to provide only the position transducer. However, we are happy to suggest sources for these types of accessories after learning more about your application. We can provide non-stock accessories for OEM and large-volume purchases or if you prefer we create a complete solution.

Q. Can the displacement cable be attached to my application with a magnet?

A. Yes, you can use a magnet attached to the cable as the connector to your application. Please contact us if you need a source for magnet connectors.

Q. I need to measure 70 inches (1.78 m) of displacement but your standard product line has a maximum range of 42.5 inches (1.08 m). Do you make a longer range product line?

A. We do not have a standard product line today that can track displacements in excess of 42.5 inches (1.08 m). However, depending on your application, we can modify our current products or refer you to other sources to meet requirements that are in excess of our standard range.

Q. Your Position Transducer catalog states that your 5-turn potentiometer for the Series 160, 161, and 162 products has a mechanical life of 5 million shaft revolutions? Is my Model 160-1705 product rated for 5 million complete cycles?

A. No. Because each complete cycle (full extraction and retraction) of the position transducer rotates the 5-turn potentiometer a total of 10 times, the position transducer has a rated life of 500,000 cycles.

ISSN 1527-5108