Application Digest

No reason to fear wear

Charles Nunziata, Executive VP, Vernitron Corp., San Diego, CA

Design engineers often choose LVDTs and magnetostrictive devices over potentiometric-based sensors for fear that the wiper/film interface will wear and become noisy. There is also concern that the cantilever wiper will lose contact with the film surface, resulting in sudden discontinuities. Neither assumption is correct, except in the extreme.

For example, the military specification for film potentiometers requires, in a rotary configuration, 25 million full-function angle cycles. In the device the specification deals with, this equates to some 175 million inches of wiper motion.

Furthermore, the specification defines failure in terms of performance degradation, not catastrophic circumstances. The designer, therefore, should define life requirement around the number of actuations expected. Under this, contacting infinite-resolution film potentiometers meet or exceed most system life requirements.

Seven grams of wiper pressure in a typical Vernitron linear motion configuration requires a minimum acceleration of 2,500Gs to separate the wiper from the film surface--a figure well above any reasonable system environment.

Additionally, Vernitron linear motion potentiometers utilize a four-wiper, individually sprung wiper set. This makes it nearly impossible to cause the wipers to lift from the film surface simultaneously. As a result, the device is extremely resistant to separation under the most stringent shock and vibration environments.

To speak with an applications engineer from Vernitron, call (813) 347-7520 .

Take the isolator out of shock isolation

John Kim, Marketing Manager, Barry Controls, Brighton, MA

When it comes to isolating shock and vibration--whether to protect delicate instruments, human operators, or the environment--design engineers have usually had to bolt on extra components to their product. These extra components add weight, take up space, increase cost, and can mechanically interfere with the performance of the product.

An alternative approach is to not attach a mechanical isolator at all, but to mold it into the structure. The bonding of the two materials (the elastomer and the structure) takes place in a multi-material molding process. The elastomer is injected simultaneously in the same mold with the rigid thermoplastic and a molecular bond occurs during cooling. Homogeneous isolation is built into components when they are formed, not later on.

The selection of the elastomer and the rigid material requires that you make several key decisions early in the design process--e.g., the selection of thermoplastics, equipment, and molding process.

Barry Controls uses a patented process called Duo-Plexx™ that employs one of nine thermoplastics which can be mixed with six thermoplastic elastomers. The goal is to best combine materials to optimize bond strength and meet application requirements such as stiffness and tensile strength. Resilient elastomers, for example, absorb energy by deflection. Damped elastomers absorb energy with less deflection but with internal heat build-up. An applications engineer can help you decide on the correct materials and molding process for your application.

To speak with a Barry Controls applications engineer, call (800) 574-5842.