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VOLUME -24 NUMBER 9
Publication Date: 09/1/2009
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Archive >  September 2009 Issue >  Special Features: PCB and Assembly > 

Ultra-Flat Membrane Potentiometers Enable Smart Engineering
Wafer-thin flexible membrane potentiometers solve numerous engineering design problems.

Membrane potentiometers have changed the way engineers think about sensing. With some only 0.5mm thick, ultra-flat membrane potentiometers have product characteristics comparable to conventional potentiometers, but provide liberal design freedom at significantly lower costs.

Today's membrane potentiometers can be used in the same applications as conventional potentiometers, but can also fit into space-constrained areas. The function can follow the form, a feature not seen very much with mechanical potentiometers.

Because most producers of membrane potentiometers offer customized products with only small tooling efforts, the costs are very competitive, even for small prototype quantities. Three additional advantages are found in the basic construction of the membrane potentiometer: the ability to be fully sealed; the possibility for a hollow shaft design; as well as numerous wiper options, including operation by hand.

Most membrane potentiometers, like the "Sensofoil" products, can be sealed at up to an IP65 (NEMA4x) rating and beyond. Conventional potentiometers require a difficult assembly of the wiper, particularly in a hollow shaft assembly. The hollow shaft design of membrane potentiometers such as "Sensofoil" is more reliable and very cost efficient. Because of the simple nature of the membrane potentiometer, certain technical adjustments are not required. The wiper for the membrane potentiometer can be as simple as a small plastic knob sliding across the surface; it requires no external electrical contact. Most membrane potentiometers can also be operated by sliding a finger over its surface and Sensofoil even offers a contactless magnetic version.

Design Characteristics
In general, a membrane potentiometer is a voltage divider; it is constructed as an open circuit that only gives output when the wiper connects the top and bottom circuits by applying pressure to the membrane surface. The spacing between the top and bottom circuit is constructed mainly of sealant adhesive, which allows sealing against external environmental factors, such as moisture or dust.

This spacing adhesive requires 2-3mm minimum width on all sides of the membrane potentiometer. The ideal width of the resistive active area should be between 3mm and 6mm, but can also be extended to 10mm or even 12mm. Linear elements can reach up to 760mm in active length, while rotary elements with a center hole can measure from 20mm diameter to approximately 450mm diameter.

The life cycle and operating temperature correspond to those of mechanical potentiometers. The Sensofoil Magnet or Sensofoil Hybrid offer more than 20 million operations, and depending on the design as much as 100 million operations are possible. The operating temperature can range from -40 to +85°C and there are systems being developed that can withstand temperatures up to 125°C. Membrane potentiometers do not need temperature compensation if they are operated in a voltage divider mode, and electromagnetic interference will not affect the system.

The standard actuation is a wiper or slider. The Sensofoil Magnet requires a defined magnet instead of a wiper. For hand tripping actuation, a "SET-function in Z-direction" is available as an option.

Due to the flat design of membrane potentiometers, they can operate in close proximity to mechanically active equipment that is being measured, and the needed installation space is minimal. The devices are available with an adhesive-covered surface and can be attached to a substrate by simply removing the protective paper backing and applying to the surface. Other options are snap-in mounting or using screws. The wiper can also be easily integrated into the application.

For small quantities, off-the-shelf wipers might be the easiest choice, while a custom designed wiper might make more sense for larger quantities or to meet space constraints.

One application might require a spring-loaded screw; others might call for a spring-plate attached to a POM/Delrin plastic. Each application can be tailored to the needs of installation space, cost and accuracy required.

If long lifecycle is required, a magnetic wiper is recommended for contactless operation. In general, a spherical wiper can be used. The typical wiper covers about 10 to 50 percent of the active width. Materials such as steel, brass, hard plastics (Delron, Delrin) are typical, but depend on the type of membrane potentiometers.

Surface Pressure
Different surface pressures are defined depending on the application, but in general 1 Newton (N) up to 6 Newton (N) is the range that is recommended for most applications, depending on whether the membrane potentiometer is configured in a standard foil layout or in a hybrid version with additional metal bonds to strengthen the structure. Standard off-the-shelf wipers are usually divided in a 1-3N and a 3-6N range.

Electrical Characteristics
The electrical characteristics will vary somewhat with the design, but are generally standard, with output comparable to a conventional potentiometer, such as a voltage divider.

In comparison to the conventional precision potentiometer, the most significant difference is the method of achieving linearity. While the conventional potentiometer might use laser-trimming to achieve linearity, membrane potentiometers, which are based on PET-Material, rely on production improvements to improve linearity. Hence, the typical offering is 2 percent for standard linear potentiometers, although FR4-based membrane devices can offer linearity as low as 0.5 percent as an option.

But more important than linearity is the repeatability and hysteresis; the accuracy of membrane potentiometers can be as good as 0.01mm over a length of 500mm, but most standard membrane potentiometers range anywhere from 0.05 to 0.1mm.

Electrical outputs are determined by the mechanical motion and stability of the wiper. As a benchmark, energy dissipation of up to 1 Watt and a dielectric strength of 500V is standard, with isolation strength of 100VDC possible as well.

Slender Outline
Because of the slender thickness of the membrane potentiometer, a typical application is linear measuring of actuators. Control valves benefit from these devices, replacing a standard that calls for gears and a rotating sensor to measure position. The structure required to install and use a membrane potentiometer is elegant in its simplicity, as well as form-friendly — eliminating the large and difficult mounting of conventional potentiometers.

Many sensing needs can be solved by using these state-of-the-art devices. An improved design, creating linear output signals right next to the moving device can sometimes increase accuracy over a conventional rotary potentiometer, which is connected by gears, often existing far from the actual motion which is being measured. While conventional mechanical potentiometers are cost intensive, membrane potentiometers can reduce these costs significantly. In many applications, both rotary and linear membrane potentiometers can be used in the same device. For example, a seat level adjustment requires linear motion, while inclining the seatback angle requires rotary motion. Similar applications are found in automotive interiors, where sliding doors, sun roofs, seats, and other applications provide very little installation space and will tolerate very little weight.

Limited amounts of space in many designs that already use a PC board provide the ideal place and need to install a membrane potentiometer. The printed circuit board can be fully populated, and afterwards, the membrane potentiometer can be attached to the board and the connectors inserted directly into the PCB. In addition, very few manufacturers can even print a conductive ink directly on FR4 and combine this with components, but the membrane potentiometer can be assembled directly onto the PCB.

Other applications, which use a membrane potentiometer mainly because of low cost, are string-pots, wire sensors and magnetic tape. In addition, membrane potentiometers can be fully integrated into a keypad for hand-operated controls. The seamless integration that is possible makes the keypad highly innovative and useful for position tracking. The integrated potentiometer can avoid installation holes — needed by a standard potentiometer — and can create a completely sealed and washable front panel surface. Because of its ultra-flat design and flexibility, combined with extremely low costs, the membrane potentiometer has opened up possible sensing applications which were previously impossible. With new and innovative materials and magnetic operation, or hybrid devices, designers and engineers can use this potentiometer in many applications. The complexity of sensor construction can be greatly reduced, simplified and integrated at far lower cost with today's membrane potentiometers.

Contact: Hoffmann + Krippner, Inc., 277 Highway 74N, Peachtree City, GA 30269 678-519-2218 fax: 678-519-3061 Web:
http://www.hoffmann-krippner.com

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