The Membrane Potentiometer
The Membrane
Potentiometer
A Competitive Alternative to Traditional Linear and Rotary Position
Sensors
Membrane potentiometers have established themselves within the
market over the past twenty years due to fascinating technological
advancements in their field. This whitepaper introduces the advantages
and disadvantages of the membrane potentiometer¡¦s
special design, and also outlines its possible applications.
* The Membrane Potentiometer: Alternative to Traditional Linear
and Rotary Position Sensors
* The Concept
* The Material
* Characteristics of several membrane potentiometers
* Frequently Asked Questions (FAQ)
* Technical Parameters
* Applications
Since the filing of its patent application in 1985, the membrane
potentiometer has experienced many interesting advancements and
has established itself as a prominent technology within the market.
Due to its flat design and voltage divider circuits, the membrane
potentiometer is applicable in the same fields as the traditional
potentiometer. This white paper first introduces the available
types of membrane potentiometers and their respective advantages
and disadvantages, and then outlines the spectrum of applications
for membrane potentiometers.
The Concept
Fundamentally, a membrane potentiometer consists of a resistive
path that is printed onto a membrane base, and a collector with
a printed short-circuit path that is applied on top of this base.
Both paths are separated by a circumferential spacer.
When pressure is applied to the collector foil, electrical contact
is simultaneously applied to the resistive path and a voltage
can be tapped into movement via the collector foil. Once the
pressure being applied to the collector foil ceases, the voltage
can no longer be tapped. This characteristic must be kept in
mind when operating the membrane potentiometer manually; however,
when a wiper or magnet is used, the voltage remains constant.
The Material
The membrane is comprised of polyethylene terephthalate (PET),
which is a thermoplastic material (plastomer) that can be molded
within a specific temperature range. Since this shaping process
is reversible, the procedure can be repeated an unlimited number
of times by cooling and reheating the material, as long as the
thermal decomposition of the PET material is not damaged due
to overheating. The critical temperature for materials used in
membrane potentiometers is approximately 73 DegC.
PET's ability to be molded has implications for membrane potentiometers.
When a wiper is pressed statically onto the collector foil that
has been heated beyond the critical temperature subsequent to
its cooling, a permanent deformation could occur at the pressure
point. Depending on the circumstances, this deformation could
cause anything from a slight change in linearity to permanent
contact between the resistance and collector path. This effect
is known as the denting effect. There are membrane potentiometers
available that operate beyond the thermoplastic point of temperature,
though. This category of membrane potentiometers is discussed
in the following section.
Denting
Effect
A membrane potentiometer is always operated mechanically with
the collector foil. When the wiper experiences prolonged periods
of standstill at high temperatures, the collector foil can become
dented; however, whether or not these dents have an impact on
the application and to what degree depends upon various factors.
These factors include the temperature and surface pressure, the
wiper¡¦s shape and the length of exposure. Appropriate
information about how to avoid this effect is usually provided
by the manufacturer.
A PET-based material can withstand temperatures of up to 100¢XC,
but static pressure should not be applied to the collector foil
at temperatures above 60¢XC. Therefore, manufacturers have
developed different technologies to exceed this limitation (refer
to ¡¥Hybrid Potentiometers¡¦). These consequences
do not usually occur when the wiper is operated dynamically and
the potentiometer is used according to specifications, though.
Adhesive
Adhesives are used in the production of membrane potentiometers
to connect their different layers, as well as to attach the potentiometer
to its customized support structure. In most cases, a single-
or double-sided acrylic adhesive with a thickness ranging from
50 to 200 £gm is used.
Commonly used transfer adhesives are suited for a temperature
range of -40DegC to 150DegC, and a service temperature of about
90 DegC. They can resist storage in water for approximately 100
hours, and are also resistant to most oils, fats, aliphatic solvents
and weak acids, as well as to fuel, salt and alkali. Furthermore,
the adhesives are usually UV resistant.
The adhesive's shear and peel strengths are quite high and depend
on the type of support structure in use (e.g. PET acryl or aluminum
acryl) and the surrounding temperature. Since the adhesive softens
as the temperature increases, it can be removed easily. Details
regarding adhesive force shall be requested from the respective
manufacturer.
Characteristics
of several membrane potentiometers
PET membrane potentiometer
In this class of membrane potentiometers, the resistance layer,
spacer, and collector foil are made of various thicknesses of
PET material, depending on the application and the manufacturer.
The thickness of the film is usually between 60µm and 190µm,
while the overall height is less than 600µm.
The PET membrane potentiometer is usually operated by using a
mechanical wiper with a surface
pressure ranging from 0.7N to 3N, but the potentiometer can also
be operated by hand. Due to the material¡¦s physical
characteristics, the operation temperature ranges between -40DegC
and 60DegC.
Kapton
membrane potentiometer
Kapton is a thermosetting polyimide (PI) that can be operated
continuously under temperatures up to 230DegC, or even as high
as 400DegC (temporarily). A PI is much more dimensionally stable
than PET and can be treated in a reflow oven. The potentiometer
layout matches the PET potentiometer¡¦s layout;
however, its production is more expensive. In terms of the denting
effect, Kapton potentiometers only show a slight improvement
when compared to PET potentiometers.
Membrane
potentiometer with SET function
A membrane potentiometer equipped with the SET function allows
the last set value to be stored¡X even when someone¡¦s
finger or wiper is no longer activating the collector foil. These
potentiometers are mainly used in applications requiring manual
operation (input systems with an integrated membrane potentiometer).
A value is set by applying slight pressure. The set function
can be realized electronically or via a special doping, which
allows for large areas up to DIN A4/US letter.
Hybrid
membrane potentiometer
Hybrid membrane potentiometers are made of a combination of different
materials, and they exhibit differences depending on the manufacturer.
* Base and spacer made of PET, collector foil made of a thicker,
foil-like material: Very
inexpensive with a slightly delayed denting effect. The operating
temperature falls into a range
between -40DegC and 65DegC. Limited use for static applications.
* PET base, collector foil made of FR4 (usually between 80µm
and 150µm thick): Considerably
delayed and reduced denting effect, but higher wiper pressure
possible (3N to 5N). Operating
temperature range between -25DegC and 75DegC.
* Base and collector foil made of FR4 with a thickness of up
to 150µm: The operation
temperature can reach up to 85¢XC, but a wiper surface pressure
of 3 to 5N is required. The
denting effect is significantly delayed.
* PET base, collector foil made of a flexible metal: This new
alternative exhibits a high level of
resistance to the denting effect; however, a wiper surface pressure
of 2 to 6N is required. The
temperature ranges between -25DegC and 85DegC.
Magnetic
membrane potentiometer
The magnetic membrane potentiometer utilizes a magnetic field
and therefore contactless connection. Different designs have
evolved and are listed below, but for almost all variants, the
magnet can provoke a tap either from the top through the collector
foil, or from the bottom through the enclosure.
* PET base, ferrite band on top of the collector: Pulled by the
magnet, the ferrite band presses
down onto the collector foil. The upper operation temperature
can be specified to be a little
higher (about 70DegC); otherwise the same characteristics as
for the PET potentiometer apply.
This technology was introduced to the market in 2004.
* PET base with a wiper between the foils; the wiper is moved
by a magnet: The magnetic
retention force can be less than what is required for a potentiometer
with a ferrite band, and the
distance between the magnet and potentiometer can be greater.
The internal wiper is abrasive,
resulting in the potentiometer¡¦s limited lifetime.
Operating the potentiometer without hysteresis
is not possible because the internal wiper is pulled by the magnet.
* PET base with inserted flexible magnetic metal strip: Only
minimal information exists on this
new technology; however, its properties are similar to the potentiometers
with a PET base and
a ferrite band on top of the collector.
* PET base with conductive ferrite band: This variant is under
development, but the magnetic
force is presumed to be as small as necessary to trigger a reed
contact.
FR4 membrane
potentiometer
Almost all models mentioned above can also be produced with a
FR4 base instead of PET. Some of the advantages of FR4 include
better overall operation, an improved resistor paste (manufacturerspecific),
and a simplified attachment of connectors or wires with the ability
to integrate the complete circuitry into the PCB.
The respective printing of the potentiometer directly onto the
PCB, the populating of the components, the soldering, and the
applying of the collector foil with integrated spacer are further
benefits. The extent of temperature resistance depends on the
material used for the collector. Different collector materials
exist in combination with FR4 PCB¡¦s, often made
of very thin PET or FR4, among them the before mentioned flexible
metal foil.
Frequently
Asked Questions (FAQ)
Is a membrane potentiometer sealed?
A membrane potentiometer is sealed, with a few exceptions. Most
sealed potentiometers, especially those featuring a smaller design,
cannot establish pressure compensation when the wiper presses
down onto the collector foil. When significant changes in air
pressure occur, a sealed potentiometer is usually only suitable
for limited use. The SENSOFOIL R HYBRID membrane potentiometer
is an exception, however. The short distance of only a few micrometers
between the resistance path and the collector path, combined
with the low level of pressure that develops within the potentiometer,
can cause contact between both layers.
In most cases, an integrated valve serves to adjust pressure,
although this usually reduces the level of tightness to IP54
or lower, depending on the design. While other solutions are
possible, they require more space.
Depending on their design, membrane potentiometers can withstand
water for a short period of time. After a while, though, the
adhesive can dissolve, since almost all adhesives are water-soluble.
Furthermore, the membrane materials used are hygroscopic.
What are the wiper's properties and conditions?
It is important for the user to understand that a membrane potentiometer
is only considered a
potentiometer when it is combined with a wiper or magnet. Only
the connection between the collector path and the resistance
layer results in a variable voltage divider.
Typical materials for wipers include delrin, brass, or nickel
silver. The user should pay attention to
the material combination with the foil. Ideally, the wiper should
be of a spherical design and
mounted with flexibility. When using a ball wiper, special attention
must be paid to its rounded
edges and parallel running. The wiper's width should be between
10% and 15% of the
resistance path¡¦s width. A longer spring deflection
should be chosen if high resistance to
shock and vibration is required.
In the case of a magnetic operation, the conditions are quite
different. The magnet's diameter can
exceed the resistance path's width. Of particular importance
are the force that is applied and the
ferromagnetic material that is selected for the ferrule.
Magnets are characterized by their energy density, a value (e.g.
N52) that is comprised of remanence (kG = kilo Gauss) and coercivity
(kOe = kilo Orsted). This value does not give information about
the starting power, which depends on the magnet¡¦s
mass, the distance to the ferrule, nor the ferrule¡¦s
material properties.
In general, the following applies to the wiper: its force has
to be great enough to operate the collector foil. The force depends
on the type of magnet in use, but usually it falls into a range
between 0.75N and 3N. When the value is given in milliTesla (mT),
it is required that the exact distance, the magnet¡¦s
direction and diameter are also specified. Generally, the manufacturer
can provide more precise recommendations.
How small
can a potentiometer be?
A potentiometer is made of three or more parts that are glued
together. The width of the required
adhesive edges has to be at least 2 mm to ensure proper bonding.
The adhesive¡¦s flow characteristics must be taken
into consideration when applying. For this reason, the bonding
surface should be 5 mm wide. The spacer should be 3 mm larger
on each side when compared to the resistance layer, resulting
in the following dimensions: adhesive edge, 2 x 5mm; resistance
layer width, 6mm; spacer edge, 2 x 3mm. These dimensions combine
to equal a total width of 22mm. A smaller width of 12 mm is only
possible if certain specifications can be altered.
Technical
Parameters
The different operational temperatures were mentioned earlier
in this white paper. Additional technical parameters that users
frequently inquire about include life cycle, repeat accuracy,
hysteresis, resolution, repeatability, and errors due to linearity.
Life cycle: The life cycle depends on the technology and wiper
being used, as well as the wiper¡¦s design and contact
pressure. Please refer to the chart for average values. The life
cycle is either stated in movements (one time travel over a minimum
of 90% of the active area) or cycles (one time travel back and
forth over a minimum of 90% of the active area). Typically, a
change in linearity of >10% is considered the end of the potentiometer¡¦s
life-cycle.
Repeat accuracy: Repeat accuracy describes the user-defined approach
of a given position that must always be carried out from the
exact same direction.
Hysteresis: Hysteresis indicates the difference in signal that
occurs when a defined position is
approached from one direction; this position is then passed and
re-approached from the opposite
direction. A potentiometer¡¦s hysteresis depends
on its mechanics. The same applies to a membrane potentiometer
¡V the wiper in use defines the hysteresis.
Resolution:
Resolution
results from the resistance layer¡¦s grit size (level
of homogeneity) and the distribution and the wiper current, among
other factors. The resolution at optimal wiper tracking is indicated
as infinite.
Repeatability: Repeatability describes
any approach of a specified position from different directions.
It demonstrates the sum of two times the resolution plus hysteresis.
Errors due to linearity: Linearity shows the deviation of a membrane
potentiometer¡¦s output voltage from a specified
theoretical function. The ¡§independent linearity
error¡¨ is the maximum deviation of the output voltage¡¦s
curve from an ideal line. This is a well-known aspect of quality,
but to most users, repeat accuracy or hysteresis may be more
important. As long as the gradient is positive, a poor linearity
can still achieve a good repeat accuracy.
Applications
This section presents several applications typical for membrane
potentiometers. The advantages and disadvantages to each application
differ by application and the potentiometer being used. Therefore,
a consultation to optimize your desired application is advisable.
Generally speaking, a membrane potentiometer with a length of
up to 700 mm is an economical alternative to a traditional potentiometer.
Rotary membrane potentiometers with diameters up to 400 mm are
also relatively inexpensive to manufacture.
Sealed
Front Panel
Membrane potentiometers are well-suited for use in fully-sealed
front panels with control elements (set point devices). The potentiometer
can either be integrated into a membrane keypad (manual operation),
or a turning knob provided with a magnet can be welded to a stud.
The magnetic potentiometer is attached to the panel¡¦s
reverse side and is activated by the magnet in the turning knob.
Flap Adjustment
For this application, electronics are usually arranged close
to the control element. Therefore, the
conductive layer can be printed directly onto the circuit board.
The collector foil is affixed after
populating the PCB. One advantage is that the conductor paths
are located on the board.
Distance
Measurement in Cylinders
Magnetic potentiometers (potentiometers operated by a wiper)
are located inside the cylinder in order
to detect linear positions.
Actuators
Membrane potentiometers can be linear or rotary, but can also
be adjusted to different shapes. Usually, the encoder can be
removed due to the high accuracy for correctly dimensioned potentiometers.
Robotic
Systems
Generally, a decision will need to be made on a case-by-case
basis; however, because of their long life-cycles, the use of
a magnetic membrane potentiometer is recommended. When exposed
to fast movements, the potentiometer must be adjusted.
Forklift
Expensive conventional potentiometers used in a controller can
be replaced by membrane
potentiometers. Additionally, switch functions can be integrated
into membrane potentiometers.
Joystick
In dirty, unstable environments, a membrane potentiometer based
on FR4 can be used to protect the resistance layer.
Ribbon
Controller
Moving a finger over a game console creates control signals while
data is simultaneously output as Midi signals.
Medical
Science
Various applications for membrane potentiometers can be found
in medical science; for example: in syringe pumps (perfusion
pumps) for extremely slow movements, in laser technology for
control and accuracy, in input systems (antimicrobial surfaces
are possible) or in prosthodontics to increase reliability.
Door and
Gate Systems
Both linear (e.g. in streetcar doors) and rotary systems (e.g.
in barriers to detect their actual position) can be used without
having to reset the systems in the event of a power outage. To
a certain extent, the encoders that are typically used can be
removed due to the membrane potentiometer¡¦s accuracy.
Summary
Because of its ultra-flat design and flexible usage, combined
with cost advantages, the membrane
potentiometer has opened up sensing possibilities which were
not possible before. With new and
innovative materials like magnetic operations or hybrid membrane
potentiometers (such as the
patented ¡§Sensofoil Hybrid¡¨ by Hoffmann
+ Krippner), designers and engineers can use this
potentiometer in many applications. Complex sensor constructions
can be reduced, simplified and integrated less expensively with
modern membrane potentiometers.
From
an article by:
Jens Kautzor
- CEO
Hoffmann + Krippner Inc
2119 Quail Meadow Ln
Frisco, TX 75034
www.hoffmann-krippner.com
April 2015
