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Temposonics
®
Magnetostrictive Linear-Position Sensors
®
SENSORS
Foolproofing Embedded Sensors
Technical Paper
551112 A
Magnetostrictive sensors
Magnetostriction as a position ­sensing technique has long been used
in industrial machines. But another variant of this technology has
emerged to handle situations that need to embed smaller sen­sors
within a product to cut size and cost. Called mag­netostrictive­core sensors, these new devices are basically con­ventional mag­netostrictive
sensors with­out their hous­ing. Gelling rid of the housing lets designers
embed them in applications such as sprayers, dispensing machines,
fastener magazines, X-Y positioners, small presses, clamps, grippers,
and many other assembly and dispensing tools.
The typical approach with embeddable sensors is to use the structure
of the tar­get appliance or machine itself to pro­tect and house the sensor, much as with any electronic component. But the closer quarters
this entails forces designers to allow for external fields and physical
fac­tors that can influence sensor readings.
The first thing to consider is construc­tion material. Magnetostrictive
sensors can read their position magnet even when­ there is a physical barrier between the position magnet and the sensor tube. This,
capability also eliminates wear. A point to note is that the material
directly between the position magnet and the rod should be nonferrous. Common architectural ma­terials that allow this include plastics,
composite, ceramics, aluminum, brass, most stainless steels, and
anything else that cannot be magnetized. Magnetic ma­terials can be
used for overall construc­tion as long as they don’t come between the
magnet and the sensor tube.
All magnetostrictive sensors should be considered a system of
matched, interact­ing components consisting of magnets, electronics
and sensing elements. The attributes of those components are selected and designed so when they act in concert, the sensor performs
Waveguide Placement
Magnetic Field
(encompasses entire magnet)
Molded Ring Magnet
S
N
W aveguide
(properly aligned, centered)
W aveguide
(limit of non-axial movement)
Magnet Orientation
OK
Not OK
90°
Not 90°
One example of an embeddable magnetostrictive sensor is the
Temposonics C-Series from MTS. Compared to industrial versions, its dimensions are smaller in every direction so it will fit into
smaller spaces. The head is 45% smaller than an industrial head,
for example, and the sensor shaft is more than 50% smaller. An
integrated connector mates with inexpensive board-style plugs for
straight or right-angle wire exits. Through holes along the axis of
the sensor head allow attachment without the need for additional
flanges. But embeddable versions of these sensors put the onus on
the design engineer to keep the position magnet orthogonal to the
waveguide. Otherwise position readings can be inaccurate.
All specifications are subject to change. Contact MTS for specifications that are critical to
your application. Go to www.mtssensors.com for the latest support documentation.
Return-signal strength
Normalized return signal
Blue tube Auto SE 180 degree bend
Bend Radius (mm)
The waveguide on embeddable magnetostriclive sensors (blue in the photo) can be curved to sense position in applications like this. But the
sensor can only tolerate so much curving - a larger radius helps improve system design margins. An example graph shows the amount of
signal loss as a function of bend radius. About 60% of the signal is lost with a bend radius of about 25 mm. That 60% loss is the lower limit
and defines the smallest bend radius possible.
its best. That’s why some magnets - including some designed by the
same manufacturer for other models of magnetostrictive sensors and any catalog magnet from an industrial supply house, probably will
not work well with the sensor element and electronics.
sensor tube moves from one side of the inner diameter to the other.
That is because the magnet’s field strength is uniform throughout its
inside diameter. However, the magnet should al­ways be orthogonal
with the sensor tube axis and as close as practical to the center of the
inside diameter.
The details of how the sensor installs are important because nearby
ferrous material can distort and shunt magnetic fields. This, in turn,
can degrade the output of the sensor. In particular, sensor linearity,
signal stability, noise susceptibility and temperature range are among
the qualities hampered by a bad installation.
A point to note is that surrounding ap­plication structures and the material used to make them can degrade or enhance the symmetry of the
magnet’s field. Manufac­turers engineer field symmetry into the sensor
magnets along both the travel axis and the orthogonal axes. Disrupting
that symmetry can cause intermittent or per­manent shifts in indicated
position. And installing the magnet in ferromagnetic materials such as
mild steel can disrupt the field and change its symmetry.
Ring magnets
Magnetostrictive sensors often use magnets configured in a donut
or ring style, with the sensor element passing through a hole in the
magnet. These ring magnets give the best overall performance. They
are more immune to installation errors than a single bar magnet.
When ring magnets must go in ferrous materials, it’s best to use
nonferrous ma­terials such as brass, aluminum, plastic, or air in the
indicated kee!>’Out areas next to the ring magnet. Ferromagnetic
mate­rials that encroach into the keel>-out ar­eas can cause shifts in
indicated position, reduced temperature range, and noise susceptibility. If snap or retaining rings keep the magnet in place, they should be
nonferrous if possible. But ferrous fasten­ ers of this sort are unlikely
to be a problem because they contain little ferrous material and the
shunting effect is usually small.
For example, motion skew between the magnet and the sensor-tube
axis is much less of a problem with ring magnets. This is because
of their uniform shape and magnetic field. And it is possible to ap­
proximate a ring magnet with a collection of discreet magnets. Though
the field is not as uniform as with a molded magnet, it still will work
fine.
Button or bar magnets
Each ring magnet has north on the in­side diameter and south on the
outer diameter. Other configurations of the magnetic field won’t work.
That includes south inside and north outside, or north ­south along
the axis of the sensor. And it’s best not to use magnets from sources
other than the sensor manufacturer un­less that manufacturer has
certified them for use. Manufacturers design magnet­ field strengths
and shapes specifically for the sensors with which they are sup­posed
to work. Most manufacturers will test for suitability magnets they
don’t make themselves.
In some installations, a ring magnet won’t fit. So manu­facturers offer
bar or button magnets to activate the sen­sor from one side of the
sensor shaft. Button, bar and seg­ment magnets have their own installation subtleties.
It’s important to keep the right distance between the surface of the
magnet facing the sensor and the sensor shaft. Manufacturer-supplied
button or bar magnets have a prescribed optimal stand-off distance
with an allow­able tolerance. At this distance sensor signals hit their
optimum. At points closer than that permissible range the sensor
can saturate. Outside the range, magnet strength maybe too weak to
generate a signal.
Most sensor makers offer ring magnets with various inner and outer
diameters. If the manufacturer’s standard and optional magnets don’t
quite fit -more common in embedded applications -consult with the
manufacturer. Be prepared to provide the maximum O.D., I.D., and
width of the space you have available for the ring mag­net. They will
model a magnet with an op­timal fit. Often you can get special magnet
molded if your volume is sufficiently high.
The sensor tube can sit anywhere in­side the inner diameter of the ring
and provide strong signals. Indicated position changes little as the
Temposonics® Sensors, Foolproofing Embedded Sensors - Technical Paper 551112A
It is particularly important to verify that button mag­nets generate
enough signal for the application at hand. Also, a minimum area around
the magnet diameter must be free of magnetic or ferrous material.
Note also that but­ton magnets are rarely marked. Make certain with a
test fixture that north faces the sensor tube.
2
MTS Sensors
For button and bar magnets, there is a general rule of thumb to use a
keep-out area having a radius of about three times the magnet radius.
The south-pole flat-magnet surface oriented away from the sensor
tube most times can be against ferrous material. In fact, sometimes
it’s bet­ter to have that area against ferrous material to act as a concentrator and throw the field out further.
Typical keep-out areas
Non-ferrous keep out zone equal to one half the width of the
magnet and equal to the I.D. and O.D. dimensions.
Routinely, ferrous materials can be fairly close to the sensor. The sensor’s tube, also called the waveguide tube, can sit within ferrous metal
structures. Of course, the area surrounding the tube where the magnet
traverses should not contain any ferrous material.
13 mm (0.51 in.) ferrous free zone
around the sensor tube
Magnet
When ring magnets must go in ferrous materials, it’s best to use
nonferrous materials such as brass, aluminum, plastic, or air in
the keep-out areas that are next to the ring magnet. Ferromagnetic
materials that encroach into the keep-out areas can cause shifts in
indicated position, reduced temperature range, and susceptibility
to noise.
The sensor head housing can be recessed completely in a metallic
structure, ferrous or nonferrous. Nonmetal­lic structures can house the
sensor head but they offer no EMI/ESD advantages. In bar or button
magnet installa­tions, the ferrous material should not come closer to
the sensor than 13 mm anywhere around the tube. And no fer­rous
material should come between the sensor tube and the magnet north
surface.
Residual magnetism
Also, a gauss meter and probe can check levels if it ap­pears the installation structure is magnetic. Generally, the keep-out areas should not
have a permeability of greater than 1.05 or fields from other sources
greater than 5 gauss (0.5 mT). Fields stronger than these levels should
be re­moved via degaussing, increasing gaps, or substituting other
materials.
In rare instances, though the keep-out areas have been observed,
residual magnetism in the structural materials can induce position
shifts, reduced linearity, reduced tem­perature range, and noise. This
can happen even in some stainless steels. Residual magnetism can
arise from cold working or machining. For example, a cylinder rod
that picks up magnetism from machining can change the mag­netic
qualities of the magnet. It may reduce the magnet’s field strength or
make it become asymmetric, degrading sensor performance.
Application specialists can be helpful for cracking par­ticularly knotty
problems. In some cases, most manufac­turers will run tests on prototype designs to determine the impact of different configurations or
changes.
A degaussing of the structure can reduce or eliminate the problem.
Degaussing coils, commonly used to degauss CRT screens, are simply
a ring coil and a core that has ac current (usually at line voltage) passing through it. When passed over a magnetic part, the ac induced field
random­izes the alignment of the magnetic domains in the material and
thus reduces the magnetism.
Any curved sensor installation needs special atten­tion. It’s possible
that the return signal will diminish as the curve radius gets smaller.
So the sensor manufacturer should help evaluate the degree of curve,
the magnet to use, and how to install the magnet. The sensor can only
tolerate a certain amount of curving and still produce a vi­able output.
It’s best to choose the largest possible radius as a way to improve the
system design margins.
Most curved sensor applications are forced to use some form of bar
or button magnet. That’s because it’s difficult to get a ring magnet to
follow a curved path when the path is defined by standoffs or a groove
in the material.
The magnet should maintain a specified standoff height along the
curve of the sensor. The magnet path should also parallel the sensor shaft. Any deviation in standoff height or path parallelism as the
magnet moves may produce a position error or reduce the effective
temperature range.
Troubleshooting
When troubles arise, a first course of action should be to individually
and incrementally raise the dimension mar­gins. This helps identify
the source and the point were the symptoms diminish or cease. This
information can help the manufacturer make recommendations about
fixes.
MTS Sensors
3
Temposonics® Sensors, Foolproofing Embedded Sensors - Technical Paper 551112A
What is magnetostiction?
Magnetostriction is a property of ferromagnetic materials such as
iron, nickel, and cobalt. These materials change size and/or shape
when sitting in a magnetic field.
This physical response of a ferromagnetic ma­terial arises because of
the presence of magnetic moments, essentially a collection of tiny
perma­nent magnets, or domains. Each domain consists of many
atoms. When a material is not magne­tized, the domains are randomly
arranged. When the material is magnetized, the domains are ori­ented
with their axes approximately parallel to one another and a change
in size or shape along that axis is a result. Interaction of an external
magnetic field with the domains also causes the magnetostrictive effect. The order of the do­mains, and thus the magnitude of the effect,
can be influenced by alloy selection, thermal anneal­ing, cold working,
and magnetic field strength.
Ferromagnetic materials used in magneto­strictive position sensors are
transition metals such as iron, nickel, and cobalt. When a material has
positive magnetostriction, it enlarges in a magnetic field; with negative
magnetostriction, the material shrinks.
Applying a magnetic field causes stress that changes the physical
properties of a magneto­strictive material. However, the reverse is also
true: Applying stress to a magnetostrictive mate­rial changes its magnetic properties (e.g., mag­netic permeability). That is, a dimensional
change in the material can lead to induced magnetic fields. Magnetostrictive sensors employ both properties to generate an ultrasonic
strain wave from the location of an external marker magnet and detect
its passage past a fixed reference point in a wave guide. By knowing
the speed of sound in the material, marker magnet position can be
determined using a time-of-flight measurement technique.
First step in the time-of-flight measurement is to apply an orthogonal
magnetic field to a magne­tostrictive wire, and a pass a current through
the wire. The magnetostrictive effect causes a twist­ing at the location
of the orthogonal magnetic field. The twisting is caused by interaction
of the orthogonal magnetic field, usually from a per­manent marker
magnet, with the magnetic field along the magnetostrictive wire, which
is present because of the current in the wire.
The current is applied as a pulse, so the mechanical twisting travels in the wire as an ultra­sonic wave. Each magnetostrictive strain
wave travels at the speed of sound in the waveguide material,
approximately at 3,000 m/sec. The po­sition magnet is attached to
whatever is being measured. The waveguide wire, enclosed within
a protective cover, is attached to the stationary part of the machine.
Movable Position
Magnet
Magnetic field encompasses entire
waveguide - generated by
interrogation pulse
Interrogation:
Return Wire
Waveguide
Magnetic Field from
Position Manget
Interaction of magnetic fields
causes waveguide to generate a
strain pulse
Pulse Output
Strain Pulse Detector
Ferromagnetic
material
Waveguide twist
Current
director
Position magnet
Magnetostriction principle of operation
Location of the position magnet can be deter­mined by starting a
counter timer when the cur­rent pulse is launched. The current pulse
causes a sonic wave to be generated at the location of the position magnet. The sonic wave travels along the waveguide until it is
detected by the pickup.
The pickup output voltage stops the counter timer. Elapsed time indicated by the timer repre­sents the distance between the position magnet
and the pickup. The frequency of the counter de­termines the resolution
of the measurement. The higher the frequency, the finer the resolution.
Finally, electronics conditions the elapsed time information into the
desired output such as a de voltage. pulse-width modulation, and
so forth.
Published date: 2007
Part Number: 12-07 551112 Revision A
MTS and Temposonics are registered trademarks of MTS Systems Corporation.
All other trademarks are the property of their respective owners.
All Temposonics sensors are covered by US patent number 5,545,984. Additional patents are pending.
Printed in USA. Copyright © 2007 MTS Systems Corporation. All Rights Reserved in all media.
®
SENSORS
UNITED STATES
MTS Systems Corporation
Sensors Division
GERMANY
MTS Sensor Technologie
GmbH & Co. KG
JAPAN
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