Whitepaper
SIMPSpad: Novel Magneto-Inductive Position Sensing Technology
Summary
SIMPSpad is a novel position sensing technology. It builds upon inductive tech-nology where a simple metallic or
resonant target may be detected, but also enables sensing through non-ferrous metals. This is achieved by adding
a layer of highly permeable soft magnetic material and a magnetic target. The magnet saturates a spot on this
layer, changing the coupling between transmit and receive coils, and gives rise to a position dependent signal.
1. Introduction
The requirement to sense position is fundamental to many applications in the au-tomotive world, and in recent
years two technology strands have dominated this cost sensitive market – inductive position sensing and
magnetic position sensing.
Inductive technology
Inductive sensing is a very flexible solution, and the most successful embodiments have been where printed
circuit board (PCB) based antenna structures interact with a simple metallic or resonant target. The development
of custom ASICs allow cost competitive solutions to be developed for a wide range of both short and long linear
as well as rotary applications. A key strength of this technology is its immunity to low frequency magnetic fields –
a requirement which is increasing with the development of hybrid and electric vehicles. However, even a thin
layer of metal between the target and sensing element will prevent an inductive sensor from working.
Magnetic technology
Magnetic sensing is generally based on Hall cells or magneto-resistive (MR) tech-nology. Here the latest
generation of ASICs are able to measure the magnetic field vector, and can be easily configured for rotary or short
(<40mm) linear sensors. These sensors can be vulnerable to external magnetic fields, and are generally not well
suited to long linear applications - arrays of sensors are possible but lead to a large increase in cost, size and
complexity. However a particular strength of these sensors is the ability to sense through non-ferromagnetic
materials – for example the aluminium walls of a gearbox.
SIMPSpad – magneto-inductive technology
Into this landscape we bring a new patented magneto-inductive technology, SIMPSpad [1], which allows our
existing inductive technology, Autopad [2], to be configured to operate using a magnet as the target, rather than
a resonant or metallic element. This complements the existing technologies, while also addressing areas
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traditionally found challenging (large external fields and high dynamic tolerances) or even previously unavailable
(long linear through metal sensing). Figure 1 illustrates this landscape.
Figure 1: Low cost position sensors, split by sensor type, and target to sensing el-ement separation, for three technologies: inductive,
magnetic and magneto-inductive.
Figure 2 shows an example of a linear SIMPSpad sensor. A thin layer of soft fer-romagnetic material, for example
Mu-metal, is attached to the surface of an inductive sensor. A magnet is then used as the moving target which
interacts with the sensor by producing a local non-uniformity, or saturation region, in the Mu-metal. It is this nonuniformity which is detected by the sensor. Section 2 will review the principles of the pure inductive sensor, while
section 3 will discuss the operation of the magneto-inductive SIMPSpad sensor.
Figure 2: Exploded view of SIMPSpad sensor
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This technology has several strengths:
•
Ability to sense through non-ferromagnetic materials.
•
Magnet to PCB sensing element separations of up to 20mm.
•
Relatively insensitive to variations of separation between magnet and sensing element.
•
Builds on existing automotive qualified inductive technology.
•
Good electromagnetic immunity, including low frequency magnetic fields.
Within the automotive sector there are many applications which would benefit from the SIMPSpad technology.
The ability to see through metallic walls make it suited for a range of engine bay applications, including gearbox,
clutch and brake sensing, while the long travel combined with high dynamic tolerance allow it to directly measure
parameters such as suspension height. These strengths also make it of interest to the industrial market for a wide
range of applications, including cylinder position measurement, linear actuators and valves.
2. Inductive foundation of technology
The implementation of SIMPSpad builds upon Autopad inductive technology. In Autopad, the fixed sensing
element, or Pad, which is generally a PCB, includes two track structures which each form transmitter coils. These
generate electromagnetic fields which are spatially varying with sine and cosine patterns as shown schemati-cally
in figure 3.
Figure 3: Example sine (in bold) and cosine coil structures
The transmitter coils are driven with a high frequency carrier, but modulated at a lower frequency in quadrature. The
fields generated by the sine and cosine coils are given by equations 1 & 2:
(
)
(
)
(
(
(1)
)
)
(2)
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Where:
c
is the carrier frequency (typically 4MHz)
m is the modulation frequency (typically 4kHz)
k is the wavenumber ( 2 /L – with L the wavelength of the transmitter coils)
z is the position along the length of the coil
A is the amplitude of the field produced by the sine or cosine coils
The resultant field is then given by the sum of these, which may be expressed as:
√
(
)
(
)
(3)
Equation 3 has two important properties: a) the magnitude of the signal is independent of position, z, along length; and
b) the phase (kz) of the modulation signal varies linearly with position z. Therefore, measurement of the phase gives an
unambiguous determination of position.
This field couples to a moving target (the Puck), which is generally a small coil made to resonate at the carrier frequency
by the addition of a capacitor (forming an LC resonant circuit). A current is induced in the target by the field from the
TXtransmit coils– the modulation phase of which is related to position. The alternating field produced by the Puck
induces a voltage inon a third coil on the PAD, the receiver coil. These structures are shown in figure 4.
Figure 4: All coil structures shown schematically, with the receive coil and the resonant Puck highlighted in bold.
An ASIC has been developed to supply the signals to the transmit coils, and then process the signal from the receive coil.
The ASIC synchronously detects this voltage signal, leaving only the modulated signal. The phase of this modulated
signal is then calculated and gives a direct measurement of the Puck position.
This technology is well established and robust, measuring linear and rotary posi-tion in many millions of products. A
sophisticated suite of tools has been developed which allows the design and modelling of sensors to be performed with
a high degree of confidence for almost any geometry.
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3. SIMPSpad
3.1 Physical principle of operation
SIMPSpad is based on the Autopad sensor technology and shares several of the same components:
1.
The same signal drive and processing, utilising identical electronics
2.
Two sinusoidal transmit coils (one sine, one cosine)
3.
A single simple loop receive coil
However, the puck is replaced by a magnet, and a layer of high permeability mate-rial is added to the pad to act as an
intermediate sensing element which both the magnet and the Pad coils interact with. In this case the layer is Mu-metal,
bonded to the PCB which carries the transmit and receive coils, and completely covering them.
Mu-metal is a soft magnetic material with the following key characteristics (see fig-ure 5):
•
High maximum relative permeability which is typically >100000.
•
Low saturation flux density of 0.75 - 0.8T.
•
Relatively low electrical conductivity >2MS/m.
•
Low hysteresis (coercivities <100mA/cm)
•
niedrige Sättigungsflussdichte von 0,75 T - 0,8 T
Figure 5: Typical B-H Curve for Mu-metal [3]
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The high relative permeability of Mu-metal makes it very effective at confining magnetic flux and its most
common use is as a magnetic shielding material. For a SIMPSpad sensor, in the absence of a magnetic target, a
significant proportion of the magnetic field generated by the transmit coils travels through the Mu-metal layer,
instead of through the air (Figure 6).
Figure 6a and b: Cross section through a sensor showing the AC magnetic flux lines from a transmit coil (2D FEMM simulation [4]). Figure 6a
(left) is without Mu-metal, and figure 6b (right) is with the addition of a layer of Mu-metal. Note how the majority of flux lines below the
sensor travel through the thin layer of the Mu-metal.
Although Mu-metal is also conductive, eEddy currents in the Mu-metal tend to be supressed due to the large
permeability (µ~100 000), and high carrier frequency (f~ MHz) leading to a small skin depth, δ, at the carrier
frequency (equation 4).
√
(4)
Therefore, the net effect presence of the Mu-metal is to enhances the coupling between the transmit and receive
coils. As this effect occurs along the whole length of the sensor, there is still no net coupling between them.
However, the low saturation flux density of Mu-metal allows the layer to be easily be locally saturated by a
permanent magnet (for example NdFeB), see Figure 7.
Figure 7a and b: Cross section through a sensor showing the effect of a perma-nent magnet on a SIMPSpad sensor. In 7a (left) without a
magnet, there is an AC magnetic field enhancement, but where the magnet saturates the Mu-metal (fig 7b, right), there is a significant
suppression of the AC magnetic field from the transmit coils due to the generation of eddy currents.
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This saturation spot locally reduces the coupling between the transmit and receive coils through two
mechanisms:
1.
The low permeability of the saturated Mu-metal presents a higher reluc-tance path and is therefore less
effective at constraining the magnetic flux from the transmit coils and coupling it to the receive coil.
2.
The decreased permeability also leads to a much larger skin depth, allow-ing significant eddy currents to
flow, further reducing the field from the transmit coils within the saturated spot.
These combine to locally reduce the coupling between the coils, giving a net im-balance between them, allowing
the position to be determined in the same way as the increased coupling from a resonant target does for Autopad
sensors.
3.2 Sensing through metal
In a pure inductive sensor, when an AC electromagnetic (EM) field encounters a metal, eddy currents are
generated and act to cancel out the EM field. There are fac-tors, such as electrical resistance, which limit how
effective the screening is but, at the high frequencies typically used by inductive sensors, the distance such fields
penetrate a conductor (the skin depth, δ, equation 4) is very small. This means that inductive sensors can only
measure through non-conductive layers.
In SIMPSpad, the DC field from the magnet passes straight through metals, unless there is a ferromagnetic
response from the material (e.g. from iron or ferrit-ic/martensitic stainless steel). This allows the magnetoinductive sensor to sense through any non-ferromagnetic material – for example, the DC magnetic field can pass
through aluminium and interact with the Mu-metal layer of the sensor.
3.3 Magnet choice
The larger and stronger the magnet the larger the possible magnet to sensor dis-tance, and the greater the
tolerance to dynamic offsets. As such, magnet size and material can be customized to a given application.
For automotive applications, cost is always a driving factor, but the majority of us-es for this sensor would be
safety critical or operationally essential. This rules out the cheapest magnetic option, ferrite, as even if the poor
high temperature stability was acceptable, these magnets suffer permanent loss of field strength at a low
temperature of -40°C and this may be experienced by vehicles in some markets.
The majority of applications would use either NdFeB (for maximum magnet to sensing element range) or plastic
bonded NdFeB, and size and shape would depend on individual requirements. For engine bay applications with
temperatures >125°C for significant periods of time SmCo would be chosen for its increased temperature stability.
Moving away from automotive into industrial gives a wider range of choice, with even ferrite being possible for
low end applications where separation is low and me-chanical tolerances are tight.
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3.4 System Modelling
The modelling problem can be broken down into two key elements:
1.
The interaction of the magnet with the Mu-metal (the DC model). This ena-bles the magnet and aspects of
the sensor geometry to be optimised.
2.
The interaction of the coils (at the carrier frequency) with the Mu-metal (the AC model), either in the
presence of the magnet, or with no magnet.
The DC Model
A number of proprietary magnetostatic codes are available which can model the interaction of a static magnet
with a thin layer of Mu-metal. We have successfully used FEMM [4] for 2D-d calculations and MAGNUM [5] for 3D calculations. However, using such codes with highly non-linear Mu-metal reduces the calculation speed as the
material is being driven well into saturation.
A faster calculation speed is desirable to facilitate efficient design optimisations, and also to integrate this model
into an efficient calculation of overall sensor linearity. The simplest approach for doing this is to discretise the
magnetic material into a collection of small volume cells, small enough so that the magnetization vector can be
considered constant across each of themcell. The magnetic field at a certain point P of the space is given by
equation 5 [6]:
⃗( )
⃗ ( )
∑ ∭ (
(⃗
)
⃗
)
(5),
Where the external field is ⃗ and the ⃗ are the magnetic vectors at each cell.
It is possible to solve for all the magnetization values, by combining equation 5 with equation 6 below:
⃗
)⃗
(
⃗
(6).
Magnetic saturation can be evaluated by using the constitutive relations:
(⃗
⃗
⃗
)
(7).
For any H higher than the saturation value:
⃗ (⃗ )|⃗
⃗
(⃗
⃗
) (8)
This is equivalent to the simple approximation that
(actually dB/dH) is assumed to have two values: the first is
characteristic of the high permeability of Mu-metal and is valid for B< ⃗ ; and for higher values of B where
saturation has occurred, a value of
is assumed. By solving these equations we come up with a sufficiently
accurate picture of how the material's magnetization arranges in response to the external field, and where the
material saturates.
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Figure 8. Example of the magnetic high speed modelling developed in house. The magnetic source is a ring magnet, which moves along a
strip of magnetic material. Below there is a plot of the saturated (dark grey) and unsaturated (white) cells for this particular configuration
(note: cell borders are shown in grey).
With the help of the model we can predict the position and size of the saturation area. This helps us to calculate
important design parameters such as magnet size and strength, gap between sensor and target and Mu-metal
dimensions.
3.5 Sensor Performance
The sensor in figure 9 below is approximately 170mm long and was developed for a specific application where
there is a requirement for an 8mm nominal separation between the magnet and the PCB and the ability to cope
with large dynamic toler-ances.
Figure 9: Example sensor – showing both the electronics and the Mu-metal layers
The sensor has a characteristic three stage output (figure 10), caused by a change in spot size as the magnet
approaches the end of the sensor. There is good agreement between theory and experiment. Development work
is underway to expand the scope and ease of use of this model still further.
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Figure 10: Model and experimental comparison of sensor output corresponding to the sensor shown in figure 9
Hysteresis is found in all magnetic systems, but the results for SIMPSpad are well within typical automotive
requirements (see figure 11): <0.05% over the full meas-urement range. Additionally, a simple linear fit to the
central region gives a linearity of better than ±0.5% for this approximately 110mm long section of travel (see
figure 12). It is possible to use the full length of travel of the sensor if the three slope characteristic is acceptable,
or can be addressed using a look-up table. Modification of the coil structures can also be used to reduce the size
of these end regions.
Figure 11: Hysteresis error for typical experimental sensor test (the jitter seen is due to noise in the experimental setup)
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Figure 12: Linearity error with linear fit to the central region of travel
The sensor is also very tolerant to offsets (figure 13) with changes of several mil-limetres in lateral magnet
position or separation from the Pad having little effect on performance.
Figure 13: Deviation in output over ±3mm variation in magnet-Pad separation
DC immunity is also a common weakness for magnetic sensors, but SIMPSpad can currently withstand 1-2 mT
with less than 1% error. Improvements currently un-der development will provide an order of magnitude
improvement (this will be the subject of a subsequent publication).
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4. Conclusion
SIMPSpad is a novel magneto-inductive position sensor well suited for the auto-motive market: it can sense
through metal with very low hysteresis; it shows better than 1% linearity performance; it is robust against offsets;
its length of travel is only limited by limits on PCB board size and it is based on proven inductive and magnetic
technology, keeping costs and risks down.
We are currently developing products and ramping up to production, with an anticipated start of production for
automotive projects involving this technology in 2015.
The features which make SIMPSpad good for automotive, also make it interesting for many application areas in
the industrial market. The shorter development and qualification time for these products will enable us to bring
the first of them to market in 2014.
Press contact: Sandra Groß
Global Head of Marketing Communications
E: [email protected]
T: +49 2389 788-175
Product information: Stefan Rühl
Head of pre-development and new technologies
E: [email protected]
T: +49 2389 788-730
5. Literature
[1] ZHITOMIRSKIY, Victor Evgenievich, “Position Sensor”, EP 1 721 130 B1, 18/06/2008.
[2] Ross Peter Jones, Richard Alan Doyle, Mark Anthony Howard, David Alun James, Darran Kreit, Colin Stuart
Sills, “Sensing apparatus and method”, EP 1 442 273 B1, 04/08/2004.
[3] B-H Curve for Mu-Metal, "Direct current magnetization curves for various magnetic materials," p. 792,
Metals Handbook, 8th edition, Volume 1, American So-ciety for Metals, 1966.
[4] FEMM: Finite Element Method Magnetics V4.David Meeker, http://www.femm.info.
[5] MAGNUM, Field Precision LLC, http://www.fieldp.com/magnum.html.
[6] Chadebec et al., IEEE transactions on magnetics, p515, 42 (4), 2006.
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