solving the challenge of stray field immunity for safety-critical applications

Solving the Challenge of Stray Field Immunity for Safety-Critical Applications

The Need for Magnetic Position Sensorswith Stray Field Immunity

Heinz Oyrer 16/10/2015

Agenda

• Sensors exposed to environmental factors

• Stray fields – definition, issues, examples

• Magnetic position sensors

• Solution options

• Unique solution, working principle

• Problem resolution and benefits

• Summary


Sensors

• Use of sensors has dramatically increased

• Number and strengths of electric fields have increased

• Exposure to environmental factors such as magnetic stray fields,

vibration and misalignment cause issues with system safety and

reliability


Sensors exposed to environmental factors

6/10/2015 Heinz Oyrer 4

What is a stray field?

• Magnetic fields are generated by magnets, motors, transformers or

any current-carrying conductors

• Stray Fields are parasitic magnetic fields as observed by a

magnetic sensor



Stray fields

Issues caused by stray fields

• High levels of electro-magnetic interference (EMI) are a strong

concern in industrial and automotive applications

• An even larger concern are:

- Increased electrification of automobiles

- Electric cars - large high current carrying wires run between the front

and back of the vehicle


Examples of Magnetic Stray fields• Electric motor

- Generates a magnetic field that effect e.g. the angle position sensor accuracy

• Drivetrain of vehicles becomes partially or wholly electrified

- Battery cable connections can be negatively impacted e.g. the position sensor in an acceleration pedal or an electronic power steering system

- Stray magnetic field from a high-voltage power line in an EV or HEV is easily large enough to affect safety-critical systems such as the brake pedal

• Huge induction fields – such as in future charging stations of electric vehicles.

• Can result in an adverse impact on all on-board sensors in a car



Stray magnetic fields in pedals


Stray magnetic fields in EPS


Stray magnetic fields in EV and HEV

Position sensor with integrated stray

field immunity achieves high accuracy

performance even near high current

carrying cables

Magnetic position sensors

• Magnetic position sensing technology is more robust and reliable

than optical encoders and contacting potentiometers

- Immune to dirt, dust, grease, moisture, and vibration

- All conditions commonly found in industrial and automotive

applications



• However, the Achilles heel with magnetic position sensor

technology is

- Needs to be sensitive to a paired target magnetic field

- But also susceptible to unintended magnetic stray fields.



• The unintended stray fields can impair the accuracy of the

magnetic position sensor’s output

- Reducing the signal to noise ratio (SNR) to unacceptable levels within

the device

- A sensor sub-system could yield inaccurate results which could lead to

reduced system performance and safety issues


Traditional solution – shielding

• Magnetic shielding

- Expensive and takes up space

- Adds to cost and size of the sensor subsystem

- Can shunt away the target magnetic field that the sensor is supposed

to be measuring

- Bares the risk of becoming magnetized over time and its performance

could vary with temperature

- Finding the effective shielding solution takes time, effort and adds to

the development cost of the sensor sub-system


Traditional solution – magnet

• Use of a stronger magnet

- A magnet with a stronger field, and/or position the magnet closer to the

magnetic sensor

- A magnet with a stronger field, drives up the sensor cost to unacceptable

levels

• Narrow the gap between the magnet and sensor

- Tendency to drive up costs as tighter mechanical tolerances are

required


A unique integrated solution

• The best solution for addressing the stray field issue is to integrate

stray field immunity circuitry into the magnetic position sensor IC

- Makes the device immune to any and all magnetic stray fields

- Architectural design features as key enabler for preventing magnetic

stray fields from interfering with sensor IC performance


Integrated stray field immunity –

how does it work?

• The first principle involves the direction of measurement

• This principle ensures that the position sensor is only sensitive to

magnetic fields that are vertical to the IC surface

• The z-direction, which is the direction of the sensor magnet.

• Horizontal magnetic field influences x and y are not measured at all.


Operation principle normal conditions


(1) The magnet is centered over the Hall sensors (2) The magnetic field is sinusoidal, the magnetic field strength increases linearly with radius => 1 turn = 1 sine wave

(3) The differential amplifiers generate one sine and cosine signal with double amplitude

(4) The Cordic transforms sine and cosine into angle and magnitude

vertical magnetic field Bz characteristics

=

)cos(*ˆ

)sin(*ˆarctan

a

aa

a

a)tan(

)cos(*ˆ

)sin(*ˆa

a

a=

a

a


how does it work?

• The second principle is based on an integrated smart algorithm, and

a sensor solution consisting of four integrated hall sensors arranged

in a circle, that together automatically reject any stray fields in the Z-

direction

• This principle is based around a differential measurement



how does it work?

• The Hall sensors constantly measure the sensor magnets rotation

and any stray fields in the Z-direction

• Through subtraction of the opposite quadrant values, the z-direction

stray fields drop out of the calculation and the result is the target

magnet’s rotation value only.


External stray fields

(1) An external magnetic field is present near the sensors

(3) The offset does not influence the differential signal

(2) The external magnetic field generates an offset on all sensors.

(4) External magnetic fields do not influence the angle

=

)cos(*ˆ

)sin(*ˆarctan

a

aa

a

a)tan(

)cos(*ˆ

)sin(*ˆa

a

a=

a

a

Differential principle


H2

H4 Hall

0

[V]

Volts

1

2

3

4

Vdiff

1

Vdiff

2

external magnetic stray field

Vdiff 1 = Vdiff 2

Patent EP0916074

Adherence to Standards

• Naturally, these unique magnetic position sensors meet the latest

standards for immunity to magnetic fields and functional safety

- ISO11452-8 - immunity to magnetic fields

- ISO26262 - functional safety



solving problems

• Stray field immunity is integrated into the magnetic sensor device.

This enables a small form factor and it allows for a cost effective

solution as external components and expensive eternal shielding are

not needed In short:


- simple

- inexpensive

- small form factor

- safer usage

- unlimited protection from stray fields


benefits 1/2

• Design engineers can be confident that the sensor is unaffected by

stray fields by design

- Save costs of developing and implementing a verified shielding design

• No need for shielding materials

- Reduces cost, size and weight of the entire system



benefits 2/2

• Verified and documented evidence to support manufacturers’

compliance programs (ISO 26262 and ISO 11542-8)

• Number of vehicles carrying powerful magnetic fields is set to grow

- Built-in immunity to magnetic interference delivers reliability and safety

for a large variety of present and future automotive and industrial

applications

- Magnetic position sensors can withstand external magnetic stray fields

far above the limits required by the most demanding car manufacturers


Magnetic stray field analysis with

Helmholtz coil

Magnetic Formulas:

B = µo * H

e.g. 1000A/m -> 1,26 mT

Magnetic stray field analysis – position errorMagnetic position sensor IC without integrated stray field immunity

-3,5

-3

-2,5

-2

-1,5

-1

-0,5

0

0,5

1

1,5

2

2,5

3

3,5

1 V

; X

; 0 H

z; 0

A/m

1 V

; X

; 0 H

z; 2

50

0 A

/m

1 V

; X

; 50

Hz;

25

00

A/m

1 V

; X

; 20

0 H

z; 1

00

0 A

/m

1 V

; Z;

0 H

z; 0

A/m

1 V

; Z;

0 H

z; 2

50

0 A

/m

1 V

; Z;

50

Hz;

25

00

A/m

1 V

; Z;

20

0 H

z; 1

00

0 A

/m

2,5

V;

X; 0

Hz;

0 A

/m

2,5

V;

X; 0

Hz;

25

00

A/m

2,5

V; X

; 50

Hz;

25

00

A/m

2,5

V;

X; 2

00

Hz;

10

00

A/m

2,5

V;

Z; 0

Hz;

0 A

/m

2,5

V;

Z; 0

Hz;

25

00

A/m

2,5

V;

Z; 5

0 H

z; 2

50

0 A

/m

2,5

V;

Z; 2

00

Hz;

10

00

A/m

4 V

; X

; 0 H

z; 0

A/m

4 V

; X; 0

Hz;

25

00

A/m

4 V

; X

; 50

Hz;

25

00

A/m

4 V

; X

; 20

0 H

z; 1

00

0 A

/m

4 V

; Z;

0 H

z; 0

A/m

4 V

; Z;

0 H

z; 2

50

0 A

/m

4 V

; Z;

50

Hz;

25

00

A/m

4 V

; Z;

20

0 H

z; 1

00

0 A

/m

Po

siti

on

Err

or

[%V

dd

]

tolerance limits in Automotive

Magnetic stray field analysis – position errorMagnetic position sensor IC with integrated stray field immunity

-3,5

-3

-2,5

-2

-1,5

-1

-0,5

0

0,5

1

1,5

2

2,5

3

3,5

1 V

; X

; 0 H

z; 0

A/m

1 V

; X

; 0 H

z; 2

50

0 A

/m

1 V

; X

; 50

Hz;

25

00

A/m

1 V

; X

; 20

0 H

z; 1

00

0 A

/m

1 V

; Z;

0 H

z; 0

A/m

1 V

; Z;

0 H

z; 2

50

0 A

/m

1 V

; Z;

50

Hz;

25

00

A/m

1 V

; Z;

20

0 H

z; 1

00

0 A

/m

2,5

V;

X; 0

Hz;

0 A

/m

2,5

V;

X; 0

Hz;

25

00

A/m

2,5

V; X

; 50

Hz;

25

00

A/m

2,5

V;

X; 2

00

Hz;

10

00

A/m

2,5

V;

Z; 0

Hz;

0 A

/m

2,5

V;

Z; 0

Hz;

25

00

A/m

2,5

V;

Z; 5

0 H

z; 2

50

0 A

/m

2,5

V;

Z; 2

00

Hz;

10

00

A/m

4 V

; X

; 0 H

z; 0

A/m

4 V

; X; 0

Hz;

25

00

A/m

4 V

; X

; 50

Hz;

25

00

A/m

4 V

; X

; 20

0 H

z; 1

00

0 A

/m

4 V

; Z;

0 H

z; 0

A/m

4 V

; Z;

0 H

z; 2

50

0 A

/m

4 V

; Z;

50

Hz;

25

00

A/m

4 V

; Z;

20

0 H

z; 1

00

0 A

/m

Po

siti

on

Err

or

[%V

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]

tolerance limits in Automotive

Summary

• Magnetic position sensors with complete stray field immunity deliver reliability and safety for many present and future automotive and industrial applications.

- Integrated stray field immunity ensures resistance to static and dynamic parasitic stray magnetic fields

- Independent of how strong or how far away the field is, the differential principle ensures that the sensor output is unaffected by any magnetic stray fields

- High accuracy measurements even in the noisiest of EMI environments

- No additional unit costs associated with the integrated stray field cancelation features

- Reduces system costs, while maintaining system sensor performance

- Eliminates the need for magnetic shielding and the use of stronger target magnets, or requiring narrow air gaps between target magnets and sensor ICs


Thank you for your attention!

• Future-proof your product with integrated stray field immunity

position sensor solutions!

For further information:

Please visit: www.ams.com/Magnetic-Position-Sensors

Email to: [email protected]


http://www.ams.com/Magnetic-Position-Sensors

mailto:[email protected]

solving the challenge of stray field immunity for safety-critical applications

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