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SensoNor TPMS Sensors

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DOCUMENT CONTENTS:

1 INTRODUCTION........................................................................................................................2 2 PURPOSE OF NOTE.................................................................................................................2 3 ABBREVIATIONS AND TERMS....................................................................................................2 3.1 ABBREVIATIONS...........................................................................................................................................2 3.2 LIST OF TERMS............................................................................................................................................2 4 SENSOR DIE DESCRIPTION.......................................................................................................3 4.1 PRESSURE SENSING ELEMENT......................................................................................................................3 4.2 ACCELERATION SENSING ELEMENT...............................................................................................................3 5 DESIGN CONSIDERATIONS........................................................................................................4 5.1 PREVENTING FOREIGN SUBSTANCES FROM ENTERING THE PRESSURE SENSOR INLET HOLE...........................4 6 CONSIDERATIONS FOR PRODUCTION AND TEST..........................................................................4 6.1 ELECTROSTATIC FIELDS...............................................................................................................................4 6.2 ACCELERATION BEAM FRACTURE PROPERTIES..............................................................................................5 7 PRODUCT LIFETIME CONSIDERATIONS.......................................................................................5 7.1 PRESSURE MEASUREMENT MEDIA................................................................................................................5 7.2 PREVENTING PRESSURE INLET HOLE FROM BEING CLOGGED.........................................................................6 7.3 RIM CLEANING.............................................................................................................................................6 7.4 SURFACTANTS.............................................................................................................................................6 8 DOCUMENT HISTORY...............................................................................................................6 9 APPENDIX A............................................................................................................................7 9.1 EXAMPLE OF ARTIFICIALLY INCREASED PRESSURE SENSOR READOUTS DUE TO LIQUID (WATER) INSIDE THE

PRESSURE SENSOR INLET HOLE VOLUME................................................................................................................7

Stamp Date Sign Rev Reference Date Doc Prepared Checked Approved

050223 RoW 00 Archive10382 050223 HWK

050223 EK 01 EM 10769 050902 050223 RoW 02 EM APPLICATION NOTE

SensoNor TPMS Sensors

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1 INTRODUCTION

Subjects mentioned in this note apply to systems using SensoNor TPMS sensors in general.

2 PURPOSE OF NOTE

This application note describes known sensor related issues regarding design, production and test of systems using SensoNor TPMS sensors.

3 ABBREVIATIONS AND TERMS

3.1 ABBREVIATIONS

TPMS

Tyre Pressure Monitoring System

3.2 LIST OF TERMS

Sensor

Pressure sensing element

Acceleration sensing element

Accelerometer Accelerometer spring Accelerometer beam Accelerometer mass

Acceleration element cavity

The complete product.

The part of the sensor that performs the pressure measurement. The pressure sensor is made in a glass – silicon – glass structure and holds a pressure sensitive membrane with buried piezo resistive elements.

The part of the sensor that performs the acceleration measurement. The acceleration sensing element is made in a glass – silicon – glass structure and holds an acceleration sensitive spring with surface piezo resistive elements.

Equal to the acceleration sensing element.

The part of the accelerometer that holds the piezo resistive elements, deflecting when the accelerometer is exposed to acceleration.

Equal to the accelerometer spring.

The part of the accelerometer that produces the deflecting force (momentum) on the spring. The accelerometer mass moves freely in the accelerometer cavity.

The volume around the accelerometer. Etched out in silicon and glass.

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4 SENSOR DIE DESCRIPTION

The sensor die consists of a pressure sensing element and an acceleration sensing element (optional). See Figure 1 for a sensor die illustration.

Figure 1 Sensor die illustration.

4.1 PRESSURE SENSING ELEMENT

The pressure sensing element is a triple stack die (glass - silicon - glass). The absolute pressure reference is given by a vacuum chamber implemented in the top glass. Buried connectors and resistors are placed in the silicon layer, giving the sensors excellent media compatibility. See Figure 2 for a cross section of the pressure sensing element.

Figure 2 Pressure sensing element illustration (not in scale).

4.2 ACCELERATION SENSING ELEMENT

The acceleration sensing element is a cantilever beam, sensitive to acceleration in the Z-direction. From a

mechanical point of view, the device can be considered as a spring - mass system. Piezo resistive elements are placed in the spring area. See Figure 3 for an acceleration sensing element illustration.

Figure 3 Acceleration sensing element illustration (not in scale).

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5 DESIGN CONSIDERATIONS

TPMS designers are advised to consider the direction of pressure sensor assembly in the wheel to prevent the occurrence of the below mentioned situations.

5.1 PREVENTING FOREIGN SUBSTANCES FROM ENTERING THE PRESSURE SENSOR INLET HOLE

Care should be taken to prevent the pressure inlet hole volume from being completely or partly filled with foreign substances.

The presence of foreign substances could lead to the following erroneous situations:

• Erroneous pressure readings. Due to centripetal forces in a TPMS application foreign substances will

act as an added mass on the pressure sensor membrane, causing erroneous pressure readings. Refer to appendix A (chapter 9) for an example.

• Erroneous pressure readings. Adhered foreign substances on the pressure sensor membrane can

change the pressure sensor qualities, like sensitivity and/or linearity, resulting in changed pressure dependant behaviour and erroneous pressure readings. This point refers especially to substances in solid state in direct contact with the membrane.

• Membrane fracture. Phase transitions of foreign substances introduce the risk for a membrane fracture.

Such damage is permanently destructive. The risk especially applies if the pressure inlet hole volume is completely filled with liquid (for example water) when the temperature goes below the freezing point.

6 CONSIDERATIONS FOR PRODUCTION AND TEST

6.1 ELECTROSTATIC FIELDS

The accelerometer of the sensor is sensitive to electrostatic fields and it is recommended to consider the need to prevent surface fields to build up on the package, as well as shielding of components during test, to avoid erroneous test results with a possible consequence of yield loss.

The lead frame itself has a shielding effect (grounded), making the surface above the lead frame the most sensitive for electrostatic fields.

Package type 1 and 7 sensors have the sensor and ASIC dies on the upper side of the lead frame and therefore the care applies especially for these sensors.

Package type 6 sensors have the sensor and ASIC dies on the lower side of the lead frame (making them more robust against electrostatics fields affecting the accelerometer). Figure 4 shows a schematic view of the

package type 6 sensor internals as an example, showing the placements of the dies. For type 1 and 7 sensors, the major difference is that the legs are bent the other way, leaving the dies on the other side of the lead frame.

Sensor die

ASIC die

Figure 4 Schematic view of the package type 6 internals.

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6.2 ACCELERATION BEAM FRACTURE PROPERTIES

The accelerometer, being essentially a mass on a beam, is susceptible to oscillations. High amplitudes can be reached if excited in the range of the resonance frequency.

Special care must be taken to prevent exposure to high loads in the lowest resonance frequency range (6 - 7 kHz). This care applies especially in Z-direction of the device. See Figure 1 for orientation.

Single crystalline silicon is perfectly elastic with very high fracture strength. Therefore, no energy is absorbed in plastic deformations and fracture will occur if load limit is exceeded. Such damage is fatal and permanently destructive.

Recent product improvements have increased the accelerometer robustness. However, in spite of these improvements, special care must still be taken.

Figure 5 shows a plot of the typical transfer-function for dynamic load-limits in Z-direction of the device.

High static loads (> 25 000 g) can also result in beam fracture, and such damage is permanently destructive in the similar way as for dynamic loads.

Experience shows that this care for the accelerometer especially should be taken for production and test. (The accelerometer does not normally see strain near load limits in application.)

Figure 5 Typical transfer-function for dynamic load-limits of the accelerometer (in Z-direction).

7 PRODUCT LIFETIME CONSIDERATIONS

7.1 PRESSURE MEASUREMENT MEDIA

SensoNor TPMS sensors are designed for air pressure measurements in TPMS applications. Experiments have shown that a media with high ability to diffuse through solid glass (helium or hydrogen) can change the absolute value of the pressure reference in the vacuum chamber, thus leading to erroneous pressure readings.

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7.2 PREVENTING PRESSURE INLET HOLE FROM BEING CLOGGED

Special care must be taken to make sure the pressure inlet hole is left open at all time, during the lifetime of the product, not clogged with any kind of material. A clogged inlet hole can affect the measurement performance and make the sensor insensitive to pressure.

7.3 RIM CLEANING

These considerations apply when the tyre is separated (removed) from the rim.

Special care must be taken during rim cleaning with pressure agents. Jet water washing equipment may

operate with internal pressures of 10000 kPa, and SensoNor TPMS sensors typically withstand burst pressures up to 2000 kPa. Dependent on the distance from the nozzle, the nozzle design and whether the water is injected directly into the hole or from an angle, this may damage the membrane. Such damage is permanently destructive.

Using a pressure washer may lead to trapped water inside the pressure sensor cavity. See chapter 5.1 and 9 for concerns.

Cleaning agents are also likely to release particles, like sand particles, and there is a chance that the inlet hole becomes clogged from particles and/or cleaning agent during the cleaning processes. This might affect the measurement performance and reliability of the device. See also chapter 7.2.

7.4 SURFACTANTS

Surfactants, like for example tyre mounting soaps as mentioned in chapter 7.3, can result in liquids passing easier into the pressure sensor inlet hole volume. Surfactants have the ability to partly remove the surface

tension for certain liquids, e.g. water. This creates also an additional risk for liquid penetration into the moulded package, which could cause reliability issues.

System developers are advised to consider the risk for tyre mounting soaps to come in contact with the sensor during TPMS lifetime and initiate preventive actions if needed.

8 DOCUMENT HISTORY

Table 1 Document history.

REV. PARAGRAPH 0

1 3.2 1 4 1 6.1 1 Figure 4

1 6.2 1 Figure 5

1 9.1

DESCRIPTION

This is the first issue of DOK230.

New chapter. Updated chapter. Updated chapter. New figure.

Updated chapter. New figure.

Updated chapter.

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9 APPENDIX A

9.1 EXAMPLE OF ARTIFICIALLY INCREASED PRESSURE SENSOR READOUTS DUE TO LIQUID (WATER)

INSIDE THE PRESSURE SENSOR INLET HOLE VOLUME

This is an example with a liquid (water) collected inside the pressure sensor inlet hole volume, describing how this can affect the pressure sensor readouts. See Figure 6 for an illustration.

Figure 6 Pressure sensing element with liquid in the pressure inlet hole volume.

The situation can result in erroneous pressure readings if the pressure inlet hole is directed against the centre of rotation in the wheel. The absolute value of the added force (applied to the membrane) is proportional to the second order of the wheel speed.

The added pressure is given by the formula:

pliquid= ρliquid×hliquid×asensor (1)

Where:

pliquid is the added pressure due to the liquid in the pressure inlet hole volume

ρliquid is the density of the liquid

hliquid is the height of the liquid column above the membrane surface asensor is the centripetal acceleration at the sensor location in the wheel

The centripetal acceleration seen by the sensor will never be the same as on the tyre surface. The centripetal acceleration at the sensor location is given by the formula:

2 vcarr

asensor=×sensor (2)

rwheelrwheel

Where:

asensor is the centripetal acceleration at the sensor location

vcar is the car speed rwheel is the wheel radius

rsensor is the distance from the wheel centre to the sensor location on the rim (approximately equal to the rim radius)

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Figure 7 illustrates a typical TPMS sensor mounting in a wheel, and vectors for rsensor and rwheel.

Figure 7 Typical TPMS sensor mounting in a wheel.

Centripetal accelerations and added pressures seen by the sensor at different car speeds can be calculated from the formulas (2) and (1) respectively. See Table 2 for a worst case scenario for a passenger car application. The value of the added pressure (applied to the membrane from the column of water) is proportional to the second order of the wheel speed.

Table 2 Added pressure readings (Pliquid) as a result of water in the pressure sensor cavity volume (worst case

scenario).

Car speed [km/h]

Centripetal acceleration seen by the

Pliquid [kPa]

sensor in the wheel [g]

40 29.5 0.87 80 118 3.47 120 266 7.82 ... … … 240 1060g 31.3 280 1450g 42.6

Calculations in Table 2 apply given that:

• • • • •

The pressure inlet hole is directed against centre of rotation.

The TPMS sensor is placed on outer side of the rim (see illustration in Figure 7) The rim size is 19-inch (extreme case) The tyre size is 265/30x19 (extreme case)

The pressure sensor cavity and inlet hole volume is completely filled with a total column of 3.0 mm water (extreme case).