Developments in Precision Power Train Sensors

109 Hitachi Review Vol. 63 (2014), No. 2 Developments in Precision Power Train Sensors

Keiji Hanzawa OVERVIEW: The fuel economy and emissions performance demands on Shinobu Tashiro vehicle power trains are becoming more stringent for reasons relating Hiroaki Hoshika to global environmental protection and the rising price of oil. There has also been a change in thinking on the measurement of emissions and Masahiro Matsumoto fuel economy toward allowing for conditions where the temperature and humidity are closer to real driving conditions. Other changes include the electrifi cation of power trains, such as in hybrid vehicles, and improvements in the running effi ciency of internal combustion engines that result in more frequent use of engine operating modes in which sensor operation is more diffi cult, such as the Atkinson cycle. Hitachi Automotive Systems, Ltd. is supporting ongoing progress in power train control by making further improvements in sensor accuracy.

INTRODUCTION Automotive power trains have made rapid progress HITACHI supplies customers around the world with on electriﬁ cation and reducing fuel consumption in a variety of systems for the driving, cornering, and recent years. This article describes advances in the braking of vehicles. By using a range of different performance of the sensors used in these power trains, sensors to determine conditions in the power train, looking at micro electromechanical system (MEMS) vehicle body movements, and what is happening air ﬂ ow sensors that reduce the error in intake pulsation, around the vehicle, these systems ensure a driving the integration of air intake relative humidity sensors experience that is safe and comfortable, and that is and pressure sensors, and the adoption of digital signal conscious of the global environment (see Fig. 1). output for sensors with network connectivity.

Air flow MAP sensor T-MAP sensor Medium/high Multi-function sensor air flow sensor pressure sensor Brake booster pressure sensor Speed sensor

Boost pressure sensor

Relative humidity sensor

Exhaust temperature sensor

Crank angle/ cam angle sensor Rotation angle sensor DPS Combined sensor Absolute velocity sensor Stroke sensor

MAP: manifold absolute pressure T-MAP: temperature-MAP DPS: differential pressure sensor Fig. 1—Hitachi Power Train Sensors. Modern vehicle power trains incorporate a variety of sensors that are used by control systems to deliver maximum environmental performance. Improvements in sensor accuracy lead directly to better control system performance.

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MEMS AIR FLOW SENSORS ratio. A consequence of this is that reverse fl ow (air Since commercializing its first hot-wire air fl ow out from the engine) is very common. fl ow sensor in 1981, Hitachi has supplied a total of In Miller type Atkinson cycle engines, for example, 200 million air fl ow sensors of various types over which use valve timing control (VTC) to signifi cantly numerous generations. These have included the delay the intake valve close (IVC) timing angle, most MEMS air fl ow sensors(1) introduced in 2005 that use of the in-drawn air is expelled again by the piston, a silicon diaphragm detection element to measure resulting in reverse fl ow and stronger pulsation. Also, air fl ow in both directions. The second generation when an engine’s EGR ratio is raised, assuming the of these sensors currently in production operate the same total volume of air-fuel mixture is supplied to the detector at a higher temperature for better anti-fouling cylinder, the amount of air is reduced by the amount and use 5-V drive to reduce power consumption. The of exhaust gas used. For the sensors that measure the third generation of sensors are currently being set for air fl ow rate, this means that even if the size of the production. They are designed for lower cost and to pulsations remains the same, they become larger in achieve high precision when operating at a high level relative terms because of the reduction in mean air of intake pulsation. fl ow (see Fig. 2). Hitachi divides the pulsation amplitude by the Sensors for Engines with High Level of Intake mean forward air fl ow to quantify it as the pulsation Pulsation amplitude ratio. If the pulsation amplitude ratio The functions and performance sought in MEMS exceeds 200%, this indicates that there is a period in air fl ow sensors depend to a large extent on advances which the air fl ow is fully reversed. As improvements in the engines in which they are used. are made in engine performance, this pulsation When engines used the Otto cycle, the intake air amplitude ratio is rising with each new vehicle fl ow remained unidirectional (into the engine) over generation (see Fig. 3). most operating conditions. Modern engines, however, As advances in engines and engine control result in have features that include reduced pumping losses a strongly pulsing fl ow in the vicinity of the air fl ow through the use of a high exhaust gas recirculation sensors, techniques for reducing error are important (EGR) ratio, and use of the Miller cycle (a variation even in the case of MEMS air fl ow sensors that can on the Atkinson cycle) to provide a high expansion cope with bidirectional fl ow.

Mechanism for large air flow pulsation in Atkinson cycle (or Miller cycle) engines Forward flow Reverse flow

Indrawn air Intake valve still is forced open midway back out. through compression stroke.

180° CA 90° CA 90° CA Movement of piston Movement Fig. 2—Flow Pulsation Start of End of Midway through End of intake stroke intake stroke compression stroke compression stroke Mechanism. As Miller type Atkinson cycle Mechanism whereby EGR causes relative amplitude of pulsation to increase engines delay the intake valve close timing until the middle EGR valve EGR Low EGR ratio component High EGR ratio of the compression stroke, a A high EGR ratio Exhaust Exhaust means less intake air. large amount of air is forced back out the inlet, resulting Inlet air Lower mean quantity component Air of intake air in large pulsations. If a large Air EGR is used, the increased Larger relative amplitude of pulsation use of recirculated exhaust gas EGR reduces the amount of air taken in, making the pulsations larger EGR: exhaust gas recirculation CA: crank angle relative to the intake air.

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Large pulsations 300 Small pulsations

200

100 Mean Pulsation amplitude ratio (%) = flow A Amplitude of pulsation B pulsation B Air flow (kg/h) Air flow Forward flow Amplitude of Mean flow A 0 Reverse flow −50 0 0.02 0.04 0.06 0.08 0.10 Time (s) Fig. 3—Flow Pulsation MEMS sensor Mechanism. HCCI throttle-less engine 1,000% Hot wire sensor For quantitative measurements, Hitachi deﬁ nes the size of High degree of downsizing Large EGR ratio (low- pulsations in terms of their pressure EGR system) amplitude as a proportion of Supercharged downsized engine Size of engine pulsations mean air ﬂ ow. This pulsation 500% Atkinson cycle amplitude ratio is increasing as

Pulsation amplitude ratio Direct injection engines become more advanced, PFI and the accuracy of air ﬂ ow Time measurement under pulsation 1980 1990 2000 2010 2020 2030 conditions is closely related to MEMS: micro electromechanical system HCCI: homogeneous charge compression ignition PFI: port fuel injection advances in engine control.

Techniques for Reducing Air Flow and reverse directions, linearity processing in which Measurement Error Due to Pulsation the fl ow rate signal is converted to its physical amount The error that occurs when inlet pulsations are before applying correction, and precise temperature large is due to a number of causes. The output signal compensation for the pulsation characteristics. of the air fl ow sensor is required to always represent By optimizing the length of the secondary channel the mean forward air fl ow. Accordingly, however (the bypass through the sensor) and the inlet for accurate the sensor may be at measuring the forward reverse fl ow (outlet) to stabilize the air fl ow in the air fl ow, the overall sensor error will be large if there is detector, Hitachi has succeeded in providing suffi cient a measurement error in the reverse fl ow component or accuracy for trouble-free engine operation even under if this is not compensated for appropriately. Pulsation high levels of pulsation, such as pulsation amplitude error has a variety of causes which are always ratios approaching 1,000% at which the sensor signal interrelated, the main ones being the fl ow detector was unable to be used in the past (see Fig. 4). having a response that is too slow, non-linearity error (due to non-linear characteristics), and turbulence MULTI-FUNCTION AIR FLOW SENSORS error (when turbulence in the air fl ow sensor itself While the predominant confi guration has long causes suction when the fl ow reverses). been to connect different single-function sensors to While this has been dealt with in the past using the engine control unit (ECU), there is also growing correction measures such as conventional signal fi lter demand for combining multiple sensors in a single processing or by the shape of the bypass channel in the multi-function package and performing mutual air fl ow sensor, these provide inadequate correction for correction to improve their overall accuracy. In air fl ows with a very high level of pulsation. 2011, Hitachi became the fi rst supplier to commence To solve this problem, Hitachi Automotive production of a multi-function air fl ow sensor that Systems, Ltd. and Hitachi Research Laboratory also included a relative humidity sensor and pressure jointly developed an application-specifi c integrated sensor (see Fig. 5). circuit (ASIC) digital signal processor (DSP) Hitachi is also working on the development of a specifically for use in air flow sensors. The air new generation of devices that use a digital interface fl ow sensor signifi cantly reduces pulsation error by to improve the accuracy of the sensor signal passed to utilizing the high-speed computing capabilities of the the ECU, incorporate network connectivity to reduce DSP to process the signal internally. This includes wire harness requirements, and provide more fl exible performing separate signal correction for the forward onboard diagnostics (OBD).

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Large pulsation error Significant reduction Error range that does not in pulsation error impede engine control

0 0 Pulsation error

Can only operate under Can operate under a wide range of pulsation levels a limited range of pulsation levels Hot wire sensor MEMS sensor

0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800 Pulsation ratio (%) Pulsation ratio (%)

Hot wire sensor Digital ASIC (DSP chip) MEMS sensor Heater Analog ASIC current Heater Heater

Hot wire drive circuit drive DSP Bridge Analog Frequency Flow rate circuit Amplifier correction Element modulation circuit temperature timer Flow Air Sensing element Signal temperature Frequency rate A/D correction

Cold wire D/A converter modulation converter DSP circuit Inlet temperature sensor Inlet Digital Air temperature adjustment temperature Program Parameter Communication sensor RAM ROM ROM circuit

ASIC: application-specifi c integrated circuit DSP: digital signal processor A/D: analog/digital D/A: digital/analog RAM: random-access memory ROM: read-only memory Fig. 4—Reducing Error in Air Flow Measurement Due to Pulsation. As there is a limit to how accurately a pulsing fl ow can be measured using analog circuits, MEMS air fl ow sensors include a special- purpose DSP for internal digital signal processing to ensure suffi cient accuracy to prevent any loss of engine control performance even when there is a very high level of fl ow pulsation.

Use of Multiple Sensors to Improve Control • Mass air flow (air flow sensor) Accuracy • Relative humidity of air Temperature at relative humidity measurement point In addition to the mass air flow traditionally (temperature and humidity sensor) • Inlet air temperature (air temperature sensor) measured by sensors, the physical properties of the • Atmospheric pressure in inlet duct (Atmospheric pressure sensor) air taken in by the engine also include such things as moisture content (water vapor) and the pressure at the Semiconductor chip measurement point. Changes in these properties will Atmospheric humidity sensor pressure sensor cause an error in the mass air fl ow measured by the Humidity Circuit detector sensor that can be as high as several percent at low board Temperature sensor fl ow rates. This is because both hot wire and MEMS A/D converter sensors work on the principle of detecting the transfer Hot wire Semiconductor (air flow sensor) Memory of heat by the air, and therefore are infl uenced by chip temperature and humidity Cold film Digital moisture-induced changes in the physical properties (air flow sensor) sensor circuit of the air. Inlet air temperature sensor Also, engine factors such as ignition performance, EGR limit, and fuel temperature are affected by the Fig. 5—Multi-function Air Flow Sensor. amount of moisture in the intake air, and these result In addition to its air fl ow sensor function, this sensor also in large changes in outcomes such as fuel consumption measures the relative humidity and the pressure in the inlet and the amount of pollutants produced. Accordingly, a duct. This allows the engine control to adapt to changes in weather, driving, and other conditions, and means that the key factor in improving environmental performance is engine control margins previously included to allow for changes to control the engine, along with exhaust gas treatment in humidity can be allocated instead to fuel economy or other and other systems, in accordance with changes in the improvements. moisture content of the intake air.

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While the existence of these errors was known in Accuracy Improvements in Sensor Signal the past, the regulatory levels were not suffi ciently Transmission tight for them to be a problem. With the market’s Improving sensor accuracy is pointless if the emphasis on low fuel consumption and steadily more accuracy is degraded during signal transmission. stringent regulations, however, semiconductor-chip- Traditionally, most sensor signals have either been based relative humidity sensors have been added to analog voltage outputs or frequency-modulated outputs multi-function air fl ow sensors to correct for the error that worked by varying a pulse frequency. However, caused by the moisture in intake air. this results in degradation of the fi nal accuracy of the Relative humidity sensors are capacitive sensors. sensor signal due to errors that occur during signal They consist of a capacitor with a dielectric made of transmission, such as fluctuations in the ground moisture-sensitive polymer with a permittivity that potential or the temperature characteristics of the varies as it absorbs water molecules from the air, the modulation reference frequency, or due to conversion extent of which can be detected from the variation error in the analog-to-digital converter (ADC) in the in the sensor’s capacitance. These relative humidity ECU that converts the analog signal to a digital value. sensors also incorporate a highly accurate temperature As a result, high-precision sensors are increasingly sensor. The absolute humidity of the air can then converting signals to digital form to avoid these be calculated from this temperature and the relative errors. Single-edge nibble transmission (SENT) humidity. communications is one example of a method for As the relative humidity varies with pressure, it converting sensor signals to digital form. It can send is possible to obtain a very accurate measurement two sensor signal channels over a single wire and can of the moisture being taken into the engine by using include use of cyclic redundancy check (CRC) error information from the built-in pressure sensor to correct detection(2). for this effect. As it is also possible to multiplex multiple signal The sensor also includes a microprocessor to channels, the technique is suitable for applications provide the fl exibility to work with various different like the multi-function air fl ow sensor that require the engine control systems, allowing output of both the output of a number of sensor signals. raw uncorrected sensor signals and the corrected The SENT protocol provides one-way (sensor to values (see Fig. 6). ECU) communications and works by converting a

Network bus MEMS air flow sensor Microprocessor (computational processing)

Air flow Convert air flow Generate transmit Network to physical value data frame. transceiver ASIC Inlet temperature Convert inlet air temperature to physical value. Frequency modulation Frequency-modulated output Inlet temperature sensor Correct inlet Output selector air flow Digital output Digital signal output Inlet temperature sensor Relative humidity Convert relative humidity to physical value. Calculate absolute moisture content. Temperature Convert temperature to physical value. Pressure sensor Pressure Convert pressure to physical value.

Fig. 6—Use of Multiple Sensors to Improve Accuracy (Multi-function Air Flow Sensor). Use of multi-function sensors can improve overall signal accuracy because it means that information from certain sensors can be used to correct other sensor signals. This example shows how the relative humidity measurement can be corrected using pressure to calculate the absolute humidity. This can then be used to correct the inlet ﬂ ow rate.

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SENT standard (SAE-J2716) Signal 1 Signal 2 12 bits 12 bits pulse (optional) Status and

4 bits 4 bits 4 bits 4 bits 4 bits 4 bits pulse Pause communication CRC/checksum 5 V Sync/adjustment

56-tick fixed length (3 μs clock tick)

0 V

Message: 462 μs to 810 μs (depends on value of data being sent)

LIN network protocol LIN frame

Header Response ECU Response space LIN network

Break Sync PID Data 1 Data 2 Data N Checksum field field field Sensor Sensor Sensor 1 2 n Inter-byte space Inter-byte space

SENT: single-edge nibble transmission CRC: cyclic redundancy check LIN: local interconnect network Sync: synchronization PID: protected identifi er ECU: engine control unit Fig. 7—Digitization of Sensor Output Signal. The ability of SENT to transmit two signals per message with a cycle time of less than 1 ms makes it a suitable protocol for digital sensor signals. Already used for angle sensors, it is anticipated that its simplicity will see it used more widely in future. LIN, meanwhile, is a low-speed network that is already widely used in vehicle electronics, and it is starting to be used in power trains because of its fl exibility and its ability to provide bidirectional communications over a single wire. conventional analog signal to digital form. In addition happening around the vehicle include improving the to SENT, the sensors currently under development safety of vehicles and their power trains, resource will also support the local interconnect network (LIN) saving, and global environmental improvement. protocol(3) (see Fig. 7). Hitachi is working on sensors that combine higher accuracy with fl exibility, such as by taking advantage of the ability to communicate REFERENCES in both directions to improve accuracy by only (1) M. Kimata et al., “The latest trend in sensors for automobiles,” activating those sensors that are required at particular CMC Publishing (Feb. 2009) in Japanese. time, as specified by a request from the ECU. (2) “SENT–Single Edge Nibble Transmission for Automotive Because converting sensor signals to digital form and Applications,” SAE-J2716 (2006). providing network capabilities improves fl exibility (3) “LIN Specifi cation Package Revision 2.1,” LIN Consortium (2006). while reducing the loss of accuracy in the signal transmission process, this is the best type of interface for precision sensors.

CONCLUSIONS This article has described advances in the accuracy of automotive sensors, including improvements in the measurement accuracy of MEMS air ﬂ ow sensors under conditions of high pulsation, the integration of multi-function sensors, and the use of digital signals for sensor output. The beneﬁ ts of using precise, high-performance sensors to determine conditions such as those in the power train, vehicle body movements, or what is

- 59 - 115 Developments in Precision Power Train Sensors ABOUT THE AUTHORS

Keiji Hanzawa Shinobu Tashiro Electronic Device Design Division, Powertrain Electronic Device Design Division, Powertrain & Electronic Control Systems Division, Hitachi & Electronic Control Systems Division, Hitachi Automotive Systems, Ltd. He is currently engaged in Automotive Systems, Ltd. He is currently engaged in the development of sensors and sensing systems. Mr. the development of sensors and sensing systems. Hanzawa is a member of The Society of Automotive Engineers of Japan (JSAE).

Hiroaki Hoshika Masahiro Matsumoto Electronic Device Design Division, Powertrain Department of Green Mobility Research, Hitachi & Electronic Control Systems Division, Hitachi Research Laboratory, Hitachi, Ltd. He is currently Automotive Systems, Ltd. He is currently engaged in engaged in the development of sensors and sensing the development of sensors and sensing systems. Mr. systems. Mr. Matsumoto is a member of The Society Hoshika is a member of The Society of Automotive of Instrument and Control Engineers (SICE). Engineers of Japan (JSAE).

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