Under the Hood - Tried and true design concepts drive Prius - Munro

D E S I G N I N S P I R A T I O N F O R T H E C R E A T O R S O F T E C H N O L O G Y

May 14, 2007

Presented by: and

From the Prius to the BlackBerry,a look inside today’s hottest systems

answers the burning question:

From the Prius to the BlackBerry,a look inside today’s hottest systems

answers the burning question:

HOW’D THEY DO THAT?HOW’D THEY DO THAT?

A S U P P L E M E N T T O E E T I M E S

Electronic Engineering Times, TechOnline 3

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MAY 14, 2007

CONTENTCover feature

6

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ONLINEView the related OnDemand seminar,“Toyota Prius exposed,” discussing the design of theskid-control module, airbag controller and the display system, at www.techonline.com/underthehood,search article ID: 199200803

Tried-and-truedesign conceptsdrive PriusBy Al Steier and Rick DeMeis

From energy efficiency tonavigation and entertainmentto safety, the Toyota Priushybrid represents the cuttingedge of automobile design. So,naturally, we had to buy oneand tear it to pieces.What wefound was at once expectedand surprising. Use this coverfeature to find out howto blend proven designtechniques and componentswith the latest in networkcontrol, power management,analog and digital design, andconsumer electronics.

under the hood: FEATURE

6 Electronic Engineering Times, TechOnline | May 14, 20076 Electronic Engineering Times, TechOnline | May 14, 2007

Upon tearing apart a Toyota Prius—spankingnew from the dealer—what strikes an engi-neer’s eye is that this first parallel drive train

gas/electric hybrid car is more like other modernvehicles than not. Ignore the large nickel-metalhydride battery tucked out of sight behind the rearseat; filter out the associated starter-motor-like

By Al Steierand Rick DeMeis

Tried-and-true designconcepts drive Prius

under the hood: FEATURE

Engine control module

Dashboarddisplay unit

Navigation control unit

Airbag control module

Skid-control module

Inverter/converter

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While we were only able to lookat a portion of the overall Priuselectronics content, the tear-down yielded some pretty cleartop-level conclusions.

First, the Toyota Prius is a fantas-tic piece of engineering designedto reduce the financial burden atthe pump and to recover energyefficiently. By combining a smallgas engine with an electric drivetrain and regenerative braking,Toyota has successfully targetedconsumers looking to lowertheir “carbon footprint.”

From a technical perspective, wefound that cautious design wasthe engineering principle, particu-larly for mission- and safety-criti-cal subsystems.While componentselection, electronics pack-aging and device complexityall became more state-of-the-art as modules for info-tainment were opened up,what’s known to work(including 10-year-oldmicroprocessor designs)gets a long lifetime in the Toyotadesign environment. Embeddedmemory likewise is preferred forcontrol systems; discrete memo-ry packages appeared in thetouchscreen interface and naviga-tion modules alone.

Finally, the teardown broughtinto stark relief the highly dis-tributed nature of auto electron-ics. Dozens of MPUs are sprin-kled wherever the local needs ofelectronics demand, communicat-ing over several buses and coop-erating to affect vehicle control,power train management, userinterface and safety functions.

—David Carey, Portelligent

IN BRIEFheavy-gage high-voltage orange cables runningalong the frame to carry current to and from theelectric traction motor for power and regenera-tive braking. Now you have a system that seems“conventional,” despite the optimized hybrid-power-train architecture.

The Prius overall appears to follow the Toyotabuild philosophy: standard modules, bits andpieces—from electronics to doors and other com-ponents—that readily fit in place, enabling thesame vehicle model to be built to the same quali-ty standard anywhere in the world.

Like any modern vehicle, the Prius packsnumerous electronic subsystems. Six, however,stood out: the inverter/converter, the user-inter-face/dash module, the engine control module,the navigation/display system, the airbag controlmodule and the anti-skid system. The functional-ity, placement, electronic content and packagingof those subsystems represent the cutting edgenot only in terms of technology but also in thecontext of reliability and power management.

ArchitectureThe Prius uses a parallel gasoline/electric powertrain, which means the car can run on electricpower alone at low speeds (and shut off the gasengine at stops), accounting for its high citymileage. The architecture incorporates twomotor generators mounted within the transaxle.One MG starts the car and recharges the high-voltage nickel metal hydride battery. The otherMG boosts the internal-combustion engine’s out-put and, conversely, regeneratively brakes thecar to reclaim energy back into the battery andoptimize brake effectiveness. Under heavy load,output from the first motor generator is sent tothe second, adding to traction power.

When running, the Prius drives similarly to aconventional car but with better low-speed accel-eration from the MGs’ torque.

Inverter/converterObviously, the module of most interest is the onecritical in making a hybrid car unique: the invert-er/converter. This unit—placed atop thetransaxle—is the electrical-system “switchingyard,” providing voltage control and switchingdc to dc, dc to ac (for the motor drive) and ac todc (to charge the battery with the gasolineengine and during regenerative braking).

The system voltage values that theinverter/converter must handle include 201 Vfrom the NiMH battery and 500 Vac system maxfor the two motor generators in the transaxle.The circuit board is pressed into a substantial

VIDEOTo view a video of this tear-down, compressing seven hoursinto one minute of streamingvideo, go to www.techonline.com/underthehood, searcharticle ID: 199201709

ALSO ONLINEAutomotive DesignLineeditor Rick DeMeis tookthe Toyota Prius fora real-world drive to seehow it performs—and how'green' it really is. He alsocompared it with theToyota Highlander hybrid.Test-drive his findings atwww.automotivedesignline.com, search articleID: 199200607

Also, to view teardownsof the Prius' skid-controlmodule, airbag controllerand dashboard displaysystem, go to www.techonline.com/underthehood

under the hood: FEATURE www.eetimes.com • www.techonline.com

10 Electronic Engineering Times, TechOnline | May 14, 2007

aluminum casting that acts as a heat sink,and the assembly is closed by a stamped

steel cover. Within the module arerobust relays and bus bars.

Engine control module The ECM looks and functions much

like those in conventional cars when pro-viding gasoline engine control (such as mod-ulating the timing and width of fuel injector

pulses) and monitoring engine and emissionssensors (including cam, crank and O2 sen-sors). But in a hybrid, it must also interfacewith other modules, such as theinverter/converter, to allow running on all-electric power at low speeds and to use thegas engine to recharge the battery pack.

Placed under the instrument panel cover,behind the glove box (and attached on acommon bracket with the climate control

Toyota’s Prius in many respects canbe considered to have two enginecontrollers: one for the traditional,

1.5-liter gasoline engine and anotherfor the electric motors used topower the car alternatively.

Starting with the petrol-powered

side of the equation, the engine con-trol module (ECM) must constantlymonitor a number of input sensors toassess the state of the engine and itsown primary inputs of fuel, air andfire.Airflow monitoring occurs by wayof an optical chopper sensor whose

output frequency is proportional toflow rate.An engineered vortex in theintake plenum creates a wake inwhich a mirrored vane flutters fasteror slower depending on airflow, withthe vane forming the mirrored reflec-tor of the chopper.An oxy-

Controllers keep dual motors humming

>>12



module), the engine control module isdifficult to access. Such positioning,

however, gets the unit out of theharsh under-hood environment,

with its higher temperatures, poten-tial moisture and higher electromag-

netic interference. (In addition, since themodule is difficult to reach, mechanics maybe less inclined to replace this costly compo-nent first when troubleshooting problems.)

Center-stack displayThe touchscreen at the top of the center stackis one of two key driver interfaces, alongwith the interface/dash module. In additionto providing navigation information andinput to navigation functions, the display

allows control of the audio system and cli-mate control, as well as updating the driveron the status of the hybrid system and inwhich mode it is running. The latter featurefascinates new hybrid operators—sometimesto the point of distraction—as they try tosqueeze out maximum mileage and trackwhich mode they are in by watching the cur-rent flows on the schematic.

The navigation system is connected to aDVD player under the driver’s seat. The datastorage device holds nav system map datathat is read in conjunction with the real-timeGPS satellite location to yield the guidanceinformation displayed on the screen. Thelocation of the GPS antenna is not obvious; itmay be located behind the touchscreen mod-

gen sensor thatmonitors for proper air/fuel mixtureis used as the input to detect eitherrich or lean conditions. Crankshaftand camshaft position, vehicle speed,throttle position, engine/intake-airtemperature, knock detect and otherengine conditions are among the addi-tional inputs to the ECM.

Output functions of the ECM areprimarily used to affect airflow, fuelinjector delivery, intake valve closureangle and spark timing as a means toclose the control loop in the engine,maintain optimal power delivery andminimize emissions. Injector solenoidsare pulse-width-modulated to controlfuel delivery volume and timing.Separately, the spark timing is drivento control detonation precisely.Ignition timing is retarded when thepiezoelectric knock sensor input indi-cates pre-detonation. Further efficien-cy refinement is achieved by extend-ing the intake valve opening (AtkinsonCycle operation) to reduce displace-ment effectively, since the intakevalves remain open partway into thecompression stroke.

The hybrid vehicle engine controlunit (HVECU) manages control of theelectrical drive plant. Heavy communi-cation with the ECM coordinates therelative contributions of gas power,electric power or, in many cases, thecombined efforts of the two systemsto provide propulsion.

As with the ECM, the HVECU has

its own set of inputs and outputs toimplement a closed-loop control sys-tem. Much of the HVECU interactionoccurs with the two motor generatorunits of the Prius (MG1 and MG2),which provide drive or recoveredenergy (the latter during regenerativebraking). Here, a motor speed/posi-tion sensor in the MG1 and MG2 areused as inputs to the HVECU, alongwith shift-level position and evenaccelerator pedal position.

While the inverter/converter unit(ICU) handles all of the electrical con-version in the system, the HVECU isinstrumental in the control of theICU, whose operation is responsiblefor energy delivery and recovery toand from MG1 and MG2.

The ECM and HVECU share com-mon attributes in their implementa-tion.Although both are housed insidethe car cabin, their physical construc-tion reflects an emphasis on reliability,with sturdy housings and protectivecoatings on the entire circuit boardassemblies.The quad flat packs andother peripheral-leaded IC devicepackaging used throughout bothengine control boards boasts a longrecord of reliability.Without anemphasis on miniaturization,“what weknow works” seems to drive techni-cal choices.

The two engine control modulesuse a common Toyota-branded NECµPD70F3155 32-bit microprocessoras the primary source of computing

power. Neither the ECM nor theHVECU contain discrete memorycomponents; the NEC processor diecontains both the volatile workingmemory and the nonvolatile ROMused to store control code.

The rest of the ECM componentsare custom to the module manufac-turer, Denso, and most appear visual-ly to implement mixed-signal inter-faces at the inputs and outputswhere sensors must be digitized andactuators driven.

A more complex set of ICs sup-ports the NEC microprocessor onthe HVECU.Two Mitsubishi 16-bitmicroprocessors are each paired witha Tamagawa AU6802N1 angle encoderand a custom Toshiba analog device,perhaps corresponding to the MG1and MG2 input interfaces.Anotherpair of Mitsubishi 16-bit controllers inthe HVECU probably manages com-munications with the ICU, ECM andskid-control module.

Custom Denso and Toyota chipsfound in the motor control andengine control modules speak to thedesign’s unique requirements formixed-signal interfaces.

It’s also worth noting that theMitsubishi controllers have die-levelcopyrights dating back as far as 1995—further evidence of the measured paceof change and conservative designpractices found in the mission-criticalelements of automotive electronics.

— David Carey

The Prius drives

like a regular

car but offers

better low-speed

acceleration.

<<10 ECM



ule to enable a good sky viewthrough the windshield.

The radio is integral to thecenter stack as well, with controlvia the touchscreen. The lightingimplementation for the radio,however, was not obvious.

All in all, the stack comprises basic, easy-to-assemble technology with few fasteners.That facilitates assembly, which in turn mini-

mizes costs as well as the opportunities forproblems (and associated warranty work).

User interface/dash moduleJust below the center windshield is a displaymodule that gives the driver speed andodometer readings; provides gear, turn andhigh-beam indicators; and contains systemstatus and warning lights. Its location servesas a quasi-heads-up display that comfortably

The inverter/converter unit (ICU) isthe ringmaster of all of the electri-cal conversion in the Prius.The

hybrid’s two motor generator unitshave distinct roles: MG1 recharges thehigh-voltage (approximately 200-V)nickel metal hydride battery pack

located in the rear of the car and alsoapplies direct power to drive theMG2 assembly. MG1 additionallyserves as the electric motor used tostart the gas engine portion of thepower train. MG2 is the primary elec-tric drive motor when energized and

performs the reverse function toserve as the power generator duringregenerative braking.

Both MG1 and MG2 are perma-nent-magnet three-phase devices, pro-viding torque when driven by acpower or providing ac output

Inverter/converter: Prius’ power broker

>>16



allows drivers to focus on the road.The upper portion of the display, compris-

ing a digital electroluminescentspeedometer, odometer, fuel

gauge and gear position indica-tor, is separate from the warn-

ing and status indicatorspositioned below. The latter

are LED-backlit behind ablack polycarbonate laser-

etched panel.On other cars, such a display would usual-

ly be one large assembly. Here, the large size

of each of the displays (23 x 10 cm for thespeedometer cluster and 20 cm wide for thewarning lights) probably dictated the dual-display configuration. The two displays areindividually prewired and then electricallyconnected to the vehicle main wiring harnessvia inline connectors pressed together duringthe module assembly. Flex circuits serve asthe interconnects between the circuit boardsand the displays.

Airbag control moduleThe airbag module governs selective deploy-

when rotatedfrom outside sources (either thegas engine or the wheel rotationduring braking).

To create the three-phase powerfor the motors, the dc battery sourcemust first be stepped from 200 to500 Vdc via a boost converter.A bankof insulated-gate bipolar transistors(IGBTs) with parallel diodes is mount-ed in bare-die form to a thermalplate, with connection to an ICU con-trol board by way of feed connectorsand ultraheavy-gauge aluminum wedgebonds for power interconnect.

With a boosted dc voltage avail-able, the inverter is responsible fordelivering the three-phase powerneeded in the MG1 and MG2 assem-blies when they are used as motors.As in the boost converter, IGBTs areused for power modulation in theinverter.Again, a dedicated assemblyplate supporting unpackaged transis-tor slices is used, with similar inter-connect to the same controller boardto which the boost converter islinked.The entire transistor/diodearray assembly used by the boostconverter and inverter is encapsulat-ed in a gooey sea of protective sili-cone gel. Four of the six legs of theinverter have small current monitorassemblies to keep tabs on powerdelivery to and from MG1 and MG2.

This same power semiconductorplate used for the inverter supports aset of diodes, again mounted as barechips, with one diode parallel to eachof the IGBTs.The diodes are usedduring regenerative braking to rectifythe ac output of the MG1 and MG2

assemblies so that after filtering andregulation (using the boost convertercircuit in reverse), the recoveredenergy resupplies the high-voltagebattery pack.

The Prius also uses an electric air-conditioning compressor motor sothat cabin cooling is maintained evenwhen running in electric mode only.A second dc/ac inverter, with circuitslocated on a second ICU controllercircuit board ringed with TO-pack-aged IGBTs, is deployed to power theelectric A/C compressor from the HVbattery pack.The A/C inverter IGBTpackages are bolted to one face ofthe substantial heat-sinking enclosureof the ICU. More on that shortly.

Since the Prius still needs a conven-tional electrical system to operateinstrumentation, cabin lighting and thelike, the ICU also supports step-downconversion from the 200-V NiMH bat-tery to the 12-V subsystem, where aconventional lead-acid battery is used.Circuits for the dc/dc converter sharespace on the same circuit board thatis believed to house the A/C compres-sor inverter, and TO-packaged devicesare again bolted to the cooling plateof the overall ICU assembly.

The boost converter, MG1 andMG2 inverter, dc/dc converter andA/C compressor inverter all operateunder the direction of the hybridvehicle engine control unit.

With the large currents involvedthroughout the ICU, cooling of powersemiconductor devices is paramount.The two sets of power compo-nents—both bare direct-mount slicesand the TO-packaged parts—are

mounted back to back on the metalcase of the ICU. Heat transfer isachieved by way of a dedicated liquidcooling loop that runs through theICU casing and is shared with the twoMG assemblies.

Semiconductor content is critical inthe ICU, but with the exception of acontrol 32-bit CPU from Renesas (theHD6437049), the IC content largelycomprises switcher and inverter con-trol components.An NEC µPC1099switching regulator controller andNEC µPC494 inverter controller joinwith Toyota custom power controldevices and a number of transistordrivers for implementing the activecircuits.Toshiba GT30J324 andRenesas 2SK1517 N-Ch MOSFETsconstitute the array of TO-packagedtransistors, and Toyota custom IGBTsand diodes are used on the bare-chippower semiconductor plate refer-enced earlier.

Inductors, transformers, filtercapacitors and a host of other pas-sives complete the component setused in this impressive example ofpower engineering.

Much as the airflow, injectors,spark plugs and valves are the con-trolled aspects of the gas engine, thedc/ac, ac/dc and dc/dc converters arethe managed elements of the electricdrive train. Batteries and motorsserve as rough parallels to the gas andmechanical pistons of the engine, andby teaming through the ECM andHVECU, the two systems create afuel-efficient source of power to getyou down the road.

— David Carey

<<14 PRIUS ICU



ment of the various airbags (front, side, side-curtain and belt pretensioners) in a crash.Inspection of this unit reveals that the frontairbags are dual-stage devices with two infla-tion rates, depending on crash severity.

The module is centrally hard-mounted ona spring steel bracket to the body directlybelow the instrument panel center stack.Located at the centerline position is a piezo-electric sensor, one of many on the vehicle

(such as at the front corners and side pillars)for crash detection. Because this location isright below a set of cup holders, a clear,vinyl-like cover is glued to the module’s die-cast aluminum housing to prevent liquidintrusion.

Closer inspection of the airbag controluncovers standard yellow connectors, denot-ing safety system circuits. All airbag squib-fir-ing circuits have shorting bars to short the

With increasing frequency, carbuyers are spending big for theconvenience of in-car naviga-

tion. Not unusual for factory-optionednavigation, the Prius’ GPS-based map-ping and guidance system adds almost$2,000 to the sticker price.

As a practical matter, what the driv-er perceives as the navigation “system”is really a composition of two distinctsubsystems, one the GPS navigationunit itself and the other a visual inter-face by which the driver can see andinteract with mapping functions.

The Prius’ visual user interface(VUI) comes by way of a Toshiba liq-uid-crystal display and associated elec-tronics located middash.The touch-screen LCD panel is an important andstandard part of any Prius boughttoday (it’s present even if the

Navigation unit bridges automotive, CE

>>20



connectors when they are disconnected forservice so that an electrostatic discharge can-

not fire off a bag.

Stability control moduleBy selectively braking individual

wheels, this module mitigates skidsand understeer during severe maneuver-

ing. The module is directly above the throt-tle (gas) pedal. But one of its key inputs is ayaw sensor placed about a foot toward therear of the car from the airbag module posi-tion. This spot is roughly at the center ofgravity of the car and determines rotation(yaw) about the vertical axis.

Other inputs to the module include steeringwheel angle (driver input), speed, brake inputsand wheel speed (from the antilock-brakingsystem sensors, via the ABS controller).

The stability control module has an unusu-al two-piece case. One half is a die-cast hous-ing whose chief feature is the many metal“posts” on its surface; they appear to be relat-ed to heat rejection. The other half of thecase is sheet metal and includes the unit’smounting brackets. But the circuit boardinside this case is only attached at the endsto bosses that are placed well away frompost-studded areas of the case. Perhaps thehousing was adapted from another unit thathad more-critical heat issues. The stabilitycontrol module circuit board has “typical” ICsand passives, with no sensors on the board.■

Al Steier is an associate at engineering analysis firm Munro

& Associates Inc. (www.leandesign.com).

Rick DeMeis is editor of Techonline’s Automotive

Designline (www.automotivedesignline.com).

navigation optionis not chosen), serving as the inter-face for energy monitoring, climatecontrol and car audio.The trend incars and aircraft today is to migratemuch of the traditional gauge andbutton user interfaces to a central-ized “glass cockpit,” and the Prius isno exception.

Communication with the enginecontrol module (ECM), hybrid vehicleengine control unit (HVECU) andother convenience control subsys-tems occurs within the VUI, allowingthe touch panel to serve many roles,depending on the selected use mode(activated via the buttons to eitherside of the LCD).While speed, gear,fuel-level and odometer functions aredelivered via a vacuum-fluorescentdisplay (VFD) assembly, the bulk ofinteraction in the Prius comesthrough the VUI.

A pair of components based on themulti-OS-capable Naviem partnershipbetween Denso and Toshiba createsthe master graphical display and con-trol interface. Joining with the Naviemdevices are 32 Mbytes of NOR flashfrom Sharp and 64 Mbytes of DDRSDRAM from Elpida to create whatcan be considered the general-pur-pose compute system responsible fortouchscreen and monitoring and com-mand I/O.The VUI box is cooledby a pair of fans, hinting at the

processor horsepower within.Pony up the extra money for the

navigation unit (NU), and theMAP/Voice and DEST mode buttonsbecome active alongside the standardVUI mode selects.With the navigationoption, a box housing the navigation-specific electronics and accompanyingDVD-ROM drive for map data getsinstalled under the driver’s seat atbuild time.The factory navigation unitsupports more than just mappinginformation, however.Voice recogni-tion allows hands-free address andnavigation command entry, and voiceprompts provide spoken driving direc-tions, among other functions.

The navigation subsystem is stand-alone, other than the visual interface.To complete the connection to theVUI for visual aspects of operation, aSony CXB1457R Gigabit VideoInterface transmitter pipes data up tothe VUI for display.A correspondingSony CXB1458R Gigabit VideoInterface receiver is found within theVUI box to bring in navigation unitinformation for that.

A GPS antenna installed under thedash cover is positioned to allow lineof sight to GPS satellites through thefront windshield, and a coax cablethen snakes its way back to the under-seat navigation box.The RF front endfor GPS constitutes quite a small por-tion of the navigation electronics,

based here on a Panasonic AN18401Adownconverter to get to a de-modu-lated GPS input signal. From there, asizable Renesas HD6473810 proces-sor is responsible for all of the GPSbaseband work needed to calculatethe present position for joining withlocal mapping data. But GPS datacrunching is only part of the requiredprocessor function, since all of thevoice recognition and voice promptingare also done in the NU.

Within the NU, 96 Mbytes of totalDDR SDRAM is found spread acrossthree 32-Mbyte Elpida chips, and1 Mbyte of Spansion NOR flashserves for local code store. Both theNU and VUI are among the few mod-ules in the Prius that use sizablestores of discrete memory. Likewise,IC packaging steps up to high-pin-count BGA devices from the QFPpackaging found in the more conserv-atively designed modules.

The mix of cabin-internal use envi-ronments, more-modern design andless-mission-critical attributes for theNU and VUI probably contributes tothe latter’s closer engineering resem-blance to traditional consumer elec-tronics (CE). Indeed, it is in infotain-ment where we see much of thegrowth in automotive electronics andconvergence with the gadgetry we’reaccustomed to seeing outside the car.

— David Carey

One of the skid

control’s main

inputs is a yaw

sensor, placed

toward the rear.

<<18 NAV UNIT

Documents

Under the Hood - Tried and true design concepts drive Prius - Munro