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12 ATZ worldwide 11/2001 Volume 103 The new Sequentronic automated transmission from Merce- des-Benz is moving the conventional gearbox into a new aera. Over the medium to long term, Sequentronic will give drivers the ease of use the demand of a transmission without sacrific- ing the excitement and sheer driveability offered by a manual gear change. The new system is also more fuel-efficient than the traditional automatic transmission with torque converter. Mercedes-Benz anticipates that, in the premium car sector, today's manual gearboxes will be replaced by the new Sequen- tronic automated transmission over the next few years. 1 Introduction The market is experiencing a boom in semi- automated transmissions, which are based on a conventional manual gearbox and have an operating interface much like a tra- ditional automatic transmission. In a semi- automated transmission – as in its manual counterpart – the tractive force is interrupt- ed as the gears are changed. Automatic transmission with a torque converter in- volves no loss of traction during gear shift- ing. Yet, despite this supposed drawback, semi-automated transmissions are gaining ground in certain sectors of the market. By Ralph Eberspächer, Klaus Heber, Reiner Pätzold and Anton Rink Sequentronic – ein automatisiertes Schaltgetriebe von Mercedes-Benz You will find the figures mentioned in this article in the German issue of ATZ 11/2001 beginning on page 1036. Sequentronic – an Automated Transmission from Mercedes-Benz

Sequentronic — an automated transmission from Mercedes-Benz

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Page 1: Sequentronic — an automated transmission from Mercedes-Benz

12 ATZ worldwide 11/2001 Volume 103

The new Sequentronic automated transmission from Merce-des-Benz is moving the conventional gearbox into a new aera.Over the medium to long term, Sequentronic will give driversthe ease of use the demand of a transmission without sacrific-ing the excitement and sheer driveability offered by a manualgear change. The new system is also more fuel-efficient thanthe traditional automatic transmission with torque converter.Mercedes-Benz anticipates that, in the premium car sector,today's manual gearboxes will be replaced by the new Sequen-tronic automated transmission over the next few years.

1 Introduction

The market is experiencing a boom in semi-automated transmissions, which are basedon a conventional manual gearbox andhave an operating interface much like a tra-ditional automatic transmission. In a semi-automated transmission – as in its manual

counterpart – the tractive force is interrupt-ed as the gears are changed. Automatictransmission with a torque converter in-volves no loss of traction during gear shift-ing.

Yet, despite this supposed drawback,semi-automated transmissions are gainingground in certain sectors of the market.

By Ralph Eberspächer,

Klaus Heber,

Reiner Pätzold and

Anton Rink

Sequentronic – ein automatisiertes

Schaltgetriebe von Mercedes-Benz

You will find the figures mentioned in this article in the German issue of ATZ 11/2001 beginning on page 1036.

Sequentronic – an Automated Transmission fromMercedes-Benz

Page 2: Sequentronic — an automated transmission from Mercedes-Benz

13ATZ worldwide 11/2001 Volume 103

In conjunction with Magneti-Marelli,Mercedes-Benz has now developed the Se-quentronic automated transmission for itsnew C-Class Sports Coupé, based on the 6-speed manual gearbox already available onthe market.

This product is aimed at drivers who en-joy changing gear, are open to new ideas,are looking for an even more dynamic dri-ving experience, want ease of use withouthaving to sacrifice the pleasure of drivingand like to be trendsetters.

2 Development Targets

Figure 1 illustrates the principal develop-ment targets.

The principal aim was to underline theorientation of the C-Class Sports Coupé tothe target group with an appropriate trans-mission providing an unusually agile anddynamic driving experience, and with theemphasis on fun. The direct link betweenthe engine and the drive provided by thedry clutch enhances this effect. At the sametime, ease of use and operating safety areincreased by means of various comfortfunctions such as the “Auto-Shift” mode foreasy cruising on country roads.

Other development targets include lowfuel consumption and low CO2 emissions,along with greater driving safety on slip-pery road surfaces.

No less important are the economic fac-tors, as the transmission can be manufac-tured in conjunction with the 6-speed man-ual gearbox already on the market. Particu-lar emphasis was placed on the ease of as-sembling the unit in the factory – in otherwords, on maintaining a limited number ofelectrical and mechanical interfaces be-tween the unit and the vehicle. These trans-mission systems are also making strong in-roads into niche markets (sports cars, etc.).

The technical design, construction andoperation of the Sequentronic transmissionare described below.

3 Technical Design

Sequentronic was developed on the basis ofthe new 6-speed manual gearbox. The aimof the project was to develop a compacttransmission unit that performs well as astand-alone unit in the subassembly planthich is able to be installed in the vehicle witha minimum number of interfaces. With theexception of the gear lever and the controlunit, all modules are located in or on thegearbox. All of the main components of thebase transmission system were incorporat-ed unchanged. The energy to power the hy-draulic control unit is supplied from anelectrically driven hydraulic pump.

In manual mode, the driver changesgear with the gear lever. The control unitcoordinates the drive elements comprisingthe engine, dry clutch and transmission.There is no mechanical connection be-tween the gear lever and transmission(shift-by-wire system). There is no clutchpedal, and the accelerator pedal can be op-erated during gear change.

The shift-by-wire concept creates a cleararea of conflict between safety, availabilityand functionality, the resolution of whichserves as a benchmark for developers.

“Auto-Shift” mode, in which the gear ischanged automatically according to the dri-ve status of the vehicle, can also be selectedat any time. The principal design data forthe subassembly is set out in Table 1.

The architecture for the Sequentronicsystem is shown in Figure 2. Its principalfeature is its strong functional integrationwith the control units for the engine anddrive system and with the instrument clus-ter.

4 Engineering Design

The individual modules of the subassemblyare shown in Figure 3. ■ 6-speed gearbox [1]The 6-speed gearbox is a fully synchronizedgearbox with a modular internal shiftmechanism and a central gear lever shaft. ■ hydraulic accumulator [2]The hydraulic accumulator supplies hy-draulic oil to the system when required.The unit comprises the piston-type accu-mulator and pressure sensor. ■ hydraulic control unit [3]The hydraulic control unit serves as an ac-tuator for the clutch and central gear levershaft. In the hydraulic control unit, all nec-essary valves (valve block 3a) and the hy-draulic cylinder (3b) are integrated into asingle physical unit. ■ hydraulic pump [4]The hydraulic pump generates the hy-draulic pressure and comprises the 12Velectric motor and a gear pump. ■ hydraulic oil reserve with air vent [5,6]The hydraulic oil reserve [5] holds the hy-draulic oil and defoams the return flow of oil.The air vent [6] equalizes the pressure in thehydraulic oil reserve by adjusting the vol-ume according to the vehicle drive status. ■ gear position sensor [7]The gear position sensor determines the po-sition of the central gear lever shaft in thegearbox. The rotary and axial movement ofthe shaft is detected by non-contacting sen-sors and transmitted as an analogue signalto the Sequentronic control unit. ■ central release bearing with clutch travelsensor [8]

The central release bearing operates theclutch pressure plate. The clutch position isdetected by the non-contacting clutch trav-el sensor and transmitted to the Sequen-tronic control unit. ■ clutch speed sensor [9]The clutch speed sensor is designed as aninductive transmitter. It senses the clutchspeed by means of a gearwheel in the gear-box.

Sequentronic is designed to be mainte-nance-free throughout its service life. Oilchanges are a thing of the past, and thiskeeps customer costs down. With innova-tive solutions in the hydraulic control unit,high system integrity and consistent use ofcommon parts across all gearbox models,Sequentronic is characterized by a low ad-ditional weight and an attractive price forthe customer compared to similar systems.

5 Operation

Mechanical Gear ChangeThe system patented by DaimlerChryslerAG comprises only one hydraulic cylinderin the hydraulic control unit for operatingthe central gear lever shaft. The basic gearchange sequence is as follows, Figure 4. ■ changing gear within a track (e.g. 1st to2nd):On detecting the driver's intention tochange gear, the solenoid valve for thestraight gears is actuated by the controlunit. The valve supplies pressurized oil tothe hydraulic cylinder. The piston rod [C2]with the piston [C4] is moved towards 2ndgear. The brake [C1] is not actuated and theS-Cam [C8] can rotate freely. The move-ment of the piston rod [C2] does not force amovement of the locating pin [C7] in thetrack guide. The gear lever is supported bythe detent [3] and makes a purely transla-tional movement. The gearshift fork [4] ismoved by the gearshift finger [7], complet-ing the movement from 1st to 2nd gear. ■ changing gear between two adjacenttracks (e.g. from 2nd to 3rd):When shifting, the S-Cam [C8] is held inplace by the brake [C1]. The integral curvecombines the translational movementwith rotation to the next track.

6 Operational Design

The operational design describes the inter-face between the driver and the Sequen-tronic system and is intended to preventmisuse and operating errors. Particularemphasis was placed on good handlingand intelligibility. The gear lever and theinstrument cluster are the central inter-faces between the driver and the system,Figure 5.

DEVELOPMENT Gearbox

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14 ATZ worldwide 11/2001 Volume 103

DEVELOPMENT Gearbox

The gear lever has three stable positions ■ position F (forward)■ position R (reverse)■ position N (neutral)and three unstable positions■ + (upshift)■ – (downshift)■ A/M (switches “Auto-Shift” mode on oroff).

The driver's gear change request ischecked for plausibility and permissibility,and the current gear is displayed in the in-strument cluster, Figure 5. Pressing A/Mswitches the “Auto-Shift” mode on or off.

If the driver needs to be alerted to a safe-ty-related situation or if a system fault isdetected, the driver is alerted by means ofthe text display in the instrument cluster.Depending on the type of fault, a warningbuzzer may also be sounded.

The selected shift direction (back to “+”),which mirrors the natural movement ofthe hand when accelerating or decelerat-ing, is designed for drivers with a particu-larly sporty driving style.

6.1 Shift Mode and Strategy –“Auto-Shift” Mode In manual mode, the driver essentially hascontrol over which gear is engaged, al-though operating errors – for example, en-gine overspeed or underspeed after chang-ing gear – are prevented.

In “Auto-Shift” mode, the gear is select-ed automatically, adjusting the shift linesto the current operating parameters. Theaim is always to select the “correct” gear interms of fuel consumption, emissions, ade-quate performance reserves and enginenoise, without switching gear too frequent-ly.

6.2 Sprint-Start FunctionThe sprint-start function was developed asa means of accelerating from a standingstart to a given vehicle speed as quickly aspossible. Under certain conditions, it allowsthe vehicle to accelerate at an increasedspeed. The individual gear changes aretherefore also completed as quickly as pos-sible. A certain level of comfort is intention-ally sacrificed.

6.3 Individual Adjustment ofthe Gear Change Sequence –Personalization In developing the Sequentronic system,particular attention was paid to the con-flicting goals of shift duration and shiftcomfort (shift jolt quality or perceived shiftduration), since the system design is suchthat the tractive force is interrupted duringthe gear change. Figure 6 shows the con-ventional range for a typical driver using

standard transmission. Unlike gearchanges in racing cars, in everyday driving– despite the legitimate desire for shortshift durations – the necessary level of com-fort must not be lost. The objective is to beable to change gear as well and as quicklyas the best driver, both in the vehicle drivestatus at any given time and consistentlyover the entire lifetime of the vehicle. Lessproficient drivers will find that everydaydriving is made significantly easier.

A targeted, linear vehicle accelerationover time is critical for optimum gear shifts.The individual shift sequence strategy forSequentronic takes account of the conflict-ing objectives mentioned above. Figure 7 il-lustrates the influencing factors in terms ofcontrolling the shift sequence.

7 Controlling the SequentronicSystem

Whereas, with standard manual gearboxes,the driver has to operate the clutch, thegear lever and the accelerator pedal in or-der to change to the desired gear, in thecase of Sequentronic these operations arecoordinated by an electronic transmissioncontroller (EGS). The operating softwarehas a critical influence on the comfort, agili-ty, safety and environmental friendlinessof the vehicle.

Since the operating software “manages”actuators such as the internal combustionengine or the clutch, which are critical tosafety, the software has to be of a very highquality. The controller design must there-fore be intrinsically safe. In this way, thehardware architecture differs fundamen-tally from that of the traditional automatictransmission system. The EGS is designedas a stand-alone control unit.

7.1 Transmission Control InterfacesThe EGS is part of the vehicle drive man-agement system and communicates withthe engine controller (EC), the drive con-troller (ESP) and the instrument cluster(CLUSTER) by means of a CAN bus. Bothsensor signals and command signals aretransmitted via this network, producing adistributed operating structure which sup-ports the construction of a redundant sys-tem architecture. Clutch speed, clutch posi-tion, shift position, track position and sup-ply pressure are read in from analogue anddigital control unit inputs.

The central interface between transmis-sion and driver is the gear lever, which isdesigned with redundant signal lines. Theclutch and shift actuator are operated byelectrohydraulic valves. A controlled pump/storage system supplies hydraulic fluids to

the hydraulic components. A serial commu-nication interface and diagnostic tool areused to code variants and to analyse faultmemories and adaptation data during com-missioning, servicing and repair work. Fig-ure 8 shows the input and output signals inthe EGS.

7.2 Development Process forthe Operating SoftwareIn a complex system, particular attention ispaid to the development process in soft-ware development. The fundamental prin-ciple is that quality should be developed,rather than be identified by testing. Anoverview of this procedure is shown in Fig-ure 9.

Development of the operating softwarebegins with an analysis of the system re-quirements and of the formal specificationfor the software architecture. This is doneusing the performance-oriented develop-ment tool “Statemate”, which provides au-tomatic hierarchical status systems and ac-tivity diagrams for describing system per-formance and structure.

In contrast to the specification stage, thebottom-up approach is used for implemen-tation. First of all, the primitive functionsare implemented and tested by means ofperformance tests. The main functions aredeveloped and validated using moduletests. The implementation phase ends withthe programming and validation of allmodules.

The individual modules can be testedand optimised with a Software in the Loop(SiL) simulator immediately after beingcoded. The test scenarios are created sys-tematically using an object-oriented proce-dure with the aid of a Classification Tree Ed-itor (CTE) and stored in a test database.

The SiL simulator requires a vehiclemodel, which is created using the develop-mental software MatrixX and which takesinto account the relevant dynamics of en-gine, transmission, connecting rods, brakes,wheels, body, actuators and sensors. Thesoftware architecture model, vehicle modeland coded operating software are linked to-gether by means of C code interfaces, sothat guided vehicle drive can be simulated.

Before being used in the vehicle, the lim-its relating to the control unit are validatedin a Hardware in the Loop (HiL) simulator,which simulates the hardware interfaces ofthe control unit. The final coordination andtesting of the entire system takes place inthe vehicle.

7.3 Architecture of the Operating SoftwareThe operating software consists of variousmain modules, Figure 10.

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Individual modules, coordinated by ahigher-level automatic status system, areable to control or regulate the “actuators” ofthe engine, clutch and transmission. Lower-level regulators calculate the output quan-tities for controlling the clutch valve andshift actuator valves. The hierarchicallystructured, modular software architecturesupports the integration and reuse of exist-ing, tried and tested software modules.

In order to ensure a good control perfor-mance, changes to system parametersmust be identified and compensated for, sothat a reproducibly good driving perfor-mance is always achieved irrespective oftemperature, load and standard spread. Thesystem parameters for the dry clutch inparticular are subject to a broad spread andhave to be taught continuously. Both therelease travel and the transferable clutchtorque are changing during this process.Standardized status variables are deter-mined from measuring signals and adapta-tion parameters and used to calculate thedrive strategies, and the link between theadaptation and drive strategies is broken.

The safety design consists of the faultdetection, fault response and emergencydriving mode program software modules.Unlike controllers for automatic transmis-sion systems, the EGS in the Sequentronicsystem has to control the engine torqueduring a gear shift. Depending on the typeof shift, the EGS increases or reduces the en-gine torque as required by the driver. In-creasing the engine torque in particular in-volves a complex safety design, which isdescribed more in detail in the followingchapter.

7.4 Safety DesignThe safety design for the Sequentronic sys-tem has three principal functions: ■ to ensure the safety of the system in theevent of driver operating error ■ to ensure the safety of the system in theevent of malfunction of one or more com-ponents ■ to ensure maximum availability of thesystem.

The safety design seeks to reconstructdefective signals, to shut down defectiveactuators, to store detected faults and asso-ciated environmental data and optionallyto activate an emergency driving modeprogram. Since automated transmissionsare ideally implemented as shift-by-wiresystems, which can both increase and re-duce the engine torque, transmission con-trollers should be designed to be inherentlysafe. Inherently safe in this context meansthat the following requirements are set forthe safety design: Simple faults must notlead to a catastrophic failure, and resetting

the control system must not lead to a cata-strophic failure either.

The fault detection and response timesare determined in such a way that a criticalsituation cannot develop for the driver. Al-though safety has priority, high availabili-ty must also be guaranteed.

To enable these requirements to be met,the safety design demonstrates the follow-ing features: ■ two-channel system for safety-relatedinputs and outputs ■ two-channel system for safety-relatedsoftware ■ two-processor design (with separatepower supplies) ■ continuous processor self-testing ■ continuous mutual processor monitoring ■ component monitoring and plausibilitytesting.

8 Engine Performance andCyclic Fuel Consumption Rates

Thanks to the greater efficiency of Sequen-tronic in comparison to the automatictransmission system, fuel consumption im-provements of up to 0.4 litres/100 km canbe achieved with the diesel engine and upto 0.2 litres/100 km with the gasoline en-gine, Figure 11. Figure 12 illustrates the cor-responding CO2 emissions. No significantfuel consumption improvements are likelyin comparison with the 6-speed manualtransmission. Table 2 compares engine per-formance values.

The debate as to whether the fastestgear shifts and hence the best accelerationvalues can be achieved with manual trans-mission or with Sequentronic is actually ir-relevant to the normal car driver because,although test drivers and professional rac-ing drivers achieve maximum values dur-ing acceleration when using manual trans-mission, even proficient normal driversvery rarely approach the values achievedwith Sequentronic.

In both transmission systems, the gearshift process takes time (engaging and dis-engaging, mechanically changing gears).The interruption to tractive force with Se-quentronic is between 200 and 400 ms, de-pending on the operating status of the ve-hicle.

9 Summary and Prospects

Sequentronic, the automated transmissionfrom Mercedes-Benz, makes an importantcontribution to enhanced customer benefit.The precision shift control fine-tunes eachgear shift to the current driving situation.

With its versatile function options, Se-quentronic offers a very sporty driving

style while also allowing comfortable cruis-ing over country roads. It also offers fuelconsumption advantages over the tradi-tional automatic transmission in bothmanual and “Auto-Shift” mode.

Future pressure to reduce fuel consump-tion and emissions and to increase ease ofuse in comparison to the manual gearboxwill allow the market share for automatedtransmissions to grow, thereby securingthe long-term future of the traditional gear-box as compared with automatic transmis-sion systems. ■