动力换档变速器的进一步发展和电控单元开发过程的重要性

Dipl.Ing.（FH）Christoph Göbel ZF Friedrichshafen AG－Operations and Technology ZF Group

1 前言

多年来，无级变速器已成功进入市场。但是多档变速器（通常在部分载荷下换档）仍然是拖拉机的标准传动方式。这就是为什么会产生将无级变速器（功能更多、操作更便捷、更舒适、自动控制等）所具有的功能扩展到多档变速器中。

这里部分挑战来自定义批量生产准备阶段的功能和通过机械、液压和电子（硬件和软件）加以实现。不断增加的控制单元、界面、（某些时候与CAN网络的联接）功能需要更具创新性的开发过程，特别是涉及软件时。

本文通过实例叙述拖拉机用多档变速器功能的扩展和说明必要的软件开发过程基本原理。

1 Introduction

For some years now，continuously variable transmissions are introduced successfully on the market.Nevertheless，multistep transmissions（usually shiftable under partial load）remain the standard drive solution for tractors.This is why ideas from continuously variable transmissions（additional functions，easier operation with increased comfort，automation，etc.）are also applied to multistep transmissions more and more.

A particular challenge here is to specify volume－production－ready functions and implement them in mechanics，hydraulics，and electronics（both hardware and software）.The growing number of control units，interfaces，and（in some cases interlinked on e.g.CAN network）functions require more innovative development processes，especially when it comes to software.

This article uses examples to describe expanded functions of multistep transmissions for tractors，and explains basic principles of the necessary software development processes.

2 T7000拖拉机传动装置、系统详细资料

带动力换档的拖拉机具有很多种用于控制各项功能的操作机构和显示元件。考虑到当今复杂拖拉机所具有的范围广泛的行驶、作业和操作功能（图1），控制系统的人机工程设计就成为一个重要因素。目前在技术方面的创新关注于这些需求并至少向以下目标之一前进：

·通过自动控制的典型程序提高驾驶舒适性

·利用人机工程设计的控制和显示设备改善操作舒适性

·提高换档品质

·通过提高最高行驶速度扩展拖拉机使用范围

·缩短响应时间以优化作业过程

·减少反作用力以提高使用寿命

以下将通过大量实例论述在不影响安全性和可靠性的前提下这些目标是如何实现的。

2 T7000Tractor Transmission，System Details

Tractors with powershift transmissions feature a large number of operating and display elements for control of the various functions.Considering the extensive range of driving，working，and operating functions of today＇s complex tractors（Figureergonomic design of the control system is a major factor.Today＇s technological innovations address these needs and pursue at least one of the following goals：

·Increasing driving comfort through automation of typical processes

·Increasing operating comfort with ergonomic design of the controls and displays

·Improving shifting comfort

·Expanding the scope of the tractor by increasing the maximum driving speed

·Optimizing working processes through shorter reaction times

·Increasing service life with smaller reaction forces

In the following，various examples are used to illustrate how these goals are achieved without compromising on safety and reliability.

图1 T7000变速器控制系统简图Fig.1 System control schema of atypical T7000transmission application

3 增加舒适性、自动操作和安全性方面的功能

3.1 自动耕地/道路切换（FRRS）

ZF公司T7000变速器可作为一个频繁切换过程自动操作的良好实例。它包括4档/6档同步啮合变速器，并带有一个两速组合式变速器用于耕地/道路工况。在过去，组合式变速器的切换必须在近乎静止状态下手动完成。而目前该过程可部分实现自动化，其优点是：

·方便驾驶员操纵

·可在某一速度范围内进行切换

·破损安全操作

组合式变速器切换步骤如下：

驾驶员按下一个预选开关向变速器电控系统提示他所要求切换的工况。

系统检查以下边界条件是否满足：

·同步啮合换挡杆是否处于空档

·离合器踏板是否踩下

·行驶速度是否处于速度限制范围内然后，相应的电磁阀被激活。位置传感器确认切换成功。

安全功能（定时锁、同步啮合变速器用电磁阀闭锁和操作失败后限制重试次数）防止了非允许的换档和损害。

3.2 自动离合器工作（换档舒适，CS）

换档舒适性和主离合器激活功能得到了改善。采用新型的比例控制后，操作过程由先前的开环控制便成为闭环控制。踩下离合器踏板操纵主离合器（或向动力换档离合器映射主离合器功能）是一个经常重复的步骤，它出现在各种主要行驶工况下（起步、倒车、变速换档、蠕动等）。尤其是在离合器结合过程中，驾驶员必须考虑所有条件（速度、负载、压力、速比）。在自动控制中，驾驶员按下一个在换挡杆上的离合器按钮激活该过程。根据上述的边界条件，电子系统估计适当的工作状态并调整离合器压力，这具有以下优点：

·更舒适的离合器结合

·较少的驾驶员工作（单手操纵离合器和换档控制）

·最佳的压力调节降低离合器和传动系统的应力

·行驶功能最优控制以提高工作性能1）

3 Functions for increasing comfort，automation and safety

3.1 Automated field to road switchover（FRRS）

The T7000transmission from ZF provides a good example of the automation of frequently recurring processes.It combines a 4－speed/6－speedsynchromesh transmission with a 2－speed group transmission for field/road operation.In the past，the synchronized switchover of the groups had to be performed manually and almost at a standstill.Now it’s established，that this procedure is partially automated，with the following benefits：

·Easier for the driver

·Switchover while driving within a certain speed range

·Failsafe operation

The steps for a group switchover are：

The driver presses a preselection key to indicate to the transmission electronics that he wishes to switch groups.

The system checks that the following boundary conditions are met：

·Synchromesh shift lever in neutral

·Clutch pedal depressed

·Driving speed within speed limit

3.3 自动动力换档（APS）

同样自动动力换档功能（APS）也已通过新型比例闭环控制加以改善。

拖拉机速度在很大程度上取决于当前的工况。驾驶员通过确定某一发动机速度、挂入合适的档位和选择合适的动力换档过程来影响速度。

自动动力换档的目的的将驾驶员从选择动力换档时机中解放出来。而是由系统根据一些变量自动完成该项任务。行驶策略系统根据速度、转速梯度、发动机负荷和发动机速度降低等估计出正确的动力换档时机。

驾驶员可根据特殊的工作状态自行选择换档阈值。自动换档系统依据不同的外界负荷和驾驶员的手动操纵（档位、组合式变速器、行驶方向等）选择正确的动力换档时机。这样，系统能够快速识别最佳换档点，并具有以下优点：

·牵引力或油耗最优的档位

·对载荷变化系统响应更快

·减少驾驶员工作

·增加舒适性

3.4 动力换档操纵品质最佳

采用比例控制阀使得动力换档操纵品质在换档舒适性方面实现尽善尽美。每个离合器的压力可单独控制。根据变速器工作条件（例如温度、负载、档位变化、速度），变速器控制器计算出最佳动力换档顺序并实施之。其它优点是提供过载保护和提高离合器转矩传递能力，其结果是倒车更快速。

3.5 电控同步啮合换档拨叉

电控换档拨叉单元是下一级自动控制。该单元的任务是根据来自变速器控制器的电信号在6个同步啮合齿轮副间换档。电控取代机械执行器后换档拨叉单元的效能大为提高。这意味着操作元件的设计变成为线控换档方法，因此更容易实现。另外无需驾驶员提供相应的操纵力而且换档过程及其精确。除了手动操作选择，依据换档策略也可自动选择。换档拨叉单元由变速器控制器控制。为完成该任务配备有两个结合/脱开电磁阀、两个比例电磁阀和一个位置传感器。

3.6 推进功率的使用和控制

推进功率是额外的预备功率，在某些工况下将后备功率接入变速器和发动机。当需要单独牵引时推进功率提供更大的功率起动性能。其意图是使用带大功率动力输出轴机械的轻型车辆。

Then，the corresponding solenoid valve is actuated.

A position sensor transmits a confirmation of successful switchover.

Safety functions（time lock，locking solenoid for synchromesh transmission，and limit to number of re－tries after failure）prevent non－permitted gear changes and damage.

3.2 Automated clutch operation （comfort shift，CS）

The familiar function of comfort shift as well as the actuation of the main clutch has been improved.With the new proportional control the processes which were previously open－loop now have closed－loop control.Pressing the clutch pedal to operate the main clutch（or mapped main clutch functionality to powershift clutches）is a frequently repeated process that occurs in many driving situations（setting off，reversing，gearshifts，inching，etc.）.Especially during clutch engagement，the driver needs to take into account a whole range of conditions（speed，load，compression，speed ratio）.In automatic control，the driver actuates this process by pressing a clutch button on the shift lever.Based on the boundary conditions described above，the electronics calculate the appropriate operating mode and modulate the clutch pressure，resulting in the following advantages：

·More comfortable clutch engagement

·Less work for the driver（clutch and gear shift controls in one hand）

·Optimum pressure modulation reduces stress on clutch and driveline.

·Optimized control of driving functions increases work performance

3.3 Automatic powershift（APS）

Also the established automatic powershift function（APS）has been improved with the new proportional closed－loop control.

The tractor speed depends to a large extent on what it is currently being used for.The driver influences the speed by determining a certain engine speed rate，engaging a suitable synchronous gear，and additionally by selecting an appropriate power－shift stage.

推进功率功能具有两个必要的特征：

1、带电控发动机控制器的现代柴油机，它能够采用不同的发动机特性（转矩曲线）。

2、智能化软件算法能够察觉除牵引工况之外所需的附加功率，例如PTO工作、空调或低效率行车条件。

变速器控制单元根据诸如尤其速度、变速器速比和各种发动机辅助驱动等工况决定采用那种功率特性曲线。这里，功率流可根据工作点而优化，这功能仅在几个控制单元相互联系的情况下才能实现。

3.7 PTO启动控制

除了车辆行驶外，取力器是拖拉机上一个相当重要的转换变速器。预选2档或4档PTO速比则是行驶开关切换过程的一部分。

事实上经改进的PTO控制相当容易。采用调节比例阀控制后，驾驶员仅感觉到有一个非常平滑的启动。借助于比例阀技术和变速器控制单元与新开发软件的交互作用使得在任何负载惯量下的启动非常平滑。低惯量的突然启动或高惯量负荷时的发动机停转已成为过去。

该功能能够保护部件并提供更高的安全工作环境。

3.8 机电驻车制动

机电驻车制动改善了操作者的安全性和舒适性。这项创新也扩展了变速器现有的功能。像动态制动和汽车应用等功能将实现自动操作，其结果是简化了手动操作的动作。

驻车制动的实施和解锁由驾驶室中的开关完成，所以驻车制动操纵杆已不再需要。当开关切断点火或离开驾驶室时，制动过程自动实施。

同时机电驻车制动为驾驶室设计提供更多的便利，因为通常采用的机械推拉式钢索被电线所替代。

4 软件开发过程的重要性

上述现代拖拉机变速器功能的开发和改进在很大程度上是由软件所控制。因此，在农业机械市场上的应用正在增大：

·实现功能的软件必须视作为一个“产品”，并且变得日益重要。

·如同硬件，“软件”产品必须经定义、设计、开发、试验和验证。这里有特殊的开发过程。

多年前ZF公司就认识到软件开发和相关过程的重要性。其结果是，公司完成了从基于工艺（技工）型企业到体系系统化、过程导向型工业的转变。

只有当合适的开发过程被确立后，再现性、可追溯性、定量和传输可靠性等各种需要才能满足。公司还必须在合适工具和过程高效管理、过程控制等方面提供支持。所有这一切的基础是由现代化高效工具所支持的精致的工作流程管理。

The purpose of the automatic powershift is to free the driver from having to select the powershift stage.Instead the system automatically takes over this task depending on a number of variables.The driving strategy system calculates the right powershift stage according to the speed，rpm gradient，engine load，and engine speed reduction.

The driver has the option of setting shift thresholds himself according to the specific working situation.The automatic system selects the right powershift stage not only in the case of different external loads，but also when the driver makes manual changes（synchromesh gear，group，direction of travel）.In this way，the system identifies the optimal operating point faster，leading to the following advantages：

·Optimum gear in terms of tractive force or fuel consumption

·Faster system response to load changes

·Less work for the driver

·Increased comfort.

3.4 Quality of powershift operations in perfection

It’s for the use of proportional controlled valves excellently due to，that the quality of powershift operations reaches perfection regarding shifting comfort.The pressure for each clutch can be controlled individually.According to the transmission operating conditions（e.g.temperature，load，gear transition，speed）the best powershifting sequence will be calculated by the transmission controller and carried out.Further advantages are the overload protection and the increased transfer capability of the clutches，which results，for example，in quicker reversing.

3.5 Electronically controlled synchromesh gearshifter

The electronically controlled gear－shifter unit represents the next grade of automation.The job of this unit is to shift all 6synchromesh gears according to electrical signals from the transmission controller.The gear－shifting unit is more efficient in case of electronically control instead of mechanic actuation.This means that the design of the oper－ating element is changed to the method shift by wire and therefore easier for realization.Further advantages are e.g.no relevant operating force necessary by the driver and highest operating preciseness.Additional to manual operating selection also automatic selection following the shift strategy is possible.The gear－shifter unit is controlled by the transmission controller.For this task two on/off solenoid valves，two proportional solenoid valves and a position sensor are used.

软件开发步骤与变速器应用场合（轿车、商用车、非公路车辆）大体无关。所有ZF软件工程师都在联合电子部门工作。这意味着他们能够充分利用各部门间的协调优势。他们特别关注功能的再利用、软件模块、程序、功能的安全性和整个工作流程。

这里给出的V型（图2）模块是软件开发过程的基础。它描述了软件开发过程中各阶段工作的详细执行顺序以及文档和相应工具方面的支持。

为确保公司软件开发工作的效率不变，我们经常进行内部和外部评估。分析和评估过程的强项和弱点，因而能够不断升级。

这里显示了ZF公司今后继续为车辆应用开发复杂软件的能力。

3.6 Usage and control of boost power

Boost power is the provision of extra power in certain operating situations by tapping into the power reserves in the transmission and the engine.The boost power provides the power take－off more performance when this would be needed for traction alone.The established intention was to use a lightweight basic vehicle in combination with powerful PTO－shaft machines.

图2 V型模块软件开发Fig.2 V－model of software development

Two features are necessary for the boost power function：

1.A modern diesel engine with an electronic engine controller that allows different engine characteristics（torque curves）to be used

2.An intelligent software algorithm which detects an additional power requirement beyond the traction work itself，e.g.PTO in operation，air conditioning system，or low－effectiveness driving conditions.

The transmission control unit determines which power characteristic curve is applied depending on conditions such as travel speed，transmission ratio，and use of various engine－driven auxiliaries.In this way，the power flow can be optimized according to the operating points，a function that is only possible with the interconnection of several control units.

3.7 PTO start－up control

Apart from the vehicle drive，the power takeoff drive is one of the most important conversion transmissions in tractors.The pre－selection for 2to 4PTO gear ratios is part of the drive switching on and off processes.

Also，the fact that the PTO control has been improved has made handling easier.With the regulated proportional valve control the driver experiences a smoother start－up.The interaction with the new software development for the transmission control unit enables a smooth start－up with the proportional valve technology for each implement inertia.Sudden start－off with low inertia or engine stalling with high inertia implements have become a thing of the past.This function protects the components and implements and meets higher safety oper－ation.

3.8 Electro－mechanical Parking Brake

The electro－mechanical parking brake improves safety and comfort for the operator.This innovation also extends the transmissions’existing range of functions.Functions like dynamic braking and auto apply will automate and，as a result，simplify a variety of actions，which have so far been manual.

The parking brake is applied and released by a switch in the driver’s cab，so the parking brake lever used so far is no longer required.The brake is automatically applied upon switching off the ignition or leaving the driver’s cab.

Also the electro－mechanical parking brake offers more flexibility for the design of the cabin because the normally used mechanical push－pull cable is replaced by electrical wire.

4 The Importance of Software Development Processes

Many of the newly developed and improved functions in modern tractor transmissions as described above are wholly or partly controlled by software.Consequently，there is an increasing realization on the agricultural machinery market of the following points：

·Software for the implementation of functions must be seen as a"product"，and is becoming increasingly important.

·Just like hardware，the"software"product must be specified，designed，developed，tested，and validated.There are specific development processes for this.

ZF recognized the importance of SW development and the associated processes many years ago.As a result，the company initiated and completed the transition from a craft－based（"artisan"）company to a systematically structured，process－oriented industrial player.

A whole range of requirements such as reproducibility，traceability，quantifiability，and delivery reliability can only be met when suitable develop－ment processes have been installed.We also have to provide support with the right tools and efficient process management，including process controlling.The basis for this is a tailored workflow management also supported by modern effective tools.

The software development procedure is mainly unrelated to the transmission application （passenger car，commercial vehicle，off－road machinery，…）.All ZF software engineers work together in the Corporate Electronics department.This means they can make the most of interdepartmental synergies.They focus in particular on function reuse，SW modules，processes，functional safety and the entire workflow.

The V－model shown here（Figure2）provides the basis for the SW development process for all applications.It describes the individual work stages to be carried out in a defined order during software development，supported by documents and appropriate tools.

To secure the sustained effectiveness of our SW development work，we regularly carry out internal and external assessments.These analyze and evaluatethestrenthsandweaknessesofthe process，so it can be continually upgraded.

This gives ZF the capacity，also in the future，to continue to develop complex SW for all vehicle applications.

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