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service self-study programme 259 ephs ­ electrically powered hydraulic steering design and function

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ephs ­ electrically powered hydraulic steering 259_033 the ephs electrically powered hydraulic steering is familiar from the lupo fsi this system is supplied by trw and by koyo the new steering system offers a wide range of advantages while at the same time retaining the excellent steering properties of the conventional hydraulic power-assisted steering advantages of the ephs improvement in comfort easy to operate when parking but steering firm at high speeds safety factor fuel savings as the energy consumption is based on demand ­ irrespective of the operating state of the internal combustion engine new important note the self-study programme describes the design and function of new developments the contents are not updated please always refer to the relevant service-literature for up-to-date inspection adjustment and repair instructions 2

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at a glance introduction 4 system overview 8 design and function 10 function diagram .20 service 21 test your knowledge .24 subject index .26 3

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introduction the pressure required for the steering servo assistance is produced by a hydraulic pump this pump is driven directly by the vehicle engine as in the conventional familiar powerassisted steering system in other words part of the engine power output is required for driving the pump at the moment at which the greatest steering servo assistance is required ­ mainly when parking ­ engine speed is at its lowest the pump capacity is designed for such a case the faster the steering rate the higher is the pump speed and thus the volume flow pump capacity which is not required at higher engine speeds is dumped via a bypass in the new steering system although the hydraulics likewise assist human steering effort the hydraulic pump ­ a gear pump ­ is driven in this case by an electric motor and is mechanically independent of the vehicle engine the hydraulic control is of identical design a new feature is the steering servo assistance based on the steering angle rate and vehicle speed a steering angle sensor is additionally integrated for this purpose in the housing of the rotary disc valve refer to fig below which transmits the steering angle rate to the electronic control the steering angle information flows along a sensor cable directly to the control unit in addition the vehicle speed is detected in the control unit during the analysis this information is supplied over the can databus power steering sensor g250 steering gea reservoir power steering control unit j500 gear pump with pump motor 259_028 4

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the steering system fitted to volkswagen models is supplied by trw and by koyo the operating principle of both steering systems is identical the steering systems differ in how they determine the steering angle rate this is already recognizable from the external shape of the steering angle sensors in order to be able to see the power steering sensor in the vehicle it is necessary to raise the vehicle and to turn the steering to full right lock see arrow in fig on right for direction of viewing 259_043 the middle illustration opposite shows a trw steering fitted in the vehicle it is recognizable from the power steering sensor which is flat and wide in design more detailed description from page 13 on 259_002 the lower illustration shows a koyo steering installed in the vehicle its power steering sensor is cylindrical in shape more detailed description from page 15 on certain components of the two steering systems differ from each other and cannot be interchanged this applies both to electrical/electronic as well as to purely mechanical components such as track rods and track rod ends 259_001 5

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introduction overview of the system with its components the servotronic warning lamp k92 after the ignition is switched on the servotronic warning lamp k92 lights up an internal test cycle takes place during this time if the warning lamp does not go out after the engine is started and after the test cycle is completed faults may be stored in this case 259_042 power steering sensor g250 the sensor is integrated in the housing of the rotary disc valve of the power steering gear it detects the steering angle and calculates the steering angle rate if the sensor fails the steering function is nevertheless assured the power steering switches over in this case to a programmed emergency running mode the steering forces required are greater malfunctions are stored in the power steering control unit j500 the illustration opposite shows the power steering sensor of the trw steering system 259_005 this illustration provides you with a view of the power steering sensor of the koyo steering system 259_027 6

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the steering angle sender g85 the steering angle sender is located on the steering column between steering column switch and steering wheel this sensor is used on vehicle models which are available only with the electronic stability programme esp in this case the power steering sensor g250 is not fitted the abs control unit j104 and the power steering control unit j500 both make use of the signal supplied by the steering angle sender transmitted over the can databus for detecting the angle by which the steering wheel is turned 259_032 the power steering control unit j500 259_010 volume flow vehicle speed 259_004 st ra eer te ing the control unit is integrated in the engine pump assembly it converts the signals for driving the gear pump in line with the steering angle rate and the vehicle speed the delivery required at a particular moment is obtained from a map stored in the control unit the control unit detects and stores faults which occur during operation a restart protection and temperature protection are integrated in the control unit an gl e 7

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system overview system overview ­ schematic diagram servotronic warning lamp k92 speedometer sender g22 motronic control unit j220 control unit with display unit in dash panel insert j285 steering angle rate signal power steering sensor g250 power steering gear vehicle speed signal hydraulic fluid reservoir pressure limiting valve can can gear pump power steering control unit j500 power steering terminal +30 power steering terminal +15 earth ­ m pump motor 8 can engine speed signal 259_006 non-return valve

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advantages of the ephs ­ electrically powered hydraulic steering 600 500 average power consumption in [watts 400 300 200 100 0 standard power-assisted steering ephs 259_007 driving on motorway driving on main road driving in town in comparison to the traditional conventional power-assisted steering system the newly developed ephs electrically powered hydraulic steering offers a multitude of advantages ­ energy savings of as much as 85 ­ more environmentally friendly system as a result of lower energy demand and reduced energy use as well as smaller quantity of fluid in hydraulic system ­ taking realistic driving cycles as a comparison fuel savings of about 0.2 l/100 km are achievable ­ improvement in active safety as the steering is easy to operate when parking but is firm at high speeds when driving solely on motorways there is a high power loss at the bypass valve in a conventional power-assisted steering system as a result of the high engine speed in other words at low steering angle rates and high engine speed the power steering pump supplies an excess volume flow the new electrically powered hydraulic steering achieves its maximum economy from the low steering angle rate when driving on motorways and from the volume flow which is matched to the vehicle speed even when driving in towns the savings are still clearly perceptible refer also to chart 9

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design and function general the ephs electrically powered hydraulic steering is a power-assisted steering system which operates in line with the steering angle rate and vehicle speed the steering hydraulics pump v119 consists of a gear pump and the electric motor restart protection the ephs electrically powered hydraulic steering features a restart protection after a fault failure or crash the restart protection if activated because of a crash can only be deactivated with a diagnostic tool if other faults exist the restart protection can be cancelled by switching the ignition off and starting the engine again it may be necessary to wait about 15 minutes in order to allow the engine pump assembly to cool down after having overheated if after this waiting period it is still not possible to override the restart protection by starting the engine this indicates there is a fault in the vehicle electrical system or the engine pump assembly is faulty in such cases it is necessary to conduct a self-diagnosis and if the engine pump assembly is faulty to replace it this steering system makes use of a gear pump integrated in the engine pump assembly in place of the hydraulic pump vane pump with which you are familiar from previous power-assisted steering systems this gear pump is driven not directly by the internal combustion engine of the vehicle but by an electric motor integrated in the engine pump assembly the electric motor runs only when the ignition is switched on and the internal combustion engine is also running signals relating to the steering angle rate vehicle speed and engine speed are transmitted to the control unit this control unit regulates the speed of the electric motor as well as the gear pump and thus the delivery and volume flow of the hydraulic fluid 10

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the engine pump assembly the engine pump assembly is a self-contained component a special bracket for the engine pump assembly is attached in the left to the engine compartment to the longitudinal member between bumper and wheelhouse the engine pump assembly is flexibly mounted in rubber bushings at the bracket 259_008 cap the engine pump assembly combines the following components ­ the hydraulic unit with gear pump pressure limiting valve and electric motor ­ the hydraulic fluid reservoir ­ the power steering control unit the engine pump assembly does not require any maintenance it is lubricated internally by the hydraulic fluid it cannot be disassembled and no provision is made for repairs a pressure line links the pump to the power steering gear the return-flow line of the hydraulic fluid merges into the reservoir rubber bushing gear pump reservoir return-flow connection pressure hose connection power steering control unit electric motor 259_009 11

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design and function the power steering control unit j500 is an integral part of the engine pump assembly incoming signals ­ engine speed internal combustion engine ­ vehicle speed ­ steering angle rate task the control unit converts the signals for driving the gear pump in line with the steering angle rate and vehicle speed extended functions ­ temperature protection for power steering ­ restart protection after faults self-diagnosis the control unit detects faults during operation and stores them in a permanent memory pump function 259_010 ignition vehicle engine pump steering servo assistance available none on off running off vehicle speed 0 running not running steering servo assistance vehicle speed low e.g parking large e.g motorway 12 steering angle rate large delivery steering servo assistance high easy to operate low firm steering high small low

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trw the power steering sensor g250 task the power steering sensor g250 is installed at the top of the steering gear and embraces the input shaft of the steering gear it determines the steering wheel angles and calculates the steering angle rate it is not an absolute angle sensor steering wheel angle in proportion to steering wheel turns analysis of signal the signal is required in the power steering control unit in order to detect the movements of the steering the larger the steering angle rate the greater is the pump speed and thus the volume flow ignoring vehicle speed substitute function if the sensor fails the power steering switches into a programme to emergency running mode the steering function remains assured although greater steering forces are required self-diagnosis 259_005 259_012 the sensor is integrated in the self-diagnosis the power steering control unit stores any malfunctions of the sensor in function 02 interrogating fault memory it is possible to detect ­ short circuit to earth ­ open circuit/short circuit to positive ­ faults j500 30 15 30 15 can l can h can l can h s can can 15 m +31 5 electric circuit g250 j500 power steering sensor power steering control unit 31 g250 31 259_013 13

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design and function the capacitive sensor a rotor mounted on the input shaft rotates between nine small plate-type capacitors this distorts the capacitance of the plate-type capacitors the sensor electronics calculates the signals steering angle and steering angle rate for the power steering control unit from this change in capacitance trw basic diagram of distortion of capacitance rotor capacitor electrodes sensor electronics 259_014 plan view input shaft with rotor rotor power steering control unit capacitor electrodes 259_015 sensor electronics 14

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koyo the power steering sensor g250 task the power steering sensor g250 is installed at the top of the input shaft of the steering gear it determines the steering wheel angles and calculates the steering angle rate it is not an absolute angle sensor steering wheel angle in proportion to steering wheel turns analysis of signal the signal is required in the power steering control unit in order to detect the movements of the steering the larger the steering angle rate the greater is the pump speed and thus the volume flow ignoring vehicle speed substitute function if the sensor fails the power steering switches into a programmed emergency running mode the steering function remains assured although greater steering forces are required self-diagnosis the sensor is integrated in the self-diagnosis the power steering control unit stores any malfunctions of the sensor in function 02 interrogating fault memory it is possible to detect ­ short circuit to earth ­ open circuit/short circuit to positive ­ faults electric circuit g250 j500 power steering sensor power steering control unit 15 31 can can 15 30 15 30 15 259_027 can l can h can l can h s j500 m +31 5 g250 31 259_029

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