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613 Outline of a new control concept for power shifting of xed step ratio automotive transmissions B Jacobson Machine and Vehicle Design Chalmers University of Technology SE 412 96 Go teborg Sweden Abstract Power shifting transmissions e g automatic transmissions are traditionally shifted from gear A to B by disengaging a clutch belonging to gear A and engaging a clutch belonging to gear B This paper tries out the idea of employing a third clutch In practice such a third clutch can be one of the clutches which is traditionally used only for other gears This means that the new concept can be used with only a control software update in an ordinary gearbox The trade oV between comfort and wear which is experienced in the traditional control concept becomes less troublesome The wear or the discomfort can be reduced to roughly half of that corresponding to a traditionally controlled upshift The work nds the largest potential in using an extra clutch which belongs to a gear higher than the high gear in an upshift The theoretical study is carried out with a general and simple model but a simulation example is added using a more detailed model of an existing gearbox Keywords power shift automatic transmission control shift quality comfort wear NOTATIONtrad newtraditional and new control concepts respectively 0dimensionless quantity not used for W A Bconstants in driving resistance expression 1peak value e g cH01in Fig 3 ccapacity of clutch i e a quantity proportional to applied external force in a clutch 1BACKGROUND Ddiscomfort de ned as time derivative of dimensionless torque Power shifting transmissions have the great advantage Jmoment of mass inertia of being able to shift gear without interruption ofm mass mechanical power ow The concept has been in auto P power motive use since the beginning of the 20th century Thes span i e ratio between ratio of two majority of power shifting transmissions have been gears implemented as traditional automatic transmissions withTtorque hydraulic torque converters and hydraulically actuatedWwear in clutch de ned as energy clutches and brakes mounted on a gear transmission ofdissipation dimensionless planetary type small positive value The mechanical phenomena of power shifting are well rotational speed angle time described in for example references 1 and 2 How to design the control of gear shifts has been studied in for example references 3 to 6 However none of Subscripts these re ects on the idea of using more clutches than clutches A and B de ned as the clutch belonging to theeengine gear before the shift and after the shift respectively in outinput and output shafts of gearbox respectively ip tpinertia and torque phase respectively 2DRIVING FORCES AND AIMS OF THIS WORK L H Xclutches for lower and higher gear and extra clutch respectively Aprobabledevelopmentofautomaticgearboxes The MS was received on 18 April 2000 and was accepted after revision involves for publication on 18 December 2000 D04300 IMechE 2001Proc Instn Mech Engrs Vol 215 Part D 614B JACOBSON a eliminating the converter in order to improve fuelthe trade oV between comfort and wear and nd a way to quantify it in a diagram see Fig 4 economy and to reduce weight and cost For an upshift a clutch for the lower gear is released b introducing more gears in order to improve fuel and a clutch for the higher gear is engaged Since trans economy emission and drivability missions are often designed with planetary gears it is This development is supported by more complex control not very obvious which clutch refers to each gear of the clutches i e control and computer engineering However reference 1 shows a way to write the equa and actuator technology Also sensor technology e g tions in a dimensionless form Then the equations can for estimation of sliding speed in the clutches is of inter be interpreted as the more simple system shown in Fig 2 est even if this has the drawback of implying more cost Then it is more intuitively clear that the shift is princi The present work aims to investigate the potential of pally just a matter of switching branches for the power using more of the gearbox clutches in a shift than just ow and that it can be done by operating the two the clutch for lower gear and clutch for higher gear This clutches involved Assume constant chassis speed out is an example of how improved control possibilities are constant and out0 1 which is almost the case during used for exploiting a potential of the mechanical part most shifts Also assume constant engine torque Te involved constant and Te0 1 which is a more serious simpli This work is conceptual and it has the following cation but it does not change the principal control limitations diYculties in the shift The bottom version in Fig 2 may therefore seem to be a special case but reference 1 1 No practical veri cation is made shows that it covers all reasonable gearbox layouts 2 The clutch for the lower gear is assumed to be con except for situations with reversed power ow i e engine trolled with perfect timing This could be considered braking on the lower gear This engine braking case is as using a one way clutch which eliminates the need covered by dimensionless equations in reference 1 but for external control it cannot be given the straightforward physical interpret 3 Only upshifts with clearly positive power ow i e no ation in Fig 2 However the present work does not engine braking are carefully analysed Downshifts claim to cover the engine braking case are discussed qualitatively in Appendix 1 In an upshift clutch L should be released and clutch H 4 The theoretical discussion uses a generalized and should be engaged Clutch L is supposed to be perfectly simple system model i e constant engine torque controlled i e it is released when starting to take torque engine torque does not vary with engine speed and load in the wrong direction This can be designed in is not reduced during gear shift constant chassis practice with clutch L as a one way clutch which elimin speed no converter and no elasticity The simulation ates the need for controlling it If clutch L is not a one example is more realistic see Section 6 way clutch there will be practical problems in disengag The principal idea is that the two control aims comfort ing it with appropriate timing which will lead to either and low wear cannot be ful lled using only one control tie up or are These phenomena are discussed in cH but adding one more control c X could decouple references 1 and 2 but they are not considered in the the aims and thereby reduce the trade oV This is present work Tie up is when too many clutches are visualized in Fig 1 engaged which forces the engine down in speed Flare is the other way around The overall control aim is to take inand Toutfrom 3TRADITIONAL SHIFT CONCEPT AND ITS point A to point B in Fig 3 First the control system TRADE OFF increases cH0and eventually reaches cH0 1 During this phase the torque phase clutch L is unloaded by In this section how power shifts are carried out tra clutch H which eventually carries the whole load ditionally will be described The presentation will onlyHowever the speeds of the transmission still remain as at the lower gear In order to change the speeds also go into as much detail as is needed to be convincing of Fig 1Visualization of the principal idea for the present work D04300 IMechE 2001Proc Instn Mech Engrs Vol 215 Part D 615OUTLINE OF A NEW CONTROL CONCEPT FOR POWER SHIFTING Fig 2Generalization and simpli cation Upper possible layout of a gearbox with planetary gear mesh Lower interpretation as ordinary gear meshes based on dimensionless version of equations from reference 1 Fig 3Reference shift based on traditionalcontrol concept using only clutches L and H Clutch L is assumed to be controlledwith perfect timing Clutch H is controlledto avoid steps in Toutand to keep a certain absolute magnitude D of dTout dt in order to have only one measure of comfort D04300 IMechE 2001Proc Instn Mech Engrs Vol 215 Part D 616B JACOBSON clutch H is further engaged to a value cH0 1 Then theSince the shift is described with a single parameter D a certain W will be found for each value of D Since thetorque on the right hand side of the engine ywheel Tin0 becomes 1 which makes the engine ywheel deceler model is simple and the control of cH0is assumed to be piecewise linear in time the following analyticexpressionate This phase is called the inertia phase and it ends when clutch H reaches zero sliding speed can be found As seen in Fig 3 it is assumed that clutch H is con trolled in such a way that the absolute magnitude of the W Wtp Wip 1 1 s 2 2D J0 2A1 1 sB 2 cH01 1 cH01 1 derivative of Toutis kept constant at D during the shift 3 This reduces the number of parameters needed to de ne the shift fully The slope D will be used as the measure where of discomfort In practice it is diYcult to nd the timing of cH0which makes it exactly 1 when the inertia phase D c H01 1 2 J0 s 1 ends Instead one has to make sure that cH0is slightly above 1 i e cH0 1 when this happens which results This means that a diagram like Fig 4 can be plotted in a discontinuous change of Toutfrom cH0 s 1 s Here in the trade oV between comfort and wear increas to 1 s when the inertia phase ends This sudden drop in ing one increases the other and vice versa An expression Toutcontributes to the discomfort but since it can be for the total shift time can also be found eliminated through good timing it is not included in the study It is also a phenomenon that cannot be dealt with ttp t ip 1 1 s D 2J 1 1 s cH01 1 4 using the new control concept In the more realistic simu lation example in Section 6 perfect timing is assumed in The total shift time is also plotted in Fig 4 Since a the end of the torque phase normal shift considering a passenger car during a similar The two measures discomfort and wear can be shift operation takes approximately 0 5 s practically identi ed in Fig 3 It is debatable exactly how to quan reasonable values of D and W are very roughly D tify discomfort but most should agree that rapid 5 5 and W 0 2 which should be noted for comparison changes or large steps in traction torque result in bad with the new shift concept in Section 5 comfort Therefore discomfort is de ned as the magni It should be mentioned that using a conventional tude of torque change clutch L instead of a one way clutch L opens up new possibilities in that Toutdoes not have to be reduced allD KdTout dtK 1 the way to 1 s in the torque phase However this involves a diYcult control problem where the engine inertia has Similarly the wear phenomena are very complex but to be balanced under are conditions i e a situation for this type of clutch it is more or less accepted that where both clutches slip in the positive direction These the heat release in the clutch lining is the main problem solutions are not treated in this paper For a certain gearbox at a certain shift operation it is found practically that wear increases with sliding time i e the heat energy and not the heat power is a good 4CONTROL DESIGN FOR NEW CONCEPT measure of wear From this reasoning the following de nition of wear is used In Fig 5 an extra clutch is drawn clutch X In this section it will be found that gear X is needed if clutch XW P TH0 relH0dt 2 Fig 4Trade oV between discomfort and wear for an upshift using the traditional control concept Plotted for span s 1 75 and dimensionless inertia J0 1 D04300 IMechE 2001Proc Instn Mech Engrs Vol 215 Part D 617OUTLINE OF A NEW CONTROL CONCEPT FOR POWER SHIFTING Fig 5Generalization and simpli cation as in Fig 2 but also using an extra clutch X is to be useful during the shift and also how it can bebetween gear L and gear H The same demands applied to the case with sXs whichcontrolled to be comparable with the traditional shift is contradictory to sX1 s Accepting the same loss energy the unloading to zero torque Equations from reference 1 torque phase could be extended in time i e the comfort extended with the clutch X give could be improved An additional positive eVect is that the energy loss would be shared between clutch H and clutch X which makes it possible to extend the torqueTL0 1 cH0 sgnA1 1 sXB cX0 5 phase further without reaching some critical value of energy loss in either of the clutches H and X Tout0 1 A1 1 sB cH0 K1 1 sXK cX0 6 However this work will not study further the use of an extra clutch in the torque phase since the main prob lem to be solved is found in the inertia phase Then it is It is to be investigated whether a clutch X can help in concluded that WH tpis identical to the traditional shift unloading the clutch L From equation 5 it is clear i e the rst term in equation 3 WH tp 1 1 s 2 2D that such unloading requires sX 1 gear X higher than and WX tp 0 gear L or sX1 s forbeen decreased to 1 s During the inertia phase clutch H the case with sX 1 leads to a new demand sX s which is brought in to stick which means reducing the engine speed from in0 1 to 1 s At the very end of the inertiabinds sXto the interval 1 sX in0 1 s it would be inconvenient if 1 sX s since the signumwhich eliminates a discontinuity in Tout which is the same assumption as for the traditional shift term would then change sign during the inertia phase Consider only the two cases In order to decelerate the engine it is required that d in0 dt Te0 Tin0 J0Te0 a sXs gear X refers to a gear higher than gear H Tin0 c H0 sgn in0 1 sX cX0 1 Te0 7 The diagrams in Fig 6 can now be drawn In order to The output torque will follow the equation have a shift that is comparable with the traditional shift a shift principally like that in Fig 7 is assumed Tout0 c H0 s sgn in0 1 sX cX0 s X 8 Fig 6Plots for xed comfort D 1 and the case where the extra clutch belongs to a higher gear than gear H sX s Upper wear of both involved clutches Lower combinations of cH01and cX01 The small circle marks the optimum point i e where the wear W min WH WX is minimized Fig 7Plots for xed comfort D 1 and the case where the extra clutch belongs to a lower gear than gear L sX 1 Upper wear of both involved clutches Lower combinations of cH01and cX01 The small circle marks the optimum point i e where the wear W min WH WX is minimized D04300 IMechE 2001Proc Instn Mech Engrs Vol 215 Part D 619OUTLINE OF A NEW CONTROL CONCEPT FOR POWER SHIFTING It can be shown using equations 1 and 2 and3 Output corresponding equations for clutch X that a datapointsforthegraphsWH cX01 and WX cX01 under the restriction of a certain D two examples are shown in Figs 8 and 9 WH ip J0 1 1 s 2 2 cH01 1 cH01 cX01 1 9 b optimal choice of cH01and cX01 i e the choice which gives the lowest max WH WX such WX ip J0 1 1 s 1 1 sX 1 1 s 2 2 curves are shown in Fig 10 The analysis behind the plot in Fig 10 assumes that the6 cX01 cH01 cX01 1 10 clutches have a limit for how much they can be applied in the case sX 1 It is formulated as cX01 5 It D AcH01 s cX01 sX 1 sB cH01 cX01 1 J0 1 1 s 11 indirectly limits cH01to approximately 13 or 14 With higher set limits better results could be achieved in the where the upper of signs and refers to sX s The typical shift from Fig 4 is marked as point A in Fig 10 If it is assumed that this sets an acceptable amount of discomfort D 5 5 the possible reduction 5TRADE OFFS IN THE NEW CONTROL of wear can be found as point B Wear is then reduced CONCEPT from W 0 2 to approximately W 0 12 On the other hand if point A sets an acceptable amount of wear W So far W and D can be calculated given values of cH01 0 2 one can nd the potential to reduce discomfort to and cX01 However it is desired to nd the limits of what point C Then the discomfort is reduced from D 5 5 is possible in the W D plane i e the trade oV curve for to D 1 4 These improvements must be considered comparison with Fig 4 Therefore the algorithm below substantial is outlined It is assumed that both clutches involved are From Fig 10 it is clear that the case sX s is the most equally sensitive to heat dissipation which means that promising one In broad description this case uses two the measure of wear is W max WH WX principles for reducing the con ict between wear and discomfort 1 Input a value of D 2 Loop over diVerent values for cX01 1 The heat energy is shared between two clutches which a for each cX01 a cH01can be found since D is automatically reduces the wear This should be intuit xed ively reasonable even without any kinematic analysis b for each cX01 WHand WXcan then also be 2 The torque of the extra clutch helps clutch H indetermined retarding the engine ywheel but with less in uenceChoose the cX01value which gives the lowest W max WH WX on output torque than clutch H i e less discomfort Fig 8Points A mark how to satisfy the requirement Tout0 1 s Points B mark the situation with the traditional control concept with the same d in dt Numerical values used for plots are s 1 75 sX 0 75 and 2 5 respectively D04300 IMechE 2001Proc Instn Mech Engrs Vol 215 Part D 620B JACOBSON Fig 9Shift layout for upshift employing an extra clutch X Fig 10Trade oV between discomfort and wear for an upshift s 1 75 and J0 1 using the new control concept Plotted for sX 0 75 and 2 5 respectively This requires a kinematic analysis in order to be fullyof the gearbox In such a case clutch X will not support clutch H in retarding the engine but it will still reduceunderstood A simple form of analysis without any equations is shown in Fig 11 the discomfort in that output torque peaks can be reduced The other extreme case when sX 0 corre sponds to when clutch X works as a brake mounted onThe extreme case when sX 2 corresponds to when clutch X works as a brake mounted on the output sidethe input side of the gearbox which only helps clutch H D04300 IMechE 2001Proc Instn Mech Engrs Vol 215 Part D 621OUTLINE OF A NEW CONTROL CONCEPT FOR POWER SHIFTING Fig 11Simple example of how clutch X helps clutch H when sXis large i e extreme version of sX s Fig 12Realist