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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 andout0=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 takeinand 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 cH01. 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= (11/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 H011)2 J0(s1)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= 11/s D + 2J(11/s) cH011 (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 TH0relH0dt(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=1cH0sgnA1 1 sXB cX0(5) phase further without reaching some critical value of energy loss in either of the clutches H and X. Tout0=1A1 1 sB cH0K1 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=(11/s)2/2D that such unloading requires sX1 (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 sX1 leads to a new demand: sX1/s, it would be inconvenient if 11=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(in01/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 (sXs). 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 (sXs 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 din/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 sXs Fig. 12Realistic model. System level and brief component description. In the equations, subscripts 1 and 2 denote left and right sides of component respectively to retard the engine and does not in?uence output7CONCLUSIONS torque. Even these extreme cases have potential to There is potential bene?t in using an extra clutch duringimprove the trade-oV between discomfort and wear. an upshift in a power shifting transmission. This is shown for a general gearbox, using a dimensionless model and analytical calculations. A more detailed 6MORE REALISTIC SIMULATION EXAMPLE model veri?es the conclusion for a realistic case, using dynamic simulation. The control for the new concept This section shows fully dynamic simulations with a does not need to be much more complex than for a realistic vehicle model, including torque converter, drive traditional shift. Automatic gearboxes contain typically shaft elasticities and tyre slip damping. The engine is ?ve or six clutches, but only two of these are traditionally modelled as following a curve of torque versus speed, used in each shift. The idea is to use the extra clutches which is a steady state engine model for a constant only by introducing new software for control. This throttle position. The control algorithm is implemented should be a fully realistic modi?cation in the case of according to Appendix 2. modern transmissions with solenoid-actuated clutches. Figure 12 shows a top level diagram of the model In the torque phase, use can be made of an extra clutch and Fig. 13 shows the gearbox model. Models are belonging to a gear between lower gear (gear L) and implemented in the modelling and simulation software higher gear (gear H). Comfort can be improved by Dymola 7. Simulation results are plotted in Fig. 14. extending the torque phase in time. This is possible with- The new shift shows less variation of torque (whichout reaching th