A METHOD OF AUTOMATIC GEAR SELECTION IN A VEHICLE POWER TRANSMISSION

Abstract: Method for automatic gear selection in a vehicle power transmission, said selection being catered for by a control system including an electronic unit (1) which, in response to signals representing vehicle speed and engine load, coacts with a programmable read-only memory PROM (28), in which there are stored, for each gear position, alternative speed limits for changing gear to neighbouring gears and also limiting values relating to speed change and engine load for selection between said alternative speed limits. To eliminate the risk of unsuitable gear selection for heavy vehicles, e.g. in conjunction with changing gear on a hill, the invention is essentially distinguished in that during travel, data on speed change and engine load are stored in a random access memory RAM (29); that the speed value most recently stored in said RAM before the initiation of the gear selection is compared, together with values stored in said RAM and relating to speed change and engine load at a certain instant before the initiation of the speed change, with values stored in said PROM and relating to speed, speed change and engine load, respectively; that said comparison results in the choice of a certain gear; that in the electronic unit (1) there is generated an output signal representing the selected gear; and that said output signal is fed to an operative means for indicating and/or engaging the selected gear.

Description: A method of automatic gear selection in a vehicle power transmission The present invention relates to a method of automatic gear selection in a vehicle power transmission, said gear selection being catered for by a control system including an electronic unit which, in response to signals represen¬ ting vehicle speed and engine load, coacts with a so-called read-only memory (PROM) in which there are stored, for each gear position, alternative speed limits for changing gear to neighbouring gears and also limiting values relating to speed change and engine load for selection between said alternative speed limits. Arrangements for automatic gear selection are already known, but such systems are primarily intended for auto a- tic gearboxes with a hydraulic torque converter. In automa¬ tic gear selection systems for purely mechanical transmis- sions, where gear changing is first enabled after the main clutch has interrupted the torque connection to the engine, problems often arise in obtaining a correct gear selection, e.g. when changing gear on a hill. Particularly with heavy vehicles, it is namely necessary that control of the engine load is accurately synchronized with the main clutch at the beginning of a gear change process. There is otherwise the risk that the vehicle is subjected to an appreciable change in speed before the torque connection is completely inter¬ rupted. Such a speed change could result in that signals fed into the gear selection system cause the selection of an unnecessarily low gear, which can be troublesome for the driver, and furthermore affect traffic rhythm negatively. The present invention has the task of providing a gear selection system where the risk of unsuitable gear selec- * tion has been eliminated. The invention is distinguished in that during travel, data on speed change and engine load are stored in a random access memory (RAM 29) ; that the speed value most recently stored in said RAM before the initiation of the gear selection is compared, together with values stored in said RAM and relating to speed change and engine load at a certain instant before the initiation of the speed change, with values stored in said PROM and relating to speed, speed change and engine load, respective¬ ly; that said comparison results in the choice of a certain gear; that in the electronic unit there is generated an output signal representing the selected gear; and that said output signal is fed to an operative means for indicating and/or engaging the selected gear. In the inventive method, the input signals generated in the gear selection system immediately prior to the commencing of the gear selection process are not used for selecting a gear. There are thus avoided undesirable varia¬ tions in the input signals, which could possibly occur immediately prior to the initiation of the gear change process. In an advantageous embodiment of the invention, values for speed change and engine load relating to an instant at least 0.5 seconds before the initiation of the gear selec¬ tion are used in the gear selection process. Other distinguishing features characterizing the inven¬ tion will be apparent from the description below and the following patent claims. The description is made with refer¬ ence to the appended drawings, where Fig. 1 schematically illustrates a system for automatic gear selection, Fig. 2 similarly illustrates a unit included in the system according to Fig. 1, Fig. 3 is a flow diagram for the gear change program, Fig. 4 is a flow diagram for a program relating to speed and acceleration calculation, and Fig. 5 is a flow diagram of the gear change routine. The exemplified arrangement is primarily intended for application in an automatic gear selection system for heavy vehicles, the selected gear being engaged in a gear change process initiated manually by the driver. A conventional mechanical transmission with a main clutch between engine and gearbox can thus be retained unchanged, which signifies JRΪ O PI that the comparatively high efficiency of the transmission can also be utilized. In an advantageous implementation of the gear selection system, the automatic gear selection may however be disengaged to enable a purely manual gear selec- tion. The system also allows casual manual operation even if the automatic gear selection system is engaged. After manual selection of a different gear, the driver has a certain time, ten seconds for example, to initiate the gear change process. The initiation may, for example, take place by the clutch being depressed, the system then sensing the movement and thereafter automatically taking care of the gear change. When the gear change is completed, this is indicated to the driver by a buzzer and/or an indication lamp, whereupon the driver can again engage the clutch. The system is thereafter prepared for a new gear selection. The gear selection system in question operates with input signals relating to a number of parameters which are schematically illustrated in Fig. 1. A signal representing the engine load is thus fed to a control unit 1 via a line 2. With an engine equipped with supercharging, said load signal is suitably proportional to the boost pressure in the engine induction system 12. In another application, the position of the vehicle accelerator pedal may constitute a measure of the engine load. Via a line 3 there is fed to the control unit a signal corresponding to the speed of a vehicle wheel, which in most cases is proportional to the speed of the vehicle. In practice, it is suitable to sense the r.p.m. of the gearbox output shaft 13. A signal respon¬ sive to the position of a clutch 14 between engine and gearbox and operable by the driver is taken to the control system via a line 4 as well as a signal via a line 5, the latter signal being responsive to whether the engine is running or not. The latter state is suitably sensed at the generator 15 conventionally connected to the engine. The input signal in a line bunch 6 represents the position of a function selector 16 operable by the driver and which includes the four following alternative positions: a first position relating to driving with manual gear selection, a second position relating to driving with automatic gear selection, a third position relating to selection of neutral gear, and a fourth position relating to selection of reverse gear. A signal from a manually operable gear lever 17 is fed to the control unit 1 via a line bunch 7, and via a line bunch 8 signals representing the state of the gearbox 18, i.e. if a gear is engaged and if so, which one. In response to said input signals, the control unit 1 generates output signals. Output signals are fed via a line bunch 9 to operative means 19 for executing the gear change, e.g. to a number of solenoid valves controlling compressed air or pressurized hydraulic oil to a number of cylinder units executing the gear change movements. Output signals are also fed via a line bunch 10 to a display unit 50 having a-buzzer or similar means for notifying the driver of the gear selection, the gear change completion or whether errors have occured in the control system. The control unit illustrated in Fig. 2 is built up around a microprocessor denoted MP 20 which, in a manner known per se, via a number of input circuits 21 is conven¬ tionally fed with input signals via the lines 2, here represented by a line bunch 22. Output circuits 23 adjust the control unit's signals on the line bunches 9, 10, here represented by a line bunch 24, to what is required for actuation of the operative means 19 and the display/buzzer unit 50. In the input and output circuits 21, 23 there are also protective circuits which protect the MP 20 against signals disturbing its function. The output circuits 23 also include an amplifier which amplifies the output signal of the control unit to a level required to actuate the solenoid valves. Via data lines, address lines and control lines, i.e. data bus 25, address bus 26 and control bus 27, the MP 20 coacts in a manner known per ≤e in the microcomputer art with said input and output circuits 21, 23 as well as with OM an external programmable read-only memory denoted PROM 28, and a random access memory denoted RAM 29, built into the MP 20. This is the case, for example, if the MP 20 is of the 6802 type (e.g. made by Motorola), which type has been found advantageous in applying the present invention. In the control unit there is also included a low- frequency oscillator 48, the pulsed output signals of which are applied via the line 49 to the input circuits 21. Each signal pulse from the oscillator 48, as with each pulse in the signal pulses sent from a speed transducer, results in that the MP 20 is -inhibited from executing an interrupt program for speed and acceleration calculation. The speed is calculated as the number of speed pulses coming in be¬ tween two ' pulses from the low-frequency oscillator 48, which corresponds to a. time period of 0.5 seconds. The pulses from the low-frequency oscillator are hereinafter called time base pulses. The speed value thus represents a mean value for said time period. The speed change is calculated as the difference between two sequentially calculated speed values. Whether the signal representing the engine load indicates a high or a low value is regis¬ tered for each time base pulse. A program is stored in the PROM 28 for automatic gear selection, and when the program is being executed, a flow diagram illustrated in Fig. 3 is followed. Said gear selection program constitutes a part of a more extensive gear change program, which has gone through a plurality of control routines before a flow path 30 follows the diagram according to Fig. 3. With the availability of manual opera- tion of the automatic gear change system, there is required a routine to decide to what extent the operating means for manual gear selection is actuated or not. Said routine is represented by a block 31 illustrated in the figure. If said operating means is activated, the block 31 begins a count-down of a time register, e.g. 10 seconds. Within this time, the driver has the possibility of utilizing the manual gear selection. If he does not avail himself of this, the system returns to automatic gear selection which, according to the flow diagram, begins at a signal proces¬ sing operation step denoted BS 32, at which an acceleration value stored in an acceleration register by means of the interrupt program is read.- In a subsequent comparing opera¬ tion step denoted JS 33, it is determined whether the speed change value constitutes a retardation value which exceeds a given predetermined retardation value, e.g. 5 π/s". If this is the case, e.g. as a result of wheel locking during braking, the program chooses to follow a flow path to an operation step denoted BS 35, at which a signal value representing great retardation is noted in a retardation notation register in the RAM 29. After BS 35 there is an operation step denoted BS 36, where selection of neutral gear is made, the gear change program thereafter being ter-. inated by an operation step denoted BS 47, where indication of the selected gear on the display unit is catered for. If said predetermined retardation value in JS 33 is not exceeded, the flow path 30 leads to a further comparing operation step denoted JS 37, at which it is determined whether a fault memory in the RAM 29 has registered a value representing a fault in respect of the speed transducer, which can take place at a run-through of another part of the gear change program. When registering a speed error in the fault memory, the program chooses to follow a flow path 38 to BS 36, where neutral gear is selected. If a speed fault has not been detected, there is a following operation step denoted JS 39 where the system checks whether great retardation is still noted in the retardation notation register. If this is not the case, the program follows a flow path 40 to a block 46, representing a routine for the actual gear selection, and in a way illustrated in Fig. 5 this routine uses the values of speed, speed change and load worked out in the interrupt program for selection of a suitable gear from tables stored in the PROM 28. If the notation "great retardation" remains in the retardation notation register, the flow path 30 leads to a block where it is determined, in a manner not described in more detail here, if the vehicle wheel subjected to great retardation has regained a stabilized state relative the substructure. If the wheel is still not stabilized, a flow path 42 leads to BS 47, where the neutral gear already selected in BS 36 is registered and indicated as the re¬ commended gear. All the registers used in the description of the flow diagram constitute a part of the RAM 29 included in the system. In Fig. 4 there is illustrated a flow path for carrying out the program for speed and acceleration calcu¬ lation. In a comparing operation step denoted JS 60, it is determined whether the signal which triggered the interrupt program is a speed pulse. If this is the case, the signal processing follows the flow path 61 to a signal processing - operation step denoted BS 62, which ensures that a speed pulse register, denoted HPR, is increased by the digit 1. The speed calculation routine is subsequently terminated by erasing in an operation step denoted BS 63 a notation made in the RAM 29, a so-called flag, relating to the interrupt routine for the speed pulse. Such a flag constitutes a command to the MP 20 to interrupt the ordinary program and instead execute a smaller separate program, which thus in this case relates to the speed calculation. If it is established at JS 60 that the incoming inter¬ rupt pulse does not constitute a speed pulse, the signal processing follows a flow path 64, and at a comparing opera¬ tion step denoted JS 65 it is determined whether the in¬ coming pulse is a time base pulse from the low-frequency oscillator 48. If this is not the case, the program follows a flow path 66, where an operation step denoted BS 67 ensures the registration of a system fault in a fault memory, subsequent to which the interrupt routine is broken off. If it is established at JS 65 that the interrupt pulse constitutes a time base pulse, the program follows a flow path 68. In an operation step denoted BS 69, the content in a first speed register, denoted KR 1, is transferred to z . O PI second speed register denoted HR 2, as well as the content in HPR to HR 1. The operation step BS 69 is terminated by zeroing HPR. In a subsequent operation step, denoted BS 70 in the flow path 68_, the register content in an acceleration register denoted ACCR 3 is transferred to an acceleration register denoted ACCR 4. The register content in an acceleration register denoted ACCR 2 is thereafter trans¬ ferred to the acceleration register ACCR 3 and finally, the content in an acceleration register denoted ACCR 1 is transferred to ACCR 2. In an operation step subsequent thereto, denoted BS 71, the speed change is calculated, i.e. the difference between the contents of HR 1 and HR 2, which is then stored in ACCR 1. In the ' flow path 68 there is then an operation step denoted BS 72, in which transfer of a number of load values registered in a corresponding number of load registers de¬ noted BR is executed. Accordingly, the content in a step BR ' 5 is tranferred to a step BR 6, the content in a step BR 4 to a step BR 5, the content in a step BR 3 to a step BR 4, the content in a step BR 2 to a step BR 3, and finally, the content in step BR 1 to a step BR 2. An operation step denoted BS 73 ensures storing of the in¬ stantaneous sensed load value in BR 1, which represents either high or low engine load. The interrupt program according to the flow path 68 is terminated at an operation step denoted BS 74 not described in more detail here, in which reduction is executed in the appropriate time value register which is utilized for providing predetermined time lags. A flow diagram illustrated in Fig. 5 for the gear selection routine is referred to in Fig. 3 as the block 46. This block includes a comparing operation step denoted JS 81, in which it is determined whether the sum of the con- tents in ACCR 2, ACCR 3 and ACCR 4 is less than zero. If this is the case, the flow path 82 is followed to a com¬ paring operation step denoted JS S3, where it is determined whether the content in any one of the calculating registers BL 2-6 represents a high load value. If this is the case, a flow path 84 leads to the selection of a suitable gear according to a table C stored in the PROM 28. On the other hand, if all the calculation registers BR indicate_ low load, a flow path 85 leads to selection according to a .table B, If it is found in the operation step JS 81 that the sum of the contents in ACCR 2, 3 and 4 are not less than zero, the program follows a flow path 86 to a comparing operation step denoted JS 87, in which it is determined whether the sum of the contents in ACCR 2, 3 and 4 is greater than a predetermined number, e.g. 8, representing 2 the speed change 0.8 m/s . If this is the case, the system chooses gear change points according to the speed limits which are given in a table A. If the sum is less than the predetermined number, the flow path 86 leads to a comparing operation step denoted JS 88 , in which it is determined whether the content in BR 2 corresponds to a high load value. If this is the case, gear change points are chosen according to table B, and if the content in BR 2 corre¬ sponds to a low load value, gear change points are chosen according to table A. In an operation step denoted BS 90, the gear is then selected by comparing the latest speed value with the speed limits in the chosen table. The selected gear is subsequently registered and indicated in BS 47, illustrated in Fig. 3. By utilizing the average value for a number of regis¬ tered past speed change values and load values, there is avoided the effect of possible variations in sensed para¬ meters immediately before a gear change process is initia¬ ted. A smooth gear selection has been achieved by utilizing parameter values relating to conditions prevailing at least 0.5 seconds ago. The speed change value utilized in JS SI and JS 87 for comparison with the limiting values stored in the PROM 28 should furthermore suitably be dependent on at least three separate speed change values stored in the RAM 29. Simi¬ larly, for a well-functioning gear selection, the compari¬ son made in JS 83 with a limiting value stored in the PROM 28 should utilize at least three separate engine load values stored in the RAM 29. OMPI Λ- WIPO

Claims: CLAIMS 1. A method of automatic gear selection in a vehicle power transmission, said gear selection being catered for by a control system including an electronic unit (1) which, in response to signals representing vehicle speed and engine load, coacts with a so-called read-only memory (PROM 28) in which there are stored, for each gear position, alternative speed limits for changing gear to neighbouring gears and also limiting values relating to speed change and engine load for selection between said alternative speed limits, characterized in that during travel, data on speed change and engine load are stored in a random access memory (RAM 29); that the speed value most recently stored in said RAM before the initiation of the gear selection is compared, together with values stored in said RAM and relating to speed change and engine load at a certain instant before the initiation of the speed change, with values stored in- said PROM and relating to speed, speed change and engine load, respectively; that said comparison results in the choice of a certain gear; that in the electronic unit (1) there is generated an output signal representing the selec¬ ted gear; and that said output signal is fed to an operative means for indicating and/or engaging the selected gear. 2. A method as claimed in Claim 1, characterized in that in the gear selection process there are used values for speed change and engine load relating to an instant at least 0.5 seconds before the initiation of the gear selec¬ tion process. 3. A method as claimed in Claim 1 or 2, characterized in that the speed change value utilized for comparison with a corresponding stored value in the PROM (28) is responsive to at least three separate speed change values stored in the RAM (29). 4. A method as claimed in Claims 1, 2 or 3, charac- terized in that at least three separate engine load values stored in the RAM (29) are utilized for comparison with a corresponding stored value in the PROM (23) . OMPI V

Inventors: MAGNUSSON KARL G, STAHL HANS V
Applicants: MAGNUSSON KARL G, SAAB SCANIA AB, STAHL HANS V
Application year: 1981
Main classification: B60K20
IPC codes: F16H59, F16H61