Switch-selectable transmission control with safety mechanism for transmission-equipped vehicles
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a switch-selectable transmission control with a safety mechanism for transmission-equipped vehicles. 
     2. Description of the Prior Art 
     FIGS. 1 and 2 show a conventional finger-touch selectable transmission that achieves gearshift by shifting the position of the gearshift fork longitudinally (as indicated by arrow (A) and laterally (as indicated by arrow (B). 
     Gearshift is done in accordance with the control signals outputted from an electric gearshift circuit d that receives selection signals from a finger-touch gear-selection switch c and controls the opening and closing of solenoid valves a1 and a2 (which permit gearshifts in direction A in FIG. 1) and b1 and b2 (which permit gearshifts in direction B in FIG. 1). 
     Such a conventional transmission control, however, allows a gearshift to be accomplished even when there is a wide gap between the vehicle speed and the rotating speed of the engine if a push-button on the gear-selection switch c is depressed. A steep downshift from high gear to low gear, for example, can give rise to an engine overrun or a clutch burst. 
     SUMMARY OF THE INVENTION 
     This invention solves the aforementioned problem. The object of this invention is to provide a switch-selectable transmission control with a safety mechanism for use with transmission-equipped vehicles which prevents hazardous gearshifts by presetting a vehicle speed limit for each gear, allowing a gearshift if the vehicle speed is within the preset limit, and forcibly maintaining the transmission in neutral, whichever gear is selected, when the vehicle speed is outside the preset limit. 
     For achieving this object, the switch-selectable transmission control for transmission-equipped vehicles according to this invention comprises a gear-selection switch, an electric gearshift circuit that controls, via a logic circuit, the opening and closing of solenoid gearshift valves in accordance with the gear selection signals from the switch, a vehicle-speed limit setter that outputs the vehicle speed limit, as an electric signal, that permits a shift to the gear that has been selected based on the signal from the switch, a vehicle-speed sensor, and a comparator circuit that determines whether the shift to the gear selected by the switch is possible or not comparing an electric signal based on the detection signal from the vehicle-speed sensor with an electric signal from the vehicle-speed limit setter, with the comparator and logic circuits connected together to constitute a safety mechanism so that the shift to the gear selected by the switch is allowed when the judgement made by the comparator circuit is positive and not allowed, with the transmission kept in neutral, when the judgement is negative. 
     Because of this feature, the switch-selectable transmission control with a safety mechanism for transmission-equipped cars of this invention prevents hazardous gearshifts by presetting a maximum vehicle speed limit for each gear, allowing a gearshift if the vehicle speed is within the preset limit, and forcibly maintaining the transmission in neutral, whichever gear is selected, when the vehicle speed is outside the preset limit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 show a conventional switch-selectable transmission control for transmission-equipped vehicles. 
     FIG. 1 schematically shows the arrangement of speed-changing gears, while FIG. 2 is an overall block diagram. 
     FIGS. 3 and 4 show a switch-selectable transmission control with a safety mechanism for transmission-equipped vehicles according to this invention. 
     FIG. 3 is an overall block diagram, while 
     FIG. 4 is a block diagram showing the principal part that is composed of an analog electric circuit. 
     FIG. 5 is a block diagram showing a second embodiment of this invention in which the principal part is made up of a digital electric circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMMENTS 
     Referring now to the accompanying drawings, preferred embodiments of this invention will be described. 
     Reference numeral 1 in FIG. 3 designates a gear-selection switch where a finger-touch selection of a transmission gear is made. Then, an electric signal Sa corresponding to the gear selection just made is supplied to an electric gearshift circuit 2. 
     Adapted to output electric signals Ba, Bb, Bc and Bd that selectively open and close solenoid valves 5 and 6 that move a select-fork in the gearshift unit (laterally or in the direction identical with the direction B in FIG. 1) and solenoid valves 3 and 4 that move the shift-fork (longitudinally or in the direction identical with the direction A in FIG. 1) respectively on receipt of the gear-selection signal Sa, the electric gearshift circuit 2 is connected to the solenoid valves 3 and 4 via OR circuits 7 and 8 which serve as logic circuits and is connected directly to solenoid valves 5 and 6. 
     The signals Ba and Bb are supplied to the solenoid valves 3 and 4 via the OR circuits 7 and 8, while the signals Bc and Bd are supplied direct to the solenoid valves 5 and 6, respectively. 
     Responding to the signal inputted from the gear-selection switch 1, the electric gearshift circuit 2 outputs signals that open or close the solenoid valves 3, 4, 5 and 6 in accordance with predetermined sequences. Ultimately, the solenoid valves 3, 4, 5 and 6 are opened or closed as shown in the following table. 
     
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         Solenoid Valve                                                   
Gear Selected                                                             
           3        4         5      6                                    
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R          Closed   Opened    Opened Closed                               
N1         Opened   Opened    Opened Closed                               
1          Opened   Closed    Opened Closed                               
2          Closed   Opened    Opened Opened                               
N2         Opened   Opened    Opened Opened                               
3          Opened   Closed    Opened Opened                               
4          Closed   Opened    Closed Opened                               
N3         Opened   Opened    Closed Opened                               
5          Opened   Closed    Closed Opened                               
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     Provision is made so that when the solenoid valves 3 and 4 are simultaneously opened, the shift-fork is held in a longitudinal mid-position (corresponding to position N1, N2 or N3 in FIG. 1) or in neutral. When the solenoid valves 5 and 6 are simultaneously opened, the select-fork is held in a lateral mid-position (corresponding to position 2, N2 or 3 in FIG. 1). 
     A safety circuit 10 compares signal Sb from the gear-selection switch 1 with vehicle-speed pulse signal Sc corresponding to vehicle speed X from the vehicle-speed sensor 9 and initiates an electric signal to maintain the transmission in neutral when the vehicle speed is not appropriate for the gearshift. 
     As shown in FIG. 4, the safety circuit 10 has a gear-shiftable vehicle-speed limit setter 11 that outputs an electric signal that indicates a gear-shiftable vehicle-speed limit corresponding to the gear selected based on the signal Sb from the gear-selection switch 1. 
     The gear-shiftable vehicle-speed limit setter 11 is provided with a gear-selection circuit 12 that selects the gear corresponding to the signal Sb from the gear-selection switch 1 and a data-setting circuit 13 that receives a signal from the gear-selection circuit 12, sets the upper limit Y i  of the vehicle speed corresponding to the selected gear, and outputs voltage Va that corresponds to the setting Y i . The output voltage Va of the data setting circuit 13 is supplied to a comparator circuit 14 as an output of the gear-shiftable vehicle-speed limit setter 11. 
     A waveform-shaping circuit 15 receives the vehicle-speed pulse signal Sc from the vehicle-speed sensor 9 that detects vehicle speed X and converts the amplitude of the vehicle-speed pulse signal Sc to an appropriate value. The signal whose peak value has been thus converted is supplied to a F-V conversion circuit (F-V converter) 16. 
     Adapted to convert into the voltage the frequency of the vehicle-speed pulse signal Sc whose amplitude has been converted, the F-V conversion circuit 16 outputs voltage Vb that is proportional to the number of waves of the pulse and supplied to the comparator circuit 14. 
     The comparator circuit 14 is an electric circuit that receives voltage Vb, which is an electric signal based on the detection signal from the vehicle-speed sensor 9, and voltage Va, which is an electric signal from the gear-shiftable vehicle-speed limit setter 11, and determines that the gearshift is possible if Vb≦Va or if the vehicle speed X is not higher than the gear-shiftable vehicle-speed limit Y i . Then, two output terminals do not send out outputs Ca and Cb, as a sign to indicate that gearshift is possible. When gearshift is possible, low-level output of negative logic is supplied to the OR circuits 7 and 8, which constitute a logic circuit, as an output of the safety circuit 10. 
     If Vb&gt;Va or the vehicle speed X is higher than the gear-shiftable vehicle-speed limit Y i , the comparator circuit 14 determines that gearshift is impossible. The two output terminals send out high-level outputs Ca and Cb to the OR circuits 7 and 8, as a sign to indicate that gearshift is impossible. 
     When either of the signal Ba from the electric gearshift circuit 2 or the signal Ca from the safety circuit 10 is supplied at high level, the OR circuit 7, which is a logic circuit, supplies high-level output Da to the solenoid valve 3 for actuation. 
     Functioning substantially like the OR circuit 7, the OR circuit 8 supplies high-level output Db to the solenoid valve 4 for actuation when either of the signals Bb and Cb is supplied at high level. 
     The comparator circuit 14 and the OR circuits 7 and 8, which are logic circuits, make up a safety mechanism. 
     When a gear is selected by operating the gear-selection switch 1, gearshift is allowed and performed accurately when the vehicle speed is not higher than the gear-shiftable vehicle-speed limit. When the vehicle speed is higher than the gear-shiftable vehicle-speed limit, on the other hand, gearshift is not allowed and the transmission is forcibly held in neutral with actuating signals sent to both of the shift-fork moving solenoid valves 3 and 4. 
     FIG. 5 shows a second embodiment of this invention in which the principal part of the switch-selectable transmission control with a safety mechanism comprises a digital electric circuit. In FIG. 5, reference characters similar to those used in FIGS. 3 and 4 designate similar parts. 
     A gear-shiftable vehicle-speed limit setter 11&#39; that constitutes a safety circuit 10&#39; in FIG. 5 receives signal Sb from the gear-selection switch 1 and outputs digital signal Ea (M bits), which corresponds to the upper limit Y i  &#39; of vehicle speed, from a data-setting circuit 13&#39;, which is a multiplexer constituting the setter 11&#39;, to a comparator circuit 14&#39;. 
     On receiving vehicle-speed pulse signal Sc, the safety circuit 10&#39; converts the amplitude of the signal Sc and supplies the converted signal to a pulse-counting circuit (pulse counter) 17 to perform pulse counting. 
     The pulse-counting circuit 17 starts counting the number of pulse signals when it receives a counting start signal and repeats the counting at given time intervals as it receives a clearance signal, both signals being supplied from a signal-generating circuit 18 through a delay circuit 19. The result of the counting is constantly outputted in M bits to a latch circuit 20. 
     The latch circuit 20 is a circuit that latches the number of pulses from the pulse-counting circuit 17 based on the latch signals sent from the signal-generating circuit (osillator) 18. Namely, the latch circuit 20 counts and latches the number of pulses that are inputted between the pulse counting start signal that is inputted in the pulse-counting circuit 17 and said latch signal. The latched signal which corresponds to the vehicle speed is supplied to the comparator circuit 14&#39; as a digital vehicle-speed signal Eb (M bits). 
     Comparing the digital signal Ea, which is a data-setting signal, and vehicle-speed signal Eb by, for example, subtraction, the comparator circuit 14&#39; lowers the level of signals Ca and Cb when Eb≦Ea and raises their level when Eb&gt;Ea. 
     Composed as described above, the second embodiment of this invention achieves substantially the same operation and result as the first embodiment described previously. 
     The data-setting circuits 13 and 13&#39; shown in FIGS. 4 and 5 may be adapted to output not only the upper limit of the vehicle speed at which gearshift is allowed but also the lower limit in the form of voltage Vb&#39; or setting data value Ea&#39;. In this case, the comparator circuits 14 and 14&#39; judge if the signal received falls between the upper and lower limits. This provision precludes the selection of high gear while the vehicle is running at low speed, thereby preventing the occurrence of knocks and other undesirable phenomena. 
     Also, a NOT circuit may be connected to the input terminal of the OR circuit or logic circuit shown in FIG. 3. When this is done, the electric gearshift circuit 2 and safety circuit 10 change the logic of their outputs. That is, low-level output is converted to high-level output and vice versa. 
     This logic changing provides a fail-safe function that assures safety of higher degree holding the transmission in neutral when the connection between the safety circuit 10 and the logic circuit is cut off. 
     A logic circuit that functions similar to the OR circuits 7 and 8 may be provided between the electric gearshift circuit 2 and each of the solenoid valves 5 and 6.