CONTROL SYSTEM FOR A CLUTCH FOR A VEHICLE

Abstract: A system for controlling clutch current of an electromagnetic clutch (2) on release of the accelerator pedal (27) of a vehicle in a low vehicle speed range. The clutch (2) is disengaged when the accelerator pedal (27) is released in the low vehicle speed range, but disengagement of the clutch (2) and reduction of an engine (1) speed are delayed so as to prevent oscillation of the transmission system.

Description: The present invention relates to a control system for an electromagnetic clutch for an automatic transmission of a motor vehicle. An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A 151038 in accordance with the preamble of claim 1. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes for clutch torque (clutch current). One of the modes is selected in accordance with a position of a selector lever and driving conditions of the vehicle to control the electromagnetic clutch. The operational modes include a start mode and a drag mode decided in accordance with operations of an accelerator pedal at a vehicle speed lower than a predetermined speed. In the start mode of the vehicle, the clutch current is increased in proportion to the engine speed or the opening degree of the throttle valve of the engine so as to smoothly engage the clutch. In the drag mode, the clutch current is reduced to zero or a small current to disengage the clutch or to provide a drag torque. More particularly, when the accelerator pedal is released in a low vehicle speed range after starting the vehicle to close the throttle valve (Figure 7a), the mode of the clutch current is immediately changed to the drag mode. Thus, as shown in Figure 7b, the clutch current becomes zero or is changed to a drag current id to disengage the clutch. The engine speed Ne gradually decreases, while speed No of an output shaft of the clutch begins to oscillate as shown in Figure 7c. The reason of the oscillation is as follows. On starting the vehicle, driving torque is applied to the drive power transmission system of the vehicle. When the accelerator pedal is released, the driving torque is exerted on the system in the reverse direction. At that time, the clutch is disengaged, so that the system is cut off, which causes the oscillation of the system. The oscillation causes the vehicle to hesitate, which is transmitted to the driver. Therefore, the driveability of the vehicle when coasting in a low speed range is adversely affected. Particularly, in an automobile with a continuously variable belt-drive transmission, since the pulley device has a large inertial mass such hesitation may easily occur. The present invention seeks to provide a control system which may improve the driving characteristics on release of the accelerator pedal in a low vehicle speed range. According to the present invention, there is provided a control system for a clutch for a motor vehicle, as outlined in claim 1. In a preferred embodiment of the present invention, the clutch is an electromagnetic clutch, and the means is provided for controlling current passing in a coil of the electromagnetic clutch. One embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a control system for an electromagnetic clutch for a motor vehicle; Figures 2a and 2b show a block diagram of a control unit according to the present invention; Figure 3 is a graph showing regions of various modes; Figure 4a is a block diagram of a main part of the system according to the present invention; Figures 4b and 4c are graphs showing variations of clutch current; Figs. 5a and 5b are flow charts showing the operation of the system; Figs. 6a to 6c are graphs showing clutch current characteristics, and Figs. 7a to 7c are graphs showing throttle valve opening degree, clutch current characteristics, and speeds of a conventional system. Referring to Fig. 1, a crankshaft 10 of an engine 1 is operatively connected to an electromagnetic powder clutch 2 for transmitting the power of the engine 1 to a continuously variable belt-drive automatic transmission 4 through a selector mechanism 3. The output of the belt-driv automatic transmission 4 is transmitted to axles 8 of vehicle driving wheels 9 through an output shaft 13, a pair of intermediate reduction gears 5, an intermediate shaft 6, and a differential 7. The electromagnetic powder clutch 2 comprises an annular drive member 2a connected to crankshaft 10 of the engine 1, a driven member 2b secured to an input shaft 11 of the transmission 4, and a magnetizing coil 2c provided in the driven member 2b. Powdered magnetic material is provided in a gap between the drive member 2a and driven member 2b. When the magnetizing coil 2c is excited by the clutch current, driven member 2b is magnetized to produce a magnetic flux passing through the drive member 2a. The magnetic powder is aggregated in the gap by the magnetic flux and the driven member 2b is engaged with the drive member 2a by the powder. On the other hand, when the clutch current is cut off, the drive and driven members 2a and 2b are disengaged from one another. In the belt-drive transmission 4, the selector mechanism 3 is provided between the input shaft 11 and a main shaft 12. The selector mechanism 3 is provided with a synchromesh mechanism comprising gears, hub, and sleeve for connecting the input shaft 11 and the main shaft 12 to selectively provide a driving position (D-range) and a reverse driving position (R-range). The continuously variable belt-drive automatic transmission 4 has the main shaft 12 and the output shaft 13 provided in parallel with the main shaft 12. A drive pulley 14 provided with a hydraulic cylinder 14a is mounted on the main shaft 12. A driven pulley 15 provided with a hydraulic cylinder 15a is mounted on the output shaft 13. A drive belt 16 engages with the drive pulley 14 and the driven pulley 15. Hydraulic cylinders 14a and 15a are communicated with an oil hydraulic control circuit 17. The hydraulic control circuit 17 is responsive to vehicle speed, engine speed and throttle valve position for controlling the amount of oil supplied to the cylinders 14a and 15a. The pulleys 14 and 15 are operated by compressive forces of cylinders so that the running diameter of belt 16 is varied to infinitely change the transmission ratio. An electronic control system for the clutch 2 and the belt-drive automatic transmission 4 has an engine speed sensor 19, and rotating speed sensors 21 and 22 for respectively sensing speeds of drive pulley 14 and the driven pulley 15. A choke switch 24 produces an output signal when a choke valve of the engine 1 is closed, and an air conditioner switch 23 produces an output signal during operation of an air conditioner. A selector lever 25 connected to the selector mechanism 3 is provided with a select position sensor 26 for sensing a drive position D and a reverse position R. An accelerator pedal switch 28 is provided for sensing the depression of an accelerator pedal 27, and a throttle position sensor 29 is provided. Output signals of the sensors and pulses of the switches are applied to an electronic control unit 20 which produces a clutch current control signal to the clutch 2 and a control signal for controlling the transmission ratio (i) and a line pressure control signal to the control circuit 17. Referring to Figs. 2a and 2b showing the control unit 20 of Fig. 1, a transmission ratio changing speed control section 30 is applied with a drive pulley speed signal N P of the sensor 21, driven pulley speed signal N S of the sensor 22, and throttle position signal O of the sensor 29 to produce the transmission ratio control signal dependent on a desired transmission ratio changing speed di/dt. A line pressure control section 31 is applied with an engine speed signal Ne of the sensor 19, throttle position signal ϑ of the sensor 29, actual transmission speed ratio signal i (N S /N P ) of the transmission ratio changing speed control section 30 to produce the line pressure control signal dependent on a desired line pressure. These control signals are applied to the control circuit 17 to control the transmission ratio and line pressure of the transmission 4. A reverse excitation mode deciding section 32 is applied with engine speed signal Ne of the sensor 19 and drive position signal of the select position sensor 26. When the engine speed Ne is below 300 rpm, or the selector level 25 is at a neutral position (N-range) or a parking position (P-range), the reverse excitation mode deciding section 32 produces a reverse excitation signal which is applied to an output deciding section 33, so that a small reverse current flows in the clutch 2 to release the clutch completely. A clutch current mode deciding section 34 is applied with signals from the reverse excitation mode deciding section 32 and accelerator pedal switch 28, and vehicle speed signal V from driven pulley speed sensor 22 for deciding driving conditions such as starting mode to produce output signals. The output signals are applied to a start mode providing section 35, drag mode providing section 36, and clutch lock-up engage mode providing section 37. The start mode providing section 35 decides clutch current of starting characteristic dependent on the engine speed Ne at ordinary start or at closing of the choke switch 24 or air conditioner switch 23. The starting characteristic is corrected by signals from the throttle valve opening degree ϑ, vehicle speed V, and driving positions of D-range and R-range. The drag mode providing section 36 decides a small drag current when the accelerator pedal 27 is released at a low speed in each drive position for providing a drag torque to the clutch 2 for the smooth start of the vehicle. The clutch lock-up engage mode providing section 37 decides a lock-up current in response to the vehicle speed V and throttle valve opening degree ϑ at each drive position for entirely engaging the clutch 2. Outputs of sections 35, 36 and 37 are applied to the output deciding section 33 to control the clutch current. A range of each mode is shown in Fig. 3. Referring to Fig. 4a, the drag mode providing section 36 is provided with a drag mode deciding section 40 applied with output signals of throttle valve opening degree ϑ, vehicle speed V, select positions of R-range and D-range. In accordance with input signals, the drag mode deciding section 40 produces output signals which are fed to a steady state deciding section 41 and a transient state deciding section 45, respectively. The steady state deciding section 41 decides the steady state of driving condition, and produces a steady state signal. A last current mode detector 44 is applied with a signal from the clutch current mode deciding section 34 and detects whether a mode of the clutch current at the last program is a start mode or not. The resultant signal is applied to the transient state deciding section 45 which produces a transient state signal. In the output deciding section 33, a steady mode current providing section 42 produces a zero current signal Io or a drag current signal Id in response to the steady state signal from the steady state deciding section 41. A transient mode current providing section 46 is applied with the transient state signal from the transient state deciding section 45 and produces a transient mode signal IT. A current detector 47 is applied with an output signal of the start mode providing section 35 and detects a clutch current Ic just before the release of the accelerator pedal. The detector 47 further operates to compare the clutch current Ic with a predetermined current IR. As shown in Fig. 4b, when the clutch current Ic is above the predetermined current IR (Ic > IR), the clutch current Ic is decreased to the predetermined current IR, and then gradually decreased therefrom to the drag current at a decreasing rate dI/dt. On the other hand, when the clutch current Ic is below or equal to the predetermined current IR (Ic ≦ IR), the clutch current Ic is gradually decreased at the decreasing rate dI/dt. It is preferable to vary the set current IR and the decreasing rate dI/dt in accordance with driving conditions. Fig. 4c shows the variations. The set current IR decreases with increase of vehicle speed V or vehicle deceleration -dv/dt, and the decreasing rate dI/dt increases with increase of the vehicle speed V or the deceleration -dv/dt. Further, the variations can be made in accordance with operation of the air conditioner. On the other hand, since the decrease of the clutch current Ic to the drag current delays, the clutch is held in partial engagement for a period. Accordingly, the load on the engine becomes larger than the output of the engine, which will cause the engine to stall. In order to absorb the load on the engine and to prevent stalling, a dashpot 48 (Fig. 1) is provided for the throttle valve to slow throttle closing to maintain the engine speed for a period. Describing the operation of the system with reference to Figs. 5a and 6a to 6c, when the selector lever is shifted to the drive position (D-range) at a time t1, the mode of the clutch current is changed to the drag mode. The drag mode is decided at the drag mode deciding section 40 of the drag mode providing section 36. In this state, a last mode is a reverse excitation mode, so that the steady mode is decided for determining a drag current Id at the steady mode current providing section 42. The drag current Id is applied to the clutch 2 through an output section 43. When the accelerator pedal is depressed at a time t2, the drag mode is changed to the start mode, so that the clutch current Ic flows from the start mode providing section 35 for starting the vehicle and increases in proportion to the engine speed Ne. Thus, the vehicle is started. When the accelerator pedal is released at a time t3 in the low vehicle speed range before the vehicle speed reaches V3 or V4, the mode of the clutch current is changed to the drag mode by the decision at the drag mode deciding section 40. In the state, the last mode detector 44 detects that the last mode is the start mode. Thus, the transient current mode in the drag mode is decided. When the start mode current detector 47 detects that the clutch current Ic is above the predetermined current IR (Ic>IR), the transient mode current providing section 46 sets the transient mode current IT to the current IR, so that the clutch current is decreased to the current IR. Fig. 5b shows an interrupt routine at the transient mode. When a flag indicating the transient mode is set, the clutch current Ic is reduced by Δ I to slowly decrease the clutch current. In the drag current range shown in Fig. 3, the above-mentioned program continues until the clutch current reaches the drag current Id at a time t4. Then the flag is reset to end the program. Accordingly, the clutch is not entirely disengaged, but maintained in partial engagement. On the other hand the engine speed Ne is kept higher than the idling speed by the dashpot 48 in synchronism with the operation of the system. Thus, the engine operation absorbs the reverse torque (load on the engine) so that the speed No of the output shaft of the clutch is prevented from rapidly decreasing and oscillating as shown in Fig. 6c. In the current zero range, the clutch current Ic is reduced to zero Io as shown by a dotted line of Fig. 6b. When the clutch current Ic is below or equal to the predetermined current IR (Ic ≦ IR), the clutch current is gradually reduced as shown by a chain line of Fig. 6b. In the preferred form of the present invention, the control system controls to delay the decrease of engine speed and clutch torque at the release of the accelerator pedal in the low vehicle speed range. Thus, preferable driving characteristics can be provided.

Claims: A control system for a clutch (2) for a motor vehicle, of the kind in which the clutch is gradually engaged when the accelerator pedal (27) of the vehicle is depressed in a low vehicle speed range below a predetermined speed (V4), the system comprising: means (28) for detecting the release of the accelerator pedal of the vehicle in the said low vehicle speed range and for producing a corresponding release signal; characterised by a control unit (20) including control means (48 and 33) responsive to the release signal for gradually reducing the speed of the engine (1) and the clutch torque. A control system according to claim 1 wherein the clutch is an electromagnetic clutch. A control system according to claim 2 wherein the control means is arranged to control the current passing through the coil (2c) of the electromagnetic clutch. A control system according to claim 3 wherein the control means includes a dashpot (48) of the engine (1). A control system according to claim 1, wherein said control unit (20) includes a clutch mode deciding section (34) responsive to the vehicle condition and the condition of the accelerator pedal to actuate one of a plurality of clutch control mode sections including a start mode section (35) and a drag mode section (36), wherein said drag mode section (36) includes a transient state deciding section (45) responsive to the position of the engine throttle valve, the vehicle speed, the selected transmission drive range and the condition of the accelerator pedal to determine a transient state condition on release of the accelerator pedal; and also comprising a start mode current detector (47) to detect the clutch current before release of the accelerator pedal and to compare said clutch current (IC) with a predetermined current (IR); a transient clutch current providing section (46) reducing said clutch current to said predetermined current (IR) when the clutch current exceedings said predetermined value (IR) and thereafter reducing said clutch current by small increments (▵I) to a drag current (Id), and an engine speed control means (dashpot) (48) to gradually reduce the engine speed on release of the accelerator pedal.

Inventors: OHKUMO, HIROYA, SAKAKIYAMA, RYUZO
Applicants: FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1987
Main classification: B60K41
IPC codes: F16D48, B60W10