Control system for a clutch for a vehicle

Abstract: A system for controlling an electromagnetic clutch for a motor vehicle during a transient state of driving conditions of the motor vehicle. An actual engagement rate signal is obtained in dependence on the speeds of the drive member and driven member of the clutch. A desired engagement rate of the clutch is obtained in accordance with the speeds of the drive member and driven member. The clutch current is controlled so as to converge the actual engagement rate and the desired engagement rate, thereby slipping the clutch in dependence on the transient state.

Description: The present invention relates to a control system for an electromagnetic clutch for an automatic transmission of a motor vehicle as defined in the preamble of the single claim. An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in the generic EP-A 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes as state modes, such as a starting mode of a vehicle, reverse excitation mode, drag mode, mode of lock-up engagement. One of the modes is selected in accordance with a position of a selector lever and driving conditions to control the electromagnetic clutch. Other prior art as GB-A 2 156 454 discloses various control systems which provide transient state modes for various transient states. The transient states include states at the changing of ranges, for example changes from a neutral range (N-range) or parking range (P-range) to drive range (D-range), high engine speed drive range (Ds-range) or reverse range (R-range), from the D-range to Ds-range, and at the changing of modes, for example changes from the starting mode to the lock-up engage mode. Further, the transient state mode may include operations such as release and depression of an accelerator pedal in starting or lock-up engage mode. In the transient state mode, clutch torque is temporarily decreased to cause the clutch to slip so as to prevent jolts during the changing operation. The clutch torque is gradually increased thereafter. In another disclosure, the clutch torque is varied with time in accordance with a particular characteristic. However, the engagement rate of the clutch during driving of the vehicle does not depend only on the clutch torque but also on the relationship between engine torque and torque-load on the clutch output shaft which depends on running resistance of the vehicle. Thus, it is difficult to cause the clutch to slip at optimum rate only by controlling the clutch torque. Accordingly, in the prior art, there remain driving condition regions where clutch torque does not provide desired engagement rates. The present invention seeks to provide a control system for a clutch in which the clutch torque is controlled in accordance with the engagement rate to provide optimum transient state characteristics. According to the present invention, there is provided a system for controlling the clutch of a motor vehicle comprising the features as defined in the single claim. Some embodiments 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 of operation; Figure 4 is a flow chart showing the operation of the system; Figure 5 is a graph showing characteristics of clutch engagement rates. Referring to Figure 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-drive 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 automatic 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), high engine speed driving position (Ds-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 at the 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 the drive positions D and D S and the 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. A driven member speed sensor 18 for sensing the speed of the driven member 2b of the clutch 2 is further 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 ϑ of the sensor 29 to produce the transmission ratio control signal dependent on a 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 signals of the select position sensor 26. When the engine speed Ne is below 300 rpm, or the selector lever 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, D S -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 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. In addition, a clutch engagement rate calculator 38 is further provided in the control unit 20 for transient state modes. At the clutch engagement rate calculator 38, which is applied with the engine speed Ne and a driven member speed Nc of the sensor 18, the calculation of engagement rate E is made by E = Nc/Ne. When E = 1, the clutch is locked-up. When the rate E is smaller than 1 (E < 1), it means that the engine speed Ne is faster than the driven member speed Nc (Ne > Nc), slipping the clutch. On the other hand, when the rate E is larger than 1 (E > 1), the engine speed Ne is smaller than the driven member speed Nc (Ne < Nc), which means that the torque is transmitted from wheels of the vehicle to the engine. Thus, the clutch is also in slipping state. The output signal of the clutch current mode deciding section 34, signals Nc, R, D, Ds and ϑ are applied to a slip mode current providing section 39 which operates to detect the transient state which the vehicle is in. The slip mode providing current section 39 is further applied with the engagement rate signal E. The slip mode current providing section 39 has an Ner-ϑ table storing values of engine speed Ner dependent on the throttle position signal ϑ at the release of the clutch. A desired engagement rate Eo is set so as to provide an optimum slipping of the clutch in accordance with the particular transient state mode, elapsed time after the start of the transient mode and driving conditions. The slip mode current providing section 39 provides an appropriate clutch current so that the actual engagement rate E coincides with the desired engagement rate Eo. The operation of the control system is described hereinafter. At the starting of the vehicle, when the accelerator pedal is depressed while the select lever 25 is positioned at D-range, the start mode providing section 35 is selected. Accordingly, the clutch torque increases with clutch current in proportion to the engine speed. When the vehicle speed reaches a clutch engaging speed V₄ shown in Fig. 3, the lock-up engage mode providing section 37 is selected to lock-up the clutch. The operation of the control system in the transient state mode when the position of the selector lever 25 is shifted from the D-range to the Ds-range during the lock-up engagement of the clutch, is described hereinafter with reference to a flowchart in Fig. 4. At the shifting of the D-range to the Ds-range, slip mode is selected. The transient state mode from D-range to Ds-range is detected at the slip mode current providing section 39. The engine speed Ner at the throttle position ϑ at the release of the clutch is derived from the Ner-ϑ table. When the Ner is larger than the driven member speed Nc (Ner > Nc), the desired engagement rate Eo is set as shown by the solid line in Fig. 5. Namely, the desired engagement rate Eo decreases to 0.7. The desired engagement rate Eo for the detected transient state is set so as to temporarily increase the slipping of the clutch in order to absorb the shock which occurs when the power of the engine suddenly increases at the Ds-range, and to gradually decrease the slipping until the clutch is locked-up again. The actual engagement rate E is compared with the desired engagement rate Eo. The program is terminated if E is equal to Eo. When the actual rate E is larger than the desired rate Eo, the clutch current Ic is reduced to increase the slipping. When the actual rate E is smaller than the desired rate Eo, the clutch current Ic is increased to decrease the slipping. Thus, the engagement of the clutch 2 varies in accordance with the desired engagement rate Eo. If the Ner is smaller than Nc (Ner < Nc), the desired engagement rate Eo is set as shown by the dotted line in Fig. 5. The actual rate E is compared so as to increase or decrease the clutch current Ic as described in the aforementioned instance. It will be understood that the clutch is controlled in a similar manner in other transient state modes. Thus in accordance with the principles of the present invention, the clutch torque is controlled in the transient state modes by varying engagement rate of the clutch. Accordingly, the clutch is controlled with an appropriate slipping characteristic at any driving condition thereby avoiding excessive or insufficient slipping. Thus, shock is effectively absorbed or entirely prevented.

Claims: A system for controlling clutch current of an electromagnetic clutch (2) for a motor vehicle being provided with a continuously variable V-belt-drive transmission and having an engine (1) connected directly to said clutch (2), said transmission (4) transmitting power of said engine (1) through an output member (2b) of said clutch (2) to axles (8,8) of vehicle driving wheels (9,9), an accelerator switch (28), a throttle position sensor (29), speed sensor (18) for detecting speed of said output member (2b), an engine speed sensor (19), and a vehicle speed sensor (22), and means for providing different modes of clutch current control; characterised by clutch engagement rate calculating means (38) responsive to said output member speed signal and said actual engine speed signal for calculating an actual engagement rate (E) as a function of said output member speed (Nc) and said actual engine speed (Ne) and for producing an actual engagement rate signal; transient state detecting means (34) responsive to said actual engagement rate signal, said vehicle speed signal (Ns) and a signal from said accelerator switch (28) for detecting a change of at least one of the operating conditions of said engine (1) and for generating a transient state signal, a desired engagement rate calculating means (39) responsive to said transient state signal (34) and the signal from said throttle position sensor (29) for setting a desired clutch engagement mode from a table of differences between a desired engine speed and said output member speed (Nc) and for producing a desired engagement rate signal; and correcting means responsive to the signal from said desired engagement rate signal for correcting said clutch current so as to maintain an optimum control of said clutch (2) in the transient state by controlling said clutch current as a predetermined function of time.

Inventors: OHKUMO HIROYA, OHKUMO, HIROYA
Applicants: FUJI HEAVY IND LTD, FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1987
Main classification: B60K41
IPC codes: F16D48, F16D37, B60K23, F16H61, B60W10, B60W30