CONTROL SYSTEM FOR A CLUTCH FOR A VEHICLE

Abstract: A system for controlling a drag torque of an electromagnetic clutch on release of the accelerator pedal of a motor vehicle with a continuously variable belt drive transmission. A detector is provided for detecting whether vehicle speed is lower than a predetermined low speed. At the low vehicle speed, the drag current is gradually reduced as the vehicle speed increases.

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 claim 1. 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 such as the start mode of the vehicle, reverse excitation mode, drag mode, and mode of lock-up engagement. One of the modes is selected in accordance with the position of a selector lever and the driving conditions to control the electromagnetic clutch. In the drag mode, a small clutch current flows in the coil of the electromagnetic clutch when the accelerator pedal is released in a low vehicle speed range thereby applying a small drag torque to a driven member of the clutch to reduce engagement clearances in the transmission. Accordingly, on starting of the vehicle or re-acceleration of the vehicle at low vehicle speed, the starting torque for driving the driven member is reduced, and hence, the power of the engine is properly transmitted to driving wheels through the transmission, so that the vehicle can be smoothly started or re-accelerated. However, in a continuously variable belt-drive transmission, the starting torque for re-acceleration increases as the vehicle speed decreases. In the system of the aforementioned prior art, the drag torque is constant regardless of the vehicle speed. Accordingly, the drag torque is not effectively exerted on the transmission upon such re-acceleration. The present invention seeks to provide a system for controlling a clutch in which the drag torque is controlled in accordance with the required starting torque in the transmission system during coasting at a low vehicle speed. According to the present invention, there is provided a system for controlling the drag torque of a clutch of a motor vehicle as defined in claim 1. 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 system for controlling an electromagnetic clutch for a motor vehicle; Figures 2a and 2b shows 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; Figure 4b is a graph showing a variation of drag current; Figure 5 is a graph showing clutch current characteristics. Referring to Figure 1, the 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 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 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 continuously vary the transmission ratio. An electronic control system for the clutch 2 and the belt-drive transmission 4 has an engine speed sensor 19, and rotating speed sensors 21 and 22 for respectively sensing rotating 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 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 singal 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 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, and actual transmission 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 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 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 reduction of clearances formed in the transmission and 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. Referring to Fig. 4a, the drag mode providing section 36 is provided with a drag deciding section 40 applied with output signals of throttle valve opening degree ϑ, vehicle speed V, drive position R-range and D-range, and clutch current mode deciding section 34. The drag mode deciding section 40 decides that the vehicle is in a drag mode when the vehicle speed is lower than a predetermined vehicle speed V₃ or V₄, as shown in Fig. 3, and applies a drag mode deciding signal to a clutch current providing section 41. The drag mode providing section 36 is further provided with a vehicle speed detecting section 43 to which a vehicle speed V is supplied. At the D-range, the vehicle speed detecting section 43 detects a vehicle speed lower than a predetermined speed V₂ so as to apply a drag current signal to the clutch current providing section 41. The clutch current providing section 41 selectively produces a signal for a zero current I O depending on the drag mode deciding signal from the section 40 and a signal for a drag current I d in accordance with the drag current signal. Either signal is applied to the output deciding section 33 through an output section 42. As shown in Fig. 4b, the drag current I d is predetermined as a function of the vehicle speed V in dependence on the starting torque of the transmission. Namely, the drag current increases with decrease of the vehicle speed. The clutch current providing section 41 has a map corresponding to Fig. 4b, and provides the drag current I d in accordance with the vehicle speed. The operation of the system will be described hereinafter with reference to Fig. 5 showing a characteristic of the clutch current. When the vehicle speed becomes lower than the speed V₄ at the release of the accelerator pedal at D-range, the drag mode deciding section 40 decides that the vehicle is in the drag mode. Accordingly, the lock-up current I L falls to a zero current I O to disengage the clutch 2. When the vehicle is further decelerated to a speed lower than V₂, the drag current I d which is decided at the current deciding section 41 in response to the output of the vehicle speed detecting section 43 flows in the coil 2c. The drag current I d increases as the vehicle speed decreases so as to provide drag torque corresponding to the starting torque. Thus, when the vehicle speed is lower than the speed V₂, the starting torque at the reacceleration is compensated by the drag torque at any vehicle speed including zero. Thus, when the accelerator pedal is depressed again, the vehicle can be smoothly reaccelerated. In the above-described embodiment, the drag torque is at maximum when the vehicle speed is zero. If the drag torque exceeds the total of internal loss of the transmission system, such as friction, and running resistance of the vehicle, the vehicle starts to creep. Accordingly, the vehicle must be braked by depressing the brake pedal, so as to keep it stationary. In order to prevent such an inconvenience, vehicle stop detecting section 44 and a small clutch current providing section 45 are provided. If the vehicle stop detecting section 44 detects that the vehicle is stationary the small clutch current providing section 45 reduces the drag current to a predetermined value (Im) as shown in Figure 5. Thus, creeping of the vehicle can be prevented. It will be understood that the present invention may be applied to other automatic clutches besides electromagnetic clutch. From the foregoing, it will be understood that the present invention provides a control system for a clutch wherein clutch torque in the drag mode is properly controlled in accordance with the starting torque dependent on vehicle speed. Thus, the vehicle can be smoothly reaccelerated while retaining the drag characteristic.

Claims: A system for controlling the drag torque of a clutch of a motor vehicle, the system comprising: first detecting means (28) for producing a release signal when the accelerator pedal is released, second detecting means (22) for producing a vehicle speed signal; third detecting means (43) for detecting whether the vehicle speed is lower than a predetermined low speed (V2) and for producing a low speed signal and a drag current signal for applying a drag torque to a driven member of the clutch; characterised by control means (41, 44, 45) responsive to the release signal and the low speed signal for producing a drag current signal and thus providing a drag torque in the clutch which varies inversely with the vehicle speed, and the drag signal and thus the drag torque is reduced to a predetermined small amount when the vehicle becomes stationary so that the vehicle may be smoothly re-accelerated from below the said low speed. A system according to claim 1 wherein the clutch is an electromagnetic clutch. A system according to claim 1 or claim 2 further comprising stop detector means (44), to reduce the torque by a predetermined amount when the vehicle becomes stationary so as to prevent wear.

Inventors: OHKUMO, HIROYA, SAKAKIYAMA, RYUZO
Applicants: FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1987
Main classification: B60K41
IPC codes: B60W10, F16H61, F16D48