GB2051264A - Expanding-pulley gear control - Google Patents

Abstract

A control system for a continuously variable ratio transmission for a vehicle incorporates expanding-pulley gearing controlled by fluid pressures in the two sheaves 26, 27 thereof. The control system incorporates a means for controlling the difference between the pressures in the sheaves in order to control the torque transmitted thereby end also includes means 52, 53 or 54 for limiting the difference between the pressures in the sheaves in order to limit the maximum torque transmitted through the transmission. <IMAGE>

Description

SPECIFICATION CVT torque limitation The invention relates to control systems for continuously variable ratio transmissions of the kind which incorporate a variator and in which the variator ratio is controlled by fluid pressure in the two sheaves thereof. A variator is constituted by a pair of sheaves, each in the form of an adjustable V-pulley, and a flexible friction drive belt or chain interconnecting the two sheaves. Adjustment of the effective diameters of the sheaves by axial adjustment of relative positions of the two pulley halves of the sheaves, varies the transmission ratio.

In a continuously variable ratio transmission of the kind which incorporates a variator, it is possible to arrange the variator in relation to other transmission components in such a way as to provide continuous variation of the ratio down to a zero ratio at which the output of the transmission remains stationary regardless of engine speed.

This is referred to as a geared neutral condition. At very low forward or reverse ratios close to the geared neutral condition, the torque multiplication available through the variator is very high. In the absence of any losses, an infinite torque multiplication would be available at an infinitely low speed. In practice the maximum torque multiplication is normally of the order of ten. If this order of torque multiplication were made available in conjunction with maximum engine torque, the torque available at the vehicle wheels would be far in excess of that required and would spin the wheels, leading to high tyre wear and instability of the vehicle. There would also be a danger of breaking various transmission components due to the excessive torque. For this reason it is desirable to provide a control system which can limit the torque output from the transmission.It is not sufficient merely to control the torque multiplication ratio because a high torque multiplication ratio is desirable for use in conjunction with low engine torque.

The requirement for torque limitation arises primarily in relation to a transmission incorporating a geared neutral, but it could also be useful in other forms of transmission. For example, it could be desirable to set a torque limiting control to a low value under the control of a driver, in order to provide gentle driving in slippery conditions.

According to the present invention there is provided a control system for a continuously variable ratio transmission of the kind incorporating a variator and in which the variator ratio is controlled by fluid pressure in the two sheaves thereof, the control system incorporating means for controlling the difference between the pressures in the sheaves in order to control the torque transmitted by the transmission and including means for limiting the difference between the pressures in the sheaves in order to limit the maximum torque transmitted through the transmission.

Preferably an open-centre valve controls the extent to which the sheaves are connected to a pressure source and to drain, the pressure difference in the sheaves is controlled by operating the open-centre valve and stop means are provided to limit the movements thereof and thus limit the torque transmitted through the transmission.

Alternatively the pressure difference in the sheaves may be limited by a pressure limiting valve arrangement connected between the two sheaves.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawing which is a hydraulic and electrical control diagram of a control system according to the invention.

A pump 11 receives hydraulic pressure fluid from a reservoir 12 through a filter 13 and supplies the fluid under pressure to a regulator valve assembly 14. The regulator valve assembly 14 includes a spring loaded pressure relief valve 1 5 through which the output from the pump 11 is connected to drain and this controls the maximum pressure from the pump. A quantity flow control valve 1 6 restricts the volume flow at the output from the regulator valve assembly by connecting excess flow to drain.

The regulated source of fluid under pressure is connected to the pressure input 1 7 of an opencentre spool valve 1 8. A drain connection 19 of the spool valve is connected to drain through a controllable pressure relief valve 21. The two controlled pressure connections 22 and 23 from the spool valve lead to hydraulic control cylinders 24 and 25 of two sheaves 26 and 27 of a variator 28.

The spool position of valve 18 is controlled by a cam 29 while the control spring load and hence the relief pressure of pressure relief valve 21 is controlled by a cam 31. The two cams 29 and 31 are arranged on a common shaft 32. A servomotor 33 drives the cams 29 and 31 through a gear box 34, a bevel gear set 35 and the shaft 32.

The electrical control circuit incorporates two speed sensors 36 and 37 which are connected to a variator ratio module 38 which calculates the instantaneous variator ratio from the two variator speeds and provides a variator ratio signal on connection 39. An input command signal indicative of a desired variator ratio signal is provided on connection 41 from a source which is indicated diagrammatically by potentiometer 42.

The actual variator ratio signal and the desired variator ratio signal are fed to a comparator/amplifier 43 which in turn provides any necessary correction signal to the servo-motor 33. In this way, the spool valve 18 is set by the control system to a position which provides the required variator ratio.

For a given variator ratio and a given level of transmitted torque there needs to be a corresponding difference in pressure of the two sheaves in order for that given variator ratio to be maintained. An increase in torque at that given variator ratio and difference in pressure of the two sheaves causes the variator ratio to change. To return the variator ratio to the original condition prior to the disturbance in transmitted torque a corresponding change in pressure difference of the two sheaves is required and is provided for by the control system. Therefore for a given variator ratio there is a relationship between transmitted torque and the difference in pressure of the two sheaves.

Due to this situation although the position of the spool valve 1 8 is responding to variator ratio and desired variator ratio signals its actual position which controls the difference in pressure of the two sheaves is a function of the transmitted torque.

As previously described, the mechanical connection to the spool of valve 1 8 also acts through cam 31 on the control of pressure relief valve 21. Cam 31 is so shaped that the relief pressure of valve 21 increases as the position of valve spool 1 9 is moved away from its balanced central position in either direction. The cam profile is such that the pressure relief valve 21 controls the pressure in drain connection 1 9 in such a way that it varies with an appropriate relationship with spool valve position, thus variator pressure difference and thus transmitted torque.

In general there is no significant pressure drop through the spool valve 1 8 from the low pressure variator cylinder 24 or 25 and the drain connection 1 9. Thus the pressure in the low pressure variator control cylinder is controlled directly by the pressure relief valve 21 so that this low pressure is a function of torque. Suitable relationships are established by means such as the cams 29 and 31 to ensure that the relationship between the pressure in the low pressure cylinder and the torque transmitted (the pressure difference) is substantially linear.

As thus far described, the control system corresponds to that described in our co-pending patent application number (GKN ref 78.177). In this embodiment of the present invention, the valve spool is provided with an abutment member 51 which is arranged to co-operate with two stops 52 and 53. The abutment 51 and stops 52 and 53 constitute a stop means to limit the movement of the spool of the spool valve 1 8. This limitation of spool valve movement automatically limits the pressure difference which can be achieved between the control cylinders 24 and 25. This limitation of pressure difference also automatically limits the torque which can be transmitted through the variator 28.The variator is still free to operate at a high torque multiplication ratio when there is a low input torque but if the input torque is increased to an excessive value with a high torque multiplication ratio, the abutment 51 comes up against stop 52 or stop 53 to limit the pressure difference applied to the sheaves 26 and 27. The operation of the stop means limiting the difference in pressure of the two sheaves thereby limits the maximum possible transmitted torque and thus the maximum possible torque that the input sheave of the transmission can absorb from the power source. Any attempt to increase the power developed by the power source results in an increase in the speed of the power source.

If the power source is an internal combustion engine the speed of the engine stabilizes at a level where the increase in power over and above what is absorbed by the transmission is absorbed by either internal losses of the engine, which increase with speed, or maximum speed limitations of the engine itself.

The stops 52 and 53 may be adjustable for setting up the transmission controls and fixed for normal operating purposes. Alternatively, the position of the stops 52 and 53 may be controllable during operation of the vehicle to enable the driver of the vehicle to limit the maximum torque which is available. For example, particularly with a high performance vehicle, a low torque limit could be introduced for an inexperienced or sedate driver or in the presence of slippery road conditions. Mechanical stops represent a simple way of explaining the invention but may not be the most practical form of the invention.

An alternative to the stop means 51, 52, 53 is shown in chain dotted outline. A pressure limiting valve arrangement 54 is connected between the two hydraulic control cylinders 24 and 25. The pressure limiting valve arrangement may for example be two independent pressure limiting valves connected in parallel in opposite senses.

This valve arrangement acts as an alternative to the control of the spool valve 1 8 to limit the pressure difference applied to the sheaves 26 and 27. The valve arrangement may be set to a fixed pressure difference or may be controllable.

A further alternative to the stop means 51, 52, 53, which may be the most practical provision in a mass produced control system, is to introduce the limitation on the movement of the valve spool into the electrical control system. For example a feed back loop from the position of shaft 32, could incorporate an electronic latch to limit the range of operation of the servo-motor 33.

Claims (5)

1. A control system for a continuously variable ratio transmission of the kind incorporating a variator and in which the variator ratio is controlled by fluid pressure in the two sheaves thereof, the control system incorporating means for controlling the difference between the pressures in the sheaves in order to control the torque transmitted by the transmission and including means for limiting the difference between the pressures in the sheaves in order to limit the maximum torque transmitted through the transmission.

2. A control system as claimed in Claim 1 in which an open-centre valve controls the extent to which the sheaves are connected to a pressure source and to drain, the pressure difference in the sheaves is controlled by operating the open-centre valve and stop means are provided to limit the movements thereof and thus limit the torque transmitted through the transmission.

3. A control system as claimed in Claim 1 in which the pressure difference in the sheaves is limited by a pressure limiting valve arrangement connected between the two sheaves.

4. A control system as claimed in Claim 1 in which an open-centre valve controls the extent to which the sheaves are connected to a pressure source and to drain, the pressure difference in the sheaves is controlled by operating the open-centre valve and an electronic control for the spool valve incorporates electronic latches to limit movements of the spool and thus limit the torque transmitted through the transmission.

5. A control system for a continuously variable ratio transmission substantially as described with reference to and as illustrated by the accompanying drawing.