Endless belt power transmission systems or components – Pulley with belt-receiving groove formed by drive faces on... – Fluid pressure actuator for adjustment of member
Reexamination Certificate
2002-07-25
2004-11-30
Charles, Marcus (Department: 3682)
Endless belt power transmission systems or components
Pulley with belt-receiving groove formed by drive faces on...
Fluid pressure actuator for adjustment of member
C474S018000
Reexamination Certificate
active
06824483
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a control system for a continuously variable transmission or CVT.
DESCRIPTION OF THE RELATED ART
Such control system and CVT are generally known, for example from the European patent publication EP-A-0.451.887. The CVT comprises a first rotatable pulley and a second rotatable pulley both provided with two pulley discs and with a piston/cylinder assembly for urging the pulley discs towards each other under the influence of a hydraulic cylinder pressure in the piston cylinder assembly. The pulley discs exert a clamping force on a drive belt, which is located between the pulley discs, such that torque transmission between the pulleys and the drive belt is enabled. The clamping force and the coefficient of friction between drive belt and pulley discs determine the maximum torque at which the torque transmission occurs virtually without mutual movement of drive belt and pulley discs, i.e. without belt slip. The ratio between the cylinder pressure in the piston/cylinder assembly of the first pulley and that in the piston/cylinder assembly of the second pulley determines the transmission ratio, i.e. the ratio between a rotational speed of the first and second pulley. The control system is capable of both clamping force control and transmission ratio control through determining the cylinder pressure for each of the pulleys. To this end the known control system is provided with a hydraulic circuit connecting the piston/cylinder assemblies to a pump and a reservoir for hydraulic medium and comprising two electronically controllable valves. The control system is furthermore provided an electronic control unit generating a respective control current for each valve in the hydraulic circuit at least based on a torque to be transmitted and on the rotational speed of the pulleys.
In the known control system the electronic control unit comprises two independently operating modules. A first module takes care of the clamping force control by setting the cylinder pressure in the piston/cylinder assembly of the first pulley through generating a first control current operating a first valve. Since the mechanical efficiency of the CVT decreases with increasing clamping force, the control system is usually arranged such that the cylinder pressure in the piston/cylinder assembly of the first pulley is maintained at a lowest possible level without belt slip occurring. Thereto the control system determines the minimum cylinder pressure in the first piston/cylinder assembly needed to prevent slip of the drive belt based on a torque to be transmitted by the transmission. In the known art the torque to be transmitted by the transmission is approximated on the safe side by adding to the actual torque level a value of 0.3 times the maximum possible torque level. Thus, the safety factor with which the actual torque level is multiplied to calculate the torque to be transmitted by the transmission is thus 1.3 at the maximum possible torque to be transmitted, but increases rapidly with a decreasing level of the torque to be transmitted. The technical effect of applying a safety factor being that belt slip is prevented in a manner and at a level practically satisfactory, even during abrupt and/or unpredicted changes in the torque to be transmitted. A second module takes care of the transmission ratio control by setting the cylinder pressure in the piston/cylinder assembly of the second pulley through generating a second control current operating a second valve.
SUMMARY OF THE INVENTION
It is an object of the invention to provide for a solution to the desire to improve the performance and efficiency of the CVT by lowering the safety factor while still effectively preventing slip of drive belt.
Although, in general the known control system provides a simple and stable CVT control, it was found that after prolonged use, drive belts can show an unexpected amount of wear having a slight adverse effect on the durability of the CVT. It is a further object of the present invention to identify the problem underlying unexpected wear of the drive belt and to provide for a solution, thereby improving the durability of the CVT.
It was found that the wear is due to the occurrence of mutual movement of drive belt and pulley discs, i.e. belt slip, which suggested that during operation of the CVT situations can occur wherein the clamping force control is unable to adequately prevent belt slip. So, even though in the known art a relatively large safety factor is applied to the value for the torque to be transmitted, belt slip still remains a problem. These practical findings suggest that it is not possible to lower the safety factor without compromising the durability of the drive belt.
From U.S. Pat. No. 5,707,314 a control system is known that is characterised by the feature that it determines a minimum cylinder pressure to prevent belt slip not only for the first pulley, but also for both for the first and the second pulley. In effect this means the clamping forces of both pulley are explicitly calculated. Based on these minimum cylinder pressures and on the ratio between the rational speeds of the first and second pulley, i.e. the transmission ratio, the control system then determines and controls both cylinder pressures to be equal, or higher than, the respective minimum cylinder pressure. With such system belt slip both on the first and second pulley may be prevent in most operational conditions.
However, according to the invention, it appeared that belt slip occurred at a high rate of change of the transmission ratio. It was furthermore found that the belt slip occurs between the pulley discs of a specific pulley and more in particular, in case of the CVT known from EP-A-0.451.887, the pulley discs of the second pulley, being the pulley used to control the transmission ratio.
According to the invention the control system and CVT having the below discussed features advantageously prevents the occurrence of belt slip and in doing so provides for a CVT with an improved durability and efficiency.
It is noted that the control system according to the invention determines both the cylinder pressure in the first and in the second piston cylinder assembly. It is common practice to use a feedback loop, wherein the actual value of a variable is fed back into a control system, when determining the variable. Therefore, it would appear that the control system according to the invention needs to be provided with two pressure sensors, one for each cylinder pressure. However, according to the invention, sufficient accuracy may be obtained, if only one cylinder pressure is measured using a pressure sensor and the other cylinder pressure is calculated from the measured cylinder pressure and the rotational speed of the pulleys as well as some transmission parameters. The cost of an additional pressure sensor thus being avoided.
Another advantage of the invention is that the approximation of the torque to be transmitted by the transmission may be made less safe compared to the known art without compromising the durability of the CVT. It was found that, when using the control system according to the invention, the torque to be transmitted by the transmission may be approximated by taking the level of the actual torque for each of the first and second pulley multiplied by a safety factor which is equal to 1.3 in a predominant part of a range of actual torque levels. The specific size of the part depends on the transmission ratio. In this manner a significant improvement of the overall efficiency of the transmission may be achieved.
According to the invention a cylinder pressure is maintained at a substantially higher level when a rate of change of the transmission ratio is relatively large than when the transmission ratio is constant. This may for example be effected by increasing the safety factor with an increasing rate of change of the transmission ratio. Since it was found that belt slip mainly occurs during a changing transmission ratio, the technical effect of this measure is that belt s
Luh Joachim
Van Der Laan Maajke
Charles Marcus
Van Doorne's Transmissie
Young & Thompson
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