Method of calculating for the speed control of a CVT

According to the preamble of claim 1 , this invention concerns a method for including the temperature in the calculation of the definite, operating-point dependent maximum and minimum adjustment speeds of a variator in a continuously variable transmission having electrohydraulic control.

A continuously variable belt-drive transmission usually consists, among others, of a starting unit, a forward/reverse drive unit, an intermediate shaft, differential hydraulic and electronic control devices and a variator. The variator usually comprises a primary and a secondary pulley, also called primary and secondary sides, both pulleys being formed by beveled pulleys disposed in pairs and provided with a torque-transmitting belt-drive element that rotates between the two pairs of beveled pulleys.

In such a transmission the actual ratio is defined by the running radius of the belt-drive element which in turn is a function of the axial position of the beveled pulleys.

Compared with a standard selector transmission, continuously variable transmissions in general have, conditioned by principle, one more degree of freedom, since aside from the selection of the ratio step to be adjusted, it is also possible to predetermine and control the adjustment speed at which the ratio is transferred from one operating point to the other.

In continuously variable transmissions having a belt-drive element (e.g. belt, chain) as the torque-transmitting part, it results from the structural design that during a change of ratio the beveled pulley pairs of the primary and secondary sides of the variator, alternately and complementary to each other, are pushed apart and together by corresponding control elements, whereby a change of the running radius of the belt-drive element acts upon the beveled pulleys thus causing a change of ratio between the primary and secondary sides.

The variator is usually hydraulically controlled. The axial displacement of the beveled pulleys produces a change of volume which, since the adjustment occurs under force and pressure control, must be compensated by the control hydraulic system through adequate flow rate changes to the respective pair of beveled pulleys.

The change of flow rate to be adjusted, via the electrohydraulic control at the same time depends directly on the actual adjustment speed of the pair of beveled pulleys.

Since the control hydraulic system as a rule is supplied via a pump dependent on rotational speed of the engine, with definitely predetermined maximum flow rate, there necessarily results a definitely stationary limit for the implementable adjustment dynamic of the variator. The variator can only be adjusted as quickly as the admitted available oil flow rate, in interplay with other control and regulating circuits or consumers.

In the structural design of the supply pump, together with assuring the necessary oil flow rate, an essential part is played by aspects such as noise and efficiency, both of which as a rule have a negative effect as the size of the pump increases. As a result, for a structural pump design, a compromise has to be implemented between the different criteria, which in relation to the operating point and the individual criteria is a less than optimum solution.

In relation to the variable adjustment speeds of the variator, this means there will always be operating states where in theory there would be possible higher adjustment gradients than momentarily admitted by the actual availability of the flow rate.

These operating states are particularly critical for a superimposed control device, since the control without the transmitting medium oil, cannot affect the behavior of the variator and thus the ratio adjustment. The consequence is instabilities which can produce disturbing oscillations of rotational speed until destroying the mechanics of the transmission.

Design-conditioned limitations (strengths of structural parts, limiting values for control pressures) on the variator constitute another aspect which likewise must constantly be taken into account to prevent damage or even destruction of the mechanics of the transmission.

A simple possible implementation would be to set for the admissible adjustment gradient empirical limiting values removed far enough from the critical values. The disadvantage is that the possible adjustment potential in this case cannot be utilized to the required extent. Besides, a generalization is hardly possible with regard to security in all operating states.

The Applicant's patent application (DE 199 08 251.0) discloses a method which by means of a physical mathematical pattern continuously calculates in every operating state the actual limiting values for the maximum possible adjustment gradient. Here are taken into consideration the special limiting conditions of the oil supply, and geometric ratios on the variator conditioned by the design.

Thus, the leakage flow rate of the different consumers such as the primary and secondary sides of the variator, S1 and S2 respectively, of the forward clutch KV, of the brake BR, and of the converter lock-up clutch WK are taken into account, according to the respectively applied pressure ps1, ps2, pkv and pwk.

The superimposed control device for adjusting a predetermined ratio set value then takes into account said limiting values when generating the correcting variables.

Herein is used a control circuit structure such as described in the Applicant's patent No. DE 196 06 311 A1. Such control circuit structures combine a physical-mathematical pattern-based linearization of the control method by means of a correcting member with a linear PID regulator. The correcting variable of the PID regulator is directly interpreted as the standard for the adjustment gradient to be set.

The problem on which this invention is based is to outline, departing from the cited prior art, a method for including temperature in the calculation and taking into consideration the define operating-point dependent maximum and minimum adjustment speeds within the scope of the ratio control of a continuously variable belt-drive transmission having electrohydraulic control.

According to the invention said problem is solved with the features of claim 1 . Other embodiments of the invention result form the sub-claims.

The invention will now be described, by way of example, with reference to the accompanying drawings in which:

FIG. 1 is a flow chart of the current invention.

It is accordingly proposed to model the hydraulic routes of the variator, of the clutches integrated in the transmission and of the lubrication (for ex., ps1 and ps2 for the variator pulleys, pkv for the forward clutch, pwk for the torque converter clutch and psm (return to the tank, lubrication)) (Step 1 ) as laminar inserts (Step 2 ) and the route of the control unit leakage (Step 3 ) as coupled laminar (Step 4 ) and turbulent (Step 5 ) hydraulic transmission elements.

The advantage of this procedure is that essentially complicated relationships of the oil circuit can be described very easily and with sufficient precision by the simplest pressure-temperature volume equations and an easy alignment to the measuring point is possible.

A more precise inclusion of the geometry in the composition of the pattern is not necessary and proceeds globally in coefficients and characteristic fields calculated from density and viscosity of the kind of oil used.

A special characteristic of the function is that the variables pressure, temperature and flow rate are accounted for in a hydraulic form as laminar, turbulent or mixed inserts. The alignment with the measurement occurs only at a measuring point.

Within the scope of the inventive method the structural, and the flow rate limiting values for the adjustable speed, are calculated with reference to a physical-mathematical pattern (Step 6 ) taking into account the temperature influences.

In this connection is used a method for extrapolation over the whole temperature range with reference to measured data in a limited temperature graph.

In addition, a fundamental calculation precept is indicated for the temperature-dependent variables (leakage flow rates).

According to an advantageous variant, for modeling the routes ps1 and ps2 for the variator pulleys, pkv for the forward clutch, pwk for the torque converter clutch and psm as laminar inserts (Steps 1 , 2 ) according to the pattern equation for laminar flow: q = k η  ( c_getr ) · p G1: 1

the flow rate q of the hydraulic route is calculated according to the invention. The pressure p is used as a set value from the flow standard of the pressure regulator and with the appertaining flow-pressure characteristic line. At a temperature point c_getr=T k the factor k corresponding to the basic viscosity characteristic line η(c_getr) is determined as k = q  ( c_getr = T k )  η  ( c_getr = T k ) p , G1: 2

wherein q(c —getr=T k ) at this operating point is measured leakage flow rate.

Since according to the pattern illustration k is a temperature-independent system constant, it is now possible to calculate via G1.1 the leakage flow rate over the whole temperature range.

To model the hydraulic route of the control unit leakage within the scope of a mixed insert composition, the flow rate q of the hydraulic route of the control unit leakage is calculated according to the equation q = k  ( ps2 , phd ) · 2 p  ( c_getr ) · ps2  phd η  ( c_getr ) G1: 3

as a function of both pressures ps 2 and phd (system pressure). Said pressures are used as set values from the flow standards of the pressure regulators and with the appertaining flow-pressure characteristic field, k is determined according to the basic viscosity characteristic line η(c_getr) and the density characteristic line η(c_getr), and with the leakage flow rate measured at this operating point q(c_getr=T k ): k  ( ps2 , phd ) = q  ( c_getr = T k ) 2 p  ( c_getr = T k ) · ps2  phd η  ( c_getr = T k ) G1: 4

Since k is a temperature-independent system constant, the leakage flow rate can be calculated over the whole temperature range via G1.3.