METHOD, APPARATUS AND USE OF THE APPARATUS FOR CONTROLLING A COMPRESSOR

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application, under 35 U.S.C. § 120, of copending international application No. PCT/EP2008/064101, filed Oct. 20, 2008, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German patent application No. DE 10 2007 062 313.7, filed Dec. 21, 2007; the prior applications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a method for controlling a compressor for delivering a pressure medium into a pressure medium system. The delivery of the pressure medium can be switched on and off as a function of a temperature signal, by measuring the compressor temperature by a temperature sensor which is arranged on the compressor. The invention further relates to a pressure medium system having a compressor and an electronic control unit which controls the ON and/or the OFF duration or the switch-off and/or the switch-on temperature of the compressor, and the use of an electronic control unit for a pressure medium system having a compressor for controlling the ON and/or the OFF duration or the switch-off and/or the switch-on temperature of the compressor.

A method and an apparatus of this kind are described in published, European patent application EP 1 253 321 A2, corresponding to U.S. Pat. No. 6,799,950.

The method for controlling a compressor which is suitable for delivering a pressure medium into a pressure medium system is used to switch the delivery of the pressure medium into the pressure medium system on and off as a function of the temperature signal of a temperature sensor. The intention is to achieve good utilization of the compressor by avoiding excessively frequent switch-on and switch-off operations. This is achieved by switching off the delivery of pressure medium when the temperature signal exceeds a temperature limit value, it being possible for the temperature limit value to be specified as a function of the backpressure caused by the pressure medium system. It is furthermore possible to allocate underlying time criteria to the compressor switch-off criteria dependent on the temperature signal by making it possible to switch off the compressor when a duration limit value is reached. It is likewise possible for the compressor to be reactivated after a switch-off operation and after cooling to a predetermined reactivation temperature and/or on expiry of a predetermined cooling time.

According to this publication, the superimposed control according to time criteria is preferably employed at low external temperatures, i.e. when the temperature at the cylinder head of the compressor rises only slowly with the compressor switched on and falls rapidly again with the compressor switched off owing to good heat dissipation to the surroundings. In this case, control according to time criteria is intended to assist in reducing the frequency with which the compressor is switched on, without reducing availability. In this case, it is intended that the electronic control unit should give preferential consideration to the time criteria for switching off and reactivating the compressor over the temperature limit value and the reactivation temperature.

According to this publication, this means that the compressor is switched off on expiry of the duration limit value, even if the temperature limit value has not yet been reached. Reactivation of the compressor is to take place only on expiry of the predefined cooling time, even if the temperature falls below the reactivation temperature in this case. If the criteria were based purely on temperature, the compressor would be reactivated when the reactivation temperature was reached, that is to say after only a relatively short cooling time. However, reactivation is to take place also in accordance with a time criterion, namely when a predefined cooling time has been reached. Only at this time is the compressor switched on again.

This later reactivation of the compressor is intended to achieve cooling to a temperature level below the reactivation temperature, leading to a corresponding lengthening of the subsequent permissible run time of the compressor. Providing underlying time criteria to the switch-off criteria dependent on the temperature signal is intended to ensure that, in the case where the compressor is cold, the internal parts of the compressor, which reach the permissible temperature more quickly than the area of the compressor in which the temperature sensor is arranged, do not exceed the critical temperature and are, as a result, better protected from temperature-related damage.

Published, non-prosecuted German patent application DE 103 54 491 A1, corresponding to U.S. patent publication No. 2007/0068182, likewise describes a method for controlling a compressor for delivering a pressure medium into a pressure medium system intended for a closed level-control system of a motor vehicle. In this method, the current compressor temperature is determined continuously, at least while the compressor is running, and the compressor is switched off at the latest when a limiting temperature is reached, the inlet pressure and the backpressure of the compressor in the closed system being taken into account for the purpose of determining the current compressor temperature. While the compressor is running, the current compressor temperature is preferably adapted by a correction value after each time unit that passes, this correction value being dependent on the difference between the backpressure and the inlet pressure.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method, an apparatus and a use of the apparatus for controlling a compressor which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, which offers an improvement in the utilization of the compressor and avoidance of frequent switch-on and switch-off operations.

Starting from the object as specified, the invention proposes a method for controlling a compressor for delivering a pressure medium into a pressure medium system, in which the delivery of the pressure medium can be switched on and off as a function of a temperature signal, by measuring the compressor temperature via a temperature sensor which is arranged on the compressor and controlling the switching on and off of the compressor via an electronic control unit. In this arrangement, provision is made for the ON duration and/or the OFF duration or the switch-off and/or the switch-on temperature of the compressor to be determined from the gradient of the compressor temperature, which is measured at short time intervals.

The invention starts from the consideration that the internal temperature of the compressor and of the components inside it, especially the pressure valve, warm up and reach a critical temperature more quickly than can be detected by the temperature sensor, which is arranged on the compressor casing and can measure only the wall temperature of the compressor, for example that of the cylinder head. The more rapid the temperature rise measured by the temperature sensor, the greater the temperature deviation between the interior of the compressor and the temperature measured by the temperature sensor, i.e. the internal temperature of the compressor rises even more quickly than the temperature at the compressor measured by the temperature sensor. If, in accordance with the invention, the compressor temperature is measured at short time intervals, it is possible to determine therefrom the gradient of the temperature profile or temperature rise per unit time and, in turn, to calculate therefrom a switch-off signal and/or a switch-on signal. This switch-off or switch-on signal can be determined as a time signal, on expiry of which the compressor is switched off and/or switched on. It is also possible to determine the switching signal as a temperature signal by determining the switch-off temperature and/or the switch-on temperature of the compressor from the gradient of the compressor temperature curve.

In order to achieve continuous adaptation of the ON and/or the OFF duration, the gradient of the compressor temperature curve is determined continuously after each switch-on and/or switch-off operation.

In accordance with the considerations underlying the invention, the ON and/or the OFF duration determined is/are shorter or the switch-off and/or the switch-on temperature is/are lower, the steeper the gradient of the compressor temperature curve. This takes account of the even more rapid rise in the internal temperature of the compressor and avoids a situation where internal components of the compressor exceed a critical temperature.

To enable continuous control to be carried out after each switch-on and/or switch-off operation, the gradient of the compressor temperature curve is preferably determined by measuring the compressor temperature at a frequency of 2 Hz.

A further refinement of the control of the compressor can be achieved if, in determining the ON and/or the OFF duration or the switch-off and/or the switch-on temperature, the measured compressor temperature at the time when the compressor is switched on and/or switched off is taken into account in such a way as to bring about an additional shortening of the ON and/or the OFF duration or a reduction in the switch-off and/or the switch-on temperature, the lower the measured compressor temperature at the time when the compressor is switched on and/or switched off.

To obtain as small as possible a difference between the internal temperature of the compressor and the compressor temperature measured by the temperature sensor, the compressor temperature is preferably measured by a temperature sensor arranged on the cylinder head of the compressor component of the compressor.

A further improvement in control can be achieved if, to determine the ON and/or the OFF duration or the switch-off and/or the switch-on temperature, the ambient temperature is taken into account in such a way as to bring about a shortening of the ON duration and/or the OFF duration or a reduction in the switch-off temperature and/or the switch-on temperature when the ambient temperature is low and vice versa.

In a similar way, it is also possible to achieve a refinement in the outcome of the method if, to determine the ON and/or OFF duration or the switch-off and/or the switch-on temperature, the speed of travel of a motor vehicle provided with an air suspension system supplied with compressed air by the compressor is taken into account in such a way as to bring about a lengthening of the ON duration and/or a shortening of the OFF duration or an increase in the switch-off and/or the switch-on temperature when the speed of travel is high and vice versa.

The above-mentioned object is furthermore achieved by a pressure medium system having a compressor and an electronic control unit which controls the ON and/or the OFF duration or the switch-off and/or the switch-on temperature of the compressor using the method in accordance with the features specified above. Furthermore the above-mentioned object is also achieved by the use of an electronic control unit for a pressure medium system having a compressor for controlling the ON and/or the OFF duration or the switch-off and/or the switch-on temperature of the compressor in accordance with the above-mentioned method features.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in a method, an apparatus and a use of the apparatus for controlling a compressor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a graph showing various temperature curves during an ON phase according to the invention; and

FIG. 2 is a graph showing various temperature curves during an OFF phase.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown curves T¹_zki and T¹_zka, T²_zki and T²_zka which show idealized temperature profiles for an internal temperature T_zki of a cylinder head and for the exterior T_zka of the cylinder head against time when these develop slowly and in parallel, with the result that the respective maximum temperatures T_max are reached substantially simultaneously, curves T¹_zki and T¹_zka applying to a new compressor, while curves T²_zki and T²_zka apply to a compressor toward the end of its service life. Thus a compressor exhibits a gain in run time toward the end of its service life as temperatures rise (FIG. 1) and a difference in the respective cooling times (FIG. 2) compared with a new compressor.

In actual operation, these curves do not follow the ideal curves T¹_zki and T¹_zka, T²_zki and T²_zka illustrated since various influences, such as the cooling conditions affecting the exterior of the cylinder head, dependent, for example, on the speed of travel and the external temperature, which may change continuously, make themselves felt.

The chain-dotted temperature curve T³_zka corresponds to a temperature profile for reduced cooling of the exterior of the cylinder head. The temperature rise is steeper and the maximum is reached earlier. However, the corresponding temperature profile for the internal temperature T³_zki of the cylinder head is considerably steeper because of the limited thermal conductivity of the cylinder head material, and the maximum of the internal temperature of the cylinder head is reached considerably earlier than the temperature maximum of the exterior of the cylinder head. Thus the compressor must also be switched off earlier to prevent the temperature critical for the interior of the cylinder head from being exceeded or to prevent it from acting for too long. According to the invention, this earlier shutdown is taken into account by programming an electronic control unit for the compressor in such a way that it determines the switch-off time or the switch-off temperature as a function of the gradient of the temperature profile of the exterior of the cylinder head. The steeper the temperature profile of the exterior of the cylinder head, the more the switch-off time is brought forward or the switch-off temperature lowered.

The temperature profile of the exterior of the cylinder head is measured by a temperature sensor with a frequency of 2 Hz arranged on the cylinder head of the compressor, and the gradient is determined from the temperature difference ΔT measured in each measuring step, and from the time interval Δt. As already explained, the control unit uses this data to calculate the switch-off time or the switch-off temperature. The gradient of the temperature profile of the exterior of the cylinder head is determined during each switch-on and/or switch-off operation in order to take account of rapid changes in the switch-on and switch-off parameters.

Similar considerations apply to the illustrated temperature profiles for the internal temperature of the cylinder head and the exterior of the cylinder head for a compressor toward the end of its service life, except for a shift in the direction of a longer compressor run time during the ON phases.

If the initial temperature T₀^Start at the beginning of an ON phase is lower than that shown in FIG. 1, the temperature of the exterior of the cylinder head also rises more slowly, i.e. with a shallower gradient, while the internal temperature of the cylinder head rises more quickly in comparison since heat dissipation to the outside is limited by the thermal conductivity of the cylinder head material. The control unit can accordingly take account of the initial temperature of each ON phase by bringing forward the switch-off time or lowering the switch-off temperature in the case of low initial temperatures T₀^Start.

FIG. 2 shows the cooling curves in the case of low-intensity and relatively high-intensity cooling of the cylinder head of the compressor, and the resulting cooling curves for the interior of the cylinder head. In this case too, the rate of cooling of the interior of the cylinder head depends on the rate of cooling of the exterior of the cylinder head, such that the interior of the cylinder head cannot with the same rapidity follow rapid cooling of the exterior of the cylinder head to a sufficiently low temperature (T_goodagain) for restarting the compressor, with the result that in this case the control unit sets a lengthening of the OFF time as a function of the gradient of the cooling curve. Here too, similar considerations apply mutatis mutandis for the case of different ambient temperatures and cooling conditions dependent on speed of travel, which are recorded by the control unit and taken into account in calculating the ON times or switch-off temperature and/or the OFF duration or the switch-on temperature.

For example, it is possible either for the speed of travel of the vehicle to be taken into account continuously or for a change in the system parameters of the temperature model to be performed if a specified speed of travel is exceeded.

The precise temperature profile for different initial conditions and changes during operation can be determined by laboratory measurements on the compressor, including measurement of the temperature of the interior of the cylinder head and/or of the critical elements, such as exhaust valves. Detailed characteristic maps derived therefrom for cooling and compressor ageing can be stored in the control unit, which then uses the maps and the instantaneous parameters and the measured temperature curve of the compressor cylinder head to calculate the respective ON duration or switch-off temperature and/or the respective OFF duration or reactivation temperature and controls the compressor accordingly.

In this way, it is possible to achieve better utilization of the compressor run time, and the control behavior of the system is improved.