The present invention relates to a device and to a method for the control of the air-conditioning system of a vehicle, in particular a motor vehicle such as an automobile, a bus, etc.
As is known, the air-conditioning systems of motor vehicles are typically provided with a closed-loop cooling circuit equipped with an evaporator and a control system capable of regulating the temperature of the air introduced into the passenger compartment.
FIG. 1 shows a control system 1 of a cooling circuit 2, which is traversed by the coolant and comprises in succession: an evaporator 3; a compressor 4, which is designed to take in, at a certain intake pressure, the coolant in the vapour phase from the evaporator 3 so as to obtain a control of the temperature of the air downstream of the evaporator 3 itself; a condenser 5, designed to receive the coolant in the vapour phase from the compressor 4; and an expansion valve 6 designed to receive the coolant in the liquid phase from the condenser 5 to supply it in dual-phase (i.e., vapour phase and liquid phase) to the evaporator 4 itself.
In particular, the compressor 4 is constituted by a compressor with externally controlled variable displacement, on which it is possible to operate by causing the displacement (defined as the working volume, where the coolant is compressed) to vary as the thermal load acting on the air-conditioning system varies.
The possibility of varying the displacement of the compressor is obtained via the electromagnetic regulation valves (not illustrated), which are driven via an external control signal SC and are designed to control the device that enables modulation of the displacement of the compressor 4.
The control system 1 moreover comprises a control device 7, which is able to generate the control signal SC of the compressor 4 in such a way as to control the temperature of the air downstream of the evaporator 3 as a function of the deviation between a reference temperature TREF set by the user by means of an externally controlled selector device 9 and an effective temperature TMIS indicating the temperature of the air present downstream of the evaporator 3. The effective temperature TMIS can be measured using a temperature sensor 10, set downstream of the evaporator 3.
The control device 7 comprises an adder block 11 having a first input designed to receive the reference temperature TREF from the selector device 9, a second input designed to receive the effective temperature TMIS from the temperature sensor 10, and an output supplying a temperature error er, given by the difference between the reference temperature TREF and the effective temperature TMIS.
The control device 7 moreover comprises a control block 12, which is designed to receive at input the temperature error er and a set of measurement parameters, such as for example Te (external temperature), RPM (engine r.p.m.), and RH (relative humidity) correlated to the exogenous disturbance, and supplies at output, according to the latter, the control signal SC.
In detail, the control block 12 comprises a compensating network of a proportional-integral (PI) type (not illustrated) and generates a control signal SC corresponding to a pulse-width modulation (PWM) signal, which drives the electromagnetic valves for regulating the stroke of the pistons, thus determining control of the displacement of the compressor 4. In the case in point, the regulation of the displacement determines a control of the intake pressure of the compressor 4 and, consequently, an indirect control of the temperature of the air downstream of the evaporator 3.
It is moreover known that, in the air-conditioning systems described above, the expansion valve 6 for supplying the coolant to the evaporator 3 is an internally controlled device, operation of which is completely independent of the control implemented on the compressor 4 by the control device 7.
The complete independence existing between the two controls determines, in certain limit conditions of operation of the compressor 4, a discordance in the control of some parameters that characterize operation of the cooling circuit 2, such as for example the intake pressure of the compressor 4 and the temperature of the air downstream of the evaporator 3, in this way causing a condition of instability of the air-conditioning system. In the case in point, during its operation, the expansion valve 6 generates a temperature disturbance ΔTEVAP of an oscillatory type, which alters the temperature of the air downstream of the evaporator 3, and leads, in certain conditions, to instability of the control.
In fact, said temperature disturbance ΔTEVAP determines an increase of the intake pressure of the compressor 4, which in certain limit conditions exceeds a threshold delimiting the condition of stability of the air-conditioning system, consequently causing a series of oscillations of the flow rate of the coolant, and of the temperature of the air downstream of the evaporator 3. The generation of said oscillations, generally referred to with the term “hunting phenomenon”, represents a major drawback in the air-conditioning systems described above in so far as it has a negative effect both on the capacity of minimizing the consumption of the air-conditioning system, and on the thermal comfort of the passenger compartment of the vehicle.