Patent Publication Number: US-6209566-B1

Title: Hydraulic control systems

Description:
FIELD OF THE INVENTION 
     The present invention relates to hydraulic control systems such as those used in the control of vehicle active suspension systems. 
     BACKGROUND OF THE INVENTION 
     It is known to provide closed loop pressure control in a hydraulic system by monitoring the hydraulic pressure at a point in a hydraulic circuit, comparing the measured pressure with a desired pressure, and controlling the electrical current to an electrically operated valve, such as a solenoid valve, to open or close the valve to adjust the pressure in the system towards the desired pressure. 
     It can be a problem with such systems that known pressure transducers have a temperature dependent characteristic, so the exact hydraulic pressure cannot be accurately measured. 
     SUMMARY OF THE INVENTION 
     The present invention provides a control system for a hydraulic valve block including an electrically operated valve, the system including an electronic control means arranged to supply an electric control current to the valve, the control current having a temperature dependent parameter, and the control means being further arranged to monitor said parameter thereby to measure the temperature of the valve. 
     The present invention further provides a hydraulic control system comprising a hydraulic circuit containing fluid and including a source of fluid pressure, an electrically operated valve for controlling the fluid pressure in a part of the hydraulic circuit, a pressure transducer for producing a pressure signal indicative of the fluid pressure in said part of the hydraulic circuit, and an electronic control means arranged to supply an electric control current to the valve to control the valve in response to signals from the pressure transducer, the control current having a temperature dependent parameter wherein the control unit is also arranged to monitor said parameter of the control current thereby to monitor the temperature of the valve, and to compensate accordingly for the effect of temperature changes on the pressure signal. 
     Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic representation of a hydraulic control system according to the invention, 
     FIG. 2 shows the output characteristic of the pressure transducer forming part of the system of FIG. 1, and 
     FIG. 3 is a diagrammatic representation of the control unit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     Referring to FIG. 1, a hydraulic circuit  10  for an active vehicle suspension system comprises a pump  12  for supplying hydraulic fluid under pressure from a reservoir  14 , and a valve block  16  for controlling the distribution of hydraulic fluid to various actuators (not shown) and the return of fluid to the reservoir  14 . The valve block has a first port  18  for receiving fluid from the pump  12  and a second port  20  for the return of fluid to the reservoir  14 . The first and second ports  18 ,  20  are interconnected by a diverter valve  22  which can allow fluid to flow from the first port  18  to the second port  20  to control the pressure at the first port as will be described in more detail below. Two further solenoid valves  24 ,  26  control the flow of fluid from the pump  12  to the actuators and from the actuators to the reservoir. These two valves basically connect and disconnect the actuators in the desired combination, and details of their operation are not relevant to this invention. A pressure transducer  28  produces a pressure signal indicative of the hydraulic pressure at the first port  18 , and a control unit controls the valves  22 ,  24 ,  26  in response to the pressure signal so as to regulate the pressure at the first port  18  to a desired level, and to connect the actuators to the first and second ports  18   20  in the desired combination. The choice of pressure produced by the diverter valve  22  is based on other inputs to the control unit  30  which are not relevant to this invention. 
     Referring to FIG. 2, the output characteristic of the pressure transducer  28  is dependent on its temperature. At a given temperature, the  10  voltage output by the transducer is directly related to the pressure being measured. As the temperature changes, the gradient of the characteristic, i.e. the change in output voltage for a given change of pressure is the same, but the absolute value of the output voltage is altered. Thus for a first low temperature T 1 , the characteristic is illustrated by the line V(P) T1 , and for a second, higher temperature T 2  the characteristic is illustrated by the line VP) T2 . The output voltage for zero pressure is referred to as the offset voltage, and the change in offset voltage with temperature is the same as the change in output voltage with temperature for any given pressure. 
     Referring to FIG. 3, the control unit can be considered as a number of  20  functional blocks. A pressure control block  32  receives a signal P d  indicative of the desired pressure at the first port  18  and another signal V(P) which is the output signal from the pressure transducer. From the difference between the measured pressure and the desired pressure it produces a signal I which indicates the current which needs to be supplied to the solenoid  22   a  of the diverter valve  22  to produce the desired pressure at the first port  18 . 
     A current control block  34  receives the signal I and also has inputs connected to a battery voltage V bat . It applies the battery voltage across the solenoid  22   a  as a pulsed signal, monitors the driving current flowing through the solenoid as a result, and modulates the pulse width so as to produce the total, or mean, current corresponding to the signal I from the pressure control block. The current control block sends a signal M/S back to the pressure control block indicative of the mark to space (or duty) ratio of the driving current. 
     Because the electrical resistance of the solenoid  22   a  is temperature dependent, the duty ratio of the solenoid driving current required to produce a given total current varies with the temperature of the solenoid. Therefore, because the valve block is a good thermal conductor, and the temperature of the pressure transducer  28  will always be approximately equal to that of the solenoid  22   a , the pressure control block can determine the temperature of the pressure transducer from the relationship between the signal I and the signal M/S. 
     Referring back to FIG. 2, in order to determine the pressure P corresponding to a transducer output voltage V, the control unit needs to know the gradient of the voltage/pressure characteristic, which is constant and can be stored in memory, and the offset voltage which is the output voltage at zero pressure. It is assumed that the offset voltage varies linearly with temperature, and the control unit is therefore arranged to record the output voltage V at a time when the temperature of the vehicle  36  is low, e.g. when it is started up, and at another time when the temperature of the vehicle  36  is high, e.g. when the engine  38  is turned off. From estimates of the temperatures at these times the relationship between offset voltage and temperature can be estimated.