Abstract:
An arrangement for monioring multiple channel solenoid currents wherein a plurality of separately controllable solenoid coils are coupled commonly by a single current measurement element to one side of a current supply.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a national stage entry International Application No. PCT/GB01/04608 filed Oct. 15, 2001, which claimed priority to Great Britain Patent Application No. 0025236.1 filed Oct. 14, 2000, the disclosures of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention is concerned with the monitoring of multiple channel solenoid currents particularly, but not exclusively, in automotive electrical and electronic control systems. 
   There are many situations in electrical and electronic control systems where there is a requirement for the current flowing in a solenoid coil to be monitored and measured. Conventionally, each channel containing an individual solenoid coil has its own current sensing element associated with it Usually, the sensing element comprises a resistive element, eg. a simple resistor disposed in series with the solenoid coil, whereby the voltage drop across the resistor is proportional to the current flowing through it, and hence proportional to the current flowing through the solenoid coil. The voltage across the resistor is conditioned and read by an analogue to digital converter (ADC). 
   The known arrangement thus has the associated cost disadvantage that individual sensing elements and conditioning are required for each channel of current to be measured, ie. for each solenoid coil to be monitored. 
   It would be advantageous to provide, particularly for automotive applications, an arrangement whereby it is no longer necessary for there to be individual sensing elements for each solenoid channel. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a plurality of separately controllable solenoid coils are coupled commonly by a single current measurement element to one side of a current supply. 
   The measurement element can, for example, be coupled to an analogue to digital converter via a signal conditioning amplifier for measurement purposes. 
   Preferably, the solenoid coils are coupled commonly by said single current measurement element to the low side of the current supply. 
   In a preferred embodiment, in order to enable the current through any one particular solenoid coil to be measured, means are included for, firstly, enabling a current measurement reading to be made only while a respective drive element for that particular solenoid coil is switched on, and, secondly, switching on the drive element for only that particular coil when the current measurement reading is made, with all other drive elements being switched off. 
   The first and second means would normally be realized by logic circuit arrangements which are implemented by hardware or software. Software implementation is preferred since the microcomputer which is present in the system for control of the braking system is available for this purpose, so that the additional hardware costs do not arise. 
   Usually the measurement of the current through the solenoid coils is required for “closed-loop” operation whereby, preferably, the duty cycle of a PWM-signal is varied to control the current through the solenoid coil. Since the present circuit arrangement has a common current measurement element, “closed-loop” operation is not possible. Therefore a so-called “calibration cycle” can be arranged to be passed through for each solenoid coil when the two before-mentioned first and second conditions can be met. During the calibration cycle, the optimum setting for the PWM duty cycle can be learned. Because the calibration cycles are repeated periodically, a reliable operation can be ensured over the whole running period, even though only “normal” control is possible. This means e.g. that in applications where valves are driven, a correct switching behaviour (“OPEN”, “CLOSED” respectively) can be guaranteed. 
   Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a circuit diagram illustrating a typical example of a known arrangement for monitoring multiple solenoid coils; 
       FIG. 2  is a circuit diagram illustrating one embodiment of an arrangement in accordance with the present invention for monitoring multiple solenoid coils; 
       FIG. 3  is an example of an electro hydraulic braking system (EHB) having a plurality of solenoid operated valves to which the present invention can be applied; 
       FIG. 4  is a circuit diagram illustrating in principle a technique for achieving the control of the timing of the solenoid energisations to achieve operation in accordance with the present invention; 
       FIG. 5  is a flow diagram illustrating operation of the circuitry of  FIG. 4 ; 
       FIG. 6  is a timing diagram corresponding to  FIGS. 4 and 5 ; and 
       FIG. 7  is a modified flow diagram showing one preferred operation. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first to  FIG. 1 , there is shown a circuit arrangement having three solenoids whose solenoid coils  10   a ,  10   b ,  10   c  are to be monitored. The solenoid coils  10   a ,  10   b ,  10   c  are disposed between a supply line  16  and ground  18  and are controlled by respective series low side switches  12   a ,  12   b ,  12   c , for example drive FETs. In parallel with each coil  10   a ,  10   b ,  10   c  is a respective recirculation diode  14   a ,  14   b ,  14   c . In order to measure the current passing through the solenoids  10   a ,  10   b ,  10   c , there is disposed between each drive FET  12   a ,  12   b ,  12   c  and ground  18  a respective resistor  20   a ,  20   b ,  20   c , the voltage drop across each resistor  20   a ,  20   b ,  20   c  being measured by a respective amplifier  22   a ,  22   b ,  22   c . The outputs of the amplifiers  22   a ,  22   b ,  22   c  lead to respective ADC inputs (not shown) for measurement purposes. 
   In the arrangement of  FIG. 2  in accordance with the present invention, identical components are given the same reference numerals. In this arrangement, the terminals of three FETs  12   a ,  12   b ,  12   c  remote from the coils  10  are connected together and coupled to ground  18  via a single common resistor  24 , the voltage across which is monitored by means of a single amplifier  26 . The output of the amplifier  26  is again passed to the input of an analogue to digital converter (ADC)  27  for measurement purposes. 
   It is emphasised that although the illustrated circuit shows three solenoid coils  10   a ,  10   b ,  10   c , this is purely by way of example and in practice there could be any number of such coils, commonly coupled to ground  18  by the single sensing resistor  24 . 
   Although the circuit illustrated uses FETs as low side drivers, in principle other drive elements, such as relays, transistors and the like, could be used. 
   The sensing element formed by resistor  24  in the circuit of  FIG. 2  detects the sum of all of the currents flowing through the solenoid coils  10   a ,  10   b ,  10   c . In order to read the current through any particular single one of the coils  10 , two conditions must be met. Firstly, the ADC reading must only be made while the respective drive element  12  for that particular solenoid channel is switched on. Secondly, only the FET  12  for the particular coil  10  being measured should be switched on when the ADC reading is made. All other drive FETs  12  must be switched off. Although not shown in  FIG. 2 , suitable control circuitry is provided to achieve these two conditions, for example as described hereinafter in connection with  FIGS. 4  to  7 . 
   The achievement of the above identified two conditions means essentially that, in order to ensure that a robust measurement can be made, only one active device can be held energized. This is normally achieved through software control of the timing of the solenoid energisations. In principle, it is possible to provide a control circuit as illustrated in the attached modified version of the present  FIG. 2  identified as FIG.  4 . Here, the output X of the device  26  is read on an input port of a microprocessor  28 . The signal X is present when one or more of the solenoids are conducting and with suitable control, as idealized in the flow chart of  FIG. 5 , the output X can be associated with a specific solenoid. The actual timing of the associated pulses can be seen from the corresponding timing diagram of FIG.  6 . 
   In practice, due to variations in the supply voltage, the period (mark/space ratio) that is required to provide a suitable energisations current for a solenoid varies such that it is likely that the actual pulse periods for all solenoids overlap. In this case, a control method is required that intentionally holds off the energisations of all but the monitored solenoid. This can be achieved by interrupting the normal pattern i.e. normal cycle, of solenoid energisations with a measurement cycle at a prescribed frequency, for example 1 in every 10 normal cycle energisations, and by holding off the energisations of all but the monitored solenoid, measurement of each solenoid being achieved by incrementing through the monitored solenoids on each and every other interruption or measurement cycle. In this control regime there would need to be an intentional disabling of the potentially active devices in order to allow current measurement of the chosen device to take place. By way of example only, the flow chart of  FIG. 7  could be included as part of the microprocessor control of the solenoid energisations that could achieve the desired objective. This control method comprises a normal cycle part which allows uninterrupted operation of the solenoids under control, and a measurement cycle part that is evoked every n&#39;th cycle that disables all solenoids then enables the chosen solenoid for subsequent measurement. After measurement, all solenoids are enabled and the chosen solenoid is incremented to the next in the measurement order. Finally the interrupt counter is reset so as to allow the normal cycle to continue. 
   In arrangements such as those described above, by having only a single sensing resistor and conditioning amplifier, a substantial cost saving can be made. 
     FIG. 3  shows an example of a typical practical situation where the use of the present invention can be of cost saving advantage.  FIG. 3  illustrates an electro hydraulic braking system where braking demand signals are generated electronically at a travel sensor  29  in response to operation of a foot pedal  30 , the signals being processed in an Electronic Control Unit (ECU)  32  for controlling the operation of brake actuators  34   a ,  34   b  at the front and back wheels respectively of a vehicle via pairs of valves  36   a ,  36   b ,  36   c ,  36   d . The latter valves are operated in opposition to provide proportional control of actuating fluid the brake actuators  34  from a pressurized fluid supply accumulator  38 , maintained from a reservoir  40  via a motor-driven pump  42 . For use, for example, in emergency conditions when the electronic control of the brake actuator is not operational for some reason, the system includes a master cylinder  44  coupled mechanically to the foot pedal  30  and by which fluid can be supplied directly to the actuators  34  in a “push-through” condition. In the push-through condition, a fluid connection between the front brake actuators  34   a  and the cylinder  44  is established by means of digitally operating, solenoid operated valves  46   a ,  46   b . Also included in the system are further digitally operating solenoid valves  48 ,  50  and  52  which respectively connect the two pairs of valves  36   a ,  36   b , the pump  42  and accumulator  38  with the two pair of valves  36   c ,  36   d . 
   The present system of monitoring solenoid coil currents can be applied to monitor the currents in the digitally operated valves  46   a ,  46   b ,  48 ,  50  and  52  by means of a common sensing element  24  and conditioning amplifier  26 . It can also be applied to the control valve pairs  36   a ,  36   b ,  36   c  and  36   d  in the event that they are not provided for a proportional control mode. 
   In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.