Patent Publication Number: US-2009237856-A1

Title: Solenoid valve drive control apparatus and method for driving a solenoid valve

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a solenoid valve drive control apparatus and to a method for driving a solenoid valve, in which a drive coil of the solenoid valve is energized and the solenoid valve is driven thereby. 
     2. Description of the Related Art 
     Heretofore, it has been known to provide a solenoid valve drive control apparatus, which stops energization of the drive coil and halts driving of the solenoid valve, at some time after the drive coil has been energized and the solenoid valve has been placed in a driven state. In such a solenoid valve drive control apparatus, because the drive coil is connected in parallel with a diode (flywheel diode), when energizing of the drive coil is stopped, a comparatively large back EMF (back electromotive force) is generated, and a current (flywheel current) caused by the back EMF flows inside of a closed circuit constituted by the drive coil and the diode. In this case, the electromagnetic energy (an electromagnetic energy corresponding to the back EMF) of the solenoid valve is consumed by the diode as heat energy, and driving of the solenoid valve is halted by reducing the current to a zero level. However, due to the presence of the diode, because the current continues to flow through the closed circuit over a comparatively long period of time, a delay in response is generated, in relation to halting the driven condition of the solenoid valve. 
     Consequently, in Japanese Laid-Open Patent Publication No. 63-297881, a proposal is offered in which a series circuit made up of the flywheel diode and a transistor is connected in parallel with the drive coil, wherein by turning OFF the transistor and interrupting the flywheel current, the time during which the flywheel current flows is shortened. Further, in Japanese Laid-Open Patent Publication No. 04-354106, a proposal is made in which a transistor that functions as the flywheel diode is connected in parallel with the drive coil, wherein the time during which the flywheel current flows is shortened by turning OFF the transistor. 
     Notwithstanding, according to the techniques proposed in Japanese Laid-Open Patent Publication No. 63-297881 and Japanese Laid-Open Patent Publication No. 04-354106, in order to turn the transistor connected in parallel with the drive coil ON and OFF, the solenoid valve drive control apparatus must be equipped with a switching circuit including the transistor and a control circuit that generates a control signal for turning the transistor ON and OFF. Therefore, the circuit structure of the solenoid valve drive control apparatus becomes complex, which makes the circuit design difficult and leads to an increase in costs. 
     SUMMARY OF THE INVENTION 
     The present invention has the object of enabling improvements in response characteristics pertaining to halting the driven state of a solenoid valve by means of a simple circuit structure. 
     More specifically, to achieve the aforementioned object, according to the present invention, in the case that a solenoid valve drive control apparatus includes a switch connected in series with a drive coil of a solenoid valve, a varistor connected in parallel with the drive coil, and a diode connected in parallel with the drive coil and the switch, after the drive coil is energized and the solenoid valve is driven in a state in which the switch is in an ON state, the switch is turned OFF. 
     In this case, since the varistor forms a voltage dependent resistor, the resistance value of which changes in accordance with the value of the voltage imposed on the varistor, when an OFF state of the switch is caused and a comparatively large back EMF is generated in the drive coil, the resistance value is decreased immediately by the back EMF, whereupon the varistor is rendered conductive. Owing thereto, a current caused by the back EMF flows inside a closed circuit constituted by the drive coil and the varistor. 
     That is, according to the present invention, because the current flows inside of a closed circuit constituted by the drive coil and the varistor, and not inside of a closed circuit made up of the drive coil and the diode as in the conventional technique, electromagnetic energy stored in the solenoid valve (i.e., electromagnetic energy corresponding to the back EMF) is consumed as heat energy in the varistor. As a result, compared to the conventional technique, the current can be reduced to a zero level in a short time period. 
     Accordingly, in the present invention, by placing the switch in an OFF state, together with rendering the varistor conductive corresponding to the back EMF, the current caused by the back EMF during the drive stop time of the solenoid valve does not flow through a diode, so that responsiveness in relation to stopping driving of the solenoid valve can be improved by means of a simpler circuit structure. 
     The solenoid drive control apparatus may further include a power source terminal through which a power source voltage is supplied to the drive coil through the switch, and a switching control means connected to the power source terminal, for controlling the ON and OFF states of the switch based on the power source voltage. 
     Owing thereto, during ON states of the switch, the power source voltage is supplied to the drive coil from the power source terminal through the switch (i.e., is energized electrically) whereupon driving of the solenoid valve is enabled, and the power supply terminal can be used in common as a terminal for supply of voltage to the switching control means, as well as a terminal for energizing of the drive coil. 
     In this case, the switch may comprise a semiconductor element having a control terminal connected to the switching control means, wherein the switching control means generates a control signal based on the power source voltage, and the semiconductor element is turned ON and OFF by the control signal, which is supplied to the control terminal from the switching control means. 
     Owing thereto, control of the ON and OFF states of the switch can easily be performed. Further, since it is sufficient for the semiconductor element simply to be a semiconductor element that is capable of being turned ON and OFF by the control signal and which is capable of supplying the power source voltage to the drive coil from the power source terminal, the solenoid valve drive control apparatus can be manufactured at a low cost. 
     Furthermore, preferably, the switching control means comprises a series circuit made up of a first resistor and a second resistor, the series circuit being connected to the power supply terminal, wherein the control terminal is connected to a connection point between the first resistor and the second resistor, and wherein the semiconductor element regards a voltage at the connection point based on the power source voltage as the control signal, and is turned ON and OFF thereby. 
     Because the switching control means is constituted by the first resistor and the second resistor, the solenoid valve drive control apparatus can be realized at a low cost and by means of a simple circuit structure. Further, since the voltage at the connection point is regarded as the control signal, the control signal can be generated easily. Moreover, because the voltage is treated as the control signal, a semiconductor element of a voltage controlled type such as an FET, or a MOSFET, can be adopted for use as the semiconductor element. 
     In addition, preferably, the varistor is rendered conductive when a voltage in a parallel circuit made up of the drive coil and the varistor becomes greater than the power source voltage. 
     As a result, while the power source voltage is being supplied from the power source terminal and through the switch to the drive coil for driving the solenoid valve, rendering the varistor conductive, and flowing of current inside the closed circuit constituted by the drive coil and the varistor can reliably be prevented. Together therewith, by turning the switch OFF, when the back EMF, which is greater than the power source voltage, is generated in the drive coil, the varistor immediately becomes conductive and the current is made to flow reliably inside the closed circuit. 
     Preferably, the varistor is a zinc oxide varistor. 
     Such a zinc oxide varistor is an electronic element that is widely available in the market and can easily be acquired. Therefore, the solenoid valve drive control apparatus can be manufactured at a low cost. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a solenoid valve drive control apparatus according to an embodiment of the present invention; 
         FIG. 2  is a time chart showing a drive control for the solenoid valve performed by the solenoid valve drive control apparatus of  FIG. 1 ; 
         FIG. 3  is a circuit diagram of a solenoid valve drive control apparatus according to a comparative example; and 
         FIG. 4  is a time chart showing a drive control for the solenoid valve performed by the solenoid valve drive control apparatus of  FIG. 3 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in  FIG. 1 , a solenoid valve drive control apparatus  10  according to the present embodiment forms a device for energizing a drive coil  12  of the solenoid valve, thereby driving the solenoid valve, including an N-channel MOSFET  14 , a P-channel MOSFET (switch, semiconductor element)  16 , a control circuit  18  constituted by a microprocessor or the like, a diode  20  that functions as a flywheel diode, a varistor  22  that utilizes a zinc oxide varistor therein, a first resistor  24 , and a second resistor  26 . 
     In this case, the MOSFET  16 , the drive coil  12  and the MOSFET  14  are connected in series between a power source terminal  28 , to which a power source voltage V is externally supplied, and a control input terminal  30 , to which a drive command signal Sa having a low potential (e.g., ground potential) is supplied. Further, the varistor  22  is connected in parallel with the drive coil  12 , and the diode  20  is connected in parallel with the series circuit made up of the MOSFET  16  and the drive coil  12 . 
     Furthermore, the first resistor  24  and the second resistor  26  are connected in a series circuit between the power source terminal  28  and the control input terminal  30 . Still further, a power source input terminal Vdd of the control circuit  18  is connected to the power source terminal  28 , a control input terminal Vss is connected to the control input terminal  30 , and a control output terminal G is connected to a gate terminal G 1  of the MOSFET  14 . 
     Accordingly, the series circuit made up of the MOSFET  16 , the drive coil  12  and the MOSFET  14 , the series circuit made up of the first resistor  24  and the second resistor  26 , and the control circuit  18 , are connected together in parallel between the power source terminal  28  and the control input terminal  30 . Further, a connection point  32  of the first resistor  24  and the second resistor  26 , which make up a switching control means  34 , is connected to a gate terminal G 2  of the MOSFET  16 . 
     Next, an explanation shall be made, with reference to  FIG. 1  and the time chart of  FIG. 2 , concerning operations (in a method for driving the solenoid valve) of the solenoid valve drive control apparatus  10  according to the present embodiment. 
     As shown in  FIG. 2 , in the solenoid valve drive control apparatus  10 , for driving the solenoid valve, energizing is carried out in a rated energizing mode with respect to the drive coil  12  (see  FIG. 1 ) for a preset first time period T 1  inside of a predetermined drive command interval T, and a power saving energizing mode is performed with respect to the drive coil  12  during a remaining second time period T 2 . 
     In actuality, in the solenoid valve drive control apparatus  10 , the drive command interval T and a predetermined OFF period (a period during which the solenoid valve is stopped) after the drive command interval T, are considered to form one cycle period, and driving of the solenoid valve is repeatedly carried out over a plurality of such cycle periods. However, in the following explanations, operation of the solenoid valve drive control apparatus  10  over one such cycle period shall be described. 
     Further, the aforementioned rated energizing mode is defined as an energizing method in which, within the first time period T 1 , a power source voltage V, which is a rated voltage of the drive coil  12 , is applied for driving (initiating movement of) the solenoid valve, wherein the power source voltage V is applied to the drive coil  12  from the power source terminal  28  through the MOSFET  16 , under a condition in which the MOSFET  14  (the 1st MOSFET in  FIG. 2 ) and the MOSFET  16  (the 2nd MOSFET in  FIG. 2 ) are both in an ON state (i.e., wherein the duty ratio of an ON state of the MOSFET  14  and the MOSFET  16  is 100%). 
     Furthermore, the aforementioned power saving energizing mode is defined as an energizing method in which, during the second time period T 2  after the first time period T 1 , while the MOSFET  16  remains in an ON state, the MOSFET  14  is repeatedly turned ON and OFF (at an OFF period T 3  and an ON period T 4 ), whereby the solenoid valve is driven (i.e., the driven state of the solenoid valve is maintained) at a reduced power, which is lower than the rated energizing power applied with respect to the drive coil  12 . 
     First, at time t 0 , when the power source voltage V is supplied externally to the power source terminal  28  and the potential of the control input terminal  30  is a drive command signal Sa having a low potential (e.g., ground potential), a drive command interval T is initiated. 
     Upon initiation of the drive command interval T, the power source voltage V is supplied (imposed) from the power source terminal  28  on the side of the power input terminal Vdd of the control circuit  18  and the first resistor  24  of the switching control means  34 , whereas on the other hand, the drive control signal Sa is supplied from the control input terminal  30  on the side of the control input terminal Vss and the second resistor  26  of the switching control means  34 . As a result, within the first time period T 1  from time t 0  to time t 1 , in the control circuit  18 , a control signal (a pulse signal having a pulse width of T 1 ) is generated at a 100% drive duty ratio, and is supplied to the gate terminal G 1  of the MOSFET  14 . Further, in the switching control means  34 , the voltage at the connection point  32 , which is a divided voltage formed by dividing the power source voltage V based on the resistance value of each of the first resistor  24  and the second resistor  26 , is regarded as the control signal, and is supplied to (imposed on) the gate terminal G 2  of the MOSFET  16 . 
     In accordance therewith, during the first time period T 1 , the MOSFET  14  is rendered conductive (turned ON) between the drain terminal D 1  and the source terminal S 1  thereof by a control signal, which is supplied to the gate terminal G 1  from the control circuit  18 . On the other hand, the MOSFET  16  is rendered conductive (turned ON) between the source terminal S 2  and the drain terminal D 2  thereof by a control signal, which is supplied to the gate terminal G 2  from the connection point  32 . As a result, the power source voltage V is applied to the drive coil  12  from the power source terminal  28  through the MOSFET  16 , whereupon the rated energizing mode with respect to the drive coil  12  is carried out to initiate movement of the solenoid valve. 
     As shown in  FIG. 1 , the path I 1  from the power source terminal  28 , through the MOSFET  16 , the drive coil  12  and the MOSFET  14  to the control input terminal  30 , indicates the path of the current that flows in the drive coil  12  during the first time period T 1 . Further, the first time period T 1  is set to a sufficient time interval, so as to enable movement of the solenoid valve to be initiated by application of the power source voltage V with respect to the drive coil  12 , whereby the movable element inside the solenoid valve is moved and attracted to the fixed iron core. 
     Next, after completion of the first time period T 1 , in the second time period T 2  from time t 1  until time t 4 , the control circuit  18  stops supplying the control signal to the gate terminal G 1  in the OFF period T 3  and supplies the control signal to the gate terminal G 1  in the ON period T 4 , repeatedly, thereby carrying out a power saving energizing mode under a PWM (pulse width modulated) control. In this case, the OFF period T 3  is defined by a time interval from time t 1  to time t 2 , whereas the ON period T 4  is defined by a time interval from time t 2  to t 3 . Accordingly, the control signal is formed by a repeating pulse signal having a duty ratio of T 4 /(T 3 +T 4 ). 
     At the OFF period T 3 , since the control signal is not supplied to the gate terminal G 1  from the control circuit  18 , the MOSFET  14  is switched from an ON state into an OFF state, between the drain terminal D 1  and the source terminal S 1  thereof, whereupon energizing of the drive coil  12  is stopped. As a result, in the closed circuit (the closed circuit indicated by path I 2  in  FIG. 1 ) constituted by the drive coil  12 , the MOSFET  16  and the diode  20 , a current caused by the electromagnetic energy of the solenoid valve, which was stored in the drive coil  12 , is made to flow, and the electromagnetic energy thereof is consumed through the diode  20 . Moreover, since the OFF period T 3  is set comparatively short, the attracted condition of the movable element to the fixed iron core is maintained, or the movable element is separated only slightly from the fixed iron core. 
     On the other hand, in the ON period T 4 , since the control signal is supplied from the control circuit  18  to the gate terminal G 1 , the MOSFET  14  is switched from an OFF state into an ON state, between the drain terminal D 1  and the source terminal S 1  thereof, whereupon energizing of the drive coil  12  is reinitiated, and the current flowing in the drive coil  12  flows along the path I 1  of  FIG. 1 . In this case, attraction of the movable element to the fixed iron core is maintained, or the movable element which has separated only slightly from the fixed iron core, or which is about to separate from the fixed iron core, is once again attracted firmly by the fixed iron core. 
     By repeatedly performing sequential operations of the OFF period T 3  and the ON period T 4  from time t 1  until time t 4  (until the drive command interval T is completed), the power saving energizing mode is carried out with respect to the drive coil  12 , while the driven state of the solenoid valve is maintained. Moreover, the second time period T 2  can be optionally set, corresponding to a desired driving time for the controlled object (fluid device) of the solenoid valve. Further, even during the second time period T 2 , since the power source voltage V is supplied in an ongoing manner from the power source terminal  28  to the switching control means  34 , the MOSFET  16  is maintained in an ON state (see  FIG. 2 ). 
     Additionally, at time t 4 , since supply of the power source voltage V from the exterior to the power source terminal  28  is terminated, and supply of the power source voltage V to the power source input terminal Vdd of the control circuit  18  and to the side of the first resistor  24  of the switching control means  34  also is stopped, generation of the respective control signals from the control circuit  18  and the switching control means  34 , and supply of such control signals to the gate terminals G 1 , G 2 , also is halted. As a result, the MOSFETs  14  and  16  are switched from the ON state into an OFF state, whereupon energizing of the drive coil  12  is stopped. By terminating electrical energizing of the drive coil  12 , a back EMF caused by the electromagnetic energy is generated in the drive coil  12 , which is greater than the power source voltage V. 
     At this time, in the varistor  22 , a zinc oxide varistor is adopted, wherein the resistance value of the varistor on which the voltage larger than the power source voltage V is imposed is immediately lowered to make the varistor conductive. Therefore, if the voltage, which is generated due to the MOSFET  16  being turned OFF in the parallel circuit made up of the drive coil  12  and the varistor  22 , is the back EMF that is greater than the power source voltage V, the resistance value of the varistor  22  is lowered immediately at time t 4 . As a result, the varistor  22  assumes a conductive state, and a current caused by the back EMF flows inside of the closed circuit constituted by the drive coil  12  and the varistor  22  (the closed circuit indicated by path I 3  in  FIG. 1 ). Accordingly, the electromagnetic energy is consumed as heat energy in the varistor  22 , resulting in the current being reduced to a zero level in a short time during the transition period T 5  from time t 4  to time t 5  (the period indicated by the slanted line, in  FIG. 2 ). As a result, the movable element separates immediately from the fixed iron core, and the solenoid valve rapidly assumes a stopped state. 
     The construction and operation of the solenoid valve and the control circuit  18  are well known (for example, as disclosed in Japanese Laid-Open Patent Publications 2007-024281 and 2007-177818), and therefore details concerning such features have been omitted from the present specification. 
     Next, explanations shall be made concerning the advantages and effects of the solenoid valve drive control apparatus  10  and the method for driving a solenoid valve according to the present embodiment. 
       FIG. 3  is a circuit diagram of a solenoid valve drive control apparatus  40  according to a conventional technique (comparative example).  FIG. 4  is a time chart showing a drive control for the solenoid valve, which is performed by the solenoid valve drive control apparatus  40 . In  FIG. 3  and  FIG. 4 , structural elements, which are the same as those shown in  FIGS. 1 and 2 , are designated by the same reference numerals, and detailed explanations of such features have been omitted. 
     In the solenoid valve drive control apparatus  40  according to the comparative example, at time t 4 , when the MOSFET  14  is turned OFF and energizing of the drive coil  12  is halted, a current caused by a back EMF generated in the drive coil  12  flows inside of a closed circuit (the closed circuit indicated by path I 2  in  FIG. 3 ) constituted by the drive coil  12  and the diode  20 . In this case, because the current flows through the diode  20 , and the electromagnetic energy of the solenoid valve is consumed inside of the closed circuit, the current continues to flow inside of the closed circuit over a comparatively long period of time (for the transition period T 6 , from time t 4  until time t 6 , as indicated by the slanted line in  FIG. 4 ), and a response delay in relation to stopping the driven state of the solenoid valve is generated. 
     In contrast thereto, according to the present embodiment, the solenoid valve drive control apparatus  10  (see  FIG. 1 ) includes the MOSFET  16  connected in series with the drive coil  12  of the solenoid valve, the varistor  22  connected in parallel with the drive coil  12 , and the diode  20  connected in parallel with the drive coil  12  and the MOSFET  16 . After the drive coil  12  is energized and the solenoid valve is driven with the MOSFET  16  in an ON state (during the drive command interval T), the MOSFET  16  is turned OFF. 
     In this case, because the varistor  22  is a voltage-dependent resistor, the resistance value of which changes in accordance with the value of the voltage imposed on the varistor  22 , when the MOSFET  16  is turned OFF and a comparatively large back EMF is generated in the drive coil  12 , the resistance value is reduced immediately by the back EMF, and the varistor  22  is rendered conductive. Therefore, the current caused by the back EMF flows within a closed circuit (the closed circuit indicated by the path I 3  in  FIG. 1 ) constituted by the drive coil  12  and the varistor  22 . 
     More specifically, in the present embodiment, the current does not flow in the closed circuit (the closed circuit indicated by the path I 2  in  FIG. 3 ) constituted by the drive coil  12  and the diode  20  as in the comparative example, but rather, flows in the closed circuit (the closed circuit indicated by the path I 3  in  FIG. 1 ) constituted by the drive coil  12  and the varistor  22 . Therefore, the electromagnetic energy (electromagnetic energy corresponding to the back EMF) stored in the solenoid valve is consumed as heat energy in the varistor  22 . As a result, in contrast to the comparative example, the current can be reduced to a zero level in a shorter period of time (T 5 &lt;T 6 ). 
     Accordingly, in the present embodiment, by preventing the current caused by the back EMF when the driven state of the solenoid valve is stopped from flowing through the diode  20 , in accordance with the OFF state of the MOSFET  16  and the conductivity of the varistor  22  corresponding to the back EMF, the responsiveness of the solenoid valve in relation to stopping driving thereof can be improved by means of a simple circuit structure. 
     Further, the solenoid valve drive control apparatus  10  includes the power source terminal  28  for supplying the power source voltage V to the drive coil  12  through the MOSFET  16 , and the switching control means  34  connected to the power source terminal  28 , for controlling ON and OFF states of the MOSFET  16  based on the power source voltage V. Therefore, during the period of time (the drive command interval T) when the MOSFET  16  is ON, the power source voltage is supplied to (i.e., energizes) the drive coil  12  from the power source terminal  28  through the MOSFET  16 , thereby enabling the solenoid valve to be driven, and the power source terminal  28  can be used in common as a terminal for supply of voltage to the switching control means  34 , as well as a terminal for energizing of the drive coil  12 . 
     Furthermore, because the MOSFET  16  is turned ON and OFF between the power source terminal  28  and the drive coil  12  based on a control signal from the switching control means  34 , control of the ON and OFF states can easily be performed, and the solenoid valve drive control apparatus  10  can be manufactured inexpensively. 
     Still further, because the switching control means  34  is constituted by the first resistor  24  and the second resistor  26 , the solenoid valve drive control apparatus  10  can be realized by means of a simple circuit structure and at a low cost. Further, since the voltage of the connection point  32  is regarded as the control signal supplied to the gate terminal G 2 , the control signal can easily be generated. Moreover, in the present embodiment, ON and OFF states performed by the MOSFET  16  have been described. However, because the voltage is taken as the control signal, even if the MOSFET  16  were replaced by other types of voltage controlled type semiconductor elements (for example, a FET), the same advantages and effects of the present embodiment could easily be obtained. 
     In addition, because the varistor  22  is rendered conductive when the voltage in the parallel circuit made up of the drive coil  12  and the varistor  22  becomes greater than the power source voltage V, while the power source voltage V is being supplied to the drive coil  12  from the power source terminal  28  through the MOSFET  16  for driving the solenoid valve, rendering the varistor  22  conductive and flowing of current in the closed circuit constituted by the drive coil  12  and the varistor  22  can reliably be prevented. Together therewith, due to the MOSFET  16  being turned OFF, when the back EMF (back electromotive force) greater than the power source voltage V is generated in the drive coil  12 , the varistor  22  immediately becomes conductive and the current is made to flow reliably inside the closed circuit. 
     Further, preferably, a zinc oxide varistor is adopted for use as the varistor  22 . Such a zinc oxide varistor is an electronic element that is widely available in the market and can easily be acquired. Therefore, the solenoid valve drive control apparatus  10  can be manufactured at a low cost. 
     The present invention is not limited to the aforementioned embodiments. It is a matter of course that various other structures or modifications could be adopted, based on the content of the present specification and drawings, as disclosed herein.