Patent Publication Number: US-8525453-B2

Title: Damper system for vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to a damper system for a vehicle constituted by including an electromagnetic damper which includes an electromagnetic motor and which is configured to generate a damping force with respect to a motion of a sprung portion and an unsprung portion toward each other and a motion thereof away from each other. 
     BACKGROUND ART 
     In recent years, there has been developed, as a suspension system for a vehicle, a so-called electromagnetic suspension system, namely, a system that comprises, as one constituent element thereof, a damper system constituted by including an electromagnetic damper which includes an electromagnetic motor and which is configured to generate a damping force with respect to a motion of a sprung portion and an unsprung portion toward each other and a motion thereof away from each other, on the basis of an electromotive force generated in the electromagnetic motor. For instance, the following Patent Literatures disclose such a system. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1 JP-A-2007-290669 
         Patent Literature 2 JP-A-2007-37264 
         Patent Literature 3 JP-A-2001-310736 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     As in the systems described in the above Patent Literatures, in the damper system constituted by including the electromagnetic damper, the electromagnetic damper generally includes a brushless DC motor as the electromagnetic motor and a drive circuit that includes a plurality of switching elements for driving the brushless DC motor. Such a damper system is relatively complicated in structure and expensive. The damper system including the electromagnetic damper is still under development and suffers from problems which arise from the basic structure of the ordinary damper system that has been proposed, such as the problem described above. Accordingly, there is plenty of room for improving the utility of the damper system. The present invention has been made in view of the situation described above. Therefore, the present invention aims at improving the utility of the damper system for a vehicle by proposing a novel structure of the damper system. 
     Solution to Problem 
     To solve the problem indicated above, a first invention provides a damper system for a vehicle which is mounted on the vehicle and which comprises an electromagnetic damper configured to generate a damping force with respect to a motion of a sprung portion and an unsprung portion toward each other and a motion thereof away from each other, 
     wherein the electromagnetic damper includes:
         an electromagnetic motor;   a motion converting mechanism configured to convert the motions of the sprung portion and the unsprung portion toward and away from each other into a motion of the electromagnetic motor and vice versa; and   an external circuit disposed outside the electromagnetic motor and including (A) a first connection passage in which an electric current is allowed to flow from a first terminal as one of two terminals of the electromagnetic motor to a second terminal as the other of the two terminals while an electric current is prohibited from flowing from the second terminal to the first terminal and (B) a second connection passage in which the electric current is allowed to flow from the second terminal to the first terminal of the electromagnetic motor while the electric current is prohibited from flowing from the first terminal to the second terminal,       

     wherein the electromagnetic damper is configured to generate the damping force that depends on an electromotive force of the electromagnetic motor, with respect to the motion of the sprung portion and the unsprung portion toward each other by permitting a generated current by the electromagnetic motor to flow through the first connection passage and with respect to the motion of the sprung portion the unsprung portion away from each other by permitting the generated current by the electromagnetic motor to flow through the second connection passage, 
     wherein the external circuit further includes: (C) a first-connection-passage-current adjuster provided in the first connection passage and configured to adjust the electric current that flows from the first terminal to the second terminal; and (D) a second-connection-passage-current adjuster provided in the second connection passage and configured to adjust the electric current that flows from the second terminal to the first terminal, 
     wherein the damper system comprises an external-circuit controller configured to control an electric current that flows in the electromagnetic motor by controlling the external circuit, 
     wherein the external-circuit controller is configured to control the generated current caused by the motion of the sprung portion and the unsprung portion toward each other by controlling the first-connection-passage-current adjuster and to control the generated current caused by the motion of the sprung portion and the unsprung portion away from each other by controlling the second-connection-passage-current adjuster, 
     wherein the external-circuit controller includes:
         (a) a main-adjuster control portion which is configured to designate the first-connection-passage-current adjuster as a main adjuster where a sprung-resonance-frequency-range component that is a component of a relative vibration of the sprung portion and the unsprung portion in a sprung resonance frequency range is a value indicative of the motion of the sprung portion and the unsprung portion toward each other, to designate the second-connection-passage-current-adjuster as the main adjuster where the sprung-resonance-frequency-range component is a value indicative of the motion of the sprung portion and the unsprung portion away from each other, and to perform, on one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the main adjuster, a first control for mainly damping the relative vibration of the sprung portion and the unsprung portion; and   (b) an auxiliary-adjuster control portion which is configured to designate the second-connection-passage-current adjuster as an auxiliary adjuster where the sprung-resonance-frequency-range component is the value indicative of the motion of the sprung portion and the unsprung portion toward each other, to designate the first-connection-passage-current adjuster as the auxiliary adjuster where the sprung-resonance-frequency-range component is the value indicative of the motion of the sprung portion and the unsprung portion away from each other, and to perform, on one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the auxiliary adjuster, a second control for assisting the damping of the relative vibration of the sprung portion and the unsprung portion by the first control.   Further, a second invention provides a damper system for a vehicle which comprises an electromagnetic damper,       

     wherein the electromagnetic damper includes:
         (i) an electromagnetic motor;   (ii) a motion converting mechanism; and   (iii) an external circuit including (A) a first connection passage, (B) a second connection passage, (C) a first-connection-passage-current adjuster, and (D) a second-connection-passage-current adjuster,       

     wherein the first-connection-passage-current adjuster and the second-connection-passage-current adjuster are constituted by respective switching elements each configured to place a corresponding one of the first connection passage and the second connection passage selectively in an electrically connected state in which an electric current flows therethrough and in an electrically shut-off state in which the electric current does not flow therethrough, 
     wherein an external-circuit controller is configured to control the generated current by the electromagnetic motor by controlling a duty ratio of each of the switching elements and is configured not to change the duty ratio of each of the switching elements that respectively constitute the first-connection-passage-current adjuster and the second-connection-passage-current adjuster, depending upon through which one of the first connection passage and the second connection passage the generated current is flowing. 
     Advantageous Effects of Invention 
     The relative vibration of the sprung portion and the unsprung portion contains a component having a relatively high frequency. Where the motion of the sprung portion and the unsprung portion toward each other (i.e., approaching motion) and the motion of the sprung portion and the unsprung portion away from each other (i.e., separating motion) alternate at very short intervals, it is difficult to change the control of a current adjuster depending upon the direction of the relative motion of the sprung portion and the unsprung portion. In the damper system according to the first invention, it is regarded that the relative vibration of the sprung portion and the unsprung portion is composed of vibration components of various frequencies, and the two current adjusters are designated as one and the other of master (main adjuster) and slave (auxiliary adjuster) on the basis of the direction of the motion indicated by a value of a component in the sprung resonance frequency range among various frequencies, so as to control the two current adjusters independently of each other. In other words, the control can be switched not on the basis of an actual direction of the relative motion of the sprung portion and the unsprung portion, but on the basis of the direction of the motion indicated by the value of the sprung-resonance-frequency-range component. Accordingly, the damper system according to the first invention is capable of effectively damping the relative vibration without suffering from the problem indicated above. In the damper system according to the second invention, the duty ratio of each of the switching elements is kept unchanged without being changed irrespective of whether or not the generated current is flowing through a corresponding one of the connection passages, and the amount of the generated current is adjustable when the generated current flows through the corresponding connection passage. Accordingly, the damper system according to the second invention is capable of effectively damping the relative vibration without suffering from the problem indicated above. Hence, the damper systems according to the present invention have high utility owing to such advantages. 
     FORMS OF INVENTION 
     There will be explained various forms of an invention which is considered claimable (hereinafter referred to as “claimable invention” where appropriate). Each of the forms of the invention is numbered like the appended claims and depends from the other form or forms, where appropriate. This is for easier understanding of the claimable invention, and it is to be understood that combinations of constituent elements that constitute the invention are not limited to those described in the following forms. That is, it is to be understood that the claimable invention shall be construed in the light of the following descriptions of various forms and preferred embodiments. It is to be further understood that any form in which one or more elements is/are added to or deleted from any one of the following forms may be considered as one form of the claimable invention. 
     In the following forms, a form in which the technical features of the forms (1), (21), and (27) are combined corresponds to claim  16 . A form in which the technical features of the forms (22) and (23) are added to claim  16  corresponds to claim  17 . A form in which the technical feature of the form (29) is added to claim  16  corresponds to claim  18 . A form in which the technical feature of the form (30) is added to claim  18  corresponds to claim  19 . A form in which the technical feature of the form (31) is added to claim  18  corresponds to claim  20 . A form in which the technical feature of the form (32) is added to claim  18  corresponds to claim  21 . A form in which the technical features of the forms (25) and (26) are added to claim  16  corresponds to claim  22 . A form in which the technical feature of the form (34) is added to claim  16  corresponds to claim  23 . A form in which the technical feature of the form (11) is added to claim  16  corresponds to claim  24 . A form in which the technical feature of the form (12) is added to claim  24  corresponds to claim  25 . A form in which the technical feature of the form (14) is added to claim  24  corresponds to claim  26 . A form in which the technical feature of the form (4) is added to claim  16  corresponds to claim  27 . A form in which the technical feature of the form (6) is added to claim  27  corresponds to claim  28 . A form in which the technical feature of the form (28) is added to claim  16  corresponds to claim  29 . A form in which the technical features of the forms (1), (21), (25), and (26) are combined corresponds to claim  30 . 
     (1) A damper system for a vehicle which is mounted on the vehicle and which comprises an electromagnetic damper configured to generate a damping force with respect to a motion of a sprung portion and an unsprung portion toward each other and a motion thereof away from each other, 
     wherein the electromagnetic damper includes:
         an electromagnetic motor;   a motion converting mechanism configured to convert the motions of the sprung portion and the unsprung portion toward and away from each other into a motion of the electromagnetic motor and vice versa; and   an external circuit disposed outside the electromagnetic motor and including (A) a first connection passage in which an electric current is allowed to flow from a first terminal as one of two terminals of the electromagnetic motor to a second terminal as the other of the two terminals while an electric current is prohibited from flowing from the second terminal to the first terminal and (B) a second connection passage in which the electric current is allowed to flow from the second terminal to the first terminal of the electromagnetic motor while the electric current is prohibited from flowing from the first terminal to the second terminal, and       

     wherein the electromagnetic damper is configured to generate the damping force that depends on an electromotive force of the electromagnetic motor, with respect to the motion of the sprung portion and the unsprung portion toward each other by permitting a generated current by the electromagnetic motor to flow through the first connection passage and with respect to the motion of the sprung portion the unsprung portion away from each other by permitting the generated current by the electromagnetic motor to flow through the second connection passage. 
     In this form, the passage in the external circuit through which the generated current by the electromagnetic motor flows in the motion of the sprung portion and the unsprung portion toward each other (i.e., approaching motion) and the passage in the external circuit through which the generated current by the electromagnetic motor flows in the motion of the sprung portion and the unsprung portion away from each other (i.e., separating motion) are made different from each other. That is, as will be later explained in detail, the damping characteristic with respect to the approaching motion of the sprung portion and the unsprung portion and the damping characteristic with respect to the separating motion thereof can be easily made different from each other by varying a resistance to the electric current that flows through the first connection passage and a resistance to the electric current that flows through the second connection passage from each other or by adjusting electric current amounts flowing through the respective first connection passage and the second connection passage. 
     The “electromagnetic motor” in this form is not particularly limited but various sorts of electromagnetic motors may be employed. In terms of simplicity of the structure of the system, it is preferable to employ an electromagnetic motor having two terminals, e.g., a brushed DC motor and a shingle-phase motor. Even in a motor configured such that a direction of its generated current does not change in accordance with the direction of the relative motion of the sprung portion and the unsprung portion, there is a way of reversing the direction of the generated current in accordance with the direction of the relative motion. In terms of simplicity of the structure of the system, however, the “electromagnetic motor” in this form is preferably configured such that the direction of the generated current in accordance with the direction of the relative motion is reversed owing to its own structure. In other words, the motor is preferably configured such that one of the two terminals which is on a high-potential side and the other of the two terminals which is on a low-potential side are switched in accordance with the direction of the relative motion. Moreover, the motor is preferably configured such that its rotational direction is reversed by switching connection between the two terminals and a high-potential-side terminal and a low-potential-side terminal of a battery. In view of the above, the “electromagnetic motor” in this form may be a brushed DC motor utilizing permanent magnets, for instance. 
     The “motion converting mechanism” in this form is not particularly limited in its structure and is configured to convert the approaching motion of the sprung portion and the unsprung portion and the separating motion thereof into the motion of the electromagnetic motor and to convert the motion of the electromagnetic motor into the approaching motion of the sprung portion and the unsprung portion and the separating motion thereof. Where the electromagnetic damper is configured to generate the damping force that depends only on the electromotive force generated in the electromagnetic motor, the motion converting mechanism converts the approaching motion and the separating motion into the motion of the electromagnetic motor. Where a portion of the electromagnetic damper composed of mechanical components such as the motion converting mechanism and the electromagnetic motor is defined as a damper main body, the damper main body is not particularly limited in its structure. Where the electromagnetic motor is configured to rotate and the damper main body is constituted by a sprung-side unit connected to the sprung portion and an unsprung-side unit which is connected to the unsprung portion and which is configured to be moved relative to the sprung-side unit in accordance with the approach and separation of the sprung portion and the unsprung portion, a screw mechanism may be employed as the motion converting mechanism, and a relative motion of the sprung-side unit and the unsprung-side unit in the vertical direction may be converted by the screw mechanism into the rotational motion of the electromagnetic motor of a rotary type. More specifically, the damper main body may be constituted as an electromagnetic shock absorber configured to be expandable and contractible and to generate a force with respect to the expansion and contraction. Further, the electromagnetic motor may be configured to rotate, the damper main body may be constituted by including an arm which extends generally in the vehicle width direction and which is rotatably connected at opposite ends thereof respectively to the sprung portion and the unsprung portion, and the damper main body may be configured such that the electromagnetic motor rotates by rotation of the end of the arm connected to the sprung portion. In such a structure, the arm may be considered as one constituent element of the motion converting mechanism. 
     (2) The damper system according to the form (1), 
     wherein the first connection passage includes a first rectifier configured to allow the electric current to flow from the first terminal to the second terminal and to prohibit the electric current from flowing from the second terminal to the first terminal, and 
     wherein the second connection passage includes a second rectifier configured to allow the electric current to flow from the second terminal to the first terminal and to prohibit the electric current from flowing from the first terminal to the second terminal. 
     This form embodies a structure by which the electric current flows in only one direction in each of the two connection passages. Each of the first and second rectifiers may be formed as a diode that allows the electric current to flow in only one direction. 
     (3) The damper system according to the form (1) or (2), wherein the external circuit is configured such that a resistance to the electric current that flows through the first connection passage and a resistance to the electric current that flows through the second connection passage are made different from each other. 
     In this form, the damping characteristic with respect to the approaching motion of the sprung portion and the unsprung portion toward each other and the damping characteristic with respect to the separating motion thereof away from each other are made different from each other, namely, the damping force with respect to the approaching motion and the damping force with respect to the separating motion are made different from each other. This form is not limited to an arrangement in which resistance values of the respective two connection passages are mutually different but may include an arrangement in which the electric current amounts flowing through the respective two connection passages are mutually different where a speed of the approaching motion of the sprung portion and the unsprung portion and a speed of the separating motion thereof are the same. In the latter arrangement, as later explained in detail, there may be provided, in each of the two connection passages, a current adjuster for adjusting the electric current flowing therethrough so as to adjust the electric current amounts flowing through the respective two connection passages, whereby the damping force with respect to the approaching motion and the damping force with respect to the separating motion can be made different from each other. Such an arrangement, however, needs a control of the current adjusters. In terms of simplification of the structure of the damper system, it is preferable to employ an arrangement in which resistors having mutually different resistance values are provided in one and the other of the first connection passage and the second connection passage. 
     (4) The damper system according to any one of the forms (1)-(3), 
     wherein the external circuit includes a battery-device connection passage in which an electric current is allowed to flow from one of the two terminals of the electromagnetic motor that is at a high potential to a high-potential-side terminal of a battery device installed on the vehicle and an electric current is allowed to flow from a low-potential-side terminal of the battery device to the other of the two terminals of the electromagnetic motor that is at a low potential, and 
     wherein the damper system is configured such that a part of the generated current by the electromagnetic motor flows through the battery-device connection passage where the electromotive force of the electromagnetic motor exceeds a voltage of the battery device. 
     In this form, at least a part of the generated power of the electromagnetic motor is regenerated to the battery device. According to the damper system of this form, the battery device is charged or the power supply to the battery device is supplemented, thereby enhancing the efficiency of the battery device. The “battery device” in this form may be a device to supply an electric power to: a power source for driving the vehicle; electrical equipment such as lamps and an audio system; or other device installed on the vehicle. The “battery device” may be a device for exclusive use of the electromagnetic damper. Further, the “battery device” may be a battery or a capacitor such as an electric double-layer capacitor. 
     (5) The damper system according to the form (4), 
     wherein the battery-device connection passage includes: 
     a first high-potential-side connection passage in which an electric current is allowed to flow from the first terminal to the high-potential-side terminal of the battery device while an electric current is prohibited from flowing from the high-potential-side terminal of the battery device to the first terminal; 
     a second high-potential-side connection passage in which an electric current is allowed to flow from the second terminal ( 102 ) to the high-potential-side terminal of the battery device while an electric current is prohibited from flowing from the high-potential-side terminal of the battery device to the second terminal; 
     a second low-potential-side connection passage in which an electric current is allowed to flow form the low-potential-side terminal of the battery device to the first terminal while an electric current is prohibited from flowing from the first terminal to the low-potential-side terminal of the battery device, and 
     a first low-potential-side connection passage in which an electric current is allowed to flow from the low-potential-side terminal of the battery device to the second terminal while an electric current is prohibited from flowing from the second terminal to the low-potential-side terminal of the battery device. 
     In this form, the structure of the battery-device connection passage is embodied. In this form, the passage through which the electric current flows is changed depending on which one of the two terminals of the electromagnetic motor is at a high potential. Where the two connection passages have the respective rectifiers as described above, it is possible to utilize the first rectifier as a constituent element of one of the first high-potential side connection passage and the first low-potential side connection passage and to utilize the second rectifier as a constituent element of one of the second high-potential side connection passage and the second low-potential side connection passage. Such an arrangement simplifies the structure of the external circuit and accordingly the structure of the damper system. 
     (6) The damper system according to the form (4) or (5), wherein the external circuit includes a battery-device-connection-passage-current adjuster configured to adjust the electric current that flows through the battery-device connection passage. 
     As the “battery-device-connection-passage-current adjuster” in this form, it is possible to employ a variable resistor, a switching element such as a transistor, and the like. This form enables adjustment of the regenerative current to the battery device which is the at least part of the generated current of the electromagnetic motor. This form may be arranged such that the battery-device-connection-passage-current adjuster is controlled on the basis of a charged amount of the battery device, for instance. (The charged amount can be considered as a residual amount or a remaining energy amount.) More specifically, this form may be arranged such that the larger the charged amount of the battery device, the smaller the regenerative current. 
     In an instance where the voltage of the battery device is lowered due to an increase in the electric power supplied from the battery device to various equipment installed on the vehicle, for instance, the electric current is likely to flow to the battery device and the regenerative current that is the part of the generated current to flow to the battery device becomes large, as compared with an instance where the voltage of the battery device is high. In other words, when the voltage of the battery device is lowered, the damping force of the electromagnetic damper becomes large, as compared with when the voltage of the battery device is high. This form can be arranged such that the damping force of the electromagnetic damper is restrained from increasing by making the regenerative current small upon the voltage decrease of the battery device by means of the above-indicated battery-device-connection-passage-current adjuster. 
     (11) The damper system according to any one of the forms (1)-(6), wherein the external circuit includes: a first resistor which is provided in the first connection passage and which functions as a resistance to the electric current that flows from the first terminal to the second terminal; and a second resistor which is provided in the second connection passage and which functions as a resistance to the electric current that flows from the second terminal to the first terminal. 
     In this form, a resistor is provided in each of the two connection passages. By appropriately setting resistance values of the two resistors, the damping characteristic with respect to the approaching motion of the sprung portion and the unsprung portion toward each other and the damping characteristic with respect to the separating motion of the sprung portion and the unsprung portion away from each other can be made respectively appropriate. Each of the “first resistor” and the “second resistor” in this form may be a fixed resistor or a variable resistor. Where the variable resistor is employed as each of the two resistors, it is possible to change the damping characteristic with respect to the approaching motion and the damping characteristic with respect to the separating motion independently of each other in accordance with the running state of the vehicle or the like, as later explained in detail. 
     (12) The damper system according to the form (11), wherein a resistance value of the first resistor and a resistance value of the second resistor are made different from each other. 
     This form realizes the above-indicated form in which the resistances to the electric currents flowing through the respective two connection passage are made mutually different, by providing the resistors having mutually different resistance values in the respective connection passages. This form is preferable in terms of simplification of the structure of the damper system as described above. 
     (13) The damper system according to the form (12), wherein the resistance value of the first resistor is made larger than the resistance value of the second resistor. 
     In this form, the damping force with respect to the approaching motion is made smaller than the damping force with respect to the separating motion. The input to the electromagnetic damper at a time when the wheel passes on a projection of the road surface is larger than the input to the electromagnetic damper at a time when the wheel passes on a depression of the road surface. According to this form, the damping force with respect to the approaching motion of the sprung portion and the unsprung portion toward each other upon passing of the wheel on the projection of the road surface is made small, thereby effectively mitigating a shock or impact applied to the sprung portion when the wheel passes on the projection. 
     (14) The damper system according to any one of the forms (11)-(13), 
     wherein the external circuit includes:
         a first-resistor bypass passage which bypasses the first resistor;   a first auxiliary resistor which is provided in the first-resistor bypass passage and which functions as a resistance to an electric current that flows through the first-resistor bypass passage;   a first open/close device configured to place the first-resistor bypass passage selectively in a state in which the electric current flows therethrough and in a state in which the electric current does not flow therethrough;   a second-resistor bypass passage which bypasses the second resistor;   a second auxiliary resistor which is provided in the second-resistor bypass passage and which functions as a resistance to an electric current that flows through the second-resistor bypass passage;   a second open/close device configured to place the second-resistor bypass passage selectively in a state in which the electric current flows therethrough and in a state in which the electric current does not flow therethrough,       

     wherein the first-resistor bypass passage is placed by the first open/close device in the state in which the electric current does not flow therethrough under normal conditions and in the state in which the electric current flows therethrough in case of a failure that the electric current does not flow through the first resistor, and 
     wherein the second-resistor bypass passage is placed by the second open/close device in the state in which the electric current does not flow therethrough under normal conditions and in the state in which the electric current flows therethrough in case of a failure that the electric current does not flow through the second resistor. 
     The resistors provided in the respective two connection passages may suffer from a failure in which the electric current fails to pass therethrough due to a break or disconnection therein or the like by heat generation. When such a failure occurs, the generated current by the electromagnetic motor does not flow through the resistors. Accordingly, no damping force is generated with respect to the motions of the sprung portion and the unsprung portion toward and away from each other, so that the vehicle stability may be deteriorated. In this form, even where any of the resistors suffer from the failure in which the electric current does not pass therethrough, the electric current flows through the bypass passage which bypasses the failure-suffering resistor and passes through the auxiliary resistor provided in the bypass passage. Therefore, this form realizes the damper system excellent in terms of failsafe operation. 
     It is not particularly limited how to detect whether or not the “failure that the electric current does not flow through the resistor” described in this form has occurred. More specifically, it is not particularly limited how to detect whether or not any of the resistors is in a state in which the electric current does not pass therethrough in spite of the fact that the electromagnetic motor is being operated. For instance, the occurrence of the failure may be judged by detection of the approaching or separating motion of the sprung portion and the unsprung portion based on a change in the rotational angle of the electric motor and at the same time by detection of the state in which the electric current does not pass through the resistor provided in the connection passage through which the generated current flows in the motion. When such a judgment is made, the open/close device may place the bypass passage that bypasses the failure-suffering resistor in a state in which the electric current flows therethrough. For making the judgment described above, there may be utilized a sensor configured to directly detect the rotational angle of the electromagnetic motor. Alternatively, the rotational angle of the electromagnetic motor may be detected indirectly by a sensor or the like configured to detect the distance between the sprung portion and the unsprung portion. Further, there may be utilized a sensor configured to directly detect the electric current passing through the resistor. Alternatively, whether or not the electric current has passed through the resistor may be detected indirectly by a sensor or the like configured to measure a potential, a voltage or the like at a certain point of the connection passage. 
     (21) The damper system according to any one of the forms (1)-(14), 
     wherein the external circuit includes: a first-connection-passage-current adjuster provided in the first connection passage and configured to adjust the electric current that flows from the first terminal to the second terminal; and a second-connection-passage-current adjuster provided in the second connection passage and configured to adjust the electric current that flows from the second terminal to the first terminal, 
     wherein the damper system comprises an external-circuit controller configured to control an electric current that flows in the electromagnetic motor by controlling the external circuit, and 
     wherein the external-circuit controller is configured to control the generated current caused by the motion of the sprung portion and the unsprung portion toward each other by controlling the first-connection-passage-current adjuster and to control the generated current caused by the motion of the sprung portion and the unsprung portion away from each other by controlling the second-connection-passage-current adjuster. 
     The “external-circuit controller” in this form may be configured so as to control the generated current by the electromagnetic motor and also a supply current from a battery device in an instance where the electromagnetic motor is connected to the battery device. Where the electromagnetic damper is configured to generate the damping force that depends mainly on the electromotive force generated in the electromagnetic motor, the external-circuit controller is for controlling a flow of the generated current by the electromagnetic motor. The “flow of the generated current” is a concept that includes a direction in which the generated current flows, an amount of the generated current and the like. 
     Each of the “first-connection-passage-current adjuster” and the “second-connection-passage-current adjuster” in this form is configured to adjust an amount of the electric current per a preset time that flows through the corresponding connection passage, and is controlled by the above-indicated external-circuit controller. That is, the external-circuit controller controls the amount of the generated current caused by the approaching motion of the sprung portion and the unsprung portion toward each other utilizing the first-connection-passage-current adjuster, thereby changing the damping force with respect to the approaching motion. Further, the external-circuit controller controls the amount of the generated current caused by the separating motion of the sprung portion and the unsprung portion away from each other utilizing the second-connection-passage-current adjuster, thereby changing the damping force with respect to the separating motion. As each of the “first-connection-passage-current adjuster” and the “second-connection-passage-current adjuster”, a variable resistor or a switching element such as a transistor is employable, for instance. That is, an arrangement in which a variable resistor is employed as each of the resistors provided in the respective connection passages in the above-indicated form can be considered as one arrangement of this form. However, for controlling the generated current caused by the relative motion of the sprung portion and the unsprung portion in accordance with the vehicle running state and the like, each current adjuster is preferably constituted by a switching element that is capable of executing pulse driving or the like, as explained below. 
     Here, there is considered a damper system in which a single connection passage is provided for connecting two terminals of an electromagnetic motor and a single current adjuster is provided in the connection passage, namely, a system in which the generated current of the motor caused by the approaching motion and the generated current of the motor caused by the separating motion flow through the common connection passage in mutually opposite directions. In such a damper system, there may arise a problem of responsiveness in the control of the current adjuster, namely, there may occur a time lag between a time point of issuance of a command from an external-circuit controller to the current adjuster and a time point of initiation of adjustment of the electric current by the current adjuster based on the command. To be more specific, the relative vibration of the sprung portion and the unsprung portion contains a component having a relatively high frequency. Where the approaching motion and the separating motion alternate at very short intervals, it is difficult to change the control of the current adjuster depending upon the direction of the relative motion of the sprung portion and the unsprung portion. 
     In contrast, in the damper system described in this form, the generated current caused by the approaching motion flows through the first connection passage while the generated current caused by the separating motion flows through the second connection passage, thereby eliminating a need of switching the control of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster depending upon the direction of the relative motion of the sprung portion and the unsprung portion. Accordingly, the relative vibration of the sprung portion and the unsprung portion can be effectively damped. As later explained in detail, the external-circuit controller may be configured to control the first-connection-passage-current adjuster and the second-connection-passage-current adjuster in accordance with the vehicle behavior, the vehicle running state and the like. By controlling the two adjusters so as to have respective different roles, the present damper system can exhibit excellent damping performance. More specifically, one of the two current adjusters may be controlled so as to damp a vibration in a sprung resonance frequency range, thereby enhancing operability and stability of the vehicle (hereinafter referred to as “operating stability” where appropriate) while the other current adjuster may be controlled so as to damp a vibration in an unsprung resonance frequency range, thereby enhancing ride comfort of the vehicle. Therefore, the system in this form ensures a good balance between the ride comfort and the operating stability which are difficult to be realized at the same time, thereby ensuring high utility. 
     (22) The damper system according to the form (21), wherein the external-circuit controller is configured to control the first-connection-passage-current adjuster and the second-connection-passage-current adjuster so as to control a damping coefficient of the electromagnetic damper. 
     In this form, the damping coefficient with respect to the approaching motion of the sprung portion and the unsprung portion toward each other is controlled by controlling the first-connection-passage-current adjuster while the damping coefficient with respect to the separating motion of the sprung portion and the unsprung portion away from each other is controlled by controlling the second-connection-passage-current adjuster. The “damping coefficient of the electromagnetic damper” described in this form is an index of an ability of the electromagnetic damper to generate the damping force and a basis of the damping force to be generated by the electromagnetic damper. In general, the damping coefficient of a damper is represented by a magnitude of the damping force with respect to the speed of the relative motion of the sprung portion and the unsprung portion. 
     (23) The damper system according to the form (22), wherein the external-circuit controller is configured to control the first-connection-passage-current adjuster and the second-connection-passage-current adjuster, such that the damping coefficient of the electromagnetic damper with respect to the motion of the sprung portion and the unsprung portion toward each other and the damping coefficient of the electromagnetic damper with respect to the motion of the sprung portion and the unsprung portion away from each other are made different from each other. 
     This form realizes the above-indicated form in which the resistances to the electric currents flowing through the respective two connection passages are made mutually different, by adjusting the electric current amounts flowing through the respective two connection passages. In an arrangement in which are combined this form and the above-indicated form wherein the resistors are provided in the respective two connection passages, it is preferable to determine a basic resistance value of each of the two connection passages in a case wherein the two current adjusters are not controlled (in a case wherein a state in which the electric currents flow through the respective connection passages), by making the resistance values of the respective two current adjusters mutually different, whereby the resistances to the electric currents flowing through the respective two connection passages are made mutually different. In the thus configured arrangement, since the resistances to the electric currents flowing through the respective two connection passages are made mutually different, the approaching motion of the sprung portion and the unsprung portion and the separating motion thereof can be respectively effectively damped even in the event of a failure in which the two current adjusters cannot be controlled. 
     (24) The damper system according to the form (23), wherein the external-circuit controller is configured to control the first-connection-passage-current adjuster and the second-connection-passage-current adjuster, such that the damping coefficient with respect to the motion of the sprung portion and the unsprung portion toward each other is made smaller than the damping coefficient with respect to the motion of the sprung portion and the unsprung portion away from each other. 
     In this form, the damping force with respect to the approaching motion is made smaller than the damping force with respect to the separating motion. This form effectively mitigates a shock or impact applied to the sprung portion by the unsprung portion that approaches the sprung portion when the wheel passes on a projection of the road surface. 
     (25) The damper system according to any one of the forms (21)-(24), 
     wherein the first-connection-passage-current adjuster and the second-connection-passage-current adjuster are constituted by respective switching elements each configured to place a corresponding one of the first connection passage and the second connection passage selectively in an electrically connected state in which an electric current flows therethrough and in an electrically shut-off state in which the electric current does not flow therethrough, and 
     wherein the external-circuit controller is configured to control the generated current by the electromagnetic motor by controlling each of the switching elements such that the electrically connected state and the electrically shut-off state are alternately and repeatedly established and by controlling a duty ratio of each of the switching elements that is a ratio determined on the basis of a time during which the electrically connected state is established and a time during which the electrically shut-off state is established. 
     In this form, each current adjuster is limited to a switching element, and the external-circuit controller is configured to execute a Pulse Width Modulation (PWM) control of the switching element. For instance, where the electromagnetic motor is a DC motor and its operational speed is proportional to a force to be generated by the motor, namely, where the damper system is configured such that the speed of the relative motion of the sprung portion and the unsprung portion is proportional to the damping force of the electromagnetic damper, the damping coefficient of the electromagnetic damper can be changed by changing the duty ratio of each switching element, namely, by changing a ratio of a time during which the electrically connected state is established with respect to a pulse pitch which is a sum of the time during which the electrically connected state is established and the time during which the electrically shut-off state is established. Therefore, this form can be the above-indicated forms in which the damping coefficient is controlled. 
     (26) The damper system according to the form (25), wherein the external-circuit controller is configured not to change the duty ratio of each of the switching elements that respectively constitute the first-connection-passage-current adjuster and the second-connection-passage-current adjuster, depending upon through which one of the first connection passage and the second connection passage the generated current is flowing. 
     This form may be considered as a form in which the duty ratio of each of the switching elements that respectively constitute the first-connection-passage-current adjuster and the second-connection-passage-current adjuster is not changed depending upon whether or not the generated current is flowing through a corresponding one of the first connection passage and the second connection passage. In this form, where the duty ratio of each switching element is kept unchanged irrespective of whether or not the generated current is flowing through a corresponding one of the connection passages, the amount of the generated current is adjusted when the generated current flows through the corresponding connection passage. According to this form, even if the approaching motion and the separating motion alternate at very short intervals, the responsiveness in the control of the switching elements explained above does not cause any problem, and the relative vibration of the sprung portion and the unsprung portion can be effectively damped. 
     (27) The damper system according to any one of the forms (21)-(26), 
     wherein the external-circuit controller includes:
         a main-adjuster control portion which is configured to designate the first-connection-passage-current adjuster as a main adjuster where a sprung-resonance-frequency-range component that is a component of a relative vibration of the sprung portion and the unsprung portion in a sprung resonance frequency range is a value indicative of the motion of the sprung portion and the unsprung portion toward each other, to designate the second-connection-passage-current-adjuster as the main adjuster where the sprung-resonance-frequency-range component is a value indicative of the motion of the sprung portion and the unsprung portion away from each other, and to control one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the main adjuster; and   an auxiliary-adjuster control portion which is configured to designate the second-connection-passage-current adjuster as an auxiliary adjuster where the sprung-resonance-frequency-range component is the value indicative of the motion of the sprung portion and the unsprung portion toward each other, to designate the first-connection-passage-current adjuster as the auxiliary adjuster where the sprung-resonance-frequency-range component is the value indicative of the motion of the sprung portion and the unsprung portion away from each other, and to control one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the auxiliary adjuster.       

     In this form, it is regarded that the relative vibration of the sprung portion and the unsprung portion is composed of vibration components of various frequencies, and one of the two current adjusters that mainly damps the relative vibration is designated as the main adjuster on the basis of a direction of the motion indicated by the value of the component in the sprung resonance frequency range among the vibration components. In other words, it is preferable that the “main adjuster” in this form have a function of damping the sprung-resonance-frequency-range component mainly for damping the sprung-resonance-frequency-range component. On the other hand, the “auxiliary adjuster” in this form can have various functions explained below in detail for aiding or assisting the main adjuster. Thus, the relative vibration of the sprung portion and the unsprung portion can be effectively damped. 
     (28) The damper system according to the form (27), wherein the main-adjuster control portion is configured to control the one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the main adjuster, such that a damping coefficient of the electromagnetic damper becomes a value suitable for damping the sprung-resonance-frequency-range component. 
     This form permits the main adjuster to have a function of damping the sprung-resonance-frequency-range component. In this form, the damping coefficient when controlling the first-connection-passage-current adjuster and the damping coefficient when controlling the second-connection-passage-current adjuster may or may not be the same. When the damping coefficients are mutually different, the damping coefficients may be determined such that the damping coefficient when controlling the first-connection-passage-current adjuster is a value suitable for damping the approaching motion while the damping coefficient when controlling the second-connection-passage-current adjuster is a value suitable for damping the separating motion. 
     (29) The damper system according to the form (27) or (28), wherein the auxiliary-adjuster control portion is configured to control the one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the auxiliary adjuster, such that a damping coefficient of the electromagnetic damper becomes a value suitable for damping an unsprung-resonance-frequency-range component that is a component of the relative vibration of the sprung portion and the unsprung portion in an unsprung resonance frequency range. 
     Since the relative vibration of the sprung portion and the unsprung portion contains a component whose frequency is higher than the sprung-resonance-frequency-range component, the direction of the relative motion of the sprung portion and the unsprung portion indicated by the value of the sprung-resonance-frequency-range component is sometimes opposite to the direction of the actual relative motion. In this instance, the generated current flows through the connection passage in which the auxiliary adjuster is disposed. Therefore, this form permits the auxiliary adjuster to have a function different from the function of the main adjuster, namely, a function of damping a component in the unsprung resonance frequency range (i.e., unsprung-resonance-frequency-range component) of the relative vibration of the sprung portion and the unsprung portion. According to this form, not only the sprung-resonance-frequency-range component, but also the unsprung-resonance-frequency-range component can be damped without changing the damping coefficients by the two adjusters. Therefore, the relative vibration of the sprung portion and the unsprung portion can be effectively damped. Here, like the above-indicated “main-adjuster control portion”, the “auxiliary-adjuster control portion” may be configured such that the damping coefficient when controlling the first-connection-passage-current adjuster is a value suitable for damping the approaching motion while the damping coefficient when controlling the second-connection-passage-current adjuster is a value suitable for damping the separating motion. 
     (30) The damper system according to the form (29), wherein the auxiliary-adjuster control portion is configured to control the one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the auxiliary adjuster, such that the damping coefficient of the electromagnetic damper becomes a value suitable for damping the unsprung-resonance-frequency-range component in a situation in which an intensity of the unsprung-resonance-frequency-range component is higher than a prescribed value. 
     In this form, an instance in which the unsprung-resonance-frequency-range component is damped by the auxiliary adjuster is specified on the basis of the intensity of the unsprung-resonance-frequency-range component. For instance, the unsprung-resonance-frequency-range component may be damped only in a situation in which the intensity of the unsprung-resonance-frequency-range component is relatively high. The “intensity of the unsprung-resonance-frequency-range component” in this form means a degree of intensity or severity of the vibration and may be judged on the basis of various factors such as an amplitude of the unsprung-resonance-frequency-range component, and the speed or the acceleration, of the relative motion of the sprung portion and the unsprung portion with respect to the unsprung-resonance-frequency-range component. The intensity of the vibration is preferably judged on the basis of a value of each of the above-described factors within a prescribed time period between the current time point and a certain previous time point that precedes the current time point, specifically on the basis of a maximum value, an effective value or the like. 
     While not belonging to this form, the above-indicated main-adjuster control portion may be configured to control one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the main adjuster, such that the damping coefficient of the electromagnetic damper becomes a value suitable for damping the unsprung-resonance-frequency-range component in a situation in which the intensity of the unsprung-resonance-frequency-range component is higher than a prescribed value. In this arrangement, both of the current adjusters are controlled to damp the unsprung-resonance-frequency-range component. Accordingly, the unsprung-resonance-frequency-range component whose intensity is high is effectively damped, thereby improving the ride comfort of the vehicle. 
     (31) The damper system according to the form (29) or (30), wherein the auxiliary-adjuster control portion is configured to control the one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the auxiliary adjuster, such that the damping coefficient of the electromagnetic damper becomes a value suitable for damping the sprung-resonance-frequency-range component in a situation in which an intensity of the sprung-resonance-frequency-range component is higher than a prescribed value. 
     In this form, both of the current adjusters are controlled to damp the sprung-resonance-frequency-range component. Accordingly, the sprung-resonance-frequency-range component whose intensity is high is effectively damped, thereby improving the operating stability of the vehicle. In a combination of this form and the above-described form wherein the auxiliary adjuster is controlled such that the damping coefficient becomes a value suitable for damping the unsprung-resonance-frequency-range component in a situation in which the intensity of the unsprung-resonance-frequency-range component is higher than a prescribed value, either one of the sprung-resonance-frequency-range component and the unsprung-resonance-frequency-range component may be damped with a higher priority than the other, in a situation in which both of the intensity of the sprung-resonance-frequency-range component and the intensity of the unsprung-resonance-frequency-range component are higher than the respective prescribed values. More specifically, the sprung-resonance-frequency-range component may be damped preferentially when the operating stability is emphasized while the unsprung-resonance-frequency-range component may be damped preferentially when the ride comfort of the vehicle is emphasized. 
     (32) The damper system according to any one of the forms (29)-(31), wherein the auxiliary-adjuster control portion is configured to control the one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the auxiliary adjuster, such that the damping coefficient of the electromagnetic damper becomes a value suitable for damping a component in a frequency range between the sprung resonance frequency range and the unsprung resonance frequency range, in a situation in which an intensity of the sprung-resonance-frequency-range component is lower than a prescribed value and an intensity of the unsprung-resonance-frequency-range component is lower than a prescribed value. 
     In general, when the damping coefficient is made large for the purpose of damping the sprung-resonance-frequency-range component and the unsprung-resonance-frequency-range component, the intensity of the vibration is undesirably increased with respect to a component in a frequency range between the sprung resonance frequency range and the unsprung resonance frequency range. According to this form, since the component of the frequency range between the two frequency ranges is damped in a situation in which both of the intensity of the sprung-resonance-frequency-range component and the intensity of the unsprung-resonance-frequency-range component are lower than the respective prescribed values, the relative vibration of the sprung portion and the unsprung portion can be effectively damped. While not belonging to this form, the above-described main-adjuster control portion may be configured to control one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the main adjuster, such that the damping coefficient of the electromagnetic damper becomes a value suitable for damping the component in the frequency range between the sprung resonance frequency range and the unsprung resonance frequency range, in a situation in which the intensity of the sprung-resonance-frequency-range component is lower than the prescribed value and the intensity of the unsprung-resonance-frequency-range component is lower than the prescribed value. 
     (33) The damper system according to any one of the forms (27)-(32), wherein the auxiliary-adjuster control portion is configured to control the one of the first-connection-passage-current adjuster and the second-connection-passage-current adjuster that is designated as the auxiliary adjuster, such that the generated current does not flow through a corresponding one of the first connection passage and the second connection passage, in a situation in which a temperature of the electromagnetic motor is higher than a threshold temperature. 
     In this form, in a case where the temperature of the electromagnetic motor becomes comparatively high, the generated current is not generated when the direction of the actual relative motion of the sprung portion and the unsprung portion is opposite to the direction of the relative motion indicated by the value of the sprung-resonance-frequency-range component. Accordingly, the load on the electromagnetic motor is reduced and the heat generation of the motor is accordingly suppressed while the relative vibration of the sprung portion and the unsprung portion is damped by the main adjuster. Where each current adjuster is constituted by the above-indicated switching element, this form is realized by controlling the duty ratio of the switching element to be 0. In this regard, where the main adjuster is controlled, in addition to the auxiliary adjuster, such that the generated current does not flow through one of the first and second connection passages in which the main adjuster is disposed, the load on the electromagnetic motor is eliminated and the electromagnetic motor can be prevented from suffering from a failure or the like with high reliability. 
     (34) The damper system according to any one of the forms (21)-(33), 
     wherein the external circuit includes:
         a first-adjuster bypass passage which bypasses the first-connection-passage-current adjuster and in which an electric current is allowed to flow from a side of the first terminal to a side of the second terminal where the electromotive force of the electromagnetic motor caused by the motion of the sprung portion and the unsprung portion toward each other exceeds a prescribed voltage; and   a second-adjuster bypass passage which bypasses the second-connection-passage-current adjuster and in which an electric current is allowed to flow from a side of the second terminal to a side of the first terminal where the electromotive force of the electromagnetic motor caused by the motion of the sprung portion and the unsprung portion away from each other exceeds a prescribed voltage.       

     In this form, where the electromotive force of the electromagnetic motor exceeds the respective prescribed voltages, in other words, where the stroke speed is high, there is generated a damping force in accordance with a certain set damping coefficient irrespective of the current adjusters. According to this form, it is possible to generate a stable damping force in a situation in which the vehicle stability is needed because of the high stroked speed. Further, according to this form, even where there occurs a failure in which the electric current does not pass through any of the current adjusters, it is possible to generate a damping force where the electromotive force of the electromagnetic motor exceeds the prescribed voltages, namely, where the stroke speed is high. Therefore, this form realizes the damper system excellent in terms of failsafe operation. Each of the first-adjuster bypass passage and the second-adjuster bypass passage may be constituted by including a Zener diode. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view illustrating an overall structure of a vehicle on which is mounted a damper system including an electromagnetic damper according to one embodiment of the claimable invention. 
         FIG. 2  is a front elevational view in cross section illustrating a spring•absorber Assy including a damper main body of the electromagnetic damper shown in  FIG. 1 . 
         FIG. 3  is a circuit diagram of an external circuit which is one constituent element of the electromagnetic damper shown in  FIG. 1  and which is provided outside an electromagnetic motor of  FIG. 2 . 
         FIG. 4  is an equivalent circuit diagram of the external circuit shown in  FIG. 3 . 
         FIG. 5  is a view showing an amplitude of a relative vibration of a sprung portion and unsprung portion and is a view showing a change of its sprung-resonance-frequency-range component with a lapse of time. 
         FIG. 6  is a view showing a relationship between rotational speed and torque of the electromagnetic motor of  FIG. 2 , in other words, a relationship between speed of relative motion of the sprung portion and the unsprung portion and damping force generated by the electromagnetic damper. 
         FIG. 7  is a flow chart showing an external-circuit-control program executed by an external-circuit controller shown in  FIG. 1 . 
         FIG. 8  is a flow chart showing an auxiliary-adjuster-duty-ratio-determining-processing sub routine executed in the external-circuit-control program of  FIG. 7 . 
         FIG. 9  is a block diagram showing functions of the external-circuit controller of  FIG. 1 . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     There will be explained in detail one embodiment of the claimable invention with reference to the drawings. It is to be understood, however, that the claimable invention is not limited to the following embodiment but may be embodied with various changes and modifications, such as those described in the FORMS OF THE INVENTION, which may occur to those skilled in the art. It is to be further understood that modified arrangements of the following embodiment may be formed utilizing the technical matters described in the FORMS OF THE INVENTION. 
     &lt;Configuration of Damper System&gt; 
       FIG. 1  schematically shows a vehicle on which is mounted a damper system including an electromagnetic damper  10  according to one embodiment of the claimable invention. The damper system is one constituent element of a suspension system mounted on the vehicle. The suspension system includes, between a body  14  of the vehicle and four wheels  12 FR, FL, RR, RL, four independent suspension apparatus corresponding to the respective four wheels  12 . Each suspension apparatus includes a spring•absorber Assy  20  in which a suspension spring and a shock absorber are united. Each of the four wheels  12  and each of the four spring•absorber Assys  20  are collectively referred to as the wheel  12  and the spring•absorber Assy  20 , respectively. Where it is necessary to distinguish the four wheels  12  from each other and to distinguish the four spring•absorber Assys  20  from each other, there are attached, to each reference numeral, a suitable one of suffixes “FL”, “FR”, “RL”, and “RR” respectively indicating a front left wheel, a front right wheel, a rear left wheel, and a rear right wheel. 
     As shown in  FIG. 2 , the spring•absorber Assy  20  is disposed between: a suspension lower arm  22  which holds the wheel  12  and which partially constitutes an unsprung portion; and a mount portion  24  which is disposed on the vehicle body  14  and which partially constitutes a sprung portion, such that the spring•absorber Assy  20  connects the suspension lower arm  22  and the mount portion  24 . The spring•absorber Assy  20  is constituted by an electromagnetic shock absorber  30  as a damper main body of the electromagnetic damper  10  and a coil spring  32  as the suspension spring disposed in parallel with the shock absorber  30 . The shock absorber  30  and the coil spring are united. 
     i) Configuration of Damper Main Body 
     The shock absorber  30  includes a ball screw mechanism  44  as a motion converting mechanism, an electromagnetic motor  46  of a rotary type (hereinafter simply referred to as “motor  46 ” where appropriate), and a casing  48  which accommodates the motor  46 . The ball screw mechanism  44  includes a threaded rod  40  in which a thread groove is formed and a nut  42  which holds bearing balls and which is screwed with the threaded rod  40 . The casing  48  rotatably holds the threaded rod  40  and is connected at its outer circumferential portion to the mount portion  24  via a vibration-damping rubber  50 . 
     The motor  46  has a motor shaft  52 . A plurality of polar bodies  60  are fixedly disposed on an outer circumferential portion of the motor shaft  52  in the circumferential direction. Each polar body  60  is formed by a core and a coil wound around the core. The plurality of polar bodies  60  constitute a rotor of the motor  46 . A pair of permanent magnets  62  each having the magnetic poles, i.e., the N-pole and the S-pole, are fixedly disposed on an inner surface of the casing  48  so as to be opposed to the plurality of polar bodies  60 . The permanent magnets  62  and the casing  48  constitute a stator of the motor  46 . The motor  46  has a plurality of commutators  64  fixed to the motor shaft  52  and brushes  66  fixed to the casing  48  so as to be in sliding contact with the commutators  64 . The motor  46  is a so-called brushed DC motor. The motor shaft  52  is connected integrally to an upper end portion of the threaded rod  40 . 
     The shock absorber  30  includes a cylinder  74  constituted by an outer tube  70  and an inner tube  72  which is fitted into the outer tube  70  and which protrudes upwardly from an upper end portion of the outer tube  70 . The outer tube  70  is connected to the lower arm  22  via a connecting bushing  76  provided at its lower end while the inner tube  72  is fixed at its upper end to the casing  48  with the threaded rod  40  inserted therethrough. At an inner bottom portion of the inner tube  72 , a nut support pipe member  78  is disposed so as to extend upright. Inside of an upper end portion of the nut support pipe member  78 , the nut  42  is fixed so as to be screwed with the threaded rod  40 . 
     The shock absorber  30  further includes a cover tube  80  which is fixed at its upper end portion to an underside of the mount portion  24  via a vibration-damping rubber  82 , such that the cylinder  74  is inserted through the cover tube  80 . A flange portion  84  functioning as an upper retainer is formed at the upper end portion of the cover tube  80 . The coil spring  32  as the suspension spring is supported or held so as to be sandwiched between the flange portion  84  and an annular lower retainer  86  provided on an outer circumferential surface of the outer tube  70 . 
     In the thus constructed shock absorber  30 , the threaded rod  40  and the nut  42  are movable relative to each other in the axial direction when the sprung portion and the unsprung portion are moved toward and away from each other, and the threaded rod  40  rotates relative to the nut  42  by the relative movement of the threaded rod  40  and the nut  42 , whereby the motor shaft  52  also rotates. As explained below in detail, the electromagnetic damper  10  also includes an external circuit  90  ( FIG. 3 ) provided outside the motor  46  and is constructed such that two terminals of the motor  46  are electrically connected by the external circuit  90 . That is, the motor  46  is rotated by an external force, so that an electromotive force is generated in the motor  46  and the motor  46  generates a motor force (torque) that depends on the electromotive force. The motor  46  is capable of giving, to the threaded rod  40 , the torque that depends on the electromotive force. Owing to the torque, it is possible to generate, with respect to the relative rotation of the threaded rod  40  and the nut  42 , a resistance force in a direction to prevent the relative rotation. In other words, the electromagnetic damper  10  is configured to permit the resistance force to act as a damping force with respect to the motion of the sprung portion and the unsprung portion toward each other (i.e., approaching motion) and the motion thereof away from each other (i.e., separating motion). 
     ii) Configuration of External Circuit 
       FIG. 3  shows a circuit diagram of the external circuit  90  that constitutes the electromagnetic damper  10 . The external circuit  90  permits an electric current to flow between a first terminal  100  and a second terminal  102  of the motor  46 . In the external circuit  90 , a point A on the first terminal side ( 100 ) and a point B on the second terminal side ( 102 ) are connected by a line AB while a point C on the first terminal side ( 100 ) and a point D on the second terminal side ( 102 ) are connected by a line CD. In the line AB, there are provided: a first diode  104  which allows an electric current to flow in a direction from the point A to the point B and which prohibits an electric current from flowing from the point B to the point A; and a second diode  106  which allows the electric current to flow from the point B to the point A and which prohibits the electric current from flowing from the point A to the point B. In the line CD, there are provided, in a direction from the point C to the point D, a first switching element [SW 1 ]  108  which is a MOS-type FET, a first resistor [R C ]  110  which is a fixed resistor, a second resistor [R S ]  112  which is a fixed resistor, and a second switching element [SW 2 ]  114  which is a MOS-type FET. A point E between the first diode  104  and the second diode  106  in the line AB and a point F between the first resistor  110  and the second resistor  112  in the line CD are electrically connected by a line EF and are grounded. 
     A point G between the first switching element  108  and the first resistor  110  in the line CD and a point H between the second resistor  112  and the second switching element  114  in the line CD are connected to a high-potential-side terminal of a battery  120  (with a nominal voltage E N : 12.0 V) as a battery device installed on the vehicle. More specifically, in a line GI which connects the point G and a point I on the side of the high-potential-side terminal of the battery  120 , there is provided a third diode  122  which allows the electric current to flow from the point G to the point I and which prohibits the electric current from flowing from the point I to the point G. In a line HI which connects the point H and the point I on the side of the high-potential-side terminal of the battery  120 , there is provided a fourth diode  124  which allows the electric current to flow from the point H to the point I and which prohibits the electric current from flowing from the point I to the point H. A third switching element [SW 3 ]  126  which is a MOS-type FET is provided on the side of the high-potential-side terminal of the battery  120 , more specifically, between the point I and the high-potential-side terminal of the battery  120 . A low-potential-side terminal of the battery  120  is grounded. A resistance  128  on the side of the high-potential-side terminal of the battery  120  shown in  FIG. 3  indicates an internal resistance of the battery  120  and is referred to as a source resistance [R B ]  128  in the following explanation. 
     The external circuit  90  further includes a first Zener diode  140  disposed in parallel with the first switching element  108  and a second Zener diode  142  disposed in parallel with the second switching element  114 . The first Zener diode  140  allows the electric current to flow from a point G′ to a point C′ while prohibits the electric current from flowing from the point C′ to the point G′. The first Zener diode  140  also allows the electric current to flow from the point C′ to the point G′ where a voltage applied thereto exceeds its breakdown voltage. The second Zener diode  142  is similar in construction to the first Zener diode  140 . More specifically, the second Zener diode  142  allows the electric current to flow from a point H′ to a point D′ while prohibits the electric current from flowing from the point D′ to the point H′. The second Zener diode  142  also allows the electric current to flow from the point D′ to the point H′ where a voltage applied thereto exceeds its breakdown voltage. 
     The external circuit  90  further includes a first auxiliary resistor  150  which is disposed in parallel with the first resistor  110  and which is a fixed resistor and a second auxiliary resistor  152  which is disposed in parallel with the second resistor  112  and which is a fixed resistor. In a line G′F′ in which the first auxiliary resistor  150  is provided, there is provided a first relay  154  configured to place the line G′F′ selectively in a state in which the electric current flows therethrough and in a state in which the electric current does not flow therethrough. In a line H′F′ in which the second auxiliary resistor  152  is provided, there is provided a second relay  156  configured to place the line H′F′ selectively in a state in which the electric current flows therethrough and in a state in which the electric currenrt does not flow therethrough. Each of the first relay  154  and the second relay  156  is configured to place the corresponding line in the state in which the electric current flows therethrough (OFF state) when its electromagnetic coil is energized and in the state in which the electric current does not flow therethrough (ON state) when not energized. 
     iii) Basic Functions of External Circuit 
     While the electromagnetic damper  10  corresponding to one of the four wheels  12  has been explained above, each of other electromagnetic dampers  10  respectively corresponding to the other three wheels  12  is similarly constructed and connected to the battery  120  as shown in  FIG. 3 . Hereinafter, there will be explained in detail basic functions of the electromagnetic dampers  10  with reference to  FIG. 4 .  FIG. 4  shows an equivalent circuit diagram in which only constituent elements of the external circuit  90  necessary for explanation are shown and the structure of the external circuit  90  is accordingly simplified. 
     In the motor  46 , the first terminal  100  is at a high potential and the second terminal  102  is at a low potential in the approaching motion of the sprung portion and the unsprung portion toward each other while the first terminal  100  is at a low potential and the second terminal  102  is at a high potential in the separating motion of the sprung portion and the unsprung portion away from each other. Accordingly, a generated current of the motor  46  flows from the first terminal  100  to the second terminal  102  via the points C, F, E, and B in the approaching motion. On the other hand, the generated current of the motor  46  flows from the second terminal  102  to the first terminal  100  via the points D, F, E, and A in the separating motion. That is, the first diode  104  functions as a first rectifier which allows the electric current to flow from the first terminal  100  to the second terminal  102  of the motor  46  while prohibits the electric current from flowing from the second terminal  102  to the first terminal  100 , and the passage CFEB in the external circuit  90  functions as a first connection passage. Further, the second diode  106  functions as a second rectifier which allows the electric current to flow from the second terminal  102  to the first terminal  100  of the motor  46  while prohibits the electric current from flowing from the first terminal  100  to the second terminal  102 , and the passage DFEA functions as a second connection passage. In the electromagnetic damper  10 , therefore, the passage through which the generated current of the motor  46  flows in the approaching motion and the passage through which the generated current of the motor  46  flows in the separating motion are mutually different. Accordingly, the damping characteristic with respect to the approaching motion and the damping characteristic with respect to the separating motion can be easily made different from each other, ensuring various advantages that will be explained in detail. 
     The first resistor  110  provided in the first connection passage CFEB functions as a resistance to the electric current that flows from the first terminal to the second terminal, and the first switching element  108  functions as a first-connection-passage-current adjuster configured to adjust the electric current that flows from the first terminal to the second terminal. The second resistor  112  provided in the second connection passage DFEA functions as a resistance to the electric current that flows from the second terminal to the first terminal, and the second switching element  114  functions as a second-connection-passage-current adjuster configured to adjust the electric current that flows from the second terminal to the first terminal. The resistance value R C  of the first resistor  110  is made larger than the resistance value R S  of the second resistor  112  (e.g., R C =2×R S ). Accordingly, a damper system is realized in which a damping force with respect to the approaching motion of the sprung portion and the unsprung portion toward each other is made smaller than a damping force with respect to the separating motion thereof away from each other in a state in which the electric current is allowed to flow between the point C and the point F and between the point D and the point F respectively by the first and second switching elements  108 ,  114 , for instance. As will be explained below, the present damper system is configured to change the damping characteristic with respect to a relative motion of the sprung portion and the unsprung portion by controlling the first switching element  108  and the second switching element  114 , under normal conditions. Further, even where there occurs a failure in which an appropriate damping characteristic cannot be attained, for instance, it is possible to mitigate a shock or impact applied to the vehicle body  14  in an instance where the wheel  12  moves toward or approaches the vehicle body  14  when the wheel  12  passes on a bump or projection on the road surface, by extablishing the above-indicted state in which the electric current is allowed to flow between CF and between DF respectively by the first and second switching elements  108 ,  114 . Accordingly, the present damper system is configured to suppress deterioration of the vehicle ride comfort at the time of occurrence of the above-indicated failure. 
     Since the motor  46  is connected to the battery  120  as described above, a part of the electric power generated by the motor  46  is regenerated into the battery  120  when the electromotive force of the motor  46  exceeds the voltage of the battery  120 . More specifically, when the sprung portion and the unsprung portion move toward each other, the generated current of the motor  46  not only flows through the above-indicated first connection passage CFEB, but also flows from the first terminal  100  to the high-potential-side terminal of the battery  120  via the line GI and flows from the low-potential-side terminal of the battery  120  to the second terminal  102 . On the other hand, when the sprung portion and the unsprung portion move away from each other, the generated current of the motor  46  not only flows through the second connection passage DFEA, but also flows from the second terminal  102  to the high-potential-side terminal of the battery  120  via the line HI and flows from the low-potential-side terminal of the battery  120  to the first terminal  102 . That is, a passage CGI in the external circuit  90  functions as a first high-potential-side connection passage in which the electric current is allowed to flow from the first terminal  100  to the high-potential-side terminal of the battery  120  while the electric current is prohibited from flowing from the high-potential-side terminal to the first terminal  100 . A passage FEB functions as a first low-potential-side connection passage in which the electric current is allowed to flow from the low-potential-side terminal of the battery  120  to the second terminal  102  while the electric current is prohibited from flowing from the second terminal  102  to the low-potential-side terminal. A passage DHI functions as a second high-potential-side connection passage in which the electric current is allowed to flow from the second terminal  102  to the high-potential-side terminal of the battery  120  while the electric current is prohibited from flowing from the high-potential-side terminal to the second terminal  102 . A passage FEA functions as a second low-potential-side connection passage in which the electric current is allowed to flow from the low-potential-side terminal of the battery  120  to the first terminal  100  while the electric current is prohibited from flowing from the first terminal  100  to the low-potential-side terminal. Accordingly, the external circuit  90  includes a battery-device connection passage which includes the first high-potential-side connection passage, the first low-potential-side connection passage, the second high-potential-side connection passage, and the second low-potential-side connection passage and in which the electric current is allowed to flow from one of the two terminals  100 ,  102  of the motor  46  that is at a high potential to the high-potential-side terminal of the battery  120  and the electric current is allowed to flow from the low-potential-side terminal of the battery  120  to the other of the two terminals  100 ,  102  of the motor  46  that is at a low potential. The above-indicated third switching element  126  is configured to adjust the electric current which flows through the battery-device connection passage and functions as a battery-device-connection-passage-current adjuster. 
     iv) External-Circuit Controller 
     In the damper system  10 , the external circuit  90  is controlled by an electronic control unit  200  (hereinafter referred to as “ECU  200 ” where appropriate) as an external-circuit controller, whereby the flow of the generated current by the motor  46  is controlled. More specifically, there are connected, to the ECU  200 , the first switching element  108 , the second switching element  114 , the third switching element  126 , the first relay  154 , and the second relay  156  which are controlled by the ECU  200 . The vehicle is equipped with four stoke sensors [St]  202  each for detecting a distance between the sprung portion and the unsprung portion for a corresponding one of the wheels  12 , temperature sensors [T]  204  each for detecting the temperature of a corresponding one of the motors  46  of the respective four electromagnetic dampers  10 , a voltage sensor [E B ]  206  for measuring the voltage of the battery  120 , and so on. These sensors are connected to the ECU  200 . The ECU  200  is configured to control the external circuit  90  on the basis of signals from the respective sensors. The characters in the above-indicated square brackets [ ] are signs when the sensors and the like are indicated in the drawings. The above-indicated distance between the sprung portion and the unsprung portion may be referred to as a “stroke” where appropriate because the distance represents an amount of expansion and contraction of the shock absorber  30 . In a ROM of the computer of the suspension ECU  200 , there are stored a program relating to the control of the external circuit  90  which will be explained, various data, and so on. 
     &lt;Control of Damper System&gt; 
     In the damper system, the external circuits  90  of the respective four electromagnetic dampers  10  can be controlled independently of each other. In the electromagnetic dampers  10 , the damping coefficients of the respective dampers  10  are controlled independently of each other, whereby the damping force with respect to the relative vibration of the sprung portion and the unsprung portion that correspond to each damper  10  is controlled. In each electromagnetic damper  10 , it is possible to control a damping coefficient C C  with respect to the approaching motion of the sprung portion and the unsprung portion toward each other and a damping coefficient C S  with respect to the separating motion of the sprung portion and the unsprung portion away from each other, independently of each other. In the electromagnetic damper  10 , in general, the generated current caused by the approaching motion flows through the first connection passage CFEB and the generated current caused by the separating motion flows through the second connection passage DFEA as explained above. Accordingly, the first switching element  108  provided in the first connection passage is controlled, so that the generated current caused by the approaching motion is controlled and the damping coefficient C C  with respect to the approaching motion (hereinafter referred to as “damping coefficient C C  at the time of approach” where appropriate) is thereby controlled. Further, the second switching element  114  provided in the second connection passage is controlled, so that the generated current caused by the separating motion is controlled and the damping coefficient C S  with respect to the separating motion (hereinafter referred to as “damping coefficient C S  at the time of separation” where appropriate) is thereby controlled. 
     Here, there is considered a damper system in which a single connection passage is provided for connecting two terminals of a motor and a single current adjuster is provided in the connection passage, for instance. In such a damper system, the generated current of the motor caused by the approaching motion and the generated current of the motor caused by the separating motion flow through the connection passage in mutually opposite directions, and the single current adjuster provided in the connection passage can adjust both of the generated currents flowing in the opposite directions. The relative vibration of the sprung portion and the unsprung portion, however, contains a component having a relatively high frequency, as shown in  FIG. 5(   a ), and the approaching motion and the separating motion may alternate at very short intervals. In such an instance, it is difficult, in view of the responsiveness in the control of the current adjuster, to switch the damping coefficients depending upon the direction of the relative motion of the sprung portion and the unsprung portion by the single current adjuster. In contrast, in the present damper system, the generated current caused by the approaching motion and the generated current caused by the separating motion flow through the respective different passages, thereby eliminating a need of switching the controls of the switching elements  108 ,  114  depending upon the direction of the relative motion of the sprung portion and the unsprung portion. In the present damper system, therefore, the damping coefficient C C  at the time of approach and the damping coefficient C S  at the time of separation can be made appropriate by controlling the two switching elements  108 ,  114  in accordance with the vehicle running state and the like, so that the relative vibration of the sprung portion and the unsprung portion can be effectively damped. There will be hereinafter explained in detail a method of determining the damping coefficient C C  at the time of approach and the damping coefficient C S  at the time of separation. 
     i) Determination of Damping Coefficients 
     a) Damping Coefficient of Main Adjuster 
     In the present damper system, it is construed that the relative vibration of the sprung portion and the unsprung portion is composed of vibrations of various frequencies. The present damper system mainly aims at damping a component in a sprung resonance frequency range (e.g., 0.1 Hz-3.0 Hz) among various frequencies. More specifically, as shown in  FIG. 5(   b ), the ECU  200  is configured to designate or determine, as a main adjuster, one of the two switching elements that adjusts the generated current caused by the relative motion of the sprung portion and the unsprung portion, on the basis of the direction of the relative motion indicated by a value of the component in the sprung resonance frequency range (i.e., the sprung-resonance-frequency-range component) and to control the main adjuster to damp the component. 
     Specifically, there is initially detected a change amount of the stroke, namely, a stroke speed Vst, on the basis of a detected value of the stroke sensor  202 . Subsequently, there is performed, on the stroke speed Vst, band-pass filter processing, namely, filter processing that allows passing of only a component having a frequency higher than 0.1 Hz and lower than 3.0 Hz, so that there is obtained a sprung-resonance stroke speed Vstb which is the sprung-resonance-frequency-range component of the stroke speed Vst. The thus obtained sprung-resonance stroke speed Vstb is judged, on the basis of its sign, to be a value indicative of the approaching motion of the sprung portion and the unsprung portion or a value indicative of the separating motion thereof. Where the sprung-resonance stroke speed Vstb is negative and accordingly is the value indicative of the approaching motion, the first switching element  108  provided in the first connection passage through which the generated current caused by the approaching motion flows is designated as the main adjuster. On the other hand, where the sprung-resonance stroke speed Vstb is positive and accordingly is the value indicative of the separating motion, the second switching element  114  provided in the second connection passage through which the generated current caused by the separating motion flows is designated as the main adjuster. 
     The main adjuster designated as described above is controlled to establish a damping coefficient suitable for damping the sprung-resonance-frequency-range component. In the present damper system, the damping coefficient C C  with respect to the approaching motion is made smaller than the damping coefficient C S  with respect to the separating motion. Accordingly, the second switching element  114  designated as the main adjuster is controlled such that the damping coefficient C S  at the time of separation becomes equal to C S1  (e.g., 5000 N·sec/m which is a value that is assumed to act directly on the wheel  12  with respect to the motion thereof), and the first switching element  108  designated as the main adjuster is controlled such that the damping coefficient C C  at the time of approach becomes equal to C C1  (e.g., 2500 N·sec/m). 
     b) Damping Coefficient of Auxiliary Adjuster 
     The ECU  200  designates or determines, as an auxiliary adjuster, the other of the two switching elements which is not the main adjuster. The ECU  200  controls the auxiliary adjuster to assist the main adjuster. The auxiliary adjuster is basically controlled to establish a damping coefficient suitable for damping a component of the relative vibration of the sprung portion and the unsprung portion in an unsprung resonance frequency range (e.g., 8.0 Hz-24 Hz), so as to damp the component. Specifically, the second switching element  114  designated as the auxiliary adjuster is controlled such that the damping coefficient C S  at the time of separation becomes equal to C S2  (e.g., 3000 N·sec/m) and the first switching element  108  designated as the auxiliary adjuster is controlled such that the damping coefficient C C  at the time of approach becomes equal to C C2  (e.g., 1500 N·sec/m). 
     It is noted here that the ECU  200  is configured to permit the auxiliary adjuster to have not only the function of damping the unsprung-resonance-frequency-range component, but also other functions on the basis of the intensity of the sprung-resonance-frequency-range component and the intensity of the unsprung-resonance-frequency-range component. More specifically, there is initially obtained, as the intensity of the sprung-resonance-frequency-range component, a maximum value of the sprung-resonance stroke speed Vstb within a prescribed time period t 0  between the current time point and a certain previous time point that precedes the current time point by t 0 . It is then judged whether or not the obtained value is larger than a set speed Vb 0 . Where the maximum value of the sprung-resonance stroke speed Vstb is larger than the set speed Vb 0 , the auxiliary adjuster is controlled, like the main adjuster, to establish the damping coefficient C S1  or C C1  for damping the sprung-resonance-frequency-range component, so as to give a higher priority to damping of the sprung-resonance-frequency-range component. 
     Where the maximum value of the sprung-resonance stroke speed Vstb is smaller than the set speed Vb 0 , the intensity of the unsprung-resonance-frequency-range component is obtained. There is initially performed, on the stroke speed Vst detected on the basis of the detected value of the stroke sensor  202 , band-pass filter processing, namely, filter processing that allows passing of only a component having a frequency higher than 8.0 Hz and lower than 24 Hz, so that there is obtained an unsprung-resonance stroke speed Vstw which is the unsprung-resonance-frequency-range component of the stroke speed Vst. Subsequently, there is obtained, as the intensity of the unsprung-resonance-frequency-range component, a maximum value of the unsprung-resonance stroke speed Vstw within the prescribed time period t 0  between the current time point and a certain previous time point that precedes the current time point by t 0 . It is then judged whether or not the obtained value is larger than a set speed Vw 0 . Where the maximum value of the unsprung-resonance stroke speed Vstw is larger than the set speed Vw 0 , the auxiliary adjuster is controlled to establish the damping coefficient C S2  or C C2  for damping the unsprung-resonance-frequency-range component, so as to damp the component, as described above. 
     Where the maximum value of the sprung-resonance stroke speed Vstb is smaller than the set speed Vb 0  and the maximum value of the unsprung-resonance stroke speed Vstw is smaller than the set speed Vw 0 , the auxiliary adjuster is controlled to establish a damping coefficient for damping a component having a frequency between the sprung resonance frequency range and the unsprung resonance frequency range, so as to damp the component (hereinafter referred to as “intermediate-frequency-range component” where appropriate). More specifically, the second switching element  114  designated as the auxiliary adjuster is controlled such that the damping coefficient C S  at the time of separation becomes equal to C S3  (e.g., 1000 N·sec/m), and the first switching element  108  designated as the auxiliary adjuster is controlled such that the damping coefficient C C  at the time of approach becomes equal to C C3  (e.g., 500 N·sec/m). 
     The thus constructed damper system permits the auxiliary adjuster to have various functions, making it possible to effectively damp not only the sprung-resonance-frequency-range component, but also the component having a higher frequency than the sprung resonance frequency range. 
     ii) Determination of Duty Ratio 
     In general, the ECU  200  controls the generated current caused by the approaching motion of the sprung portion and the unsprung portion toward each other by controlling the first switching element  108  of the external circuit  90 , so as to control the damping coefficient C C  at the time of approach. The ECU  200  also controls the generated current caused by the separating motion of the sprung portion and the unsprung portion away from each other by controlling the second switching element  114 , so as to control the damping coefficient C S  at the time of separation. The ECU  200  performs a Pulse Width Modulation (PWM) control on the switching elements  108 ,  114 . More specifically, in the PWM control, a pulse pitch is made constant which is obtained by adding a pulse-on time t ON  during which each of the connection passages that correspond to the respective switching elements  108 ,  114  is in the electrically connected state and a pulse-off time t OFF  during which each connection passage is the electrically shut-off state, and a duty ratio r D  (=t ON /t ON +t OFF )) that is a ratio of the pulse-on time to the pulse pitch is controlled. That is, the ECU  200  controls the generated current by the motor  46  by controlling the duty ratio r D  of each of the switching elements  108 ,  114 , so as to control the damping coefficient C of the electromagnetic damper  10 . 
     There will be next explained a relationship between the duty ratio r D  of each of the switching elements  108 ,  114  and the damping coefficient C of the electromagnetic damper  10 . The motor  46  of the electromagnetic damper  10  is the brushed DC motor as described above. Where the electric current that flows in the motor  46  is I, the torque to be generated is Tq, the rotational speed is w, and the voltage generated between the two terminals  100 ,  102  is E, the following relationships are established:
 
 E=α·ω   (1)
 
 Tq=α·I   (2)
 
Here, “α” is a motor constant (a torque constant, a conter-electromotive-force constant) of the motor  46 .
 
     Initially, a situation is considered in which the motor  46  is rotated in a state wherein each of the connection passages that correspond to the respective switching elements  108 ,  114  is in the electrically connected state, namely, wherein the duty ratio r D  is 1.0 and in which the electromotive force E of the motor  46  is not larger than the nominal voltage E N  of the battery  120 . In this situation, the generated current of the motor  46  flows through the first connection passage CFEB in the approaching motion while flows through the second connection passage DFEA in the separating motion, and the amounts of the generated current are determined by the following formulas:
 
Approaching motion:  I=E/R   C   (3)
 
Separating motion:  I=E/R   S   (4)
 
Here, in an instance where the switching elements  108 ,  114  are controlled under the duty ratio r D , the amounts of the generated current in that instance are as follows:
 
Approaching motion:  I=r   D   ·E/R   C   (3′)
 
Separating motion:  I=r   D   ·E/R   S   (4′)
 
The above-indicated formula (1) is substituted into the formulas (3′) and (4′), and “I” obtained by the substitution is substituted into the above-indicated formula (2), so that the following formulas are obtained:
 
Approaching motion:  Tq=r   D ·α 2   /R   C ·ω  (5)
 
Separating motion:  Tq=r   D ·α 2   /R   S ·ω  (6)
 
The damping coefficient C of the electromagnetic damper  10  is represented by a magnitude F of the damping force with respect to the speed Vst of the relative motion of the sprung portion and the unsprung portion, in other words, represented by the torque Tq of the motor  46  with respect to the rotational speed ω of the motor  46 . That is, the damping coefficient C C  with respect to the approaching motion and the damping coefficient C S  with respect to the separating motion are indicated as follows:
 
 C   C   =r   D ·α 2   /R   C   (7)
 
 C   S   =r   D ·α 2   /R   S   (8)
 
     Next, a situation is considered in which the electromotive force E of the motor  46  exceeds the nominal voltage E N  of the battery  120 . In this situation, the generated current of the motor  46  flows through the first connection passage and the battery-device connection passage in the approaching motion while flows through the second connection passage and the battery-device connection passage in the separating motion, and the magnitude of the generated current is determined by the following formulas: 
                     Approaching   ⁢           ⁢   motion   ⁢     :     ⁢           ⁢   I     =         E   /     R   C       +       (     E   -     E   N       )     /     R   B         =         (       1   /     R   C       +     1   /     R   B         )     ·   E     -       E     N   ⁢               /     R   B                   (   9   )                 Separating   ⁢           ⁢     motion   :           ⁢   I       =         E   /     R   S       +       (     E   -     E   N       )     /     R   B         =         (       1   /     R   S       +     1   /     R   B         )     ·   E     -       E   N     /     R   B                   (   10   )               
Here, in an instance where the switching elements  108 ,  114  are controlled under the duty ratio r D , the amount of the generated current in that instance is as follows:
 
Approaching motion:  I=r   D ·[(1 /R   C +1 /R   B )· E−E   N   /R   S ]  (9′)
 
Separating motion:  I=r   D ·[(1 /R   S +1 /R   S )· E−E   N   /R   B ]  (10′)
 
The above-indicated formula (1) is substituted into the formulas (9′) and (10′), and “I” obtained by the substitution is substituted into the above-indicated formula (2), so that the following formulas are obtained:
 
Approaching motion:  Tq=r   D ·[α 2 ·(1 /R   C +1 /R   B )−α· E   N /( R   B ·ω)]·ω  (11)
 
Separating motion:  Tq=r   D ·[α 2 ·(1 /R   S +1 /R   B )−α· E   N /( R   B ·ω)]·ω  (12)
 
     Accordingly, the damping coefficient C C  with respect to the approaching motion and the damping coefficient C S  with respect to the separating motion are indicated as follows:
 
 C   C   =r   D ·[α 2 ·(1 /R   C +1 /R   S )−α· E   N /( R   B ·ω)]  (13)
 
 C   S   =r   D ·[α 2 ·(1 /R   S +1 /R   B )−α· E   N /( R   B ·ω)]  (14)
 
     Accordingly, the ECU  200  controls the damping coefficient C C  at the time of approach by controlling a duty ratio r DSW1  of the first switching element  108  while controls the damping coefficient C S  at the time of separation by controlling a duty ratio r DSW2  of the second switching element  114 . More specifically, the damping coefficients each as a target are determined according to the manner described above, and the duty ratio r D  as a target is determined according to the following formulas so as to establish the target damping coefficients: 
                     r     DSW   ⁢           ⁢   1       =       ⁢       C   C     *     /     (       α   2     /     R   C       )                     ⁢     (     E   ≤     E   N       )                 =       ⁢       C   C     *     /     [         α   2     ·     (       1   /     R   C       +     1   /     R   B         )       -     α   ·       E   N     ⁡     (       R   B     ·     V   st       )           ]                     ⁢     (     E   &gt;     E   N       )                               r     DSW   ⁢           ⁢   2       =       ⁢       C   S     *     /     (       α   2     /     R   S       )                     ⁢     (     E   ≤     E   N       )                 =       ⁢       C   S     *     /     [         α   2     ·     (       1   /     R   S       +     1   /     R   B         )       -     α   ·       E   N     ⁡     (       R   B     ·     V   st       )           ]                     ⁢     (     E   &gt;     E   N       )                 
The switching elements  108 ,  114  are controlled to be opened and closed under the respective duty ratios determined as described above, so that the damping coefficient of the electromagnetic damper  10  is changed. Whether or not the electromotive force E is higher than the nominal voltage E N  of the battery  120  is judged on the basis of the stroke speed Vst that is proportional to the rotational speed ω of the motor  46 , according to the relationship of the formula (1). That is, the judgment is made depending upon whether the stroke speed Vst is larger than a value V 0  (=K·E N /α, K: a constant between motor rotational speed ω and stroke speed Vst) that corresponds to the nominal voltage E N  of the battery  120 .
 
       FIG. 6  shows the damping coefficients C C *, C S * determined as described above in the present damper system, in other words, the target duty ratios r DSW1 , r DSW2  of the switching elements  108 ,  114 . In the present damper system, the ratio between the damping coefficient C C1  of the first switching element  108  and the damping coefficient C S1  of the second switching element  114  with respect to the sprung-resonance-frequency-range component and the ratio between the resistance value R C  of the first resistor  110  and the resistance value R S  of the second resistor  112  are made equal to each other. Accordingly, the duty ratio r DSW1  where the first switching element  108  establishes the damping coefficient C C1  and the duty ratio r DSW2  where the second switching element  114  establishes the damping coefficient C S1  may be the same duty ratio, i.e., r 1 , in a range in which the electromotive force E is lower than the nominal voltage E N  of the battery  120 . This is true of the damping coefficients C C2 , C S2  with respect to the unsprung-resonance-frequency-range component and the damping coefficients C C3 , C S3  with respect to the intermediate-frequency-range component, and the duty ratio with respect to the unsprung-resonance-frequency-range component is made equal to r 2  while the duty ratio with respect to the intermediate-frequency-range component is made equal to r 3 . 
     iii) Other Control of Auxiliary Adjuster 
     The ECU  200  permits the above-described auxiliary adjuster to have functions other than those described above depending upon situations. For instance, in an instance where the remaining energy amount of the battery  120  has been decreased, it is desirable to increase a regenerative current to the battery  120 . To this end, where an actual voltage E B  of the battery  120  detected by the voltage sensor  206  becomes lower than a set voltage E 0  and the stroke speed Vst becomes larger than a value V 1  that corresponds to the actual voltage E B , the duty ratio r D  of the switching element designated as the auxiliary adjuster is made equal to 1.0 for maximizing the magnitude of the regenerative current to the battery  120 . 
     In an instance where a relatively large input from the unsprung portion continues and the motor  46  is suffering from a large load, there is a risk that the motor  46  may be damaged. Where the temperature T of the motor  46  detected by the temperature sensor  204  is higher than a set temperature T 0 , it is estimated that the load on the motor  46  is large. In this case, the duty ratio r D  of the switching element designated as the auxiliary adjuster is made equal to 0, thereby reducing the load on the motor  46 . 
     iv) Adjustment of Regenerative Current to Battery 
     The voltage of the battery  120  varies to a certain degree. Where the electric power supplied from the battery  120  to electrically loaded portions mounted on the vehicle becomes large, for instance, the voltage of the battery  120  is lowered. In this case, the generated current of the motor  46  tends to readily flow to the battery  120 , and the regenerative current to the battery  120  is larger than that in a case where the voltage of the battery  120  is high. That is, in the case where the voltage of the battery  120  is lowered, the damping force of the electromagnetic damper  10  becomes larger as compared to the case where the voltage of the battery  120  is high. In view of this, in the present damper system, the third switching element  126  is controlled so as to reduce the regenerative current when the voltage of the battery  120  is lowered, thereby suppressing an increase in the damping force of the electromagnetic damper  10 . More specifically, where the actual voltage E B  of the battery  120  detected by the voltage sensor  206  becomes lower than the nominal voltage E N , the duty ratio r DSW3  of the third switching element  126  as a target is calculated according to the following formula:
 
 r   DSW3   =E   B   /E   N  
 
The third switching element  126  is controlled so as to be opened and closed under the thus determined duty ratio, so that the regenerative current is decreased and the damping force of the electromagnetic damper  10  is restrained from being increased.
 
v) How to Deal with Failures
 
     There will be next explained how to deal with failures occurred in the external circuit  90 . As failures in the external circuit  90 , there may occur a failure, for instance, in which the electric current does not pass through the first resistor  110  or the second resistor  112  due to a break or disconnection therein or the like by heat generation. When such a failure occurs, the generated current does not flow through the corresponding connection passage in which the failure-suffering resistor is disposed. Accordingly, no damping force is generated with respect to the relative motion of the sprung and unsprung portions that corresponds to the connection passage, so that the vehicle stability is deteriorated. In the present damper system, the first relay  154  is placed in the ON state when such a failure occurs in the first resistor  110  while the second relay  156  is placed in the ON state when such a failure occurs in the second resistor  112 . 
     Whether or not the failure that the electric current does not pass through the first and second resistors  110 ,  112  is judged on the basis of the stroke sensor  202  and two Schmitt triggers  220 ,  222  which are disposed between the external circuit  90  and the ECU  200 . The Schmitt trigger  220  is configured to perform an output where the potential of the point G in the external circuit  90  exceeds a first threshold and to stop the output where the potential of the point G becomes lower than a second threshold that is lower than the first threshold. Another Schmitt trigger  222  is configured such that the output state and the non-output state are switched therebetween depending upon the potential of the point H in the external circuit  90 . Where there is no output from the Schmitt triggers  220 ,  222  to the ECU  200  in spite of detection of the stroke speed higher than a speed corresponding to the first threshold of the Schmitt triggers  220 ,  222  by the stroke sensor  202 , it is judged that there has occurred a failure in which the electric current does not pass through the first resistor  110  or the second resistor  112 . Which one of the first resistor  110  and the second resistor  112  is suffering from the failure is judged by the sign of the detected stroke speed. 
     As described above, in the present damper system, the generated current caused by the approaching motion of the sprung portion and the unsprung portion toward each other flows through the first auxiliary resistor  150  in place of the first resistor  110  even where the failure in which the electric current does not pass through the first resistor  110  occurs. Further, the generated current caused by the separating motion of the sprung portion and the unsprung portion away from each other flows through the second auxiliary resistor  152  in place of the second resistor  112  even where the failure in which the electric current does not pass through the second resistor  112  occurs. Accordingly, even where the failure in which the electric current does not pass through the firs resistor  110  or the second resistor  112  occurs, it is possible to execute the normal control described above, and the vehicle stability is not deteriorated. The passage GG′F′ in the external circuit  90  functions as a first-resistor bypass passage which bypasses the first resistor  110  while the passage HH′F′ in the external circuit  90  functions as a second-resistor bypass passage which bypasses the second resistor  112 . 
     As another failure in the external circuit  90 , there may occur a failure in which, in the circuit diagram shown in  FIG. 4 , the electric current does no pass through the first switching element  108  or the second switching element  114 , more specifically, a failure the switching element is placed in the open state due to a fault of the switching element per se or a fault of the ECU  200 . When such a failure occurs, the generated current does not flow through the connection passage in which the failure-suffering switching element is disposed. Accordingly, no damping force is generated with respect to the relative motion of the sprung and unsprung portions that corresponds to the connection passage, so that the vehicle stability is deteriorated. In the present damper system, even where the first switching element  108  suffers from such a failure, the electric current flows through the first Zener diode  140  disposed in parallel with the first switching element  108  where there is generated an electromotive force that exceeds a breakdown voltage of the first Zener diode  140 . Hence, the damping force can be generated upon receiving a large input from the unsprung portion. Further, even where the second switching element  114  suffers from such a failure, the second Zener diode  142  disposed in parallel with the second switching element  114  functions in a manner similar to the first Zener diode  140 . 
     In other words, the external circuit  90  includes a first-adjuster bypass passage which bypasses the first switching element  108  and in which the electric current is allowed to flow from a side of the first terminal  100  to a side of the second terminal  102  only where the electromotive force of the motor  46  caused by the approaching motion of the sprung portion and the unsprung portion toward each other exceeds a prescribed voltage, and the passage C′G′G in the external circuit  90  functions as the first-adjuster bypass passage. Further, the external circuit  90  includes a second-adjuster bypass passage which bypasses the second switching element  114  and in which the electric current is allowed to flow from a side of the second terminal  102  to a side of the first terminal  100  only where the electromotive force of the motor  46  caused by the separating motion of the sprung portion and the unsprung portion away from each other exceeds a prescribed voltage, and the passage D′H′H in the external circuit  90  functions as the second-adjuster bypass passage. 
     The above-indicated two Zener diodes  140 ,  142  are configured such that the generated current flows therethrough when the electromotive force of the motor  46  exceeds the breakdown voltages of the respective Zener diodes  140 ,  142  even if the failure in which the electric current does not pass through the first switching element  108  or the second switching element  114  has not occurred. That is, in a situation in which the vehicle stability is needed upon a large input from the unsprung portion, it is possible to generate a stable damping force without requiring the control of the switching elements  108 ,  114 . 
     &lt;Control Flow of External Circuit&gt; 
     The control of the external circuit  90  described above is executed such that an external-circuit control program indicated by a flow chart of  FIG. 7  is repeatedly implemented by the ECU  200  at short time intervals (e.g., several milliseconds), with an ignition switch of the vehicle placed in an ON state. Hereinafter, there will be briefly explained the flow of the control referring to the flow chart. The external-circuit control program is executed for each of the four electromagnetic dampers  10  provided for the respective four wheels  12 . In the following description, there will be explained processing by the program performed on one electromagnetic damper  10 , for the interest of brevity. 
     In the control program, in step  1  (hereinafter abbreviated as “S 1 ” and other steps are similarly abbreviated), the stroke speed Vst is obtained on the basis of the detected value of the stroke sensor  202 . S 1  is followed by S 2  in which the band-pass filter processing for the sprung resonance frequency range is performed on the stroke speed Vst, and the sprung-resonance stroke speed Vstb which is the sprung-resonance-frequency-range component of the stroke speed Vst is calculated. Subsequently, in S 3 , it is judged, on the basis of the sign of the sprung-resonance stroke speed Vstb, which one of the approaching motion and the separating motion is indicated by the value of the sprung-resonance-frequency-range component of the relative motion of the sprung portion and the unsprung portion. 
     Where it is judged in S 3  that the sprung-resonance stroke speed Vstb is negative and indicates the approaching motion, S 4  is implemented to designate the first switching element  108  as the main adjuster and designate the second switching element  114  as the auxiliary adjuster. S 4  is followed by S 5  in which the duty ratio r DSW1  of the first switching element  108  designated as the main adjuster is determined according to the above formula so as to establish the damping coefficient C C1  with respect to the sprung-resonance-frequency-range component. S 5  is followed by S 6  in which processing for determining the duty ratio of the auxiliary adjuster is executed for the second switching element  114  designated as the auxiliary adjuster. 
     On the other hand, where it is judged in S 3  that the sprung-resonance stroke speed Vstb is positive and indicates the separating motion, S 7  is implemented to designate the second switching element  114  as the main adjuster and designate the first switching element  108  as the auxiliary adjuster. S 7  is followed by S 8  in which the duty ratio r DSW2  of the second switching element  114  designated as the main adjuster is determined according to the above formula so as to establish the damping coefficient C S1  with respect to the sprung-resonance-frequency-range component. S 8  is followed by S 9  in which processing for determining the duty ratio of the auxiliary adjuster is executed for the first switching element  108  designated as the auxiliary adjuster. 
     The above-described processing for determining the duty ratio of the auxiliary adjuster is executed such that an auxiliary-adjuster-duty-ratio-determining-processing sub routine indicated by a flow chart of  FIG. 8  is implemented. In the processing, S 21  is initially implemented to judge whether or not the temperature T of the motor  46  detected by the temperature sensor  204  is higher than the set temperature T 0 . Where the temperature T of the motor  46  is higher than T 0 , S 22  is implemented in which the duty ratio of the switching element designated as the auxiliary adjuster is made equal to 0, thereby reducing the load on the motor  46 . 
     Where the temperature T of the motor  46  is not higher than T 0 , S 23  is implemented to judge whether or not the actual voltage E B  of the battery  120  detected by the voltage sensor  206  is lower than the set voltage E 0 . Where the actual voltage E B  of the battery  120  is lower than the set voltage E 0 , S 24  is implemented to judge whether or not the stroke speed Vst is larger than a value V 1  that corresponds to the actual voltage E B  of the battery  120 . Where the stroke speed Vst is larger than V 1 , S 25  is implemented in which the duty ratio of the switching element designated as the auxiliary adjuster is made equal to 1.0. In this instance, a part of the generated current of the motor  46  is regenerated to the battery  120 . Accordingly, the regenerative current flowing to the battery  120  is maximized by setting the duty ratio of the auxiliary adjuster at 1.0. 
     On the other hand, where the actual voltage E B  of the battery  120  is lower than the set voltage E 0  or where the stroke speed Vst is larger than V 1 , S 26  is implemented to obtain the maximum value of the sprung-resonance stroke speed Vstb within the prescribed time period t 0  between the current time point and the certain previous time point that precedes the current time point by t 0 , and it is judged whether or not the value is larger than the set speed Vb 0 . Where the maximum value of the sprung-resonance stroke speed Vstb is larger than the set speed Vb 0 , S 27 -S 29  are implemented in which the duty ratio of the auxiliary adjuster is determined according to the above formula so as to establish the damping coefficient C S1  or C C1  with respect to the sprung-resonance-frequency-range component. 
     On the other hand, where the maximum value of the sprung-resonance stroke speed Vstb is smaller than the set speed Vb 0 , S 30  is implemented in which the band-pass filter processing for the unsprung resonance frequency range is performed on the stroke speed Vst obtained in S 1 , so as to calculate the unsprung-resonance stroke speed Vstw which is the unsprung-resonance-frequency-range component of the stroke speed Vst. Subsequently, S 31  is implemented to obtain the maximum value of the unsprung-resonance stroke speed Vstw within the prescribed time period t 0  between the current time point and the certain previous time point that precedes the current time point by t 0 , and it is judged whether or not the value is larger than the set speed Vw 0 . Where the maximum value of the unsprung-resonance stroke speed Vstw is larger than the set speed Vw 0 , S 32  is implemented in which the duty ratio of the auxiliary adjuster is set at r 2  so as to establish the damping coefficient C S2  or C C2  with respect to the unsprung-resonance-frequency-range component. On the other hand, where the maximum value of the unsprung-resonance stroke speed Vstw is smaller than the set speed Vw 0 , S 33  is implemented in which the duty ratio of the auxiliary adjuster is set at r 3  so as to establish the damping coefficient C S3  or C C3  with respect to the intermediate-frequency-range component. After the duty ratio of the auxiliary adjuster has been determined according to a series of processing described above, one execution of the external-circuit control program is ended. 
     &lt;Functional Structure of ECU&gt; 
     Functions of the above-described ECU  200  are schematically shown in the functional block diagram of  FIG. 9 . According to the functions described above, the ECU  200  includes: an adjuster-role designating portion  250  configured to designate the two switching elements  108 ,  114  as one and the other of the main adjuster and the auxiliary adjuster, such that the two switching elements  108 ,  114  take the respective roles; an main-adjuster control portion  252  configured to control one of the first switching element  108  and the second switching element  114  that is designated as the main adjuster; and an auxiliary-adjuster control portion  254  configured to control one of the first switching element  108  and the second switching element  114  that is designated as the auxiliary adjuster. In the ECU  200  of the present damper system, the adjuster-role designating portion  250  is constituted by including a portion that executes the processing in S 1 -S 4  and S 7  of the external-circuit control program. The main-adjuster control portion  252  is constituted by including a portion that executes the processing in S 5  and S 8  of the external-circuit control program. The auxiliary-adjuster control portion  254  is constituted by including a portion that executes the processing in S 6  and S 9  of the external-circuit control program, namely, a portion that executes the auxiliary-adjuster-duty-ratio-determining-processing sub routine. The ECU  200  further includes a regenerative-current control portion  260  configured to control the regenerative current by controlling the third switching element  126  as the battery-device-connection-passage-current adjuster, for suppressing a fluctuation of the damping force when a part of the generated current is regenerated to the battery  120 . 
     REFERENCE SIGNS LIST 
       10 : electromagnetic damper  12 : wheel  14 : vehicle body  20 : spring•absorber Assy  22 : lower arm (unsprung portion)  24 : mount portion (sprung portion)  30 : shock absorber (damper main body)  32 : coil spring (suspension spring)  40 : threaded rod  42 : nut  44 : ball screw mechanism (motion converting mechanism)  46 : electromagnetic motor  52 : motor shaft  60 : polar body  62 : permanent magnet  64 : commutator  66 : brush  90 : external circuit  100 : first terminal  102 : second terminal  104 : first diode (first rectifier)  106 : second diode (second rectifier)  108 : first switching element [SW 1 ] (first-connection-passage-current adjuster)  110 : first resistor [R C ]  112 : second resistor [R S ]  114 : second switching element [SW 2 ] (second-connection-passage-current adjuster)  120 : battery (battery device)  122 : third diode  124 : fourth diode  126 : third switching element [SW 3 ] (battery-device-connection-passage-current adjuster)  128 : source resistance [R B ]  140 : first Zener diode  142 : second Zener diode  150 : first auxiliary resistor  152 : second auxiliary resistor  154 : first relay (first open/close device)  156 : second relay (second open/close device)  200 : electronic control unit (ECU, external-circuit controller)  202 : stroke sensor [St]  204 : temperature sensor [T]  206 : voltage sensor [E B ]  220 ,  222 : Schmitt trigger  250 : adjuster-role designating portion  252 : main-adjuster control portion  254 : auxiliary-adjuster control portion  260 : regenerative-current control portion 
     Passage CFEB: first connection passage Passage DFEA: second connection passage Passage CGI: first high-potential-side connection passage Passage FEB: first low-potential-side connection passage Passage DHI: second high-potential-side connection passage Passage FEA: second low-potential-side connection passage Passage GG′F: first-resistor bypass passage Passage HH′F: second-resistor bypass passage Passage C′G′G: first-adjuster bypass passage Passage D′H′H: second-adjuster bypass passage 
     R C : resistance value of the first resistor R S : resistance value of the second resistor C C : damping coefficient at the time of approach C S : damping coefficient at the time of separation C C1 , C S1 : damping coefficient with respect to the sprung-resonance-frequency-range component C C2 , C S2 : damping coefficient with respect to the unsprung-resonance-frequency-range component C C3 , C S3  damping coefficient with respect to the intermediate-frequency-range component Vst: stroke speed Vstb: sprung-resonance stroke speed Vstw: unsprung-resonance stroke speed r DSW : duty ratio of SW 1  r DSW2 : duty ratio of SW 2  α: motor constant E N : nominal voltage of the battery E B : actual voltage of the battery T: motor temperature