Power transmission device

A power transmission device 1 includes a variable stiffness unit 41, which has variable stiffness, receives a torque from a motor A2, and transmits the torque to an output unit B, a variable viscosity coefficient unit 42, which has variable viscosity, receives the torque from the motor A2, and transmits the torque to the output unit B, and a controller A4 which modifies the stiffness of the variable stiffness unit 41 and the viscosity of the variable viscosity coefficient unit 42.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power transmission device which transmits motive power transmitted from a driving element to a driven element.

2. Description of the Related Art

In recent years, as in the case where a robot comes in contact with an obstacle, it is desired that a joint disposed between links of the robot is flexible as a power transmission element in order to prevent the robot from being damaged even in the case where an impact is applied to the robot.

In view of the above circumstances, there is known an actuator with an elastic member disposed between a driving element and a driven element of a robot (U.S. Pat. No. 5,650,704). This actuator prevents an impact from being directly transmitted to the driven element or the driving element by the elasticity of the elastic member even in the case where the impact occurs in the driving element or the driven element. In the case where a joint has flexibility in this manner, a controlled object (for example, the motion of a joint or the like) oscillates more easily by improving a control response. Therefore, the oscillation of the controlled object is suppressed by a feedback control on the basis of information detected by various sensors or the like.

In the case where various sensors or the like are placed in an abnormal state, however, an appropriate feedback control might not be able to be performed and by extension the oscillation might not be able to be suppressed appropriately. Accordingly, it is conceivable to suppress the oscillation by giving viscosity to the joint even in the case where it is impossible to suppress the oscillation by the feedback control.

Meanwhile, in such a case where a precise operation is required, it is preferable that a joint is stiff in some cases. Specifically, if the stiffness of a joint is able to be varied, appropriate control can be performed in various situations. In order to satisfy this requirement, it is conceivable to use a member whose stiffness is variable such as a nonlinear spring as an elastic member.

Generally, however, the following relational expression is already known in a spring-damper system:

[MATH⁢⁢1]h·ω=c2⁢m⁢⁢h·km=c2⁢m(1)
where h is a damping constant, ω is an angular frequency, k is an elastic coefficient, m is the mass of a load, and C is a viscosity coefficient.

According to the expression (1), supposing that the mass m of a load and the viscosity coefficient C are constant, the damping constant h varies if the elastic coefficient (stiffness) k representing the elasticity of the elastic member is varied. The damping constant h represents a damping rate of vibration in a process in which a vibrating load converges.

The control processing is performed for each previously-determined period. Therefore, the control processing is able to be easily performed if the vibration of a load in a control period at the present time is able to be predicted from the vibration of the load in the previous control period. In other words, the control processing is able to be easily performed when the damping rate (and by extension the damping constant h) is constant than when it is variable. Accordingly, when the damping constant h is constant in the expression (1), the mass m of the load is constant, and therefore it is necessary to vary the viscosity coefficient C according to a change in stiffness k.

In the case where a joint has viscosity as well as elasticity as described above, it is desirable to vary the viscosity coefficient C as well as the stiffness k. Additionally, there is no problem even if the value of the damping constant h fluctuates as long as within the range where the control is able to be easily performed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a power transmission device capable of effectively suppressing the oscillation of a controlled object in the case where a power transmission element such as the aforementioned joint has a variable stiffness and also has viscosity.

According to an aspect of the present invention, there is provided a power transmission device which transmits motive power transmitted from a driving element to a driven element, including: a first element having variable stiffness and configured to receive the motive power from the driving element and to transmit the motive power to the driven element; a second element having variable viscosity and configured to receive the motive power from the driving element and to transmit the motive power to the driven element; a first modification unit configured to modify the stiffness of the first element; and a second modification unit configured to modify the viscosity of the second element (First aspect of the invention).

According to the first aspect of the invention, the motive power of the driving element is transmitted to the driven element via the first element and the second element. Specifically, the second element having viscosity as a power transmission element is provided on a power transmission path between the driving element and the driven element, thereby enabling the vibration of the driven element to be mechanically converged at the time of the control of the driven element. Moreover, the first element has stiffness able to be modified by the first modification unit and therefore the driven element is able to be flexibly controlled according to the situation.

Moreover, the second element has viscosity able to be modified by the second modification unit and therefore, for example, in the case where the first modification unit modifies the stiffness, the second modification unit is able to modify the viscosity of the second element according to the modified stiffness. Thereby, even in the case where the stiffness of the first element is modified according to the situation, the modification of the viscosity of the second element enables the prevention of, for example, a remarkable change in the damping rate of the vibration of the driven element.

Accordingly, in the execution of control processing, the vibration of a load in the control period at the present time is easily predicted from the vibration of the load in the previous control period, thereby facilitating the control processing. In this manner, the oscillation of the controlled object can be effectively suppressed by using the power transmission element whose stiffness and viscosity are able to be modified by appropriately modifying the stiffness and viscosity.

In the first aspect of the invention, preferably the second modification unit modifies the viscosity according to the stiffness modified by the first modification unit so that damping of vibration of the driven element is predetermined damping (Second aspect of the invention). Thereby, the viscosity of the second element is modified according to the stiffness of the first element modified by the first modification unit so that the damping of the vibration of the driven element is predetermined damping. This enables the damping of the driven element to be predicted in advance in each control period, thereby facilitating the control processing and suppressing the oscillation more effectively.

In the first aspect of the invention, preferably the power transmission device is configured to be able to switch between a transmission state in which the motive power is transmitted to the driven element and a non-transmission state in which the transmission is disconnected by modifying the stiffness of the first element or the viscosity of the second element (Third aspect of the invention). Thus, the modification of the stiffness or viscosity causes a switch to the non-transmission state in which the motive power is not transmitted to the driven element, thereby preventing the driven element from being mechanically driven. Moreover, the second element has viscosity and therefore only a switch to the non-transmission state enables the oscillation of the driven element to be mechanically suppressed without any other control for suppressing the oscillation.

In the third aspect of the invention, preferably the power transmission device further includes: a drive source which transmits motive power to the driving element; and a control unit which controls the drive source and is configured to perform the control to modify the stiffness of the first element and the viscosity of the second element, wherein the control unit includes: a first determination unit configured to determine whether the drive source is able to be normally controlled; and a second determination unit configured to determine whether the transmission state is set, wherein: the control unit performs a control to achieve the non-transmission state in the case where the determination result of the first determination unit is negative; the control unit controls the drive source so that a difference between a displacement of the driven element and a displacement of the driving element is equal to or less than a predetermined value in the case where the determination result of the first determination unit is affirmative and the determination result of the second determination unit is negative; and the control unit controls the motive power transmitted to the driven element in the case where the determination result of the first determination unit is affirmative and the determination result of the second determination unit is affirmative (Fourth aspect of the invention).

Thereby, the non-transmission state is set in the case where the first determination unit determines that the drive source is not able to be normally controlled. This causes a state where motive power is not transmitted to the driven element. If this state occurs, the second element has viscosity and therefore, even in the case where the drive source is not controlled, the oscillation of the driven element is able to be suppressed and converged.

Moreover, generally at the time of transition from the non-transmission state to the transmission state, the transmission state needs to be set within the range of a previously-defined difference in a relative displacement between the driven element and the driving element. Therefore, it is necessary to set the transmission state after bringing the difference between the displacement of the driven element and the displacement of the driving element close to the defined difference. Accordingly, if the difference in the relative displacement between the driven element and the driving element is greater than the defined difference when the control unit or the like determines to set the transmission state, it is necessary to bring the difference in the relative displacement to a level equal to or less than the defined difference before setting the transmission state.

In the present invention, however, in the case where the control unit is able to control the drive source normally and the non-transmission state is set, the driving element is controlled so that the difference between the displacement of the driven element and the displacement of the driving element is equal to or less than the predetermined value. Therefore, immediately after the control unit or the like determines to set the transmission state, the transmission state is able to be set. Consequently, the time for control processing is able to be reduced.

Furthermore, in the case where the first determination unit determines that the drive source is able to be normally controlled and the second determination unit determines that the transmission state is set, the motive power transmitted to the driven element is controlled. This enables the control of the operation of the driven element.

In the first aspect of the invention, preferably the first element and the second element each include a conductive polymer actuator and the first modification unit and the second modification unit are each configured as a voltage application unit which applies a voltage to the conductive polymer actuator (Fifth aspect of the invention). The stiffness and viscosity of the conductive polymer actuator are able to be modified by applying a voltage to the conductive polymer actuator. Therefore, the use of the conductive polymer actuator simplifies the structure of the power transmission device in comparison with the case of preparing “a system which modifies the stiffness” and “a system which modifies the viscosity” separately.

In the third aspect of the invention, preferably the first element and the second element each include a conductive polymer actuator formed in a tubular shape; the first modification unit and the second modification unit are each configured as a voltage application unit which applies a voltage to the conductive polymer actuator; the driven element is at least partially disposed in a hollow portion of the conductive polymer actuator; and the conductive polymer actuator is configured in such a way that the space of the hollow portion decreases in response to the application of the voltage from the voltage application unit and that the inner wall surface of the hollow portion comes in contact with the driven element in the case where the applied voltage is equal to or greater than a predetermined voltage (Sixth aspect of the invention).

Thereby, the stiffness and viscosity of the conductive polymer actuator vary by applying the voltage to the conductive polymer actuator. Therefore, the use of the conductive polymer actuator simplifies the structure of the power transmission device in comparison with the case of preparing “a system which modifies the stiffness” and “a system which modifies the viscosity” separately.

Furthermore, the conductive polymer actuator is configured in such a way that the space of the tubular hollow portion decreases (for example, decreases in a radial direction or the like) in response to the application of the voltage. In the case where the voltage applied to the conductive polymer actuator is equal to or greater than the predetermined voltage, the inner wall surface of the hollow portion comes in contact with the driven element. Thereby, the motive power transmitted to the conductive polymer actuator is transmitted to the driven element (the transmission state in which the motive power is transmitted from the driving element to the driven element is set).

In other words, it is possible to switch between the transmission state and the non-transmission state by selecting whether or not to set the voltage applied to the conductive polymer actuator to a value equal to or greater than the predetermined voltage. In the case where the inner wall of the hollow portion of the conductive polymer actuator is assumed to be an input-side clutch plate and the region of the driven element which comes in contact with the inner wall is assumed to be an output-side clutch plate, it is possible to assume that a clutch system is achieved by using the conductive polymer actuator and the driven element.

As described hereinabove, the control unit is able to modify the stiffness and the viscosity and to control the transmission state and the non-transmission state only by controlling the voltage applied to the conductive polymer actuator.

In the third aspect of the invention, preferably the first element and the second element each include a conductive polymer actuator formed in a tubular or pillar shape; the first modification unit and the second modification unit are each configured as a voltage application unit which applies a voltage to the conductive polymer actuator; a convex portion is provided on the outer wall of the conductive polymer actuator; and the conductive polymer actuator is configured so as to contract in the longitudinal direction of the conductive polymer actuator in response to the application of the voltage from the voltage application unit and in such a way that the convex portion comes in contact with the driven element in the case where the applied voltage is equal to or greater than a predetermined voltage (Seventh aspect of the invention).

Thereby, similarly to the sixth aspect of the invention, the stiffness and viscosity of the conductive polymer actuator vary in response to the application of the voltage, thereby achieving a simple structure of the power transmission device.

Furthermore, the conductive polymer actuator is configured so as to contract in the longitudinal direction of the tubular or pillar shape in response to the application of the voltage. In the case where the voltage applied to the conductive polymer actuator is equal to or greater than the predetermined voltage, the convex portion comes in contact with the driven element by moving in the longitudinal direction. Thereby, the motive power transmitted to the conductive polymer actuator is transmitted to the driven element (the transmission state in which the motive power is transmitted from the driving element to the driven element is set).

In other words, it is possible to switch between the transmission state and the non-transmission state by selecting whether or not to set the voltage applied to the conductive polymer actuator to a value equal to or greater than the predetermined voltage. In the case where the convex portion of the conductive polymer actuator is assumed to be an input-side clutch plate and the region of the driven element which comes in contact with the convex portion is assumed to be an output-side clutch plate, it is possible to assume that a clutch system is achieved by using the convex portion and the driven element.

As described hereinabove, the control unit is able to modify the stiffness and the viscosity and to control the transmission state and the non-transmission state only by controlling the voltage applied to the conductive polymer actuator.

In the third aspect of the invention, preferably the first element and the second element each include a conductive polymer actuator formed in a tubular or pillar shape; the first modification unit and the second modification unit are each configured as a voltage application unit which applies a voltage to the conductive polymer actuator; the driven element is connected to one end of the conductive polymer actuator; and the conductive polymer actuator is configured so as to contract in the longitudinal direction of the conductive polymer actuator in response to the application of a voltage from the voltage application unit and in such a way that the driven element comes in contact with the driving element in the case where the applied voltage is equal to or greater than a predetermined voltage (Eighth aspect of the invention).

Thereby, similarly to the sixth and seventh aspects of the invention, the stiffness and viscosity of the conductive polymer actuator vary in response to the application of the voltage, thereby achieving a simple structure of the power transmission device.

Furthermore, the conductive polymer actuator is configured so as to contract in the longitudinal direction of the tubular or pillar shape in response to the application of the voltage. In the case where the voltage applied to the conductive polymer actuator is equal to or greater than the predetermined voltage, the driven element comes in contact with the driving element by moving in the longitudinal direction of the conductive polymer actuator formed in the tubular or pillar shape. Thereby, the motive power transmitted to the conductive polymer actuator is transmitted to the driven element (the transmission state in which the motive power is transmitted from the driving element to the driven element is set).

In other words, it is possible to switch between the transmission state and the non-transmission state by selecting whether or not to set the voltage applied to the conductive polymer actuator to a value equal to or greater than the predetermined voltage. In the case where the region of the driven element which comes in contact with the driving element is assumed to be an output-side clutch plate and the region of the driving element which comes in contact with the driven element is assumed to be an input-side clutch plate, it is possible to assume that a clutch system is achieved by using the region of the driving element and the region of the driven element in contact with each other.

As described hereinabove, the control unit is able to modify the stiffness and the viscosity and to control the transmission state and the non-transmission state only by controlling the voltage applied to the conductive polymer actuator.

In the first aspect of the invention, preferably the first modification unit includes a first actuator, the first element is configured as a variable stiffness unit which varies in stiffness in a direction perpendicular to the driving direction by being at least partially pressed in response to driving of the first actuator, the second modification unit includes a second actuator, and the second element is configured as a variable viscosity coefficient unit which varies in viscosity in a direction perpendicular to the driving direction by being at least partially pressed in response to driving of the second actuator (Ninth aspect of the invention). Thus, the first actuator is able to vary the stiffness of the first element and the second actuator is able to vary the viscosity of the second element, thereby enabling the oscillation of the controlled object to be effectively suppressed.

In the ninth aspect of the invention, preferably the first actuator and the second actuator are piezoelectric elements each formed in a tubular or pillar shape, the first element is a nonlinear spring, the second element includes a third element in which a convex portion is provided and a fourth element in which a concave portion along the shape of the convex portion is provided and filled with a viscous liquid, motive power is transmitted from the driving element to one of the third element and the fourth element, motive power is transmitted from the other of the third element and the fourth element to the driven element, and the second element is configured so that driving of the piezoelectric element increases an area in which the convex portion comes in contact with the viscous liquid (Tenth aspect of the invention). Thus, the nonlinear spring is moved by driving the piezoelectric element, thereby enabling the stiffness of the nonlinear spring to be modified. Moreover, the third element is moved by driving the piezoelectric element, thereby bringing the convex portion close to the bottom of the concave portion so as to increase the contact area between the convex portion and the viscous liquid. This enables an increase in the viscosity coefficient of the convex portion and the viscous liquid. The stiffness and the viscosity are able to be modified by driving the piezoelectric elements in this manner, thereby enabling the oscillation of the controlled object to be effectively suppressed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a conceptual diagram of a power transmission device1of the exemplary embodiment. As illustrated inFIG. 1, the power transmission device1mainly includes a motor A2, an input gear G2, an intermediate member4, a driven-side clutch plate6, a controller A4, and an output unit B.

The motor A2is a so-called electric motor, which generates a torque which rotates a motor output shaft A2aconnected to the motor A2about the axis of the motor output shaft A2aby being supplied with electric power from a power supply which is not illustrated. Moreover, the motor A2is connected to a motor encoder A3which detects a rotation angle of the motor A2. A drive gear G1is fixed to the motor output shaft A2a. The drive gear G1rotates along with the motor output shaft A2a.

The input gear G2is configured to engage with the drive gear G1, and the input gear G2rotates by the rotation of the drive gear G1. At this time, the input gear G2rotates while reducing the speed of the rotation of the motor A2. In other words, the drive gear G1and the input gear G2function as speed reducers.

Moreover, the input gear G2is provided with an input-side torque sensor (for example, a strain gauge or the like)5disposed therein, which detects a torque output from the input gear G2.

The intermediate member4includes a variable stiffness unit41, a variable viscosity coefficient unit42, a characteristic modification unit43, a threaded shaft44, a nut45, and a drive-side clutch plate46. In this condition, the variable stiffness unit41is a nonlinear spring whose stiffness varies according to a displacement. Moreover, the variable viscosity coefficient unit42is a nonlinear damper whose viscosity coefficient varies according to a displacement.

Furthermore, the characteristic modification unit43is a drive source which rotates the threaded shaft44according to an applied voltage (hereinafter, referred to as “characteristic modification voltage”). The characteristic modification unit43is supplied with electric power from a power supply (not illustrated) for use in driving the characteristic modification unit43. The nut45moves in the horizontal direction ofFIG. 1along the axial direction of the threaded shaft44by the rotation of the threaded shaft44which is caused by driving the characteristic modification unit43.

At this time, the characteristic modification unit43moves the nut45in the rightward direction if the characteristic modification voltage increases, moves the nut45in the leftward direction if the characteristic modification voltage decreases, and stops the movement of the nut45if the characteristic modification voltage is constant. Unless the characteristic modification voltage is applied to the characteristic modification unit43, the characteristic modification unit43moves the nut45to the leftmost side.

Moreover, the nut45moves in the rightward direction ofFIG. 1, thereby pressing the variable stiffness unit41and the variable viscosity coefficient unit42, by which the variable stiffness unit41and the variable viscosity coefficient unit42are displaced in the rightward direction ofFIG. 1by a distance by which the nut45moves in the rightward direction. Further, the nut45moves in the leftward direction ofFIG. 1, thereby releasing the rightward pressing force and the variable stiffness unit41and the variable viscosity coefficient unit42are displaced in the leftward direction ofFIG. 1by a distance by which the nut45moves in the leftward direction. The above movement of the nut45varies the stiffness of the variable stiffness unit41and the viscosity coefficient of the variable viscosity coefficient unit42. In other words, the stiffness of the variable stiffness unit41and the viscosity coefficient of the variable viscosity coefficient unit42vary according to the characteristic modification voltage.

FIG. 2Aillustrates the voltage characteristic of the variable stiffness unit41with the horizontal axis representing the voltage and the vertical axis representing the stiffness. As illustrated inFIG. 2A, the variable stiffness unit41has a characteristic that the stiffness of the variable stiffness unit41increases with an increase in a voltage applied to the characteristic modification unit43of the intermediate member4. In this situation, the voltage-stiffness characteristic curve has a slope steepened with an increase in the voltage.

FIG. 2Billustrates the voltage characteristic of the variable viscosity coefficient unit42with the horizontal axis representing the voltage and the vertical axis representing the viscosity coefficient. As illustrated inFIG. 2B, the variable viscosity coefficient unit42has a characteristic that the viscosity coefficient of the variable viscosity coefficient unit42increases with an increase in a voltage applied to the characteristic modification unit43of the intermediate member4. In this situation, the voltage-viscosity-coefficient characteristic curve has a slope getting smaller with an increase in the voltage up to a predetermined voltage and maintained constant at the predetermined voltage and higher voltages. InFIGS. 2A and 2B, the stiffness and the viscosity coefficient do not increase at a certain voltage and higher voltages.

As described by using the aforementioned expression (1), the voltage characteristics of the stiffness and the viscosity coefficient are decided so that the damping rate of vibration on the driven side (for example, the output unit B) is constant (note, however, that the damping rate is not strictly constant, but constant at a certain minimum desired or required level for enabling the control processing to be easily performed as described above). Specifically, the characteristics of the stiffness and the viscosity coefficient are decided so that the damping rate is constant for the respective voltage values.

In addition, the controller A4decides the voltage value corresponding to a desired stiffness when acquiring the desired stiffness on the basis of the characteristic as illustrated inFIG. 2A. The characteristics illustrated inFIGS. 2A and 2Bare previously decided according to an experiment or the like (decided at the stage of forming the variable stiffness unit41and the variable viscosity coefficient unit42), and therefore the controller A4only needs to decide the voltage value so as to obtain the desired stiffness, which enables the viscosity coefficient of the variable viscosity coefficient unit42to be modified to a value that makes the damping rate constant.

Here, “to modify the stiffness and the viscosity coefficient according to the characteristics previously determined so that the damping rate is constant” as described above corresponds to “the second modification unit modifies the viscosity according to the stiffness modified by the first modification unit so that damping of vibration of the driven element is predetermined damping” in the present invention.

Here, althoughFIG. 1illustrates the power transmission device as if the variable stiffness unit41applied a force according to the “stiffness” and the “displacement” in the horizontal direction ofFIG. 1and the variable viscosity coefficient unit42applied a force according to the “viscosity coefficient” and the “speed” in the horizontal direction ofFIG. 1, actually each of the variable stiffness unit41and the variable viscosity coefficient unit42applies the force according to the rotational direction of the drive-side clutch plate46(or the driven-side clutch plate6). The reason why the power transmission device is illustrated as inFIG. 1is because the illustration is complicated if the illustration is made in such a way that the force is applied according to the rotational direction of the drive-side clutch plate46, which makes the illustration to be hard to understand. In this manner,FIG. 1is a diagram illustrating the concept of the power transmission device1of the exemplary embodiment in a simplified manner.

The drive-side clutch plate46is connected to the variable stiffness unit41and the variable viscosity coefficient unit42. Thereby, the drive-side clutch plate46rotates about the central axis common to the input gear G2when the input gear G2rotates since a torque output from the input gear G2is transmitted via the variable stiffness unit41and the variable viscosity coefficient unit42. Therefore, the output torque of the input gear G2is transmitted to the drive-side clutch plate46according to the stiffness of the variable stiffness unit41and the viscosity coefficient of the variable viscosity coefficient unit42.

Additionally, a displacement encoder B1is fixed to the nut45. The displacement encoder B1is a distance sensor which detects a distance between the nut45and the drive-side clutch plate46. Specifically, it is possible to detect where the nut45is located on the basis of an output from the displacement encoder B1. Furthermore, it is possible to detect (or estimate) the stiffness of the variable stiffness unit41and the viscosity coefficient of the variable viscosity coefficient unit42according to the position of the nut45.

If the characteristic modification voltage is less than a predetermined voltage V1, the drive-side clutch plate46and the driven-side clutch plate6are not in contact with each other (specifically, the clutches are not connected to each other: hereinafter, this state is referred to as “non-transmission state”). If the characteristic modification voltage is equal to or greater than the predetermined voltage V1, the drive-side clutch plate46and the driven-side clutch plate6are in contact with each other (specifically, the clutches are connected to each other: hereinafter, this state is referred to as “transmission state”). Hereinafter, when both of the drive-side clutch plate46and the driven-side clutch plate6are specified, terms “clutches6and46” may be used in some cases.

A surface where the drive-side clutch plate46and the driven-side clutch plate6come in contact with each other is formed so as to generate a great frictional force. Accordingly, when the drive-side clutch plate46and the driven-side clutch plate6are in contact with each other, the torque generated at the time of rotation of the drive-side clutch plate46is transmitted to the driven-side clutch plate6, by which the driven-side clutch plate6rotates about the central axis common to the drive-side clutch plate46.

Additionally, the driven-side clutch plate6is provided with an output-side torque sensor (for example, a strain gauge or the like)7disposed therein, which detects the torque transmitted to the driven-side clutch plate6.

Moreover, the output unit B, which is connected to a load, is connected to the driven-side clutch plate6. As the load, various loads may be used according to the uses or the like of the power transmission device1. For example, if the power transmission device1is an arm composed of two links for use in lifting up various objects and the intermediate member4is a joint disposed between the links, the output unit B is a link on the driven side and the load is an object lifted up by the link.

Since the power transmission device1is configured as described above, the clutches6and46are placed in the transmission state when a characteristic modification voltage equal to or greater than the predetermined voltage V1is applied to the characteristic modification unit43of the intermediate member4. Therefore, the torque output from the motor A2is transmitted to the driven-side clutch plate6and by extension to the output unit B via the intermediate member4. On the other hand, if the voltage applied to the characteristic modification unit43of the intermediate member4is zero or the applied characteristic modification voltage is less than the predetermined voltage V1, the clutches6and46are placed in the non-transmission state, and therefore the torque output from the motor A2is not transmitted to the driven-side clutch plate6and by extension to the output unit B.

The above description, “the transmission state and the non-transmission state are able to be switched to each other according to whether the characteristic modification voltage, which modifies the stiffness and the viscosity coefficient, applied to the characteristic modification unit43is equal to or greater than the predetermined voltage V1” corresponds to “the power transmission device is configured to be able to switch between a transmission state in which the motive power is transmitted to the driven element and a non-transmission state in which the transmission is disconnected by modifying the stiffness of the first element or the viscosity coefficient of the second element” in the present invention.

In addition,FIGS. 2A and 2Bdo not illustrate the case where the characteristic modification voltage is less than the predetermined voltage V1regarding the characteristics of the stiffness and the viscosity coefficient because the clutches6and46are placed in the non-transmission state at voltages less than the predetermined voltage V1. Specifically, in the non-transmission state, the stiffness and the viscosity coefficient of the intermediate member, which is disposed on the power transmission path between the drive side and the driven side, do not make sense and therefore are not illustrated. Actually, even if the characteristic modification voltage is less than the predetermined voltage V1, the characteristics of the stiffness and the viscosity coefficient are previously defined.

The controller A4includes “one or a plurality of electronic circuits which perform arithmetic processing of a central processing unit and the like” and “one or a plurality of storage devices composed of a ROM, a RAM, and the like.” Moreover, the controller A4receives outputs from the motor encoder A3, the displacement encoder B1, the input-side torque sensor5, and the output-side torque sensor7. The controller A4controls the motor A2and the intermediate member4(the characteristic modification unit43inFIG. 1) on the basis of the received information.

More specifically, the controller A4supplies the motor A2with electric current appropriate to a torque command value at the present time (in the present control period) so that the torque output from the motor A2becomes a target torque. Moreover, the controller A4applies a voltage appropriate to a stiffness command value (i.e., the characteristic modification voltage) at the present time (in the present control period) to the intermediate member4so that the stiffness of the intermediate member4becomes a target stiffness.

Moreover, the controller A4also has functions of a first determination unit A41which determines whether the motor A2is able to be normally controlled and a second determination unit A42which determines whether the transmission state is set.

Here, the variable stiffness unit41corresponds to “the first element” in the present invention, the variable viscosity coefficient unit42corresponds to “the second element” in the present invention, and the controller A4corresponds to “the first modification unit,” “the second modification unit,” “the voltage application unit,” and “the control unit” in the present invention. Additionally, the motor A2corresponds to “the driving element” and “the drive source” in the present invention.

Subsequently, the control processing performed by the controller A4will be described with reference toFIG. 3. The controller A4performs the control processing ofFIG. 3at predetermined intervals (of 10 [ms], for example). The controller A4, first, acquires the state of the power transmission device1in the first step ST1. Here, as the state of the power transmission device1, there are a normal state and an abnormal state. The normal state means a state where all of “sensors and the like such as the motor encoder A3, the displacement encoder B1, the input-side torque sensor5, and the output-side torque sensor7” normally operate. The abnormal state means a state where the sensors and the like abnormally operate.

Whether there is abnormality in the sensors and the like is determined according to whether the values detected by the sensors and the like are outside a predetermined range (in other words, whether the values seem to be not detected in the normal operation). Such values are previously decided according to an experiment or the like and are stored in the storage device of the controller A4.

If any one of the sensors and the like has an abnormality and if information acquired by the sensor having the abnormality is able to be estimated from information detected by a normal sensor, the state may be considered to be normal. In this case, an estimated value is used, instead of the output from the sensor having the abnormality.

For example, if the input-side torque sensor5has an abnormality, it is possible to estimate a torque output from the input gear G2from the electric current used when the electric power is supplied to the motor A2. For “the electric current supplied to the motor A2,” a value obtained from a current sensor (not illustrated) is used. More specifically, first, “the output torque of the motor A2” is obtained by multiplying “a torque constant (the coefficient of an output torque to the supplied electric current) defined by the motor A2” by “the electric current supplied to the motor A2.”

Then, “the torque transmitted to the input gear G2” is obtained by multiplying “the obtained output torque of the motor A2” by “a reduction ratio defined by the drive gear G1and the input gear G2.” Furthermore, “the torque output from the input gear G2” is obtained by subtracting “a frictional force generated when the teeth on the drive gear G1engage with the teeth on the input gear G2” from “the obtained torque transmitted to the input gear G2.”

Moreover, if the output-side torque sensor7has an abnormality, it is possible to estimate a torque transmitted to the driven-side clutch plate6from respective outputs of the input-side torque sensor5and the displacement encoder B1. More specifically, first, a shear stress of the variable viscosity coefficient unit42is obtained by multiplying “the viscosity coefficient of the variable viscosity coefficient unit42” by “a variation per unit time of the output from the displacement encoder B1.” Here, the term “a variation per unit time” means a difference between “the output from the displacement encoder B1in the present control period” and “the output from the displacement encoder B1in the previous control period.”

It is, then, possible to estimate the torque transmitted to the driven-side clutch plate6by subtracting the shear stress of the variable viscosity coefficient unit42obtained in the above from “the torque output from the input gear G2and output from the input-side torque sensor5.” If both of the output-side torque sensor7and the input-side torque sensor5have an abnormality when the torque transmitted to the driven-side clutch plate6is estimated, “the torque output from the input gear G2” may be estimated from “the electric current supplied to the motor A2” as described above. In this case, the torque transmitted to the driven-side clutch plate6is estimated from “the electric current supplied to the motor A2” and “the output from the displacement encoder B1.”

Moreover, as another method of estimating the torque transmitted to the driven-side clutch plate6, the following method is conceivable. First, the controller A4calculates “stress accumulated in the variable stiffness unit41” by multiplying “the stiffness of the variable stiffness unit41” by “the output from the displacement encoder B1.” Then, the controller A4estimates “the torque transmitted to the driven-side clutch plate6” by adding “the shear stress of the variable viscosity coefficient unit42” obtained as described above to “the calculated stress.”

The estimation of the torque transmitted to the driven-side clutch plate6as described above is performed only in a state where the drive-side clutch plate46is in contact with the driven-side clutch plate6and the two clutch plates46and6do not vary in the relative rotation angle (i.e., a state of no occurrence of slip), and thus the estimation is not performed in a state where slip occurs between the two clutch plates46and6(a state of so-called half-clutch).

In this manner, the controller A4is able to estimate the torque output from the input gear G2and the torque transmitted to the driven-side clutch plate6, and therefore the power transmission device1does not need to be provided with the input-side torque sensor5and the output-side torque sensor7.

Subsequently, the controller A4proceeds to step ST2to determine whether the state acquired in step ST1is the abnormal state. If the state is determined to be abnormal, the control or the like of the motor A2might not be able to be appropriately performed. Accordingly, the controller A4proceeds to step ST3to set the electric current or voltage supplied to the motor A2and the characteristic modification unit43of the intermediate member4to zero. Here, the processing of step ST2corresponds to the processing performed by “the first determination unit” in the present invention.

Thereby, the motor A2stops the driving and the voltage applied to the characteristic modification unit43of the intermediate member4becomes zero, by which the clutches6and46are placed in the non-transmission state. Therefore, even if the torque is transmitted to the driven-side clutch plate6before the processing of step ST3is performed, the clutches6and46are placed in the non-transmission state by performing the processing of step ST3. Specifically, the mechanical connection is closed, thereby causing a state where the torque output from the motor A2is not transmitted to the output unit B side (that is, the transmission of the torque is mechanically disconnected). Therefore, it is possible to prevent the power transmission device1from performing an unexpected operation which is caused by unexpected values of the voltages supplied to the motor A2and the intermediate member4due to a control or the like based on the information detected by a sensor which does not operate normally and therefore to improve the safety of the power transmission device1. After the end of the processing of step ST3, this control processing terminates.

Here, the processing of step ST3corresponds to “perform a control to achieve the non-transmission state in the case where the determination result of the first determination unit is negative” in the present invention.

If the state is determined to be normal in step ST2, the controller A4proceeds to step ST4to acquire a stiffness command value of the variable stiffness unit41of the intermediate member4, which is decided by processing (not illustrated) of the controller A4. The stiffness command value of the variable stiffness unit41of the intermediate member4is a value for use in controlling the stiffness of the variable stiffness unit41of the intermediate member4to be the value concerned. The stiffness command value is appropriately decided by the controller A4according to the operation of the power transmission device1. For example, if a joint is required to be stiff such as a case of accurately driving the arm of the power transmission device1, the controller A4sets the stiffness command value to a large value in order to increase the stiffness of the power transmission device1. In addition, if a joint is required to be flexible for the reason such as softening the effects of an unintended impact applied to the arm or the like of the power transmission device1, the controller A4sets the stiffness command value to a smaller value.

Upon the completion of the processing of step ST4, the controller A4proceeds to steps ST5and ST6. Specifically, in order to execute steps ST5and ST6in parallel, the controller A4branches the thread, executes step ST5(or ST6) in the branched thread, and executes step ST6(or ST5) in the thread which has run before the branching. Thereby, steps ST5and ST6are executed in parallel.

In step ST5, the controller A4decides a voltage applied to the characteristic modification unit43of the intermediate member4so that the stiffness of the variable stiffness unit41of the intermediate member4is equal to the stiffness command value. In this decision, the voltage applied to the characteristic modification unit43is decided by acquiring the voltage corresponding to the stiffness command value according to the “voltage-stiffness” map as illustrated inFIG. 2A. Upon the completion of the processing of step ST5, a control signal is output so that the voltage acquired in step ST5is applied to the characteristic modification unit43of the intermediate member4and this control processing ends in the thread where the processing of step ST5has been performed.

In step ST6, the controller A4estimates the stiffness of the variable stiffness unit41of the intermediate member4and the viscosity coefficient of the variable viscosity coefficient unit42. The controller A4estimates the stiffness and the viscosity coefficient by acquiring the stiffness of the variable stiffness unit41and the viscosity coefficient of the variable viscosity coefficient unit42corresponding to the voltage applied to the characteristic modification unit43of the intermediate member4at the present time according to the “voltage-stiffness” map and the “voltage-viscosity coefficient” map as illustrated inFIGS. 2A and 2B.

Upon the completion of the processing of step ST6, the controller A4proceeds to step ST7to determine whether the clutches6and46are in the transmission state. In the driven-side clutch plate6, a torque occurs due to the transmission state of the clutches6and46. On the other hand, if the clutches6and46are in the non-transmission state, the torque transmitted to the drive-side clutch plate46is not transmitted to the driven-side clutch plate6and therefore the torque detected by the output-side torque sensor7is zero. In this situation, the output from the output-side torque sensor7might be a value greater than zero due to a measurement error or the like of the output-side torque sensor7.

Therefore, in this step ST7, the controller A4determines that the clutches6and46are in the transmission state if the torque detected by the output-side torque sensor7is greater than a predetermined value (the predetermined value is zero or a value greater than zero with consideration for the above error) and determines that the clutches6and46are in the non-transmission state if the torque is equal to or smaller than the predetermined value. The predetermined value is previously decided according to an experiment or the like and stored in the storage device of the controller A4. Here, the processing of step ST7corresponds to “the second determination unit” in the present invention.

If it is determined that the clutches6and46are not connected to each other in step ST7, the controller A4proceeds to step ST8to perform a rotation follow-up control. In the rotation follow-up control, the motor A2is controlled so that a difference between the rotation angle of the driven-side clutch plate6and the rotation angle of the drive-side clutch plate46is equal to or less than a predetermined value. This enables an immediate change to the transmission state at the time of a change of the clutches6and46from the non-transmission state to the transmission state.

Usually, when the clutches6and46are connected to each other, it is necessary to place the clutches6and46in the transmission state within a range of a difference in a relative displacement between the previously-defined driven element and driving element. By previously performing the above rotation follow-up control, the controller A4is able to set the transmission state immediately after determining that the transmission state is to be made and by extension to reduce the time for the control processing.

The rotation follow-up control is performed, for example, in the case of restarting the driving of the output unit B after temporarily interrupting the driving of the output unit B for some reason during driving of the output unit B with the motor A2.

Here, the processing of step ST8corresponds to “control the drive source so that a difference between a displacement of the driven element and a displacement of the driving element is equal to or less than a predetermined value in the case where the determination result of the first determination unit is affirmative and the determination result of the second determination unit is negative” in the present invention.

If it is determined that the clutches are connected to each other in step ST7, the controller A4proceeds to step ST9to perform a feedback torque control. Here, the feedback torque control is to control an output torque of the motor A2so that the torque transmitted to the output unit B reaches a target torque decided by processing (not illustrated) of the controller A4.

Well-known various control processes are applicable to the feedback torque control. For example, Japanese Patent Application Laid-Open No. 2011-115878 describes a technique of controlling a driving element in the case where an elastic element is disposed between the driving element (the motor A2) and the driven element (the output unit B). In step ST9, for example, a control described in this gazette is performed.

Upon the completion of the processing of step ST8or step ST9, the controller A4ends this control processing.

Here, the processing of step ST9corresponds to “control the motive power transmitted to the driven element in the case where the determination result of the first determination unit is affirmative and the determination result of the second determination unit is affirmative” in the present invention.

In addition, it is possible to determine whether the clutches6and46are in the transmission state from the voltage applied to the characteristic modification unit43of the intermediate member4. The drive-side clutch plate46, however, might not be in contact with the driven-side clutch plate6(in other words, the non-transmission state is set), even in the case where the voltage applied to the characteristic modification unit43of the intermediate member4is equal to or greater than the predetermined voltage V1, due to an individual difference in the intermediate member4or ambient surrounding (for example, temperature, etc.).

For such a case, if the voltage applied to the intermediate member4is equal to or greater than the predetermined voltage V1when it is determined that the clutches6and46are not placed in the transmission state in the processing of step ST7, the predetermined voltage V1may be corrected (hereinafter, this correction is referred to as “correction processing”). More specifically, in the correction processing, the controller A4sets the voltage applied to the intermediate member4at the time when the clutches6and46shift from the non-transmission state to the transmission state to a new predetermined voltage V1.

As described above, the controller A4controls the motor A2and the intermediate member4.

As described hereinabove, in the power transmission device1of the exemplary embodiment, the torque of the motor A2is transmitted to the output unit B via the intermediate member4. Specifically, the power transmission device1is provided with the variable viscosity coefficient unit42having viscosity as a power transmission element on the power transmission path between the motor A2and the output unit B, thereby enabling the vibration of the output unit B to be mechanically converged at the time of controlling the output unit B. Moreover, the stiffness of the variable stiffness unit41is able to be modified by the control of the controller A4, thereby enabling the output unit B to be flexibly controlled as the situation demands.

Moreover, the controller A4modifies the stiffness and the viscosity coefficient according to the voltage characteristics as illustrated inFIGS. 2A and 2Band therefore is able to make the damping rate of the vibration of the output unit B constant. This enables the controller A4to easily predict the vibration of the output unit B in the control period at the present time from the vibration of the output unit B in the previous control period, which makes the control processing easy. In this manner, a power transmission element whose stiffness and viscosity coefficient are variable is used and the stiffness and the viscosity coefficient are appropriately modified, thereby enabling the oscillation of the controlled object to be effectively suppressed.

First Embodiment

Subsequently, specific embodiments of the power transmission device of the exemplary embodiment will be described. A first embodiment is described, first.

FIGS. 4A,4B, and4C are diagrams illustrating a power transmission device11of the first embodiment. The power transmission device11includes a motor A2, a controller A4, a speed reducer2, an outer frame3, an intermediate member401, an input-side torque sensor5, an output-side torque sensor7, and an output unit B. The motor A2is an electric motor which outputs a torque rotating a motor output shaft A2aby being supplied with electric power in the same manner as the description of the exemplary embodiment.

For the speed reducer2, for example, a Harmonic Drive® or the like is used. In the speed reducer2, there is disposed an input-side torque sensor5formed in a substantially columnar shape. The input-side torque sensor5is a torque sensor with a strain gauge. The input-side torque sensor5detects a torque which is output from the speed reducer2according to a strain of the input-side torque sensor5and outputs an electrical signal appropriate to the magnitude of the torque.

The input-side torque sensor5is connected to the outer frame3. The outer frame3is formed in a substantially cylindrical shape, with a substantially cylindrical intermediate member401fixed to a hollow portion thereof. The intermediate member401may be formed in other shapes. For example, the shape may be a quadrangular prism or the like with a through-hole provided in the longitudinal direction (at this time, the outer frame3is formed in such a way that the inner wall of the outer frame3comes in contact with the outer wall of the intermediate member401). In this specification, it is assumed that the “tubular shape” includes this kind of shape as well as a circular cylindrical shape.

The intermediate member401is formed so as to be reduced in the space of the hollow portion in a state where a voltage is applied. More specifically, as illustrated inFIGS. 4B and 4C, the hollow portion of the intermediate member401has a small diameter in a state where a voltage is applied by the control of the controller A4(FIG. 4C), in comparison with a state where the voltage is not applied (FIG. 4B). Here, the controller A4corresponds to “the voltage application unit” in the present invention.

The reduction in space of the hollow portion is achieved by forming the intermediate member401as exemplified inFIGS. 5A and 5B.FIG. 5Aillustrates a state where the voltage is not applied to the intermediate member401, andFIG. 5Billustrates a state where the voltage is applied to the intermediate member401. As illustrated inFIG. 5A, the intermediate member401includes an anode P, a cathode M, and a conductive polymer actuator E. These are laid in the order of “the cathode M→the conductive polymer actuator E→the anode P” (hereinafter, this stacked structure is collectively referred to as “stack M, E, P”). If a potential difference occurs between the anode P and the cathode M, the stack M, E, P expands on the anode P side of the conductive polymer actuator E and contracts on the cathode M side of the conductive polymer actuator E.

The above stack M, E, P is formed so as to incline from the cathode M side toward the anode P side in a direction from the radial outside to the radial inside. Thereby, if a potential difference occurs between the anode P and the cathode M, the stack M, E, P contracts on the cathode M side and expands on the anode P side as illustrated inFIG. 5B, and therefore the inclination of the stack M, E, P becomes gentler (where the inclination angle is an angle between “the radial direction of the intermediate member401” and “the direction perpendicular to the stacking direction of the stack M, E, P”). Thus, the diameter of the hollow portion of the stack M, E, P inFIG. 5Bis smaller than the diameter (indicated by a broken line inFIG. 5B) of the hollow portion of the stack M, E, P inFIG. 5A.

At this time, as the potential difference between the anode P and the cathode M is greater, the inclination becomes gentler (the angle of the inclination becomes smaller). Additionally, as the inclination is gentler, the diameter of the hollow portion of the stack M, E, P (and by extension the intermediate member401) becomes smaller.

Moreover, the conductive polymer actuator E varies in the stiffness and the viscosity coefficient by the application of a voltage. The conductive polymer actuator E used for the intermediate member401of the first embodiment has the characteristics of the stiffness and the viscosity coefficient to the voltage as illustrated inFIGS. 2A and 2B.

In the hollow portion of the intermediate member401, an output-side torque sensor7formed in a substantially columnar shape is disposed. The output-side torque sensor7, which is a torque sensor with a strain gauge in the same manner as the input-side torque sensor5, detects a torque transmitted to the output-side torque sensor7according to the strain and outputs an electrical signal appropriate to the magnitude of the torque.

The output-side torque sensor7is formed in such a size that the wall surface (hereinafter, referred to as “output-side wall surface”)7aof the output-side torque sensor7does not come in contact with the wall surface (hereinafter, referred to as “intermediate member wall surface”)401aof the hollow portion of the intermediate member401when the voltage is not applied to the intermediate member401and is formed in such a size that the output-side wall surface7acomes in contact with the intermediate member wall surface401awhen the voltage applied to the intermediate member401is equal to or greater than the predetermined voltage V1. In addition, the “intermediate member wall surface4a” and the “output-side wall surface7a” are formed so that a large frictional force occurs when these are in contact with each other.

Moreover, the output unit B to which a load is connected is fixed to the output-side torque sensor7.

Due to the above configuration, the intermediate member wall surface401acomes in contact with the output-side wall surface7awhen a voltage equal to or greater than the predetermined voltage V1is applied to the intermediate member401. Thereby, the torque output from the motor A2is transmitted to the output unit B via the intermediate member401. At this time, the motor A2, the intermediate member401, and the output unit B rotate about the central axis common thereto. On the other hand, when no voltage is applied to the intermediate member401or a voltage less than the predetermined voltage V1is applied to the intermediate member401, the intermediate member wall surface401ais not in contact with the output-side wall surface7aand therefore the torque output from the motor A2is not transmitted to the output unit B.

In this manner, whether the torque is transmitted to the output unit B is decided according to whether or not the voltage equal to or greater than the predetermined voltage V1is applied to the intermediate member401. Specifically, it is possible to consider that a clutch system is formed by the intermediate member wall surface401aand the output-side wall surface7a.

Here, the intermediate member401of the first embodiment corresponds to the intermediate member4of the exemplary embodiment, and the conductive polymer actuator E corresponds to the variable stiffness unit41and the variable viscosity coefficient unit42of the exemplary embodiment. In addition, the intermediate member wall surface401aof the first embodiment corresponds to the drive-side clutch plate46of the exemplary embodiment. Moreover, the output-side wall surface7aof the first embodiment corresponds to the driven-side clutch plate6of the exemplary embodiment.

Furthermore, in the output unit B, there is disposed an encoder B11for use in detecting a rotation angle relative to the outer frame3. Thereby, the encoder B11detects a relative displacement (rotation angle) of the outer frame3relative to the output-side wall surface7a(and by extension the output unit B). Accordingly, when the intermediate member wall surface401ais in contact with the output-side wall surface7a, the relative displacement therebetween is zero, and therefore the displacement detected by the encoder B11indicates a displacement in the twist direction of the intermediate member401appropriate to the torque transmitted to the intermediate member401. Here, the encoder B11of the first embodiment corresponds to the displacement encoder B1of the exemplary embodiment.

As described above, the power transmission device1of this embodiment uses the conductive polymer actuator as the intermediate member401and therefore is able to modify the stiffness and the viscosity coefficient thereof.

Second Embodiment

Subsequently, a power transmission device12of a second embodiment of the present invention will be described with reference toFIGS. 6A,6B, and6C. The power transmission device12of the second embodiment differs from the power transmission device11of the first embodiment in the configurations of the intermediate member and the output side. In this embodiment, as illustrated inFIGS. 6A,6B, and6C, an intermediate member402is composed of a conductive polymer actuator and is formed in a substantially columnar shape. Therefore, the conductive polymer actuator varies in the stiffness and the viscosity coefficient by the application of a voltage. The intermediate member402may be formed in, for example, a pillar or tubular shape or the like as well as a substantially columnar shape.

Moreover, an input-side clutch plate462which functions as a clutch plate is fixed to the left-hand columnar end portion of the intermediate member402. The input-side clutch plate462is formed so as to have a diameter larger than the diameter of the intermediate member402. Here, the input-side clutch plate462corresponds to the “convex portion” in the present invention.

The intermediate member402is configured as exemplified inFIG. 7A. The intermediate member402is formed by stacking a plurality of stacks M, E, P each of which is composed of a cathode M, a conductive polymer actuator E, and an anode P laid in the order of “the cathode M→the conductive polymer actuator E→the anode P” via an insulating layer I. At this time, these are stacked along the columnar axial direction of the intermediate member402. Thereby, in the case where a potential difference occurs between the cathode M and the anode P, the intermediate member402gets shorter in the stacking direction (axial direction) due to the expansion and contraction of the conductive polymer actuator E as exemplified inFIG. 7B, in comparison with the case where the potential difference between the cathode M and the anode P is zero. At this time, the intermediate member402gets shorter in the stacking direction (axial direction) as the potential difference increases.

Therefore, the intermediate member402expands and contracts in the stacking direction according to whether or not a voltage is applied as illustrated inFIGS. 6A,6B, and6C.FIG. 6Billustrates a state where the voltage is not applied to the intermediate member402andFIG. 6Cillustrates a state where the voltage is applied to the intermediate member402. As illustrated in these diagrams, the intermediate member402is configured so as to contract in the columnar axial direction (the horizontal direction ofFIGS. 6A,6B, and6C) by the application of the voltage.

Moreover, the power transmission device12has an output-side torque sensor72in which a hole larger than the diameter of the intermediate member402is provided in the central portion. The intermediate member402is disposed in the hole of the output-side torque sensor72. At this time, the input-side clutch plate462is disposed so as to be located to the left of the output-side torque sensor72inFIGS. 6A,6B, and6C. Further, an output unit B to which a load is connected is fixed to the output-side torque sensor72.

Moreover, the surface of the input-side clutch plate462, particularly a right-hand surface inFIGS. 6A,6B, and6C (hereinafter, referred to as “input-side clutch plate friction surface”)462a, and a surface of the output-side torque sensor72, particularly a left-hand surface inFIGS. 6A,6B, and6C (hereinafter, referred to as “output-side friction surface”)72a, are configured so as to generate a large frictional force when these surfaces come in contact with each other. In other words, the output-side torque sensor72functions also as a driven-side clutch plate.

In addition, when a voltage equal to or greater than the predetermined voltage V1is applied to the intermediate member402, the intermediate member402contracts in the axial direction, by which the input-side clutch plate friction surface462acomes in contact with the output-side friction surface72a. Moreover, if the voltage applied to the intermediate member402is less than the predetermined voltage V1, the intermediate member402expands in the axial direction, by which the input-side clutch plate friction surface462aseparates from the output-side friction surface72a.

Since the power transmission device12is configured as described above, it is possible to select whether to transmit the motive power of the motor A2as a driving element to the output unit B as a driven element by adjusting the voltage applied to the intermediate member402.

Here, the intermediate member402of the second embodiment corresponds to the intermediate member4of the exemplary embodiment and the conductive polymer actuator E corresponds to the variable stiffness unit41and the variable viscosity coefficient unit42of the exemplary embodiment. Further, the input-side clutch plate462of the second embodiment corresponds to the drive-side clutch plate46of the exemplary embodiment. Moreover, the output-side friction surface72aof the second embodiment corresponds to the driven-side clutch plate6of the exemplary embodiment.

Furthermore, in the output-side torque sensor72, there is disposed an encoder B12for use in detecting a rotation angle relative to the input-side torque sensor5. Thereby, the encoder B12detects a relative displacement (rotation angle) of the input-side torque sensor5relative to the output-side torque sensor72(and by extension the output unit B). Accordingly, when the input-side clutch plate friction surface462ais in contact with the output-side friction surface72a, a relative displacement therebetween is zero. Therefore, the displacement detected by the encoder B12indicates a displacement in a twist direction of the intermediate member402appropriate to the torque transmitted to the intermediate member402. Here, the encoder B12of the second embodiment corresponds to the displacement encoder B1of the exemplary embodiment.

Third Embodiment

Subsequently, a power transmission device13of a third embodiment of the present invention will be described with reference toFIGS. 8A,8B, and8C. The power transmission device13of the third embodiment is the same in the configurations of a motor A2, a motor output shaft A2a, a motor encoder A3, and a speed reducer2as the power transmission device12of the second embodiment.

In the power transmission device13of the third embodiment, a columnar input-side torque sensor5is fixed to the speed reducer2. Additionally, a disk-like input-side clutch plate463is fixed to an end portion opposite to the end portion to which the speed reducer2of the input-side torque sensor5is fixed. The input-side clutch plate463is formed so as to have a diameter larger than the diameter of the input-side torque sensor5.

Moreover, the power transmission device13has a substantially cylindrical output-side torque sensor73. The diameter of the hollow portion of the output-side torque sensor73is larger than the diameter of the input-side torque sensor5and smaller than the diameter of the input-side clutch plate463. Then, the input-side torque sensor5is inserted and disposed into the hole of the output-side torque sensor73. At this time, the input-side clutch plate463is disposed so as to be located to the left of the output-side torque sensor73inFIGS. 8A,8B, and8C.

Furthermore, one end of each of three intermediate members403, which is formed in a substantially columnar shape, is fixed to the output-side torque sensor73. The other end of each of the three intermediate members403is fixed to the output unit B to which a load is connected. Each of the three intermediate members403is configured so as to expand and contract in the axial direction when a voltage is applied in the same manner as the intermediate member402of the second embodiment. The number of intermediate members403may be any other number as well as three. In addition, the intermediate member403may be formed in other shapes such as other pillar or tubular shapes as well as the columnar shape.

Moreover, the surface of the input-side clutch plate463, particularly a right-hand surface inFIGS. 8A,8B, and8C (hereinafter, referred to as “input-side clutch plate friction surface”)463a, and a surface of the output-side torque sensor73, particularly a left-hand surface inFIGS. 8A,8B, and8C (hereinafter, referred to as “output-side friction surface”)73a, are configured so as to generate a large frictional force when these surfaces come in contact with each other. In other words, the output-side torque sensor73functions also as a driven-side clutch plate.

In addition, when a voltage equal to or greater than the predetermined voltage V1is applied to the intermediate member403, the intermediate member403contracts in the axial direction, by which the input-side clutch plate friction surface463acomes in contact with the output-side friction surface73a. Moreover, if the voltage applied to the intermediate member403is less than the predetermined voltage V1, the intermediate member403expands in the axial direction as illustrated inFIG. 8B, by which the input-side clutch plate friction surface463aseparates from the output-side friction surface73a.

Since the power transmission device13is configured as described above, it is possible to select whether to transmit the motive power of the motor A2as a driving element to the output unit B as a driven element by adjusting the voltage applied to the intermediate member403.

Here, the intermediate member403of the third embodiment corresponds to the intermediate member4of the exemplary embodiment and the conductive polymer actuator E corresponds to the variable stiffness unit41and the variable viscosity coefficient unit42of the exemplary embodiment. Further, the input-side clutch plate463of the third embodiment corresponds to the drive-side clutch plate46of the exemplary embodiment. Moreover, the output-side friction surface73aof the third embodiment corresponds to the driven-side clutch plate6of the exemplary embodiment.

Furthermore, in the output-side torque sensor73, there is disposed an encoder B13for use in detecting a rotation angle relative to the input-side torque sensor5. Thereby, the encoder B13detects a relative displacement (rotation angle) of the input-side torque sensor5relative to the output-side torque sensor73(and by extension the output unit B). Accordingly, when the input-side clutch plate friction surface463ais in contact with the output-side friction surface73a, a relative displacement therebetween is zero. Therefore, the displacement detected by the encoder B13indicates a displacement in a twist direction of the intermediate member403appropriate to the torque transmitted to the intermediate member403. Here, the encoder B13of the third embodiment corresponds to the displacement encoder B1of the exemplary embodiment.

Fourth Embodiment

Subsequently, a power transmission device14of a fourth embodiment of the present invention will be described with reference toFIGS. 9A,9B,10A,10B, and10C. The power transmission device14of the fourth embodiment is the same in the configurations of a motor A2, a motor output shaft A2a, a motor encoder A3, and a speed reducer2as the power transmission device11of the first embodiment.

As illustrated inFIG. 9A, the input-side torque sensor54is formed in a substantially columnar shape. The speed reducer2is fixed to one end of the input-side torque sensor54, and three nonlinear springs414are fixed to the other end of the input-side torque sensor54. The number of the nonlinear springs414is not limited to three, but may be any other number. Furthermore, as illustrated inFIG. 10A, a concave portion541, which has a circular shape when viewed along the normal line direction to the surface of the input-side torque sensor54, is provided at the other end of the input-side torque sensor54. In addition, the concave portion541is filled with grease542as a viscous liquid.

An input-side clutch plate464formed in a disk shape is fixed to an end portion opposite to the end portion to which the input-side torque sensor54of the three nonlinear springs414are connected. Further, a substantially columnar output-side torque sensor74is disposed in a state where an output-side friction surface74a, which is one surface of the output-side torque sensor74, faces the surface opposite to the surface to which the nonlinear springs414of the input-side clutch plate464are fixed (hereinafter, the surface is referred to as “input-side clutch plate friction surface”)464a. At this time, the input-side clutch plate friction surface464aand the output-side friction surface74aare disposed apart from each other.

One end of each of three piezoelectric elements434, which is formed in a substantially pillar shape, is connected to the surface opposite to the output-side friction surface74aof the output-side torque sensor74. Moreover, the other end of each of the three piezoelectric elements434is connected to the output unit B. The number of the piezoelectric elements434is not limited to three, but may be any other number. In addition, the piezoelectric element may have any other various shapes such as a pillar or tubular shape or the like.

The piezoelectric element434has a piezoelectric body Z, which deforms when a voltage is applied thereto, with the piezoelectric body Z sandwiched between a cathode M and an anode P so as to be stacked. In the second and third embodiments, the stack M, E, P is formed in such a way that the conductive polymer actuator E is sandwiched between the cathode M and the anode P. In the fourth embodiment, however, there is used a stack M, Z, P in which the conductive polymer actuator E of the stack M, E, P is replaced with the piezoelectric body Z. Further, the piezoelectric element434is formed by stacking a plurality of stacks M, Z, P in the fourth embodiment, in the same manner as in the second and third embodiments.

The stack M, Z, P in the fourth embodiment configured as described above gets longer in the stacking direction since the piezoelectric body Z gets longer in the stacking direction when a potential difference occurs between the cathode M and the anode P. Therefore, the piezoelectric element434gets longer in the columnar axial direction of the piezoelectric element434(expands in the rightward direction ofFIG. 9AandFIG. 9B) by applying a voltage to the piezoelectric element434(more specifically, by generating a potential difference between the cathode M and the anode P constituting the piezoelectric element434). At this time, the piezoelectric element434gets longer as the potential difference increases.

The application of a voltage to the piezoelectric element434decreases the distance between the input-side clutch plate friction surface464aand the output-side friction surface74a, which have been disposed apart from each other. Then, when the voltage applied to the piezoelectric element434becomes equal to or greater than the predetermined voltage V1, the input-side clutch plate friction surface464acomes in contact with the output-side friction surface74aas illustrated inFIG. 9B. In this state, a transmission state is achieved in which the torque transmitted to the input-side clutch plate464is transmitted to the output-side torque sensor74. Since the power transmission device14is configured as described above, it is possible to select whether to transmit the motive power of the motor A2as a driving element to the output unit B as a driven element by adjusting the voltage applied to the piezoelectric element434.

Moreover, if the voltage applied to the piezoelectric element434becomes greater than the predetermined voltage V1, the nonlinear spring414is biased in the rightward direction ofFIG. 9AandFIG. 9B. Thereby, the stiffness of the nonlinear spring414increases. The nonlinear spring414used in the fourth embodiment is configured so as to vary in the stiffness in the twist direction (the rotational direction of the motor output shaft A2a) by being displaced in the pressing direction (the rightward direction ofFIG. 9AandFIG. 9B). Therefore, the nonlinear spring414varies in the stiffness to the output torque of the motor A2transmitted to the input-side torque sensor54by being biased by the piezoelectric element434.

Moreover, on the surface opposite to the input-side clutch plate friction surface464aof the input-side clutch plate464, there is provided a convex portion464bprojecting toward the input-side torque sensor54. The convex portion464bis formed in an arc shape in which a part of the radial cross section of the cylindrical shape missing. The tip of the convex portion464bis inserted into a concave portion541.

The concave portion541is, as described above, formed in a circular shape when viewed along the normal line direction of the surface of the input-side torque sensor54. In this condition, the central point of the circular shape of the concave portion541is identical with the central point of the rotating shaft of the input-side torque sensor54. Thereby, even in the case of an occurrence of a change in the relative angle between the input-side torque sensor54and the input-side clutch plate464, it is possible to prevent the concave portion541and the convex portion464bfrom interfering with each other in the rotational direction.

The convex portion464bdoes not need to have a shape with a part of the cylindrical shape missing, but may be formed in a “cylindrical shape (the radial cross section is ring-shaped).” Moreover, even in the case of an occurrence of a change in the relative angle between the input-side torque sensor54and the input-side clutch plate464, the convex portion464bmay have any form unless the concave portion541and the convex portion464binterfere with each other in movements in the rotational direction.

FIG. 10Billustrates a state where no voltage is applied to the piezoelectric element434andFIG. 10Cillustrates a state where a voltage equal to or greater than the predetermined voltage V1is applied to the piezoelectric element434. As illustrated inFIG. 10B, a part of the side surface of the convex portion464bis in contact with the grease542. Therefore, in the case of an occurrence of a change in the relative rotation speed between the input-side torque sensor54and the input-side clutch plate464, a viscous force to the rotational direction occurs according to the area of the contact.

The application of the voltage equal to or greater than the predetermined voltage V1to the piezoelectric element434moves the convex portion464btoward the bottom side of the concave portion541(in the rightward direction ofFIG. 10C). This increases the contact area between the convex portion464band the grease542. The increase in the contact area as described above increases the viscous force in comparison with the viscous force before the increase in the contact area. More specifically, if a voltage equal to or greater than the predetermined voltage V1is applied to the piezoelectric element434, the viscous force increases with an increase in the voltage.

As described hereinabove, the controller A4is able to vary “the stiffness of the nonlinear spring414” and “the viscosity coefficient between the convex portion464band the grease542” according to the voltage applied to the piezoelectric element434.

Here, the nonlinear spring414of the fourth embodiment corresponds to the variable stiffness unit41of the exemplary embodiment. The convex portion464band the grease542of the fourth embodiment correspond to the variable viscosity coefficient unit42of the exemplary embodiment. Moreover, the piezoelectric element434of the fourth embodiment corresponds to the characteristic modification unit43of the exemplary embodiment. Furthermore, the input-side clutch plate464of the fourth embodiment corresponds to the drive-side clutch plate46of the exemplary embodiment and to the third element of the present invention. Moreover, the input-side torque sensor corresponds to the fourth element of the present invention. Furthermore, the output-side friction surface74aof the fourth embodiment corresponds to the driven-side clutch plate6of the exemplary embodiment.

Further, in the input-side clutch plate464, there is disposed an encoder B14for use in detecting a rotation angle relative to the input-side torque sensor54. This causes the encoder B14to detect a relative displacement (rotation angle) of the input-side torque sensor54relative to the input-side clutch plate464. Therefore, the displacement detected by the encoder B14indicates a displacement in the twist direction of the nonlinear spring414. Here, the encoder B14of the fourth embodiment corresponds to the displacement encoder B1of the exemplary embodiment.

Although the convex portion464bis provided in the input-side clutch plate464and the concave portion541is provided in the input-side torque sensor54in the fourth embodiment, the arrangement is not limited thereto. For example, the convex portion may be provided in the input-side torque sensor and the concave portion may be provided in the input-side clutch plate. In this case, the input-side clutch plate corresponds to the fourth element of the present invention and the input-side torque sensor corresponds to the third element of the present invention.

Moreover, in the fourth embodiment, the convex portion464band the concave portion541(the third element and the fourth element) are provided between the input-side clutch plate464and the motor A2(closer to the driving element than the clutch system) and the piezoelectric element434(piezoelectric element) is provided between the output-side torque sensor74and the output unit B (closer to the driven element than the clutch system). The arrangement, however, is not limited thereto. For example, the third element and the fourth element may be provided in positions closer to the driven element than the clutch system and the piezoelectric element may be provided in a position closer to the driving element than the clutch system.

The conductive polymer actuator is slow to operate in comparison with the drive source of the motor such as an electric motor. Accordingly, it is difficult to use the conductive polymer actuator for a fast control. The conductive polymer actuator, however, has various advantages such as “a large generative force per unit weight or unit volume,” “lightweight,” “a simple drive structure which enables a reduction in size,” “no driving sound due to operation on the molecular level (or, if any, the sound is not large enough to be noise),” and “able to be driven at a low voltage.”

Therefore, as long as the conductive polymer actuator is used for a purpose in which the reaction rate of the conductive polymer actuator is permissible, the merit of using the conductive polymer actuator is large. For example, for a use in which switching is performed between the transmission state and the non-transmission state, such as a clutch, fast control of one [ms] or the like is unlikely to be a requirement.

Moreover, the conductive polymer actuator varies in the stiffness and the viscosity coefficient by the application of a voltage.

Therefore, the present inventor focuses attention on this point, and has arrived at a conclusion of using the conductive polymer actuator as a member whose stiffness and viscosity coefficient are varied and as a member which fulfills a clutch function, instead of as a drive source for use in moving the load, in the first to third embodiments. Thereby, it is possible to achieve a power transmission device which is advantageous in weight and size, in comparison with a power transmission device in which a conventional clutch system is disposed.

Moreover, since the conductive polymer actuator has variable stiffness and viscosity coefficient, the structure of the power transmission device is able to be simplified, with a less number of parts, thereby enabling a reduction in weight and size, in comparison with a power transmission device having a mechanism for use in varying the stiffness of the elastic member and a mechanism for use in varying the viscosity coefficient of the viscous member separately.