Force sensing means

Background of the invention

The present invention relates to force sensing means, and more particularly to force sensing means for a driving device which employs a shape memory alloy.

A driving system for driving a load is generally constructed of the load, a force transmission unit and a driving portion, the load being coupled to the driving portion by the force transmission unit. The driving system of this type comprises means to sense a force acting on the driving portion, for such purposes as protection against an overload acting on the driving portion, etc. The sensing is performed with a force sensor such as load cell. In this case, the force sensor needs to be assembled in a part of the force transmission unit. Therefore, the structure of the force transmission unit has been complicated, and it has been difficult to render the driving device small in size and light in weight.

Moreover, the force is detected with a tension, a distortion stress, or the like which acts directly on the force sensor. In case of sensing a great force, therefore, the structural strength of the force sensor itself has needed to be sufficiently high, and the force sensor has tended to become large in size.

On the other hand, as the driving system is for driving the load as stated above, there is one employing a shape memory alloy. Examples are disclosed in British Patents No. 1554738 and No. 1573204. When, in the driving system of this type, it is intended to control a force acting on the driving portion, the installation of a force sensor is necessitated, and the construction of the driving system becomes complicated and large in size, likewise to the foregoing.

Summary of the invention

An object of the present invention is to provide force sensing means for an actuator which employs a shape memory alloy.

Another object of the present invention is to provide small-sized and light-weight force sensing means in an actuator which employs a shape memory alloy.

The present invention has taken note of the fact that a force to be generated by a shape memory alloy varies depending upon the temperature or resistance and the displacement of the shape memory alloy, and is characterized in that force information for controlling a force acting on an actuator and controlling an overload acting on the shape memory alloy is detected on the basis of the information of the temperature or resistance of the shape memory alloy constituting the actuator and the displacement of the actuator.

Other objects, advantages and features of the present invention will become apparent from embodiments to be described below.

Brief description of the drawings

• Figure 1 is a diagram showing the arrangement of one embodiment of the means of the present invention;
• Figure 2 is a graph of the displacement-force characteristics of a shape memory alloy for use in the embodiment shown in Figure 1;
• Figures 3 and 4 are diagrams each showing an example of arrangement of a force detector which constitutes the means of the present invention shown in Figure 1;
• Figures 5 to 8 are diagrams each showing another example of an actuator as well as a temperature detector which constitutes the means of the present invention shown in Figure 1;
• Figure 9 is a diagram showing the arrangement of another embodiment of the means of the present invention;
• Figure 10 is a graph of the displacement-force characteristics of a shape memory alloy for use in the embodiment shown in Figure 9;
• Figure 11 is a diagram showing the arrangement of still another embodiment of the means of the present invention;
• Figure 12 is a graph of the displacement-force characteristics of a shape memory alloy for use in the embodiment shown in Figure 11;
• Figure 13 is a diagram showing the arrangement of one embodiment of the means of the present invention;
• Figure 14 is a graph of the displacement-force characteristics of a shape memory alloy for use in the embodiment shown in Figure 13;
• Figure 15 is a graph of displacement-resistance characteristics associated with Figure 14;
• Figures 16 to 18 are diagrams each, showing another example of an actuator as well as a resistance detector for use in the embodiment shown in Figure 13;
• Figures 19 to 21 are diagrams each showing an example of arrangement of a force detector for use in the embodiment shown in Figure 13;
• Figure 22 is a diagram showing the arrangement of another embodiment of the means of the present invention;
• Figure 23 is a graph of the displacement-force characteristics of a shape memory alloy in the embodiment of Figure 22;
• Figure 24 is a diagram showing the arrangement of still another embodiment of the means of the present invention; and
• Figure 25 is a graph of the displacement-force characteristics of a shape memory alloy in the embodiment of Figure 24.

Detailed description of the preferred embodiments

Now, embodiments of the present invention will be described with reference to the drawings.

Figure 1 shows the arrangement of one embodiment of the means of the present invention. In this figure, numeral 9 designates a load. This load 9 is coupled through a force transmission unit 8 to an actuator 7 which is made of a shape memory alloy 6 such as NiTi. Numeral 15 indicates a driver which is connected across the shape memory alloy 6.

The shape memory alloy 6 is joule heated by current I generated by the driver 15, to shrink and to drive the load 9 through the force transmission unit 8.

The displacements of the actuator 7· is detected as a displacement signal V_x by a displacement detector 10 which is constructed of a potentiometer or the like connected to the force transmission unit 8. In addition, the temperature T of the shape memory alloy 6 is detected as a temperature signal V_t by a temperature detector 13.

The temperature detector 13 is constructed of a temperature sensor 11, such as a thermocouple or thermistor, which is disposed in contact with the shape memory alloy 6, and a processor 12 which converts the output of the temperature sensor 11 into the temperature signal V_t. A force detector 14 calculates a force F_e acting on the actuator7, on the basis of the displacement signal V_x and the temperature signal V_t, and provides a force signal V_f proportional to the force F_e. This force signal V_f is used as, for example, a control signal for the force acting on the shape memory alloy 6.

Next, the operation of the above-stated embodiment of the means of the present invention will be described.

Figure 2 shows the displacement―force characteristics of the shape memory alloy 6. In this figure, the axis of abscissas represents the displacement x of the shape memory alloy 6, and the axis of ordinates the force F_sm which the shape memory alloy 6 generates. The shape memory alloy 6 exhibits a low stiffness as indicated by a broken line T, in the figure, in the low temperature state in which its crystal assumes the martensite phase, while it exhibits a high stiffness as indicated by a solid line T_h in the figure, in the high temperature state in which its crystal assumes the austenite phase. Accordingly, when an external force as indicated by a load curve T in the figure is exerted from the load 9 on the shape memory alloy 6, the displacementx becomes x, at the lowtemperature T,, and the rise of temperature to the high temperature T_h causes the shape memory alloy 6 to shrink with the displacement x changed to x_h.

Regarding the actuator 7, load 9 and displacement detector 10 shown in Figure 1, solid lines illustrate the high temperature state, and broken lines the low temperature state.

From the above and on the basis of the characteristics of Figure 2, the force F_sm can be expressed as a function of the displacement x and the temperature T as indicated by Equation (1):

Further, in the arrangement shown in Figure 1, assuming that the loss of the force in the force transmission unit 8 be negligible, the external force F_e can be expressed by Equation (2):

Thus, Equation (3) is obtained from Equations (1) and (2):

From this equation (3), the force F_e can be calculated with the displacement x and the temperature T.

The force detector 14 executes the operation of Equation (4) based on Equation (3):

Next, examples of arrangements of the force detector 14 in the embodiment shown in Figure 1 will be described in detail.

Figure 3 shows one example of arrangement of the force detector 14. It is so arranged that an address A is determined with the displacement signal V_x and the temperature signal V_t by an address converter 16, that data D is selected on the basis of the address A by a table circuit 17, and that the force signal V_f based on the data D is provided by an output port 18. The table circuit 17 is constructed of, for example, a ROM (read only memory), which stores as the data D of the address A the value of V_f which has been obtained by the calculation with Equation (4) from the values of the displacement signal V_x and the temperature signal V_t corresponding to the address A.

Figure 4 shows another example of arrangement of the force detector 14. This example of arrangement approximates the operation of Equation (4) into Equation (5):

This example is constructed of operation units 19, 20 and a multiplier 21. The operation unit 19 calculates the value of the n-th power of the difference between the temperature signal V_t and a constant P_o. The operation unit 20 calculates the value of the m-th power of the displacement signal V_x.

Further, the product between the outputs of the operation units 19 and 20 is operated by the multiplier 21, to obtain the force output V_f.

In a case where n=1 and m=1 hold, the operation unit 19 is constructed of a subtracter, and the operation unit 20 is not required and the displacement signal V_x is directly applied to the input b of the multiplier 21.

According to the foregoing embodiment, the output of the actuator 7 can be detected on the basis of the temperature of the shape memory alloy, and a driving device in which the actuator itself is furnished with a force sensing function can be constructed.

The detected force signal is compared with a desired value, whereby the force acting on the actuator can be controlled. It can also be used as a signal for controlling an overload acting on the shape memory alloy.

Although, in this embodiment, the position detector 10 is connected to the force transmission unit 8, it is obvious that it may well be connected to the actuator 7. It is also obvious that the position detector 10 may well be one capable of measuring the displacement of the actuator in non-contacting fashion.

Although, in this embodiment, the temperature sensor 11 of the temperature detector 13 is disposed in contact with the shape memory alloy 6, obviously a non-contacting temperature sensor such as infrared sensor may well be employed.

It is to be understood that the outputs of the displacement detector 10, temperature detector 13 and force detector 14 may be either analog signals or digital signals.

It is also to be understood that any of the displacement detector 10, temperature detector 13 and force detector 14 may be constructed of an analog circuit, a digital circuit, an analog and digital hybrid circuit, or software based on a microcomputer.

It is also to be understood that the shape memory alloy 6 to construct the actuator 7 may be in any shape such as a wire, coil or plate.

It is also to be understood that the heating of the shape memory alloy 6 by the driver 15 may be, not only the joule heating, but also any of induction heating, heating by an electromagnetic wave such as laser radiation, heating by a chemical reaction, etc.

Next, there will be described other embodiments of the actuator and the temperature detector therefor which constitute the present invention.

Figure 5 shows another example of the actuator as well as the temperature detector. An actuator 22 is joined with a shape memory alloy 23 at both the end parts A and B, and it is constructed of members 24 and 25 of a conductor different from the shape memory alloy 23. The juncture B is provided with a constant temperature unit 26 which is formed of a Peltier effect device or the like for holding the temperature of this juncture B at a constant temperature To. Wires 24a and 25a of an identical material are connected to the respective conductor members 24 and 25. Across the wires 24a and 25a, a potential difference E,, arises into which a thermo-electromotive force E_t based on the temperature T of the node A and a thermoelectromotive force -E_to based on the temperature To of the node B are added. A temperature detector 27 receives the potential difference E,_n, and calculates and delivers a temperature signal V_t of a value proportional to the temperature T.

Since, according to this embodiment, the temperature can be detected by the thermoelectromotive force effect of the shape memory alloy 23, the actuator need not be especially provided with a temperature sensor.

Figure 6 shows still another example of the actuator as well as the temperature detector. An actuator 28 is constructed of a shape memory alloy 29. Joined to the shape memory alloy 29 are a wire 30 which is made of the same material asthe shape memory alloy, and a wire 31 which is made of a conductor different in property from the shape memory alloy 29. A temperature detector 32 is constructed of a constant temperature unit 34 which is disposed at the node B' between the wire 30 and a wire 33 made of the same material as that of the wire 31 and which holds the temperature of the node B' at a constant temperature To, and a processor 35 which receives a potential difference E_1n developing across the wires 31 and 33 and being the total of the thermoelectromotive forces of a node A' and the node B' and which calculates and delivers a temperature output V_t proportional to a temperature T.

According to this embodiment, likewise to the example shown in Figure 5, the actuator can be endowed with the function of a temperature sensor. Further, the actuator need not be provided with a constant temperature unit, and the miniaturization of the actuator and the simplification of the structure are facilitated.

Figure 7 shows a modification of the embodiment illustrated in Figure 6, and in Figure 7, parts assigned the same numerals as in Figure 6 are similar portions. A temperature detector 36 is constructed of a temperature sensor 37 which is joined to the node B' in order to detect the temperature To of the node B', and a processor38 which corrects the potential difference E_ln across the wires 31 and 33 on the basis of the output of the temperature sensor 37 and which calculates and delivers the temperature output V_t proportional to the temperature T of the node A' shown in Figure 5.

According to this modification, the temperature detector need not be provided with a constant temperature unit, and the miniaturization and simplification of the temperature detector are facilitated.

Figure 8 shows still another modification of the embodiment shown in Figure 6. In Figure 8, parts assigned the same numerals as in Figure 6 are similar portions.

An actuator 39 is made of the shape memory alloy 29, to which wires 301-30n identical in property to the wire 30 shown in Figure6andwires 311-31n identical in property to the wire 31 shown in Figure 6 are alternately joined. The pairs of the adjacent wires 301-30n and 311-31 n are respectively connected to temperature detectors 321-32n each of which is identical in construction to the temperature detector 32 shown in Figure 6. Temperature outputs V_t1-V_tn from the processors 321-32n, which are proportional to the temperatures T₁-T_n of nodes A₁―A_n, are applied to an operation unit 40. The operation unit 40 provides the temperature output V_t which is proportional to the average value of the temperature outputs V_n―V_tn.

According to this modification, the average temperature of the shape memory alloy can be detected, and the reliability and precision of the processing to succeed the temperature detectors can be enhanced.

Next, another embodiment of the means of the present invention will be described with reference to Figures 9 and 10.

Figure 9 shows an arrangement diagram of another embodiment of the means of the present invention. In this figure, parts assigned the same numerals as in Figure 1 are similar portions. An actuator 41 is so constructed that the shape memory alloy 6 and an elastic unit 42 such as spring are coupled through a connection 44. The actuator 41 drives a load through a force transmission unit 43 which is connected to the connection 44. The displacement detector 10 is connected to the connection 44. A force detector 45 calculates a force F_e acting on the actuator 41, with the displacement signal V_x and the temperature signal V_t, and provides the force signal V, proportional to the force F_e.

Next, the operation of the above embodiment of the means of the present invention will be described.

Figure 10 shows the displacement-force characteristics of the shape memory alloy 6 similarly to Figure 2. Shown by a dot-and-dash line in the figure is a load curve T_A which represents the force acting on the shape memory alloy 6. Since the elastic unit 42 is coupled to the shape memory alloy 6 with the distortion of an initial displacement x_o given in advance, it pulls the shape memory alloy 6 with an elasticity F_sp. Further, in a case where the force F. is applied through the force transmission unit 43, the shape memory alloy 6 is pulled by the resultant force of the elasticity F_sp and the force F_e.

From this fact, Equation (6) holds among the force F_sm, the elasticity F_sp and the force F_e:

That is, as understood by referring to Figure 10, the displacement x of the actuator settles to x, at the low temperature T,. As the temperature rises to the high temperature Th, the force F_sm increases, and the displacement shifts to x_h. The elasticity F_sp of the elastic unit 42 can be expressed as a function of the displacement x as indicated by Equation (7) by way of example:

Here, k denotes a spring constant.

As indicated by Equation (1), the force F_sm can be expressed as the function of the displacement x and the temperature T. From Equation (6), accordingly, the force F_e can be expressed as a function of the displacement x and the temperature T as indicated by Equation (8):

The force detector 45 is constructed so as to execute the operation of Equation (9) based on Equation (8) and to calculate the force output V_f:

Since this embodiment is constructed so as to determine the displacement x owing to the balance between the force Fs_m of the shape memory material 6 and the elasticity F_sp of the elastic material 42, the displacement x can be increased with decrease in the force F_e based on the load, and the displacement of the actuator 41 can be changed in both the directions of decrease and increase.

It is obvious in this embodiment that the displacement detector 10 may be connected, not only to the connection 44, but also to either of the shape memory alloy 6 and the force transmission unit 43. It is also obvious that the shape memory alloy 6 may well be connected directly to the elastic material 42. It is also obvious that the elastic material 42 may well be connected to the force transmission unit 43. It is also obvious that the elastic unit 42 may be of any material or form generating an elasticity versus a displacement, such as a helical spring, leaf spring or air spring. Although, in the arrangement of this embodiment, the tensions of the shape memory alloy 6 and the elastic unit 42 are opposed, obviously an arrangement may well be employed in which the pressures, bending moments, twisting moments or the like of them are opposed.

Now, still another embodiment of the means of the present invention will be described with reference to Figures 11 and 12.

Figure 11 shows the arrangement of still another embodiment of the means of the present invention. In this figure, parts assigned the same numerals as in Figure 9 are similar portions. An actuator 49 is so constructed that the shape memory alloy 6 and a shape memory alloy 47 are coupled through a connection 46. The actuator 49 drives a load through a force transmission unit 48 which is connected to the connection 46. The displacement detector 10 is connected to the connection 46.

Drivers 15 and 50 cause currents 1₁ and 1₂ to flow through the shape memory alloy members 6 and 47 for joule heating, respectively.

Here, the drivers 15 and 50 are controlled so as to decrease the current 1₂ in case of increasing the current 1₁ and to increase the current I₂ in case of decreasing the current I₁.

The temperature detector 13 delivers a resistance signal V_t1 which is proportional to the temperature T₁ of the shape memory alloy 6. A temperature detector 51, which has an arrangement similar to that of the temperature detector 11, delivers a temperature signal V_t2 which is proportional to the temperature T of the shape memory alloy 47. A force detector 52 calculates a force F_sm1 generated by the shape memory alloy 6, with the displacement signal V_x and the temperature signal V_t1, and provides a force output V_f1 proportional to the force F_sm1. A force detector 53 calculates a force F_sm2 generated by the shape memory alloy 47, with the displacement signal V_x and the temperature signal V_t2, and provides a force output V_f2 proportional to the force F_sm2. A subtracter 54 delivers a force output V_f which is proportional to the difference between the force outputs V_f1 and V_f2.

Next, the operation of the above embodiment of the means of the present invention will be described with reference to Figure 12.

Figure 12 shows the displacement-force characteristics of the shape memory alloy 6 similarly to Figure 2. In Figure 12, a dot-and-dash line and a two-dot chain line illustrate load curves T₁ and T₂ which represent the force acting on the shape memory alloy 6. The shape memory alloy 47 is coupled to the shape memory alloy 6 with the distortion of an initial displacement x_o given in advance. Therefore, the shape memory alloy members 6 and 47 pull each other with stiffnesses corresponding to their respective temperatures. Further, in a case where a force F_e is applied through the force transmission unit 48, the shape memory alloy 6 is pulled by the resultant force between the force F_sm2 of the shape memory alloy 47 and the force F_e.

From this fact, Equation (10) holds among the forces F_sm1, F_sm2 and F_e:

That is, as understood by referring to Figure 11, in a case where the shape memory alloy 6 is at a low temperature T_l1 and the shape memory alloy 47 is at a high temperature T_h2, the displacementx settles to x,. As the temperature of the shape memory alloy 6 shifts to a high temperature T_h1 and simultaneously that of the shape memory alloy 47 to a low temperature T,₂, the displacement shifts to x_h.

By applying Equation (1), the force F_Sm2 can be expressed as Equation (11):

As to the force F_sm1, Equation (1) can be applied as it is:

From the above, the force F_e can be expressed in accordance with Equations (10), (11) and (12) as follows:

The force detector 52 executes the operation of Equation (14) based on Equation (12), and the force detector 53 executes the operation of Equation (15) based on Equation (11), so as to calculate the force outputs V_f1 and V_f2:

Further, the difference between the force outputs V_f1 and V_f2 is calculated by the subtracter 54 on the basis of Equation (13), to produce the force output V_f which is proportional to the force F_e.

Since this embodiment is constructed so as to determine the displacement x owing to the balance between the force F_sm, of the shape memory alloy 6 and the force F_sm2 of the shape memory alloy 47, the displacement of the actuator 49 can be changed in both the directions of decrease and increase.

With a construction in which the displacement--force characteristics of the shape memory alloy 6 and the shape memory alloy 47 are equalized, output characteristics which are uniform in both the directions of the displacement of the actuator are attained.

It is obvious in this embodiment that the ^-displacement detector 10 may be connected, not only to the connection 46, but also to either of the shape memory alloy 6 and the force transmission unit 48. It is also obvious that the shape memory alloy members 6 and 47 may well be directly connected. It is also obvious that the shape memory alloy 47 may well be connected to the force transmission unit 48. It is also obvious that the shapes and materials of the shape memory alloy members 6 and 47 may well differ from each other. Although, in the arrangement of this embodiment, the tensile forces of the shape memory alloy members 6 and 47 are opposed, obviously an arrangement may well be employed in which the pressures, bending moments, twisting moments or the like of them are opposed. In the one and other embodiments described above, obviously the temperature detector may well be arranged so as to detect the temperature of the shape memory alloy on the basis of a physical quantity changing with the temperature of the shape memory alloy, for example, electric resistance or magnetic reluctance.

The foregoing embodiments detect the force information of the shape memory alloy on the basis of the temperature and displacement thereof. There will now be described a case where the force information of the shape memory alloy is detected on the basis of the electrical resistance value and displacement thereof.

Figure 13 shows one embodiment of the means of the present invention, in which parts of the same symbols as in Figure 1 are identical portions. In this embodiment, the point of difference from the embodiment shown in Figure 1 consists in comprising a resistance detector 55 which detects the resistance R of the shape memory alloy 6 as a resistance signal V_r, and a force detector 56 which calculates a force F_e acting on the actuator 7, with the displacement signal V_x and the resistance signal V_r, and which provides a force signal V_f proportional to the force F_e.

Next, the operation of the above embodiment of the means of the present invention will be described.

Figure 14 shows the displacement-force characteristics of the shape memory alloy 6. Since it is similar to Figure 2, it shall not be described in detail.

Figure 15 shows the displacement―resistance characteristics of the shape memory alloy 6 corresponding to the displacement-force characteristics in Figure 14. The axis of abscissas in this figure represents the displacement x of the shape memory alloy 6, while the axis of ordinates represents the resistance R of the shape memory alloy 6. In this figure, characteristics indicated by a solid line and a broken line are those corresponding to the high temperature Th and the low temperature T in Figure 14 respectively. As seen from this figure, the resistance R exhibits monotonously increasing characteristics versus the displacement x. In addition, the gradient of the displacement-resistance characteristic becomes greater as the temperature T is higher. That is, the resistance R exhibits monotonously increasing characteristics versus the temperature T, too.

Thus, owing to the characteristics in Figure 14, the force F_sm can be expressed as a function of the displacement x and the temperature T as indicated by Equation (16):

Besides, owing to the characteristics in Figure 15, the resistance R can be expressed as a function of the displacement x and the temperature T as indicated by Equation (17):

Here, when the temperature T in Equations (16) and (17) is eliminated as a common parameter and then the results is arranged, Equation (18) is obtained:

That is, the force F_sm can be expressed as a function of the displacement x and the resistance R.

Further, assuming in the arrangement of Figure 13 that the loss of the force in the force transmission unit 8 be negligible, the external force F_e can be expressed by Equation (19):

From Equations (18) and (19), accordingly, the following equation (20) is obtained:

From this equation (20), the force F_e can be calculated with the displacement x and the resistance R.

The force detector 56 executes the operation of Equation (21) based on Equation (20):

Next, the constituents of the embodiment shown in Figure 13 will be described in detail.

Figures 16, 17 and 18 show examples of arrangements of the resistance detector 55.

Figure 16 shows one example of arrangement thereof. In this example, a constant current I_r is caused to flow to the shape memory alloy 6 by a constant current unit 58 which is supplied with power by a power source 57, and the voltage drop E_r across the shape memory alloy 6 due to the resistance R thereof is amplified by an amplifier 59, to provide the resistance signal V_r. The voltage E_r is proportional to the resistance R as seen from Equation (22):

Therefore, if the amplifier 59 has a linear characteristic, it can produce the resistance signal V_r proportional to the resistance R.

Figure 17 shows another example of arrangement of the resistance detector. In this example, a current I_d is caused to flow through the shape memory alloy 6 by a constant voltage source 60, the voltage drop E_r across the shape memory alloy 6 due to the resistance R thereof is held constant, the current I_d is further caused to flow through a resistance R_d, and the voltage drop E_d across the resistance is applied to an operation unit 61, to provide a resistance output V_r. The current I_d is obtained in accordance with Equation (23), while the voltage drop E_d is obtained in accordance with Equation (24) by employing Equation (23):

The operation unit 61 is arranged so as to deliver the inverse number of the voltage drop E_d as the resistance output V_r. It can produce the resistance output V_r proportional to the resistance R as indicated by the following equation (25) obtained from Equation (24):

Figure 18 shows still another example of arrangement. A constant voltage is applied by a constant voltage source 62 to a series connection circuit consisting of a capacitance C and the resistance R of the shape memory alloy 6, and the variation of the voltage drop E, across the capacitance C with time is detected by a time constant circuit 63, to provide a resistance signal V_r. By way of example, when the time constant circuit 63 is constructed of a monostable multivibrator, the resistance signal V, is obtained as a pulse width which is determined by the time constant between the capacitance C and the resistance R.

Next, examples of arrangements of the force detector 56 will be described with reference to Figures 19, 20 and 21.

Figure 19 shows one example of arrangement thereof. An address A is determined with the displacement signal Vx and the resistance signal V_r by an address converter 64, data D is selected on the basis of the address A by a table circuit 65, and a force signal V_f based on the data D is provided by an output port 66. The table circuit 65 is constructed of, for example, a ROM, which stores as the data D of the address A the value of the force signal V_f obtained by the calculation with the aforementioned Equation (21) from the values of the displacement signal V_x and the resistance signal V, corresponding to the address A.

Figure 20 shows another example of arrangement of the force detector. It corresponds to a case where the operation of Equation (21) is approximated to the following Equation (26):

More specifically, the resistance signal V, and a constant value ―P₁ produced by a constant generator 67 are added by an adder 68, and the output (V_r-P₁) of the adder 68 and the displacement signal V_x are multiplied by means of a multiplier 69, to become a force output V_f.

Figure 21 shows still another example of arrangement of the force detector. This example corresponds to a case where the operation of Equation (21) is approximated to following equation (27):

More specifically, the resistance signal V_r multiplied by P₃ by means of a multiplier 70, the displacement signal V_x multiplied by P₄ by means of a multiplier 71 and a constant value -P₅ produced by a constant generator 72 are added by an adder 73, to become a force signal V_f.

Since the foregoing embodiment is constructed as thus far described, the output of the actuator can be detected from the resistance value of the shape memory alloy 6. As a result, a driving device can be constructed in which the actuator itself is furnished with a force sensor function.

Although, in the above embodiment, the position detector 10 is connected to the force transmission unit 8, it is obvious that it may well be connected to the actuator 7. It is also obvious that the position detector 10 may well be one capable of measuring the displacement of the actuator in non-contacting fashion. It is to be understood that the outputs of the displacement detector 10, resistance detector 55 and force detector 56 may be either analog signals or digital signals. It is also to be understood that any of the displacement detector 10, resistance detector 55 and force detector 56 may be constructed of an analog circuit, a digital circuit, an analog and digital hybrid circuit, or software based on a microcomputer. It is also to be understood that the shape memory alloy 6 to construct the actuator 7 may be in any shape such as a wire, coil or plate. It is also to be understood that the heating of the shape memory alloy 6 by the driver 15 may be, not only the joule heating, but also.any of induction heating, heating by an electromagnetic wave such as laser radiation, heating by a chemical reaction, etc.

Now, another embodiment of the means of the present invention will be described with reference to Figures 22 and 23.

Figure 22 shows the construction of another embodiment of the means of the present invention. In this figure, parts assigned the same numerals as in Figure 9 are similar portions.

When compared with the embodiment shown in Figure 9, this embodiment comprises a resistance detector 74 similar to the resistance detector 55 shown in Figure 13 and a force detector 75 similar to the force detector 56 shown in Figure 13. The force detector 75 calculates the force F_e acting on the actuator 41, with the displacement signal V_x and the resistance signal V_r, and delivers the force signal V_f proportional to the force F_e.

Next, the operation of the above embodiment of the means of the present invention will be described with reference to Figure 23.

Figure 23 shows the displacement-force characteristics of the shape memory alloy 6 similarly to Figure 10. In Figure 23, a dot-and-dash line indicates a load curve T_A which represents the force acting on the shape memory alloy 6.

Since the elastic unit 42 is coupled to the shape memory alloy 6 with the distortion of an initial displacement X_o given in advance, it pulls the shape memory alloy 6 with the elasticity F_sp.

Further, in a case where the force F_e is applied to the actuator 41 through the force transmission unit 43, the shape memory alloy 6 is pulled by the resultant force between the elasticity F_sp of the elastic unit 42 and the force F_e.

From this fact, Equation (28) holds among the force F_sm, elasticity F_sp and force F_e:

Referring to Figure 23, the displacement x of the shape memory alloy 6 settles x_l at the low temperature T_l, and as the temperature rises toward the high temperature T_h, the force F_sm increases and the displacement shifts toward _Xh.

The elasticity F_sp of the elastic unit 42 can be expressed as a function of the displacement x as indicated by Equation (29) by way of example:

Here, k denotes a spring constant.

In addition, the force F_sm can be expressed as the function of the displacement x and the resistance R as indicated by Equation(18).

Thus, from Equation (28), the force F_e can be expressed as a function of the displacement x and the resistance R as indicated by the following Equation (30):

In this way, the force detector 75 executes the operation of the following Equation (31) based on Equation (30) and calculates the force output V_f:

Since this embodiment is constructed so as to determine the displacement x owing to the balance between the force F_sm of the shape memory alloy 6 and the elasticity F_sp of the elastic unit 42, the displacement x can be increased with decrease in the force F_e based on the load, and the displacement of the actuator 41 can be changed in both the directions of decrease and increase.

It is obvious in this embodiment that the displacement detector 10 may be connected, not only to the connection 44, but also to either of the shape memory alloy 6 and the force transmission unit 43. It is also obvious that the shape memory alloy 6 may well be connected directly to the elastic unit 42. It is also obvious that the elastic unit 42 may well be connected to the force transmission unit 43. It is also obvious that the elastic unit 42 may be of any material or form generating an elasticity versus a displacement, such as a helical spring, leaf spring or air spring. Although, in the arrangement of this embodiment, the tensions of the shape memory alloy 6 and the elastic unit 42 are opposed, obviously an arrangement may well be employed in which the pressures, bending moments, twisting moments or the like of them are opposed.

Now, still another embodiment of the means of the present invention will be described with reference to Figures 24 and 25.

Figure 24 shows the arrangement of still another embodiment of the means of the present invention. In this figure, parts assigned the same numerals as in Figure 22 are similar portions.

In this embodiment, the actuator 41 is constructed by coupling the shape memory alloy 6 and a shape memory alloy 76 through a connection 77. The actuator 41 drives a load through a force transmission unit 78 which is connected to the connection 77. The displacement detector 10 is connected to the connection 77. Drivers 15 and 79 are respectively connected to the shape memory alloy members 6 and 76 so as to cause currents 1₁ and 1₂ to flow through the shape memory alloy members 6 and 76 for joule heating.

Here, the drivers 15 and 79 are controlled so as to decrease the current I₂ in case of increasing the current 1₁ and to increase the current I₂ in case of decreasing the current I₁. A resistance detector 80 connected to the shape memory alloy 6 provides a resistance signal V, which is proportional to the resistance R₁ of the shape memory alloy 6. A resistance detector 81, which is similar in arrangement to the resistance detector 80 and which is connected to the shape memory alloy 76, provides a resistance signal V,₂ which is proportional to the resistance R₂ of the shape memory alloy 76. A force detector 82 calculates a force F_sm1 generated by the shape memory alloy 6, with the displacement signal V_x and the resistance signal V_r1, and provides a force output V_f1 proportional to the force F_sm1. On the other hand, a force detector 83 calculates a force F_sm2 generated by the shape memory alloy 76, with the displacement signal V_x and the resistance signal V_r2 and provides a force output V_f2 proportional to the force F_sm2. A subtracter 84 produces a force output V_f which is proportional to the difference between the force outputs V_f1 and V,₂.

Next, the operation of the above embodiment of the means of the present invention will be described with reference to Figure 25.

Likewise to Figure 12 explained before, Figure 25 shows the displacement-force characteristics of the shape memory alloy 6. In this figure, a dot-and-dash line and a two-dot chain line indicate load curves T₁ and T₂ which represent the force acting on the shape memory alloy 6. The shape memory alloy 76 is coupled to the shape memory alloy 6 with the distortion of an initial displacement X_o given in advance. Therefore, the shape memory alloy members 6 and 76 pull each other with stiffnesses corresponding to their respective temperatures.

Further, in a case where the force F_e is applied through the force transmission unit 78, the shape memory alloy 6 is pulled by the resultant force between the force F_Sm2 of the shape memory alloy 76 and the force F_e.

From this fact, Equation (32) holds among the forces F_sm1, F_sm2 and F_e:

More specifically, referring to Figure 25, in a case where the shape memory alloy 6 is at the low temperature T_l1 and that 76 is atthe high temperature T_h2, the displacement x of the actuator 41 settles to X₁. As the temperature of the shape memory alloy 6 shifts toward the high temperature T_h1 and that of the shape memory alloy 76 simultaneously shifts toward the low temperature T,₂, the displacement x shifts toward X_h·

By applying Equation (16), the force F_sm2 can be expressed as Equation (33):

Likewise, by applying Equation (17), the resistance R₂ can be expressed as Equation (34):

From Equations (33) and (34), accordingly, the force F_sm2 can be expressed as a function of the displacement x and the resistance R as indicated by the following Equation (35):

Regarding the force F_sm1' Equation (18) can be applied as it is:

Thus, from Equations (32), (35) and (36), the force F. can be expressed as:

The force detector 82 executes the operation of the following Equation (38) based on Equation (36), and the force detector 83 the operation of the following Equation (39) based on Equation (35), to calculate the force outputs V,_i and V_f2 respectively:

Further, the subtracter 84 calculates the difference of the force outputs V_f1 and V_f2 on the basis of Equation (37) and provides the force output V_f proportional to the force F_e.

Since this embodiment is constructed so as to determine the displacement x of the actuator 41 owing to the balance between the force Fsm1 of the shape memory alloy 6 and that F_srn2 of the shape memory alloy 76, the displacement of the actuator 41 can be changed in both the directions of decrease and increase.

With a construction in which the displacment--force characteristics of the shape memory alloy members 6 and 76 are equalized, uniform output characteristics are attained in both the directions of the displacement of the actuator 41.

It is obvious in this embodiment that the displacement detector 10 may be connected, not only to the connection 77, but also to either of the shape memory alloy 6 and the force transmission unit 78. It is also obvious that the shape memory alloy 6 and the shape memory alloy 76 may well be directly connected. It is also obvious that the shape memory alloy 76 may well be connected to the force transmission unit 78. It is also obvious that the shapes and materials of the shape memory alloy members 6 and 76 may well differ from each other. Although, in the arrangement of this embodiment, the tensile forces of the shape memory alloy members 6 and 76 are opposed, obviously an arrangement may well be employed in which the pressures, bending moments, twisting moments or the like of them are opposed.

As set forth above, according to the present invention, an actuator can be furnished with the function of a force sensor, and hence, means capable of detecting an external force based on a load can be provided without assembling any force sensor in a power transmission system independently.