Piezoelectric actuator having strain gage

Strain gages (81, 82, 83) are adhered to protective layers (31, 32) and an active portion (2) of a laminated-structure (10) of the piezoelectric actuator, respectively. Since it is possible to measure a total amount of displacement of the protective layers (31, 32) and the active portion (2), the measured amount of displacement of the laminated structure (10) is accurate even if there are difference in strain due to temperature and stress between them. The same effect can be obtained for such actuator whose laminated structure (10) further includes any member such as temperature compensating member (19) of metal whose elastic modulus is different from that of the laminated structure.

BACKGROUND OF THE INVENTION 
The present invention relates to an actuator and, particularly, to a 
laminated-type piezoelectric actuator having strain gages. 
A piezoelectric actuator which utilizes the piezoelectricity of crystal has 
been used widely in a high precision positioning mechanism since it can 
control a minute mechanical displacement at high speed. Among others, a 
laminated-type piezoelectric actuator composed of a laminated structure of 
piezoelectric films of piezoelectric ceramics, etc., each having thickness 
of several tens microns and having a thin film internal electrode provided 
thereon is featured by its capability of generating large force with 
relatively low drive voltage and has been widely utilized in the fields of 
high precision positioning control of a semiconductor manufacturing 
apparatus such as exposure apparatus, control of valves of a mass 
flowmeter, thickness control in an extruder of plastic film and optical 
axis control of an optical device, which require high precision control of 
minute displacement in the order of micron or less. 
Since an amount of displacement of such laminated-type piezoelectric 
actuator depends upon strain generated in the respective piezoelectric 
ceramics layers correspondingly to a drive voltage applied externally 
across the respective piezoelectric ceramic layers through external 
electrodes, it is basically possible to control the amount of displacement 
of the piezoelectric actuator by controlling magnitude of the drive 
voltage. In the case of actuator utilizing piezoelectric effect, however, 
a relation between drive voltage and strain of respective piezoelectric 
material (ceramic) layers is usually not linear but exhibits hysteresis. 
Therefore, when the piezoelectric actuator is used in these fields to 
precisely control displacement in the order of micron or less, it is 
necessary to detect an actual displacement of the piezoelectric actuator 
and feed it back to the drive voltage. A construction of such 
piezoelectric actuator having a strain gage for detecting an amount of 
displacement of the actuator is disclosed in Japanese Utility Model 
Laid-open No. Sho 61-140661. 
FIG. 1 is a schematic perspective view of a conventional piezoelectric 
actuator having a strain gage. The piezoelectric actuator itself includes 
an active portion 2 shown in FIG. 2. Usually, the piezoelectric actuator 
includes an upper protective layer 31 and a lower protective layer 32 
provided on opposite ends of the active portion 2, respectively, as shown 
in FIG. 2 for reasons of use. In FIG. 2, the active portion 2 is 
constituted with an alternatively laminated structure of a plurality of 
piezoelectric ceramics layers 4 and a plurality of internal electrode 
layers 5. The internal electrode layers 5 are connected to external 
electrodes 71 and 72 alternatively to form a pair of comb electrodes 
having electrode fingers arranged interdigitally in cross section, so that 
the external electrodes 71 and 72 function as opposing electrodes opposing 
each other through the piezoelectric ceramics layers 4. In FIG. 2, a drive 
voltage applied between the external electrodes 71 and 72 through leads 
711 and 712 is applied between adjacent internal electrode layers 5 to 
form an electric field across each piezoelectric ceramics layer 4 
sandwiched between the internal electrodes. With such electric fields, the 
respective piezoelectric ceramics layers 4 generate strain in a direction 
perpendicular to a plane of the piezoelectric ceramics layer, that is, in 
a thickness direction or laminating direction. The piezoelectric actuator 
derives the strain as a change of length of the laminated-structure in a 
thickness direction, that is, a displacement, and transmits it externally. 
In the piezoelectric actuator shown in FIG. 1, a strain gage 8 is attached 
onto a side surface of the actuator on which there is no external 
electrode is provided. When the drive voltage is applied between the 
external electrodes 71 and 72, the piezoelectric actuator is extended in 
the laminating direction thereof by a predetermined amount due to 
piezoelectric effect. With such expansion of the piezoelectric actuator in 
the thickness direction thereof, a resistor of the strain gage 8 is 
subjected to tensile force and its resistance value is increased. An 
amount of the resistance change is detected through leads 9 and which is 
linearly related to an amount of expansion or contraction of the 
piezoelectric actuator, that is, an amount of displacement. Therefore, by 
preliminarily measuring a relation between the amount of displacement of 
the piezoelectric actuator and the amount of resistance change of the 
strain gage at respective drive voltages and detecting the amount of 
resistance change of the strain gage 8 when a predetermined voltage is 
applied between the external electrodes 71 and 72, it is possible to know 
the amount of displacement of the piezoelectric actuator. Further, when 
the amount of resistance change of the strain gage 8 is inconsistent with 
an aimed setting value, it is possible to control the amount of resistance 
change to the aimed displacement by controlling the drive voltage 
correspondingly to a deviation of the amount of resistance change from the 
aimed value. 
As mentioned above, the amount of expansion or contraction, that is, the 
amount of displacement, of the laminated-type piezoelectric actuator can 
be controlled precisely by detecting an actual amount of displacement by 
means of the strain gage attached to the side surface of the laminated 
structure and feeding back a difference between the detected displacement 
and the aimed displacement to the drive voltage. 
However, in case where a laminated-type piezoelectric actuator is to be 
used practically, in addition to the laminated active portion for 
generating displacement corresponding to the externally applied drive 
voltage, portions such as protective layers for protecting the active 
portion and temperature compensating members of metal for improving 
accuracy of displacement control, which do not produce displacement with 
the drive voltage, are required in the laminated structure. In such 
conventional actuator as mentioned above, there may be a difference 
between a total amount of actual displacement of the whole piezoelectric 
actuator and an amount of displacement detected by the strain gage, upon 
which it may become impossible to control displacement precisely. This 
will be described in detail below. 
As mentioned previously, FIG. 2 shows the cross section of the 
laminated-type piezoelectric actuator having the above-mentioned 
protective layers. In FIG. 2, the piezoelectric actuator includes a 
laminated-structure 10 of the active portion 2 and the protective layers 
31 and 32 as its basic components. 
The active portion 2 is constituted with the alternatively laminated 
structure of the piezoelectric ceramics layers 4 and the internal 
electrode layers 5 and displacement thereof is generated in the laminating 
direction by the drive voltage applied to the external electrodes 71 and 
72 and hence to the respective piezoelectric ceramics layers 4 through the 
respective internal electrodes 5. 
The protective layers 31 and 32 are provided to protect the active portion 
2 electrically and mechanically against external force. That is, although 
the basic function of the piezoelectric actuator as an electromechanical 
transducer is obtained by the active portion 2, it is desirable, in order 
to use it on a practical device, that an outermost portion of the 
laminated-structure 10 in the laminating direction is of an insulating 
material since it can be adapted to an associated device even if the 
latter is formed of not insulating material but metal material. Further, 
since each piezoelectric ceramics layer 4 of the active portion 2 is as 
thin as several tens microns and strength of the electric field generated 
in the piezoelectric ceramics layer 4 by the drive voltage in the order of 
150 V applied thereto is very large, the active portion 2 must be 
protected mechanically against external mechanical shock in such a way 
that the piezoelectric ceramics layer or layers 4 are not cracked or 
damaged. For this purpose, it least the protective layers 31 and 32 are 
provided on an upper and lower ends of the active portion 2 and, 
therefore, each protective layer should be thick enough to provide desired 
protection. For example, the thickness, that is, length in the laminating 
direction, of each protective layer may be in the order of 2 mm for the 
active portion 2 having length of 12 mm in the laminating direction. In 
view of easiness of fabrication of the protective layers 31 and 32, each 
protective layer is usually formed by laminating thin layers of the same 
material as that of the piezoelectric ceramics layer 4 constituting the 
active portion 2. 
In the laminated-type piezoelectric actuator constituted as mentioned 
above, when external force exerted on the laminated structure 10 in the 
strain generating direction, that is, the laminating direction, is varied, 
elastic strain of the protective layers 31 and 32 is different from that 
of the active portion 2 even if they have the same piezoelectric 
characteristics, since the active portion 2 is subjected to electric field 
while the protective layers are not. That is, an actual amount of 
displacement of this laminated structure is a sum of strain of the active 
portion 2 and strain of the protective layers 31 and 32. In the 
piezoelectric actuator shown in FIG. 1, however, strain of only the active 
portion 2 is detected. Therefore, the detected displacement is 
inconsistent with the actual displacement. Further, since the length of 
the protective layers 31 and 32 is not negligible with respect to the 
length of the active portion 2 as mentioned previously, the difference 
between the actual displacement and the detected displacement is very 
important. 
On the other hand, when temperature of the laminated structure 10 is 
changed by change of ambient temperature and/or heat generated by an 
operation of the piezoelectric actuator, detected displacement also 
becomes inconsistent with actual displacement if coefficient of linear 
expansion or the active portion 2 is different from that of the protective 
layers 31 and 32. 
It may be considered, in order to flatten temperature characteristics of 
displacement amount of the piezoelectric actuator, to further provide a 
temperature compensating member on the upper or lower portion of the 
laminated structure 10. Such temperature compensating member is a block of 
such as stainless steel whose coefficient of linear expansion is opposite 
in sign to that of the laminated-structure 10 formed of piezoelectric 
material whose coefficient of linear expansion is negative and functions 
to compensate for variation of displacement of the piezoelectric actuator 
due to thermal expansion thereof. Length of the temperature compensating 
member in the laminating direction may be preferably in the order of about 
4 mm for the laminated-structure 10 having length of 16 mm. Even in such 
piezoelectric actuator having such temperature compensating member, a 
detected displacement also becomes inconsistent with an actual 
displacement when elastic strain of the active portion 2 is different from 
that of the protective layers 31 and 32, resulting in degraded accuracy of 
displacement control. 
That is, when a piezoelectric actuator includes, in addition to an active 
portion which generates displacement corresponding to a drive voltage, 
portions whose elastic strains and/or strains due to thermal expansion are 
different each other, preciseness of displacement control according to the 
conventional technique tends to be degraded when external force in a 
laminating direction and/or temperature is changed. Such degradation of 
displacement control accuracy also occurs by change of drive voltage when 
the active portion is constituted with a plurality of portions made of 
materials whose piezoelectric properties are different from each other. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a laminated-type 
piezoelectric actuator having strain gages, with which there is no 
degradation of accuracy of displacement control even when drive voltage 
therefor, external force to be exerted thereon and/or temperature thereof 
is changed. 
A piezoelectric actuator according to the present invention which has a 
laminated structure including an active portion formed by alternatingly 
laminating a plurality of ceramics layers having piezoelectric 
characteristics and a plurality of internal electrode layers, for 
generating strain in a direction perpendicular to the ceramics layers and 
the internal electrode layers according to a drive voltage externally 
applied thereto and a plurality of other portions, at least one of strain 
in the direction caused by drive voltage applied to the 
laminated-structure, elastic strain in the direction and strain caused by 
thermal expansion in the direction of adjacent ones of the active portion 
and the plurality of the other portions being different from each other, 
is characterized by means fixedly associated with the active portion and 
the plurality of the other portions, respectively, for detecting strains 
generated in the direction in the active portion and the plurality of the 
other portions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 3, a laminated-structure 10 has the same structure as 
that shown in FIG. 2 and is fabricated by a method which will be described 
with reference to FIG. 2. 
First, green ceramics sheets are obtained by slip-casting slurry of a 
mixture of powder of piezoelectric ceramics such as lead titanate or lead 
titanate zirconate, a binder and an organic solvent. One surfaces of the 
green ceramics sheets are screen-printed with a conductive paste 
containing powder of a mixture of silver and palladium or platinum powder 
dispersed therein to form internal electrodes. Then, a plurality of the 
green ceramics sheets having the conductive paste thereon are laminated 
and green ceramic sheets printed with no conductive paste are disposed on 
the uppermost and lowermost green ceramics sheets to form a laminated 
piezoelectric ceramics body. The green ceramics sheets having the internal 
electrodes printed are adapted to form an active portion 2 and the green 
ceramics sheets having no internal electrodes are adapted to form an upper 
and lower protective layers 31 and 32, respectively. After the laminated 
piezoelectric ceramics body is heated to 100.degree. C. and intimately 
stacked under pressure, the binder contained therein is decomposed and 
removed at about 500.degree. C. and, then, sintered at about 1000.degree. 
C., resulting in a laminated-structure including the protective layers 31 
and 32 and the piezoelectric ceramics layers 4 and the internal electrode 
layers 5 laminated alternatingly with the piezoelectric ceramics layers. 
Thereafter, odd numbered internal electrode layers 5 exposed on one side 
surface of the sintered body are electrically insulated by insulators 6 
and even numbered internal electrode layers 5 exposed on the other side 
surface of the sintered body are electrically insulated similarly such 
that the internal electrode layers 5 which are not insulated appear 
alternatingly on the both surfaces of the sintered body. The internal 
electrode layers 5 which are not insulated and exposed on the both side 
surfaces of the sintered body are connected electrically to external 
electrodes 71 and 72, respectively, resulting in a laminated structure 10. 
The sintered body of the laminated-structure 10 which includes the 
piezoelectric ceramics layer 4 having the conductive paste forms an active 
portion 2 which generates strain due to piezoelectric effect caused by 
electric field generated by an application of a drive voltage thereto and 
an upper and lower portions of the laminated structure 10 which include 
the piezoelectric ceramics layer having no conductive paste form an upper 
and lower protective layers 31 and 32, respectively, which do not generate 
strain due to the drive voltage. A cross sectional area of the laminated 
structure 10 in this embodiment in a direction perpendicular to the 
laminating direction is 5 mm.times.5 mm and length thereof in the 
laminating direction is 16 mm. Length of the active portion 2 is 12 mm and 
lengths of the protective layers 31 and 32 are 2 mm, respectively. 
In the first embodiment shown in FIG. 3, strain gages 81, 82 and 83 each 
having resistance value of 120 .OMEGA. and gage rate of 2 are attached to 
surfaces of the protective layer 31, the active portion 2 and the 
protective portion 32 , respectively. Gage leads 9 of these strain gages 
are separately provided. Each of the strain gages used in the first 
embodiment has a base of polyimide sheet 0.1 mm thick and is adhered to 
the actuator by an adhesive of epoxy, phenol or cyanoacrylate type under 
pressure of 2 kgf/cm.sup.2. 
Change rates of resistance of these strain gages 81, 82 and 83 are 
proportional to magnitude of strain in the respective portions (the 
protective layers 31 and 32 and the active portion 2). In this embodiment, 
in order to measure such changes of resistances, these strain gages are 
connected to Wheatstone bridges 101, 102 and 103 each of 120 .OMEGA., 
respectively, as shown in FIG. 4. The Wheatstone bridges are commonly 
connected to a constant voltage source 11, from which strain signals 121, 
122 and 123 whose magnitudes are proportional to coefficient of strain 
detected by the strain gages are obtained. The detection of resistance 
change of the strain gage can be done by any of other known methods than 
the D.C. bridge method mentioned above. 
An amount of displacement of a thing when the latter is deformed is a 
product of length and strain rate of the thing and a total amount of 
displacements of a plurality of series-connected things is a sum of 
displacements of the respective things. Therefore, the total displacement 
can be obtained by knowing lengths and strain rates of these things. 
In this embodiment in which the strain gages 81, 82 and 83 have identical 
resistance values and identical strain rates and the Wheatstone bridges 
101, 102 and 103 connected to the common constant voltage source 11, the 
magnitudes of the strain signals 121, 122 and 123 are analogous to the 
strain rates detected by the strain gages 81, 82 and 83, respectively. In 
this embodiment, the protective layers 31 and 32 and the active portion 2 
are 2 mm, 2 mm and 12 mm long in the laminating direction, respectively. 
Therefore, an operating circuit 13 amplifies the strain signals 121, 122 
and 123 with amplification factors equal to ratios of lengths of the 
portions to which the respective strain gages 81, 82 and 83 are attached 
and adds them, resulting in a displacement signal 14 proportional to the 
total displacement of the laminated-structure 10. 
In this embodiment, a magnitude y of the displacement signal 14 obtained by 
the operating circuit 13 is represented by 
EQU y=k.times.(2a+2b+12c) 
where a, b and c are magnitudes of the respective strain signals 121, 122 
and 123 and k is an arbitrary constant. 
By supplying the displacement signal 14 thus obtained to a feedback control 
circuit 15 having a comparison operation circuit, a displacement 
corresponding to a control signal 16 assigning an aimed drive voltage 
value and having no hysteresis can be obtained. 
In an example, when the bridge resistances are 120 .OMEGA., respectively, 
the voltage value of the constant voltage source is 2 V and the voltage 
values of the respective strain signals are -0.1 V to +0.1 V, a change of 
1 mV is obtained. By amplifying the amount of change of 1 mV, the 
displacement signal 14 of -5 V to +5 V is obtained. By setting the control 
signal voltage to -5 V to +5 V and comparing it with the displacement 
signal 14 and amplifying a result of comparison in the feedback control 
circuit 15, a drive voltage of the piezoelectric actuator in a range from 
0 to 100 V is obtained. 
Comparing the piezoelectric actuator using the laminated-structure of this 
embodiment with a conventional piezoelectric actuator using the same 
laminated-structure as that of the present invention, a deviation of 1 
.mu.m or less between the measured displacement and the actual 
displacement was observed in the present actuator when an external force 
of 100 kgf is exerted thereon while, in the conventional actuator, it was 
5 .mu.m under the same conditions. 
The Wheatstone bridges and the operating circuit in this embodiment can be 
simplified by connecting the three strain gages in series as shown in FIG. 
5 which is a perspective view of a second embodiment of the present 
invention. In FIG. 5, strain gages 81 and 82 are adhered to surfaces of 
protective layers 31 and 32 of a laminated-structure 10 which is the same 
as that of the first embodiment and a strain gage 83 is adhered to a 
surface of an active portion 2 of the laminated-structure 10. Resistance 
value of the strain gage 83 is 90 .OMEGA. and resistance values of the 
strain gages 81 and 82 are 15 .OMEGA., respectively, and gage rates of the 
strain gages 81, 82 and 83 are 2 commonly. A series connection of these 
three strain gages 81, 82 and 83 is equivalent to a single strain gage of 
120 .OMEGA.. Characteristics of this single strain gage will be described. 
Assuming that a strain gage having resistance value R and gage rate K is 
adhered to a portion of a thing having length L and the resistance R of 
the strain gage is changed by .DELTA.R as a result of displacement 
.DELTA.L of the thing deformed at strain rate .epsilon., the following 
equation is established: 
##EQU1## 
Therefore, in such case as this embodiment where the strain gages are 
adhered to the respective three portions, that is, the protective layers 
31 and 32 and the active portion 2, respectively, resistance changes 
.DELTA.R of the respective strain gages become proportional to 
displacement .DELTA.L of the respective portions by making a product of 
the resistance value R and gage rate K of the strain gage proportional to 
the length L of the thing to which the strain gage is adhered, that is, by 
making a value of L(R.times.K) in each portion constant. By connecting 
these three strain gages in series, displacement of the laminate-structure 
10 can be measured by measuring the resistance change of the series 
connected strain gages. In this embodiment, the strain gages are selected 
according to this principle. 
Although, in the first embodiment, three bridge circuits are necessary for 
the respective strain gages in addition to the operation circuit for 
operating the three strain signals, it is enough in the second embodiment 
to provide the single bridge circuit without requiring any operation 
circuit. 
FIG. 6 shows a third embodiment of the present invention. In FIG. 6, a 
laminated-structure 10 is the same as that used in the first embodiment. 
Since protective layers 31 and 32 are made of the same material and have 
the same cross sectional areas and the same Young's modulus, strains of 
them against external force are equal. Further, since thermal expansion 
coefficients of them are equal, strains thereof against temperature change 
are also equal. Since, therefore, their strains are always equal, it is 
possible to remove either one of the strain gages. In order to make 
resistance change of a single strain gage, in FIG. 6, the strain gage 82, 
matched with those of the protective layers 31 and 32, the strain gage 82 
has a resistance value calculated on the basis of a sum of lengths of the 
protective layers 31 and 32. In this embodiment, the strain gage 82 is 
attached to only the protective layer 32. The resistance values of the 
strain gage 82 and the strain gage 83 are 30 .OMEGA. and 90 .OMEGA., 
respectively, and their gage rates are commonly 2. 
FIG. 7A shows a fourth embodiment of the present invention. Contrary to the 
third embodiment in which the strain gages 82 and 83 are attached 
separately as shown in FIG. 6, a single strain gage 87 formed by 
series-connecting gage portions 181 and 182 on a common base 17 as shown 
in FIG. 7B is used in the fourth embodiment to thereby improve the 
reliability of connection and reduce manufacturing steps. Resistance 
values of the gage portions 181 and 182 are 30 .OMEGA. and 90 .OMEGA., 
respectively, for a laminated-structure 10 which is the same as that used 
in the first embodiment and their gage rates are commonly 1.98. 
In a fifth embodiment shown in FIG. 8, a strain gage 88 having resistance 
value of 120 .OMEGA. is attached to a boundary area between an active 
portion 2 and a protective layer 32 of a laminated-structure 10 which is 
the same as that used in the first embodiment. The boundary area to which 
the strain gage 88 is attached covers the protective layer 32 by a 
distance L1 and the active portion 2 by a distance L2 where L1:L2=3:1. 
This is equivalent to a case where strain gages having resistance values 
30 .OMEGA. and 90 .OMEGA., respectively, are connected in series. 
According to the fifth embodiment, the same effect as that obtained in the 
third or fourth embodiment can be obtained with a single strain gage 
having a single gage portion. Further, since identical strain gages have 
identical gage rates, it is enough to consider only the ratio between L1 
and L2. 
A sixth embodiment shown in FIG. 9 is constituted by adhering a temperature 
compensating member 19 formed of stainless steel (15 ppm/.degree.C.) and 4 
mm long to the structure of the second embodiment shown in FIG. 5 and 
adhering a strain gage 84 to the temperature compensating member 19. In 
this embodiment, since a laminated-structure 10 contracts by 0.06 .mu.m 
for temperature increase of 1.degree. C., a total thermal expansion 
thereof as a piezoelectric actuator is substantially zero. By connecting 
strain gages 81, 82, 83 and 84 having resistance values 72 .OMEGA., 12 
.OMEGA., 12 .OMEGA. and 24 .OMEGA., respectively, in series, it is 
possible to correct inconsistency of aimed displacement of the temperature 
compensating member 19 and the laminated-structure 10 due to difference in 
elastic strain when external force is changed. That is, in the 
conventional piezoelectric actuator, it is impossible practically to 
attach such temperature compensating member for improving its temperature 
characteristics thereto since strains of the temperature compensating 
member and the laminated structure become much different from each other 
when external force is changed. This embodiment exhibits correct 
displacement for not only temperature change but also change of external 
force. For example, in the conventional piezoelectric actuator having no 
temperature compensating member, a controllable range of displacement with 
respect to an applied voltage is in the order of 3 .mu.m at 50.degree. C. 
In this embodiment, however, a controllable range is 0.5 .mu.m or less at 
50.degree. C. and a deviation when external force of 100 kgf is exerted is 
1.5 .mu.m or less. 
As described hereinbefore, the piezoelectric actuator according to the 
present invention is composed of a plurality of portions whose strains 
caused by drive voltage applied, elastic strains and strains due to 
thermal expansion are different from each other and strain gages for 
detecting strains of the respective portions. 
Thus, according to the present invention, it is possible to detect strains 
of the respective portions even when external force is changed and/or 
temperature is changed, unlike the conventional piezoelectric actuator in 
which only strain due to drive voltage is detected. Thus, by detecting a 
displacement of the piezoelectric actuator as a whole on the basis of 
strains detected in the respective portions and feeding it back to the 
drive voltage, it is possible to substantially improve the control 
accuracy of displacement when external force and/or temperature is 
changed, compared with the conventional piezoelectric actuator. Further, 
the accuracy of displacement amount can be maintained even in a 
piezoelectric actuator including a laminated structure composed of a 
plurality of portions formed of materials having different piezoelectric 
characteristics.