Latching piezoelectric relay

An improved piezoelectric relay based on a bimorph actuator is described. The relay maintains its state in the absence of power thereto. The relay operates by changing the residual polarization of the piezoelectric strips used in the bimorph actuator.

The present invention relates to piezoelectric relays and more particularly 
to latching piezoelectric relays. 
Relays having an actuator or bender consisting of a piezoelectric bimorph 
element are well known to the prior art. Such relays typically employ a 
finger-like bimorph actuator which is constructed from two thin 
rectangular sheets of piezoelectric material bonded to the opposite sides 
of a center shim which is typically a brass sheet. One end of the bimorph 
actuator is typically immobilized in a clamp while the other end, which 
carries a relay contact, is free to move in response to electrical fields 
generated in the bimorph by maintaining predetermined potential 
differences between electrodes which are included in the bimorph actuator. 
The face of each sheet which is opposite that bonded to the center shim is 
typically plated with a conductive material to form an electrode. One such 
electrode is formed on the top surface of the bimorph, and a second such 
electrode is formed on the bottom surface of the bimorph. The electric 
fields in question are generated by the application of a potential between 
the center shim which forms a third electrode and these surface 
electrodes. 
Relays of this type must be continually powered to maintain the state of 
the relay. To be useful, the relay must have at least two states, one in 
which the contacts are closed and one in which the contacts are separated. 
At least one of these states requires that power be applied to the bimorph 
element. In a typical prior art piezoelectric relay, the bimorph element 
has one of the contacts in question on the end thereof. When electric 
fields are applied to be piezoelectric sheets which make up the bimorph 
element, this contact is moved so as to contact the second contact which 
is typically mounted on a fixed surface and positioned so as to engage the 
contact on the bimorph element when the bimorph element is deflected from 
a resting position. When the electric fields are removed from the bimorph 
element, the bimorph element returns to a neutral position and the 
contacts are separated. Hence, to maintain the contacts in the closed 
position, power must be continually applied to the bimorph element or some 
form of mechanical latching apparatus must be included in the relay to 
prevent the contacts from separating when power is removed from the 
bimorph element. 
Both of these methods of maintaining the contacts in the closed position 
are unsatisfactory. Providing a source of uninterrupted power is often not 
possible at an acceptable cost. In addition, maintaining power on the 
bimorph element increases the cost of operation of the relay. Finally, 
satisfactory mechanical latches are expensive to construct and decrease 
the reliability and lifetime of the relay. 
Broadly, it is an object of the present invention to provide an improved 
piezoelectric relay. 
It is a further object of the present invention to provide a piezoelectric 
relay which does not require that power be maintained to the relay or that 
mechanical latch elements be employed in order to maintain the state 
thereof. 
These and other objects of the present invention will become apparent to 
those of ordinary skill in the relay arts from the following detailed 
description of the invention and the accompanying drawings. 
SUMMARY OF THE INVENTION 
The present invention comprises a bimorph actuated relay constructed from a 
cantilever mounted piezoelectric bimorph element having first and second 
planar piezoelectric strips. The position of the free end of said bimorph 
element is determined by altering the the residual polarization of at 
least one of the first or second planar piezoelectric strips in response 
to predetermined control signals. In the preferred embodiment, the 
residual polarization is altered by applying electrical signals to 
electrodes which sandwich each of the piezoelectric strips. To depolarize 
a piezoelectric strip, an AC signal is applied across the strip to create 
an electric field in the strip which alternates in direction. To polarize 
a piezoelectric strip, a pulsating signal is applied across the strip 
which creates a pulsating electric field which always points in the same 
direction. Alternatively, a piezoelectric strip may be polarized by 
applying a constant voltage across the strip.

DETAILED DESCRIPTION OF THE INVENTION 
The advantages of the present invention will become more apparent after a 
discussion of a typical prior art piezoelectric bimorph relay element 112 
shown in FIG. 1. 
FIG. 1 is a cross-sectional view of a piezoelectric relay 100 constructed 
from a bimorph element 112 which is actuated by switching voltages applied 
to the top and bottom electrodes thereof. The bimorph element 112 is 
mounted in a cantilever manner over a mounting surface 114 by attaching 
one end to a raised portion 113 on the mounting surface 114. The free end 
of bimorph element 112 includes a first electrical contact 116 which is 
electrically isolated from bimorph element 112. The contact 116 is brought 
into contact with a second electrical contact 118 when the free bimorph 
end on which the first electrical contact 116 is mounted moves toward the 
surface 114. 
The bimorph element 112 typically consists of two planar strips of 
piezoelectric material 120 and 122 which are bonded to three planar 
electrodes 124, 126 and 128. The top electrode 124 and the bottom 
electrode 128 are typically constructed by electroless plating of a 
conducting material such as nickel on the corresponding piezoelectric 
strips although other methods of bonding the electrodes to the surface of 
the piezoelectric strips will be apparent to those skilled in the art. 
The center electrode 126 is typically a brass shim connected to plated 
electrodes on the surfaces of the piezoelectric strips which are adjacent 
to said shim. For clarity, these plated electrodes are omitted from FIG. 
1. Center electrode 126 may also be constructed by depositing a metallic 
layer between two sheets of piezoelectric ceramic material such as lead 
zirconate titanate prior to firing the ceramic material. 
Each of the strips of piezoelectric material 120 and 122 is polarized such 
that the application of an electrical field across the strip will result 
in a change in the length of the strip. If a potential is applied across 
only one of the strips, bimorph element 112 will bend causing the free end 
thereof to move in a direction which is perpendicular to the surface of 
the piezoelectric strips. The direction of motion will be toward the strip 
whose length decreased. 
This polarization is typically accomplished by applying voltages between 
the two electrodes on each side of the piezoelectric sheet while cooling 
the piezoelectric sheet in question from a temperature above the Curie 
point of the piezoelectric material to a temperature below said Curie 
point. Alternatively, the polarization of some piezoelectric materials may 
be carried out at room temperature by applying significantly larger 
potentials across the piezoelectric strips then those needed if the 
materials are heated above their Curie point. The polarizing electric 
fields cause micro domains in the piezoelectric material to become 
aligned. 
The length of each piezoelectric strip depends on the degree of 
polarization of that strip. If the unpolarized length of the piezoelectric 
strip is L.sub.0, then after the above described polarization, the strip 
will have a new length, L.sub.r. In general, L.sub.r will be less than the 
L.sub.0 but greater than the minimum length for the piezoelectric strip, 
since the degree of polarization which remains after the polarizing 
electric fields are removed from the piezoelectric strip is less than the 
maximum degree of polarization for the piezoelectric strip. To obtain the 
maximum degree of polarization, an electric field must be maintained 
across the piezoelectric strip. 
After polarization, the length of each piezoelectric strip may be changed 
by applying an electric field to that strip which either increases or 
decreases the polarization of the strip. The direction of the applied 
electrical field relative to the direction of polarization determines 
whether the length of the strip will increase or decrease. If the electric 
field produced by the potentials on the electrodes is in the same 
direction as the electric field used to polarize the piezoelectric strip, 
the piezoelectric strip will decrease in length because it will become 
further polarized. However, when the electric field is removed, the length 
of the piezoelectric strip will return to L.sub.r. 
If the direction of the applied electric field is opposite to that used to 
polarize the piezoelectric strip, the piezoelectric strip will become 
partially depolarized. This will cause the piezoelectric strip to increase 
in length. Provided the applied electric field is small, the piezoelectric 
strip will return to its initial polarization state and length, L.sub.r 
when the electric field is removed. If, however, the electric field is 
large, the length of the piezoelectric strip when the field is removed 
will greater than L.sub.r, because the piezoelectric strip will have 
become partially depolarized. However, it is important to note, that the 
length of the piezoelectric strip will not return to L.sub.0. If the 
applied electric field is too large, the piezoelectric strip will merely 
become polarized in the opposite direction. 
Referring to bimorph element 112, the polarization of the piezoelectric 
material in strip 120 is in the same direction as that of the material in 
strip 122 as indicated by the arrows 121 and 123 which point in the 
direction of the electric fields used to polarize the piezoelectric strips 
in question. The electric fields used to actuate the relay are typically 
generated by the application of an electrical potential between the 
electrodes 124 and 126 simultaneously with the application of the opposite 
potential between the electrodes 126 and 128. This potential causes one of 
the strips to shorten and the other to elongate; hence, this mode of 
driving the bimorph will be referred to as the "push-pull" driving mode. 
As a result of the application of these potentials, the bimorph will 
either bend toward the surface 114 or away from said surface depending on 
the direction the electrical fields generated. Typically, one direction is 
used to close the relay contacts, the other is used to move the contacts 
away from each other. In principle, this second motion can be used to 
cause a second set of contacts 130 and 132 to close thus implementing a 
single pole double throw relay. 
A driving circuit for switching the voltages is indicated at 190 in FIG. 1. 
Such circuitry is conventional in the electronic arts and hence has not 
been shown in detail. Driving circuit 190 connects each electrode 
connected thereto to one of two potentials V.sub.1 or V.sub.2. The choice 
of which potential is connected to each electrode is determined by a 
control signal on an input line 191. 
In prior art piezoelectric relays, every attempt is made to avoid permanent 
depolarization of the piezoelectric strips. Depolarization of one or both 
of the piezoelectric strips leads to a phenomenon which is often referred 
to as "sag". When no electric fields are applied to the bimorph element, 
the bimorph element is said to be in its neutral position. In the neutral 
position, both of the piezoelectric strips are equally polarized, having 
lengths equal to L.sub.r. If one of the piezoelectric strips becomes 
partially depolarized, its length will increase. Hence, the position of 
the bimorph element in the neutral position will change. As a result, the 
contacts may be brought together when they are not supposed to contact 
each other or the load rating of the relay altered. To avoid sag, prior 
art relays have limited the electric fields applied to the strip which is 
driven in the "push" direction, i.e., the anti-polarization direction, or 
even refrained from driving either strip in this direction. Both of these 
strategies impose design constraints on the relays which increase the 
amount of piezoelectric material needed to provide a relay with a given 
voltage and current rating. 
One problem with prior art piezoelectric relays is that such relays will 
not maintain contact between contacts 116 and 118 if the potentials are 
removed from electrodes 124, 126, and 128. As noted above, when power is 
removed from bimorph element 112 it returns to the neutral position in 
which the relay contacts 116 and 118 are separated. Hence, relay 100 
requires that power be maintained on electrodes 124, 126, and 128 at all 
times. This is not always practical. In addition, the operating cost of 
relay 100 is increased by the need to supply power to driving circuit 190 
at all times. 
In contrast to prior art designs, the present invention utilizes the 
depolarization of one of the piezoelectric strips to provide a relay which 
maintains its state when power is removed therefrom. For the purposes of 
this discussion, the degree of polarization of the piezoelectric strips 
which remains when power is removed therefrom will be referred to as the 
residual polarization of these strips. In prior art piezoelectric relays, 
the degree of residual polarization is not altered during the operation of 
the relay. In the present invention, the residual polarization is altered 
each time the state of the relay is changed. 
One embodiment of a relay according to the present invention is shown at 
200 in FIG. 2. Relay 200 includes a bimorph element 202 mounted in a 
cantilever manner over a surface 204 by spacing bimorph element 202 from 
surface 204 using a spacer 206. A contact 208 is affixed to the free end 
of bimorph element 202. Contact 208 is caused to touch a second contact 
209 mounted on surface 204 when bimorph element 202 bends toward surface 
204. 
Bimorph element 202 is constructed from two planar strips of piezoelectric 
material shown at 210 and 212. When both of these piezoelectric strips are 
polarized, bimorph element 202 is in the neutral position which is the 
position shown in FIG. 2. If one of these piezoelectric strips were to 
become depolarized, the length of that strip would increase which, in 
turn, would cause bimorph element 202 to bend in a direction away from the 
depolarized piezoelectric strip. Hence, bimorph element 202 may be caused 
to bend toward surface 204 by depolarizing piezoelectric strip 210 while 
maintaining piezoelectric strip 212 in the polarized state. And, bimorph 
element 202 may be caused to bend toward surface 220 by depolarizing 
piezoelectric strip 212 while maintaining piezoelectric strip 210 in the 
polarized state. This motion may be used to further separate contacts 208 
and 209 and/or to make contact between a second set of contacts 222 and 
224. 
The residual polarization of the piezoelectric strips of the present 
invention are preferably altered by applying appropriate voltages to 
planar electrodes bonded to the surfaces of the piezoelectric strips. For 
the purpose of this discussion, each of the piezoelectric strips shown in 
FIG. 2 has two planar electrodes bonded to the surfaces thereof. 
Piezoelectric strip 210 is sandwiched between planar electrodes 232 and 
234. Piezoelectric strip 212 is sandwiched between planar electrodes 236 
and 238. 
To polarize a given piezoelectric strip, a constant electric field is 
generated in said piezoelectric strip by applying a constant potential 
difference between the electrodes which sandwich said strip. The potential 
difference must be above a minimum value which depends on the 
piezoelectric material from which the piezoelectric strip is constructed 
and the thickness of that strip. For example, if the piezoelectric strip 
is constructed from lead zirconate titanate, the potential difference must 
be sufficient to create an electric field of at least 1.1.times.10.sup.4 
volts/cm. 
To depolarize a piezoelectric strip, an alternating electric field is 
generated in said piezoelectric strip by applying an AC voltage between 
the electrodes which sandwich the piezoelectric strip in question. The 
magnitude and frequency of the AC voltage needed to depolarize the 
piezoelectric strip will depend on the piezoelectric material and its 
thickness. The frequency of the AC voltage must be sufficiently high to 
prevent all of the micro domains in the piezoelectric strip from becoming 
aligned during a single cycle of the AC voltage. At the same time, the 
frequency must be sufficiently low to allow at least some of the domains 
to become aligned with the electric field during each cycle of the AC 
voltage. For example, a lead zirconate titanate piezoelectric strip having 
a thickness of 0.007 inches may be depolarized by applying an AC voltage 
of 300 volts at a frequency of 1 Hz. 
Hence, to set relay 200 such that contacts 208 and 209 are touching, 
piezoelectric strip 210 is connected to a power source which provides an 
AC voltage across said strip, and piezoelectric strip 212 is connected to 
a power source which provides a constant potential across said strip. 
These potentials are applied for a period of time which is sufficient to 
cause piezoelectric strip 210 to become depolarized and piezoelectric 
strip 212 to become polarized. The potentials may then be removed, and 
bimorph element 202 will remain bent such that contacts 208 and 209 are 
touching, since the residual polarizations of the piezoelectric strips 
will have been changed. Similarly, contacts 208 and 209 may be separated 
by applying a polarizing potential to strip 210 either with or without a 
depolarizing AC potential being applied to piezoelectric strip 212. If a 
depolarizing potential is not applied to piezoelectric strip 212, the 
final resting position of bimorph element 202 when the potentials are 
removed from the electrodes sandwiching the piezoelectric strips will be 
the neutral position. If a depolarizing potential is applied to 
piezoelectric strip 212, the final resting position will be such that 
contacts 222 and 224 are touching. 
Although the operation of the present invention has been described in terms 
of a constant polarizing potential being used to polarize one of the 
piezoelectric strips, it will be apparent to those skilled in the 
piezoelectric arts that a piezoelectric strip may also be polarized by 
applying a pulsating potential across the piezoelectric strip in question. 
So long as the peak magnitude of the potential difference is sufficiently 
large and the average direction of the electric field generated in the 
piezoelectric strip by said pulsating potential difference is in the same 
direction, the piezoelectric strip will become at least partially 
polarized. It is preferred, however, that the direction of the electric 
field be always in the same direction since this results in more complete 
polarization. 
By using a pulsating electric field for polarizing the piezoelectric 
strips, a relay according to the present invention which has fewer 
electrodes than relay 200 shown in FIG. 2 may be constructed. Such a relay 
is shown in FIG. 3 at 300. Relay 300 is similar to relay 100 shown in FIG. 
1 in that it is constructed from a bimorph element 312 which is mounted in 
a cantilever manner over a surface 314 by spacing bimorph element 312 from 
said surface with a spacing element 313. When the free end of bimorph 
element 312 moves toward surface 314, a connection is made between two 
electrical contacts 316 and 318. 
Bimorph element 312 is constructed from two piezoelectric strips 320 and 
322 which are sandwiched between three electrodes 324, 326, and 328. When 
the free end of bimorph element 312 moves toward surface 314 in response 
to the state of polarization of piezoelectric strips 320 and 322 being 
changed, electrical contact is made between two relay contacts 316 and 
318. 
A driving circuit for changing the residual polarizations of piezoelectric 
strips 320 and 322 is shown at 350. Driving circuit 350 may be used to 
generate an electric field having a direction which alternates in one of 
the piezoelectric strips while simultaneously generating a pulsating 
electric field having an electric field always in the same direction in 
the other of the piezoelectric strips. 
Driving circuit 350 includes three half-bridge circuits 360, 362, and 364. 
Each half-bridge circuit is used to connect one of the electrodes shown in 
FIG. 3 to a potential selected from two potentials shown as ground and V 
in FIG. 3. The identity of the potential so selected is specified by a 
signal level on a control line connected to each half-bridge circuit. The 
control line for circuit 360 is shown at 361. The control line for circuit 
362 is shown at 363, and the control line for circuit 364 is shown at 365. 
The transistors used to construct the half bridge circuits are preferably 
high voltage transistors since the voltages needed to change the 
polarizations of the piezoelectric strips may be as high as several 
hundreds of volts. 
The voltages applied to control lines 361, 363, and 365 are determined by a 
control signal on line 370 and by a square wave generator 375. The output 
of the square wave generator is applied to control line 363. Either the 
complement of the output of square wave generator 375 or a constant 
potential V.sub.c is applied to control line 361 by NAND gate 381. The 
choice of which signal is applied to control line 361 is specified by 
control signal 370. Similarly, either the complement of the output of 
square wave generator 375 or a constant potential V.sub.c is applied to 
control line 365 by NAND gate 382. The choice of which signal is applied 
to control line 365 is specified by control signal 370. When V.sub.c is 
applied to control line 361, the complement of the output of square wave 
generator 375 is applied to control line 365 and vice versa. 
Consider the case in which bimorph element 312 is to be bent toward surface 
314. To accomplish this, piezoelectric strip 320 must be depolarized and 
piezoelectric strip 322 must be polarized. Driving circuit 350 
accomplishes this by placing a constant potential on electrode 328, a 
square wave on electrode 326, and the complement of said square wave on 
electrode 324. This potential pattern is illustrated in FIG. 4. The 
potentials on electrodes 328, 326, and 324 are shown at 404, 408, and 406, 
respectively. The potential on the control line 370 is shown at 402. The 
electric fields generated in piezoelectric strips 320 and 322 by the 
potentials on electrodes 328, 326, and 324 are shown in FIG. 5 at 412 and 
410, respectively. The electric field strength, E, is proportional to the 
potential V on line 372 shown in FIG. 3. 
Since the electric field generated in piezoelectric strip 320 alternates in 
direction, piezoelectric strip 320 will be depolarized, and its length 
will increase. The electric field generated in piezoelectric strip 322 
does not change direction; hence piezoelectric strip 322 will become 
polarized and its length will be less than that of piezoelectric strip 
320. As a result, bimorph element 312 will bend toward surface 314. 
Once piezoelectric strips 320 and 322 are depolarized and polarized, 
respectively, power may be removed from the driving circuit and the state 
of relay 300 will remain unchanged. That is, contacts 316 and 318 will 
remain touching. Power may be removed from driving circuit 350 by 
disconnecting line 372 from the power source which maintains that line at 
V. 
To separate contacts 316 and 318, the complement of the above control 
signal value is provided on line 370. This results in electrode 324 being 
held at the constant potential of V while electrodes 326 and 324 are 
subjected to a square wave and the complement thereof, respectively. As a 
result, piezoelectric strip 320 will become polarized, and piezoelectric 
strip 322 will become depolarized. Thus piezoelectric strip 320 will 
become shorter than piezoelectric strip 322, and bimorph element 312 will 
bend upward. 
The potential patterns and resulting electric fields are shown in FIGS. 6 
and 7, respectively. The potentials on electrodes 328, 326, and 324 are 
shown at 404', 408', and 406', respectively. The potential on the control 
line 370 is shown at 402'. The electric fields generated in piezoelectric 
strips 320 and 322 by the potentials on electrodes 328, 326, and 324 are 
shown in FIG. 7 at 412' and 410 , respectively. 
Once piezoelectric strips 320 and 322 are polarized and depolarized, 
respectively, power may be removed from the driving circuit and the state 
of relay 300 will remain unchanged. That is, contacts 316 and 318 will 
remain separated. As noted above, power may be removed from driving 
circuit 350 by disconnecting line 372 from the power source which 
maintains that line at V. 
It will be apparent to those skilled in the art that the bimorph element of 
the present invention may be used as an actuator in other apparatuses than 
relays. The bimorph element can, in principle, be used for any purpose 
requiring a mechanical arm having two positions which are to be set by 
electrical control signals. 
Accordingly, an improved piezoelectric actuator and relay have been 
described in which the state of the relay is maintained even in the 
absence of power being applied thereto. Modifications of the present 
invention will be apparent to those of ordinary skill in the relay arts; 
hence the scope of the present invention is to be limited only by the 
following claims.