JPH0471858A - Driving device of piezoelectric element - Google Patents

Abstract

PURPOSE:To realize low power consumption and low heating and besides to prevent needless displacement of a piezoelectric element by a method wherein electric charges accumulated in the piezoelectric element are effectively returned to a power source, and the electric charges remaining in the piezoelectric element are effectively released in non-excitation. CONSTITUTION:In the case where, for instance, the invention is applied to a driving circuit of a piezoelectric actuator of a dot printer, a circuit as given in the figure is formed, a control circuit 55 intercepts a transistor (Tr)2 by indication of drive, and makes Tr1 conduct. After accumulating enough charges in a piezoelectric element P, Tr1 is intercepted, and Tr3 is made to conduct. A current made to flow to a coil 53 is damped, the stand by Tr3 is intercepted for displacement time necessary for printing, and Tr2 is made to conduct. Though charges accumulated in the element P flow through a closed loop, consumption as heat hardly exists. All electric energy accumulated in the element P is accumulated in a coil L as magnetic energy adjacent to the coil 53 after specific time. When Tr2 is intercepted at this instant, all the accumulated electric energy is returned to a dc power source 51 via a diode D1 by action of the coil 53.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a drive device for a piezoelectric element, and more particularly to a drive device for a piezoelectric element that can realize low power consumption, low heat generation, and low cost.

[Prior Art] A piezoelectric element can resonate at a predetermined frequency while repeating charging and discharging together with a coil.

However, in some cases, such as when used to drive a print wire in a printer, it is necessary to maintain the displacement state of - times of displacement for a predetermined period of time. As a drive device in such a case, for example, a device disclosed in Japanese Patent Application Laid-Open No. 63130357 is known.

This device is constructed as shown in FIG. That is, piezoelectric element 1
02 is connected to the power supply terminal of the power supply 101 via the emitter-collector of the switching transistor 103. This switching transistor 103 has a drive signal V
2 is input, a power supply voltage is applied to the piezoelectric element 102. Further, a diode 110 is connected in parallel to the piezoelectric element 102 in such a direction that a voltage is applied in the opposite direction when the switching transistor 103 is in an on state. Further, one end of a coil 105 is connected to the non-ground terminal of the piezoelectric element 102, and the other end of the coil 105 is connected to a power terminal of the power source 101 via a diode 109.

This diode 109 is oriented to allow current to flow only from the piezoelectric element 102 to the power supply terminal side.

A connecting line between the diode 109 and the coil 105 is connected to the collector of the switching transistor 106. The emitter of this switching transistor is connected to ground, and the base of the switching transistor 108
connected to the collector. The emitter of this switching transistor 108 is connected to the piezoelectric element 102 via a resistor.
connected to the non-ground terminal of the The emitter of the switching transistor 108 is connected to the collector of the switching transistor 107, and the emitter is connected to ground. Also, this switching transistor 1
The release signal ■3 is input to the base of 07.

In the device as described above, when the drive signal v2 becomes negative, the switching transistor 103 becomes conductive. Then, the electric charge from the power supply terminal is charged to the piezoelectric element 102 via the switching transistor 103, and the piezoelectric element 102 is displaced.

When canceling the displacement of the piezoelectric element 102, the drive signal V
2 is set low, and the release signal V3 is set high. Then, the supply of new charge is cut off, and the switching transistor 107 becomes conductive. Then, the base voltage of the switching transistor 108 becomes Ov. On the other hand, since the emitter of the switching transistor 108 is connected to the charged piezoelectric element, a predetermined voltage is applied thereto. Therefore, a predetermined voltage is applied between the base and emitter of the switching transistor 108, and the switching transistor 108 becomes conductive.

Therefore, a voltage equivalent to the terminal voltage of the piezoelectric element 102 is applied to the base of the switching transistor 106, and the switching transistor 106 becomes conductive. Therefore, the piezoelectric element 102 (here equivalent to a capacitor) and the coil 105 form a resonant circuit arranged in series.

Therefore, the current and voltage flowing through coil 105 are
It becomes a sine wave with a predetermined period determined by the inductance of the piezoelectric element 102 and the capacitance of the piezoelectric element 102, and its phase difference is 90 degrees if the pure resistance component in the circuit is ignored.

Also, at this time, there is almost no pure resistance in the circuit, so there is no loss in the circuit. Since the voltage applied to the coil 105 is sinusoidal in this way, the voltage applied to the coil becomes zero when a quarter of the period has elapsed after the release signal becomes high. At this time, since the emitter of the switching transistor 108 becomes OV, the switching transistor 108 is turned off, and at the same time, the switching transistor 106 is also turned off. Then coil 105
All of the current accumulated in the diode 109 flows into the power supply. Therefore, the discharged charges are not dissipated by heat, achieving low heat generation and low power consumption.

[Problems to be Solved by the Invention] A predetermined load is connected to such a piezoelectric element, and such a load has a relatively large mass. Further, such a piezoelectric element is biased in a direction to release the displacement by an elastic member such as a leaf spring. Furthermore, since the piezoelectric element is susceptible to stress in the tensile direction, some devices are constructed so that the movable element or the piezoelectric element is mechanically separated when the excitation is released and the piezoelectric element is expanded or contracted. Therefore, when the excitation is released, the movable element returns due to the leaf spring after a minute delay after the piezoelectric element expands and contracts.

Therefore, the movable element collides with the piezoelectric element a short time after the excitation is released, and charges are excited in the electrodes of the piezoelectric element.

However, when using the drive device configured as above,
Both terminals of the piezoelectric element are electrically cut off because the switching transistor 6 is turned off. Therefore, charges generated at both terminals are accumulated in the piezoelectric element. Therefore, this accumulated charge causes a slight displacement in the piezoelectric element. For this reason, for example, when a piezoelectric element is used in a printing wire drive mechanism in a printer, the printing wire becomes slightly protruded, causing problems such as interfering with printing ribbon feeding.

The present invention has been made to solve the above problems,
The purpose of this is to effectively return the electric charge accumulated in the piezoelectric element to the power supply to achieve low power consumption and low heat generation, and to effectively release the electric charge that remains in the piezoelectric element when not excited. The purpose is to prevent unnecessary displacement.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a method in which one terminal A is electrically connected to a terminal B of a piezoelectric element, and the other terminal C is electrically connected to a power source. a coil electrically connected to a terminal C of the coil and the other terminal of the piezoelectric element to configure a resonant circuit with the piezoelectric element and the coil; and release of displacement of the piezoelectric element. The present invention is characterized in that it includes a control means that turns on the switch means when instructed to do so, and shuts off the switch means after a predetermined period of time has elapsed.

[Operation] In the present invention having the above configuration, when the piezoelectric element is to be displaced, the first switch means is made conductive by the control means. Then, charge flows from the power supply into the piezoelectric element through the first switching means, and the piezoelectric element is displaced. This conduction time is continued until at least sufficient charge is accumulated in the piezoelectric element.

On the other hand, when the displacement of the piezoelectric element is to be canceled, the second switch means is brought into conduction by the control means while the first switch means is cut off. In this state, the piezoelectric element and the coil constitute a resonant circuit,
The charge stored in the piezoelectric element repeatedly flows out through the coil and is stored again in the piezoelectric element. The control means waits until the electric charge is flowing through the coil, and then, when it is indicated that a predetermined period of time has elapsed after the second switch means is turned on, the control means cuts off the second switching means. do. The current flowing through the coil then flows into the power source.

[Example] Hereinafter, an example in which the present invention is applied to a drive circuit for a piezoelectric element that is a drive source of a piezoelectric actuator that drives a print wire of a print head for an impact-type de-sotoblink will be described in detail based on the drawings. Explain.

A piezoelectric actuator consists of a large number of piezoelectric elements P shown in FIG.
Laminated piezoelectric element 10 made up of layers laminated along a straight line direction
It is equipped with As shown in FIG. 2, the laminated piezoelectric element 10 is supported by two frames 12.degree. There is.

A movable member 16 and a temperature compensating member 18, each having a rectangular parallelepiped shape, are fixed to both end faces of the laminated piezoelectric element 10, respectively.

One side surface of the mover 16 parallel to the stacking direction is connected to the frame 1 through a pair of leaf springs 20 and 22 that are stacked on top of each other.
2, and the back surface of the temperature compensating material 18 opposite to the surface to which the laminated piezoelectric element 10 is fixed is opposed to the surface 26 of the frame 12. The movable element 16 and the leaf spring 20, and the leaf spring 22 and the surface 24 of the frame 12 are respectively fixed to each other, but the leaf springs 20, 22 are brought into sliding contact along these surfaces.

Further, a pin 28 is fixed to the frame 12 to contact the temperature compensation material 18 and bring it closer to the movable element 16. Thereby, the laminated piezoelectric element 10 is attached to the frame 12.14 with a slight compressive force remaining in the lamination direction. Therefore, when a voltage is applied to the laminated piezoelectric element 10 and the laminated piezoelectric element 10 extends in the lamination direction, it moves in the positive direction (upward in the figure) relative to the leaf spring 20 or the leaf spring 22, and on the other hand, When the voltage is removed from the laminated piezoelectric element 10 and the laminated piezoelectric element 10 contracts, the leaf spring 20 moves in the opposite direction to the leaf spring 22.

Note that even if the voltage is completely removed from the laminated piezoelectric element 10,
A residual strain in the positive direction remains in the laminated piezoelectric element 10, and this residual strain becomes smaller as the temperature of the laminated piezoelectric element 10 increases. Therefore, even if the magnitude of the voltage applied to the laminated piezoelectric element 10 is controlled to be constant and the amount of displacement of the laminated piezoelectric element 10 is controlled to be constant, if the temperature is high, the laminated piezoelectric element 10
The maximum displacement position cannot reach the normal position, and the higher the temperature, the greater the unreached distance remaining between the normal position and the maximum displacement position. The temperature compensating material 18 is provided to avoid such a situation. The temperature compensating material 18 expands more as its temperature increases, and the temperature compensating material 18 is arranged in series in the direction of displacement of the laminated piezoelectric element 10. In other words, by compensating for the unreached distance of the laminated piezoelectric element 10 with the expansion length of the temperature compensating material 18, the maximum displacement position of the laminated piezoelectric element 10 is prevented from changing due to temperature changes. There is.

The frame 14 is made of an elastically deformable plate having a longitudinal shape longer than the laminated piezoelectric element 10, and connects a portion of the frame 12 close to the temperature compensation material 18 and the movable element 16. The function of frame 14 will be explained later.

A holding member 34 having a groove 32 extending linearly is fitted to the end of the pair of leaf springs 20 and 22 opposite to the end on the side of the laminated piezoelectric element 10 (the upper end in the figure). There is. The width of the groove 32 is made larger than the sum of the plate thicknesses of the leaf springs 20.22, and each leaf spring 20.22 and the side surface of the groove 32 facing the leaf spring 20.22 are fixed to each other. Holding member 34
An arm 36 extends out from the holder, and a printing wire 38 is fixed to the tip of the arm 36. This printing wire 38 is opposed to the printing paper via a printing ribbon.

Therefore, when the laminated piezoelectric element 10 is extended and the leaf spring 20 slides upward relative to the leaf spring 22, and the holding member 34 is rotated counterclockwise approximately about the center line of the groove 32 in the figure. , the printing wire 38 is pressed against the printing paper via the printing ribbon, and dots are printed on the printing paper. When the laminated piezoelectric element 10 contracts from this state, the holding member 34 is rotated clockwise, and as a result, the printing wire 3
8 returns to the non-active position. The non-active position of the printing wire 38 is located at a low-repulsion rubber stopper 4 fixed to the frame 14.
It is defined by the arm 36 coming into contact with 0.

As is clear from the above description, the piezoelectric actuator uses a pair of leaf springs 20, 2 to control the displacement of the laminated piezoelectric element 10.
2, it is expanded by the holding member 34 and arm 36 and transmitted to the printing wire 38.

Note that when the printing wire 38 is pressed against the printing paper, a moment is generated in the movable element 16 in a direction that causes the movable element 16 to rotate counterclockwise in the figure approximately around its center, and the laminated piezoelectric element 10 may be bent in a direction that intersects with the stacking direction. However, in this embodiment, when the laminated piezoelectric element 10 extends, the frame 14 elastically extends accordingly, and as a result, a moment is generated in the movable element 16 in the direction of rotating it clockwise. The directional moments cancel each other out, making it possible for the laminated piezoelectric element 10 to expand and contract linearly without bending.

Next, a drive circuit for the laminated piezoelectric element 10 is shown in FIG. In this embodiment, a DC power supply 51 with an output voltage E, a transistor T r 1 , a coil 53 and a piezoelectric element P are connected in series, and the negative electrode side of the DC power supply 51 and the piezoelectric element P
The electrode side that should serve as the negative electrode is grounded. The forward direction of the transistor Trl is the forward direction from the positive electrode side of the DC power supply 51 to the electrode side that should become the positive electrode of the piezoelectric element P (hereinafter referred to as the forward direction of the circuit).

Furthermore, the connection point between the transistor Trl and the coil 53 is grounded via the transistor Tr2.

The forward direction of the transistor Tr2 is the direction from the connection point between the transistor Tr1 and the coil 53 toward the ground point. Diodes DI and D2 are connected in parallel to transistors Tri and Tr2, respectively, and the forward direction of each diode DI and D2 is opposite to the forward direction of the transistor to which each diode is connected in parallel. .

The positive electrode side of the DC power supply 51 and the electrode side of the piezoelectric element P that should become the positive electrode are connected by a diode D3, and the forward direction of the diode D3 is opposite to the forward direction of the circuit. Further, a transistor Tr3 is connected in the opposite direction to the diode D3. Further, a diode D4 is connected in parallel to the piezoelectric element P, and its forward direction is a forward direction from the electrode side of the piezoelectric element P that should be the negative electrode to the electrode side that should be the positive electrode. Note that a large number of piezoelectric elements P constituting the laminated piezoelectric element 10 are connected in parallel to each other.

Transistors Trl, Tr2. Switching between the cutoff state and the conduction state of Tr3 is performed by a transistor control circuit 55 (hereinafter simply referred to as control circuit 55). This control circuit 55 is also responsible for the overall control of the impact type printer in which this device is mounted, and is composed of a microcomputer. This control circuit 55 includes a timer 55a
Built-in.

Next, control of the control circuit 55 will be explained. Figure 3 shows
2 is a flowchart showing an excerpt of the control of driving a predetermined printing element for printing a predetermined dot of the printer. In addition to the control shown in this flowchart, this control circuit simultaneously drives other printing elements and controls other printers.

First, when the printing element is instructed to be driven, the control circuit 55 turns off the transistor Tr2 (S1), and immediately thereafter turns on the transistor Tri (S2). Then, the charge generated by the DC power supply 51 flows into the piezoelectric element P via the transistor Trl and the coil 53. This piezoelectric element P is therefore displaced, and this displacement causes the printing wire 38 to protrude.

This printing wire 38 presses the printing paper placed on the platen through the printing ribbon.

The control circuit 51 waits until a sufficient charge is accumulated in the piezoelectric element P (S3). Then, due to the action of the coil 53, the current continues to flow through the closed current loop via the diode D3 and the transistor Trl, and the current flowing through the coil 53 is maintained.

After a sufficient charge is accumulated in the piezoelectric element P, the control device 51 turns off the transistor Trl and turns on the transistor Tr3 (S4). Then, a current continues to flow through the coil 53 due to the action of the coil 53, and the current flows through the diode D3. DC power supply 51. It continues to flow in the closed current loop via diode D2 and eventually attenuates.

During this time, a small amount of current is consumed by the piezoelectric element P, but this current is transferred from the DC power supply 51 to the transistor Tr3.
is supplied to the piezoelectric element P via.

That is, during this period, the displacement of the piezoelectric element P remains maintained.

The control circuit 55 waits until the current flowing through the coil 53 is attenuated and the displacement time necessary for printing (S5), and then the control circuit 55 shuts off the transistor Tr3 and turns on the transistor Tr2 after a slight delay. is made conductive (S6). Then, the charge accumulated in the piezoelectric element flows through a closed current loop via the coil 53 and the transistor Tr2. At this time, there is almost no pure resistance in this closed current loop, so there is almost no heat consumption of electrical energy. All of the electrical energy resulting from the charges accumulated in the piezoelectric element P is accumulated in the coil as magnetic energy near the coil 53 after a predetermined period of time to be described later.

Then, the control circuit 55 waits until this moment in step S7.
), the transistor Tr2 is cut off (S8). Then, because the current in the coil 53 tends to continue flowing due to the action of the coil 53, this current is returned to the DC power source E via the diode D1. In other words, all the electrical energy stored in the coil is returned to the DC power source 51.

The control circuit 55 waits until the charge accumulated in the piezoelectric element P is returned to the DC power supply 51 as described above.
makes the transistor Tr2 conductive again (S
10). This is for the reason shown below.

Although the piezoelectric element P is expanded and contracted by the process of S6, the printing wire returns from the protruding position to the retracted position due to the rebound on the platen and the elasticity of the leaf spring. Therefore, compressive stress is applied to the piezoelectric element P with a minute delay from S6. For this reason, the piezoelectric element P is slightly displaced and a charge is generated on the electrode.

In addition, vibrations etc. during driving of the printing element P other than the above-mentioned printing are transmitted to the piezoelectric element P, and stress caused by the vibration is also transmitted to the piezoelectric element P.
It takes. Therefore, a charge is generated in the piezoelectric element P. If these charges are not released, they will accumulate in the piezoelectric element P, displacing the piezoelectric element P, and causing the print wire to slightly protrude from the print head surface. Therefore, after sufficient time has elapsed for the charge accumulated in the piezoelectric element P in the SIO to be returned to the DC power supply, the control circuit 55 again turns on the transistor Tr2. This transistor Tr2 is kept conductive until the next printing wire is driven (Sl). Next, the waiting times of 87 and S9 will be explained.

First, when transistor Tr2 is closed (time t-0),
A closed circuit consisting of the piezoelectric element P (capacitance C), the coil 53 and the transistor Tr2 is formed, and the piezoelectric element P
Due to the electrostatic energy stored in the piezoelectric element P
A current flows through the coil 53. The time when this transistor is closed is set to t-〇, and the transistor Tr is connected to the coil 53.
If the current flowing from the 2 side to the piezoelectric element P side is i (t), then by assuming that there is no resistance component in the transistor Tr2 etc., the following equation can be obtained from Kirchhoff's second law from the above-mentioned closed circuit. I can do it.

However, here, L is the inductance of the coil 53, and C
represents the capacitance of the piezoelectric element P.

Now, if the charge of the piezoelectric element P is q (t), then
q (t) becomes the following formula.

q(t)-J 1(t)dt (2)i
(t) −dq(t) (3)
dt By substituting these equations (2) and (3) into equation (1), the following equation is obtained.

L Q(t) 10' Q(t) −0(4)dt”
c Solve this differential equation (4) to find q (t). In order to solve this equation, a differential operator p is introduced. This differential operator p is expressed by the following equation.

p - d (5) dt By substituting equation (5) into equation (4), the following equation is obtained.

Ll) 2+ 1-0 Solving equation (6) yields the following equation.

However, j is an imaginary unit. If the constant A1°A2 is introduced here, q(t) will be expressed by the following equation.

(Jofi 1 By equations (9), (10), and (11),
Equation (8) is transformed as follows.

9(t)−Acos ωOt +Bs1n ωo
t (12) By substituting this equation (12) into equation (3), the following equation is obtained.

1(t)=ωo [Bcos (AJOt −A
sin ωot Next, consider the initial conditions at time tQ. Before time tQ, charge is stored in the piezoelectric element P in order to perform printing. At this time, the potential difference between both ends of the piezoelectric element P is equal to the power supply voltage E. Since the capacitance of the piezoelectric element P is C, the charge stored in the piezoelectric element P at time t-Q is expressed by the following equation.

Now, further define the following equation.

A −A , +A 2 B =j(A + A 2 ) Furthermore, since the transistor Tr2 is closed at time tQ, current is not yet flowing through the coil 53 at time 1-0. Therefore, the current flowing through the coil 53 at time 1-0 is expressed by the following equation.

(14), (15) and t-0 are replaced by (12) (
13) By substituting into the equation, the constants A and B become as follows.

A-CE (1B)B
-0(17) By giving equations (16) and (17) to equations (12) and (13), the following equation is obtained.

q(t)=CE cos ωo t
(18) i(t) --(1,10CEs1n ωQ
t (19) On the other hand, if the potential difference between both ends of the piezoelectric element P is v (t), then v (t) is expressed by the following equation.

By substituting equation (18) into equation (20), the following equation is obtained.

v(t)−Ecos ω,) t
(21) Therefore, when the transistor Tr2 is closed at time t-0, the potential difference v (
t) takes the value expressed by equation (21) until it reaches zero.

On the other hand, the time when the potential difference v (t) between both ends of the piezoelectric element P reaches zero can be obtained by solving the equation (21) with the left side set to zero.

Ecos ω□ t -0 (22) 2.2.2 2 ωo, 2 ω0゜Here, the first time that the potential difference v (t) between both ends of the piezoelectric element P reaches zero from time 1-0 is (24) From the formula, it can be seen that the time 1 π is −・. The 2 ω0 value of ω0 was defined in equation (11), so by substituting this value, we get
Obtained at time t-''m.

If the transistor Tr2 is opened at this time t"i, the current 1 (t) due to the current energy of the coil 53 cannot flow through the transistor Tr2. Therefore, the current 1 (t) due to the current energy of the coil 53 cannot flow through the transistor Tr2. Current begins to flow in the closed circuit.Time t is defined as the time when transistor Tr2 is opened, that is, as in the following equation.

t , -W ヱ (25) On the other hand, from the above-mentioned closed circuit, the following equation holds true according to Kirchhoff's second law.

E--L di (t) (26) t Solving equation (26) yields the following equation.

1(t)-1dt+K (27)(2
Considering the initial conditions of equation (7), it is sufficient to substitute tl of equation (25) for t of equation (19). Therefore, the value 1(tl) of i (t) at time t is as follows.

-ω, CE (2g) Using equation (28), if we eliminate the constant K in equation (27), we get 1-1. It can be seen that the subsequent i (t) is as shown in the following equation.

1(t)=-E(t t + ) +ωo CE
(29) Next, from equation (29), t, which is i (t) - 0, is calculated as follows.

Mato ω. LC+t.

−fτC+l+ (30) Therefore, time 1
-1. When the transistor Tr2 is opened, it takes time LC to regenerate the current energy of the coil 53 to the DC power supply 51.

During these operations, the potential difference V(1) across the piezoelectric element P and the current i (t) flowing through the coil 53 are as shown in FIG. 4(a).

Next, the time during which the transistor Tr2 is closed is “fl”
The case where it is not F will be explained.

First, a case where the transistor Tr2 is opened at 1fE- for a longer time will be described.

Assuming that the transistor Tr2 is closed at time 1-0, it will be exactly the same as the above example until time "flF", so
No particular explanation will be given here.

When time t-ij becomes positive, the potential difference between both ends of the piezoelectric element P becomes zero. Then, the diode D4 closes, so that a closed circuit is composed of the coil 53, the switching element Tr2, and the diode D4.

If the transistor Tr2 and the diode D4 are ideal switching elements, there is no factor that consumes the current energy of the coil 53, so the potential difference v (t) across the piezoelectric element P and the current i (t) of the coil 53 are as follows. It becomes like the solid line in FIG. 4(b).

However, in reality, transistor Tr2. Diode D4. Since the coil 53 etc. have a resistance component, the fourth
As shown by the broken line in Figure (b), while the transistor Tr3 is closed, the current energy of the coil 53 is consumed and converted into thermal energy. However, this converted thermal energy is minute, and the increase in heat generation from the drive circuit of this embodiment is also minute, so this does not pose a major problem.

If it is closed longer than that, as shown by the solid lines in Fig. 4 (a) and (b), if the element is ideally used, the current i (t) of the cocoil L after LC from the time when transistor Tr2 is opened. becomes zero.

Therefore, it is sufficient to close the transistor Tr2 again LC after the time when the transistor Tr2 was opened.

Next, a case will be described in which the transistor Trl is closed at a time other than the time when the current i (t) flowing through the coil 53 becomes zero due to individual differences in the capacitance C of each piezoelectric element P or the reactance of the coil 53.

First, a case will be described in which the transistor Tr2 is closed before the current i (t) flowing through the coil becomes zero.

When the transistor Tr2 is closed, the current i (t) flowing through the coil 53 changes. It flows through a closed circuit composed of a transistor Tr2 and a diode D4. These coils 53. If the transistor Tr1 and the diode D4 are ideal elements and have no resistance component, the transistor Tr2
The amount of current that was flowing through the coil 53 at the time when the transistor Tr1 was closed continues to flow while the transistor Tr1 is closed.

However, since these elements usually have resistance components, the current energy of the coil 53 is converted into thermal energy by these resistance components, and the heat is radiated from these elements. However, since the transistor Trl is open for a certain period of time and a certain amount of the current energy of the coil '53 is regenerated to the DC power supply E, the current 1 (1) already flowing through the coil is small, so these elements generate heat. Since the amount is small, it is not a particular problem. On the other hand, coil 5
A case will be described in which the transistor Tr2 is closed after a certain amount of time has passed after the current i (t) flowing through the transistor Tr2 becomes zero.

The piezoelectric element P is compressed by the inertial force of the expansion mechanism connected to the piezoelectric element P, and the voltage generated in the piezoelectric element P is minute, and the component that occurs during a short period of time is even minute, so If these mechanisms were used, for example, in a dot matrix printer, they would not be displaced by any significant amount. When this voltage is applied and the transistor Trl is closed, the coil 53. Since it is discharged by the resistance components of the transistor Tr2 and the diode D4, the transistor Tr is discharged after a certain period of time after the current i (t) flowing through the coil becomes zero.
No problem occurs even if 2 is closed.

As explained above, if the transistor Tr2 is closed again after a period of time LC has elapsed after opening the transistor T2, the current flowing through the coil 53 may have not yet reached zero, or may have already reached zero and a certain amount of time has elapsed. It's a good thing regardless of how much time has passed.

The case where the period is closed will be explained.

When the transistor Tr2 is opened at the time t-t1, the current 1 (t
l) becomes as follows from equation (19).

--(JQ CEs1n w(, t + (31) On the other hand, when transistor Tr2 is opened at time t-t1,
The current energy in coil 53 causes coil 53. Diode D1. A current flows through a closed circuit consisting of the DC power supply 51 and the piezoelectric element P. Regarding this closed circuit, the following formula holds from Kirchhoff's second law.

By introducing q(t tl), the following equation is obtained.

E = L Q (t tt ) Tokichi q (t - t +
) (33) Solve the differential equation for di2 to find q (t). First, the transient term qt(t tt) is expressed by the homogeneous equation (
4) This is the solution to the equation. Therefore, from equation (12), it becomes as follows.

q(t-t1) -Acos tt)6 (t-
tt )+Bs1n (JJO(t-tl)
(34) On the other hand, the steady solution qs is as follows.

qs-CE (35) Therefore, the general solution q(t tt) is as follows from equations (34) and (35).

q(t-t+)-qs+qt(t-t, )-C
E+Acos ω□ t +Bsjn ωOt
(38) Using equation (3) in equation (36), the following equation is obtained.

j (t-t + )-ωo IBcos (JQ (t
−t, ) Asin ω0 (t−t, )l
(37) Time 1-1. From the initial conditions, we get the following.

CEcos ωot + =CE+A
(38) −ω(ICEs1n ωo t + −ω
□ B From (39) (3g) and equation (39), we get the following.

^ -CE (cos ωQ t-1)
(40)B --CEs
in ω□ t + (41)(
By substituting equations (40) and (41) into equations (3B) and (37), we get the following.

q(t-t+)-CE+CE(cos ωOt+-1)cos
ωo(t-tt)−CEsin(IJOt+
sinω0 (t-tt) (42)1(
t-t,) −-ωr) CEs1n ω0 (t-tt)-ω(
, CE(cosωo t + −1) sinωo (t
-tt ) (43) The following equation is obtained from equations (42) and (20).

v(t-t+)=E+E(cos ω□t+-1)cosωo
(t-tt)-EsjnωOt 1 sjnω
o (t-tl) (44) Therefore, the potential difference v (t) between both ends of the piezoelectric element P and the current 1 of the coil 53
(t) becomes as shown in FIG. 4(C). Here v(t-t
The following equation holds true at the time t-t2 when 1) becomes minimum.

The left side of equation (45) can be transformed as follows.

j(t−t, ) −ω□ CEs1nωQtleOSωo(t2t+)+
ω□ CE (1-cosω□ t + ) sin '
L'O(t2-tt)--ωOCE[' sin (
AJOltl”(t 2-tl)1+1slnωo
1t2-tt )11+(JJOCEs1n (JJO
(t2-tl)=-ωQ CE[' sin CIJO
I(t2-tl)+t1] +1slnωo i(t2-tt)"t+1m
ωOCEs1n (AJO(tl −tl )−(JJ
OCEs1n ω0t2 −2ω. CEcos['ωo i(t 2 −tl
) 10t 2 11sin['ω01(t 2-t
l )-t 2 )]-2ω□ CEcosωO[t
l −' t + 1, Sin 1 (IJOt + 2 (4B) Now, the following equation holds true.

ω0≠0 (47)C≠
0 (48)E≠O
(49) Therefore, from equations (45) and (4B), it becomes as follows.

cosω01t2-yoshitt1 sin l ω□ t 1-0 Therefore, one of the following two equations will hold true.

cosωo(tl-'t+)-0(51)sin I
(JJ□t + -0 Now, what I am telling you here is to
, /TE, the following equation holds true.

0<(IJOtl<π (53)
Therefore, equation (52) does not hold true, and equation (51) holds true. Therefore, it becomes as follows.

ωo (12- and tel-π (54)tl
−111+” ) 2 ω0 Next, as shown in FIG. 4(d), at time t-t2, the value of the potential difference v(t-tl) between both ends of the piezoelectric element P becomes zero.
, we will find.

The value of v(t tl) at t''-tl is as follows.

v(t-t,) t-tl E+E(cos tL) □ t l-1) cosωg
(tl −tl )−Esin ω□ t 2 11
stn(t2-tl)E +Ecosωot+ ・eOsωo (tl =t+
)-Ecosωo (tl-tl) -Esinω012 ・sinωo (tl-tl)
E + ' Ecos [ωt) lt+ −(t 2−t+
month] + ' Ecos [ωo lt+ ” (t 2-1
．． )l]-Ecos ωo (tl-tl) -1Ecos[ωo lt+ -(t2-h)l]
+ I Ecos [(1,10th +(t 2
-ts))ko+Ecos ωOt 2-Ec
os ωo (tl −tl )E −2Esin ′(JJO ・1 Sln ω0 (t 2 ”(t 2−1+ )1ft 2−
t 2-tl )]・1 ” Sin ωot+ 2 2 ωo2 #S1n ωot+ -E-2E9stn 11sin ωOt +-E
−2Esin ωo t ,
(5B) Since equation (56) becomes zero, it becomes as follows.

E-2Esin (IJOt + -0(57)(49
), equation (57) becomes as follows.

E sin ω01l= ■ From equation (53), it is as follows.

It takes (2 + t)J time for the current i (t) at voltage τ 3 to become zero, but as shown in FIG.
is closed only during i7'ET, then the time t
-iJ and then open the transistor Tr2, as shown in Fig. 4 (e
), the potential difference v (t) across the piezoelectric element reaches zero.

By the way, let us consider the value of t2 when the transistor Tr2 is opened at time t-''J.

Substituting equation (59) into equation (55) yields the following.

By the way, if the transistor Tr2 is closed for a period of one η7 as shown in FIG. Since the next drive cannot be performed until the current i (t) of the coil 53 becomes zero, a drive time shorter than "fE"6 is useful for speeding up the drive of the piezoelectric element P. .

As explained above, in the present invention, the time of iI "[,
It is not necessary to keep the transistor Tr2 closed; it is sufficient to keep it closed for at least -f' time.

That is, after opening the switching element Tr2, the current 1 (t) in the coil L becomes zero after i months=time.

By the way, the ratio of the above-mentioned positive time f and this fr〒 is approximately 1:1.047. As mentioned earlier, this time does not need to be exactly fET, so if switching element Tr2 is opened and then closed again after a period of time LC has elapsed,
This is true regardless of whether the current flowing through the coil has not yet reached zero or has already reached zero and a certain amount of time has passed.

Note that the present invention is not limited to the above configuration, and various other modifications are possible. For example, MOS field effect transistors may be used as the transistors Tri to Tr3 to further reduce power consumption. Further, in this embodiment, the timer is used in S7 and S9 to measure the progress of the standby period, but as shown in the prior art, when the current value is detected and the current value becomes zero, the process of 88 is performed. You may do so.

C Effects of the Invention] As is clear from the detailed description above, in the present invention, the charges accumulated in the piezoelectric element are efficiently released after the excitation of the piezoelectric element is released, so that unnecessary charges are not accumulated in the piezoelectric element. It never happens. Therefore, no unnecessary displacement occurs in the piezoelectric element. Therefore, in a piezoelectric actuator using the piezoelectric element driving device described above, it is possible to achieve low power consumption and low heat generation, and to perform stable driving.

[Brief explanation of drawings]

FIGS. 1 to 4 show an embodiment of the present invention. FIG. 1 is a diagram showing the electrical configuration of the device of this embodiment, and FIG. 2 is a diagram showing the electrical configuration of the device of this embodiment. FIG. 3 is a front view of the printing element of the impact type dot printer, FIG. 3 is a flowchart showing an excerpt of the control of the control circuit, and FIG. 4 is a diagram showing changes in current and voltage. Further, FIG. 5 is a circuit diagram showing a conventional device. In the figure, Tr2 is a transistor constituting a switch means;
53 is a coil, and TC is a control circuit constituting a control means.

Claims (1)

[Claims] 1. A coil whose one terminal A is electrically connected to the terminal B of the piezoelectric element and whose other terminal C is electrically connected to a power supply; a switch means that can be electrically connected to the other terminal D of the piezoelectric element to configure a resonant circuit with the piezoelectric element and the coil; and a switch means that is electrically connected when the displacement of the piezoelectric element is instructed to be released. and control means for turning on the switching means again after a predetermined period of time has elapsed, and turning on the switching means again after the electrical energy held in the coil is returned to the power source. Device.