JPH04164652A - Piezoelectric element drive circuit - Google Patents

Abstract

PURPOSE:To expand or shrink a piezoelectric element at a desired timing by a required amount by providing a charging means for charging the piezoelectric element, a discharging means for discharging it, a bypass means, and a regenerative means. CONSTITUTION:When a charge switch S1 is turned ON, a charging current ICP flows to a piezoelectric element Cp through a direct current power source E, the charge switch S1, a coil L, and a diode D4. A terminal voltage VCP of the piezoelectric element Cp is raised to expand the piezoelectric element Cp. When the terminal voltage VCP becomes higher than a power source voltage VE, a diode D1 is electrified the terminal voltage VCP is kept at approximately the same value as the power source voltage VE, and the expansion of the piezoelectric element Cp is stopped at an amount corresponding to the power voltage VE. When the charge switch S1 is turned OFF, an electromotive force is generated in the coil L, a diode 2 is electrified, and the electromotive force generated in the coil L is regenerated in a condenser C1. When a switch S2 is turned ON, a transistor TR1 is electricfied the terminal voltage VCP of the piezoelectric element Cp is lowered, the piezoelectric element Cp is shrunk, a diode D5 is electrified, the terminal voltage VCP is kept at approximately 0V, and the piezoelectric element Cp is shrunk to a state before charging.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a piezoelectric element drive circuit that charges and discharges charges to a piezoelectric element to expand and contract the piezoelectric element.

[Prior Art] Various devices using piezoelectric elements as actuators have been known, such as a dot impact printer in which a print head is driven by expansion and contraction of a piezoelectric element.

This type of device is equipped with a drive circuit to drive the piezoelectric element, but conventional drive circuits (a resonant circuit is formed by the front piezoelectric element and a coil, and the piezoelectric element is expanded and contracted by the resonant voltage) However, when the piezoelectric element is expanded and contracted by resonance between the piezoelectric element and the coil, the timing of expansion when the piezoelectric element reaches its maximum expansion state, and the contraction timing when the piezoelectric element reaches its maximum contraction state. (The resonant frequency is determined by the other resonant frequency.) In addition, the resonant frequency changes depending on the usage environment such as temperature and changes in the capacitance of the piezoelectric element, so it may not be possible to expand or contract the piezoelectric element at the desired timing. .

Therefore, the applicant (Yo) proposed a charging circuit that charges a piezoelectric element through a coil, and a charging voltage of the piezoelectric element by the charging circuit (
= expansion amount of the piezoelectric element) exceeds a predetermined level, the charge circuit includes a bypass circuit that returns the charge from the charging circuit to the power source, and a discharge circuit that discharges the charge charged in the piezoelectric element. proposed a drive circuit in which the extension timing and contraction timing could be controlled by external commands (for example, Japanese Patent Application Laid-Open No. 11927/1999).
No. 6).

[Problems to be Solved by the Invention] The proposed drive circuit ('L) can expand and contract the piezoelectric element at a desired timing, and the amount of expansion can be kept constant, so that the print head, etc. This makes it possible to drive the drive member stably at all times, but since the discharge circuit consumes the charge stored in the piezoelectric element through a resistor, etc., there are problems such as inefficient use of the power supply. There was a problem in that the circuit components generated a large amount of heat, necessitating cooling components such as heat sinks.

Therefore, it is an object of the present invention to provide a piezoelectric element drive circuit that can expand and contract a piezoelectric element by a desired amount at a desired timing and that can efficiently use a power source.

[Means for Solving the Problems] That is, the piezoelectric element drive circuit of the present invention, which has been made to achieve this object, connects the piezoelectric element and a DC power source via a coil in response to an external charging command, and generates a piezoelectric element. a charging means for charging the element; a discharging means for discharging the piezoelectric element by causing a current to flow through the coil in the same direction as when charging, using the electric charge accumulated in the piezoelectric element due to the charging, and discharging the piezoelectric element according to an external discharge command; bypass means connected between the power supply and the piezoelectric element to cause current to flow in a predetermined direction so that the charging voltage of the piezoelectric element does not exceed the power supply voltage by tongue-and-groove charging; and a bypass means for causing the charging means and the discharging means to complete the charging or discharging operation. The present invention is characterized in that it includes a regeneration means for regenerating the electromotive force generated in the coil when this occurs to the DC power source.

[Function] In the piezoelectric element drive circuit of the present invention configured as described above, when a charging command is input from the outside, the charging means connects the piezoelectric element and the DC power source via the coil to charge the piezoelectric element. At the same time, the bypass means causes current to flow in a predetermined direction so that the charging voltage of the piezoelectric element does not exceed the power supply voltage,
Return the electric charge charged in the piezoelectric element to the DC power supply (as a result, the piezoelectric element expands by a predetermined amount determined by the power supply voltage)
.

On the other hand, when a discharge command is input from the outside, the discharging means uses the charge accumulated in the piezoelectric element to cause current to flow through the coil in the same direction as when charging, discharging the piezoelectric element (as a result, the piezoelectric element returns to the state before charging). shrink). In addition, during charging and discharging, current flows through the coil, so when charging or discharging is finished, electromagnetic energy is generated in the coil, and this electromotive force is transferred to the DC power supply through regeneration means. will be regenerated.

The piezoelectric element is discharged by passing current through the coil in the same direction as when charging.This is to enable the electric charge charged in the piezoelectric element to be efficiently regenerated into the DC power supply.In other words, the coil By applying a discharge current to
The electric charge charged in the piezoelectric element is converted into electromagnetic energy in the coil, and after the discharge is completed, it can be regenerated into the DC power source via the regeneration means, and the direction of the discharge current to the coil is the same as that during charging. This causes a problem that occurs when a discharging current is passed through the coil in the opposite direction to that used during charging.In other words, when a current is passed in the opposite direction to the coil, the coil generates heat due to hysteresis loss, and power is wasted. This prevents such problems.

[Example] Embodiments of the present invention will be described below with reference to the drawings.

First, FIG. 1 is an electrical circuit diagram showing the configuration of a piezoelectric element drive circuit according to an embodiment.

As shown in the figure, the piezoelectric element drive circuit of this embodiment (the DC power supply E in which the front capacitor C1 is connected in parallel and the negative terminal is directly connected to the piezoelectric element cp) is in an ON state when a charging signal is input from the outside. So, coil L, diode D4
A charging switch S serving as a charging means that connects the positive terminal of the DC power source E and the positive terminal of the piezoelectric element cp to charge the piezoelectric element cp with a positive charge via the piezoelectric element cp of the coil L; A diode D1 is provided between the side end and the positive terminal of the DC power supply E, and a diode D1 is provided as a bypass means to enable current to flow from the side of the piezoelectric element CP in the direction of the DC power supply E, and a discharge signal is input from the outside. When the piezoelectric element cp is turned on, the positive charge accumulated in the piezoelectric element cp is transferred to the PNP transistor TRI, the diode D3. A discharge switch S2 as a discharging means for discharging through the coil L, a diode D5 connected in parallel to the piezoelectric element CP to prevent the potential of the positive terminal of the piezoelectric element CP from becoming negative during discharge, and a charging switch S1 or a diode D2 as a regeneration means for regenerating the electromotive force generated in the coil to the DC power source E through the diode D when the discharge switch S2 is switched from ON to OFF;
It consists of

The charging switch S1 and the discharging switch S2 (shown as open/close switches in the figure above, but are actually FETs)
Alternatively, it is configured by a switching element such as a transistor. Also, the base of transistor TR1 is connected to resistor R1.
It is connected to the end of the piezoelectric element cp side of the coil L through the coil L, and when the discharge switch S2 is turned on, it is connected to the negative terminal of the DC power supply E through the resistor R1 and the discharge switch S2, and is turned on. .

In the piezoelectric element drive circuit of this embodiment configured as described above (as shown in Table 2), when a charging signal is not input from the outside and the charging switch S1 is turned on (time tl
), a charging current of 1 cP does not flow in the path of DC power supply E - charging switch S1 - coil diode D4 - piezoelectric element Cp - DC power supply E, and the terminal voltage VCP of piezoelectric element Cp rises.
As a result, the piezoelectric element Cp begins to expand.

Then, at a time t2), the terminal voltage VCP of the piezoelectric element cp becomes higher than the power supply voltage VE due to the action of the coil L.
The diode D1 conducts, the charging switch St coils, and a closed circuit is formed by the diode D1.The charging current 1cP becomes almost 0, and the terminal voltage VC of the piezoelectric element Cp increases.
P is maintained approximately at power supply voltage VE. As a result, the piezoelectric element Cp stops expanding at a predetermined amount determined by the power supply voltage VE. In FIG. 2, 1 represents the current value of the coil L, and charging to the piezoelectric element Cp stops. However, current continues to flow through the closed circuit consisting of charging switch Sl, coil L, and diode D].

Next, the charging signal input stops and the charging switch S1 is turned on.
When switched to the OFF state (time t3), an electromotive force is generated in the coil L due to the electromagnetic energy accumulated in the coil L due to the above-mentioned energization, but this electromotive force in turn makes the diode D2 conductive, causing the diode D2. A closed circuit consisting of a coil L, a diode D1, and a DC power source E is formed, and the electromotive force generated in the coil is regenerated to a capacitor C connected in parallel to the DC power source E.

On the other hand, as described above, when the piezoelectric element cp is charged, the piezoelectric element cp
is in the extended state, when a discharge signal is input from the outside and the discharge switch S2 is turned on (time t4), the transistor TRI becomes conductive, and the piezoelectric element Cp - the transistor TRl - the diode D3 - the coil L - the discharge switch S
2-A discharge current -ICP does not flow in the path of the piezoelectric element Cp, and the terminal voltage VCP of the piezoelectric element cp decreases, and as a result, the piezoelectric element Cp begins to contract.

When the terminal voltage VCP of the piezoelectric element cp reaches approximately Ov (time t5), the diode D5 becomes conductive, and the diode D5. Transistor TRI.

Diode D3. A closed circuit is formed by the discharge switch s2.The discharge current -ICP becomes O, and the terminal voltage VCP of the piezoelectric element cp is maintained at approximately Ov.

As a result, the piezoelectric element Cp contracts to the state before charging.

Note that during this discharging, a current flows through the coil L in the same direction as during charging, and even after the piezoelectric element cp has finished discharging, the current continues to flow through the closed circuit.

Next, when the input of the discharge signal is stopped and the discharge switch S2 is switched from ON to OFF (time t6),
Similar to when charging switch $1 is switched from ON to OFF, an electromotive force is generated in the coil, but this electromotive force causes diode D2 to conduct, causing diode D2.
A closed circuit consisting of a diode DI and a DC power source E is formed, and the electromotive force generated in the coil L is regenerated to a capacitor C connected in parallel to the DC power source E.

As described above, in the piezoelectric element drive circuit of this embodiment (the charging voltage of the upper piezoelectric element is made almost the same as the power supply voltage VE), the piezoelectric element can always be expanded at a constant rate. Since the piezoelectric element can be controlled arbitrarily by independent control signals (charging signal, discharging signal) from the Used in the print head drive circuit of impact printers ('i), it is possible to always obtain stable print quality.

In addition, in this example, since the piezoelectric element is charged and discharged via the coil, and current is always passed through the coil in the same direction, the power other than that required for expanding and contracting the piezoelectric element is transferred to the power source. Moreover, the power consumption in the coil can be minimized.Therefore, the efficiency of power usage can be significantly improved.Also, since the power consumption in the circuit components is low, excess heat generation can be avoided. Therefore, the entire drive circuit including the power supply can be made smaller.

Note that in place of the transistor TRI, another switching element may be used, and the switching element may be turned on when the discharge signal is on, and turned off when the discharge signal is off.

[Effects of the Invention] As detailed above, the piezoelectric element drive circuit of the present invention not only allows the piezoelectric element to be expanded and contracted by a desired amount at a desired timing, but also minimizes power usage efficiency. This makes it possible to drive the piezoelectric element efficiently.

Furthermore, since no heat is generated in the coil or the like due to charging and discharging, there is no need to provide a special member for heat radiation. Therefore,
The entire drive circuit including the power supply can be downsized.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram showing the piezoelectric element drive circuit of the embodiment,
FIG. 2 is a time chart explaining the operation.

Claims (1)

[Claims] Charging means for charging the piezoelectric element by connecting the piezoelectric element and a DC power source via a coil in response to an external charging command; A discharging means is connected between the DC power supply and the piezoelectric element, and a discharging means is connected between the DC power source and the piezoelectric element, and the piezoelectric element is discharged by causing a current to flow through the coil in the same direction as when charging due to the accumulated charge, and the charging voltage of the piezoelectric element is increased by charging. bypass means for flowing a current in a predetermined direction so as not to exceed the power supply voltage; and regeneration means for regenerating the electromotive force generated in the coil when the charging means and discharging means finish charging or discharging operations into the DC power supply. A piezoelectric element drive circuit comprising: