JP2793753B2 - Wire dot print head - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a capacitance sensor built in a wire dot print head used in an impact printer.

[0002]

2. Description of the Related Art Some impact printers use a wire dot print head having a capacitance sensor for detecting the displacement of an armature by a sensor electrode facing the armature.

For example, Japanese Patent Application Laid-Open No. 1-24 filed by the present applicant
Japanese Patent No. 4869 is one of them. According to that publication,
A sensor for detecting the displacement of the print wire, a timing circuit for generating a timing signal based on the output of the sensor, and a head driver for driving the print wire in accordance with the timing circuit; The timing of collision with the print medium and the like are detected by a sensor, and the head driver controls the print wire in accordance with the output of the sensor. Therefore, regardless of the thickness of the print medium,
It is disclosed that printing can always be performed with an optimum printing force.

[0004] The displacement of the printing wire is detected by the amount of change in the capacitance between the armature 132 and the sensor electrode 13 facing the armature 132, as shown in FIG.
By the way, the core 1 for sucking and releasing the armature 132
Armature yo through armature 132 from 35
The magnetic flux passes through the arm 141, and the armature 132 causes a change in the number of magnetic fluxes crossing the armature yoke 141 when attracting and releasing the armature 132, and the armature 132 and the armature yoke 141. The magnetic flux passing through the armature 132 and the armature yoke 141 is substantially symmetrical as shown by the arrow in FIG. 9 so that the generated potentials are reversed and cancelled. In addition, almost no potential is generated on the surface of the armature 132 due to electromagnetic induction, and the sensor electrode 13 facing the armature 132 does not capture the noise voltage.

[0005]

However, when the armature adjacent to the armature to be detected and the core facing the armature are also energized at the same time, as shown in FIG. 10, magnetic flux from the adjacent armature is generated. It goes around the armature to be detected through the armature yoke. As a result, the magnetic flux density of the armature to be detected becomes asymmetric, and
The potential due to the crossing of the armature and the magnetic flux due to the release and attraction of the armature is also asymmetric, and a potential is generated on the armature surface. This potential is trapped by the sensor electrode facing the armature as a noise voltage, causing a problem of deteriorating the S / N ratio.

SUMMARY OF THE INVENTION An object of the present invention is to provide a wire dot print head capable of improving the S / N ratio of a capacitance sensor and performing printing with an optimum printing force.

[0007]

In order to achieve the above object, in a wire dot print head according to the present invention, a noise cancel electrode disposed around a sensor electrode, and a voltage from a sensor electrode and a noise cancel electrode. And a differential input sensor circuit for erasing the noise voltage by inputting the same.

[0008]

The differential input sensor circuit of the wire dot print head configured as described above eliminates a noise voltage by inputting a voltage from the noise cancel electrode and the sensor electrode disposed around the sensor electrode. Therefore, according to the present invention,
The noise caused by the potential generated when the armature crosses the magnetic field whose magnetic flux density is not symmetrical is removed to reduce the noise of the capacitance sensor.
It is possible to provide a wire dot print head capable of improving the / N ratio and performing printing with an optimum printing force.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the drawings. Elements common to the drawings are denoted by the same reference numerals. FIG. 1 is a sectional view showing the structure of the embodiment. On one plate surface side of the base plate 1, a permanent magnet 2, a yoke 3, a spacer 4, a bias leaf spring 5, an armature yoke 6, a printed circuit board 7 for a capacitance sensor, a guide frame. And a cap 9 is laminated on the other plate surface side of the base plate 1 and the periphery thereof is fixed by a clamp 10. An armature 11 is provided at a free end of a cantilevered biasing leaf spring 5 radially arranged toward the center, and a base of a printing wire 12 is fixed to a tip end of the armature 11. The distal end of the wire 12 is guided by a guide frame 8 to a platen (not shown) so that it can protrude.

A core 13 is provided on one plate surface side of the base plate 1 so as to be built in the annular permanent magnet 2 and the yoke 3 and opposed to the armature 11. A bobbin 15 around which a coil 14 is wound is mounted. The coil 14 is connected to a printed circuit board 17 for a wire drive fixed to the other plate surface of the base plate 3 via an insulating member 16. The sensor electrode 18 is provided on the printed circuit board 7 for the capacitance sensor so as to face each armature 11. FIG. 3 is a schematic pattern diagram showing the electrode surface of the printed circuit board for the capacitance sensor.
FIG. 4 is a schematic component layout diagram showing the mounting surface of the printed circuit board for the capacitance sensor. The sensor electrode 18
As shown in FIG. 3, on the electrode surface of the printed circuit board 7 for the capacitance sensor, it is provided radially so as to face the armature 11,
As shown in the enlarged view, a noise canceling electrode 19 having an area approximately equal to that of the sensor electrode 18 is provided around the sensor electrode 18 as shown in the enlarged view. Further, a shield pattern is formed around the noise canceling electrode 19 so as to prevent crosstalk with the adjacent sensor electrode 18.
20 is provided. Printed circuit board for capacitance sensor 7
A sensor integrated circuit 22 is mounted on the wiring surface, and is connected to via holes 18a and 19b connected to the sensor electrode 18 and the noise canceling electrode 19 on the electrode surface by a pattern (not shown).

FIG. 2 is a circuit diagram of the differential input sensor according to the embodiment. The noise potential generated on the surface of the armature 11 also occurs on the surface of the armature yoke 6 adjacent to the armature 11. Therefore, the sensor voltage accompanied by the noise potential generated on the surface of the armature 11 is captured by the sensor electrode 18, and the noise potential on the surface of the armature 6 is captured by the noise cancel electrode 19. Then, the noise voltage is input to the sensor integrated circuit 22 to remove the noise voltage. Armature
A variable capacitor is constituted by the sensor 11 and the sensor electrode 18,
Armature yoke 6 and noise canceling electrode 19
Constructs a capacitor. The armature 11 is grounded via the via leaf spring 5 and the armature yoke 6. Assuming that the capacitances of the variable capacitors are Cx and Cy (Farad), they are charged from the power supply + Vs (volts) via the resistors R1 and R2 (ohm), respectively, and (+ Vs)
× Cx, (+ Vs) × Cy (cron) charges Qx, Q
Store y.

A high impedance circuit is provided between the resistor R1 and the sensor electrode 18 and between the resistor R2 and the noise canceling electrode 19 as an emitter follower circuit 25,2.
6 is connected to the input unit. Emitter follower circuit 25
Is composed of transistors Tr1 and Tr2 and a resistor R3 (ohm). The input section of the emitter follower circuit 25 is the base terminal of the transistor Tr1, the emitter terminal is connected to the base terminal of the transistor Tr2, and the collector terminals are respectively connected to the power supply + Vcc (volt). The power supply + Vcc is set higher than the power supply + Vs. The emitter terminal of the transistor Tr2 is grounded via a resistor R3.

Similarly, the emitter follower circuit 26 comprises transistors Tr3 and Tr4 and a resistor R4 (ohm). The input section of the emitter follower circuit 26 is the base terminal of the transistor Tr3, the emitter terminal is connected to the base terminal of the transistor Tr4, and the collector terminals are respectively connected to the power supply + Vcc. The emitter terminal of the transistor Tr3 is grounded via a resistor R4. The portions between the transistors Tr2 and Tr4 and the resistors R3 and R4 are output portions of the emitter follower circuits 25 and 26, respectively, and are connected to the non-inverting input terminal and the inverting input terminal of the differential amplifier 27.

Next, the operation will be described. When the coil 14 is not energized, the magnetic flux of the permanent magnet 2 causes an arc.
The armature 11 is attracted to the core 13 and deflects the biasing leaf spring 5. When the coil 14 is energized, a magnetic flux is generated in the coil 14 so as to cancel the magnetic flux of the permanent magnet 2, and
The armature 11 is released from the core 13 toward the sensor electrode 18 by the spring force of the biasing leaf spring 5.

When the coil adjacent to the coil 14 to be energized is not energized, when the armature 11 is released from the core 13, the distance between the armature 11 and the sensor electrode 18 changes and the static electricity changes. The capacitance changes. Since the rate of change is represented by dCx / dt, the rate of change of charge Qx is dQx / dt.
/ Dt = (+ Vs) × dCx / dt, which is the charging / discharging current Is (ampere). Since the emitter follower circuit 25 is provided as a high impedance circuit on the terminal side of the sensor electrode 18, the charging / discharging current Is is almost equal to the resistance R1.
And the voltage at the input terminal of the emitter follower circuit 25 becomes (+ Vs) -R1 * Is (volt). Then, the voltage is input from the output terminal of the emitter follower circuit 25 to the non-inverting input terminal of the differential amplifier 27 as a voltage V1 (volt).

At this time, the magnetic flux densities of the lines of magnetic force flowing from the armature 11 toward the armature yoke 6 are symmetric as shown in FIG. 9, and when the armature 11 is released, the magnetic flux density is reduced. Since the change in the number of intersecting chains is also symmetric, the potential of electromagnetic induction generated on the surface of the armature 11 is canceled out, and the sensor electrode 18 does not capture a noise voltage. At this time, the noise canceling electrode 19 is also
The emitter follower circuit 26
Becomes almost 0 (volt), and is input from the output terminal of the emitter follower circuit 26 to the inverting input terminal of the differential amplifier 27 as voltage V2 = 0 (volt).
As a result, the voltage output from the output terminal of the differential amplifier 27 has a voltage characteristic diagram as shown in FIG.

On the other hand, when the coil adjacent to one of the coils is also energized, the magnetic flux density of the lines of magnetic force flowing from the armature 11 to the armature 6 is asymmetric as shown in FIG. When the armature 11 is released and attracted, the number of lines intersecting with the magnetic flux changes asymmetrically, so that the potential of electromagnetic induction generated on the surface of the armature 11 is not canceled out, and the sensor electrode 18 reduces the noise voltage. Capture.

Therefore, the voltage input to the non-inverting input terminal of the differential amplifier 27 has a voltage characteristic diagram as shown in FIG.

The potential of the electromagnetic induction generated on the surface of the armature 11 is changed by the mutual induction to the armature 6.
And the noise canceling electrode 19 captures it. Therefore, the voltage input to the inverting input terminal of the differential amplifier 27 has a voltage characteristic diagram as shown in FIG. As a result, the voltage output from the output terminal of the differential amplifier 27 has a voltage characteristic diagram as shown in FIG.

When current is also supplied to the coils on both sides adjacent to each other, the magnetic flux flowing from the adjacent coils is equal, so that the magnetic flux is balanced and no noise voltage is generated.

[0021]

Since the present invention is configured as described above, the following effects can be obtained. By removing the noise voltage captured by the sensor electrode and the noise canceling electrode by the differential input sensor circuit, the noise caused by the potential generated by the armature crossing the magnetic field having an unsymmetric magnetic flux density can be removed. S / N of capacitance sensor
It is possible to provide a wire dot print head capable of improving the ratio and performing printing with an optimum printing force.

Further, since a shield pattern is provided around the noise canceling electrode, no crosstalk occurs between the noise canceling electrode and the adjacent sensor electrode.

[Brief description of the drawings]

FIG. 1 is a sectional view showing the structure of an embodiment.

FIG. 2 is a circuit diagram of a differential input sensor according to the embodiment.

FIG. 3 is a schematic pattern diagram showing an electrode surface.

FIG. 4 is a schematic component layout diagram showing a mounting surface.

FIG. 5 is an output waveform diagram from a differential amplifier.

FIG. 6 is an input waveform diagram to a non-inverting input terminal of the differential amplifier.

FIG. 7 is a diagram of an input waveform to an inverting input terminal of the differential amplifier.

FIG. 8 is a detailed view of a wire driving unit.

FIG. 9 is a magnetic flux state diagram when a current is applied to the core alone.

FIG. 10 is a magnetic flux state diagram when current is supplied to adjacent cores.

[Explanation of symbols]

 11, 132 Armature 13, 18 Sensor electrode 19 Noise cancel electrode 20 Shield pattern 22 Sensor integrated circuit 27 Differential amplifier

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Naoji Akutsu 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd. (56) References JP-A-64-53860 (JP, A) JP-A-59-2864 (JP, A) JP-A-1-244869 (JP, A) JP-A-62-9530 (JP, U) (58) Fields investigated (Int. Cl. ⁶ , DB name) B41J 2 / 30

Claims (3)

(57) [Claims] 1. A wire dot print head having a capacitance sensor for detecting a displacement of an armature by a sensor electrode facing the armature, the wire dot printhead being disposed around the sensor electrode and surrounding the armature.
To the armature yoke placed close to
Noise canceling electrode provided to form a sensor
And a differential input sensor circuit for inputting a voltage from a sensor electrode and a noise canceling electrode to eliminate a noise voltage. 2. The wire dot print head according to claim 1, wherein a shield pattern is provided around said noise canceling electrode. 3. The differential input sensor circuit according to claim 1, wherein the sensor electrode and the noise canceling electrode are connected to respective inputs of a differential amplifier via a high impedance conversion circuit. Wire dot print head.