Inkjet head and inkjet printer

1. An inkjet head, comprising:

a pressure chamber configured to house ink;

an actuator configured to be arranged corresponding to the pressure chamber and to generate a different displacement resulting from an applied voltage between a first displacement generated when hysteresis in a last time drive direction is the same as a present drive direction and a second displacement generated when hysteresis in the last time drive direction is reverse to a present drive direction;

a plate configured to have a nozzle communicating with the pressure chamber; and

a driving circuit configured to drive the actuator,

wherein the driving circuit applies to the actuator a first auxiliary pulse for contracting a volume of the pressure chamber and a second auxiliary pulse for generating an amplitude of pressure vibration of the ink opposite to an amplitude of pressure vibration of the ink resulting from the first auxiliary pulse, wherein the first and second auxiliary pulses do not eject an ink drop from the nozzle, and the second auxiliary pulse is applied prior to the first auxiliary pulse, and then applies to the actuator a drive signal which contains an expansion pulse for expanding the volume of the pressure chamber and a contraction pulse for contracting the volume of the pressure chamber to eject the ink from the pressure chamber once or for multiple times, and

wherein the second displacement resulting from the first auxiliary pulse assists to boost an ejection speed of a first ink drop prior to a second ink drop in the multiple ink drops.

2. The inkjet head according to claim 1 , wherein the second displacement is larger than the first displacement.

3. The inkjet head according to claim 1 , wherein a stored charge on the actuator generating the second displacement is larger than that on the actuator generating the first displacement.

4. The inkjet head according to claim 1 , wherein the driving circuit outputs the expansion pulse as a second auxiliary pulse signal.

5. The inkjet head according to claim 1 , wherein the driving circuit sets a pulse center interval between the first and second auxiliary pulse signals as a resonance period of the pressure chamber with the ink.

6. The inkjet head according to claim 1 , wherein the driving circuit equalizes a pulse width of first and second auxiliary pulse signals.

7. The inkjet head according to claim 1 , wherein an auxiliary pulse signal is applied in a case in which a number of the continuous ink drops is equal to or smaller than N (N is equal to or greater than 1 and equal to or smaller than “the maximum number of the drops−1” to form one pixel).

8. An inkjet printer, including:

an inkjet head; and

a conveyance mechanism configured to convey an image recording medium to a printing position by the inkjet head,

wherein the inkjet head further comprising:

a pressure chamber configured to house ink;

an actuator configured to be arranged corresponding to the pressure chamber and to generate a different displacement resulting from an applied voltage between a first displacement generated when hysteresis in a last time drive direction is the same as a present drive direction and a second displacement generated when hysteresis in the last time drive direction is reverse to a present drive direction;

a plate configured to have a nozzle communicating with the pressure chamber; and

a driving circuit configured to drive the actuator,

wherein the driving circuit applies to the actuator a first auxiliary pulse for contracting a volume of the pressure chamber and a second auxiliary pulse for generating an amplitude of pressure vibration of the ink opposite to an amplitude of pressure vibration of the ink resulting from the first auxiliary pulse, wherein the first and second auxiliary pulses do not eject an ink drop from the nozzle, and the second auxiliary pulse is applied prior to the first auxiliary pulse, and then applies to the actuator a drive signal which contains an expansion pulse for expanding the volume of the pressure chamber and a contraction pulse for contracting the volume of the pressure chamber to eject the ink from the pressure chamber once or for multiple times, and

wherein the second displacement resulting from the first auxiliary pulse assists to boost an ejection speed of a first ink drop prior to a second ink drop in the multiple ink drops.

9. The inkjet printer according to claim 8 , wherein the second displacement is larger than the first displacement.

10. The inkjet printer according to claim 8 , wherein a stored charge on the actuator generating the second displacement is larger than that on the actuator generating the first displacement.

11. The inkjet printer according to claim 8 , wherein the driving circuit outputs the expansion pulse as a second auxiliary pulse signal.

12. The inkjet printer according to claim 8 , wherein the driving circuit sets a pulse center interval between first and second auxiliary pulse signals as a resonance period of the pressure chamber with the ink.

13. The inkjet printer according to claim 8 , wherein the driving circuit equalizes a pulse width of first and second auxiliary pulse signals.

14. The inkjet printer according to claim 8 , wherein an auxiliary pulse signal is applied in a case in which a number of the continuous ink drops is equal to or smaller than N (N is equal to or greater than 1 and equal to or smaller than “the maximum number of the drops−1” to form one pixel).