Determining the operational status of a printhead

1. A system for determining the operational status of a nozzle in an inkjet printhead having a piezoelectric actuator configured to cause the ejection of ink through the nozzle, the system comprising:

a driving circuit configured to apply a driving signal to the piezoelectric actuator during a first time period; and

a sensing circuit configured to measure the current within the piezoelectric actuator as a function of time during a second time period after the first time period and before a third time period during which residual pressures oscillate in the piezoelectric actuator;

wherein the system is configured to determine the operational status of the nozzle in dependence on the measured current as a function of time during the second time period without dependence on a current measured during the third time period; and

wherein the driving signal is held constant for the duration of the second time period.

2. A system as claimed in claim 1 , wherein the measured current as a function of time during the second time period is a slope of the measured current.

3. A system as claimed in claim 1 , wherein the operation status of the nozzle is in dependence on the time taken for the measured current as a function of time to reach a predetermined condition during the second time period.

4. A system as claimed in claim 3 , wherein the predetermined condition is one of: a threshold current value; a maximum current value; when the gradient of the measured current as a function of time is equal to zero; or when the gradient of the measured current as a function of time is equal to zero for the first time during the second period.

5. A system as claimed in claim 4 , wherein the predetermined condition is the threshold current value and the system further comprises a comparator configured to compare the measured current to the threshold current value.

6. A system as claimed in claim 3 , wherein the second time period is separated from the first time period by an intermediate time period.

7. A system as claimed in claim 6 , wherein the time taken for the measured current to reach the predetermined condition is measured from the end of the intermediate period.

8. A system as claimed in claim 3 , wherein the time taken for the measured current to reach the predetermined condition is measured from the start of the second period.

9. A system as claimed in claim 3 , wherein the system further comprises a counter configured to measure the time taken for the measured current to reach the predetermined condition.

10. A system as claimed claim 3 , wherein the system further comprises a low noise amplifier.

11. A system as claimed in claim 3 , wherein the operational status of the nozzle is determined by a logic processor.

12. A system as claimed claim 3 , wherein the operational status of the nozzle is determined using one or more of a set of rules, an algorithm and a look-up table.

13. A system as claimed in claim 3 , wherein the operational status is determined as one or more of normally jetting, deviated jetting, partially blocked, fully blocked and containing an air bubble.

14. A system as claimed in claim 3 , wherein the driving signal is not applied to the actuator during the second time period.

15. An inkjet printhead comprising:

a nozzle;

a piezoelectric actuator configured to cause the ejection of ink through the nozzle; and

a system according to claim 3 .

16. A method for determining the operational status of a nozzle in an inkjet printhead, the method comprising:

applying a driving signal to a piezoelectric actuator during a first time period;

measuring the current within the piezoelectric actuator as a function of time during a second time period after the first time period and before a third time period during which residual pressures oscillate in the piezoelectric actuator;

holding the driving signal constant for the duration of the second time period; and

determining the operational status of the nozzle in dependence on the measured current as a function of time during the second time period without dependence on a current measured during the third time period.

17. A method according to claim 16 , wherein the measured current as a function of time is a time taken to reach a predetermined condition during the second time period.

18. A method according to claim 16 , wherein the measured current as a function of time is a slope of the measured current.