Temperature control in printheads having thermal actuators

1. A method of controlling the temperature of an ink jet printhead, said printhead including an array of nozzles formed on a substrate, each nozzle including a thermal actuator which is thermally connected to and relatively displaceable from the substrate, a portion of the thermal actuator being heated to cause displacement of the thermal actuator whereby ejection of ink through a nozzle opening of an associated nozzle is caused, said heated portion of the thermal actuator being thermally insulated from portions of the thermal actuator in physical contact with said ink in said associated nozzle, said method including the steps of:

(a) obtaining at least one first temperature signal representing a temperature of a part of said substrate;

(b) comparing one of the at least one first temperature signal and a derivative thereof with a predetermined threshold;

(c) if said one of the at least one first temperature signal and a derivative thereof is below said predetermined threshold, supplying energy to at least one of said actuators so as to heat at least part of the substrate and thereby heat the ink via the substrate until said one of the at least one temperature signal and a derivative thereof is above said predetermined threshold,

wherein step (c) does not cause the ejection of ink from said printhead and further wherein said thermal actuator is a thermally actuated lever arm.

2. A method as claimed in claim 1 further including the steps of:

(aa) obtaining a second temperature signal representing an ambient temperature surrounding said printhead;

(ab) setting said predetermined threshold based on to be said second temperature signal.

3. A method as claimed in claim 2 wherein said predetermined threshold is based on a predetermined operational factor amount, said operational factor amount being dependent on said second temperature signal.

4. A method as claimed in claim 1 further including the step of:

(e) monitoring said printhead temperature whilst printing and if said one of the at least one first temperature signal and a derivative thereof falls below said predetermined threshold, reheating said printhead until said one of the at least one first temperature signal and a derivative thereof is above said predetermined threshold.

5. A method as claimed in claim 1 wherein said step (b) includes constantly obtaining said at least one first temperature signal whilst heating said printhead.

6. A method as claimed in claim 1 wherein said step (c) includes applying a series of short electrical pulses to said at least one thermal actuator, each being insufficient to cause the ejection of ink from said printhead.

7. A method as claimed in claim 1 wherein each of said pulses supplies less than 500 nJ to each thermal actuator.

8. A method as claimed in claim 1 , wherein said lever arm pivots about a post connected to the substrate.

9. A method as claimed in claim 8 , wherein said lever arm is pivoted about said post by heating one end of said lever arm.

10. A method as claimed in claim 9 , wherein said heating is performed by passing a current through one of said layers.

11. A method as claimed in claim 8 , wherein said lever arm includes two conductive layers adjacent said post, and said lever arm is pivoted about said post by heating of one of said layers.

12. An ink jet printhead as claimed in claim 8 , wherein said lever arm includes an end paddle for ejecting ink through said nozzle opening.

13. A method as claimed in claim 1 , wherein said lever arm includes an end paddle for ejecting ink through said nozzle opening.

14. An ink jet printhead including:

an array of nozzles formed on a substrate, each nozzle including a thermal actuator that is thermally connected to and relatively displaceable from the substrate, a portion of the thermal actuator being heated to cause displacement of the thermal actuator whereby ejection of ink on demand through a nozzle opening of an associated nozzle is caused, said heated portion of the thermal actuator being thermally insulated from portions of the thermal actuator in physical contact with said ink in said associated nozzle;

at least one temperature sensor that generates a respective a first temperature signal representing a temperature of a part of said substrate;

a temperature determination unit that receives said first temperature signal;

an ink ejection drive unit connected to said temperature determination unit and to said printhead;

wherein, for said at least one temperature sensor, said temperature determination unit compares one of the first temperature signal and a derivative thereof with a predetermined threshold and if one of the first temperature signal and a derivative thereof is below said predetermined threshold outputs a heating signal to the ink ejection drive unit,

and wherein on receipt of said heating signal, said ink ejection drive unit supplies energy to at least one of the thermal actuators to heat at least part of said substrate and thereby heat the ink via the substrate, the energy supplied to said at least one of the thermal actuator being insufficient for the ejection of ink,

and wherein said thermal actuator is a thermally actuated lever arm.

15. A ink jet printhead as claimed in claim 14 wherein said at least one temperature sensor comprises a series of spaced apart temperature sensors formed on said printhead.

16. A ink jet printhead as claimed in claim 14 wherein said array of nozzles are divided into a series of spaced apart segments with at least one temperature sensor per segment.

17. A ink jet printhead as claimed in claim 14 wherein said ink ejection drive unit applies a series of short electrical pulses to said at least one thermal actuator.

18. A ink jet printhead as claimed in claim 14 wherein each of said pulses supplies less than 500 nJ to said at least one thermal actuator.

19. An ink jet printhead as claimed in claim 14 wherein said lever arm pivots about a post.

20. An ink jet printhead as claimed in claim 19 , wherein said lever arm is pivoted about said post by heating one end of said lever arm.

21. An ink jet printhead as claimed in claim 20 , wherein said heating is performed by passing a current through one of said layers.

22. An ink jet printhead as claimed in claim 19 , wherein said lever arm includes two conductive layers adjacent said post, and said lever arm is pivoted about said post by heating of one of said layers.