Fluid ejection device with integrated ink level sensor

1. A fluid ejection device comprising:

a slot formed in a fluid ejection die;

an integrated fluid level sensor to sence a fluid level in a chamber that is fluid communication with the slot; and

a fluid clearing device disposed within the chamber to clear the chamber of fluid.

2. The fluld ejection device of claim 1 , wherein the fluid clearing device comprises a clearing resistor.

3. The fluid ejection device of claim 2 , wherein the clearing resistor comprises a circuit of four resistors surrounding a sense capacitor plate of the fluid level sensor, each resistor adjacent to and aligned parallel with a different side of the sense capacitor plate.

4. The fluid ejection device of claim 2 , wherein the fluid level sensor comprises multiple sensors to sense fluid levels in multiple chambers in fluid communication with the slot, the fluid ejection device further comprising a shift register to select between the multiple sensors for output onto a common data line.

5. The fluid ejection device of claim 4 , wherein the multiple sensors comprise four sensors around a single slot, each of the four sensors located at a different end-corner of the slot.

6. The fluid ejection device of claim 4 , further comprising a sense capacitor plate in each sensor, wherein each sense capacitor plate is a minimum safe distance of about 40 to about 50 microns from an end of the slot.

7. The fluid ejection device of claim 4 , further comprising a controller to control activation of the clearing resistor circuit and to control the shift register to select between the multiple sensors for output onto the common data line.

8. The fluid ejection device of claim 7 , further comprising a machine-readable medium storing code representing instructions that when executed by the controller of the fluid ejection device cause the controller to:

initiate operation of multiple sensors to sense a fluid level at multiple areas of the fluid ejection device; and

control a shift register on the fluid ejection device to multiplex outputs from the multiple sensors onto a common data line.

9. The fluid ejection device of claim 8 , wherein the instructions futher cause the controller to determine the fluid level using the outputs from the multiple sensors.

10. The fluid ejection device of claim 9 , wherein determining the fluid level comperises averaging the multiple outputs from the multiple sensors.

11. The fluid ejection device of claim 1 , wherein operation of the fluid level sensor comprises:

placing a charge on a sense capacitor at a memory node M 1 ;

coupling M 1 to a second memory node M 2 , to share the charge between the sense capacitor and a reference capacitor the shared charge causing a reference voltage Vg at M 1 , M 2 , and a transistor gate;

determining a resistance across the transistor drain to source; and

comparing the resistance to a reference value to determine a fluid level.

12. The fluid ejection device of claim 11 , wherein operation of the fluid level sensor further comprises:

applying a voltage Vp to M 1 to place the charge on the sense capacitor; and

simultaneously applying Vp to a node Mp to prevent a parasitic capacitance charge from developing between M 1 and Mp.

13. The fluid ejection device of claim 1 , wherein the fluid clearing device comprises a clearing resistor, and

the device farther comprising:

a processor-readable medium storing code representing instructions that when executed by a processor the of the fluid ejection device cause the processor to:

activate the clearing resistor circuit to purge fluid from the chamber;

apply a pre-charge voltage Vp to the sense capacitor within the chamber to charge the sense capacitor with a charge Q 1 ;

share charge Q 1 between the sense capacitor and the reference capacitor, causing a reference voltage Vg at a gate of the evaluation transistor; and

determine a resistance from drain to source of the evaluation transistor that results from Vg.

14. The fluid ejection device of claim 13 , wherein the instructions further cause the processor to:

provide a delay after activating the clearing resistor circuit to enable fluid from a fluid slot to flow back into the chamber prior to applying the per-charge voltage Vp.

15. The fluid ejection device of claim 1 , wherein the fluid sensor comprises:

a sense capacitor whose capacitance changes with the fluid level in the chamber;

a switch T 2 to apply a voltage Vp to the sense capacitor, placing a charge on the sense capacitor;

a switch T 3 to share the charge between the sense capacitor and a reference capacitor, resulting in a reference voltage Vg; and

an evaluation transistor configured to provide a drain to source resistance in proportion to the reference voltage.

16. The fluid ejection device at claim 1 , further comprising a parasitic elimination circuit to eliminate intrinsic parasitic capacitance of the fluid level sensor.

17. A fluid ejection device comprising:

a slot formed in a fluid ejection die;

an integrated fluid level sensor to sence a fluid level in a chamber that is in fluid communication with the slot; and

a clearing resistor circuit disposed within the chamber to clear the chamber of fluid.

18. The fluid ejection device of claim 17 , wherein the clearing resistor circuit comprises four resistors surrounding a sense capacitor plate of the fluid level sensor, each resistor adjacent to and aligned parallel with a different side of the sense capacitor plate.

19. The fluid ejection device of claim 17 , wherein the fluid level senor comprises multiple sensors to sense fluid levels in multiple chambers in fluid communication with the slot, the fluid ejection device further comprising a shift register to select between the multiple sensors for output onto a common data line.

20. The fluid ejection device of claim 19 , wherein the multiple sensors comprise of four sensors around a single slot, each of the four sensors located at a different end-corner of the slot.