Patent ID: 8251471

Claim:
An apparatus comprising: droplet ejection devices each comprising an element to change a volume of a fluid chamber of one of the droplet ejection devices, the element having an electrical capacitance, each droplet ejection device being associated with a plurality of charging resistors; and control circuitry to effect uniform velocities of droplets ejected from at least two different ones of the droplet ejection devices by providing respective charge voltages or charge currents to the volume changing elements to individually control a charge on each volume changing element; wherein for each droplet ejection device, the control circuitry provides the respective charge voltage or charge current by selecting a first charging resistor associated with the droplet ejection device to charge the electrical capacitance at a first rate followed by deselecting the first charging resistor to maintain the charge on the electrical capacitance at a first value for a first period of time, followed by selecting a second charging resistor associated with the droplet ejection device to charge the electrical capacitance at a second rate to increase the volume of the fluid chamber, followed by deselecting the second charging resistor to maintain the charge on the electrical capacitance at a second value for a second period of time, followed by selecting a first discharging resistor associated with the droplet ejection device to discharge the electrical capacitance at a third rate, followed by selecting a second discharging resistor associated with the droplet ejection device to discharge the electrical capacitance at a fourth rate to decrease the volume of the fluid chamber, wherein the first rate, the second rate, the third rate, and the fourth rate are all different rates.