Piezoelectric Printhead Capacitance Compensation via Current Waveform Control
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Solution Overview
Problem
Piezoelectric inkjet printheads face issues with temperature sensitivity and long-term degradation, leading to non-uniform performance due to changes in capacitance, which affect drop weight and velocity, and are often inadequately managed by existing methods that either decrease printing performance or increase costs.
Innovation Solution
A closed-loop control system that monitors and adjusts the current delivered to each piezoelectric element by altering the rise and fall times of the current waveform, using a circuit that senses the current and adjusts the resistance or FET turn-on times to maintain consistent performance despite capacitance changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the driving voltage is fixed, then the control is simple, but the capacitance increase with temperature causes current increase and overheating
Solution Approach 1:
The patent implements a feedback control system that continuously monitors the capacitance of the piezoelectric element and adjusts the drive voltage accordingly. The controller measures capacitance changes and modifies the drive voltage to maintain constant power delivery, preventing temperature runaway while keeping the control system relatively simple.
Solution Approach 2:
The patent dynamically changes the drive voltage parameter based on measured capacitance variations. As capacitance increases with temperature, the system reduces the drive voltage to compensate, maintaining stable operating conditions without requiring complete system redesign.
2Power
If the capacitance increases with temperature, then the piezo-element pumping performance strengthens, but the drop weight and velocity continue to increase adversely affecting printer performance
Solution Approach 1:
The system uses feedback control to monitor capacitance changes and adjust the drive voltage in real-time. This ensures that the piezoelectric element delivers consistent pumping performance despite temperature variations, maintaining uniform drop weight and velocity throughout the printing process.
Solution Approach 2:
The patent applies preliminary anti-action by anticipating capacitance increases due to temperature rise and preemptively adjusting the drive voltage downward. This prevents the adverse effects of excessive pumping strength before they occur, maintaining consistent print quality.
3Power
If the current increases due to capacitance increase, then more power is delivered to the piezo-element, but the piezo-element and fluid heat up causing thermal runaway
Solution Approach 1:
The patent implements feedback control that monitors capacitance as an indicator of temperature rise and adjusts the drive voltage accordingly. This prevents thermal runaway by reducing power delivery when capacitance increases, while still allowing full power delivery when conditions are normal.
Solution Approach 2:
The system uses the capacitance change itself as the sensing mechanism and automatically adjusts the drive parameters without external temperature sensors. The piezoelectric element's own electrical property changes trigger the compensatory action, creating a self-regulating system.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution ensures uniform performance of piezoelectric elements and ink ejection nozzles throughout a print job and the life of the printhead, effectively controlling temperature and pumping strength, thereby improving printer performance and reducing overheating.
Implementation Method 1
a piezoelectric material actuator includes a flexible piezoelectric material sheet that deforms in response to an applied electric field, generating pressure pulses inside a fluid-filled chamber to eject fluid droplets
Implementation Method 2
sensing a current driving a piezoelectric element... from the sensed current, it is determined that a capacitance of the piezoelectric element is changed
Implementation Method 3
the rise time of the current driving the piezoelectric element is altered to compensate for the changed capacitance
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
In an embodiment, a method of compensating for capacitance change in a piezoelectric element of a fluid ejection device includes sensing a current driving a piezoelectric element, determining from the current that capacitance of the piezoelectric element has changed, and altering a rise time of the current driving the piezoelectric element to compensate for the changed capacitance.