Capacitive Ink Level Sensor Using Charge Sharing
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Solution Overview
Problem
Existing ink level sensing techniques for inkjet printers face challenges in accuracy and cost, with current methods often leading to premature replacement of ink cartridges and potential low-quality prints due to inadequate ink supply indications.
Innovation Solution
A capacitive, charge-sharing ink level sensor integrated on a thermal inkjet printhead employs a sample and hold technique using a sense capacitor and a reference capacitor, with a current source measuring the drain-to-source resistance of an evaluation transistor to determine ink levels, improving accuracy and cost-effectiveness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional ink level sensing techniques are used, then the system can detect ink levels, but the measurement precision is insufficient leading to inaccurate ink level indications
Solution Approach 1:
The patent replaces conventional mechanical or simple capacitive sensing mechanisms with a sophisticated charge-sharing circuit that uses transistor-based voltage division. The sense capacitor couples the ink level detection to the gate of a transistor, and the charge-sharing mechanism with a reference capacitor creates a precise voltage ratio that accurately reflects ink level, achieving measurement precision comparable to more complex mechanical systems but with integrated circuit simplicity.
Solution Approach 2:
The patent changes the electrical parameters of the sensing system by using a transistor's gate voltage to control drain current, which in turn controls a DAC output voltage. By measuring the ratio of voltages (Vout/Vref) rather than absolute values, the system achieves high measurement precision that is immune to variations in power supply voltage or capacitor tolerances, thereby improving both measurement precision and reliability.
2Loss of substance
If accurate ink level sensing is implemented, then ink waste is reduced, but the device complexity increases
Solution Approach 1:
The patent merges multiple functions into a single integrated circuit: the sense capacitor, reference capacitor, evaluation transistor, current source, and voltage ratio measurement logic are all combined in one chip. This integration achieves accurate ink level detection (reducing ink waste) without the device complexity of separate mechanical sensors, capacitors, and measurement circuits, as everything is fabricated together using standard semiconductor processes.
Solution Approach 2:
The charge-sharing circuit automatically performs the measurement function: the sense capacitor naturally charges through the ink level-dependent coupling, then automatically shares charge with the reference capacitor through the transistor gate, and the voltage ratio self-calibrates against the reference voltage. This self-service mechanism reduces ink waste through accurate detection without requiring complex external control circuitry or calibration procedures.
3Device complexity
If conventional capacitive sensing is used, then the circuit is simple, but the measurement precision is insufficient due to charge leakage
Solution Approach 1:
The patent substitutes the direct capacitance measurement approach with a charge-sharing voltage ratio measurement. Instead of measuring capacitance directly (which is prone to leakage errors), the system shares charge between capacitors and measures the resulting voltage ratio through a transistor's gate. This substitution maintains circuit simplicity while achieving high measurement precision by measuring voltage rather than charge directly.
Solution Approach 2:
The patent changes the measurement parameter from absolute capacitance value to voltage ratio (Vout/Vref). By measuring the ratio of voltages rather than absolute capacitance, the system becomes immune to charge leakage effects, capacitor tolerance variations, and power supply fluctuations. This parameter change maintains circuit simplicity while dramatically improving measurement precision and reliability.
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
The solution provides precise ink level detection, reducing waste and preventing low-quality prints by accurately indicating ink levels, thus optimizing ink usage and printer performance.
Implementation Method 1
a sense capacitor to which a charge is shared between the sense capacitor and a reference capacitor to cause a reference voltage
Implementation Method 2
an evaluation transistor to provide a drain to source resistance in proportion to the reference voltage at its gate
Data Source
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Figure 3~4
AI summary
In an embodiment, a method of sensing an ink level includes applying a pre-charge voltage Vp to a sense capacitor to charge the sense capacitor with a charge Q1, sharing Q1 between the sense capacitor and a reference capacitor, causing a reference voltage Vg at the gate of an evaluation transistor, and determining a resistance from drain to source of the evaluation transistor that results from Vg.