Voltage Nulling Pressure Sensor Preamp for Safe Current Limiting
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Solution Overview
Problem
Solid-state electronic piezoresistive pressure sensors used in medical procedures face challenges in operating safely within current limits, as they often exceed safe patient exposure levels, necessitating careful design to restrict currents to below 10 μA or 50 μA to prevent cardiac arrhythmia and other issues.
Innovation Solution
A pressure sensing circuit with a Wheatstone bridge configuration, incorporating current and voltage limiting elements, and an amplification circuit that nullifies bridge output voltage to minimize self-heating and noise, allowing for reduced resistor sizes and safe patient current levels, while maintaining accurate pressure and temperature measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If maximum current is increased to improve signal-to-noise ratio and reduce self-heating, then measurement accuracy improves, but patient safety is compromised due to risk of cardiac arrhythmia
Solution Approach 1:
The patent introduces a current limiting resistor as an intermediary component between the excitation voltage source and the pressure sensor. This resistor mediates the current flow to ensure it remains within safe patient exposure limits while still allowing adequate current to reach the sensor for accurate measurement. The resistor value is specifically designed to limit current to below 10 μA during normal operation and below 50 μA during single-fault operation, thus protecting the patient while maintaining measurement capability.
Solution Approach 2:
The patent employs feedback control to dynamically adjust the excitation voltage parameter based on the actual current flowing through the sensor. By monitoring the current and modifying the voltage accordingly, the system maintains optimal operating conditions for the sensor while ensuring patient safety. This dynamic parameter adjustment allows the system to operate at the maximum safe current level, improving signal-to-noise ratio without compromising patient safety.
2Object-affected harmful factors
If current limiting resistors are added to restrict patient current exposure, then patient safety is improved, but signal-to-noise ratio deteriorates due to increased resistance in the measurement path
Solution Approach 1:
The patent implements a feedback control mechanism that monitors the actual current flowing through the pressure sensor and adjusts the excitation voltage accordingly. This feedback loop compensates for the voltage drop across the current limiting resistor, ensuring that the sensor receives adequate excitation current for accurate measurement while the overall system current remains within safe limits. The feedback action effectively cancels out the noise introduced by the current limiting resistor.
Solution Approach 2:
The patent replaces passive current limiting methods (simple resistors) with an active control system that uses feedback to regulate current. This substitution allows the system to achieve current limiting without the significant voltage drops and noise associated with high-value resistors. The active control mechanism adjusts the excitation voltage in real-time to maintain optimal sensor operation while ensuring patient safety.
3Object-affected harmful factors
If excitation current is reduced to below 10 μA or 50 μA to meet safety standards, then patient safety is improved, but self-heating of resistors is reduced improving measurement accuracy
Solution Approach 1:
The patent employs dynamic adjustment of the excitation voltage based on real-time current measurement. The feedback control system continuously adapts the voltage to maintain the current at the maximum safe level, ensuring that the sensor operates with optimal signal-to-noise ratio while keeping patient exposure within safety limits. This dynamic operation allows the system to extract maximum information from the sensor without causing excessive self-heating.
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 improves signal-to-noise ratio and ensures safe patient current levels, simplifying device design and manufacturability by allowing operation within safe current limits, enhancing accuracy and reducing self-heating and parasitic leakages.
Implementation Method 1
solid-state electronic piezoresistive pressure sensor, based on the piezoresistive effect manifested in semiconductors (or metal strain gauges). The piezoresistive effect is a change in the electrical resistivity of a semiconductor when mechanical strain is applied.
Implementation Method 2
the sensors are generally more accurate at lower current flow, as self-heating of the resistors and parasitic leakages are reduced
Data Source
AI summary
Pressure sensors that can be reliability operated with the maximum current flowing through the device restricted to 10 uA or below, or below 50 uA in a single-fault condition. This can provide at least a reduced need for the final medical device architect to consider potential risks from excessive current to the patient, simplifying the design and manufacturability of the medical device. An additional benefit is that the sensors are generally more accurate at lower current flow, as self-heating of the resistors and parasitic leakages are reduced, if the signal-to-noise problem is resolved.