3-Wire Sensor Circuit Input Protection Accuracy
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
3-wire resistive measurements face accuracy issues due to resistance mismatches in lead wires and protection resistors, leading to tradeoffs between cost, protection, and accuracy, especially when additional resistors are added for protection against shorting.
Innovation Solution
The solution involves relocating current sources to the lead wires and using additional protective devices to eliminate measurement errors caused by protection resistors, ensuring that only lead wire resistance mismatches affect the measurement, thereby maintaining the simplicity of a 3-wire configuration while improving accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If additional protection resistors are added to protect the measurement device from shorting, then device protection is improved, but measurement accuracy deteriorates due to resistance mismatches between the two legs
Solution Approach 1:
The patent introduces a differential voltage measurement approach where the voltage difference between the two legs is measured. This intermediary measurement method eliminates the effect of protection resistor mismatches by measuring the differential voltage across the sensor element, thereby maintaining both protection and accuracy.
Solution Approach 2:
The patent changes the measurement parameter from absolute voltage to differential voltage between the two legs. By measuring the voltage difference rather than absolute voltages, the system becomes insensitive to common-mode variations caused by protection resistor mismatches, resolving the accuracy-protection tradeoff.
2Measurement precision
If the resistance in both legs is reduced to improve matching, then measurement accuracy is improved, but power dissipation increases and input protection decreases
Solution Approach 1:
The differential measurement approach acts as an intermediary that allows accurate measurement without requiring reduced resistance in the legs. The system measures voltage differences that are independent of the absolute resistance values, thereby maintaining both accuracy and adequate protection without excessive power dissipation.
3Reliability
If secondary protection such as Zener diodes is inserted to provide current sink at higher voltages, then device protection is improved, but cost increases and leakage currents increase which degrades measurement accuracy
Solution Approach 1:
The differential voltage measurement serves as an intermediary that eliminates the need for secondary protection elements like Zener diodes. By measuring the voltage difference across the sensor element, the system inherently protects against over-voltage conditions without introducing leakage currents that would degrade measurement accuracy.
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 approach eliminates measurement errors from protection resistors, maintaining the simplicity of 3-wire measurements while ensuring high accuracy by minimizing the impact of protection resistor mismatches, thus enhancing the overall measurement precision without increasing power dissipation or circuit complexity.
Implementation Method 1
connecting a current sensing device (e.g., a reference resistor) to provide at the third terminal a signal that is indicative of the current in the third lead wire of the sensor
Implementation Method 2
connecting a first protective device between the first lead wire of the sensor and the first terminal; connecting a second protective device between the second lead wire of the sensor and the second terminal
Data Source
AI summary
A method and a circuit take a measurement of a sensor having first, second and third lead wires by: (a) providing first, second and third terminals for voltage measurements; (b) connecting a current sensing device (e.g., a reference resistor) to provide a signal at the third terminal that is indicative of the current in the third lead wire of the sensor; (c) connecting a first protective device between the first lead wire of the sensor and the first terminal; (d) connecting a second protective device between the second lead wire of the sensor and the second terminal; (e) connecting a first current source to the first lead wire of the sensor; (f) connecting a second current source to the second lead wire of the sensor; and (g) measuring a first voltage across the first and second terminals and a second voltage across the third terminal and the voltage reference.