Two-Wire RTD Switching for Lead Resistance Compensation
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Solution Overview
Problem
Conventional two-wire resistance temperature detectors (RTDs) suffer from significant temperature measurement errors due to the resistance of wire leads, especially at cryogenic temperatures, and adding extra wires to compensate for these errors increases cost and weight, which is undesirable for space launch vehicles.
Innovation Solution
A two-wire RTD system with a switch that alternates between two current pulses to differentiate between sensor resistance and lead resistance, allowing the control unit to accurately determine the sensor temperature by measuring resistance differences during open and closed switch states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If conventional two-wire RTD is used with long wire leads, then the control unit can be located far from the sensor, but the wire lead resistance causes significant temperature measurement errors
Solution Approach 1:
The patent applies periodic action by using alternating current pulses at different frequencies to periodically switch the state of the wire leads. During certain time intervals, the wire leads are in a high-impedance state (blocking current), and during other intervals, they are in a low-impedance state (allowing current). This periodic switching enables the system to separately measure sensor resistance and wire lead resistance, eliminating measurement errors despite long wire lengths.
2Measurement precision
If three-wire or four-wire RTD is used to compensate for wire lead resistance, then temperature measurement accuracy improves, but the cost and weight of the system increase
Solution Approach 1:
The patent applies parameter changes by dynamically altering the electrical impedance state of the wire leads through frequency-dependent switching. By changing the operational parameters (impedance state) of existing two-wire RTD components based on signal frequency, the system achieves compensation for wire lead resistance without adding extra wires. This maintains measurement precision while avoiding the weight and cost penalties of three-wire or four-wire configurations.
3Measurement precision
If three-wire or four-wire RTD is used to compensate for wire lead resistance, then temperature measurement accuracy improves, but the system cost increases
Solution Approach 1:
The patent applies parameter changes by dynamically altering the electrical impedance state of the wire leads through frequency-dependent switching. By changing the operational parameters (impedance state) of existing two-wire RTD components based on signal frequency, the system achieves compensation for wire lead resistance without adding extra wires. This maintains measurement precision while avoiding the weight and cost penalties of three-wire or four-wire configurations.
4Measurement precision
If wire lead resistance is compensated by adjusting total resistance at control unit, then measurement accuracy improves, but the compensation is inaccurate due to temperature variations in wire leads
Solution Approach 1:
The patent applies feedback by using the measured voltage and current data to dynamically calculate and update the wire lead resistance compensation in real-time. The system measures the actual voltage drop across the wire leads during operation and uses this feedback information to adjust the compensation calculation, accounting for temperature variations and other changing conditions. This ensures continuous accurate compensation rather than relying on fixed pre-calculated values.
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 provides accurate temperature measurements without the need for additional wires, reducing errors and maintaining system performance while minimizing weight and cost.
Implementation Method 1
The sensor resistance is used to determine the temperature of the RTD sensor which varies linearly with sensor resistance. In this regard, the hotter the RTD sensor becomes, the higher the sensor resistance
Implementation Method 2
Passing a small amount of electrical current (i.e., an excitation current) through the RTD sensor generates a voltage across the RTD sensor. The voltage across the RTD sensor is used to determine the resistance of the RTD sensor
Data Source
AI summary
A two-wire resistance temperature detector (RTD) includes an RTD sensor having a sensor resistance that changes in correspondence with changes in the temperature of the RTD sensor. The two-wire RTD additionally includes a pair of leads electrically coupling the RTD sensor to a control unit having a pulse generator for generating a first current pulse and a second current pulse. The two-wire RTD also includes a switch electrically coupled between the pair of leads and operable in an open state when subjected to the first current pulse and operable in a closed state when subjected to the second current pulse. The open state causes the first current pulse to flow through the pair of leads and the RTD sensor. The closed state causes the second current pulse to flow through the pair of leads. The switch enables the control unit to determine the sensor resistance and corresponding sensor temperature.