Current-Sensing Circuit with Variable Gain Operational Amplifier
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Solution Overview
Problem
Current-sensing circuits using operational amplifiers for inverter current sensing suffer from errors due to variations in supply voltage and require expensive microcomputers with high arithmetic performance to process pulse-wave output voltages, leading to increased costs and reduced accuracy.
Innovation Solution
A current-sensing circuit with a shunt resistor, operational amplifier, average voltage generating unit, and arithmetic unit, where the operational amplifier's supply voltage is set higher than the arithmetic unit's, averaging the output voltage to a lower value, allowing for more affordable and accurate current sensing by reducing errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the output voltage of the operational amplifier is directly input to the microcomputer for arithmetic operation, then the current sensing resolution is improved, but an expensive microcomputer with high arithmetic performance is required
Solution Approach 1:
The patent applies preliminary action by performing the arithmetic operation in advance within the operational amplifier circuit itself, before the voltage signal reaches the microcomputer. The operational amplifier performs division by a constant value (equivalent to multiplication by a scaling factor) through its feedback resistor network, converting the pulse-wave output into a scaled version that the microcomputer can process with standard arithmetic capabilities. This eliminates the need for expensive high-performance microcomputers while maintaining sensing resolution.
Solution Approach 2:
The patent substitutes the mechanical/computational system (microcomputer with high arithmetic performance) with an electronic analog system (operational amplifier circuit). Instead of relying on the microcomputer to perform complex arithmetic operations on pulse-wave signals, the operational amplifier circuit performs the arithmetic transformation electronically in the analog domain, reducing the computational burden on the microcomputer and allowing the use of more affordable microcomputers.
2Measurement precision
If the gain of the operational amplifier is increased to improve current sensing resolution, then the current sensing accuracy is improved, but the output voltage becomes too high for the microcomputer to process accurately
Solution Approach 1:
The patent applies dynamics by making the gain of the operational amplifier variable rather than fixed. The gain is dynamically adjusted based on the magnitude of the current command value (reference voltage). When the current command value is large, the operational amplifier operates with lower gain to keep the output voltage within the microcomputer's acceptable range. When the current command value is small, the operational amplifier operates with higher gain to improve sensing resolution. This dynamic gain adjustment resolves the contradiction between sensing accuracy and voltage compatibility.
Solution Approach 2:
The patent changes the parameter of gain dynamically based on operating conditions. The gain of the operational amplifier is not fixed but varies according to the current command value. This parameter change allows the system to optimize both current sensing resolution and voltage level compatibility across different operating conditions, eliminating the need to choose between high gain for accuracy or low gain for voltage compatibility.
3Measurement precision
If a comparator is added to change the gain based on the current command value, then the current sensing accuracy is improved, but additional circuit components increase the cost
Solution Approach 1:
The patent applies universality by making the operational amplifier perform multiple functions: it serves as both the voltage amplification device and the gain control mechanism. The operational amplifier's gain is controlled by the current command value through its feedback network, eliminating the need for a separate comparator and gain control circuit. This multi-functionality reduces the number of circuit components while maintaining current sensing accuracy.
Solution Approach 2:
The patent merges the functions of voltage amplification and gain control into a single operational amplifier circuit. Instead of using a comparator to detect the current command value and then switching gain states, the operational amplifier directly incorporates the gain control functionality through its feedback resistor network, which is influenced by the current command value. This merging of functions eliminates additional components and simplifies the circuit configuration.
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 configuration enables a cost-effective current-sensing circuit with improved accuracy by using a more affordable arithmetic unit and allowing detection of zero vector regions, enhancing the precision of current sensing in air-conditioning devices.
Implementation Method 1
a shunt resistor (R1) connected in series with a path of the current from the inverter (106)
Implementation Method 2
an operational amplifier (12) amplifying voltage across both ends of the shunt resistor (R1) with a predetermined gain
Data Source
Figure 1
Figure 2
Figure 3
AI summary
The current-sensing circuit 1 includes a shunt resistor R1 connected in series with a path of current from an inverter 106, an operational amplifier 12 amplifying voltage across both ends of this shunt resistor R1 with a predetermined gain, a low-pass filter 18 averaging output voltage of this operational amplifier 12, and an arithmetic unit 50 calculating the current flowing through the path based on output voltage of the low-pass filter 18. The supply voltage of the operational amplifier 12 is set higher than the supply voltage of the arithmetic unit 50. The output voltage of the operational amplifier 12 is averaged at the low-pass filter 18 to a lower value than the supply voltage of the arithmetic unit 50.