Q-Axis Current Control for Three-Phase AC Motor Stability
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Solution Overview
Problem
Conventional control devices for three-phase alternating current motors in NC machine tools face challenges in achieving high command following performance while maintaining high disturbance suppression performance and stability, particularly in the q-axis current control loop, leading to issues with vibration and reduced control stability during acceleration and deceleration.
Innovation Solution
The control device incorporates a q-axis current controller that calculates an acceleration q-axis current command value and adds proportional compensation using the IP coefficient KIP·iqf*·Gqp, along with a coefficient/gain setting unit to dynamically adjust KIP based on the q-axis current error, ensuring high command following performance and stability similar to I-P control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional q-axis current control is used in NC machine tools, then basic motor control is achieved, but command following performance deteriorates during acceleration and deceleration
Solution Approach 1:
The controller performs preliminary differentiation of the q-axis current command value to predict future current requirements. By calculating the derivative dq*/dt in advance and using it to generate the compensation term Gqp·dq*/dt, the system proactively compensates for command changes before they fully manifest, improving command following performance during acceleration and deceleration phases
Solution Approach 2:
The invention creates a copied version of the q-axis current command value by differentiating it to obtain dq*/dt. This copied derivative information is then scaled by gains Gqp and KIP and added to the original control signal, effectively creating a enhanced control signal that incorporates both the original command and its rate of change information
2Speed
If proportional gain Gqp is increased to improve command following performance, then response speed improves, but mechanical resonance vibrations increase
Solution Approach 1:
The controller dynamically adjusts the effective proportional gain by multiplying Gqp by the IP coefficient KIP, which varies based on operating conditions. During acceleration and deceleration when command following is critical, KIP increases the effective gain to Gqp·KIP. During steady-state operation, KIP reduces the gain to minimize resonance vibrations. This dynamic adjustment allows the system to optimize performance across different operating phases
Solution Approach 2:
The invention changes the control parameter by introducing the derivative term dq*/dt and scaling it with the product of gains Gqp and KIP. This parameter modification allows the controller to respond more aggressively to command changes without permanently increasing the base proportional gain, thereby improving response speed while controlling resonance through selective application of the enhanced gain only when needed
3Object-generated harmful factors
If additional filters are added to suppress mechanical resonance vibrations, then vibration reduction is achieved, but control loop complexity increases
Solution Approach 1:
The invention replaces the need for mechanical resonance filters with an electronic control solution. Instead of adding physical filter components to the control loop, the system uses electronic differentiation and proportional compensation through the term Gqp·KIP·dq*/dt to achieve vibration suppression. This substitution maintains a simpler control loop structure while effectively reducing mechanical resonance vibrations
Data Source
AI summary
A control device, which is configured to control current of a three-phase alternating current motor by d-q axis control, includes a q-axis current controller configured to receive, as one input, an acceleration torque command value τf calculated based on a position command value X, and output a controller output voltage Δeq to be added to a q-axis voltage command value vq*. The q-axis current controller calculates, based on the acceleration torque command value τf, an acceleration q-axis current command value iqf*, which is a current command value corresponding to the acceleration torque command value τf, and adds, to the controller output voltage Δeq, a value obtained by multiplying the acceleration q-axis current command value iqf* by an IP coefficient KIP indicating a ratio of switching between PI control and I-P control and by a q-axis proportional gain Gqp.