Motor Position Observer for Low-Speed Sensorless Rotor Detection
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Solution Overview
Problem
Existing sensorless control methods for synchronous motors face challenges in accurately determining rotor position at low speeds and standstill, particularly due to errors in back emf estimation and inductance measurement, which increase with motor speed and load, leading to inefficient torque production and acoustic noise.
Innovation Solution
A combined approach using inductance detection at low speeds and emf estimation at high speeds, where motor parameters are calculated and updated, allowing for parallel operation of both methods with weighted control signals based on motor speed, and incorporating non-linear modeling to improve accuracy and reduce errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If back emf estimation method is used for rotor position detection, then position detection is possible at high speeds, but the method cannot be used at low speeds or standstill due to insufficient induced emf
Solution Approach 1:
The patent combines two different sensorless control schemes: back emf estimation method for high-speed operation and inductance detection method for low-speed operation. The controller selectively switches between or blends these two methods based on the motor's operating speed, thereby achieving accurate position detection across the entire speed range from standstill to high speed.
Solution Approach 2:
The patent changes the detection method parameter based on operating conditions. At low speeds where back emf is insufficient, the system transitions to inductance detection which relies on measuring changes in stator winding inductance rather than induced voltage, thus adapting the detection principle to match the available signal levels at different speeds.
2Speed
If inductance detection method is used for rotor position detection, then position data is available at low speed and standstill, but practical implementation is difficult requiring precise measurement of rate of change of current
Solution Approach 1:
The patent introduces an intermediary computational approach where the controller calculates inductance changes by measuring current at different inverter voltage states and eliminating the effects of back emf and resistive voltage drop through mathematical processing. This intermediary calculation layer simplifies the direct measurement requirements while maintaining accuracy.
Solution Approach 2:
The system uses feedback from multiple current measurements taken at different inverter states to continuously update and refine the position estimate. By taking two readings of the rate of change of current in two different inverter voltage states, the system feedbacks the position information back to the controller for continuous correction and improvement of detection accuracy.
3Measurement precision
If inductance measurement is performed by applying static voltage vector and measuring current change, then position data is obtained, but the measurement time increases requiring inverter to be held in fixed voltage vector condition
Solution Approach 1:
The patent employs periodic sampling of current at specific inverter voltage states during normal motor operation. Instead of holding the inverter in a fixed state for extended measurement, the system periodically inserts measurement pulses or uses existing switching states to capture current data, thereby reducing the time penalty while maintaining measurement accuracy through strategic timing of measurements.
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 solution enables robust and accurate rotor position estimation and torque optimization from low to high speeds, reducing errors and acoustic noise, and allows for efficient operation of synchronous motors, including hybrid stepping motors, by interpolating motor parameters and using Rogowski coils for precise current measurements.
Implementation Method 1
A Rogowski coil can be used: a Rogowski coil inherently performs magnetic differentiation of the current in a conductor linking the coil. A single sample of the voltage across the Rogowski coil provides measurement of the rate of change of current linking the coil.
Implementation Method 2
Methods which use the back emf induced in the stator windings created by rotation of permanent magnet fluxes or by variation in phase winding inductances while current flows in the winding
Data Source
AI summary
A control system for an electrical motor comprises a rotor, a stator having a plurality of phase windings, and an inverter for applying voltage to the plurality of phase windings by connecting individual phase windings to a first or second voltage level. The control system is configured to measure a first rate of change of current in a first phase winding, of said plurality of phase windings, connected to the first voltage level, to measure a second rate of change of current in a second, different phase winding connected to the first voltage level, and to calculate a difference between the first and second rate of change of current. The control system is further configured to use the calculated difference to obtain data related to a position of the rotor.