Abstract
Sensorless control technology for switched reluctance motors (SRMs) is a critical research direction aimed at enhancing system reliability and reducing costs. This paper provides a systematic review of sensorless control strategies across different speed ranges. In the low-speed region, the focus is on pulse voltage injection methods, which extract rotor position information through high-frequency signal excitation, and unsaturated region flux linkage modeling approaches that estimate position by leveraging the linear relationship between flux linkage and phase current. In the medium-to-high-speed region, the review covers rotor-characteristic-position-based detection methods, position observer techniques, and the application of intelligent algorithms including neural networks and fuzzy logic in position estimation. Research indicates that while each method demonstrates strong adaptability under specific operating conditions, achieving high-precision estimation across the entire speed range and enhancing parameter robustness remain key challenges for future studies. Development trends suggest that deep integration of hybrid strategies and intelligent algorithms will further advance the performance of sensorless control systems.
Keywords
Introduction
The Switched Reluctance Motor (SRM) is a type of special electric motor characterized by its simple and robust structure, low manufacturing cost, ease of maintenance, and excellent fault tolerance capabilities.1–9 Its unique doubly salient structure (where both stator and rotor feature salient poles), the design incorporating windings only on the stator without windings or permanent magnets on the rotor, and its inherent capability for wide speed regulation range, demonstrate significant application potential in fields such as electric vehicle propulsion, industrial automation equipment, aerospace auxiliary systems, household appliances, and power tools.10–25 The operating principle of the SRM is based on the principle of minimum reluctance. The controller applies excitation currents to the stator phase windings in a specific logical sequence, generating reluctance torque by utilizing the characteristic that the air gap permeance varies with the rotor position.26–36 Consequently, high-performance control of the SRM fundamentally relies on the precise angular coordination between the stator phase currents and the rotor position, necessitating the real-time and accurate acquisition of rotor position information.37–46
Conventional SRM control architectures typically employ mechanical position sensors—such as optical encoders, resolvers, or Hall-effect sensors—mounted directly on the motor shaft to acquire real-time rotor position signals. Core control strategies, including Current Chopping Control (CCC) for low-speed, high-torque scenarios, Angle Position Control (APC) for high-speed efficiency optimization, and fundamental voltage Pulse Width Modulation (PWM) control, are critically dependent on the precision of these feedback signals to ensure accurate commutation timing and phase current regulation.47–65
However, the incorporation of traditional mechanical position sensors also introduces a series of significant drawbacks. These disadvantages manifest primarily in increasing the overall cost and complexity of the drive system. The position sensors themselves and their associated interface circuits directly increase material and manufacturing costs. Furthermore, the additional sensor installation and cable connections increase system complexity and introduce potential points of electrical or mechanical failure. More critically, physical position sensors are often a weak link in system reliability. In harsh industrial environments—characterized by high temperature, high humidity, pervasive dust, oil contamination, strong vibrations, and intense electromagnetic interference—their performance and lifespan may deteriorate drastically. Once these sensors fail, it can lead to the complete collapse and shutdown of the entire motor control system, posing a significant threat to system safety.
Given the problems associated with mechanical position sensors, especially in application contexts emphasizing cost-effectiveness, high reliability, resistance to harsh environments, and miniaturization/integration, the development of SRM control technology that does not rely on physical position sensors has become highly necessary and urgent.66–75 Sensorless control technology has emerged precisely to address this challenge. Its core concept is to utilize inherent and easily measurable electrical variables during SRM operation, real-time and accurately estimating the instantaneous rotor position and speed information through the design and application of advanced signal processing algorithms, state observers, or intelligent computational models, thereby completely replacing the function of physical sensors.75–86
The development of high-performance sensorless control techniques for SRMs holds significant theoretical value and broad practical significance. It can effectively reduce the overall system cost, enhancing product market competitiveness by eliminating the sensor and its interface circuitry. More crucially, it substantially improves the overall reliability and robustness of the system, eliminating the potential for failure due to physical damage to sensors. This enables the drive system to withstand harsher and more variable operating conditions.87–110
Given their critical role in cost-effective and robust drive systems, SRM sensorless control strategies have emerged as a persistent research hotspot in power electronics and electrical drives, catalyzing a wide array of technical solutions rooted in diverse physical principles and algorithmic innovations. This review systematically surveys and categorizes recent advancements in this field, providing a critical analysis of the underlying methodologies. The structure is organized as follows: First, mainstream sensorless techniques will be classified based on operational speed ranges, including low-speed and high-speed phases. Subsequently, a detailed analysis of sensorless control methods for different speed ranges will be provided, covering their fundamental principles and specific implementation schemes. Finally, the review will summarize persistent technical bottlenecks and challenges in current research, along with a forward-looking perspective on future development trends and research directions. This work intends to offer valuable references and insights for researchers, engineers, and readers interested in technological advancements in SRM sensorless control.
The composition of SRM drive system
Composition of SRM system
Figure 1 illustrates the key components of a typical sensorless drive system for a SRM. At its core, the SRM itself functions as the primary element for energy conversion. Its operation is powered by a DC source via a power converter. Driven by PWM signals generated by the controller, this power converter precisely switches current into each phase winding of the SRM to produce driving torque.111–123 The intelligent core of the system resides in the controller. This component not only executes the core sensorless control algorithm but also performs real-time analysis of the electrical signals—such as phase currents or phase voltages—obtained from the SRM. By identifying inherent electromagnetic characteristics within these signals (e.g. inductance variations), the controller continuously and indirectly estimates the rotor’s precise position and speed information, completely eliminating reliance on external physical position sensors. Based on these estimations, the controller accurately generates the required PWM command signals. This enables closed-loop regulation of the power converter, ultimately ensuring smooth, efficient, and stable operating performance for the SRM across diverse operating conditions.124–130

A sensorless drive system for SRMs.
Figure 2 details the three core operating modes governing phase current dynamics, which are instrumental for sensorless control: the Exciting Mode establishes current rise, the Freewheeling Mode maintains continuity, and the Demagnetizing Mode facilitates rapid current collapse. While these modes are fundamental to SRM drives, their primary relevance to position estimation lies in their ability to shape the current transient—serving as the excitation carrier for high-frequency injection at low speeds and defining the observation window for back-Electromotive Force (EMF)/flux linkage detection at high speeds. This topology not only achieves energy recuperation but also utilizes its steep current decay slope to accelerate phase current collapse for precise turn-off timing control, which is crucial for enhancing torque response speed and mitigating torque ripple. Through coordinated switching of the power devices, these three modes significantly influence the phase current’s transient response characteristics, providing the essential electromagnetic parameters from which sensorless algorithms extract critical rotor position features.131–145

Topology of asymmetric half-bridge converter: (a) conduction mode, (b) freewheeling mode, and (c) demagnetization mode.
Mathematical model of SRM
The motion control of the SRM can be achieved by energizing the phase windings according to specific excitation rules. Taking a three-phase SRM with a 12/8 pole structure as an example, the evolution of its electromagnetic states during rotation through an angular range equivalent to one pole pitch is clearly illustrated in Figure 3. Figure 3(a) depicts the initial state: the rotor salient pole is perfectly aligned with the center of the stator slot (also referred to as the stator tooth center). After the rotor rotates 22.5° counterclockwise, it reaches the second state shown in Figure 3, where the rotor salient pole becomes accurately aligned with the center of the stator pole. It is particularly noteworthy that within the rotor angular motion range from 0° to 22.5°, the inductance value of the corresponding phase winding is not constant. As the rotor position changes, continuously reducing the magnetic circuit reluctance (increasing permeance), the phase inductance gradually rises from its minimum value, reaching its peak when the rotor reaches the position aligned with the center of the stator salient pole (approximately 22.5°). Subsequently, the inductance exhibits a declining trend as the rotor continues rotating, until it enters the third state denoted in Figure 3 after rotating approximately another 22.5°. Furthermore, through precise finite element simulation methods, the static characteristic curves of flux linkage and torque versus rotor angular position under different phase current conditions can be obtained for the SRM. These key relationships are also intuitively reflected in Figure 3(b) and (c).146–155

SRM structure topology of a three-phase 12/8 poles and electromagnetic properties 255 : (a) rotor electrical angle, (b) flux linkage characteristic, and (c) torque characteristic.
Figure 3 systematically elaborates the core physical constraints and flux linkage modeling challenges inherent in SRM operation. As an energy conversion device governed by electromagnetic induction laws, the electrical dynamics of each winding in its multi-phase structure are strictly controlled by the electromagnetic equations, represented as (1). The key issue lies in the SRM’s doubly salient structure and variable air-gap characteristics, which cause the magnetic circuit to exhibit high non-linearity. This makes the flux linkage ψ(i, θ) a strongly coupled function of both phase current i and rotor position angle θ—a non-linear distortion further exacerbated by magnetic saturation effects under specific positions and high-current operating conditions.
where ψ k , e k , and t represent the flux linkage, induced electromotive force and time of the kth phase winding, respectively. And k = 1, 2, …, m.
Although mathematical expressions can be used to approximate this flux linkage model, traditional fitting methods face significant theoretical limitations in achieving accuracy across the entire operating range: Firstly, it is difficult to precisely characterize multi-dimensional non-linear boundaries; Secondly, real-time calculations are prone to state drift induced by noise interference.
In engineering practice, the three-dimensional look-up table (3D-LUT) method is widely adopted as the core strategy for high-precision flux linkage acquisition. This method constructs a discrete dataset of ψ(i, θ) in advance, covering the full range of currents, position angles, and temperature conditions, based on finite element simulation or experimental measurement data. Continuous flux linkage estimates are then output through real-time coordinate mapping and interpolation calculations. This strategy essentially involves trading off storage space for computational complexity, thereby avoiding the numerical instability associated with solving non-linear equations online. It provides a reliable flux linkage observation foundation for sensorless algorithms, ultimately supporting the coordinated optimization of predictive current control and position observers.156–165
There exists an intrinsic coupling relationship among flux linkage, rotor position, and current, as mathematically expressed in (2):
where L k and i k represent the phase inductance and phase current, respectively. θ ph is rotor position.
The voltage balance equation is expressed by (3):
where U k and R k represent the phase voltage and phase resistance, respectively.
The mechanical equilibrium equation is expressed by (4):
where T e and T L represent the electromagnetic torque generated by the motor and load torque, respectively. J and D are the constant parameters of the moment of inertia and viscous friction coefficient. ω is the actual speed of the motor.
Figure 3 reveals the multidimensional coupling characteristics and technological evolution logic of the SRM control system. At its core lies the construction of a dynamic mapping framework between physical quantities, where the interactions among electromagnetic torque, flux linkage, inductance, and rotor position parameters not only form the theoretical foundation for sensorless estimation algorithms but also profoundly empower the global optimization of motion control architectures. In the speed control domain, algorithm design directly governs the motor’s dynamic performance, with CCC suppressing torque ripple through current threshold constraints, APC expanding high-efficiency operating ranges via turn-on/turn-off angle modulation, and voltage chopping control (VCC) along with direct torque control (DTC) enhancing response speed from power modulation and flux linkage trajectory tracking perspectives, respectively, while torque sharing function (TSF)-based instantaneous torque control achieves smooth output across all operating conditions through multiphase torque coordination.166–180
Higher-dimensional stability demands drive the fusion of signal fault tolerance and sensorless innovation, creating a deeply coupled closed-loop relationship where the position observer provides real-time rotor spatial state feedback while control algorithm outputs serve as excitation carriers for position estimation methods, forming a critical robustness barrier against sensor failures or noise interference.181–194
The collaborative solving of electromagnetic-mechanical equations further unlocks performance leaps, with the strongly nonlinear, position-sensitive nature of flux linkage and inductance spurring a paradigm shift from analytical equation-based fitting to 3D magnetic digital twins that leverage full-range flux linkage-current-position surface interpolation to overcome traditional modeling limitations in saturation regions. This methodological evolution systematically reshapes sensorless architectures by reducing physical sensor dependence while enhancing position-solving accuracy and dynamic disturbance rejection in complex operating conditions, highlighting how multiphysics coupling across electromagnetic, mechanical, and control domains enables SRM systems to achieve self-adaptive optimization under cost, reliability, and performance constraints, with the integration of data-driven modeling and model-based control representing the next frontier for high-performance sensorless SRM drives.195–208
An overview of recent developments in sensorless control of SRM
This study systematically reviews existing position estimation strategies to clarify technological evolution trajectories, reveal core characteristics, and identify future directions. All selected methods are derived from experimentally validated literature, with their performance being quantitatively evaluated through metrics such as speed regulation range or estimation error, thereby establishing an empirical benchmark for optimizing sensorless control architectures. The comparative analysis demonstrates how different approaches address fundamental challenges in SRM control-including nonlinear magnetic saturation effects, parameter sensitivity, and real-time computational constraints-while highlighting emerging trends that integrate data-driven techniques with traditional model-based methods to achieve robust position estimation across diverse operating conditions. Throughout this review, the operational speed ranges are classified based on the availability of back-EMF information. Unless otherwise specified, low-speed operation refers to conditions below 15%–20% of the motor’s base speed. Medium-high-speed operation corresponds to speeds above this threshold, extending up to the maximum speed limit of the drive system.
Sensorless control strategies for low-speed operation
In the SRM starting and extremely low-speed operating range, conventional back-EMF methods become ineffective. The sensorless control strategies in this regime primarily rely on extracting the motor’s inherent magnetic characteristics as position information sources. By applying detection signals to non-excited phases or utilizing subtle features in phase currents, these methods identify electrical quantities sensitive to rotor position, thereby enabling accurate position estimation. This approach achieves zero-speed starting and stable low-speed operation without dependence on mechanical sensors.209–215
Pulse injection method
The pulse injection method is a key strategy for achieving sensorless control of switched reluctance motors in low-speed and zero-speed ranges. Its core idea involves artificially injecting short-duration high-voltage pulse signals into non-conducting phases. Due to the highly nonlinear magnetization characteristics caused by the doubly salient structure of switched reluctance motors, the saturation level of the core material changes significantly with rotor position. When a high-voltage pulse is applied to a non-conducting phase winding, if the magnetic circuit of that phase is in an unsaturated state, the current will rise slowly; conversely, if the magnetic circuit is near saturation, the current will rise rapidly. Therefore, by precisely detecting and analyzing the current response induced by the injected pulse within a short time, the transient inductance information of that phase winding can be extracted, thereby deducing the real-time position of the rotor pole relative to the stator pole of that phase. Unlike back-EMF-based techniques, this approach exploits the inherent nonlinearity of the salient-pole magnetic circuit as its physical basis for position detection. By actively injecting probing signals to excite the motor’s transient inductance characteristics, it overcomes the limitations of traditional methods at standstill and extremely low speeds, ensuring reliable startup and operation in low-speed regimes. Typical implementations encompass voltage pulse train injection and current response feature extraction, often supplemented by position lookup tables. The practical viability of pulse injection strategies has been extensively validated on hardware platforms beyond laboratory simulations. Notably, these methods have been successfully deployed in electric vehicle powertrains and light electric vehicles (LEVs), where the high signal-to-noise ratio at standstill ensures reliable startup under heavy inertial loads. Furthermore, industrial implementations utilizing high-power converters have demonstrated robust performance in driving centrifugal compressors and pumps, confirming that the injected torque ripple does not significantly compromise the overall drivetrain efficiency in practical applications.
Cai et al. 216 proposed an innovative sensorless starting scheme that employs a dual-inductance dynamic threshold algorithm for the initial phase selection process. This algorithm accurately identifies the optimal starting phase by dynamically adjusting the decision threshold through real-time comparison of inductance characteristics among the three-phase windings (specifically manifested as inductance value differences between each pair of phases). Guo et al. 217 innovatively proposed a three-current-threshold detection method to address the specific limitations of low-cost resistive current sampling schemes. This method solves the core problem of traditional approaches being unable to detect phase current during demagnetization by setting three critical current thresholds: the first threshold precisely determines the turn-on moment, the second threshold reliably identifies the turn-off moment, while the newly added third threshold (set as a minimal current value close to zero) specifically detects whether the demagnetization current has completely decayed to zero. This triple-threshold mechanism not only preserves the structural simplicity of conventional dual-threshold methods but also enables the system to immediately trigger new current pulse injections after the current sampling blind zone ends through the introduction of demagnetization state monitoring. Guo 218 proposed an innovative high-precision injected current sampling scheme to address the critical issue of controller noise severely affecting current sampling accuracy in cost-sensitive applications. Based on the traditional direct-drive low-speed sensorless control framework, this study achieved significant improvements in current sampling accuracy through three key technological breakthroughs: First, a quantitative analysis model of traditional sampling errors was established, revealing the intrinsic relationship between noise interference and position estimation errors. Second, a novel dual-channel current sampling circuit was designed, using differential signal processing technology to reduce sampling noise to 23% of conventional single-channel solutions. Finally, the proposed pulse current peak prediction algorithm based on curve intersection method effectively compensated for time delay errors caused by discrete sampling.
Xiao et al. 219 addressed the inherent limitations of traditional fixed-amplitude pulse injection methods in low-speed sensorless control by proposing an intelligent pulse modulation scheme based on terminal sliding mode control (TMSC). Conventional approaches, which employ constant-amplitude pulse voltage injection, result in nonlinearly varying induced currents in unexcited phases with excessive amplitudes. This not only generates significant negative torque but also severely compromises control performance. To resolve this issue, the authors innovatively developed a pulse amplitude regulator based on TMSC. This regulator dynamically adjusts the pulse voltage amplitude through a nonlinear control law, maintaining the induced current at minimal levels throughout the entire unexcited phase period. The distinctive advantage of this solution lies in its complete elimination of adverse effects from motor parameter uncertainties—requiring neither pre-acquired flux linkage characteristics of the switched reluctance motor nor compromising practical engineering feasibility.
The methodological architecture proposed in this paper is fully presented in the overall block diagram shown in Figure 4. This integrated system optimizes low-speed operational performance by regulating pulse injection amplitude through a terminal sliding mode controller (TSMC). The left side of the diagram comprises the signal acquisition module, which detects the idle-phase current peak in real time and compares it with a reference value to generate an error signal. The central TSMC core receives this error signal along with approximate inductance values, dynamically adjusting the pulse voltage amplitude through a nonlinear control law to drive the PWM module for generating adaptive pulses. The right side employs a regional phase-locked loop (RPLL) estimator, utilizing the calculated inductance values to estimate rotor position and speed, which are fed back to the speed-current dual-loop controller to form closed-loop control. This architecture replaces traditional fixed-amplitude injection with online TSMC regulation, enhancing the signal-to-noise ratio at aligned positions while reducing copper loss and torque ripple at unaligned positions. It operates without reliance on motor magnetic characteristic parameters, acquiring key parameters solely through a self-tuning process, demonstrating strong robustness and universality. Notwithstanding these advancements, a fundamental trade-off persists in pulse injection strategies between estimation accuracy and injected torque ripple; while adaptive modulation schemes (e.g. TMSC) effectively suppress negative torque, they inevitably increase algorithmic complexity compared to fixed-amplitude injection methods, necessitating careful selection based on application-specific torque ripple tolerance.

Overall block diagram of the TSMC based pulse injection scheme.
Unsaturated inductance model method
The unsaturated inductance model method represents one of the core modeling strategies for sensorless control of SRMs at low speeds. Its fundamental principle leverages the distinctive characteristic that winding inductance depends predominantly and uniquely on rotor mechanical position when the motor operates in the non-saturated region. Through in-depth analysis of the doubly salient magnetic circuit structure and reluctance variation patterns, this model analytically establishes a relatively simple explicit mathematical relationship between winding inductance and rotor angle as the foundation for position observers. During operation, the system continuously monitors or calculates the inductance value of the phase under observation, then substitutes this identified inductance value into the predefined inductance-position relational model to directly derive real-time rotor position information. This method offers distinct advantages including clear model structure, high computational efficiency, and ease of engineering implementation, making it particularly suitable for low-speed operation scenarios featuring light loads, low current levels, and gradual speed variations. Frequently employed as either a robust complement or simplified alternative to other algorithms in specific operational ranges, it also serves as a fundamental research tool for investigating intrinsic inductance-position relationships. The reliance on inductance linearity makes this method particularly suitable for medium-to-low power industrial drives where magnetic saturation is less pronounced. Published experimental results have shown that this approach maintains acceptable position estimation accuracy (within 2°–3°) even during load transients, making it a cost-effective solution for fan and blower applications where extreme precision is not the primary requirement.
Guo et al. 220 proposed an innovative full-cycle unsaturated inductance offline automatic measurement method. This method achieves automatic rotor position adjustment without mechanical fixtures or position sensors through an original single-phase current chopping (SPCC) and dual-phase current chopping (DPCC) combined control strategy. During the measurement phase, the system simultaneously injects high-frequency pulses into all three-phase windings, precisely identifying each phase’s inductance characteristics using the current slope difference (CSD) method, and establishes a complete mapping relationship between unsaturated inductance and rotor position through Clarke transformation and arctangent operations. When transitioning to normal operation, this method only requires injecting a single high-frequency pulse into the idle phase, enabling rapid rotor position calculation through real-time inductance measurement and linear comparison with pre-stored characteristic curves, significantly improving computational efficiency.
However, due to the existence of magnetic saturation regions, the above method may increase rotor position estimation errors. To address the problems caused by magnetic saturation interference, many scholars have turned their attention to the idea of unsaturated inductance reconstruction.
Sun et al. 221 proposed an innovative model reshaping and self-calibration scheme. The study first conducted an in-depth analysis of the influence mechanism of magnetic saturation on the position estimation accuracy of traditional linear inductance models. Based on this analysis, two complementary technical approaches were developed: leveraging the constant-sum characteristic of phase inductances in unsaturated states for three-phase 12/8-pole SRMs, a virtual inductance construction strategy was designed to effectively circumvent nonlinear effects in magnetic saturation regions through algebraic transformations; to enhance the scheme’s applicability to multi-phase srms, an innovative real-time equivalent position triangle was established to identify position errors caused by magnetic saturation through geometric relationships, along with the development of a self-calibration technique based on inductance linearity.
Cai et al. 222 aims to solve the saturation effect problem caused by high currents during SRM startup, thereby achieving reliable sensorless starting. This method combines two innovative algorithms: unsaturated inductance reconstruction based inductance vector coordinate transformation (UIR-IVCT) and incremental inductance partitioning (IIP) to enable continuous rotor position estimation under saturated conditions while providing failure tolerance capability during single- or two-phase faults.
The core methodological flow is clearly presented in the overall block diagram Figure 5, which depicts the hybrid strategy’s architecture. First, the incremental inductance measurement equation (5) serves as the starting point, calculating incremental inductance through high-frequency pulse injection and current slope difference to circumvent saturation effects. While equation (5) provides a precise analytical expression for incremental inductance, its practical implementation in digital controllers relies heavily on high-resolution timers and fast ADCs to capture the current slopes (di/dt) within microseconds. The subtraction operation amplifies noise sensitivity, necessitating careful tuning of sampling intervals in hardware. The UIR-IVCT algorithm achieves initial positioning and continuous position estimation, supplemented by IIP algorithm for failure scenarios. Position estimation is represented by (6), which computes rotor position using orthogonal signals after inductance vector coordinate transformation.
where Linc is the incremental inductance, Ubus is the bus voltage, V T and V D represent the voltage drops across the power switch and diode, respectively.
where θest is the estimated rotor position, L α and L β are derived from the normalized inductance vector.

Hybrid sensorless starting architecture combining UIR-IVCT and IIP algorithms.
This hybrid strategy demonstrates complementarity: UIR-IVCT relies on three-phase inductance data for high-precision estimation, while IIP requires only a single inductance threshold to enable failure tolerance, thereby enhancing overall startup process reliability. However, the efficacy of unsaturated inductance models remains highly susceptible to magnetic saturation and temperature drift; although reconstruction techniques (e.g. UIR-IVCT) mitigate these effects, their performance still relies heavily on the assumption of a known linear region, limiting their robustness under heavy-load or high-temperature operating conditions.
Beyond these approaches, some scholars have significantly improved rotor position estimation accuracy by incorporating regional RPLL technology.
Xiao et al. 223 addressed the dependency of traditional low-speed sensorless control schemes on offline magnetic characteristic measurements by proposing an innovative adaptive position estimation method. The core of this method lies in the development of RPLL technology, which achieves precise position estimation through the coordinated operation of two key phases: during the initialization phase, the system employs a self-tuning process to automatically acquire unsaturated inductance characteristics, eliminating the need for offline measurements required by conventional methods; during operation, it utilizes idle-phase inductance information combined with the RPLL algorithm to achieve full-cycle rotor position tracking. The solution specifically designs a reversible speed detection mechanism based on heterodyne principles, effectively resolving the failure issues of traditional methods during speed reversal. Theoretical analysis demonstrates that this algorithm guarantees local stability and convergence.
Song et al. 224 proposed a novel sensorless control strategy for SRMs operating under heavy-load and low-speed conditions, with its core innovation combining wide-range modeling of idle-phase inductance and a proportional-gain-decreasing second-order generalized integrator (SOGI-FLL-P). Conventional methods often fail in position estimation due to saturation effects, while piecewise position reconstruction causes abrupt changes at connection points, leading to speed fluctuations. To address these issues, the authors first inject high-frequency pulses into idle phases and calculate unsaturated inductance based on current slope differences, as expressed by (7):
where L un is the unsaturated inductance, L ph is the phase inductance.
This formula avoids saturation zone effects and provides the foundation for position estimation. Further, a third-order polynomial is utilized to establish the inductance-position mapping model, as expressed by (8):
where i = A, B, or C. a1, a2, a3, and a4 are coefficients of the function.
The core control architecture is fully presented in the overall block diagram (Figure 6). The left side of the diagram shows high-frequency pulse injection into idle phases to generate current responses, with the inductance model outputting initial position values; the central core is the SOGI-FLL-P module, which simultaneously achieves position filtering and speed estimation through dynamically attenuated proportional gains, eliminating phase lag and abrupt changes inherent in traditional differential methods; the right-side closed-loop control generates drive signals through a PI speed regulator and hysteresis current controller, supporting heavy-load transient scenarios.

SOGI-FLL-P-based control architecture.
Sensorless control strategies for medium-high speed operation
During medium-high speed operation, the back-EMF signal generated by SRMs becomes sufficiently strong, serving as a crucial information source for high-precision sensorless control. The core strategy focuses on effectively extracting and utilizing flux linkage or back-EMF characteristics of phase windings, with mainstream technical approaches including reconstructing flux linkage trajectories through real-time integration of measured phase voltage and current, then estimating position based on their inherent relationship with rotor position, and designing high-performance state observers that leverage electromagnetic dynamic equations to achieve real-time rotor position/speed identification and tracking through closed-loop error feedback between model outputs and actual measurements. Concurrently, an efficient rotor-characteristic-position-based method finds widespread application. This approach capitalizes on the periodic variation of SRM phase inductance with rotor angle, which exhibits distinct maximum and minimum points. By continuously monitoring instantaneous inductance or derived physical quantities of each phase winding, these characteristic inductance variation points are detected and captured. These characteristic points serve as natural “anchors” for positioning, enabling continuous position estimation across the full electrical angle range when combined with speed information for interpolation or when working synergistically with the aforementioned state observers. Medium-high speed solutions demonstrate relatively strong parameter robustness, high precision, and fast dynamic response, significantly reducing the need for additional signal injection. They form the core foundation for reliable operation of modern SRM sensorless systems in medium-high speed ranges.225–233
A. Rotor-characteristic-position-based method
In the medium-high speed operating range, rotor-characteristic-position-based sensorless strategies directly capture transient physical quantity mutation signals generated when the rotor passes specific geometric positions. These mutations typically manifest as zero-crossings or extremum points in the rate of change of phase winding inductance or derived physical quantities, exhibiting high sensitivity and determinacy. The system continuously detects and precisely timestamps these mutation events, correlating them with instances when the rotor crosses predefined critical angular positions. Subsequently, by incorporating the motor’s current average operating speed, continuous rotor position signals can be generated through linear or simple model-based position interpolation between adjacent characteristic points. This strategy eliminates the need for complex model identification and integration operations, demonstrates high tolerance to parameter variations, and delivers excellent execution efficiency, making it particularly suitable as a cost-effective implementation solution for position feedback in medium-high speed ranges.
Cai et al. 234 proposed a sensorless method based on phase inductance slope characteristics. The core of this method lies in real-time detection of negative zero-crossings in the rate of change of motor phase winding inductance (specifically, the particular zero point where phase inductance slope transitions from positive to negative), enabling the system to accurately capture the critical position information when rotor and stator teeth become perfectly aligned. By utilizing these periodically captured alignment positions as reference points and combining them with timing logic during motor operation, the method further achieves estimation of instantaneous rotor angle and rotational speed. The advantage of this approach is its utilization of distinct characteristic points in inductance slope for position capture, providing a reliable foundation of rotor position and speed information for sensorless control.
Sun et al. 235 proposed a medium-high speed sensorless control strategy based on unaligned rotor position estimation. This scheme achieves position estimation by detecting negative zero-crossings of phase reluctance slope, employing high-frequency pulse injection for CCC mode and conduction angle optimization for APC mode respectively. The study also designed a smooth mode-switching strategy and a fault-tolerant solution based on faulty-phase skipping, validating the effectiveness of the method on a six-phase SRM experimental platform.
Zhou et al. 236 addressed the limitations of traditional methods in demagnetization zone sensitivity, aligned position estimation accuracy, and fault tolerance by integrating phase inductance gradient detection and idle-phase inductance threshold comparison techniques. The core innovation of this solution lies in adopting a demagnetization inductance slope negative zero-crossing (D-ISNZ) detection method to avoid misestimation in the main excitation region, combined with a single inductance threshold (SIT) remedial strategy to achieve robust control across wide speed ranges. Experiments conducted on a three-phase 12/8 SRM validate its effectiveness under full-speed operation (including heavy-load conditions) and phase-loss faults, requiring no prior magnetic characteristic data or complex computations, significantly enhancing system reliability. The phase inductance is calculated as follows:
The real-time inductance value when current is greater than zero is obtained by integrating the difference between phase voltage and resistive voltage drop, then dividing by current; when current equals zero, inductance is set to zero to avoid invalid calculations. This formula provides fundamental inductance data for position estimation, whose accuracy directly affects the subsequent slope detection and threshold comparison processes. The inductance slope is calculated as follows:
where L k (n + 1) and L k (n) are the calculated phase inductances at the (n + 1)th and nth sampling instants, respectively, and T s is the inductance sampling period.
By differencing inductance values at adjacent sampling instants and dividing by the sampling period Δt, the rate of inductance change is quantified. This formula serves as the core of the D-ISNZ method, enabling detection of negative zero-crossings in inductance slope during the demagnetization region to determine aligned positions, thereby avoiding false triggers caused by current fluctuations or saturation in traditional methods.
Figure 7 illustrates the overall control architecture of this method. As shown in Figure 7, the left section represents the high-frequency pulse injection module, which calculates inductance through idle-phase current responses; the central core algorithm module incorporates D-ISNZ detection and SIT remedial strategies, adaptively selecting estimation methods based on demagnetization current states; the right section constitutes the closed-loop control module, generating drive signals through a PI speed regulator and hysteresis current controller to support dynamic speed adjustment and fault tolerance. The block diagram highlights multi-module collaboration: D-ISNZ prioritizes aligned position detection, switches to SIT upon failure, and achieves healthy-phase data fusion under phase-loss conditions through fault flags, ensuring continuous position estimation.

Synthetic inductance detection strategy architecture.
B. Position-observer-based method
Currently, position-observer-based strategies represent a focal point in sensorless control research. This approach treats the motor itself as a dynamic system and constructs its mathematical model as the core of the observer. Operating in parallel with the actual motor, the observer receives measured voltage and current as inputs, and outputs optimal estimates of internal states—including rotor position and speed—in real time. Its essence lies in a closed-loop correction mechanism that continuously compares observer outputs with actual measurements, utilizing this error to dynamically adjust the observer’s internal states and force convergence toward true values. This model-based feedback correction mechanism endows the algorithm with exceptional dynamic tracking performance and robust disturbance resistance, effectively suppressing interference from parameter variations and measurement noise. As such, it stands as a mainstream advanced solution for achieving high-performance, high-precision sensorless operation. Observer-based techniques have demonstrated significant potential in high-power traction applications, particularly in scenarios demanding wide speed ranges. Field tests on electric haulage shearers and mining machinery have confirmed that flux-linkage observers can operate reliably in high-temperature and high-dust environments where mechanical sensors typically fail. Additionally, the compatibility of these observers with digital signal processors (DSPs) has facilitated their integration into commercial variable frequency drives, bridging the gap between academic research and industrial deployment.
Shao et al. 237 addressed issues such as low model utilization and discrete position estimation in characteristic-flux-based sensorless drive methods by proposing a position-adaptive dual-loop observer based on a nonlinear decoupling model. The solution first establishes a nonlinear decoupled flux linkage model to reconstruct continuously linearized position-dependent functions; subsequently, a position adaptation law is designed to enable the reconstructed function to track the actual rotor position in real time. Through coordinated regulation of the dual-loop structure, continuous position estimation is achieved while effectively suppressing high-frequency noise interference.
Sun et al. 238 addressed the challenges of dynamic response and estimation accuracy in sensorless control of SRMs by proposing a composite control strategy based on an improved sliding mode controller. The solution achieves performance enhancement through two key innovations: first, a time-varying-function-based improved sliding mode speed controller (TVSMSC) was designed to effectively suppress internal parameter disturbances and external load interference, while significantly reducing initial peaks during startup through a novel reaching law; second, a sliding mode position observer (SMPO) based on an inductance model was developed to achieve real-time precise estimation of rotor position and speed. Experimental results demonstrate that the proposed strategy ensures strong system robustness while significantly reducing speed overshoot and transient response time, while maintaining high position estimation accuracy. This approach of combining improved control algorithms with advanced observers provides an effective solution for high-performance sensorless control of SRMs.
Xiao et al. 239 achieved full-cycle position estimation independent of magnetic characteristics through a quadrature flux estimator (QFE). Traditional methods relying on pre-measured flux linkage lookup tables face challenges such as time-consuming offline measurements, substantial storage requirements, and parameter sensitivity. This study utilizes the band-pass characteristics of QFE to eliminate DC offsets and harmonic components in flux linkage, transforming highly nonlinear flux signals into clean quadrature sine-cosine position signals. These signals are then processed by a three-phase phase-locked loop (PLL) to extract rotor position and speed. This method requires no prior magnetic characteristic data.
The core of this method lies in flux linkage calculation and quadrature signal generation. The real-time flux linkage calculation formula is expressed as follows:
where θON and θOFF are the turn-on and turn-off angles, respectively.
This piecewise function defines the flux linkage calculation method within the excitation interval: during the turn-on angle to turn-off angle range, real-time flux linkage is obtained by integrating the difference between phase voltage and resistive voltage drop; in non-conduction regions, flux linkage is set to zero to avoid invalid calculations. The integration process effectively suppresses current ripple effects, providing raw signals for subsequent QFE processing. The innovation of this formula lies in relying solely on circuit parameters rather than magnetic characteristic parameters, establishing the foundation for magnetic-characteristic-independent estimation. The formula for quadrature flux generation is expressed as follows:
where λf denotes the fundamental flux, λfq denotes the quadrature of the fundamental flux λf, λmag is the magnitude of fundamental flux.
The transformation into quadrature flux signals is not merely a mathematical manipulation; it is an engineering strategy to convert a nonlinear tracking problem into a linear PLL problem. This significantly simplifies the software architecture and enhances noise immunity compared to direct arctangent calculations of raw flux linkage data.
This pair of quadrature signals completely strips the DC offsets and harmonics from the flux linkage, encoding rotor position information in the phase rather than the amplitude, thereby simplifying position estimation to a standard phase-locked loop (PLL) problem. The breakthrough of this formula lies in transforming the nonlinear estimation problem of SRMs into a linear system tracking problem, eliminating the dependency on flux-linkage-position-current three-dimensional models.
Figure 8 illustrates the overall control architecture of this method. The block diagram reveals the complete process from signal acquisition to position estimation: the left section shows the flux linkage calculation module, which obtains raw flux linkage through voltage and current sampling; the central core innovation is the QFE module, which receives flux linkage signals and outputs filtered quadrature flux linkage; the right section constitutes the position generation module, where a three-phase PLL converts quadrature flux linkage into position and speed signals. The diagram highlights three key characteristics: first, full-period capability, achieving continuous 360° estimation through three-phase signal splicing; second, adaptive ability, feeding estimated speed back to QFE for center frequency adjustment; third, dynamic compensation, where output positions undergo lead compensation to eliminate digital control delays.

Control architecture of the magnetic-characteristic-free sensorless strategy based on QFE.
This system overcomes the dependency on magnetic characteristics inherent in traditional methods by leveraging the frequency-selective properties of QFE, combined with logical triggering design to address the discontinuous flux linkage issue in SRMs. Ultimately, it achieves accuracy comparable to magnetic-characteristic-based methods without requiring any offline measurements, providing a universal solution for SRM sensorless control.
C. Intelligent-algorithm-based methods
Intelligent-algorithm-based sensorless strategies aim to address the challenges posed by the highly nonlinear and model-imprecise nature of SRMs. These methods circumvent the reliance on precise analytical models by leveraging data-driven tools such as neural networks, fuzzy logic, or support vector machines. Through offline or online learning, they directly construct complex nonlinear mappings from easily measurable electrical quantities to rotor position. Well-trained intelligent models exhibit strong nonlinear fitting capabilities and fault tolerance, effectively suppressing disturbances and parameter variations. This approach offers a promising pathway for robust position estimation under complex operating conditions. However, their performance heavily depends on the quality and completeness of training data, and they often face engineering implementation challenges due to high computational resource demands.240–250 Despite concerns regarding computational complexity, recent advancements in edge-AI hardware have enabled the real-time deployment of neural network estimators. Experimental validations on automotive-grade SRM prototypes have shown that optimized neural networks can achieve position estimation errors comparable to traditional model-based methods while offering superior resilience to parameter variations, paving the way for next-generation autonomous driving actuators.
Cai et al. 251 addressed the strongly nonlinear relationship between rotor position and electromagnetic parameters in SRMs by proposing a neural network modeling method based on measured flux linkage characteristics. Through comparative analysis of improved backpropagation neural network (BPNN) and radial basis function neural network (RBFNN) modeling performance, it was found that the BPNN utilizing the Levenberg-Marquardt algorithm achieves higher position prediction accuracy while maintaining a compact network structure.
Yalavarthi and Singh 252 proposed an optimized artificial neural network (ANN)-based sensorless control strategy for SRMs, aiming to address the modeling challenges and position sensor dependency caused by nonlinear saturation characteristics. The core innovation lies in utilizing the Levenberg-Marquardt (LM) backpropagation learning algorithm to train the ANN model, enabling rotor position estimation through real-time flux linkage data and eliminating the dependency on pre-stored magnetic characteristic lookup tables (LUTs) inherent in traditional methods. The ANN structure adopts a single hidden layer design, optimizing the number of neurons to reduce both training and validation errors while improving position estimation accuracy. The control strategy integrates advanced angle control (AAC), achieving speed regulation by adjusting conduction and turn-off angles instead of traditional hysteresis current control to reduce switching losses.
The core methodology of this paper focuses on ANN output computation and the LM weight update mechanism. The ANN output formula is expressed as follows:
where wjo represents the weight vector from the hidden layer to the output layer, xhj denotes the output of hidden layer neurons, and bo is the output layer bias.
This formula defines the computation process of the neural network output layer, where the output directly corresponds to the estimated rotor position value. Its accuracy depends on the number of hidden layer neurons and the quality of training data. The formula demonstrates how the ANN maps nonlinear inputs to position signals without requiring an explicit magnetic characteristic model. The LM weight update formula is expressed as follows:
where J is a Jacobian matrix and the coefficient is μ ≥ 0.
Although the LM algorithm in equation (14) offers rapid convergence, the matrix inversion operation imposes a heavy computational burden. Deploying this neural network observer on low-cost microcontrollers (MCUs) often requires approximation or simplification, which is a critical consideration for commercial SRM drives.
This formula serves as the core of the LM algorithm, used for iteratively updating ANN weights and biases. When μ = 0, the algorithm degenerates to the Gauss-Newton method, ensuring rapid convergence; when μ is large, it approaches gradient descent, enhancing stability. This formula guarantees that ANN training achieves high precision with a mean square error (MSE) of 2 × 10−4 degrees within 1000 epochs, overcoming the local minima problem inherent in traditional stochastic gradient descent (SGD).
Figure 9 illustrates the overall control architecture of the proposed method. The block diagram integrates position estimation, speed control, and power conversion modules: the left section contains the ANN position estimator, which receives phase current and flux linkage as inputs and outputs rotor position; the central section comprises the speed control loop, where a PI regulator compares reference speed with estimated speed to generate current references; the right section features the advanced angle control (AAC) module, which calculates advance angles to derive conduction and turn-off angles, driving a split DC converter (SDC) to achieve single-pulse mode (SPM) operation and reduce switching losses. The diagram highlights the synergy between ANN and AAC: the ANN provides high-precision position feedback, enabling AAC to dynamically adjust commutation angles and ensure speed tracking accuracy. Simultaneously, the architecture supports a startup algorithm that detects initial position through diagnostic pulses, smoothly transitioning to ANN estimation mode to enhance system reliability.

ANN-based control architecture.
Despite their powerful nonlinear fitting capabilities, the practical deployment of neural-network-based estimators is often constrained by the “black-box” nature, which complicates stability analysis, and the high computational burden associated with real-time matrix operations, making them less favorable than model-based observers for cost-sensitive or safety-critical applications.
Comparison of major sensorless control strategies for SRMs
To provide a clear quantitative benchmark for evaluating the diverse sensorless strategies discussed herein, Table 1 summarizes the key performance indicators across representative methodologies. This comparative overview highlights the inherent trade-offs between estimation accuracy, computational complexity, and robustness to parameter variations, offering researchers a quick reference to select appropriate algorithms based on specific application constraints
Quantitative performance comparison of major sensorless control strategies for SRMs.
Comparative perspectives: Sensorless control in other traction motors
While this review primarily focuses on SRM sensorless control, it is instructive to contextualize recent advancements within the broader family of traction motors, particularly Permanent Magnet Synchronous Motors (PMSMs) and Interior Permanent Magnet (IPM) motors, which dominate the electric vehicle market. Unlike SRMs that rely on reluctance torque and saliency-based position estimation, PMSMs often utilize permanent magnet flux for back-EMF observation. However, recent research in PMSM control shares conceptual parallels with SRM advancements. For instance, similar to the push for full-speed-range sensorless operation in SRMs, PMSM research has explored static current error elimination algorithms in predictive current control to enhance estimation robustness under low-speed conditions. 253 Furthermore, the design philosophy for PMSMs has evolved toward topology-oriented multi-objective optimization using AutoML-based surrogate models to balance electromagnetic performance, efficiency, and cost. 254 Although the physical principles differ—with SRMs favoring structural simplicity and PMSMs favoring high power density—both motor types face common challenges in terms of parameter sensitivity and real-time computational burdens for high-performance transportation electrification. This cross-domain comparison highlights that while SRMs offer distinct advantages in harsh environments due to their rugged structure, the algorithmic innovations in PMSM control provide valuable insights for further refining SRM sensorless strategies.
Conclusion and future directions
Conclusion
In the field of sensorless control for SRMs, position sensing strategies exhibit distinct technical boundaries and complementary characteristics across different speed ranges. For low-speed operation, pulse voltage injection methods extract rotor position information by applying high-frequency excitation signals and detecting current responses, offering high signal-to-noise ratios but potentially introducing additional vibration and acoustic noise. Methods based on unsaturated region flux linkage modeling utilize nonlinear mapping relationships between flux linkage and position to achieve position estimation, though their accuracy heavily depends on motor parameter precision and is susceptible to temperature drift and saturation effects. In medium to high-speed ranges, rotor-characteristic-position-based strategies enable position sensing by detecting phase inductance variations, featuring simple structures but limited dynamic performance. Position-observer-based methods achieve position tracking through constructed motor dynamic models, demonstrating good robustness but relying on model accuracy. Meanwhile, intelligent-algorithm-based approaches establish nonlinear mapping relationships through data-driven methods, showing strong adaptability in complex operating conditions but facing challenges such as high computational complexity and insufficient generalization capability. Future research should focus on hybrid position observation schemes that integrate multiple strategies, combining parameter adaptive compensation and intelligent optimization algorithms to achieve high-precision, high-reliability sensorless control across the entire speed range. This direction will simultaneously enhance system disturbance resistance and dynamic response performance.
Future directions
While hybrid strategies remain a promising avenue, future research must transcend simple algorithmic stacking and address the fundamental bottlenecks of SRM sensorless control with technically specific solutions. We identify three critical directions for future investigation:
Physics-informed neural networks (PINNs) for magnetic characteristic-free modeling
Instead of purely data-driven black-box neural networks, future work should focus on PINNs that embed the Maxwell equations and inductance saturation constraints into the loss function. This approach promises to eliminate the reliance on offline 3D-LUT measurements while ensuring physical consistency, directly tackling the core weakness of current model-based methods.
Cross-domain robustness synthesis with PMSM control techniques
Drawing insights from recent advancements in PMSM control, future SRM observers should incorporate parameter self-calibration mechanisms. Specifically, the concept of AutoML-based surrogate modeling, successfully applied in IPM topology optimization, should be adapted to optimize the hyperparameters of sliding mode observers or PLLs in SRMs, enhancing robustness against temperature drift and manufacturing tolerances.
Edge-AI implementation and mixed-signal analog/digital architectures
To overcome the high computational burden of intelligent algorithms, future research should explore mixed-signal hardware implementations. Deploying critical nonlinear estimation tasks on dedicated analog circuits while reserving digital cores for logic control can drastically reduce latency and power consumption, making high-precision sensorless control viable for cost-sensitive applications.
These directions shift the focus from merely improving estimation accuracy to enhancing the system-level viability and parameter autonomy of SRM drives in harsh electrified transportation environments.
Footnotes
Handling Editor: Aarthy Esakkiappan
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was Funded by Sci&Tech Program of Huai’an (HAG202302) and Major Projects of the Basic Science (Natural Science) Research Program in Jiangsu Higher Education Institutions (24KJA470002).
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
