# An Overview of Position Sensorless Techniques for Switched Reluctance Machine Systems
The following Boolean search terms and modifiers were employed: switched reluctance *AND sensorless OR position estimation. This initial search yielded 432 papers. Articles were limited to peer-reviewed journal articles in English. Titles and abstracts were read to narrow the list of studies and ensure they met the following criteria: the study had to be an experimental manipulation under field or laboratory conditions linking SRM. Finally, according to these criteria, \(311\) studies were retained for our systematic review. Current signal, rotor position, and voltage signal are important feedback signals in the control of SRM drive system. The waveform of the current is especially important for speed/torque control [46; 47; 48; 49] and position sensorless control [50; 51]. In different speed ranges, the current output of the motor is very different. As the speed of the output increases, the current gradually enters the form of a single pulse. At low speed, the current will be chopped, and the output of torque and rotational speed will be controlled by controlling the range of the chopper. In the medium and high speed segments, the motor needs to output higher power, and the effective value of the current needs to be increased. The motor will control the output of speed and torque by controlling the turn-on angle and turn-off angle, i.e., control the power output by the motor. Position sensorless technology has attracted much attention and has been widely studied. Such technology can improve the stability of switched reluctance motors to adapt to complex application environments. The sensorless technology has been rapidly promoted via the development of power electronics technology, finite element simulation technology [39; 40; 41; 42; 43], flux linkage measurement methods, and control theory. New position sensorless methods are constantly being proposed, which is more coincidental with the requirements of higher position detection accuracy, wider speed regulation range, and better versatility [52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62]. Figure 2 shows the classification of position sensorless methods. In this paper, the classification is based on whether or not the methods require a priori parameters of the motor to build a model. Position sensorless methods are mainly divided into three broad categories: magnetic model-based methods [63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104; 105; 106; 107; 108; 109; 110; 111; 111; 112; 113; 114; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124; 125; 126; 127; 128], magnetic model-free methods [129; 130; 131; 132; 133; 134; 135; 136; 137; 138; 139; 140; 141; 142; 143; 144; 145; 146; 147; 148; 149], and hybrid detection methods [150; 151; 152; 153; 154; 155; 156; 157; 158; 159; 160; 161; 162; 163; 164; 165; 166]. These methods have their own unique advantages, which are driving the sensorless technology to be more efficient. Figure 1: Number of studies published before Jan. 2021 that experimentally examined the position sensorless methods for SRM. The main work of this paper is to sort out the rotor position estimation method of SRM from the perspective of technology development. The main contribution is to classify different types of sensorless methods and summarize them, demonstrate the process of development of sensorless methods, and essentially explore and categorize a multitude of methods. A comparative analysis is made addressing the feasibility, generality, and speed regulation range of these methods. The sensorless methods with outstanding performance and future research directions are screened out.