## 1 Introduction  
Milli/microbots refer to small-scale robots with dimensions from micrometers to millimeters. They have potential to be used in performing related operations in a limited workspace in vivo. Compared with traditional surgeries, medical milli/microbots can easily enter the internal human body and complex organ lesions and reduce unnecessary trauma to the patient's body tissue and shorten the postoperative recovery period [1; 2; 3; 4]. Due to these advantages, the development of milli/microbot technology is of great significance in the minimally invasive diagnosis and diagnostic medical procedures where milli/microbot can be designed to perform minimally invasive surgery on cell tissue, vascular tissue, human intestine, and so on. The special application environment of medical milli/microbot and its small scale mean that the choice of movement control is a big challenge. Passive actuation that relies on direct interaction with the environmental media in the human body, such as early capsule robots move to rely on gastrointestinal peristalsis, is clearly not applicable to the field of minimally invasive diagnosis due to its inefficiency and difficulty in control. Wireless external field energized active actuation is currently the most suitable remote control method including magnetic actuation, electric actuation, microwave actuation, laser actuation, and ultrasonic wave actuation [5]. Compared with other external field energy sources, magnetic actuation has been proven safer and easier to control in the experimental environment because the magnetic field is capable of response in a short period and only has an influence on the magnetic medium placed therein [6]. Besides, cell polarization and thermal effects can be ignored in low-frequency magnetic fields [7; 8]. Inspired by the way microbes move in nature, the general magnetic field actuation method can be divided into three types [9]. The first method uses a combination of magnetic field direction and magnetic field gradient for the actuation of the magnetic medium milli/microrobot, as shown in Figure 1a[10]; The second method uses a uniform magnetic field with a rotating change to the actuation of a magnetic milli/microrobot with a spiral structure, as shown in Figure 1b[11; 12; 13]; The third method uses a uniform magnetic field with an oscillating change to the actuation of a magnetic milli/microrobot with a flexible tail, as shown in Figure 1c[14]. Studies have shown that under ideal conditions, the rotating magnetic field method and the oscillating magnetic field way have similar peak performance [9]. In addition, it is easier to generate the desired external magnetic field with the latter two methods. To achieve the same function such as targeted drug delivery, the design, and manufacture of the milli/microrobot in Figure 1a does not need to consider the motion and attitude conversion services for the milli/microrobot in the body. This is because the irregular structure and complex milli/microrobot can achieve the expected motion through the design and modification of the control algorithm. However, this is an attribute that is not available in the latter two actuation modes. The control mode in Figure 1a requires that the external magnetic field generating device can generate desired gradient and torque in the workspace; this satisfies the movement and rotation of the milli/microrobot, which is a basic requirement that must be met by the system described herein. There has been a lot of researches on varying magnetic field generating devices, which can be mainly summarized into two major categories. The first type of varying magnetic field system can be achieved by changing the relative position between the permanent magnet system and the workspace. [PERSON] et al. proposed using a six-degree-of-freedom manipulator to control a permanent magnet to actuate the capsule endoscope to achieve a three-degree-of-freedom closed-loop position and two-degree-of-freedom open-loop control [15].