# A Novel Dual-Permanent-Magnet Mechanical Antenna for Pipeline Robot Localization and Communication
###### Abstract  
The demand for pipeline inspection has promoted the development of pipeline robots and associated localization and communication technologies. Among these technologies, ultra-low-frequency (30-300 Hz) electromagnetic waves have a significant advantage because of their strong penetration, which can penetrate metal pipe walls. Traditional low-frequency transmitting systems are limited by the size and power consumption of the antennas. In this work, a new type of mechanical antenna based on dual permanent magnets was designed to solve the above problems. An innovative amplitude modulation scheme that involves changing the magnetization angle of dual permanent magnets is proposed. The ultra-low-frequency electromagnetic wave emitted by the mechanical antenna inside the pipeline can be easily received by the antenna outside to localize and communicate with the robots inside. The experimental results showed that when two N38M-type Nd-Fe-B permanent magnets with a volume of 3.93 cm\({}^{3}\) each were used, the magnetic flux density reached 2.35 nT at 10 m in the air and the amplitude modulation performance was satisfactory. Additionally, the electromagnetic wave was effectively received at 3 m from the 20# steel pipeline, which preliminarily verified the feasibility of using the dual-permanent-magnet mechanical antenna to achieve localization of and communication with pipeline robots. mechanical antenna; ultra-low frequency; amplitude modulation; communication 20232  
Conventional low-frequency antennas generate electromagnetic waves through an energized coil [14; 15; 16], and the power loss and volume of equipment are inevitably large. Permanent-magnet-type mechanical antennas have recently received wide attention in the field of underwater and underground communications. They can produce a strong magnetic field by using a small permanent magnet of high magnetic energy density [17], which provides a new method for the localization of and communication with pipeline robots [18]. According to their structure, permanent-magnet-type mechanical antennas can be mainly classified into three types. (a) Modulation is realized by changing the rotational speed of the permanent magnets [19; 20; 21; 22]. For example, the frequency of the signal can be modulated by adjusting the speed of a three-phase induction motor. This type is easy to implement, but the power consumption is large and the bandwidth is narrow due to the inertia of the magnets. (b) The permanent magnet rotation speed is maintained as constant and a modulator is added to achieve the signal modulation. For example, when placing a cylinder with a coil wrapped around a spherical permanent magnet rotating at a constant speed, the amplitude of the magnetic field can be modulated by controlling the coil current [23; 24]. The phase of the magnetic field can be modulated by changing the position of two pairs of orthogonal bow-tie-shaped shielding layers [25; 26]. Such schemes do not require the speed of the motor to be changed, reducing the power consumption. However, the additional modulator increases the complexity of the system. (c) The permanent magnet is maintained at rest and signal modulation is realized through a special modulator. For example, the frequency of the magnetic field can be modulated by varying the rotational speed of a louvered rotary shutter structure made from soft magnetic materials which periodically shields the magnetic field generated by the magnets [27]. This type reduces the requirement for mechanical strength of the system, as the permanent magnets are stationary. However, it has high demands regarding the structure and performance of the modulator. Practical applications of the permanent-magnet mechanical antenna require the design of a suitable signal-modulation scheme considering the size, power consumption, and communication bandwidth of the antenna. The common modulation schemes for mechanical antennas are frequency modulation (FM), amplitude modulation (AM), and phase modulation (PM).