# A Design Method for Multijoint Explosive-Proof Manipulators by Two Motors
## 1 Introduction
These manipulators are essential, and can work more effectively and skillfully than humans. Artificial limb and rehabilitation manipulators are always driven by motors. In recent years, the transmission method of motor-wire-actuator has been widely used in artificial limbs [15] and palm rehabilitation [16]. Robot manipulators used in surgery adopt a motor-reducer-actuator structure because of its high precision. A manipulator can also be integrated with aerospace craft or satellites to maintain and aid in work in space. These manipulators must be anti-radiation and high-temperature-resistant [17]. If an explosive-proof manipulator adopts the hydraulic drive method, a hydraulic pump and hydraulic fluid must be included, which will dramatically increase the power consumption of its robot chassis. The pneumatic drive method will exhaust work gases, which can change the explosive environment [18]. When the pneumatic drive method is used, there is an unpredictability problem introduced by the work gases. Therefore, the motor drive method is preferred. Every joint of manipulators and robots are usually driven directly by a motor. Therefore, the motor will move together with the manipulator arm [19]. In the explosive environment, every motor must be enveloped in an explosive-proof box made of steel. This will make the drive unit very heavy, and requires the adoption of a motor with greater power. Then, the steel box will become bigger with the increment of the motor dimensions. As a result, the manipulator will become very large and heavy in order to be explosive-proof. On the other hand, the electric wires which are used to connect every joint's motor to the battery and controller are fixed on the manipulator, and will destroy the battery and result in an explosion when the wire is fired or in a high-temperature environment. So far, there are two robots which are assembled with an explosive-proof manipulator. One is the V2 robot [20], and one is its new edition prototype [21]. Both use motors to directly drive the joint. If the manipulator uses the motor-wire drive method [22; 23], the motors used for drive joints can be assembled at the end of it and the motors can be separated from the manipulator [23]. In this way, the manipulator can improve its work capacity by reducing its mass. However, the angular displacement of the joint is determined by the storage capacity of the wire. If the joint needs to rotate unlimitedly, the problem of wire storage becomes very difficult. The characteristics of the motor-pulley transmission method [24; 25] applied in manipulators are similar to the motor-wire structure, but its angular displacement has no limits. Both of the transmission methods can separate the motors from the manipulator, however, the number of motors is still the same as the number of joints. When there are many joints, it results in motors which are too numerous to be assembled on the robot chassis because of the limited space. This paper introduces a novel design method for manipulators which can search and rescue in the explosive environment, especially in the underground coal mine environment. The manipulators adopted in this design method can ensure the security of the explosive environment. Finally, the paper describes the prototype that was developed and the experiments that were run to evaluate it. The prototype manipulator not only separates the motors from the manipulator, but also uses only two motors to drive three joints. One motor is a power motor which supplies the power for joint rotation. The other motor is a switch motor which is used to switch power among joints. The number of joints in the manipulator is determined by the dimensions of cams, not by the number of motors. Power switching among joints is realized by the switch motor and power switch units (PSUs) which are integrated into every joint. Under this design method, every joint can maintain its status independently of whether other joints are driven or not.