## 1 Introduction
### Back-Support Exoskeleton  
With the development of bionic devices, more and more wearable devices have emerged to help people carry loads. one of these devices is wearable exoskeleton [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. Exoskeleton is a wearable device supporting the human to generate the physical power required for manual tasks [11]. The exoskeleton was designed to help soldiers carry heavier equipment. With the improvement of the overall technology level of exoskeleton, more and more industrial fields (e.g., Assist medical workers carry patients or medical equipment, stooped work and construction work) require wearable exoskeletons especially back-support exoskeletons to help the transporter make efforts in the process of handling and reducing the risk of low back injury. Back-support exoskeletons can be classified as \"active\" and \"passive\" [12]. The passive exoskeleton consists mainly of springs or dampers that store energy generated by human motion and release it when needed, without an external power source [11]. The active exoskeleton is composed of one or more actuation units, whose main function is to drive the mechanical structure through the actuators during the movement, and output torque with the human body joint [13]. On the one hand, notable research prototypes in passive exoskeleton include the PLAD [14], the BNDR [15] and SPEXOR [16; 17]. There are also some passive exoskeleton products (e.g., BackX [18] and Leavo [11]) in the industrial field because the drive structure is easy to realize. [PERSON] et al. measured the effects of Leavo on muscle activity with manual lifting. The test results showed that Leavo could reduce the activity of the lumbar muscles during the process of lifting by 35-38%, and increase the carrying endurance time from 3.2 min to 9.7 min [11]. However, the elastic forces in passive devices are generated by passive elements only, these forces cannot be adjusted during operation and have an upper limit, which may limit the versatility of this class of exoskeletons. The active exoskeleton, on the other hand, is difficult to provide a comfortable experience due to its heavy drive structure. Since the increased metabolic energy carried by the extra mass of the limb cannot be offset by the metabolic energy saved by the exoskeleton, this is why it has been studied in a laboratory setting for a long time. With the development and progress of actuators, more and more high-power density actuators have been applied to active exoskeletons. [PERSON] et al. designed an active exosuit to assist the human body in walking and running, using the different paths of the center of mass in the vertical direction of running and walking to distinguish the two gaits [19], but will not provide any help during manual handling tasks. ATOUN designed a waist active exoskeleton named Model A, which is an active, upper body exoskeleton for the waist and lower back intended to aid users during lifting and carrying objects. By driving the thighs and upper body back during bending motions to relieve the waist and lower back, this can help lift to 10 Kg [20]. Since the back-support active exoskeleton usually adopts the drive structure in the hip joint, the back-support exoskeleton is a kind of hip active exoskeleton. In most active back-support exoskeletons (e.g., the Model A [20], the Muscle Suit [21], the HAL Lumbar Support [22] and the WSAD [23]), the body is assisted by pushing the thighs and upper body. The work presented in this paper provides a research prototype that can further advance the state of hip active exoskeleton and controllers. By cooperating with the human hip joint to output torque, the hip active exoskeleton is designed to assist the carrier in reducing the muscle activity of the lower back during the rise phase of the transport and reduce the energy consumption in the reciprocating transport. The goal is to design a wearable exoskeleton platform that can effectively assist in manual material handling.