## 1 Introduction  
Interest in the use of robotic devices for assisting both healthy and disabled people with various activities has been growing over the past two decades. Robotic exoskeletons have been developed to augment or assist the physical functions of humans in industry as well as daily life. These are often controlled by advanced technologies developed specifically for human-robot interaction (HRI). Physical HRI is capable of decoding and reacting to a human's motion-based intentions through physical contact with the wearer.  
Advancements in robotic exoskeleton technologies were demonstrated by a number of wearable robots: BLEEX [1] showed success in augmenting human load-carrying performance in field testing for military applications, while iHAS [2], Noonee [3], Robo-mate project [4], and BobyWeight Assist devices [5] have done the same for industry applications, and the Hybrid Assist Limb [6] shows promise for rehabilitation. Following these achievements, a new modular type [3; 4; 5] of human-assistive device was developed to alleviate the physical workloads placed on workers.  
The market for robotic exoskeletons is expected to increase with society's aging demographics. In industry, robotic exoskeletons may be used to assist physically impaired individuals working for prolonged periods, helping to improve their productivity, safety, and quality of work. These factors will reduce production costs as well as the risk of injury [6].  
A number of new technologies in physical HRI have been introduced to address existing safety issues. Highly back-drivable mechanisms can accommodate safe interaction with human operators [7; 8].  
Implementing passive elements [9; 10] and pneumatic actuators [11; 12] into device design enables improved mechanical compliance in a robot. The design and control of a variable stiffness actuator can also help to actualize fast and safe physical HRI [13; 14]. Alternative approaches for modifying physical HRI systems have also been proposed, including a knee brace based on a magnetorheological actuator by [PERSON] et al. [15], an electrorheological fluid actuator for a knee orthotic device [16], a quasi-passive knee exoskeleton using a cable-driven series elastic actuator by [PERSON] et al. [17], a knee-assistive device driven by a series elastic actuator by [PERSON] et al. [18], and multiple variable stiffness actuators proposed by [PERSON] et al. [19].  
These existing designs, however, have encountered difficulties controlling motion generation (corresponding to the wearer's intention) and power assistance (reducing stress on the knee). Moreover, these devices require a large power capacity, as the actuator is driven continuously while the wearer is walking or sitting. This causes energy consumption problems over the course of long-term operation.  
In contrast to existing devices, this study proposes an alternative four-bar mechanism combining passive and active actuators so that this device can assist the wearer's body weight using a small torque input in sitting and can provide no resistance in walking. This work designs and proposes a knee-assistive robotic exoskeleton (KARE-1) for application to the industrial field, which can completely support a worker's body weight in various sitting postures.  
To this end, it is worth noting that, differently from the previous work [20], three representative kinds of four-bar mechanism-based assistive devices were analyzed and compared to optimize torque generation and reduce energy consumption. Simulation results of the proposed system exhibited the requisite dynamic properties, including torque, displacement, and force amplification. Finally, the effectiveness of the proposed approach was verified experimentally using a real assistive robot. Industry applicability was further confirmed by measuring muscle activation during testing with a human subject.