Lower Extremity Exoskeletons and Active Orthoses: Challenges and State-of-the-Art
Aaron M. Dollar, Member, IEEE, and Hugh Herr, Member, IEEE
Abstract—In the nearly six decades since researchers began to explore methods of creating them, exoskeletons have progressed from the stuff of science ﬁction to nearly commercializedproducts. While there are still many challenges associated with exoskeleton development that have yet to be perfected, the advances in the ﬁeld have been enormous. In this paper, we review the history and discuss the state-of-the-art of lower limb exoskeletons and active orthoses. We provide a design overview of hardware, actuation, sensory, and control systems for most of the devices that have beendescribed in the literature, and end with a discussion of the major advances that have been made and hurdles yet to be overcome. Index Terms—Exoskeleton, lower extremity, orthosis, orthotics, rehabilitation, robotics, walking, wearable.
I. INTRODUCTION ESIDES mention in early patents and science ﬁction , research in powered human exoskeleton devices began in the late 1960s, almost in parallelbetween a number of research groups in the United States and in the former Yugoslavia. However, the former was primarily focused on developing technologies to augment the abilities of able-bodied humans, often for military purposes, while the latter was intent on developing assistive technologies for physically challenged persons. Despite the differences in the intended use, these two ﬁelds facemany of the same challenges and constraints, particularly those related to portability and interfacing closely to a human operator. For this reason, we address both of these applications. For the purposes of this review, an exoskeleton is deﬁned as an active mechanical device that is essentially anthropomorphic in nature, is “worn” by an operator and ﬁts closely to his or her body, and works inconcert with the operator’s movements. In general, the term “exoskeleton” is used to describe a device that augments the performance of an able-bodied wearer. The term “active orthosis” is typically used to describe a device that is used to increase the ambulatory ability of a person suffering from a leg pathology. Occasionally, however, the term “exoskeleton” is also used to describe certainassistive devices, particularly when they encompass the majority of the lower limbs.
We will focus our review on exoskeleton and orthotic devices for the lower limbs, and only cover devices that operate in parallel with the human legs, as opposed to devices such as the Spring Walker  that operate in series with the wearer. For active orthoses, we will limit our scope to devices that providesome means of augmenting power at one or more joints of the lower extremities. This includes both adding and dissipating power, as well as the controlled release of energy stored in springs during various phases of gait. Along these lines, we do not include devices whose active components simply lock and unlock joints of an orthosis, nor systems that are purely a hybrid of a passive orthotic braceand a method of a functional electrical stimulation (FES) control. Finally, exoskeletons used for therapy that are not portable and do not stand-alone mechanically (e.g., treadmill-based devices such as the Lokomat ) are not discussed, as these are not subject to the vast number of constraints associated with portable devices. We attempt to cover all of the major developments in the areasdescribed before, particularly focusing on the initial development of the different concepts, and less on similar devices built for research purposes. When available, we describe the results of any quantitative evaluation of the effectiveness of the exoskeleton and orthotic devices in performing their intended tasks; however, there are surprisingly few instances of such studies being reported. We...