Exoskeleton products have a variety of applications not just limited to medical healthcare or military purposes. Number of widely recognized organisations are agnizing the applications of exoskeleton products in a working environment to improve their workforce productivity by reducing physical work time strain upon an employee’s body and health and thereby improving their morale and capacity to perform quality work. Essentially such exoskeletons are being developed specifically to provide back support to the employees who are working long hours in a traditional office setup or at a physical workplace like an automotive plant or logistics and construction sites. These devices are intended to reduce lower back-pain and injuries by providing support and assistance while performing activities requiring physical effort. Major industries like automotive, aerospace, logistics, construction, healthcare and agriculture are carefully calibrating the implications of these devices in a workplace setup and notably taking efforts to implement them in their workforce.
A number of prototypes are already in development phases and soon to be available for commercial use. Now when it comes to the actual aspect of how to design and develop these devices for a particular task there are three technological aspects which need to be taken into account which are essential for the proper implementation of these devices, namely
- Actuation, the component of an exoskeleton that produces forces or torques.
- Force transfer to the user, which is a function of exoskeleton’s structures and attachments.
- Control strategies, i.e., how exoskeletons can make use of sensory information to adjust the provided forces/torques during operation in order to offer the most appropriate assistance profiles.
This article specifically focuses on how exoskeletons provide a portion of the torque required to perform physical tasks, like lifting or maintaining a stoop posture. These devices are also often known as back support, lift assist, lumbar support, hip orthosis and spinal exoskeleton devices. They are built around the concept that forces/torques are applied in the sagittal plane, between the user’s torso and thighs, to assist with the extension of the back and/or hip joint thereby providing the user with a certain portion of torque required to perform physical tasks.
Exoskeleton devices generate forces/torques either by employing passive components, like springs, or through an active powered actuator, like electric motors. A passive exoskeleton can store and/or dissipate energy provided by the user, while an active exoskeleton has the capability to introduce additional energy from external energy sources, like a battery, on demand (Toxiri, 2018).
Existing passive back support exoskeletons employ elastic elements of different types, like coil or gas springs, elastic bands and flexible beams, for different kinds of components in the structure of the exoskeleton enabling the wearer to extract the necessary amount of force or torque with minimal amount of physical exertion. These exoskeleton devices provide support and assistance by amplifying the torque created by the wearer and then channeling it back in the form of physical force needed to perform the desired physical task at hand. However, the assistance level and the profile of these exoskeletons are decided during its design phase and are often difficult to be adjusted during its operation. An active exoskeleton on the other hand employs actuators whose actions are controlled by a computer program based on sensory information during its operation. Most active exoskeletons work on electric motors but examples of pneumatic actuation also exists, thereby they are often considered more versatile than the passive ones. Electric motors are used in combination with reduction gears to achieve the necessary forces or torques, but geared motors often introduce an undesirable scenario of dynamics for the user essentially because of added friction and large resulting inertia. To address this drawback a mechanical arrangement of elastic components with electric motors is proposed. An additional approach has also been employed in which electromagnetic clutches are capable of decoupling the actuator from the exoskeleton itself when no additional assistance is required, this approach helps in reducing the energy consumption and undesirable actuator dynamics. This approach is also often termed as quasi-passive or semi-active devices.
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There are further two types of exoskeleton devices namely, soft and rigid exoskeleton devices. Soft exoskeletons do not have any rigid structures and essentially contribute to compression forces on the lower back almost similar to the effects of a para spinal muscle contractions. Whereas, rigid exoskeletons are built with rigid frames that transmit forces perpendicular to the limb.
Soft exoskeletons, also exosuits, are devices consisting of garments worn on body segments adjacent to the joint that is assisted, for example the thigh and shank for a knee exosuit. Assistance is generated by using the garments to pull two body segments together via a cable or strap. Joint flexion and extension is achieved separately, each by a dedicated cable or strap. Rigid exoskeletons are built with hard articulated structures that connect actuators to garments worn by the user. These rigid structures often run parallel to the body segments and apply forces perpendicularly. They also tend to use the space lateral to the user’s body, increasing the lateral footprint in some scenarios. Thereby in continuation these rigid exoskeletons support back extension by pulling the upper torso backwards via backpack like shoulder straps and the hip extension is supported by forces on the thighs, which are applied either by pushing the front of the thigh or by pulling from the back of the leg. Now, there are also exoskeletons which employ both soft and rigid components thereby exploiting the best of these two types of exoskeleton structures. In this particular type of device a particular component, like a carbon fiber rod or beam, acts both as a force generator and as a structure to transfer force between a user’s body parts. In some devices a flat spring can also be combined with an artificial pneumatic muscle in order to achieve the same desired force or torque.
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Finally, coming down to the aspect of how to control the force of an exoskeleton device, also known as control strategies, the process of how to modulate the assistive action provided by an active exoskeleton. This is achieved by mapping user intent, often measured by physical sensors, into desired patterns of assistance. The essential hurdle a control strategy effectively tries to overcome is how to generate appropriate reference signals to control the speed, torque or the impedance of the actuated joints over time. Typically there are two types of control approaches. The indirect control approach, which relies on the measurements from the device or the environment, for example joints motion or interaction forces, and the direct control approach in which the volitional information is captured from the user itself, for example bio signals such as electromyography.
In an indirect approach motions of relevant body segments can be measured with the help of IMU’s, Inertial Measurement Units, which is capable of measuring, among other quantities, the inclination of a body segment with respect to gravity, or encoders, joints angle, integrated into the exoskeleton. Whereas, in a direct control approach volitional information can be acquired using the surface electromyography (EMG). This technique essentially requires the use of electrodes in direct contact with the skin surface at a location corresponding to the target muscle(s), specifically the EMG signal carries information about the level of muscle activation in the respective region. Thereby both these methods enable the user to control the amount of force or torque generated through these devices.
For even more comprehensive references and information regarding the topic you can refer to the journal article Back Support Exoskeletons for Occupational Use: An Overview of Technological Advancements and Trends by Stefano Toxiri et al., July 2018 (Toxiri, 2018).
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