**3. Variable stiffness actuators**

Rehabilitation is known as the process of regaining the deceived somatic talents as a result of an illness or accident, all of which are necessary for survival, quality of life, and living together with their families and society. With the advancement of technological development, specialized mechanisms and devices are used more frequently to resolve some of the issues related with physical interaction between humans and robots. Also these mechanisms or devices operate in clinical environments; some of them are designed to provide mobility for daily usage. Namely, exoskeletons are the wearable types of these mechanisms. Among military usage, civilian purposes, and industrial applications, exoskeletons are for rehabilitation

**5**

*Safe Human-Robot Interaction Using Variable Stiffness, Hyper-Redundancy, and Smart Robotic…*

or acquisition of lost actions of people with disabilities. Commonly, similar to other mechatronic systems, exoskeletons comprise sensing, control, and actuation

*Schematic representation of series elastic actuators (SEA) or variable stiffness actuators (VSA).*

fully while maintaining a lightweight mechanical structure [18].

nisms are presented as possible solutions to these problems.

within a specific range (see **Figure 3**).

**3.1 Operational principles of VSAs**

torque (τ) and position (θ) is linear as follows: *<sup>k</sup>* = \_

Since rehabilitation is a human-centered therapy to overcome the impairments of the motor functions, it is required to be for exoskeletons to provide safe interaction and mimicking human motions [13]. Compliance is a prerequisite for safety, which can be maintained by software or hardware solutions. Software-based solutions allow controlling impedance [14] by implementing control techniques on rigid joint structures [15]. On the other hand, hardware-based solutions imply flexible joint structures with passive compliance [16, 17]. Also, the interface surface between the patient and the mechanism is covered by soft materials to increase comfort. In addition, adequate amount of force/torque should be supplied to perform the predefined tasks success-

Safety is a trending topic within industrial robotic applications. To increase precision, robot joints are designed as stiff as possible; however, Pratt et al. [19] proposed to connect motor and load with elastic components. Consequently, passive compliance is obtained, but it is comprehended that single stiffness value is not suitable for different robotic tasks. Variable stiffness actuators (VSAs) or in general variable impedance actuators (VIAs) are able to adjust the stiffness/impedance

Besides the need of excessive number of human therapists for ordinary rehabilitation techniques, they are time-consuming for labors [20]. Furthermore, these techniques are deficient to measure the performance of rehabilitation outputs for objective analysis. Inflexibility due to different level of treatments, namely, impersonal aims, can be counted as another drawback for traditional rehabilitation therapy methods. In this section, VSA-based exoskeleton/rehabilitation mecha-

Conventional robot joints are designed to track a motion profile and try to keep the position against external effects after reaching the goal position. On the contrary, in the SEA mechanism, there are elastic elements between the load and the motor, which allow the external influences to change in the joint position. The elastic element herein has constant output stiffness (k), and the relation between

*d*

τ = *k*Δθ (2)

*<sup>d</sup>* (1)

*DOI: http://dx.doi.org/10.5772/intechopen.92693*

components.

**Figure 3.**

*Safe Human-Robot Interaction Using Variable Stiffness, Hyper-Redundancy, and Smart Robotic… DOI: http://dx.doi.org/10.5772/intechopen.92693*

#### **Figure 3.**

*Schematic representation of series elastic actuators (SEA) or variable stiffness actuators (VSA).*

or acquisition of lost actions of people with disabilities. Commonly, similar to other mechatronic systems, exoskeletons comprise sensing, control, and actuation components.

Since rehabilitation is a human-centered therapy to overcome the impairments of the motor functions, it is required to be for exoskeletons to provide safe interaction and mimicking human motions [13]. Compliance is a prerequisite for safety, which can be maintained by software or hardware solutions. Software-based solutions allow controlling impedance [14] by implementing control techniques on rigid joint structures [15]. On the other hand, hardware-based solutions imply flexible joint structures with passive compliance [16, 17]. Also, the interface surface between the patient and the mechanism is covered by soft materials to increase comfort. In addition, adequate amount of force/torque should be supplied to perform the predefined tasks successfully while maintaining a lightweight mechanical structure [18].

Safety is a trending topic within industrial robotic applications. To increase precision, robot joints are designed as stiff as possible; however, Pratt et al. [19] proposed to connect motor and load with elastic components. Consequently, passive compliance is obtained, but it is comprehended that single stiffness value is not suitable for different robotic tasks. Variable stiffness actuators (VSAs) or in general variable impedance actuators (VIAs) are able to adjust the stiffness/impedance within a specific range (see **Figure 3**).

Besides the need of excessive number of human therapists for ordinary rehabilitation techniques, they are time-consuming for labors [20]. Furthermore, these techniques are deficient to measure the performance of rehabilitation outputs for objective analysis. Inflexibility due to different level of treatments, namely, impersonal aims, can be counted as another drawback for traditional rehabilitation therapy methods. In this section, VSA-based exoskeleton/rehabilitation mechanisms are presented as possible solutions to these problems.

#### **3.1 Operational principles of VSAs**

Conventional robot joints are designed to track a motion profile and try to keep the position against external effects after reaching the goal position. On the contrary, in the SEA mechanism, there are elastic elements between the load and the motor, which allow the external influences to change in the joint position. The elastic element herein has constant output stiffness (k), and the relation between torque (τ) and position (θ) is linear as follows:

$$k\_{\perp} = \frac{d\tau}{d\theta} \tag{1}$$

$$
\pi \quad = \; k \Delta \theta \tag{2}
$$

where Δθ denotes the deflection of the elastic element. VSA mechanisms are designed to have nonlinear τ-θ relation yielding variable output stiffness as given in Eq. 3:

$$d\tau \, = \, f(\Theta)d\theta \,\tag{3}$$

where f(θ) is the nonlinear stiffness function. Regarding the change in stiffness, the reaction of the joint for external effects can be adjusted according to the desired task. The stiffness adjustment mechanisms are classified in three main categories in [21]: (i) spring preload, (ii) changing transmission between load and spring, and (iii) physical properties of the spring. The system, in which a couple of springs and motors run in a reciprocal manner, namely, antagonistic springs, is the first type. The working principle resembles biological musculoskeletal system in the first category. To obtain a linear stiffness variation, two quadratic springs are utilized in [22] by using a cam mechanism with a linear helical spring. In [23], the importance of quadratic springs in the design of VSAs is shown. The second type provides nonlinear torque-position relation by changing the distance between rotation center, the linear spring connection points, and/or tip point [24]. The last kind exploits natural characteristics of linear springs. In [25], nonlinearity of helical springs under bending and compression determines stiffness variation. Moreover, mechanism in [26] specifies the number of active coils of helical spring which results a change in stiffness. General schematic representations of the first two types are given in **Figure 4**.

VSAs are generally actuated by conventional electrical motors; however, in [27] stiffness variation is obtained by a pneumatically artificial muscle. Hobby servo motors are another alternative to conventional motors which is used in a modular VSA design to lower the cost [28]. Similar to actuation units, elastic components can vary in different mechanisms. Although it is not implemented in an actuator, a nonlinear spring mechanism in [29] includes rubber, and in [30], a timing belt is introduced as the source of elasticity.

VSAs are superior to conventional actuators according to energy efficiency under various working conditions and performing highly dynamic task. Energy-efficient gait is performed by using compliant actuators because of the energy storage capability of the elastic element. However, when the environment or the walking speed is changed, natural dynamics of the mechanism is expected to maintain efficiency. In [31], running motion energy cost of a legged robot, *Edubot*, decreases about 40% when it is compared to fixed stiffness legged robots. In addition, dynamic tasks that cannot be accomplished with classical robots can be done by these mechanisms due

**7**

*Safe Human-Robot Interaction Using Variable Stiffness, Hyper-Redundancy, and Smart Robotic…*

Modern studies towards medical mechatronic systems are performed as interdisciplinary collaboration conducted with physicians, therapists, and scientists among engineering community. Recent approach to these systems serves the emergence of new perspectives beyond the advantages of robot-assisted therapy. Principally, practical studies can be divided into two parts depending on the intention of

Upper body exoskeleton applications are mostly focused on the upper limb and elbow parts. A torque-driven and lightweight exoskeleton called *Limpact* is proposed to sustain therapeutic aid for upper limb rehabilitation in [33, 34]. Suitable dimensions for wearable functionality and impairment quantification can be taken into account as further characteristics as well as rotational hydro-elastic actuator as being a new type of SEA. There also exist different control modes such as compliant impedance and stiff admittance. In [16, 17], a 4-DOF wearable passive exoskeleton mechanism for elbow rehabilitation, named as NEUROExos, driven by a variable impedance antagonistic actuator, is presented. The double-shell link structure of NEUROExos contributes to ergonomics, and the joint position and stiffness are controlled separately by passive compliant actuation system. The experiments conducted including a human subject show that the increase in the joint stiffness causes smaller angular error during the motion in the reference trajectory. This is a result of ensuring the proper torque transmission relation between the human and exoskeleton. AVSER [35] is another study towards elbow rehabilitation using an active variable stiffness exoskeleton. Within AVSER, there is an active variable stiffness elastic actuator (AVSEA) composed of two DC motors, one for controlling joint position and the other one is for varying stiffness that is produced by a leaf spring. The effective length of the leaf spring, which is controlled by AVSEA, affects the motion characteristics of AVSER, which can be active or passive. Human-included experiments are conducted using the data gathered from two encoders for motor and elbow angles, a linear potentiometer for linear spring deflection and two active electrodes for electromyogram (EMG) signals. The results display the compatibility of AVSER for active-passive elbow rehabilitation tasks with its capabilities of stiff-

Lower body assistance can be in forms of full support to the legs, or it can affect only dysfunctional part such as the ankle or knee. Exoskeletons and rehabilitation mechanisms are widely used in the lower limb to regain locomotion of disabled or patients who have difficulty with walking. In addition to gait assistance, standing up motion is provided with the help of exoskeletons. A brace about the foot which is called as ankle-foot orthosis (AFO) is a usual treatment for drop-foot gait. A variable impedance actuator with force and position sensors is assembled to an AFO in [36]. It is shown that during different phases of the gait, adjusting impedance values increases the benefits of AFOs. Sit-to-stand task is a torque demanding task especially for knee joints. In [37], a lever arm mechanism based on VSA for knee exoskeleton is presented, and design methodology is explained in detail. Moreover, the effects of different stiffness values are evaluated for standing task. It is not only necessary to supply sufficient torque to knee joint but also to understand the intention of the user. Instead of splitting task into phases to control stiffness or impedance in [28], muscle activity of the patient is collected via EMG in order to detect patient

to the energy storage feature. An optimal control strategy is implemented to the mechanism to maximize ball-throwing distance in [32]. Benefits of stiffness adjustment is presented in the study by comparing variable and fixed stiffness perfor-

mances. A detailed analysis of the VSA designs can be found in [21].

*DOI: http://dx.doi.org/10.5772/intechopen.92693*

**3.2 Example applications of VSA**

mechanisms to the upper body and lower body.

ness adjusting, safety, and energy efficiency.

**Figure 4.**

*Stiffness adjustment types: (a) spring preload; (b) changing transmission between load and spring.*

to the energy storage feature. An optimal control strategy is implemented to the mechanism to maximize ball-throwing distance in [32]. Benefits of stiffness adjustment is presented in the study by comparing variable and fixed stiffness performances. A detailed analysis of the VSA designs can be found in [21].
