**3.2 Example applications of VSA**

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 mechanisms to the upper body and lower body.

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 stiffness adjusting, safety, and energy efficiency.

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

**Figure 5.**

*Variable stiffness neck joint. (a) Components of the mechanism and (b) reduced kinematic model [25].*

intention and correct stiffness values in [29]. Along with functional design of VSAs in rehabilitation mechanisms, researchers are inspired from the human muscle structure and designed full lower limb orthosis to improve impaired gait of patients by using pneumatic [38] and wire-based artificial muscles [20]. Furthermore, VSAs are expected to mimic human joint behaviors and have a great potential [39, 40] to be used in rehabilitation purposes. In [25] a VSA which resembles a human neck joint is presented. The schematic representation of the mechanism is given in **Figure 5**. Cable-driven lightweight structure brings simplicity. Also, there is no additional hardware other than a helical spring for stiffness variation. Although the middle shaft restricts the motion due to its revolute-revolute-prismatic (RRP) structure, actuation principle is similar to parallel mechanisms.

All in all, robot-assisted rehabilitation studies and applications are still attractive research areas. Human motion imitation for mechanisms used in rehabilitation is emphasized for successful results. To this end, VSAs are comprised within rehabilitation systems. More information about the latest progress can be found in [41–43]. These findings reveal that robot-assisted technologies will result in less human labor time consumption with increasing quality of observable rehabilitation outputs.

#### **4. Smart robotic skins**

For connecting the robot to its environment, visual sensor channels are usually preferred and widely applied. The tactile sensor applications are limited to certain locations, which tends to be the tip of the device especially for robotic catheters. In this part of the chapter, a large-area application for the tactile sensors to form an artificial skin on the robotic catheter is covered. The large-area, skin-like applications of tactile sensors can empower the robotic catheters to have better perception output during diagnosis/palpation while helping to obtain higher safety levels during operations.

#### **4.1 Tactile sensors for medical robotics**

In one of the recent surveys on state-of-the-art tactile sensing for minimally invasive surgery (MIS) [44], it is clearly stated that the best place to include sensing elements in MIS device is on the instrument shaft inside the patient's body. The force sensors on the tip of the endoscopic tools are not strongly suggested, because

**9**

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

the space is very limited. Incorporating a tip sensor involves either having a larger gripper or manufacturing of extremely small transducers. The general overviews on the tactile sensors without a specific focus on use in the surgical robotics can be seen in [45–48]. In [46], the focus of the overview was extended to electronic skin technologies, whereas in [45] the effective utilization of the tactile skin takes the contact condition into special consideration. Some overviews focus on the wearable features [47], and the others explain the difficulties in the development of tactile sensor units emphasizing its complexity involving multiple transduction ways [48]. In order to develop tactile sleeves/sheaths for MIS endoscopic robotic platforms, a broader perspective of current tactile technology development is needed. Although the application is very different and does not contain any tactile modalities, in [49] a flexible and wearable skin for health monitoring interface is reported. These types of advanced skin patches can even be used for scheduled drug delivery [50]. Some of the relevant studies can be found from soft robotics literature. For example, in [51] a shape-tracking algorithm using polyvinylidene fluoride (PVDF)-based sensors on the hyper-flexible beams is used. Although the beam is in 2D, the proposed method can be extended to 3D providing a spatial ego-motion tracking for flexible endoscopic robots. The research [52] reports a discrete piezo-ceramic sensor array embedded in soft substrate, therefore offering a solution to accuracy problems in film-based piezo material but at the same time providing some compliance and stretchable behavior. Although the piezo-electric transduction is very widely used, there are also alternative methods based on optical modality. For example, in [53], a large-area sensor for pressure measurement was suggested using organic fieldeffect transistors (OFETs). Similarly, in [54] an optical principle is used to measure data through employing fiber Bragg grating and waveguides inside the compliant substrate material. The waveguide approach is also used in [55] but employing

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

PDMS as the substrate this time.

using the structural computation.

**4.2 Example application of tactile sensors as robotic skin**

Herein, an example application is presented from AvH Project, MEDICARE [1], together with the measurement methodology it uses. The manufacturing of the tactile sleeve is achieved using multiple layers of silicone substrate in an additive manner to embed the piezo sensors in the desired depth and location. The silicone substrate was selected as Eco-flex 00-10 because of its relatively easy vacuuming and curing procedures. In addition to these advantages, the mechanical properties of Eco-flex are very close to the human tissue, and it is relatively low-cost. The distance between pressure sensors is large in this setup; however, ideally, they can be arranged with 4 mm separation in each active cell. The data cables connecting the sensors to the data acquisition circuit are soldered carefully, and meandering shapes are given to the bare wires to prevent fractures within the substrate when the sleeve moves with the backbone. It must be stated that using off-the-shelf sensors limits the stretchability of the sensing areas; still, the sleeve remains flexible enough to be wrapped around a robotic backbone. The tactile sleeve is produced in a flat sheet (**Figure 6a**) having slanted edges and was connected to the backbone in cylindrical form (**Figure 6b**) in the second step. The slanted angles at the edges allowed connecting the sleeve without having a bulk on the connection line. As it can be seen in **Figure 6**, the silicone sleeve features a ripple structure on the outer surface. This structure is a first attempt to increase the perception capacity of embedded sensors

The measurement methodology of the sleeve when the outer surface of the silicone sleeve contacts with a rough surface, the ripples would help create a highfrequency interpretation of the surface properties in the sensor output. Although *Safe Human-Robot Interaction Using Variable Stiffness, Hyper-Redundancy, and Smart Robotic… DOI: http://dx.doi.org/10.5772/intechopen.92693*

the space is very limited. Incorporating a tip sensor involves either having a larger gripper or manufacturing of extremely small transducers. The general overviews on the tactile sensors without a specific focus on use in the surgical robotics can be seen in [45–48]. In [46], the focus of the overview was extended to electronic skin technologies, whereas in [45] the effective utilization of the tactile skin takes the contact condition into special consideration. Some overviews focus on the wearable features [47], and the others explain the difficulties in the development of tactile sensor units emphasizing its complexity involving multiple transduction ways [48]. In order to develop tactile sleeves/sheaths for MIS endoscopic robotic platforms, a broader perspective of current tactile technology development is needed. Although the application is very different and does not contain any tactile modalities, in [49] a flexible and wearable skin for health monitoring interface is reported. These types of advanced skin patches can even be used for scheduled drug delivery [50]. Some of the relevant studies can be found from soft robotics literature. For example, in [51] a shape-tracking algorithm using polyvinylidene fluoride (PVDF)-based sensors on the hyper-flexible beams is used. Although the beam is in 2D, the proposed method can be extended to 3D providing a spatial ego-motion tracking for flexible endoscopic robots. The research [52] reports a discrete piezo-ceramic sensor array embedded in soft substrate, therefore offering a solution to accuracy problems in film-based piezo material but at the same time providing some compliance and stretchable behavior. Although the piezo-electric transduction is very widely used, there are also alternative methods based on optical modality. For example, in [53], a large-area sensor for pressure measurement was suggested using organic fieldeffect transistors (OFETs). Similarly, in [54] an optical principle is used to measure data through employing fiber Bragg grating and waveguides inside the compliant substrate material. The waveguide approach is also used in [55] but employing PDMS as the substrate this time.

#### **4.2 Example application of tactile sensors as robotic skin**

Herein, an example application is presented from AvH Project, MEDICARE [1], together with the measurement methodology it uses. The manufacturing of the tactile sleeve is achieved using multiple layers of silicone substrate in an additive manner to embed the piezo sensors in the desired depth and location. The silicone substrate was selected as Eco-flex 00-10 because of its relatively easy vacuuming and curing procedures. In addition to these advantages, the mechanical properties of Eco-flex are very close to the human tissue, and it is relatively low-cost. The distance between pressure sensors is large in this setup; however, ideally, they can be arranged with 4 mm separation in each active cell. The data cables connecting the sensors to the data acquisition circuit are soldered carefully, and meandering shapes are given to the bare wires to prevent fractures within the substrate when the sleeve moves with the backbone. It must be stated that using off-the-shelf sensors limits the stretchability of the sensing areas; still, the sleeve remains flexible enough to be wrapped around a robotic backbone. The tactile sleeve is produced in a flat sheet (**Figure 6a**) having slanted edges and was connected to the backbone in cylindrical form (**Figure 6b**) in the second step. The slanted angles at the edges allowed connecting the sleeve without having a bulk on the connection line. As it can be seen in **Figure 6**, the silicone sleeve features a ripple structure on the outer surface. This structure is a first attempt to increase the perception capacity of embedded sensors using the structural computation.

The measurement methodology of the sleeve when the outer surface of the silicone sleeve contacts with a rough surface, the ripples would help create a highfrequency interpretation of the surface properties in the sensor output. Although

#### **Figure 6.**

*Robotic sheath for endoscopic hyper-redundant platform developed in AvH project by Dr. Boyraz at Leibniz University of Hannover: (a) flat; (b) wrapped around the robotic platform, featured in [2].*

being very simple, the surface ripple structures can be elaborated to include multiscale ripples in a fractal manner to interpret different surface structures having different frequency in the vibration pattern.
