Development of sEM G-Driven Assistive Devices

2017 3rd International Conference on Control, Automation and Robotics

Development of sEM G-Driven Assistive Devices Based on Twisted String Actuation

Mohssen Hosseini, Roberto Meattini, Gianluca Palli and Claudio Melchiorri DEI - Department of Electrical, Electronic and Information Engineering Universita di Bologna, Viale Risorgimento 2, Bologna, Italy e-mail: {mohssen.hosseini.roberto.meattini2.gian1uca.palli.c1audio.melchiorri}@unibo.it

Abstract-A

twisted

string actuation

module with

an

in­

tegrated force sensor based on optoelectronic components is presented in this paper. This solution is especially suited for very compact, light-weight and wearable robotic devices, such as exoskeletons, but is appropriate for various robotic applications. An in-depth presentation of the proposed actuation module, the description and the basic sensor working principle of the integrated force sensor are portrayed and discussed. Exten­ sive experimental measures have been carried out to verify the actuation module compliant frame design and the related finite element analysis results. Therefore, the proposed actuation module has been used for the implementation of a simple assistive

Fig. 1. Conceptual view of an elbow exoskeleton based on the twisted string actuation module.

application where surface-electromyographic signals are used to detect the user's muscle activity in order to control the support action provided by the actuator, thus reducing hislher effort.

Keywords -Twisted String Actuation, Sensors and

A light-weight, low cost and compact linear transmission system is achieved through the Twisted String Actuation (TSA) concept [6]. The usually demanding requirements for the implementation of miniaturized and highly-integrated mechatronic devices are fulfilled with the correct choice of string parameters (in particular the string radius and length) and a rotative electric motor. The slender structure of TSA also makes it particularly appropriate for wearable devices. A crucial role is performed by the measurement of the actuation force in the control of this actuation system, as reported in [7]. Previous studies on integrated actuation module for TSA are elaborated upon in [8], [9] and [10].

Actua­

tors, Wearable Robotics, Assistive technology and rehabilitation engineering, EMG.

I. INTRODUCTION Research laboratories worldwide are developing an avant­ garde era of robots which are much more advanced than their predecessors in terms of cognitive capabilities and are able to constantly adapt to diverse environments. These new robots have been designed specifically for physical interaction with unstructured environments and humans such as servoactuated prostheses and exoskeletons. An exoskeleton is a wearable robotic system usually composed by an external structural mechanism with joints and links corresponding to those of the human body, provided with a suitable actuation system to support the user's movements. The force and torque mea­ surements are used to reinforce and conform to human goals in these systems. Exoskeletons are applicable in a number of fields, especially in rehabilitation and haptic applications, and ultimately all populations, both disabled and healthy, can benefit from their functions. Lately, the use of exoskeletons for the hand and lower/upper-limb support applications has drastically in­ creased [1]-[3]. In [4], an upper-limb power-assist exoskeleton by pneumatic muscle actuation with two metal joints is introduced. An integrated cable-driven, low-cost and light­ weight wearable upper body orthotics system to be worn over the upper body to generate effective torques in order to move the arm through a set of assistive motions was developed in [5].

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The design of a novel TSA module is demonstrated in this paper with the main goal of developing wearable assistive and rehabilitation systems. Its original structure makes the TSA well suited if force is applied directly to the users limbs, without external rigid structures supporting the limb and/or the actuation, as schematized in Fig. 1. The design of the proposed wearable assistive and rehabilitation system aims to: i) remove all rigid joints to develop a lighter and more comfortable system that can easily be applied to any user; ii) reduce the weight, size and mechanical complexity of the exoskeleton, avoiding complex regulation mechanisms, to lower the costs while improving its reliability and affordability; iii) design a modular actuation system that can be utilized for implement­ ing different assistive movements. The main components and structure of the proposed TSA module which is characterized by an integrated force sensor and embedded acquisition and control electronics are portrayed in Fig. 2. A complete overview of the actuation module structure and the integrated force sensor based on optoelectronic com-

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Acrualion Frame

Encoder

Shah

Load

Encoder

DC Motor (Rotative)

Fig. 2. Schematic representation of the TSA structure.

ponents, called light fork, are explained in this paper. The motor module structure is manufactured in ABS plastic by 3D printing. The actuation module is equipped a low-cost DC motor and with an optical encoder for output shaft position measurement. In order to validate the actuation module design, the Finite Element Analysis (FEA) together with experimental measures has been executed. Two experiments have been performed in dynamic conditions, and the results have been verified through a reference strain-gauge based force sensor to confirm the properties of the proposed TSA module. Finally, as an application example, an assistive device able to support the user while carrying loads with hislher arm was created using the TSA module together with a surface­ ElectroMyoGraphic (sEMG) signals acquisition system and the preliminary results are reported here. The paper is organized as follows. Section II describes the mechanical design of the TSA modules and the working principle of the integrated force sensor. In Section III the experimental evaluation of the TSA module are presented ,while in Section IV the results about the use of the proposed device for the implementation of wearable assistive systems are reported. Finally, the conclusion of the presented work and description of the future research activities are presented in Section V. II.

Fig. 3. Detail of the twisted string actuation module.

on the sensor's body as a consequence of the application of an external force. B. TSA Module Design

Figure 2 represents the schematic view of the designed TSA module that is composed, from left to right, by: i) a connection element to connect the module to the supporting frame; ii) the force sensor to measure the actuation load; iii) a frame hosting the DC motor, the output shaft where the twisted strings are connected and all the electronics; iv) the twisted string itself connecting the motor module with the load, and the load itself represented in Fig. 2 as a translating mass. The design and basic principles of TSA can be found in [7]. A couple of strings are attached to a rotative electrical motor and twisted on one end, while on the other end the strings are connected to a linear moving element, i.e. the load. The rotation produced by the electrical motor reduces the overall string length. The rotative motion of the electric motor is thus converted to a linear motion on the other side of the strings. A detailed 3D view of the TSA module design is portrayed in Fig. 3. Its mechanical structure is manufactured by rapid prototyping in ABS plastic. On the opposite side with respect to the twisted strings, a pair of axial-symmetric compliant beams has been integrated in the TSA module frame. An adequate compliance, required for converting the force exerted by the TSA module into a proper frame deformation and thus to the implementation of the force sensor, is provided to the structure thanks to these beams behaving as a linear spring. The force sensor's presence in the TSA module is critical to successfully measure the force the actuator applies to the load. Fig. 3 demonstrates the location of the force sensor in the proposed TSA module, between the frame connection point on the robot structure and the frame hosting the DC motor, i.e. on the opposite side of the twisted strings with respect to the rotative motor. Fig. 4 is a photo of the TSA module prototype developed through this project. Figure 4(a) also highlights the TSA module embedded controller based on an Arduino NANO board. In the TSA module, an optoelectronic device is subsequently used to detect the frame deformation and convert it back to the applied force causing the deformation. As shown in Fig. 4, a DC motor as well as an optical encoder for motor angular position sensing is found in the module frame, while the output shaft is supported by a combined axial-radial bearing at the point of the twisted string connection to both reduce the friction and prevent the transmission force from

SYS TEM DESCRIP TION

The presented system is designed with the goal of work­ ing together with rehabilitation and assistive applications, as schematically reported in Fig. 1, where a light assistive device is obtained by connecting the TSA module with integrated force sensor to the forearm of the user. The TSA module is mounted on a orthopedic shoulder support strap to allow the TSA to work in a biarticular configuration [11], providing help on the whole arm and not on the elbow only. The designed TSA module fits very well with this kind of application due to its light and compact structure and the ability of acting similarly to human muscles, as will be shown in the experiments described in Sec. IV. A. Sensor Working Principle

As the optoelectronic sensor is described in detail in [12], only a brief description of its main features is reported here. The main concept is to have a LED illuminating a photodiode (PD ) , where the current flowing through the PD can be modulated by means of a mechanical component that partially intercepts the light emitted by the LED. The position of the mechanical component depends on the deformations occurring

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Fig. 6. Finite Element Analysis of the compliant frame.

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Fig. 7.

(a) Upper view of the TSA module prototype (b) Lateral view of the and control electronics. TSA module prototype.

10

20

30

40 Force IN]

50

60

70

80

Deformation of the motor module in the direction of the transmis­ sion:comparison between experimental values and FEA.

The TSA module structure is devised to demonstrate a symmetric deformation along the direction of the measured force (i.e. the x-axis). The proposed actuation module design has been verified through both experimental measurements and FEA. The FEA was performed at the maximum load condition, i.e. with a load of 80 N, see Fig. 6. This limit is not established by the TSA capabilities, but rather by the control system to preserve the twisted string. The TSA module structure has been designed to sustain a maximum load 4 times higher than the previously mentioned force range to prevent damage to the mechanical structure in case of overload, even if the verification of the frame deformation is not significant in that case. The comparison between the measured deformation and FEA is shown in Fig. 7. The result evidences that the mea­ sured deformation is slightly smaller than the FEA, which can be influenced by the manufacturing process. The maximum deformation with the maximum load of 80 N is �Xmax c::= 2.34. 10-4 m indicating the compliant frame deformation is within the goal working region �d