rticle
Cobot withPrismatic Compliant Joint Intended for Doppler Sonography
Juan Sandoval 1,*, MedAmine Laribi 1, Saïd Zeghloul1, Marc Arsicault 1 and Jean-Michel Guilhem 2
1 Department of GMSC, PprimeInstitute, CNRS, ENSMA, University of Poitiers, UPR 3346 Poitiers, France;med.amine.laribi@univ-poitiers.fr (M.A.L.); said.zeghloul@univ-poitiers.fr(S.Z.); marc.arsicault@univ-poitiers.fr (M.A.)
2 Private Practice, 4 rue deCoumasaout, Foix, 09000 Toulouse, France; jean-michel.guilhem@orange.fr
* Correspondence:juan.sebastian.sandoval.arevalo@univ-poitiers.fr; Tel.: +33-5-4949-6538
Received: 31January 2020; Accepted: 12 March 2020; Published: 16 March 2020
Abstract: This paper deals with acollaborative robot, i.e., cobot, coupled with a new prismatic compliant joint(PCJ) at its end-effector. The proposed collaborative solution is intended forDoppler sonography to prevent musculoskeletal disorders issues. On one hand,the Doppler sonographer’s postures are investigated based on motion capture useduring the arteries examination. This study highlighted that configurationsadopted by angiologists lead to the musculoskeletal disorder. On the otherhand, the proposed PCJ with variable stiffness gives an intrinsic compliance tothe cobot handling the probe. This feature allows preserving the human safetywhen both human and cobot share a common workspace. The effectiveness of theproposed solution is experimentally validated through a 7-DoF Franka Emikarobot virtually coupled with the PCJ, during the execution of a trajectoryperformed during a Doppler ultrasound exam. The impact force criterion isconsidered as a safety performance.
Keywords: intrinsic compliance; variablestiffness mechanism; safe human–robot interaction; cobot; doppler sonography;motion capture
1. Introduction
The use of collaborative robots, i.e.,cobots, emerges as a solution to improve the task execution of those taskswhere human is required. The cobots can coexist with humans in a shared commonworkspace and cooperate with them to accomplish the desired tasks. While arobot can magnify the human capabilities, such as its force, speed orprecision, a human can bring a global knowledge and his experience to jointlyexecute the tasks [1].
The ultrasound scan is a noninvasivemedical technique that creates in real time a two- or three-dimensional imageof organs using high-frequency sound waves reflection. It has become a key ofmedical decision-making. The Doppler echography is an ultrasound technique usedto evaluate blood flow through arteries and veins (mainly used for abdomen,legs, arms, neck exams). It is widely used to detect blockages to blood flow(clots), narrowing of vessels, tumors, and vascular malformations. During theultrasound examination, the sonographer handles the transducer (ultrasoundprobe) and carries out the investigation by moving the probe over the patient’sbody. At the same time, the operator monitors the ultrasound station and uses akeyboard to access the software facilities in order to control the image.Several studies conducted in the past decade have highlighted work-relatedmusculoskeletal disorders (WRMD) and repetitive stress injury (RSI) amongsonographers with almost
80% of them suffering ofthose problems. Static and uncomfortable postures are main causes of thoseWRMD, which can lead to pain, sickness, as well as long-term di****lity [2–6].
Robotics 2020,9, 14; doi:10.3390/robotics9010014 http://www.mdpi.com/journal/robotics
Multiple tele-operated solutions have beendeveloped in the past with the aim of providing medical facilities togeographically isolated patients [7–14]. The robotized ultrasound system, using a master-slavearchitecture, constitute a solution to this issue. The master interface isoperated by the sonographer who remotely controls the probe located on theslave robot [7–9]. In the main cases of teleoperation approach, the ultrasound probeis positioned by a robot, with the operator, the robot controller, and anultrasound image processor having shared control over its motion [10–12]. Nowadays, one ofthe commercial solutions has been developed with the aim of reducing WRMD andRSI: Medirob Ergo. This solution, used for cardiac sonography, is based on a6-degrees-of-freedom (DoF) serial robot that moves the ultrasound probe. Theexpert remotely controls the robot using a 3D mouse (generally used for CAD) withoutany haptic feedback on the force applied on the patient. More recently, a softrobotic ultrasound imaging system is used to improve sonographers ergonomics [12] and a bespokerobotic ultrasound manipulator has been designed to ensure the patient’s safety[13].
In this paper, a new solution is developedconsisting of a tele-operated ultrasound solution for Doppler echographyequipped with a collaborative robot as slave and haptic device as masterinterface.
Based on the standard ISO/TS 15066published in 2016 for collaborative robots, safety is the most important issueto guarantee before establishing collaborative tasks between human and robot,where a high risk of collisions between them is palpable and may result inhuman damages. In this context, research efforts are focused on the design ofsolutions to reduce the energy transferred by the robot in case of collision,decreasing the risk of injury for the human [15]. On this way, some basicsolutions have been proposed. For instance, Park et al. introduced the use of aviscoelastic covering in the robot’s body to reduce the impact forces [16]. Fritzsche et al.proposed monitoring the contact forces by providing the robot’s body with atactile sensor used as an artificial skin [17]. Furthermore, several controlapproaches have been proposed to provide the robot with a compliant behaviorwhile it executes a task. These compliant control strategies typically make itpossible to assign a dynamic relationship between the robot and theenvironment, enabling the interaction behavior to be con-trolled by properlyselecting the dynamic parameters [18]. On the other hand, mechanical solutions have also been proposedto provide an intrinsic compliance to the robot, as the one proposed by Wang etal. [13]based on the use of customized spring-loaded ball clutch joint. These solutionsare highly recommended by the standard ISO/TS 15066 as a risk reductionmeasure. Among these compliant mechanisms, the variable stiffness actuators(VSA) allow introducing an intrinsic compliance to the robot joints [19]. These mechanismsare capable of providing adjustable stiffness to the joints, which can beadjusted according to the needs.
The SISCob (Safety Intelligent Sensor forCobot) project, funded by the French National Research
Agency (ANR), aims at developing a newintelligent and modular device mimicking the functions of biologicalarticulations and their synergy for collaborative robots. On this context, anovel safety prismatic compliant joint (PCJ) with variable stiffness has beendeveloped and its behavior in a cobot is presented in this paper. Differenthuman safety criteria are proposed to study the effectiveness of a compliantmechanism, such as the head injury criterion (HIC) [20] or the head impact power(HIP) criterion [21]. In robotics, other safety criteria can also be employed, such asthe measures of robot displacements, e.g., velocities or accelerations, and themeasure of the impact force.
The paper is organized as follows. Section 2 introduces therobot-assisted Doppler sonography. We firstly present the medical gesture studyphase of the sonographer thanks to the motion capture system. The protocolsetup, as well as experimental results are also detailed in the second part ofthis section. The interactions with the patient during echo-Doppler examinationis investigated through effort measurement. In Section 3, we present the structure, aswell as the kinematic model of the PCJ. Curves obtained from both the simulationmodel and the prototype of the PCJ are presented. In Section 4, we describe thedynamic model of a torque-controlled robot coupled with the proposed compliantmechanism, as well as the control approach implemented to execute cartesiantasks. A case study allowing to compare the safety performance of a rigid-bodyrobot vs. a robot using the PCJ is presented in Section 5. Conclusions aboutthe effectiveness of the proposed mechanism in terms of human safety areprovided in the last section.
2. Robot-Assisted Doppler Sonography2.1. Gesture Analysis during Echo-Doppler Examinations
The sonography’s gesture analysis has beenperformed by an angiologist during Doppler ultrasound examinations on realpatients in the same conditions as in medical office. Hence, multiple organshave been scanned: Carotid, legs, and abdomen. The experimental analysis wasmade using a motion capture system (Qualisys) and has been repeated severaltimes. The system uses a set of high-resolution cameras to detect reflectivemarkers to study angiologist gestures (Figure 1). The software Qualisys trackmanager (QTM) allowing us to record, visualize, construct, and export the 3Dposition of each marker has been used. Then, we have recorded the motions ofmarkers while they were making real ultrasound examinations. The frequency ofmotion acquisitions was fixed to a hundred images per second. Our motioncapture method is based on the experience of the biomechanics communityespecially for the choice of marker sets and segment reference definition [22]. Using QTMsoftware, we have been able to reconstruct each motion by regrouping markersinto
“segments” as shown inFigure 1.A segment is defined as a set of markers of the same solid. One of thesesegments is a representation of the ultrasound probe.
Figure 1. Motion capture during sonographyexamination: (Left): Markers locations on angiologist, (right) carotid and legexaminations with the probe reconstruction model with segments.
Multiplereflectivemarkershavebeenplacedontheultrasoundprobe,aswellasontheangiologist’s body to evaluate the positions and orientationsduring the examinations (Figure 2).
Figure 2. Reflectivemarkers on probe.
Based only on posture observations, theexpert is far away from its neutral positions and out of their joint comfortzone. In addition, the gesture during the sonography examinations can bedescribed to be repetitive. All these observations approve the uncomfortablepostures that could cause musculoskeletal disorders.
The gesture analysis using motion capturewill approve the musculoskeletal disorders issue. This study is focused oncompute of the member orientations of the angiologist and mainly the right arm,the pelvis, and the head.
The maximum values of the joint orientationangles are considered, which corresponds to the worst postures. These jointangles are compared to reference angles of comfort zone defined in ISO 11226,ISO 11228-3, and NF EN 1005-4 norms. A sample data on a head rotation and anarm motion are given on Figure 3. These values, joint angles for the neck torsion, and wrist joint,show that the angiologist is all the time out of the comfort zone described bythe norms.
Figure 3. Necktorsion (left) and wrist joint angle (right).
In addition to the gesture, the interactionforce between the probe and the skin has been studied. The forces applied onthe probe during an examination have been evaluated using force sensitiveresistors (FSR) installed on shell, composed of two parts: A fixed one and amoving one, surrounding the probe (Figure 4). One part of the shell istightened to the probe body (no motion allowed). The other part encapsulatesthe probe with a small gap (filled with low-density foam), which allows smallmovements between the two parts. Two FSR sensors are installed between fixedand moving shell in order to evaluate forces applied by the angiologist ontothe patient. The shells have been designed to match the shape of the probeallowing the expert to manipulate it with a similar grasp as the classic probe.
Figure 4. Instrumentedprobe—Force measurement.
The applied force is maximum duringabdominal examination. Indeed, the angiologist has to apply a large force tofind abdominal aorta. Figure 5 shows force measurement during the aorta investigation. The forceapplied depends on the patient, more force is needed to find abdominal aorta ona fat patient, with a large stomach. These patients are those who usually needDoppler examination.
Figure 5. Appliedforce during abdominal examination.
As an outcome of this section, one canconclude that the posture of angiologist experts, studied during a classicalDoppler ultrasound exam by using a motion capture system, are out of comfortzones. The angiologist needs also to apply large forces on the probe in contactwith patient skin. These static postures, out of comfort zones, foster theoccurrence of musculoskeletal disorders.
In the next section, a tele-operated systemis proposed to assist the angiologist in order to prevent and alleviate theseinconveniences.
2.2. Teleoperated System for Doppler Sonography
The robotics team of Prime Institutedevelops teleoperation platforms based on collaborative robots. These platformsare mainly developed for medical applications, such as Doppler sonography orsurgical applications [23].
The tele-operated system for Dopplersonography is composed of slave station and a master station (Figure 6). The master stationconsists of a 6-DoF haptic device composed of a Novint Falcon interface linkedto a virtual probe instrumented with an inertial measurement unit. The slavestation consists of a 7-DoF collaborative robot, i.e., a Franka Emika. This isa torque-controlled collaborative robot including a torque sensor on each jointand whose maximum payload is 3 kg. The slave robot handles an ultrasound probe,linked to a Doppler sonography station. The master device controls motions ofthe slave robot and gives haptic feedback to the angiologist. ROS-basedframework is used to establish the data exchanges between the master device andthe robot.
Figure 6. (a) Tele-operation chain using a6-degrees-of-freedom (DoF) master device and a 7-DoF collaborative slave robot;(b) Doppler ultrasound testperformed by the angiologist using the tele-operation platform.
The proposed master device increases thethree translational DoF of a classic commercial Falcon interface by attaching avirtual probe to the end-effector using a universal joint. The virtual probe,called holder, includes an inertial measurement unit (IMU) allowing to computethe three DoF of rotation. This modification allows making a 6-DoF device withhaptic feedback along the translational axes.
The slave device is a Franka Emika with7-DoF torque-controlled robot useful to coexist with human in a shared commonworkspace.
Cobot withPrismatic Compliant Joint Intended for Doppler Sonography
Juan Sandoval 1,*, MedAmine Laribi 1, Saïd Zeghloul1, Marc Arsicault 1 and Jean-Michel Guilhem 2
1 Department of GMSC, PprimeInstitute, CNRS, ENSMA, University of Poitiers, UPR 3346 Poitiers, France;med.amine.laribi@univ-poitiers.fr (M.A.L.); said.zeghloul@univ-poitiers.fr(S.Z.); marc.arsicault@univ-poitiers.fr (M.A.)
2 Private Practice, 4 rue deCoumasaout, Foix, 09000 Toulouse, France; jean-michel.guilhem@orange.fr
* Correspondence:juan.sebastian.sandoval.arevalo@univ-poitiers.fr; Tel.: +33-5-4949-6538
Received: 31January 2020; Accepted: 12 March 2020; Published: 16 March 2020
Abstract: This paper deals with acollaborative robot, i.e., cobot, coupled with a new prismatic compliant joint(PCJ) at its end-effector. The proposed collaborative solution is intended forDoppler sonography to prevent musculoskeletal disorders issues. On one hand,the Doppler sonographer’s postures are investigated based on motion capture useduring the arteries examination. This study highlighted that configurationsadopted by angiologists lead to the musculoskeletal disorder. On the otherhand, the proposed PCJ with variable stiffness gives an intrinsic compliance tothe cobot handling the probe. This feature allows preserving the human safetywhen both human and cobot share a common workspace. The effectiveness of theproposed solution is experimentally validated through a 7-DoF Franka Emikarobot virtually coupled with the PCJ, during the execution of a trajectoryperformed during a Doppler ultrasound exam. The impact force criterion isconsidered as a safety performance.
Keywords: intrinsic compliance; variablestiffness mechanism; safe human–robot interaction; cobot; doppler sonography;motion capture
1. Introduction
The use of collaborative robots, i.e.,cobots, emerges as a solution to improve the task execution of those taskswhere human is required. The cobots can coexist with humans in a shared commonworkspace and cooperate with them to accomplish the desired tasks. While arobot can magnify the human capabilities, such as its force, speed orprecision, a human can bring a global knowledge and his experience to jointlyexecute the tasks [1].
The ultrasound scan is a noninvasivemedical technique that creates in real time a two- or three-dimensional imageof organs using high-frequency sound waves reflection. It has become a key ofmedical decision-making. The Doppler echography is an ultrasound technique usedto evaluate blood flow through arteries and veins (mainly used for abdomen,legs, arms, neck exams). It is widely used to detect blockages to blood flow(clots), narrowing of vessels, tumors, and vascular malformations. During theultrasound examination, the sonographer handles the transducer (ultrasoundprobe) and carries out the investigation by moving the probe over the patient’sbody. At the same time, the operator monitors the ultrasound station and uses akeyboard to access the software facilities in order to control the image.Several studies conducted in the past decade have highlighted work-relatedmusculoskeletal disorders (WRMD) and repetitive stress injury (RSI) amongsonographers with almost
80% of them suffering ofthose problems. Static and uncomfortable postures are main causes of thoseWRMD, which can lead to pain, sickness, as well as long-term di****lity [2–6].
Robotics 2020,9, 14; doi:10.3390/robotics9010014 http://www.mdpi.com/journal/robotics
Multiple tele-operated solutions have beendeveloped in the past with the aim of providing medical facilities togeographically isolated patients [7–14]. The robotized ultrasound system, using a master-slavearchitecture, constitute a solution to this issue. The master interface isoperated by the sonographer who remotely controls the probe located on theslave robot [7–9]. In the main cases of teleoperation approach, the ultrasound probeis positioned by a robot, with the operator, the robot controller, and anultrasound image processor having shared control over its motion [10–12]. Nowadays, one ofthe commercial solutions has been developed with the aim of reducing WRMD andRSI: Medirob Ergo. This solution, used for cardiac sonography, is based on a6-degrees-of-freedom (DoF) serial robot that moves the ultrasound probe. Theexpert remotely controls the robot using a 3D mouse (generally used for CAD) withoutany haptic feedback on the force applied on the patient. More recently, a softrobotic ultrasound imaging system is used to improve sonographers ergonomics [12] and a bespokerobotic ultrasound manipulator has been designed to ensure the patient’s safety[13].
In this paper, a new solution is developedconsisting of a tele-operated ultrasound solution for Doppler echographyequipped with a collaborative robot as slave and haptic device as masterinterface.
Based on the standard ISO/TS 15066published in 2016 for collaborative robots, safety is the most important issueto guarantee before establishing collaborative tasks between human and robot,where a high risk of collisions between them is palpable and may result inhuman damages. In this context, research efforts are focused on the design ofsolutions to reduce the energy transferred by the robot in case of collision,decreasing the risk of injury for the human [15]. On this way, some basicsolutions have been proposed. For instance, Park et al. introduced the use of aviscoelastic covering in the robot’s body to reduce the impact forces [16]. Fritzsche et al.proposed monitoring the contact forces by providing the robot’s body with atactile sensor used as an artificial skin [17]. Furthermore, several controlapproaches have been proposed to provide the robot with a compliant behaviorwhile it executes a task. These compliant control strategies typically make itpossible to assign a dynamic relationship between the robot and theenvironment, enabling the interaction behavior to be con-trolled by properlyselecting the dynamic parameters [18]. On the other hand, mechanical solutions have also been proposedto provide an intrinsic compliance to the robot, as the one proposed by Wang etal. [13]based on the use of customized spring-loaded ball clutch joint. These solutionsare highly recommended by the standard ISO/TS 15066 as a risk reductionmeasure. Among these compliant mechanisms, the variable stiffness actuators(VSA) allow introducing an intrinsic compliance to the robot joints [19]. These mechanismsare capable of providing adjustable stiffness to the joints, which can beadjusted according to the needs.
The SISCob (Safety Intelligent Sensor forCobot) project, funded by the French National Research
Agency (ANR), aims at developing a newintelligent and modular device mimicking the functions of biologicalarticulations and their synergy for collaborative robots. On this context, anovel safety prismatic compliant joint (PCJ) with variable stiffness has beendeveloped and its behavior in a cobot is presented in this paper. Differenthuman safety criteria are proposed to study the effectiveness of a compliantmechanism, such as the head injury criterion (HIC) [20] or the head impact power(HIP) criterion [21]. In robotics, other safety criteria can also be employed, such asthe measures of robot displacements, e.g., velocities or accelerations, and themeasure of the impact force.
The paper is organized as follows. Section 2 introduces therobot-assisted Doppler sonography. We firstly present the medical gesture studyphase of the sonographer thanks to the motion capture system. The protocolsetup, as well as experimental results are also detailed in the second part ofthis section. The interactions with the patient during echo-Doppler examinationis investigated through effort measurement. In Section 3, we present the structure, aswell as the kinematic model of the PCJ. Curves obtained from both the simulationmodel and the prototype of the PCJ are presented. In Section 4, we describe thedynamic model of a torque-controlled robot coupled with the proposed compliantmechanism, as well as the control approach implemented to execute cartesiantasks. A case study allowing to compare the safety performance of a rigid-bodyrobot vs. a robot using the PCJ is presented in Section 5. Conclusions aboutthe effectiveness of the proposed mechanism in terms of human safety areprovided in the last section.
2. Robot-Assisted Doppler Sonography2.1. Gesture Analysis during Echo-Doppler Examinations
The sonography’s gesture analysis has beenperformed by an angiologist during Doppler ultrasound examinations on realpatients in the same conditions as in medical office. Hence, multiple organshave been scanned: Carotid, legs, and abdomen. The experimental analysis wasmade using a motion capture system (Qualisys) and has been repeated severaltimes. The system uses a set of high-resolution cameras to detect reflectivemarkers to study angiologist gestures (Figure 1). The software Qualisys trackmanager (QTM) allowing us to record, visualize, construct, and export the 3Dposition of each marker has been used. Then, we have recorded the motions ofmarkers while they were making real ultrasound examinations. The frequency ofmotion acquisitions was fixed to a hundred images per second. Our motioncapture method is based on the experience of the biomechanics communityespecially for the choice of marker sets and segment reference definition [22]. Using QTMsoftware, we have been able to reconstruct each motion by regrouping markersinto
“segments” as shown inFigure 1.A segment is defined as a set of markers of the same solid. One of thesesegments is a representation of the ultrasound probe.
Figure 1. Motion capture during sonographyexamination: (Left): Markers locations on angiologist, (right) carotid and legexaminations with the probe reconstruction model with segments.
Multiplereflectivemarkershavebeenplacedontheultrasoundprobe,aswellasontheangiologist’s body to evaluate the positions and orientationsduring the examinations (Figure 2).
Figure 2. Reflectivemarkers on probe.
Based only on posture observations, theexpert is far away from its neutral positions and out of their joint comfortzone. In addition, the gesture during the sonography examinations can bedescribed to be repetitive. All these observations approve the uncomfortablepostures that could cause musculoskeletal disorders.
The gesture analysis using motion capturewill approve the musculoskeletal disorders issue. This study is focused oncompute of the member orientations of the angiologist and mainly the right arm,the pelvis, and the head.
The maximum values of the joint orientationangles are considered, which corresponds to the worst postures. These jointangles are compared to reference angles of comfort zone defined in ISO 11226,ISO 11228-3, and NF EN 1005-4 norms. A sample data on a head rotation and anarm motion are given on Figure 3. These values, joint angles for the neck torsion, and wrist joint,show that the angiologist is all the time out of the comfort zone described bythe norms.
Figure 3. Necktorsion (left) and wrist joint angle (right).
In addition to the gesture, the interactionforce between the probe and the skin has been studied. The forces applied onthe probe during an examination have been evaluated using force sensitiveresistors (FSR) installed on shell, composed of two parts: A fixed one and amoving one, surrounding the probe (Figure 4). One part of the shell istightened to the probe body (no motion allowed). The other part encapsulatesthe probe with a small gap (filled with low-density foam), which allows smallmovements between the two parts. Two FSR sensors are installed between fixedand moving shell in order to evaluate forces applied by the angiologist ontothe patient. The shells have been designed to match the shape of the probeallowing the expert to manipulate it with a similar grasp as the classic probe.
Figure 4. Instrumentedprobe—Force measurement.
The applied force is maximum duringabdominal examination. Indeed, the angiologist has to apply a large force tofind abdominal aorta. Figure 5 shows force measurement during the aorta investigation. The forceapplied depends on the patient, more force is needed to find abdominal aorta ona fat patient, with a large stomach. These patients are those who usually needDoppler examination.
Figure 5. Appliedforce during abdominal examination.
As an outcome of this section, one canconclude that the posture of angiologist experts, studied during a classicalDoppler ultrasound exam by using a motion capture system, are out of comfortzones. The angiologist needs also to apply large forces on the probe in contactwith patient skin. These static postures, out of comfort zones, foster theoccurrence of musculoskeletal disorders.
In the next section, a tele-operated systemis proposed to assist the angiologist in order to prevent and alleviate theseinconveniences.
2.2. Teleoperated System for Doppler Sonography
The robotics team of Prime Institutedevelops teleoperation platforms based on collaborative robots. These platformsare mainly developed for medical applications, such as Doppler sonography orsurgical applications [23].
The tele-operated system for Dopplersonography is composed of slave station and a master station (Figure 6). The master stationconsists of a 6-DoF haptic device composed of a Novint Falcon interface linkedto a virtual probe instrumented with an inertial measurement unit. The slavestation consists of a 7-DoF collaborative robot, i.e., a Franka Emika. This isa torque-controlled collaborative robot including a torque sensor on each jointand whose maximum payload is 3 kg. The slave robot handles an ultrasound probe,linked to a Doppler sonography station. The master device controls motions ofthe slave robot and gives haptic feedback to the angiologist. ROS-basedframework is used to establish the data exchanges between the master device andthe robot.
Figure 6. (a) Tele-operation chain using a6-degrees-of-freedom (DoF) master device and a 7-DoF collaborative slave robot;(b) Doppler ultrasound testperformed by the angiologist using the tele-operation platform.
The proposed master device increases thethree translational DoF of a classic commercial Falcon interface by attaching avirtual probe to the end-effector using a universal joint. The virtual probe,called holder, includes an inertial measurement unit (IMU) allowing to computethe three DoF of rotation. This modification allows making a 6-DoF device withhaptic feedback along the translational axes.
The slave device is a Franka Emika with7-DoF torque-controlled robot useful to coexist with human in a shared commonworkspace.
