Design and Implementation of a Bespoke Robotic Manipulator for Extra-corporeal Ultrasound

Summary

This paper introduces the design and implementation of a bespoke robotic manipulator for extra-corporeal ultrasound examination. The system has five degrees of freedom with lightweight joints made by 3D printing and a mechanical clutch for safety management.

Abstract

With the potential for high precision, dexterity, and repeatability, a self-tracked robotic system can be employed to assist the acquisition of real-time ultrasound. However, limited numbers of robots designed for extra-corporeal ultrasound have been successfully translated into clinical use. In this study, we aim to build a bespoke robotic manipulator for extra-corporeal ultrasound examination, which is lightweight and has a small footprint. The robot is formed by five specially shaped links and custom-made joint mechanisms for probe manipulation, to cover the necessary range of motion with redundant degrees of freedom to ensure the patient's safety. The mechanical safety is emphasized with a clutch mechanism, to limit the force applied to patients. As a result of the design, the total weight of the manipulator is less than 2 kg and the length of the manipulator is about 25 cm. The design has been implemented, and simulation, phantom, and volunteer studies have been performed, to validate the range of motion, the ability to make fine adjustments, mechanical reliability, and the safe operation of the clutch. This paper details the design and implementation of the bespoke robotic ultrasound manipulator, with the design and assembly methods illustrated. Testing results to demonstrate the design features and clinical experience of using the system are presented. It is concluded that the current proposed robotic manipulator meets the requirements as a bespoke system for extra-corporeal ultrasound examination and has great potential to be translated into clinical use.

Introduction

An extra-corporeal robotic ultrasound (US) system refers to the configuration in which a robotic system is utilized to hold and manipulate a US probe for external examinations, including its use in cardiac, vascular, obstetric, and general abdominal imaging¹. The use of such a robotic system is motivated by the challenges of manually holding and manipulating a US probe, for instance, the challenge of finding standard US views required by clinical imaging protocols and the risk of repetitive strain injury^2,3,4, and also by the needs of US screening programs, for instance, the requirement for experienced sonographers to be on-site^5,6. With emphases on different functionalities and target anatomies, several robotic US systems, as reviewed in earlier works^1,7,8, have been introduced since the 1990s, to improve different aspects of US examination (e.g., long-distance teleoperation^9,10,11,12, as well as robot-operator interaction and automatic control)^13,14. In addition to the robotic US systems used for diagnostic purposes, robotic high-intensity focused ultrasound (HIFU) systems for treatment purposes have been widely investigated as summarized by Priester et al.¹, with some recent works^15,16 reporting the latest progress.

Although several robotic US systems have been developed with relatively reliable technologies for control and clinical operation, only a few of them have been successfully translated into clinical use, such as a commercially available tele-ultrasound system¹⁷. One possible reason is the low level of acceptance for large-size industrial-looking robots working in a clinical environment, from the point of view of both patients and sonographers. Additionally, for safety management, the majority of the existing US robots rely on force sensors to monitor and control the applied pressure to the US probe, while more fundamental mechanical safety mechanisms to limit the force passively are usually not available. This may also cause concerns when translating into clinical use as the safety of robot operation would be purely dependent on electrical systems and software logic.

With the recent advancements of 3D printing techniques, specially shaped plastic links with custom-made joint mechanisms could provide a new opportunity for developing bespoke medical robots. Carefully designed lightweight components with a compact appearance could improve clinical acceptance. Specifically for US examination, a bespoke medical robot aimed at being translated into clinical use should be compact, with enough degrees of freedom (DOFs) and range of motion to cover the region of interest of a scan; for example, the abdominal surface, including both the top and sides of the belly. Additionally, the robot should also incorporate the ability to perform fine adjustments of the US probe in a local area, when trying to optimize a US view. This usually includes tilting movements of the probe within a certain range, as suggested by Essomba et al.¹⁸ and Bassit¹⁹. To further address the safety concerns, it is expected that the system should have passive mechanical safety features which are independent of electrical systems and software logic.

In this paper, we present the detailed design and assembly method of a 5-DOF dexterous robotic manipulator, which is used as the key component of an extra-corporeal robotic US system. The manipulator consists of several lightweight 3D-printable links, custom-made joint mechanisms, and a built-in safety clutch. The specific arrangement of the DOFs provides full flexibility for probe adjustments, allowing easy and safe operations in a small area without colliding with the patient. The proposed multi-DOF manipulator aims to work as the main component that is in contact with patients and it can be simply attached to any conventional 3-DOF global positioning mechanism to form a complete US robot with fully active DOFs to perform a US scan.

Protocol

1. Preparation of Each Link, End-effector, and Additional Components

1. Print all the links (L₀, L₁, L₂, L₃, and L₄) and the end-effector as shown in Figure 1, with acrylonitrile butadiene styrene (ABS) plastic, polylactic acid (PLA) plastic, or nylon, using a 3D-printing service. Use the .STL files provided in the Supplementary Materials when printing.
   NOTE: Changes in shape and scale of each part can be made based on the provided files. The inner profile of the end-effector can be changed to fit different US probes.
2. Print all the required additional components as shown in Figure 2 in nylon, using a 3D-printing service. Refer to the Table of Materials for the required number of each component. Use the .STL files provided in the Supplementary Materials when printing.
3. Polish all the printed plastic parts with polishing tools if necessary. Remove any supporting materials left from 3D printing, if necessary.
   NOTE: Some structures in the provided end-effector design are for a force sensor, which is not a part of the protocol reported here and will not be used for the assembly. The force sensor design concept has been reported in previous work²⁰; thus, it is not covered in this paper.

2. Assembly of Joint 1

NOTE: The assembly of joint 1 (J₁) is based on Figure 3.

1. Place the four small, geared stepper motors (with 20-teeth spur gears attached) into the mounting cavities of L₀ and mount them with screws.
2. Place the two 37 mm OD bearings into the bearing housings of L₀ and secure the 120-teeth spur gear (Type A) onto the hexagon key of L₁.
3. Insert the shaft on L₁ into the shaft hole on L₀ with the four small driving spur gears and the large, driven spur gear engaged, and assemble the shaft collar to secure and retain the shaft.

3. Assembly of Joint 2

NOTE: The assembly of joint 2 (J₂) is based on Figure 4.

1. Place the four small, geared stepper motors (with 20-teeth spur gears attached) into the mounting cavities of L₁ and mount them with screws.
2. Attach the two 120-teeth spur gears (Type B) to the two 37 mm OD bearings and position them into the gear cavities of L₁, with the 120-teeth spur gear (Type B) engaged with the 20-teeth spur gears mounted on the motors. Unscrew and re-screw the motor if necessary to allow the easy positioning of the two 120-teeth type-B spur gear.
3. Align L₁ and L₂ and insert the bearing and the ball-spring pairs into the clutch holes in L₂. With the two round clutch covers aligning and pushing the spring into the clutch mechanism for preloading, insert an M6 bolt into the bores of L₁ and L₂.
4. Rotate the assembly to the other side and repeat steps in 3.3 for this side. Secure the assembly by attaching a nut to the M6 bolt.

4. Assembly of Joint 3

NOTE: The assembly of joint 3 (J₃) is based on Figure 5.

1. Place the two small, geared stepper motors (with 20-teeth spur gears attached) into the mounting cavities of L₂ and mount them with screws.
2. Place the 37 mm OD bearing into the bearing housing of the 120-teeth spur gear (Type C) and place the 32 mm OD bearing into the bearing housing of L₃.
3. Secure the large spur gear into the hexagon keyhole of L₃ (additional screws can be used if necessary) and insert the shaft on L₂ into the bores on the large spur gear and L₃, with the small and the large spur gears engaged.

5. Assembly of the Driving Mechanism of Joint 4

NOTE: The assembly of joint 4 (J₄) is based on Figure 6.

1. Place the two small, geared stepper motors into the mounting cavities of L₃ and mount them with screws. Place the 8 mm OD bearings into the bearing housings of L₄.
2. Mount the 20-teeth long spur gear onto the two small stepper motors.

6. Assembly of the Driven Mechanism of Joint 4 and Joint 5

NOTE: The assembly of joint 4 (J₄) is based on Figure 6 and joint 5 (J₅) is based on Figure 7.

1. Position the driven 144 teeth bevel gear onto the extrusion of L₄.
2. Place the two small, geared stepper motors (with 18-teeth bevel gears attached) into the mounting cavities of L₄ and mount them with screws. Finally, insert the M5 shaft into the shaft hole of L₃ and L₄ after the two links are aligned. Ensure the built in driven gear structures on L₄ matches with the 20 teeth long spur gear.
3. Insert the end-effector into the keyway of the large bevel gear and vertically position the end-effector with the end-effector collar screwed onto it. 

Results

Following the protocol, the resulting system is a robotic manipulator with five specially shaped links (L₀ to L₄) and five revolute joints (J₁ to J₅) for moving, holding, and locally tilting a US probe (Figure 8). The top rotation joint (J₁), with gear mechanisms actuated by four motors, can rotate the following structures 360°, to allow the US probe to point toward different sides of the scanning area, s...

Discussion

Unlike many other industrial robots that have been translated into medical applications, the proposed robotic manipulator described in the protocol was specifically designed for US examinations according to clinical requirements for the range of motion, application of force, and safety management. The lightweight robotic manipulator itself has a wide range of movements sufficient for most extra-corporeal US scanning, without the need for large movements of the global positioning mechanism. As the closest mechanical struc...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
3D-printed link L0	3D printing service	1	As shown in Figure 1, with the STL file provided
3D-printed link L1	3D printing service	1	As shown in Figure 1, with the STL file provided
3D-printed link L2	3D printing service	1	As shown in Figure 1, with the STL file provided
3D-printed link L3	3D printing service	1	As shown in Figure 1, with the STL file provided
3D-printed link L4	3D printing service	1	As shown in Figure 1, with the STL file provided
3D-printed end-effector	3D printing service	1	As shown in Figure 1, with the STL file provided
20-teeth spur gear	3D printing service	12	0.5 module, 5 mm face width, with mounting keyway, as shown in Figure 2, with the STL file provided
18-teeth bevel gear	3D printing service	2	0.5 module, 5 mm face width, with mounting keyway, as shown in Figure 2, with the STL file provided
120-teeth spur gear (Type A)	3D printing service	1	0.5 module, 6 mm face width, with mounting keyway, bearing housing, and bore, as shown in Figure 2, with the STL file provided
120-teeth spur gear (Type B)	3D printing service	2	0.5 module, 6 mm face width, with detent holes, bearing housing, and bore, as shown in Figure 2, with the STL file provided
120-teeth spur gear (Type C)	3D printing service	1	0.5 module, 6 mm face width, with mounting key, bearing housing, and bore, as shown in Figure 2, with the STL file provided
20-teeth long spur gear	3D printing service	1	0.5 module, 21.5 mm face width, with mounting keyways, as shown in Figure 2, with the STL file provided
144-teeth bevel gear	3D printing service	1	0.5 module, 7 mm face width, with mounting keyways, as shown in Figure 2, with the STL file provided
Bearing (37 mm O.D and 30 mm I.D)	Bearing Station Ltd., UK	5	Bearing size and supplier can be varied
Bearing (12 mm O.D and 6 mm I.D)	Bearing Station Ltd., UK	2	Bearing size and supplier can be varied
Bearing (32 mm O.D and 25 mm I.D)	Bearing Station Ltd., UK	1	Bearing size and supplier can be varied
Bearing (8 mm O.D and 5 mm I.D)	Bearing Station Ltd., UK	2	Bearing size and supplier can be varied
Plastic/metal shaft (6 mm O.D, 70 mm long)	TR Fastenings Ltd., UK	1	e.g. Could be an M6 bolt and a nut
Plastic/metal shaft (5 mm O.D, 70 mm long)	TR Fastenings Ltd., UK	1	e.g. Could be an M5 bolt and a nut
Ball-spring pairs	WDS Ltd., UK	4	Numbers of ball-spring pairs could varied to adjust the triggering force of the clutch
Clutch covers	3D printing service	2	104 mm O.D, 5mm face width, 6 mm bore, as shown in Figure 2, with the STL file provided
3D-printed shaft collar	3D printing service	1	35 mm O.D and 30 mm I.D, 8mm face width, as shown in Figure 2, with the STL file provided
3D-printed end-effector collar	3D printing service	1	As shown in Figure 2, with the STL file provided
Small geared stepper motors	AOLONG TECHNOLOGY Ltd., China	14	Part number: GM15BYS; Internal gear ratio 232:1 or 150:1, all acceptable