Additive Manufacturing for New Robotic Devices
The use of robotic assistance is of increasing interest in interventional MRI. A robotic device can bring accuracy and the ability to perform needle remote manipulation in this highly constrained environment. Obviously, robot-assisted procedures need to be safe for both the patient and the radiologist. From the designer point of view, the introduction of MR-compatible robotic assistance is then a real challenge. Other major design constraints furthermore exist, such as the compatibility of the devices with the imaging device, and their compactness so it can be integrated in a convenient way in the operating room.
We investigate the use of additive manufacturing (i. e. 3D printing) in order to develop devices that overcome such constraints. Our approach is to more specifically consider now available multimaterial additive manufacturing (MMAM). The MMAM process we work with is described here, as well as the design of a multimaterial (MM) compliant joint we introduced called the HSC joint. Using such a joint, new unibody mechanisms can be envisioned, that are presented below. The ongoing research shows that MMAM leads today to design alternatives and robotic architectures of great interest for the field in interventional radiology.
The manufacturing process: Description and properties
In this project, multimaterial additive manufacturing is considered. A commercial process called Polyjet is employed (link), developed by Stratasys, USA. It allows the manufacturing of parts that integrate volumes of different materials with different properties. Therefore, in addition to the freedom of shapes inherent to additive manufacturing processes, it also proposes a freedom in the choice of materials, enlarging the boundaries in terms of possible designs.
In our work, we choose to take advantage of the combination of a rigid material with a rubber-like one. The first one is ensuring the structural stability of the system, while the second one is enabling the use of large deformations with low stiffness in the system. Those two materials have been extensively studied in order to identify their behaviors and use a model of their behavior. Further information can be found in .
The Helical Shape Compliant (HSC) joint
We first focused on the development of a revolute joint. Being compliant, this joint requires no assembly and can therefore be easily integrated in a structure in order to obtain a unibody mechanism. The application field imposes mechanisms with large ranges of motion and low rotation stiffness. A volume of rubber-like material is introduced in the center of the joint, combined with an helical shape. This particular shape is ensuring that for every force exerted on the moving volume, at least two regions of rubber-like material are submitted to compression. As a result, the stiffness is improved because of the incompressible behavior of this material. In the end, this particular design is showing the following properties:
- A large and controllable range of motion (up to 180°) with physical stops
- A low rotational stiffness combined with improved ones in the other directions
- The absence of backlash, wear and friction because of the compliant feature
- A good compactness regarding our application area.
The enclosed video is providing more details about the particular geometry, as well as a caption of a prototype in movement.
Because of its compliant nature, this joint is presenting an internal rotational stiffness. We have investigated compensation strategies. The proposed solution is employing a pre-stressed volume of rubber-like material as a spring in order to counterbalance the internal stiffness of the HSC joint . A significant reduction has been observed experimentally, as shown in the enclosed video. Moreover, the proposed solution is compatible with an integration in a unibody structure.
From the HSC joint to unibody robotic systems
With the considered manufacturing process, several HSC joints can easily be combined and integrated in a structure in order to create a mechanism. Based on the requirements, the development of this mechanism is achieved using a CAD model. Its manufacturing is then straightforward since no assembly is required: « Click and Print ».
For our application, a so-called Remote Center of Motion mechanism has been designed. Its purpose is to kinematically ensure the rotation of the needle around its insertion point, while having a safe zone around this point for tissue integrity. The proposed mechanism is based on two arms placed in a parallel way. Each arm is composed of a parallelogram structure. Cables are used to perform a remote actuation in order to ensure MR compatibility. As demonstrated in the enclosed video, by using MMAM, we have developed a system that is:
- Fully compatible with any imaging device: MRI, Xray…
- Compact and light enough to be patient-mounted (100g, 7cm height and 10cm of base diameter)
- With larges ranges of motion
- The 3D-printer is funded by the Investissements d’Avenir program (Labex CAMI & Equipex ROBOTEX) under references ANR-10-EQPX-44 and ANR-11-LABX-0004.
- PhD grant from French ministry of research (A. Bruyas, 20012-2015)
- A. Bruyas, F. Geiskopf, L. Meylheuc, P. Renaud, Combining Multi-material Rapid Prototyping and Pseudo-Rigid Body Modeling for a New Compliant Mechanism, In proceedings of the IEEE International Conference on Robotics and Automation (ICRA), Hong Kong, 2014. FINALIST FOR THE BEST AUTOMATION PAPER AWARD.
- A. Bruyas, F. Geiskopf, P. Renaud, Towards Statically Balanced compliant Joints using Multimaterial 3D Printing, In proceedings of the ASME International Design Engineering Technical Conferences (DETC2014-34532), Buffalo, NY, USA, 2014. BEST INTERACTIVE PRESENTATION AWARD.
- A. Bruyas, F. Geiskopf, P. Renaud, Design and Modeling of a Large Amplitude Compliant Revolute Joint: the Helical Shape Compliant Joint, ASME Journal of Mechanical Design, 2015, to appear.
- National TV channel ARTE Future, presentation of the project: ("Potential of 3D printing in the medical field") (In French)