Design of a portable control platform for rod-driven continuum parallel robots

This project designed and implemented a portable, modular and scalable platform together with a control system. A G-code specification was created for programming the robot, along with a command-line application to interact with the system. A versatile mounting solution and the linear actuators that move the rods were also developed.

Context

Parallel robots are characterized by multiple closed kinematic chains that connect and move mobile platforms, in contrast to serial robots, which operate with an open kinematic chain. Recent robotics research is exploring new alternatives in parallel robots, pushing the limits of the field and opening up new possibilities.

Continuum robots in particular stand out for their lightness and flexibility, and are able to move into new fields such as medicine or the exploration of hard-to-reach places. These robots are classified by their design, distinguishing between extrinsic and intrinsic actuation. Extrinsic actuation transmits motion from the base of the robot along its structure, while intrinsic actuation uses actuators within the robot's structural frame to produce motion. Rod-driven robots are a notable example of extrinsically actuated robots.

Current challenges in developing continuum parallel robots include actuator miniaturization, integration with rigid robots and the exploration of smart materials, among others. Modeling efforts focus on representing interaction environments, improving real-time implementations and standardizing simulation environments. Current control challenges include ensuring precise manipulation and adapting to dynamic environments through advanced sensing technology and adaptive strategies.

Structure of rod-driven robots

Rod-driven continuum parallel robots are a relatively new class of robots. To design the platform, it was necessary to bound the dimensions and general structural characteristics of these robots. The target robot is a smaller-scale version of a design previously developed by other researchers, chosen as a reference for its versatility, simplicity and wide range of motion.

The most common materials for this type of robot are fiberglass rods and steel wires. These materials are used because of their high stiffness and their ability to withstand significant bending without plastic deformation or fracture. Steel alloys such as AISI 302, commonly used in springs, exemplify these properties. This was the material chosen, because it is commercially available and, despite its fairly high modulus of elasticity, large deflections can be reached with the diameters in which it is usually sold.

The robot consists of two segments, both made of wires, vertebrae and joints between the vertebrae and the wires. The joints are closed and cylindrical, and besides connecting the components they also constrain the motion of the rods so the robot can be controlled. Like the reference model, the structure has the shape of a truncated cone, which provides stability and stiffness.

Linear actuators

One characteristic of parallel robots is their use of linear actuators. In fact, that is where their name comes from, and this robot shows it clearly, since it moves through linear actuators arranged in parallel: three per segment, one for each wire. However, commercial linear actuators have a fixed, non-adjustable stroke, which is a major limitation when building a portable, flexible platform for different variations of rod-driven robots. Fortunately, continuum robots open up a new range of possibilities, since what is actually needed is to move a wire linearly, which is very similar to what a 3D printer extruder does.

A linear actuator was developed, inspired by the drive mechanism of 3D printer extruders but with smaller dimensions. This model was designed to be manufactured by 3D printing. The design also took into account the geometric constraints needed to miniaturize the actuators. A DC gear micromotor with a Hall-effect magnetic encoder was used to count and control its revolutions.

Platform

The platform assembly integrates an array of actuators into a frame that holds the structure and the electronics needed to control the platform. The frame has six actuators and a hexagonal acrylic base, and the control unit is housed in a box. It is 30 cm tall and the whole platform weighs about 1 kg.

The electronics box uses an Arduino MEGA 2560, with the connections made on a breadboard. It has a power plug, a serial port, an emergency stop button, a power button, a 5-pin connector for a joystick and two IDC ports to connect the unit to the linear actuators. The microcontroller receives commands from a computer over the serial port and is powered by a 9 V supply connected to the H-bridges, which in turn power the motors and control their polarity and speed.

Control system interface

One of the project's goals was to devise a protocol for controlling the robot. Although the robot has a joystick input, this would not be enough to control the robot in three-dimensional space. Moreover, the platform was conceived to adapt to any user requirement, which highlights the need for such a protocol. The control system is direct, meaning that the distance or position of each actuator is specified. A G-code-based specification was developed for sending commands, since G-code is a widely used industry standard that also suits the platform's direct control, adaptability and simplicity.

Tests were carried out to determine the transfer function and tune the PID controller used to control the position of the linear actuator motors. In addition to the microcontroller firmware, a command-line application was developed to communicate with the platform and send commands in real time, either manually or from a G-code instruction file. This way, users can program the robot and adapt it to their needs.

Results and conclusions

Developing the robotic platform brought together mechanical design, control theory and software development. Testing showed that the design and prototype could lay the groundwork for experimenting with and controlling rod-driven continuum parallel robots. Some difficulties arose during construction, but the main goal of designing a flexible, programmable robotic system was achieved.

The difficulties in fabricating the linear actuators highlighted the need for a precise manufacturing method and iterative design improvements. The fabrication method used, additive manufacturing by 3D printing, introduced some manufacturing defects that affected the quality and performance of the linear actuators. This should be taken into account in future iterations aimed at optimizing the performance and reliability of these actuators. The electronics box should also be improved, since a breadboard and jumper wires are not the most suitable choice for a reliable, robust control unit: they are error-prone and hard to assemble.

The analog control method using a joystick was not implemented, and some firmware features were not fully completed. However, the established architecture provides a solid foundation for extending functionality in both software and hardware. The modular nature of the system makes it easy to add sensors, actuators and other peripherals. The firmware was designed so that new commands and features can be added with minimal reconfiguration. This built-in flexibility is expected to support future developments and applications.