Isira Naotunna, Master Thesis

Advisor: Assoc. Prof. Theeraphong Wongratanaphisan

Description of Project:

Researchers have been developing fully autonomous robotic systems that can perform specific tasks while safely interacting with an unknown environment. Such mobile manipulator systems are very flexible in different environments to perform various activities such as; logistics and object handling in a manufacturing environment, fruit harvesting in the agriculture industry, elderly care, and domestic services, military, etc. This research will develop and program a mobile manipulator robot that exhibits autonomous and intelligent behavior to perform a fetch and carry task.

The proposed system has three main sub-systems; i.e, manipulator system (Baxter), the mobile robot, and the Vision system. All these systems will be communicated through the Robot Operating System by connecting to a host computer. These tasks were achieved by a comprehensive study on robot kinematics, computer vision, object detection, navigation, localization, and path planning.

To validate the system, an experimental setup was arranged to perform the fetch and carry task, using a box with dimensions in the range of maximum grasping width 155cm. In this particular setup, the robot navigates through the various paths of the constructed map while searching for the target box. Once the target box is identified, the Baxter robot reaches the target box and picks it up using its left arm. Finally, the robot arrives at the goal position and places the box at the goal location.

Hardware Development

Baxter mobile manipulator system

The overall system proposed in this research consists of a Baxter dual-arm mobile manipulator system that mounts on a differential drive two-wheeled mobile robot. Baxter robot is approximately 3 feet tall, and weighs approximately 75Kg and has two 7-DOF arms attached to it. Baxter has a vacuum gripper in the left arm and a parallel gripper in the right arm. Angle position and joint torque sensing are available with both arms. There are three integrated cameras located at the end effectors of each joint and the robot head.

The mobile robot system is controlled by Arduino MEGA 2560, which is communicated with the NVidia TX1 via the ROS Serial package. The system is driven using a couple of MY1016Z3 brushed DC motors, each controlled separately using ELMO-VIO 25/60 motor control units. The motor power is transmitted to the wheels via chain drives with the speed ratio of the motor to wheel 4.9:1. Since the motor controller requires an analog control signal to perform smooth motor controlling, MCP4922 Digital to Analog Converter (DAC) is used to generate the analog control signal from Arduino. The converter establishes an SPI communication between itself and Arduino with a frequency of 10MHz. Motor feedback is acquired using AMS’s ASP5147P absolute magnetic encoders communicate with Arduino via Serial Peripheral Interface (SPI) with the frequency of 1MHz. Readings from the encoder are recorded every 1ms using the Arduino timer 1 interrupts. Both the encoders and the Digital to the analog converter use the serial peripheral interface to communicate with the Arduino.

The vision system uses an Intel RealSense D435i depth camera and an Intel RealSense T265 tracking camera. In addition to that, Baxter’s right arm camera is used for the object manipulation process. Both RealSense cameras are mounted at the front frame of the mobile base. The two RealSense cameras are connected to the Nvidia TX1 via a USB hub, and Intel RealSense Linux drivers are installed to Nvidia TX1 to communicate with the cameras. Since all the vision tasks related to autonomous navigation and visual servoing will be carried out in the ROS environment, RealSense ROS wrapper libraries are used to collaborate RealSense cameras with ROS Simultaneous Localization and Mapping (SLAM) and navigation.

The entire system is operated using a 12V DC voltage that is powered by two 12V lead-acid batteries. Although the system runs on DC voltage, an AC voltage is required to operate the Baxter robot. It is achieved using a MeanWell TS1000 inverter.

Software Development

A cluster of software run inside the Nvidia TX1 development board along with an Ubuntu 18.04 operating system and a set of Jetpack 4.2 drivers installed. To operate the system, the Nvidia TX1 has to establish communication within all three subsystems simultaneously. ROS melodic software is used for this purpose. Baxter and Arduino SDKs are installed into the ROS workspace for programming and control. The vision system is communicated through RealSense SDK 2.35 and RealSense ROS wrapper.

Methodology

SLAM and Navigation

The mobile manipulator system is developed to autonomously navigate through a map while searching the target and transport the object to the desired goal location. Concept of the Simultaneous Localization and Mapping (SLAM) and navigation techniques are used to perform the autonomous navigation task. In this research, the ros_rtabmap package is used to perform the SLAM task.

Real-Time Appearance Based mapping which is shortly known as the RTAB-Map is a graph-based SLAM approach. In this approach, the map generation and the localization are done using the data collected from the D435i depth camera. RTAB-Map uses a process called loop closure detection to perform the localization by determining whether the robot has seen a location before based on the vision data saved in the RTAB-Map database. When the robot travels to alien areas in its environment, the map is expanded, and the number of images in the database increases. This causes the loop closure to take a long time along with the complexity increasing linearly. Therefore, RTAB-Map is optimized to handle the large data set to perform long-term SLAM by using the front-end and back-end SLAM approach. RTAB-Map can be used to generate a 3D map of the environment using 3D point cloud data and the RGBD data provided by the D435i depth camera, and odometry data provided by the T265 tracking camera. After creating the map using RTAB-Map, a “map server” package is used to save the data of the 2D map to be used for navigation.

ROS Navigation stack configuration

Navigation

Map-based navigation is done using a configured ROS navigation stack according to the system requirement. The RealSense D435i camera is the main observation source of the navigation system. The RGBD data is used to perform the localization with the RTAB-Map while obstacle detection is done using the point cloud data and the fake laser scan data generated from the depth images of the D435i camera.

Localization and Path planning

RTAB-Map provides the localization during the navigation by finding the loop closure. The loop closure detector uses a bag-of-words approach to determine if a new image detected by the D435i camera is from a new location or from a location that has been already visited. This is done by matching the real-time image data with the image data saved in the rtabmap.db database during the map construction. Once a loop closure is found, the odometry is corrected and the real-time point cloud data is aligned to the map.

Path planning contains two sublevels known as the global planner level and the local planner level. For the global planner, the ROS global_planner library is used. The global_planner library is developed based on Dijkstra’s and A* algorithms. By experimenting, the TEB (Timed Elastic Band Approach) local planner is selected as the local planner of the navigation stack of the developed mobile manipulator system.

Development of Mobile Manipulation Strategy and system operation

The Aruco marker-based mobile manipulation technique is used in this system. Aruco marker is a binary square with a black background and a white generated pattern that uniquely identifies it. In this experiment, the Aruco markers are placed on the top and the front side of each box. Using these Aruco markers the position and orientation of the target box location are calculated. To detect the markers and estimate the pose a ROS python library is implemented based on the OpenCV Aruco library with the cv_bridge ROS package.

During the mobile manipulator system operation, the mobile manipulator navigates through the map by following the pre-defined waypoints on the map. When the robot reaches a pre-defined waypoint in the map, the robot starts to search the Aruco markers using the D435i camera. Once the marker is detected the robot reaches the target location and picks the object based on the aruco marker pose estimation. Then, the robot carries the object to the final goal location and place it at the final goal location. Finally, the robot reaches the initial position and terminates the operation.

Conclusion

• This system can be introduced as a reasonable and straightforward solution for basic mobile manipulation tasks in various industrial and domestic environments.
• Poor lighting conditions may cause a problem for the localization.
• In an empty area, the system cannot perform a precise localization due to the lack of data provided by the 3D camera to the RTAB-Map node. . However, the odometry provided by the RealSense T265 tracking camera reduces this error.
• The system cannot deal with dynamic obstacles present on the left, right, and rear sides of the robots. It may cause unnecessary collisions.

Future Works

• Introducing laser scanners will improve system navigation within a small area.
• The system can be further developed by introducing complex object manipulation techniques such as grasp planning, redundancy avoidance, and complex geometry manipulation.
• Introducing a close kinematic chain for this system will improve the system’s efficiency, allowing it to perform more sophisticated tasks.
• By introducing a more powerful computer to the system, the Aruco detection and the navigation can be performed at the same time which will reduce the total operation time thus increasing efficiency.

Acknowledgment:

This research was financially supported by The Royal Golden Jubilee Ph.D. Program (RGJPHD) Scholarship no. PHD/0101/2552 1.M.CM/52/G.1. The work was performed at the Motion and Control Laboratory, Chiang Mai University, Thailand.

Publication

Chawalit Khanakornsuksan and Theeraphong Wongratanaphisan ”Optimal Design of a Hybrid Cable Driven Parallel Robot for Desired Trajectory and Force”, on 2018 International Conference on Mechatronic, Automobile, and Environment Engineering (ICMAEE 2018), Chiang Mai, Thailand, 7-9 July 2018

Chawalit Khanakornsuksan and Theeraphong Wongratanaphisan ”Design Optimization of a Suspended Cable-Driven Gait Generator using Three Cables”, on Joint Symposium on Mechanical-Industrial Engineering and Robotics (MIER 2017), Chiang Mai, Thailand, 16-17 November 2017

Chawalit Khanakornsuksan , Pinyo Puangmali and Theeraphong Wongratanaphisan,”Design of a Hybrid Cable- driven Foot Platform Device”, proceeding on The  5^th TSME International Conference on Mechanical Engineering, Chiang Mai, Thailand, 17-19 December 2014

Chawalit Khanakornsuksan, Ph.D. student

Assoc. Prof. Theeraphong Wongratanaphisan

Description of Project:

Parallel Kinematic Manipulators (PKM) have gained wide attention over past decades. Many researchers have developed PKMs to overcome some limitation of traditional serial robots. However, there are major drawbacks of PKMs such as small workspace, low dexterity and singularities. These are challenging problems that need to be addressed during the design. Cable Driven Parallel Robots (CDPRs) are a special class of PKMs whose actuated limbs are cables instead of rigid-linked actuators. All cable are attached to a mobile platform which hosts robot’s end-effector and the other ends are wound in fixed reels installed on a fixed base. Positions and orientations of the mobile platform can be attained by controlling the length of those actuating cables. The benefits of the cable actuation are its simple form of producing force, light weight, and relatively large actuating range resulting in large workspace. One of the promising applications of CDPRs is for the design of locomotion device as it can provide wide motion range with very light weight.

Objective:

As the cable-driven parallel robot has great potential for use in many applications, this study aim explorer that possibility. The purpose of the study is two fields:

1. To introduce the new gait mechanism concept which is driven by cables in an end-effector type configuration. Conceptually, the user places their feet on a moving platform which is driven by a number cables during the gait motion. This machine can generate 3 DOF motion of the moving platform: ( x- and y- position and orientation in a vertical plane). The robot would be designed to suit rehabilitation task which is to produce a smooth gait motion for user with different physical parameter such as height and weight.
2. Derive a method to optimally design the cable-driven gait generating mechanism. The outcome of study should allow us to indicate, in an optimal sense, what the locations of the cable attachment point on the moving platform and ground should be. The design targets are defined in terms of reducing cable tensions under applied external forces from user weight during the motion. Uncertainties due to external forces applied on the platform will be taken into account.

Scope of the study

1. Analyze the gait motion and propose a suitable configuration of cable-driven gait generating mechanism which will be a platform for study. The study focuses on a non-redundant type cable-driven robot.
2. Optimally determine the location of cable attachment points using PSO optimization technique.
3. The motion of the mechanism is confined to a vertical plane.

Assumptions

1. The weight of the cable will be neglected. Hence it will be assuming in the kinematic and force analyses that the cables are adjustable length linear elements.
2. The moving platform is considered as a rigid body.

Optimal Design of a Hybrid and Fully Cable Driven Parallel Robot

In this thesis, two new automated gait mechanisms based on a cable-driven parallel kinematic mechanism are introduced: 1) a hybrid cable driven robot with 2 cables and one rigid moving link and 2) a 4-cable planar parallel robot (4CDPPR). Both mechanisms can generate three degrees of freedom (DOF) gait pattern motion of a moving platform (two translational and one rotational motion in a vertical plane) for normal walking gait.

The first proposed mechanism, called the “hybrid cable driven planar parallel robot (HCDPPR)” , Fig. 1, was designed with two cable and one rigid link in order to maintain the number of degrees of actuation equal the number of degrees of freedom. The cables were designed to take only the tension force and the rigid link can take both tension and compression loads. A method was proposed to optimally locate the cable attachment points on ground and moving platform while keeping the cable tensions minimized in optimal sense. With the current design configuration, the results suggest that the cable attachment points on the ground should be at the waist level and the cable attachment points to the foot support should be below the joint of the connecting rod. In this configuration, constraints on optimized parameters were also be imposed in order to avoid cable and user interference.

The second proposed mechanism employed only cables as actuating members. A method for the gait generating dimensional synthesis of cable driven planar parallel robots mechanism was proposed, Fig. 2. Two configurations have been considered. Firstly, a suspended 3-cable mechanism which makes use additional force from gravity as a virtual cable downward force. To optimally locate the attachment point, Particle Swarm Optimization (PSO) algorithm has been used to minimize cable tensions. The results showed that the optimal cable attachment locations are usually very high above ground which made structure of the mechanism very large and unrealistic. Due to the limit of space for installation, the range of cable attachment parameters are reduced to realistic values. In addition, to avoid body interference, the safety boundary constraints are included in the optimization algorithm.

Realistically, the mechanism must be designed to take into account the range of unpredictable external forces that can act on the platform. The concept of graphical representation by polygon is applied to optimization algorithm to find the minimal polygon’s shape which can cover the desired wrench. The 4CDPPR system was used to show the behavior of the algorithm concept. For the whole range of motion, two wrench boxes were used to represent two different level of external force range (high and low) separated by two regions. The high-force region corresponds to the stance phase and the low force region represents the swing phase. In the optimization process we first randomly select the optimized parameters that a wrench closure workspace (WCW) exists. This will ensure that solutions for the polyhedra to enclose the wrench boxes can be found, Fig. 3. Then the PSO algorithm, Fig. 4, was set to begin finding the optimal parameters. It was shown that the proposed method that solutions in wrench feasible workspace (WFW) may be found by relaxing the size of the polyhedra. The size of polyhedra is related to the magnitude of maximum cable tension used in the objective function. By reducing the size of the feasible polyhedra, the optimal tension force can be obtained. Finally, our intuition is that the same approach can be applied to the dimensional synthesis of other prescribed trajectories as well as fully constrained systems with arbitrary number of cables.

Acknowledgment:

This research was financially supported by The Royal Golden Jubilee Ph.D. Program (RGJPHD) Scholarship no. PHD/0101/2552 1.M.CM/52/G.1. The work was performed at the Motion and Control Laboratory, Chiang Mai University, Thailand.

Publication

Chawalit Khanakornsuksan and Theeraphong Wongratanaphisan ”Optimal Design of a Hybrid Cable Driven Parallel Robot for Desired Trajectory and Force”, on 2018 International Conference on Mechatronic, Automobile, and Environment Engineering (ICMAEE 2018), Chiang Mai, Thailand, 7-9 July 2018

Chawalit Khanakornsuksan and Theeraphong Wongratanaphisan ”Design Optimization of a Suspended Cable-Driven Gait Generator using Three Cables”, on Joint Symposium on Mechanical-Industrial Engineering and Robotics (MIER 2017), Chiang Mai, Thailand, 16-17 November 2017

Chawalit Khanakornsuksan , Pinyo Puangmali and Theeraphong Wongratanaphisan,”Design of a Hybrid Cable- driven Foot Platform Device”, proceeding on The  5^th TSME International Conference on Mechanical Engineering, Chiang Mai, Thailand, 17-19 December 2014

Pongsiri Kuresangsai, Ph.D. student

Advisor: Prof. Dr. Matthew O.T. Cole

Description of Project:

Conventional mechanisms, used to transfer energy, forces or generate desired motion, are constructed by connecting links using joints with rolling or sliding parts. This introduces effects such as backlash and friction, which can compromise the accuracy at micrometer scales. To overcome these effects, monolithic mechanisms may be constructed having compliant flexure-based joints that can achieve motion with higher repeatability and fidelity, even to sub-micron levels. Such compliant mechanisms have a number of other advantages. They require a reduced number of parts and can be made by single-process manufacturing such as 3-D printing or electric discharge machining. Constructing a mechanism as a single piece can also reduce time and costs. Flexure-based joints suffer less from wear or contamination and do not require lubrication. Even though flexure-jointed mechanisms can give high accuracy motion, the range of motion is limited by the elastic limit of the joints under deflection. Also the dynamic and kinematic behavior is more complex. For such mechanism, the analysis and optimization of designs is challenging.

Objective:

1. To develop and verify non-linear dynamic models of the proposed mechanism
2. To consider the design and application of controllers based on linearized models of the mechanism
3. To investigate enhancing dynamic performance by utilizing non-linear dynamic models of the mechanism within the controller design procedure
4. To implement and test controllers on the experimental system to verify and compare performance

Research overview

Many researchers have considered the design of flexure-based mechanism having workspace in micrometer scale. For widening the range of applications of flexure mechanism, we focus on designing flexure mechanism having large workspace in cm-scale but with compact mechanism that can operate at high accuracy for a large number of motion cycles.

A case study was undertaken involving the optimized design of a mechanism inspired by a having conventional five-bar linkage 2 DOF in XY plane and with 2 actuators. We use uniform beam joint to replace revolute joint because it is relatively simple to model and design and has good characteristics to achieve increased working area with low stress concentration.

In any equilibrium position for the flexure mechanism, the platform will rotate due to acting forces and bending moment applied through the connecting flexures. It is therefore desirable to design a mechanism for which rotation of the platform is as small as possible.

To undertake design optimization for flexure jointed mechanisms the use of nonlinear FEA models is possible. This can give high accuracy prediction of motion but is very computationally intensive. Therefore, a novel modelling approach having high accuracy even at large deformation configurations was developed. This approach was extended and applied for design optimization of a complete mechanism. The aim was to prevent rotation of the platform as much as possible. To validate the suitability of the proposed flexure modelling approach, result have been compared with nonlinear finite element method. The FEM results

then agreed closely with modelling approach, although a high accuracy FEM computation took approximately two hours compared with less than 10 seconds. Experimental results for  the X-Y stage confirmed the effectiveness of the optimized design.

In the proposed work, this flexure modelling approach will be extended for use in dynamic models of the whole mechanism. Verification of the model can be made by frequency response testing approach. As the dynamic model will be complex and have some uncertainty a robust control approach will be required.

Acknowledgment:

This work was partially funded by the Chiang Mai University Mid-Career Research Fellowship program and Center of Mechatronic System and Innovation. Pongsiri Kuresangsai was supported by the RGJ-PhD scholarship program under the Thailand Research Fund and by a Ph.D. Studentship and TA/RA Scholarship from the Graduate School, Chiang Mai University. 

by Amnad Tongtib, Ph.D. student

Advisor: Assoc. Prof. Dr. Theeraphong Wongratanaphisan

 Description of Project:

Patients who cannot move part of body from accident, medical surgery, injury form sport playing, spinal cord injury or clogged arteries, require recovering. From this reason, the patient will be rehabilitated by physical therapist. In rehabilitation, the patient generally requires close care from a physical therapist. However, the rehabilitation tasks are repetitive over lengthy time. This causes physical therapists to be tired. From this reason, concept of using robot(s) to assist physical therapists is appealing.
Rehabilitation with robots can be divided into two main area: leg and arm. The leg rehabilitation includes hip, knee and ankle part and the arm rehabilitation includes shoulder, elbow, forearms, wrists and finger part. There are two main types of rehabilitation robot: exoskeleton robot and end-effector type robot. The exoskeleton robot employs serial mechanical structure that align with part of body to be treated, leg or arm. The joints of the robot must be aligned with rotation of joints of the body to generate the motion. For example, if we want to treat legs (hip and knee), we will use two-link serial mechanism attached to the leg. The motor will be attached to align with the hip joint and the other motor will align with the knee joint. Then, the robot will operate with pre-specified joint-space motion. In the end-effector type, the end-effector of the robot is attached to the body part and take this part to move in pre-specified path in three dimensional space. Advantage of this type is that the robot does not necessary to attach to many locations on the body part as in exoskeleton type. Therefore, the patient feels more comfortable. In this project, the parallel robot for arm rehabilitation will be analyzed, designed and built.

Objective:

1. To analyzes and design parallel robot for arm rehabilitation.
2. To study and design smooth path generation of the robot for arm rehabilitation.
3. To study algorithm for designing a 3-legs parallel robot with optimization technique.

 Gait training robot Overview:

           In establishing arm rehabilitation robot, mechanism of the 3- parallel robot is used to build the robot. Moving platform connected to fixed base with three link chains (or leg), each leg consist of two rigid body (l_li and l_ui) which connected by revolute joint, end of leg connected to moving platform with gimbal joint (spherical joint is replaced with gimbal joint because range of motion of gimbal more than spherical joint) and the other end connected to base with two actuators. The robot has six actuators which are brushless DC motors. Two motors in each leg are installed at the fixed base with the perpendicular direction. At center of moving platform, we install the handle to catch the patient’s hand. Mechanism of the robot is shown in Figure 1.

 

 Robot Structure Design:

The gait training robot is designed as exoskeleton type, which is a two-leg robot with four DOFs as shown in Fig. 2. Each hip and knee joint is a revolute joint, which can flexion and extension movements in the sagittal plane. The mechanical ranges of the hip and knee joints are from -45 to 35 degrees and 0 to 65 degrees respectively. The robot is equipped with a ball screw actuator to drive each joint. Joint motion of each joint is calculated form  measuring signals of a potentiometer as installed at each joint shaft and an encoder as installed at the end of each joint’s actuator.       There are three load cells at cuffs for each robot leg in order to measure interaction forces from a subject leg. The interaction forces are used to make compliance into the robot control between the training.

Moreover, the robot can be adjusted parts for fitting each subject body as follows. The upper and lower robot leg lengths can be adjusted from 350 to 480 mm and 290 to 380 mm respectively. The pelvic width and back and pelvic cushions can be also adjusted for the subject’s comfortable. The cuffs can be adjusted for appropriating each subject leg.

 

Publications:

• Amnad Tongtib and Theeraphong Wongratanaphisan “Concept for Using a 6-DOF Parallel Manipulator In Passive Upper Limb Rehabilitation”; The 12th International Convention on Rehabilitation Engineering and Assistive Technology 14-16 July 2018 in Shanghai, China.
• Amnad Tongtib and Theeraphong Wongratanaphisan “Kinematic analysis and Workspace Analysis of a 3- Parallel Robot’’; Joint Symposium on Mechanical-Industrial Engineering, and Robotics 2017(MIER2017) 16-17 November 2017 in Chiang Mai, Thailand
•  Amnad Tongtib, Pinyo Puangmali and Theeraphong Wongratanaphisan “Kinematic analysis of a 3- Parallel Robot’’; The 7th TSME International Conference on Mechanical Engineering, 13-16 December 2016 in Chiang Mai, Thailand.

Acknowledgment:

This project is partially supported by 1) Mid-Career Researcher Grant, Chiang Mai University and 2) the Center Mechatronic System and Innovation, Chiang Mai University.

By Wasin Wongkum
Advisor: Assoc. Prof. Theeraphong Wongratanaphisan

Delta Robot

Delta Robot is a popular parallel robot which has 3 degree-of-freedom. It is used widely for packaging industry. The advantages of the Delta robot are high strength, high agility, low inertia and high precision making it suitable for high-speed & high-precision applications. Typically, parallel robot has complicated kinematic. However, the kinematic of Delta robot is quite simple.

Most Delta robots are symmetric in its structure i.e., each legs is installed on the base 120 degrees apart. For this the workspace of the robot is symmetric. However, in some applications, it may occur that the task space are not necessarily symmetric. This project focuses on synthesis of an asymmetric Delta robot.

Objective

• To study effects of the kinematics and workspace of the Delta robot which has different arm length and arm angles.
• To synthesize an asymmetric Delta robot optimized for a certain workspace.

by Prapon Ruttanatri, Ph.D. student

Advisor:  Assoc. Prof. Dr. Matthew O.T. Cole

Description of project

Active control of structural vibration in machining processes to prevent dynamic instabilities and chatter is the main issue for this project. A controller design is proposed based on a multi-mode description of flexible structure dynamics combined with cutting process equations that capture time-delayed feedback effects. A method for robust delayed-state feedback control synthesis is presented based on Lyapunov-Krasovskii functionals (LKFs).

Objective

To develop and compare controller design methodologies for active control of regenerative vibration in machining processes in order to improve/increase operating regions for stable cutting. The main focus is on the use of modern robust controller design methodologies and new techniques for controller design based on combined models of machine structure flexibility and cutting force generation mechanisms.

Hardware-in-the-loop evaluations overview:

The experimental system used for this study (Fig. 1) has been designed to exhibit two main structural resonances and is dynamically similar to an AMB-equipped milling spindle that exhibits significant cutting tool flexibility. The main structure is a flexure-pivoted beam connected to an end-mass through another flexure pivot. Two non- contact electromagnetic actuators are used to apply forces to the structure. Hardware-in-the loop simulation and controller testing for machining vibration can be undertaken based on the configuration shown in the block diagram Fig. 2. In this scheme, actuator 2 is an auxiliary actuator used to apply a simulated cutting force that is computed in real-time using a cutting force model implemented within the control hardware. This also requires real-time measurement of the “tool” displacement. Actuator 1 is the control actuator and is used to apply the stabilizing control forces. Note that, as the cutting force model is implemented digitally on the control hardware, the cutting model can be selected freely, subject to limits of actuator force capacity and bandwidth. Displacement of the structure can be measured by two non-contact sensors positioned close to the actuators. Additional vibration measurements are obtained by strain gauges at the flexure locations. Integration of strain sensing in spindle bearing guides has been considered for the purpose of tool deflection compensation. However, in the present case the signals provide dynamic state information related to tool deflection that can be used for feedback control of vibration. Displacement measured at the cutting location is used for calculation of the simulated cutting force but is not used for feedback. Full state information is obtained by combined use of the displacement and strain sensors located away from the “tool” end.

Publication:

• Prapon Ruttanatri, Matthew O.T. Cole and Radom Pongvuthithum, “Stabilizing Active Vibration Control of Machining Processes Based on Lyapunov-Krasovskii Functionals for Time-Delay Systems”, The 7th TSME International Conference on Mechanical Engineering, Chiang Mai, Thailand, page no. 152, December 13-16, 2016
• Prapon Ruttanatri, Matthew O.T. Cole and Radom Pongvuthithum, “Structural vibration control using delayed state feedback via LMI approach – with application to chatter stability problems” SAGE

Acknowledgement:

This work was partly supported by the RGJ-PhD scholarship program under the Thailand Research Fund (grant number PHD/0089/2553).

by Boonruk Suchaitanawanit, Ph.D. student

Advisor: Assoc. Prof. Matthew Cole

Time-Optimal Control

Nowadays many engineering operations require rapid movement with high precision. To acquire maximal speed of motion, an extreme value of actuation effort is employed which could lead to excitation of the flexible mode causing undesired vibration. The time interval can be minimized by a bang-bang control. However, with correct timing of switching action. vibration can be minimized. In this research, we focused on finding the correct timing for the switching action whist minimizing the interval of motion. Real world applications of time-optimal control includes motion of the flexible structures are such as hard-disk drive, crane and spacecraft structures.

 Structural Optimization

Conventionally, mechanical structure design and controller design are separate procedure. By combining two processes together, a better performance of the system could be achieved. A structure whose parameters are compatible with the tasks, might achieve better time performance the one that are not. The selected case study in this research involves tuning a stiffness of the flexible structure to match with the travelling distance associate with time-optimal motion.

Objective

The objective of the project is to develop a new mathematical strategy based on the convexity of the reachable set in order to solve time-optimal control problem. The proposed algorithm can guarantee the true optimality of the control solution since it solves directly for co-state variables which satisfy the necessary conditions rather than searching for switching time and later verify the optimality by using Pontryagin’s Minimum Principle. Figure 1 shows the example of the reachable set of 3 states linear model of the flexible structure.

The system along with time-optimal control solutions acquired by the proposed algorithm will be tuned to achieve even faster motion. The tuning algorithm uses iterative scheme to alter the structural parameters and control profile in order to reduce the motion interval. By exploiting the continuity of the reachable set the iterative tuning method based on the steepest descent gradient searching is able to move toward the local minimum.

The obtained solution was verified and applied on the experimental rig (fig. 2). Here linear translation of the crane-like structure with flexible armature is to provide vibration free rest-to-rest motion. Solution are shown in figure 3. The rig has tunable stiffness mechanism which allows natural frequency and damping ratio of the flexible link to be tuned for verifying the simulation results.

Publication

• B. Suchaitanwanit and M.O.T Cole, “An Algorithm to Obtain Control Solutions Achieving Minimum-Time State Transfer of Linear Dynamical System Based on Convexity of The Reachable Set”, In Proceeding of the 4^th International Conference on Intelligent Systems, Modeling and Simulation, Vol. 1, pp. 340-345, 2013.
• B. Suchaitanwanit and M.O.T Cole, “Fundamental Limit of Performance in Minimum-Time Motion Control due to Structural Flexibility”, In Memoirs of Muroran Institution of Technology, Vol. 6, pp. 7-12, 2013.