Parallel robots exhibit good performance in terms of rigidity, accuracy, and dynamic characteristics. However, parallel robots have complex configurations and their dynamic model is highly nonlinear, and conventional PID controllers are not sufficiently robust for their motion control. In this paper, we have investigated the intelligent control of a hydraulically driven parallel robot based on the dynamic model and two control schemes have been developed: 1) Fuzzy-PID self tuning controller composed of the conventional PID control and with Fuzzy logic; 2) Adaptive neuro-fuzzy inference system-PID (ANFIS-PID) self tuning of the gains of the PID controller. The two controllers are used to track a straight line. The obtained results confirm the theoretical findings, i.e., the Fuzzy–PID and ANFIS-PID self tuning controller can reduce more tracking errors than the conventional PID controller. Amongst these methods, ANFIS has provided the best results for controlling robotic manipulators as compared to the conventional control strategies. Finally, simulated results that demonstrate the robot behaviors are presented. 

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ANFIS and Fuzzy Tuning of PID Controller for Trajectory Tracking of a Flexible Hydraulically Driven Parallel Robot Machine

Mazin Al-saedi Flexible multibody dynamics and intelligent control of a hydraulically driven hybrid redundant robot machine Lappeenranta 2014 144 pages Acta Universitatis Lappeenrantaensis 577 Diss. Lappeenranta University of Technology ISBN 978-952-265-594-3, ISBN 978-952-265-595-0 (PDF), ISSN-L 1456-4491, ISSN 1456-4491. The assembly and maintenance of the International Thermonuclear Experimental Reactor (ITER) vacuum vessel (VV) is highly challenging since the tasks performed by the robot involve welding, material handling, and machine cutting from inside the VV. The VV is made of stainless steel, which has poor machinability and tends to work harden very rapidly, and all the machining operations need to be carried out from inside of the ITER VV. A general industrial robot cannot be used due to its poor stiffness in the heavy duty machining process, and this will cause many problems, such as poor surface quality, tool damage, low accuracy. Therefore, one of the most suitable options should be a light weight mobile robot which is able to move around inside of the VV and perform different machining tasks by replacing different cutting tools. Reducing the mass of the robot manipulators offers many advantages: reduced material costs, reduced power consumption, the possibility of using smaller actuators, and a higher payload-to-robot weight ratio. Offsetting these advantages, the lighter weight robot is more flexible, which makes it more difficult to control. To achieve good machining surface quality, the tracking of the end effector must be accurate, and an accurate model for a more flexible robot must be constructed. This thesis studies the dynamics and control of a 10 degree-of-freedom (DOF) redundant hybrid robot (4-DOF serial mechanism and 6-DOF 6-UPS hexapod parallel mechanisms) hydraulically driven with flexible rods under the influence of machining forces. Firstly, the flexibility of the bodies is described using the floating frame of reference method (FFRF). A finite element model (FEM) provided the Craig-Bampton (CB) modes needed for the FFRF. A dynamic model of the system of six closed loop mechanisms was assembled using the constrained Lagrange equations and the Lagrange multiplier method. Subsequently, the reaction forces between the parallel and serial parts were used to study the dynamics of the serial robot. A PID control based on position predictions was implemented independently to control the hydraulic cylinders of the robot. Secondly, in machining, to achieve greater end effector trajectory tracking accuracy for surface quality, a robust control of the actuators for the flexible link has to be deduced. This thesis investigates the intelligent control of a hydraulically driven parallel robot part based on the dynamic model and two schemes of intelligent control for a hydraulically driven parallel mechanism based on the dynamic model: (1) a fuzzy-PID self-tuning controller composed of the conventional PID control and with fuzzy logic, and (2) adaptive neuro-fuzzy inference system-PID (ANFIS-PID) self-tuning of the gains of the PID controller, which are implemented independently to control each hydraulic cylinder of the parallel mechanism based on rod length predictions. The serial component of the hybrid robot can be analyzed using the equilibrium of reaction forces at the universal joint connections of the hexa-element. To achieve precise positional control of the end effector for maximum precision machining, the hydraulic cylinder should be controlled to hold the hexa-element. Thirdly, a finite element approach of multibody systems using the Special Euclidean group SE(3) framework is presented for a parallel mechanism with flexible piston rods under the influence of machining forces. The flexibility of the bodies is described using the nonlinear interpolation method with an exponential map. The equations of motion take the form of a differential algebraic equation on a Lie group, which is solved using a Lie group time integration scheme. The method relies on the local description of motions, so that it provides a singularity-free formulation, and no parameterization of the nodal variables needs to be introduced. The flexible slider constraint is formulated using a Lie group and used for modeling a flexible rod sliding inside a cylinder. The dynamic model of the system of six closed loop mechanisms was assembled using Hamilton’s principle and the Lagrange multiplier method. A linearized hydraulic control system based on rod length predictions was implemented independently to control the hydraulic cylinders. Consequently, the results of the simulations demonstrating the behavior of the robot machine are presented for each case study. In conclusion, this thesis studies the dynamic analysis of a special hybrid (serialparallel) robot for the above-mentioned special task involving the ITER and investigates different control algorithms that can significantly improve machining performance. These analyses and results provide valuable insight into the design and control of the parallel robot with flexible rods.

Flexible multibody dynamics and intelligent control of a hydraulically driven hybrid redundant robot machine

The invention relates to a hydraulic drive system of a four-foot walking robot. The hydraulic drive system comprises a servo cylinder assembly, wherein the servo cylinder assembly is respectively connected with a hydraulic power device and an oil return line by pipelines. The invention has the following advantages: 1. the power-mass ratio is large, thereby enhancing the weight loading capability of the robot; 2. the fuel can be conveniently filled, and the cruising power is high; 3. the robot has faster dynamic response capability, thereby being beneficial to the stability of the robot; 4. the robot has better speed rigidity, thereby reducing the effect of the load change on dynamic characteristics; 5. the allocation of the joint driving power is more convenient, thereby being capable of intensively supplying the system driving power to specified joints in a very short time to meet the driving requirements for special movement; and 6. the hydraulic cylinders can be easily arranged, thereby enabling the leg structure of the robot to be more simple and compact.

Hydraulic drive system of four-foot walking robot

Coordination dynamics and model-based neural network synchronous controls for redundantly full-actuated parallel manipulator

A hybrid five-bar mechanism is a special parallel mechanism that consists two different types of driving actuators: a uncontrollable constant velocity (CV) motor and a controllable servo motor. The control for such a system is still a challenge because of the coupling behavior between the CV motor and the servomotor through a closed loop mechanism. In this paper, the hybrid five-bar mechanism is revisited with a new solution: position domain control for its ideal application in this hybrid five-bar mechanism. It is the first time to solve the challenge of controlling hybrid five-bar mechanisms. A position domain PD control developed by the authors is applied to control the servomotor that follows the master motion of the CV motor in an active way to eliminate the effect of the CV motor on the contour performance. Simulation results for different contour tracking demonstrate that the position domain control obtained much better contour tracking performances compared to previous developed complicated time domain controller.

Revisiting hybrid five-bar mechanism: Position domain control application

The invention provides an electro-hydraulic driver of a control valve, which comprises a hydraulic driving system, a pressure sensor, a flow sensor, a position sensor and a control unit; wherein, the hydraulic driving system is used for driving a valve core of the control valve and comprises a bidirectional driving cylinder, a directional control valve, a three-position four-way reversing valve, an energy accumulator, a servo motor and a bidirectional gear pump; the pressure sensor and the flow sensor are installed on a fluid output pipeline, are used for detecting the pressure and the flow of the output fluid in the pipeline and are connected with the control unit; the position sensor is used for detecting the valve core position of the control valve to be controlled and is connected with the control unit The electro-hydraulic driver realizes that hydraulic oil can flow out of an oil outlet cavity and directly enter an oil inlet cavity without passing through an oil tank, and has simple and compact structure; the motor and the oil pump do not need to continuously run but operate intermittently, which can not only save energy but also prolong the service life of the system, thus realizing stable operation for extra-long time and quick response of the control valve with high reliability

Electro-hydraulic driver of control valve

Parallel robots have very good performance in terms of rigidity, strength-to-weight ratios and positioning accuracy. A 2-DOF redundantly actuated parallel robot is taken as the object of study in this paper. As far as trajectory tracking performance, controller is essential. In order to improve the trajectory tracking performance, a neural network PID (proportion integral differential) controller is adopted. The backpropagation (BP) neural network is used to adjust the PID parameters. A simple proportion differential (PD) controller is selected to compare with the proposed controller. A circular motion control of the end-effector is studied using the two control methods. Simulation results show that the neural network PID controller is more effective than the PD controller in terms of reducing tracking errors, and it is a practical and valid alterative to parallel robots.

Research on trajectory tracking of a parallel robot based on neural network PID control

A new approach for modeling and simulating of spiral bevel gears is proposed. Based on generating method, the actual cutting and adjusting process of Gleason spiral bevel gears was simulated. The intersections of tool tracing lines caused by every two adjacent edges in the cutting process were used as points on the tooth flank. Thus a mathematical model of spiral bevel gears was developed. Machine-tool settings and machine constants were calculated and adjusted by implementing the above model in a Visual Basic code. Tooth profile was reconstructed by using the flank points obtained from the program and three-dimensional solid models for spiral bevel gears were established. Then dynamic interference detection was carried out by using the computer software Pro/E. Based on the interference detection results, machining-tool settings can be adjusted until satisfying gear pair is acquired. The developed approach is illustrated by an example.

Modeling and Simulating of Spiral Bevel Gears Based on Actual Cutting Process

Whether an electro-hydraulic control valve for the mixed-gas pressure control in a combined cycle power plant (CCPP) of a steel mill meets the challenge of the gas turbine depends on its dynamic response. To obtain the dynamic characteristics of the electro-hydraulic control valve before it is manufactured, a dynamic design approach based on the physical simulation model is presented. The electro-hydraulic actuator of the control valve is redesigned according to the proposed design approach. The results show that the intercoupling effect of the flow fluctuations and the pressure pulsations can result in the velocity oscillation of the electro-hydraulic actuator. By using a proper accumulator whose frequency of absorbing the pressure pulsations is equal to the flow-fluctuation frequency, the intercoupling effect is weakened, and the velocity oscillation of the electro-hydraulic actuator decreases obviously. The results are of significance for the optimization design of the electro-hydraulic control valve.

Dynamic Design of Electro-hydraulic Control Valve Based on Physical Simulation Model

The invention discloses a large-flow coal gas pressure regulating valve. The regulating valve mainly utilizes the variation of the inputted pressure signals to regulate flow by driving the valve plug of the regulating valve to move by a hydraulic cylinder. The regulating valve has a high-frequency vibration mechanism, which can generate high-frequency vibration with variable frequency, amplitude value, vibration signal direction and signal mode. The regulating valve is provided with a discharge chamber, and a pressure balance chamber is disposed in the valve plug. The regulating valve has intelligent malfunction self-diagnostic function, and can realize automatic regulation of zero point and stroke according to the state of the regulating valve. The invention has the advantages of simple structure, good bias resistance, fast execution speed, optimized response sensitivity, and no influence by tar oil clogging; and is especially suitable for rapid pressure regulation of large-flow coal gas in cycle power generation project in steel mill, and also suitable for regulation and control of fluid in thermal power generation plant, nuclear power plant and chemical industry.

Yong Wang

Papers

Electro-hydraulic driver of control valve

Research on trajectory tracking of a parallel robot based on neural network PID control

Modeling and Simulating of Spiral Bevel Gears Based on Actual Cutting Process

Dynamic Design of Electro-hydraulic Control Valve Based on Physical Simulation Model

High volume gas pressure regulating valve