Force Tracking Error Research Articles

Development of Multidimensional Loading Material Testing Machine

A multidimensional loading material testing machine (MLMTM) based on Stewart Platform is proposed in this paper. The kinematic and dynamic characteristics of MLMTM are analyzed, including the influence of universal joint in each limb on dynamic modeling. The dynamic model is established via Kane method, and the control system is founded based on classic PID strategy. Four experiments are performed to examine the validity of proposed dynamic model and control system of the MLMTM, including two uniaxial experiments and two biaxial experiments. Results of these experiments are summarized as well as the capacity of force tracking and load errors are analyzed. The experimental results demonstrate the validity of dynamic model via Kane method and illustrate that the proposed MLMTM is capable of performing multidimensional loading tests.

Intelligent Robotic Gripper Control Strategy

Although, on-off control robot gripper is widely employed in pick-and-place operations, it can not be applied in fragile or soft objects handling. Here, an intelligent gripper is designed with embedded distributed control structure for overcoming the uncertainty of grasped object mass and soft/hard features. An efficient model-free intelligent fuzzy sliding mode control strategy is employed to design the position and force controllers of gripper, respectively. Experimental results of pick-and-place soft and hard objects with grasping force auto-tuning and anti-slip control strategy are shown by pictures to verify this distributed system performance. The position and force tracking errors are less than 1 mm and 0.1 N, respectively.

Bouc–Wen model-based real-time force tracking scheme for MR dampers

A Bouc–Wen model-based control scheme is presented which allows tracking the desired control force in real-time with magnetorheological (MR) dampers without feedback from a force sensor. The control scheme estimates the MR damper force by parallel computing of several Bouc–Wen models with different constant currents as inputs and for the actual MR damper displacement and velocity, respectively. Based on the estimated forces and the desired control force the MR damper current is determined by a piecewise linear interpolation scheme. The model-based feed-forward control scheme is numerically and experimentally validated. If the desired control force is not constrained by the pre-yield region, residual force at 0 A and force at maximum current, the very small force tracking error ≤0.0015 in the simulation is caused by the control-oriented simplification of the linear interpolation scheme. The tests reveal that the real-time control scheme is numerically stable and the force tracking error of ≤0.078 represents an acceptable accuracy.

H ∞ state‐feedback control of bilateral teleoperation systems with asymmetric time‐varying delays

In this study, a novel controller synthesis method is proposed for the bilateral teleoperation systems with asymmetric time-varying delays. The main idea of the proposed method is to deal with the control problem for a bilateral teleoperation system under asymmetric time-varying delays as a control problem for a system with multiple input time delays, and then to find the state-feedback control law to stabilise the system with guaranteed H ∞ performance on the defined control output. With intentionally introducing the pretend time delays to the (local) state variables of the master or the slave, the teleoperation controller design problem is converted to a controller synthesis problem for a system with multiple input time delays. Then the state-feedback controllers are designed for both the master and the slave by defining an appropriate Lyapunov-Krasovskii functional candidate and solving linear matrix inequalities. The designed controllers guarantee the closed-loop system stability and system transparency performance in terms of the optimised H ∞ norm on the position and force tracking errors between the master and the slave. At last numerical simulations are used to validate the effectiveness of the proposed method. The results show that the designed controllers can effectively stabilise the present teleoperation system with good transparency performance under time-varying delays.

Decentralised adaptive fuzzy control of coordinated multiple mobile manipulators interacting with non‐rigid environments

Many robotic applications involve the tasks in which multiple coordinated mobile manipulators system interacts with rigid or non-rigid environments. In this study, we study the topic of multiple mobile manipulators grasping a rigid common object in contact with a deformable working environment, for which few investigations have been made. To solve the difficulty. We first develop the decentralised dynamics of the inter-connected system, including the dynamics of the mobile manipulators, the internal forces between end-effectors and the object and the interaction forces between the object and the environments. Then, we design decentralised adaptive fuzzy control based on impedance approach for coordinated multiple mobile manipulators using Lyapunov synthesis. Rigourous stability and performance analysis have been established, and simulation are presented to figuratively demonstrate that the motion/force-tracking errors converge to zero, whereas the internal force-tracking error remains bounded and can be made arbitrarily small.

Voltage based linearization of a reluctance actuator for high-precision applications

A single-phase C-core reluctance actuator is linearized using a combined analog and digital control, and magnetic field measurements. The analog stage contains a high-bandwidth sensing coil voltage tracking controller which rejects the electric and magnetic circuit disturbances and makes the plant behave as an idealized transformer between the driving coil and the sensing coil on a wide frequency range. Since this is the natural high frequent behavior of the actuator, very low sensing coil voltage tracking errors are achieved. Furthermore, in combination with flux feedforward and drift stabilizing low bandwidth Hall probe feedback, this circuitry achieves very low reluctance force tracking errors, i.e. very small linearization errors and low stiffness inherent to voltage-based control schemes. The performance is experimentally verified on a 1DoF measurement rig with 1 nm position and less than 50 mN force resolution, where linearization errors smaller than 0.05 N for challenging 200 N second order force profiles are measured.

Output feedback domination approach for finite-time force control of an electrohydraulic actuator

Force control of a single-rod electrohydraulic actuator is investigated. The model considers the friction effect of the actuator. First, a non-linear force controller using a feedback domination approach, which can globally finite-time stabilise the force tracking error system without the friction parameter uncertainties, is proposed. Then, another feedback domination controller is proposed to cope with the friction parameter uncertainties. To estimate the unmeasurable states, an observer with finite-time convergence is developed. Rigorous stability analysis is given. Compared with the conventional output feedback backstepping control method, the proposed output feedback domination control scheme provides a faster convergence rate and a higher tracking accuracy for force tracking. Numerical simulation results demonstrate the effectiveness of the proposed control design.

Frequency and damping adaptation of a TMD with controlled MR damper

This paper describes the new concept of a semi-active tuned mass damper with magnetorheological damper (MR-STMD). The real-time controlled MR damper force emulates controlled damping and a superimposed controllable stiffness force to augment or diminish the force of the passive spring stiffness which enables us to control the MR-STMD natural frequency. Both the damping and natural frequency are tuned according to Den Hartog’s formulae to the actual dominant frequency of the main structure irrespective of whether it is a resonance or a forced frequency. The MR-STMD is experimentally validated on the Empa bridge with a 15.6 m main span for different added masses to shift its resonance frequency −12.2% and +10.4% away from its nominal value. The experimental results are compared to those obtained when the MR-STMD is operated as a passive TMD that is precisely tuned to the nominal bridge. The comparison shows that the MR-STMD outperforms the TMD both in the tuned and all de-tuned cases by up to 63%. Simulations of the MR-STMD concept point out that the proposed semi-active control algorithm is most suitable for MR-STMDs due to the small amount of clipped active forces. A sensitivity analysis demonstrates that the real MR-STMD could be even more powerful if the force tracking errors in the MR damper force due to the current driver and MR fluid dynamics and remanent magnetization effects could be further reduced. The MR-STMD under consideration represents the prototype of the 12 MR-STMDs that have been running on the Volgograd Bridge since late fall 2011.

Adaptive Control of Teleoperation Systems With Linearly and Nonlinearly Parameterized Dynamic Uncertainties

Existing work concerning adaptive control of uncertain teleoperation systems only deals with linearly parameterized (LP) dynamic uncertainties. Typical teleoperation system dynamics, however, also posses terms with nonlinearly parameterized (NLP) structures. An example of such terms is friction, which is ubiquitous in the joints of the master and slave robots of practical teleoperation systems. Uncertainties in the NLP dynamic terms may lead to significant position and force tracking errors if not compensated for in the control scheme. In this paper, adaptive controllers are designed for the master and slave robots with both LP and NLP dynamic uncertainties. Next, these controllers are incorporated into the 4-channel bilateral teleoperation control framework to achieve transparency. Then, transparency of the overall teleoperation is studied via a Lyapunov function analysis. Simulation studies demonstrate the effectiveness of the proposed adaptive scheme when exact knowledge of the LP and NLP dynamics is unavailable.

Design and control of integrated pneumatic dexterous robot finger

Based on flexible pneumatic actuator (FPA), bending joint and side-sway joint, a new kind of pneumatic dexterous robot finger was developed. The finger is equipped with one five-component force sensor and four contactless magnetic rotary encoders. Mechanical parts and FPAs are integrated, which reduces the overall size of the finger. Driven by FPA directly, the joint output torque is more accurate and the friction and vibration can be effectively reduced. An improved adaptive genetic algorithm (IAGA) was adopted to solve the inverse kinematics problem of the redundant finger. The statics of the finger was analyzed and the relation between fingertip force and joint torque was built. Finally, the finger force/position control principle was introduced. Tracking experiments of fingertip force/position were carried out. The experimental results show that the fingertip position tracking error is within ±1 mm and the fingertip force tracking error is within ±0.4 N. It is also concluded from the theoretical and experimental results that the finger can be controlled and it has a good application prospect.

A novel adaptive hybrid force-position control of a robotic manipulator

A new hybrid force-position control method for uncertain robotic manipulator interacting with its environment is presented. First, the system dynamical model in the compliance frame is derived from the usual joint frame model and leads to two sets of equations due to the constraint associated to the contact surface. Next, the two dynamics are separately used for the synthesis of position and force tracking controllers. For the position control part, the design method consists of an estimated-parameters dependent coordinate transformation and a control law derived from a backstepping procedure. The force control law has two aspects: first it compensates the dynamical interaction between the end-effector motion and the force induced by the environment and secondly, it imposes a desired force using a proportional-like equation. Finally, a parameter-adaptation algorithm is derived from a stability criteria and dependent both on the position and force tracking errors. Simulation results on a four-degree of freedom robotic system tracking a triangle while maintaining a constant contact force prove the effectiveness of our solution.

Adaptive robust coordinated control of multiple mobile manipulators interacting with rigid environments

In this paper, we consider multiple mobile manipulators grasping a common object in contact with a rigid surface, and propose a new version of adaptive robust control extended to the actuator level for multiple mobile manipulators carrying a common object in a cooperative manner. The proposed controls are robust not only to parametric uncertainties including mass variation and electrical parameters but also to external disturbances. Simulation results are presented to validate that the motion/force tracking errors converge to zero whereas the internal force tracking error remains bounded and can be made arbitrarily small.

A Neuro-Sliding Mode Control Scheme for Constrained Robots with Uncertain Jacobian

The joint robot control requires to map desired cartesian tasks into desired joint trajectories, by using the ill-posed inverse kinematics mapping. In order to avoid inverse kinematics, the control problem is formulated directly in task space to gives rise to cartesian robot control. In addition, when the robot is constrained due to its kinematic mappings yields a stiff system and an additional complexity arises to implement cartesian control for constrained robots. In this paper, an alternative approach is proposed to guarantee global convergence of force and position cartesian tracking errors under the assumption that the jacobian is not exactly known. A neuro-sliding mode controller is presented, where a small size adaptive neural network compensates approximately for the inverse dynamics and an inner control loop induces second order sliding modes to guarantee tracking. The sliding mode variable tunes the online adaptation of the weights. A passivity analysis yields the energy Lyapunov function to prove boundedness of all closed-loop signals and variable structure control theory is used to finally conclude convergence of position and force tracking errors. Experimental results are provided to visualize the expected performance.

Adaptive Jacobian position/force tracking control of free-flying manipulators

This paper solves the problem of position/force tracking control of a free-flying space manipulator with uncertain kinematics and dynamics. A free-flying manipulator interacting with an uncertain compliant surface is considered. To cope with the uncertainties arising from free-flyer’s kinematics, dynamics and surface stiffness and position, an adaptive Jacobian controller is devised. The convergence of the force and position tracking errors is proved based on Lyapunov stability analysis. Numerical simulation is presented to show the performance of the controller.

Robust adaptive control of coordinated multiple mobile manipulators

In this paper, we investigate robust adaptive controls of multiple mobile manipulators carrying a common object in a cooperative manner with unknown inertia parameters and disturbances. First, a concise dynamics consisting of the dynamics of mobile manipulators and the geometrical constraints between the end-effectors and the object is developed for coordinated multiple mobile manipulators. Subsequently, we design robust adaptive controls where parametric uncertainties are compensated by adaptive update techniques and the disturbances are suppressed. The controls ensure that the output tracking errors of the system converge to zero whereas the internal force tracking error remains bounded and can be made arbitrarily small. Throughout this paper, feedback control design and stability analysis are performed via explicit Lyapunov techniques. Simulation studies on the control of coordinated two wheels driven mobile manipulators show the effectiveness of the proposed scheme.

Force Tracking Impedance Control with Variable Target Stiffness

In this paper, a novel force tracking impedance control strategy is presented in which target stiffness is varied on-line to regulate the desired contact force without any knowledge of the environment. Humans can control contact force by adjusting their arm stiffness. The contact force can be either increased by making one's arm stiffer or decreased by reducing the arm stiffness. Furthermore, humans can keep the force tracking error within a certain range without any knowledge of environmental parameters as long as how much force they exert on the object is known to them. Analogously, the proposed control scheme achieves a contact force regulation control by adjusting the target stiffness of the impedance control. The new force tracking impedance control scheme does not require estimating environment stiffness or locations since the controller is adapted only based on the previous force tracking error between the desired and real contact force. Stability of the proposed scheme is discussed with a quadratic Lyapunov function. Extensive simulation studies with a 7 degree of freedom (DoF) robot manipulator using full arm dynamics are conducted to demonstrate the validity of the proposed scheme.

Adaptive Tracking Control of Coordinated Nonholonomic Mobile Manipulators

In this paper, adaptive control of multiple mobile manipulators carrying a common object in a cooperative manner with unknown inertia parameters and disturbances has been investigated. firstly, A complete dynamics consisting of the dynamics of mobile manipulators and the object is presented for coordinated multiple mobile manipulators. Then, adaptive control has been designed for compensating parametric uncertainties and suppressing disturbances. The control ensures that the output tracking errors of the system converge to zero whereas the internal force tracking error remains bounded and can be made arbitrarily small. Simulation studies show the effectiveness of the proposed scheme.

Nonlinear H∞ Control of a Bilateral Nonlinear Teleoperation System

In this paper, we present a nonlinear H∞ control technique for the bilateral teleoperation of a nonlinear master-slave system. The proposed controller guarantees robust stability in the presence of uncertainty in operator and environment impedances. The guidelines to include nonlinear intervening tool between master and slave robot are suggested. The proposed technique enables adjusting weighting of position and force tracking error functions for both amplitude and frequency domain. To solve the corresponding partial differential equation, called HJI, an approximation method based on Taylor series expansion of the solution is used. A numerical simulation demonstrates that the linear controller tends to instability in contact tasks while the third order approximated nonlinear controller yields desired performance.

Multiple-arm space free-flying robots for manipulating objects with force tracking restrictions

Multiple Impedance Control (MIC) is an algorithm that enforces designated impedance at various levels, i.e. on the manipulated object, all cooperating manipulators, and the moving platform of a robotic system. In this paper, a force tracking strategy inspired by a human control system is added to the MIC algorithm and the general formulation is revised to fulfill a desired force tracking strategy for object manipulation tasks. The stability analysis of the MIC algorithm based on the Liapunov Direct Method, besides error analysis, shows that a good tracking of cooperative manipulators and the manipulated object is guaranteed. Next, using MAPLE and MATLAB tools, a system of three manipulators mounted on a space free-flying robot is simulated. The task is moving an object based on given trajectories which come across an obstacle, to examine the performance of the developed control law. The results show that, even in the presence of both external disturbances and an impact due to collision with the obstacle, the response of the MIC algorithm is smooth. Moreover, based on the embedded force tracking strategy, the contact force is confined to follow a desired trajectory. Also, it is shown that decreasing the values of the controller mass matrix elements results in reducing both the object position and force tracking error.

SMOOTH SLIDING MODE CONTROL FOR CONSTRAINED MANIPULATOR WITH JOINT FLEXIBILITY

This paper presents a smooth sliding mode control plus backstepping method for the motion/force control of a constrained robotic manipulator with flexible joints. The control scheme is presented based on that overall system parameters are with uncertainties whereas only the constrained force, positions and velocities of links and rotors are measurable. The smooth sliding controller can achieve zero motion and force tracking errors. The simulation results for a planar robot illustrate the expected satisfactory performance.