Sensorless Force Control Research Articles

A Cable-Based Haptic Interface with a Reconfigurable Structure

Abstract Cable robots have been used as haptic interfaces for several decades now, with the most notable examples being the SPIDAR and its numerous iterations throughout the years, as well as the more recent IPAnema 3 Mini manufactured by Fraunhofer IPA. However, these robots still have drawbacks, particularly their high number of cables required to maintain a high workspace-to-installation-space ratio. Using a hybrid-structure cable robot (HSCR) could prevent some collisions that occur between the cables and the user's body. More specifically, some applications requiring multimodal feedback could benefit from the flexibility that a reduced number of cables offers. Therefore, this paper presents a novel SPIDAR-like VSCR and its sensor-less force control method based on motor current. The purpose of this work is to clarify the advantages that a variable structure can provide for haptic interaction. In this regard, experimental results regarding the device's workspace and its force feedback capabilities are presented. Additionally, since real-time high-frequency updates are required for haptic display, we provide additional data regarding the control algorithm's runtime. Lastly, another experiment was conducted to study changes in user performance when using both the variable and the usual cable configuration. The results showed that feedback accuracy is maintained, and there are no drawbacks to using hybrid configurations.

Enhanced Estimation of Clamping-Force for Automotive EMB Actuators Using a Switching Extended State Observer

The automotive intelligence and electrification require the braking system to cooperate with the controller to achieve regenerative and active braking functions. Hence, the range and driving safety of the electric vehicles are improved. Electromechanical Brake (EMB) systems completely abandon the fluid and hydraulic elements and replace the traditional hydraulic braking system with a wire harness and electromechanical parts. The braking torque adjustment is achieved entirely by controlling the four-wheel EMBs. At present, reliability and fault tolerance are the key technical issues associated with EMBs. To eliminate the need of any clamping-force sensor due to its vulnerability and high cost, in this paper, a switching extended state observer (SESO) is presented to provide rapid force estimation with higher precision. The substrates of the internal linear and nonlinear structures have been unified into a single form by using an equivalent gain method. The SESO is effective to detect the initial contact between the braking disk and pad by estimating the load torque. It is experimentally verified that the developed force sensorless control strategy is accurate and superior to the existing techniques.

LADRC-based Sensorless Force Control for Robotic Joint Considering Static Friction

LADRC-based Sensorless Force Control for Robotic Joint Considering Static Friction

Fingertip Sensorless Force Control by Tendon-driven Two-joint Robotic Finger

In this paper, we designed an articulated robot finger with a round-belt twist mechanism combining a wire drive and a Twisted round-belt Actuator (TbA). This robot finger has the following two main features. First, we designed new mechanism，Variable-pitch Screw Module (VSM) to apply stretch to the round belt while adding twists to them at the same time. Second, we replaced acceleration sensors with encoders to stabilize output signals of joint angles. We explain the mechanism of this robot finger which are mentioned above. We show the results of experiments of fingertip force measurement. Finally, we show the results of a force-following control experiment for stepwise target values.

Fingertip sensor-less force control by articulated two-fingered robotic hand using twisted round-belt actuator

Fingertip sensor-less force control by articulated two-fingered robotic hand using twisted round-belt actuator

Bilateral force sensorless control based on finite‐time adaptive sliding‐mode‐assisted disturbance observer

The cover image is based on the Original Article Bilateral force sensorless control based on finite-time adaptive sliding-mode-assisted disturbance observer by Dapeng Tian et al., https://doi.org/10.1049/cth2.12198.

Bilateral force sensorless control based on finite‐time adaptive sliding‐mode‐assisted disturbance observer

Bilateral teleoperation systems are widely used to compensate for image spin and in many other applications. Good transparency (position tracking and force feedback) is an important performance index in the control design of bilateral teleoperation systems. This study aims to achieve good transparency for teleoperation systems using force sensorless by proposing an alternative to direct force measurement. Our proposed method is a finite-time adaptive sliding-mode-assisted disturbance observer, which combines disturbance observer with adaptive sliding mode control. The finite-time stability of adaptive sliding-mode-assisted disturbance observer is analysed using the Lyapunov theory. Moreover, a bilateral control law based on adaptive sliding-mode-assisted disturbance observer is designed. The proposed method not only improves the system transparency with both force feedback and position tracking, but is also chatter-free. Experiments are conducted using bilateral brushless DC motors in an experimental setup to demonstrate the effectiveness of the proposed method.

Force Sensorless Admittance Control for Teleoperation of Uncertain Robot Manipulator Using Neural Networks

In this paper, a force sensorless control scheme based on neural networks (NNs) is developed for interaction between robot manipulators and human arms in physical collision. In this scheme, the trajectory is generated by using geometry vector method with Kinect sensor. To comply with the external torque from the environment, this paper presents a sensorless admittance control approach in joint space based on an observer approach, which is used to estimate external torques applied by the operator. To deal with the tracking problem of the uncertain manipulator, an adaptive controller combined with the radial basis function NN (RBFNN) is designed. The RBFNN is used to compensate for uncertainties in the system. In order to achieve the prescribed tracking precision, an error transformation algorithm is integrated into the controller. The Lyapunov functions are used to analyze the stability of the control system. The experiments on the Baxter robot are carried out to demonstrate the effectiveness and correctness of the proposed control scheme.

Sensor-less Force Control by Articulated Two-fingered Robotic Hand using Fingertip Force Estimation

Sensor-less Force Control by Articulated Two-fingered Robotic Hand using Fingertip Force Estimation

Robust state dependent Riccati equation variable impedance control for robotic force-tracking tasks

Industrial robots are increasingly used in highly flexible interaction tasks, where the intrinsic variability makes difficult to pre-program the manipulator for all the different scenarios. In such applications, interaction environments are commonly (partially) unknown to the robot, requiring the implemented controllers to take in charge for the stability of the interaction. While standard controllers are sensor-based, there is a growing need to make sensorless robots (i.e., most of the commercial robots are not equipped with force/torque sensors) able to sense the environment, properly reacting to the established interaction. This paper proposes a new methodology to sensorless force control manipulators. On the basis of sensorless Cartesian impedance control, an Extended Kalman Filter (EKF) is designed to estimate the interaction exchanged between the robot and the environment. Such an estimation is then used in order to close a robust high-performance force loop, designed exploiting a variable impedance control and a State Dependent Riccati Equation (SDRE) force controller. The described approach has been validated in simulations. A Franka EMIKA panda robot has been considered as a test platform. A probing task involving different materials (i.e., with different stiffness properties) has been considered to show the capabilities of the developed EKF (able to converge with limited errors) and controller (preserving stability and avoiding overshoots). The proposed controller has been compared with an LQR controller to show its improved performance.

Disturbance Suppression Considering Thrust Constant Fluctuation and Restoring Force of Flat Cable for Precise Force Control

Force sensorless control with voice coil motor (VCM) is thought to be challenging because its thrust constant fluctuation characteristic and the flexible flat cable connected to the moving coil degrade the estimation accuracy. Modeling methods of thrust constant fluctuation have been proposed in previous studies. However, these methods were time consuming, easily affected by friction, and did not consider cable disturbance. This article presents an identification and compensation methodology for nonlinear internal disturbances in a VCM to accomplish precise force control with a linear VCM stage. In this article, a novel compensator is constructed on basis of these deterioration mechanisms. To realize the compensator, the deteriorations had to be obtained. To that end, a novel approach to identify the parameter fluctuation and cable restoring force is developed, and the compensator is applied to sensorless force control. The proposed approaches are verified through experiments using the prototype of a linear VCM stage.

Object Transportation With Force-Sensorless Control and Event-Triggered Synchronization for Networked Uncertain Manipulators

This article proposes a distributed control method for networked manipulators to cooperatively transport an unmodeled object without force measurement. First, we design an adaptive neural network control algorithm, based on the wavelet neural network, to estimate unknown system dynamics. A novel task-space synchronization control is studied with triggering condition for the network under time-varying communication delays, to improve the cooperative quality while reducing the network burden. The proposed control method ensures uniform ultimate boundedness of velocity/position tracking errors and boundedness of the internal force with the avoidance of Zeno behavior. Moreover, the robustness of networked uncertain manipulators is guaranteed. Finally, a numerical simulation and experiments are carried to demonstrate the performance and efficiency of the proposed control algorithm.

Fine Sensorless Force Control Realization Based on Dither Periodic Component Elimination Kalman Filter and Wide Band Disturbance Observer

This paper proposes a new force estimation approach based on the dither periodic component elimination (DPCE) Kalman filter and the disturbance observer (DOB) for realization of a fine sensorless force control system under the existence of static friction. A dither signal is inserted to the desired reference signal to eliminate the effect of static friction of a ball-screw system. The DOB estimates the periodical torque caused by dither signal. The DPCE Kalman filter uses the periodical torque for the estimation of the high-order torque responses to achieve the torque estimation with the elimination of periodic component. The problem of noise in the force sensation and the effect of static friction are reduced effectively by using the DPCE Kalman filter. Therefore, the bandwidth of the force sensing is increased and the wide band DOB is achieved. As a result, this paper has accomplished the fine sensorless force control realization. The force control experiments of a ball-screw system are carried out to verify the validity and effectiveness of the proposed method.

Modeling and sensorless force control of novel tendon-sheath artificial muscle based on hill muscle model

Muscle as the actuator of human body has good biological properties. Various types of mechanical structures called artificial muscles were put forward to simulate muscle characteristics. However, the defects of artificial muscles such as high stiffness, nonlinearity, short stroke and low power density make them not well applied in robotic transmission systems. To overcome these shortages, this paper develops a type of artificial muscle actuated by a motor and tendon-sheath system based on Hill muscle model. The series and parallel elasticity in the model were simplified as linear springs in this structure which can increase the compliance and the abilities of shock-absorbing. Due to the linear series spring, the distal force of the system can be easily achieved by measuring the elongation of the spring with the use of encoders in the joints instead of installing force sensors. A model of the single tendon-sheath artificial muscle is established based on the coulomb friction model. Then, a series of experiments are conducted to validate the transmission model. After that, a method of sensorless force control is proposed based on the muscle's force-elongation model. The results indicate high accuracy of the transmission model (R-square > 0.99, Max error < 1.9 N) and good performance of force control.

Finite-Time Disturbance Observer for Robotic Manipulators.

Robotic manipulators may be subject to different types of disturbances such as unknown payloads, unmodeled dynamics, and environment interaction forces. Observing these unknown disturbances in robotic manipulators is fundamental in many robotic applications such as disturbance rejection and sensorless force control. In this paper, a novel disturbance observer (DOB) is introduced based on the insights from the finite-time observer (FTO) and robot dynamics. Different from the traditional DOBs, this new observer can provide the capability to track the disturbance within a finite time. The performance of the presented observer is verified by two kinds of typical disturbances for a two-link manipulator with a comparison with several existing DOBs. The simulation results show the rapidity and accuracy of the proposed FTO.

Sensorless force control of tip reaction force for planar operation of a wheeled inverted pendulum robot

Sensorless force control of tip reaction force for planar operation of a wheeled inverted pendulum robot

Sensorless force estimation and control of Delta robot with limited access interface

Purpose Delta robot is a parallel robot specifically designed for high-speed pick and place tasks. However, sometimes they are asked to perform additional assembling and squeezing actions, which is beyond the capability of position-controlled Delta robots. Force sensors may be expensive and add mass to the system. Therefore, the purpose of this paper is to study sensorless force control of Delta robots using limited access interface. Design/methodology/approach Static force analysis is performed to establish a relation between joint torques and external forces. The joint torques are observed from signals provided by motor drivers. A distributed mass model is proposed to compensate the gravity of upper arms and forearms. To minimize the effect of backlash and nonlinear frictions brought by gearboxes, model parameters are calibrated in two separated modes: “LIFTING” and “LOWERING”. Finally, a hybrid force estimation model is built to deal with both cases simultaneously. Surrogate model-based force control law is proposed to increase the force control loop rate and handle the force control problem for discrete position-controlled Delta robots. Findings The results show that the force estimation model is effective and mode separation can significantly improve the accuracy. The force control laws indeed stabilize the robot in desired states. Originality/value The proposed solution is based on position-controlled commercial Delta robot and requires no additional force sensor. It is able to extend Delta robots’ capability and meet requirements of emerging complex tasks.

Experimental dynamic identification and model feed-forward control of Novint Falcon haptic device

This paper addresses the dynamic identification and model feed-forward control implementation of a Novint Falcon haptic device. Firstly, inverse and forward kinematic problem of the device is obtained. Subsequently, equations describing the robot’s dynamics is derived using Newton–Euler approach which includes friction of active joints. The unknown dynamic parameters including mass and moment of inertia of constituting parts along with dry and viscous friction coefficients is identified practically by a white-box identification approach using LS technique. Furthermore, the torque digital scale coefficient which Novint Falcon’s firmware uses is identified. To the end of obtaining some of regressors a novel numerical-based differentiation approach is presented. Having an experimentally verified dynamic model in hand, the model is used in a model feed-forward impedance controller in order to omit the effect of natural dynamics of the robot, including active joints frictions from displayed impedance. Experimental tests reveal that the implemented force sensor-less controller performs 26% more accurate in terms of output force comparing device’s official controller.

Sensorless force control for dielectric elastomer transducers

Multilayer stack-transducers made of dielectric elastomers generate considerable tensile forces and deformations when they are electrically stimulated. Thus, due to their capacitive behavior, they are for example energy efficient substitutes for conventional electromagnetic drives and enable various further completely new applications. Within this contribution, we present a sensorless force control for dielectric elastomer stack-transducers driven by a bidirectional flyback-converter. This force control for example can be used as interface for superimposed application-oriented controls. First of all, the properties of dielectric elastomer stack-transducers and the flyback-converter as well as its control structure are summarized. Due to the characteristic behavior of the power converter, the design of a sliding mode force controller is carried out based on an analytical model of the coupled electromechanical system containing the transducer and driving electronics. Using this model, the transducer force can be determined with the measured driving voltage and deformation so that an explicit force measurement is not required. In a first step, a two-point controller is developed for the variable structure control that is afterward extended in order to improve the control quality, for example, by adapting the inner power converter control. Finally, the experimental validations with a prototype stack-transducer and bidirectional flyback-converter demonstrate the feasibility as well as the precision and dynamics of the developed force control.

Sensorless Force Control with Observer for Multi-functional Upper Limb Rehabilitation Robot

Sensorless Force Control with Observer for Multi-functional Upper Limb Rehabilitation Robot