The quadruped robot locomotion based on force control

In teleoperation tasks, communication delays between master and slave sides negatively affect the stability and transparency of closed loop system, and make it difficult to maintain a desired contact force. In order to improve the performance of contact force control in teleoperation under large time delays, a shared compliant control method is proposed in this paper. On the master side, the operator issues a desired contact force command according to the feedback of the contact stiffness identified on line besides motion commands. And on the slave side, a local contact force controller is designed using an adaptive Smith predictor, so as to shares control load with the operator. Experimental results show that this method can improve the force control performance, lower the difficulty of operation, and help the operator complete contact tasks with proper forces.In teleoperation tasks, communication delays between master and slave sides negatively affect the stability and transparency of closed loop system, and make it difficult to maintain a desired contact force. In order to improve the performance of contact force control in teleoperation under large time delays, a shared compliant control method is proposed in this paper. On the master side, the operator issues a desired contact force command according to the feedback of the contact stiffness identified on line besides motion commands. And on the slave side, a local contact force controller is designed using an adaptive Smith predictor, so as to shares control load with the operator. Experimental results show that this method can improve the force control performance, lower the difficulty of operation, and help the operator complete contact tasks with proper forces.

BackgroundSuccessful object manipulation relies on the ability to form and retrieve sensorimotor memories of digit forces and positions used in previous object lifts. Past studies of patients affected by Parkinson's disease (PD) have revealed that the basal ganglia play a crucial role in the acquisition and/or retrieval of sensorimotor memories for grasp control. Whereas it is known that PD impairs anticipatory control of digit forces during grasp, learning deficits associated with the planning of digit placement have yet to be explored. This question is motivated by recent work in healthy subjects revealing that anticipatory control of digit placement plays a crucial role for successful manipulation.Methodology/Principal FindingsWe asked ten PD patients off medication and ten age-matched controls to reach, grasp and lift an object whose center of mass (CM) was on the left, right or center. The only task requirement was to minimize object roll during lift. The CM remained the same across consecutive trials (blocked condition) or was altered from trial to trial (random condition). We hypothesized that impairment of the basal ganglia-thalamo-cortical circuits in PD patients would reduce their ability to anticipate digit placement appropriate to the CM location. Consequently, we predicted that PD patients would exhibit similar digit placement in the blocked vs. random conditions and produce larger peak object rolls than that of control subjects. In the blocked condition, PD patients exhibited significantly weaker modulation of fingertip contact points to CM location and larger object roll than controls (p<0.05 and p<0.01, respectively). Nevertheless, both controls and PD patients minimized object roll more in the blocked than in the random condition (p<0.01).Conclusions/SignificanceOur findings indicate that, even though PD patients may have a residual ability of anticipatory control of digit contact points and forces, they fail to implement a motor plan with the same degree of effectiveness as controls. We conclude that intact basal ganglia-thalamo-cortical circuits are necessary for successful sensorimotor learning of both grasp kinematics and kinetics required for dexterous hand-object interactions.

SUMMARYThis paper addresses the problem of contact force control for multiple contacts distributed over multiple links in a robot. This is of importance when performing complex tasks in unstructured environment, particularly in humanoid robot applications. The proposed multicontact control framework provides a new way of defining the operational space coordinates, which facilitates the specification of multiple contact control. The contact force space on multiple links is constructed as an operational space for the highest priority task. Motion control, given lower priority, can be executed using the rest of degree of freedom within the null-space of the force control. The dynamic control structure, then, provides a means to control each contact force and motion independently. This dynamic decoupling enables each contact force controller to utilize linear control theories. In particular, the contact force controllers adopt full state feedback control and estimation methods to produce robust performance with respect to modeling and parameter uncertainties. The effectiveness of the multiple contact control framework was demonstrated using a PUMA560 manipulator, with multiple contacts on the end-effector and third link. The demonstrated tasks involved controlling each of the contact forces with null-space motion.

A compliant landing strategy for a trotting quadruped robot on unknown rough terrains based on contact force control is presented. Firstly, in order to lower the disturbance caused by the landing impact force, a landing phase is added between the swing phase and the stance phase, where the desired contact force is set as a small positive constant. Secondly, the joint torque optimization of the stance legs is formulated as a quadratic programming (QP) problem subject to equality and inequality/bound constraints. And a primal-dual dynamical system solver based on linear variational inequalities (LVI) is applied to solve this QP problem. Furthermore, based on the optimization results, a hybrid motion/force robust controller is designed to realize the tracking of the contact force, while the constraints of the stance feet landing angles are fulfilled simultaneously. Finally, the experiments are performed to validate the proposed methods.

In this study, the stable detection principle of a MEMS hardness sensor with “reference plane” structure is theoretically analyzed and demonstrated with experimental results. Hardness measurement independent of contact force instability is realized by the optimum design of the reference plane. The fabricated devices were evaluated, and a “shore A” hardness scale (JIS K 6301 A) was obtained as the reference in the range from A1 to A54 under a stable contact force. The contact force dependence on hardness sensor signals was effectively reduced by 96.6% using our reference plane design. Below 5 N contact force, the maximal signal error of hardness is suppressed to A8. This result corresponds to the detection capability for fat hardness, even when the contact force is unstable. Through experiments, stable detection of human body hardness has been demonstrated without any control of contact force.

Dynamic walking is often required in a quadruped robot for the purpose of quick locomotion. It is generally known that the walking pattern of animals is changed to the trot or the pace in middle speed locomotion. Trot walking was recently studied by several groups because of its stability. In the trot, diagonal legs form pairs. While one pair of legs provides support, the other pair of legs swings forward in preparation for the next step. In the trot of the quadruped robot with point-support feet, a rotation around a diagonal line formed by the supporting leg can't be controlled directly by using torques at the articular joints. In this paper, the 3D behavior in trot walking is analyzed from the viewpoint of foot force, duty factor and the roll angle of the body. The effects of the stance and the center of gravity of the body are also examined experimentally using our quadruped robot 'COLT-2.'

Soft robots are increasingly being explored and developed in various settings that demand safe and adaptable interactions between robots and their environments. In addition, soft robots exhibit passive compliant behavior and generate continuous deformations when engaging with the environment. This imposes challenges on achieving active, on-demand interaction force control, especially when feedback force-sensing devices are not available. Consequently, there is a need to explore new model-based force control paradigms for soft robots. In this article, we propose a (quasi-)static force control approach for soft robots based on compliance modeling, avoiding the necessity for feedback control loops or extensive training data collection. The proposed approach can deliver contact force control along three Cartesian axes when the robot is actuated into various configurations. The compliance matrix is derived from the robot configuration, which allows the calculation of desired deflection displacements needed to generate on-demand forces. The resulting force control is achieved by solving inverse kinematics problems based on these deflection displacements. The efficacy of our proposed controller is validated through experiments with both one- and two-segment pneumatic-driven soft continuum robots. The results demonstrate effective static force control performance, with mean control errors below 5% of the desired peak forces.

As the application range of present day industrial robots is expanded beyond pick‐and‐place operations, spray painting and spot welding, there is an increasing need for the control of contact forces between the manipulator end point and environment. One important force control task is robotic drilling in hazardous environments. This paper addresses the analysis and characterisation of forcecontrolled robotic drilling, generation of a control strategy in the light of specifications obtained from the characterisation of this task, and the completion of a drilling operation with robot manipulator under contact force control. The experimental results demonstrate that a robot manipulator can perform drilling if enough contact thrust‐force is provided between a workpiece and drill, and is controlled properly. It follows that the analysis and characterisation of a task, especially investigation of the dynamic interactions between the tool carried by a manipulator and a workpiece, strategy generation, and controller design proceed simultaneously in order to develop a force control system for a specific task.

This paper investigates the role of precise estimation of contact points in force control. This analysis is motivated by scenarios in which robots make contacts, either voluntarily or accidentally, with different parts of their body. Control paradigms that are usually implemented in robots with no tactile system, make the hypothesis that contacts occur at the end-effectors only. In this paper we try to investigate what happens when this assumption is not verified. First we consider a simple feedforward force control law, and then we extend it by introducing a proportional feedback term. For both controllers we find the error in the resulting contact force, that is induced by a hypothetic error in the estimation of the contact point. We show that, depending on the geometry of the contact, incorrect estimation of contact points can induce undesired joint accelerations. We validate the presented analysis with tests on a simulated robot arm. Moreover we consider a complex real world scenario, where most of the assumptions that we make in our analytical derivation do not hold. Through tests on the iCub humanoid robot we see how errors in contact localization affect the performance of a parallel force/position controller. In order to estimate contact points and contact forces on the forearm of the iCub we do not use any model of the environment, but we exploit its 6-axis force/torque sensor and its sensorized skin.

A new method of generating an optimal approach velocity of a manipulator to its environment is presented in order to control the collision forces properly. First, analysis of a contact motion shows the necessity of contact control not only after the collision, as in previous studies, but also before it. Next, using a model of the force-controlled end-effector and its environment, forces generated at the contact are formulated as outputs of an autonomous system of which the initial condition is determined by the approach velocity. The optimal approach velocity is defined as the velocity which minimizes a performance index : the integration of the square deviation of contact forces from an equilibrium forces in the control of contact forces. A proportional relation between the optimal approach velocity and the contact force reference is derived analytically based on a mass-damper-spring model of the force-controlled end-effector and its environment. Results in simulation and experiment are presented to prove the effectiveness of the method.

In this paper, a compliance control strategy based on ZMP preview control for quadruped walking gait locomotion under unperceived disturbances is proposed. The preview control is designed to plan the CoM trajectory to realize the desired ZMP. When the robot suffers from interference of environmental disturbances, we introduce an adjusted expected ZMP to suppress the change of the ZMP. The force distribution module calculate desired contact force by eliminating interaction force between the stance legs and the force controller make the contact force track the desired force. Simulation results demonstrate the effectiveness of our compliance control strategy based on ZMP preview control as the quadruped robot successfully walking under unperceived disturbances.

This paper presents a control scheme to maintain consistent contact force using a handheld robot during soft tissue interaction with imaging instruments such as probe-based confocal laser endomicroscopy (pCLE). The handheld robot incorporates a 6-DOF miniature micromanipulator and a force sensor for the control. Given force set for clearly visualizing tissue on contact, the handheld robot can maintain such a tiny force by actively controlling its tool tip using the micromanipulator. It allows keeping locating the tool tip within the depth-of-focus region of the imaging instrument regardless of actual force on the soft tissue engaged by hand motion delivered to the handpiece of the robot. The contact force is regulated primarily by controlling the depth of the tool tip from the tissue surface. This procedure is accomplished by controlling the position of the tool tip via closed-loop force control. To validate the control scheme, we evaluated force tracking performance at the tool tip given a sinusoidal force input with an amplitude of 0.1 N for various frequencies, while firmly fixing the robot. As a result, the handheld robot could maintain the target force with an RMS error of less than 7.8 mN. The handheld task holding the robot on top of a single contact point was also evaluated, which yields an RMS error of 22.6 mN along the Z-axis of the sensor. Finally, the robot-aided scanning over the surface of the soft tissue was performed using the handheld robot, and the error was significantly reduced by 31.6% on average compared to unaided operation.

The surface following by a robot arm is investigated in this paper. The goal is to present control strategies so that the arm can follow a surface at a given speed with desired contact force. First, a discussion on the use of direct-drive manipulators with decoupled and invariant dynamics is given. These are shown to be suitable for such force control applications. In this case, the controller design in either joint space or end effector space would involve nonlinearities due to kinematics only. This would reduce the computation burden and allows for high performance since the kinematic parameters can be estimated with better accuracies than dynamic parameters. Second, to have a better understanding of some fundamental characteristics of force control, such as the effects of actuator dynamics, environment/sensor stiffness on the performance, experiments were done using a one degree-of-freedom direct-drive arm. The use of a high gain inner velocity loop is shown to give the force control good command following and disturbance rejection characteristics. The direct-drive manipulator shows a superior performance over conventional systems as a result of fast actuator dynamics, low friction and no mechanical backlash. The experiments are conducted to investigate the force speed of response and steady state behavior, impact control and surface following performance.

Position control was applied to industrial robots when they carry out tasks such as handling, stacking, welding. However, carrying out complex tasks like grinding, polishing and deburring requires more. In these cases, the contact force, between the manipulator's end effector and the working environment, also needs to be controlled to achieve compliance. The surface tracking based on force control is the key technology for industrial robots to perform tasks like grinding, polishing. Force control requires robots to sense the information of the external environment. In this paper, a Six-Axis Force/Torque Transducer is used to acquire the contact force between the tool and the surface. Then we use the constant contact force control algorithm, together with the orientation adjustment strategy, to keep the contact force constant and ensure the tool vertical to the contact surface to realize the surface tracking. Experiments show that the scheme is efficient, and has a certain guiding significance for manipulator in industrial production tasks where force control is required.

In this paper, model-based precision force and position control of an ionic polymer metal composite (IPMC) is presented. A 23.8 mm × 3.4 mm × 0.16 mm IPMC strip was used as an actuator in a cantilever configuration. Open-loop force and position responses of an IPMC are not repeatable, and hence closed-loop precision control is of critical importance to ensure proper functioning, repeat-ability and reliability. After feedback controllers were designed and implemented with empirically obtained fourth-order plant transfer functions, the overshoot decreased from 460 to 2.8 per cent and the settling time was reduced from 37.5 to 3.22 s in force control. In position control the overshoot decreased from 333 to 20.3 per cent and the settling time was reduced from 21.5 to 2.56 s. Microscale precision force and position control capabilities of the IPMC actuator were also demonstrated experimentally. An 8 μN force resolution was achieved with a force noise of 0.5μN r.m.s., and the position resolution was 6 μm with a position noise of 2.5 μm r.m.s. The maximum force and tip displacement achieved with the IPMC actuator under closed-loop control were 2 mN and 5 mm respectively. The IPMC actuator could follow various commanded force and position trajectories such as sinusoidal and trapezoidal position profiles, and a velocity profile with a 3 mm/s maximum velocity. A novel hybrid force and position control strategy demonstrated its utility in practical micromanipulalion applications where the actuator force must be limited to prevent damaging micro-objects. High-precision control of the IPMC at low force level proved its potential for micromanufacturing and micromanipulation applications such as robotic and biomedical microgrippers.