This paper studies the precision motion control of single-rod hydraulic actuators with time-varying unknown inertia loads. The swing motion control of a robot arm driven by a single-rod hydraulic actuator (a scaled down version of industrial backhoe loader arm) is used as a case study. During the swing motion, the effective mass acting on the hydraulic actuator varies greatly with the swing angle and thus time-varying. Furthermore, due to the change of the load of the robot arm in operations, the effective mass is also unknown. A discontinuous projection based adaptive robust controller is constructed to take into account this strong dependence of the effective mass on the fast changing swing angle and the unknown payload of the arm. The resulting ARC controller is able to take into account not only the effect of parametric uncertainties coming from the payload and various hydraulic parameters but also the effect of uncertain nonlinearities such as uncompensated friction forces and external disturbances. The ARC controller guarantees a prescribed output tracking transient performance and final tracking accuracy while achieving asymptotic output tracking in the presence of parametric uncertainties. In addition, the zero output tracking error dynamics for tracking a large class of time-varying trajectories is shown to be globally uniformly asymptotically stable. Comparative experimental results are presented to show the effectiveness and the high performance nature of the proposed control algorithm.

The lack of position or velocity feedback is a very common problem particularly on construction equipment such as excavators and backhoes. The extra cost of adding position/velocity sensors is prohibitive as well as the high likelihood of sensor failure in such a harsh environment. These factors make the possibility of improving control performance with limited sensory feedback very attractive. This paper studies the velocity control of a single-rod double actuating hydraulic cylinder utilizing a programmable valve with only cylinder pressure feedback. The boom motion control of a scaled-down model of an industrial backhoe loader arm is used as a case study. The programmable valve used in this study is a unique combination of five proportional cartridge valves connected in such a way that the meter-in and meter-out flows can be independently controlled by four of the valves as well as a true cross port flow controlled by the fifth valve. The programmable valve decouples the control of the meter-in and meter-out flows providing tremendous control flexibility to control the cylinder motion while decreasing the pump energy required by utilizing the potential and kinetic energy of the load. This paper demonstrates the potential energy savings possible through the use of the programmable valve. The paper also proves that significant improvements in the velocity tracking performance of hydraulic cylinders can be attained with only pressure feedback.

A new perspective on dealing with the noise sensitive problem of repetitive control algorithms is given in the paper. It is firstly shown that, in continuous-time domain, what the conventional repetitive learning algorithm does is equivalent to adapting all the values of the periodic uncertainties over one period. Such an endeavor means very high bandwidth of the learning algorithm as an infinite number of parameters need to be adapted, which puts a great demand on microprocessor memory in implementing the algorithms. At the same time, such a formulation also makes the algorithm very sensitive to noise as it treats the values of the periodic uncertainties over the same period totally independent from each other, just like a random noise. Based on this new perspective on the noise sensitive problem of repetitive algorithm, a simple remedy is provided for the recently proposed adaptive robust repetitive control (ARRC) design by recognizing the physical dependence of the values of the periodic uncertainties over the same period and using certain known basis functions to capture these physical dependence. By doing so, only the amplitudes of these known basis functions need to be adapted on-line. The net results are that, not only the number of the parameters to be adapted is reduced drastically, but also the noise sensitive problem of the conventional learning algorithm is overcome. The precision motion control of a linear motor drive system is used as an application example. The comparative experimental results demonstrate that, with the new adaptive robust repetitive control design, not only the noise sensitive problem of repetitive learning is completely eliminated, but also a much improved tracking performance is achieved due to the built-in extrapolation capability of the basis functions used.

Compared to conventional robot manipulators driven by electrical motors, hydraulic robot arms have richer nonlinear dynamics and stronger couplings among various joints (or hydraulic cylinders). This paper focuses on the physical model based coordinated adaptive robust control (ARC) strategies that explicitly take into account the strong coupling among various hydraulic cylinders (or joints). In our recent studies, two such methods were proposed to avoid the need of acceleration feedback in doing ARC backstepping designs. The first method uses an observer to recover the state needed for the ARC backstepping design. The second method utilizes the property that the adjoint matrix and the determinant of the inertial matrix can be linearly parametrized by certain suitably selected parameters and employ certain over-parametrizing techniques. Theoretically, both the resulting ARC controllers guarantee a prescribed output tracking transient performance and final tracking accuracy while achieving asymptotic output tracking in the presence of parametric uncertainties only. This paper focuses on the comparative studies of these two methods under various practical constraints. Extensive simulation results which are based on a three degree-of-freedom (DOF) hydraulic robot arm are presented to illustrate the advantages and drawbacks of each method.

This paper studies high performance robust motion control of linear motors that have a negligible electrical dynamics. A discontinuous projection based adaptive robust controller (ARC) is constructed. Since only output signal is available for measurement, an observer is designed to provide exponentially convergent estimates of the unmeasurable states. This observer has an extended filter structure so that online parameter adaptation can be utilized to reduce the effect of the possible large nominal disturbance. Estimation errors that come from initial state estimates and uncompensated disturbances are effectively dealt with via certain robust feedback at each step of the ARC backstepping design. Theoretically the resulting controller achieves a guaranteed transient performance and a prescribed final tracking accuracy. In the presence of parametric uncertainties only, asymptotic output tracking is also achieved. This scheme is implemented on an epoxy core linear motor. Extensive experimental results are presented to illustrate the effectiveness and the achievable control performance of the proposed scheme.

This paper studies local valve control of the electro-hydraulic system. The sluggish response of hydraulic control valve usually becomes the bottleneck of whole system performance. Although fast valves (e.g. high-bandwidth servo-valves) are available, they are far more expensive than slow valves (e.g. proportional directional control valves). To improve the performance of proportional directional control valves, three different types of controllers are synthesized. Firstly, based on the pole zero cancellation technique, an open loop compensator is designed which requires the accurate valve dynamic model information; Secondly, a full state feedback adaptive robust controller (ARC) is synthesized, which effectively takes into account the effect of parametric uncertainties and uncertain nonlinearities such as friction force and flow force. Finally, an output feedback ARC controller is synthesized to address the problem of unmeasurable states. Theoretically, the proposed ARC controllers guarantee a prescribed output tracking transient performance and final tracking accuracy while achieving asymptotic output tracking in the presence of parametric uncertainties. Comparative experimental results are obtained to show the advantages and limitations of each method.

In recent years, many works focusing on improving motion tracking accuracy of hydraulic actuators have been done by developing nonlinear control algorithms. Model nonlinearity compensation, such as valve deadband, nonlinear friction force, and flow leakages, is one of the key points in the control design. However, the inherently nonlinear valve flow characteristic is usually treated as linear for simplicity, which might lead to undesirable flow rate deviation and reduce the tracking precision especially when the cylinder moves at a low speed. In this study, an online valve flow nonlinearity compensation method is developed to improve the motion tracking precision of hydraulic actuators. Specifically, polynomials are used to mimic the nonlinear valve flow characteristic between the valve control input, pressure difference, and flow. The coefficients in polynomials are synthesized based on the offline identification experiments while the lumped discrepancy is updated online in the proposed control design. The adaptive robust control approach is also used to develop the closed-loop motion controller, which can handle the parametric uncertainties and uncertain nonlinearities in the electro-hydraulic system. Comparative experiments are conducted on an independent metering system with a differential cylinder and the results show the improved tracking precision with nonlinear valve flow comparing to the linear one, verifying the effectiveness of the proposed approach.

Pump control systems are widely used, because of their energy saving properties and their high efficiency. Nevertheless, pump control systems generally have a low accuracy, because of the nonlinear pump flow and other nonlinearities of hydraulic systems. Furthermore, due to the underlaying high order dynamics, pump control systems have low response times, which is one of the main drawbacks. Servo motor-pump direct driven systems are commonly used in today’s industry. The application of servo motor pumps allow a reliable performance with quick response times. For low speeds, however, servo motor pumps do not achieve a high accuracy due to the pump flow deviation. The goal of this paper is to present a controller for a servo motor-pump direct driven system, which improves the accuracy especially at low pump speeds. For that purpose, the controller uses a nonlinear flow mapping, which maps the desired pump flow and pump pressure to the voltage input for the motor-pump. Additionally, the controller uses a backstepping method, which tracks the position as a first step and regulates the pressure as a second step. Comparison experiments to a controller without nonlinear flow mapping show that the controller designed in this paper has improved performances and tracking accuracy.

In this paper, a strictly defined new orthogonal global task coordinate frame (NGTCF) based on the false position method is proposed for precision contouring control of biaxial systems. In contrast to the existed global task coordinate frame (GTCF), the value of the normal coordinate in NGTCF directly represents the contour error, rather than the first-order approximation. Moreover, different from the conventional GTCF just suitable for contours with explicit shape functions, the proposed NGTCF can be utilized in various complex contours. The false position method is adopted to calculate the curve coordinates of actual points in NGTCF. Then an adaptive robust controller (ARC) is designed to deal with the effects of strong coupling of the system dynamics in the task space and modeling uncertainties. The proposed NGTCF-based ARC contouring control strategy is tested on a linear motor driven biaxial industrial gantry. Experiments under different contouring tasks with high-speed and large-curvature are conducted to verify the effectiveness of the proposed method, and the experimental results confirm that the excellent contouring performance of the proposed approach can be achieved.

The independent metering approach makes the two chamber pressure states of the hydraulic actuator completely controllable, which offers the opportunity for energy conservation by reducing the working pressure. However, there still remains a great deal of throttling losses since the entire flow rate is throttling controlled with the valves. In order to avoid throttling losses, the direct pump controlled system might be an option, but the relatively slow dynamic response restricts its application in occasions with response and precision demands. To realize further energy conservation and simultaneous high precision tracking performance, this paper proposes a brand new hardware configuration which integrates the direct pump control, independent metering, and energy reuse methods and takes advantages of their respective strengths. The adaptive robust control approach is applied to deal with the nonlinear control problems. Comparative simulations are done with the focuses on energy consumption and tracking performance.

To perform complicated tasks in the unstructured environment, teleoperation robot systems integrating human operators in the control loop have been investigated by researchers recently. Currently, most of the existing researches on the teleoperation robot systems focus on stability-guaranteed issues in the presence of time delays existing in the communication channel between the master and slave site. However, the high accurate performance-oriented motion tracking in the slave site is still a challenging issue due to the following facts: with the coupling dynamics existing in the joints, the dynamics of the slave robot behaves complexly significantly when the degree of freedoms (DoFs) increase, which makes the model compensation extremely difficult in the controller design. Next, parameters variations such as payload changes and effects of modeling errors exist in the actual dynamics of the slave robot manipulator. In this paper, to achieve a high accurate motion tracking performance in the slave site, a teleoperation robot system is configured in which the slave robot is a Baxter manipulator with 7 DoFs. The complicated dynamic model of the Baxter manipulator with 7 DoFs is first established and then further formulated in a linear parametric form for the purpose of online parameter adaptations. In this way, an adaptive robust control (ARC) strategy is developed to deal with the payload variations and modelling errors effectively. The simulation results on the configured teleoperation robot systems indicate that a higher accurate motion tracking performance can be achieved by the developed ARC controller in the presence of payload variations and modelling uncertainties in the Baxter manipulator with 7 DoFs.

Pump control hydraulic systems have been widely used in industry by the advantages of no throttling loss and overflow loss as well as high power-to-volume ratio. However, the characteristics of high order dynamics, high nonlinearities and disturbances make the accurate position control of those systems very challenging. And to implement the controllers easily, some dynamics such as servo motor loop are usually ignored in most of existing methods, which may lead to the limitation of closed-loop bandwidth and disturbance rejection ability. In this paper, adaptive robust control (ARC) algorithm is utilized in a pump control electro-hydrualic system. The ARC guarantees the stability and high performance in the presence of model uncertainties and nonlinear disturbances. For the high-order of the hydraulic system, a modified three-step backstepping method is constructed which is covering the whole electro-hydraulic system. The servo motor-pump dynamics is taken into considered in the three-step adaptive backstepping controller design. Theoretical control performance based on Lyapunov functions and the simulation results proved that the control strategy this paper proposed achieved high performance in spite of the nonlinearities and uncertainties.

The control objective of exoskeleton for human performance augmentation is to minimize the human machine interaction force while carrying external loads and following human motion. This paper addresses the dynamics and the interaction force control of a 1-DOF hydraulically actuated joint exoskeleton. A spring with unknown stiffness is used to model the human-machine interface. A cascade force control method is adopted with high-level controller generating the reference position command while low level controller doing motion tracking. Adaptive robust control (ARC) algorithm is developed for both two controllers to deal with the effect of parametric uncertainties and uncertain nonlinearities of the system. The proposed adaptive robust cascade force controller can achieve small human-machine interaction force and good robust performance to model uncertainty which have been validated by experiment.

This paper presents an inversion-based adaptive robust tip tracking control strategy that results in high tracking performance while guaranteeing the robustness for a class of linear motor driven single-link flexible beam. To address the non-minimum phase characteristics of the system, the inversion control is achieved by applying output redefinition technique as well as defining the new output at the tip of the flexible beam. In addition, the adaptive robust control (ARC) is introduced to deal with the parameter uncertainties and model uncertainties and to guarantee transient and steady state tracking performance. Comparative simulations and actual experiments based on linear motor driven flexible beam system are carried out to verify the effectiveness of the proposed control strategy.

Steerable flexible needles with bevel-tip are designed for many medical diagnoses and treatments. In this paper we present a new kinematic model for the bevel-tip flexible needle. Based on the analysis of needle deflection, the procedure of needle insertion can be decomposed of n sub-procedures, which are independent on the depth of insertion. In each sub-procedure, the tip is steered though the base to generate three motions with respect to the body-frame Ox i y i z i : the rotation about z i , the translation along z i and the rotation about y i . The kinematics of flexible needle with bevel-tip is presented using the Denavit-Hartenberg method. The inverse kinematics of needle insertion is derived from the kinematic equation of flexible needle. The solution sets are discussed for the insertion consisting of consecutive rotation and insertion control actions. Furthermore, how to select suitable insertion position and orientation is discussed.

The magnetorheological (MR) control valve is a major component in MR fluid systems to achieve controllable pressure drop or damping characteristics in practice. However, the optimal design of MR control valve is fairly complex due to large extent of design parameters from both magnetic flux generation and mechanical flow characteristics, as well as different requirements or constraints in practical applications. In this paper, the analytical electro-mechanic-magnetic coupling model of the MR control valve with annular-radial flow path is firstly investigated to quantitatively predict the relationship between design parameters and achievable performances such as pressure drop and dynamic range etc.. And then comparison results based on analytical analysis and finite element method are presented to validate the effect model utilized in MR valves. Consequently, a performance-oriented optimization of MR control valves with annular-radial flow path in a non-dimensional design concept is developed through minimizing reciprocal of dynamic range and identifying several optimal internal design parameters subject to predefined constraints under fairly less quantity of combined external design parameters.Finally, the inherent sensitivity of achievable performances with respect to external design parameters is analyzed to provide practical instructions for appropriate specification of the MR control valve.

Time optimal trajectory planning under various hard constraints plays a significant role in simultaneously meeting the requirements on high productivity and high accuracy in the fields of both machining tools and robotics. In this paper, the problem of time optimal trajectory planning is first formulated. A novel back and forward check algorithm is subsequently proposed to solve the minimum time feed-rate optimization problem. The basic idea of the algorithm is to search the feasible solution in the specified interval using the back or forward operations. Four lemmas are presented to illustrate the calculating procedure of optimal solution and the feasibility of the proposed algorithm. Both the elliptic curve and eight profile are used as case studies to verify the effectiveness of the proposed algorithm.