Additional Feedforward Term Research Articles

Cascade Control for Two-Axis Position Mechatronic Systems

The current paper proposes an extension for two controller design procedures for a two-axis positioning mechatronic system, followed by a comparison between them. As such, the first method consists in formulating an optimization problem in terms of linear matrix inequalities (LMIs) in order to impose the location of the closed-loop poles, considering an uncertain model of such a system. The uncertain model is treated using various forms of linear differential inclusions (LDIs), namely, polytopic LDI (PLDI) and diagonal norm-bound LDI (DNLDI). Additionally, the problem regarding the command signal constraints is characterized in terms of LMIs. The imposed structure of the controller is a cascade one, with a PI controller for the position loop and a P controller for the velocity loop, having an additional feedforward term. On the other hand, the second method consists in designing a cascade controller with an inner P controller, as in the previous method, the outer controller being a fractional-order IλIDλD (FO–ID) controller. In terms of degrees of freedom, both methods present four degrees of freedom for each axis. The presented controller design procedures will be applied for a numerical example of such a positioning system, and a comparison of the obtained performance metrics will be performed.

Design and Implementation of a T-Type Based Topology for Grid-Connected Current-Source Inverters in PV Applications

This paper presents a novel non-isolated 5-level inverter for interfacing renewable sources to the power grid. It consists of a modified T-type converter with an HERIC-based AC decoupling strategy. The 5-level output voltage is filtered by a passively damped LCL filter, resulting in a high power density structure. Experimental results on a 250 (W) prototype are presented to validate the operating principle of the proposed topology and show a high efficiency over the entire output power range. Besides, the injected current complies with the IEEE 1547-2018 specifications regarding harmonic content until the 10th harmonic, using a classical PI controller with an additional feedforward term.

Unified Control Parameterization Approach for Finite-Horizon Feedback Control With Trajectory Shaping

This article presents control parameterization as a unifying framework for designing a linear feedback control law that achieves the finite-time transfer of output as well as trajectory shaping. Representing control input as a linear combination of independent basis functions allows wide variability in the resultant feedback control laws through the selection of the number and types of basis functions. Given an array of basis functions that meets the trajectory shaping necessities, the unified design approach proceeds with the determination of the coefficients so that the predicted trajectory attains the desired output at the final time. The input evaluated with the coefficients found at each instance essentially turns out to be a linear state feedback policy with an additional feedforward term and time-dependent gains, which is appropriate for practical use. The unified control parameterization approach lends itself well to missile guidance applications with the expandability and direct trajectory shaping capability that it provides. To emphasize the expandability of the framework, this study revisits the trajectory shaping guidance laws from the control parameterization viewpoint and shows how the notion of specifying input basis functions not only generalizes various existing methods but also enables further extensions. Furthermore, an application to integrated guidance and control illustrates the strength of the design process in handling the shaping requirements more directly through the construction of appropriate basis.

Improved Widrow–Hoff Based Adaptive Control of Multiobjective PV-DSTATCOM System

This article presents an adaptive control algorithm for a multiobjective grid-tied solar photovoltaic (PV)-distribution static compensator. The multiobjective system extracts the maximum energy available across the solar PV array and delivers it to the load and the grid connected at the common point of interconnection (PIC). In addition to this, the system reduces the losses on the distribution network and improves the quality of the voltage at the PIC by maintaining the grid currents balanced with THD value less than 5%. The perturb and observe-based maximum power point tracking method is used to extract the maximum energy, while the voltage source converter (VSC) is controlled by the presented improved Widrow-Hoff algorithm. This control algorithm improves the grid current power quality by estimating the fundamental component of the load current. It uses two weight to track the desired signal as compared to the other existing similar algorithms, which improves its tracking accuracy, reduces the steady-state error, and oscillation form the desired output signal. The same algorithm is used for synchronization of the VSC with the distribution network. Hence, it eliminates the need for another algorithm for the synchronization, which further improves its performance in dynamic conditions. An additional feed-forward term corresponding to the solar PV array (IPVFF) power is added in the VSC control to improve its dynamic response in the severe atmospheric conditions. The performance of the system is validated through simulated and test results in various dynamic conditions.

Cooperating manipulator control with feedforward dynamic compensation

In this paper the transport of an object by two cooperating manipulators is simulated and performed experimentally. The controller used for each manipulator is the sum of two parts: (a) an off-line calculated feedforward part, which includes compensation of the manipulator and object rigid-body dynamics and (b) an on-line computed PID feedback part. The simulation problem is handled by first computing the open chain forward dynamics of each manipulator using an efficient O(n) recursive multibody dynamics algorithm. The closed chain problem is then solved through addition of a corrective joint acceleration term to the open chain solution, which is computed from the constraint forces. In the experiment an additional feedforward term is utilized, which takes into account viscous and Coulomb friction in the manipulators’ joints. The simulation and experiment are carried out using two PUMA 560 manipulators to move a steel cylinder along a sine wave path. Tracking performance results are given in Cartesian space for the simulation and experiment.

An iterative scheme for learning gravity compensation in flexible robot arms

Mimicking the case of rigid robot arms, the set-point regulation problem for manipulators with flexible links moving under gravity can be solved by either model-based compensation or PID control. The former cannot be applied if an unknown payload is present or when model parameters are poorly estimated, while the latter requires fine and lengthy tuning of gains in order to achieve good performance on the whole workspace. Moreover, no global convergence proof has been yet given for PID control of flexible robot arms. In this paper, a simple iterative scheme is proposed for generating exact gravity compensation at the desired set-point, without the knowledge of rigid or flexible dynamic model terms. The control law starts with a PD action on the error at the joint level, updating at discrete instants an additional feedforward term. Global convergence of the scheme is proved under a mild condition on the proportional gain and a structural property on the arm stiffness, which is usually satisfied in practice. The proposed learning scheme is also extended to the direct control of the end-effector (tip) position. Experimental results are presented for a two-link robot with a flexible forearm moving on a tilted plane.

Learning gravity compensation in robots: Rigid arms, elastic joints, flexible links

AbstractThe setpoint regulation problem for robotic manipulators is a basic task that can be solved either by PID control or by model‐based gravity compensation. These approaches are commonly applied both to rigid arms and to robots with flexible links and/or elastic joints. However, PID control requires fine and lengthy tuning of gains in order to achieve good performance over the whole workspace. Moreover, no global convergence proof has yet been given for this control law in the case of flexible links or elastic joints. On the other hand, a constant or even a configuration‐dependent gravity compensation is only an approximate solution when an unknown payload is present or when model parameters are poorly estimated. In this paper a simple iterative scheme is proposed for generating exact gravity compensation at the desired setpoint without the knowledge of dynamic model terms. The resulting control law is shown to be global asymptotically stable for rigid arms as well as for manipulators with elastic joints or flexible links. Starting with a PD action on the error at the joint (i.e. motor) level, an additional feedforward term is built and updated at discrete instants. Convergence of the scheme is proved under a mild condition on the proportional gain, related to a bound on the gravity terms. In the presence of concentrated or distributed flexibility a structural property of the joint or of the link stiffness is further required, largely satisfied in practice. Simulation results are given for a three‐link rigid arm and experimental results are also presented for a two‐link robot with a flexible forearm.

An Iterative Scheme for Learning Gravity Compensation in Flexible Robot Arms

Mimicking the case of rigid robot arms, the set-point regulation problem for manipulators with flexible links moving under gravity can be solved by either model-based compensation or PID control. The former cannot be applied if an unknown payload is present or when model parameters are poorly estimated, while the latter requires fine and lengthy tuning of gains in order to achieve good performance on the whole workspace. Moreover, no global convergence proof has been yet given for PID control of flexible robot arms. In this paper, a simple iterative scheme is proposed for generating exact gravity compensation at the desired set point, without the knowledge of rigid or flexible dynamic model terms. The control law starts with a PD action on the error at the joint level, updating at discrete instants an additional feedforward term. Global convergence of the scheme is proved under a mild condition on the proportional gain and a structural property on the arm stiffness, which is usually satisfied in practice. Experimental results are presented for a two-link robot with a flexible forearm moving on a tilted plane.

Extended Bandwidth Zero Phase Error Tracking Control of Nonminimal Phase Systems

This paper describes a new feedforward algorithm for accurate tracking control of nonminimal phase systems. Accurate feedforward calculation involves a prefilter design using the inverse system model. Nonminimal phase systems cause problems with this prefilter design, because unstable zeros become unstable poles in the inverse model. The zero phase error tracking control algorithm (ZPETC) consists of a substitution scheme, which removes the unstable zeros. This scheme introduces a small gain error, which increases with frequency, but no phase error. This paper investigates additional properties which give more insight into the ZPETC algorithm, and allow to improve it. The improved algorithm is based on the same substitution scheme as ZPETC, but adds additional feedforward terms to compensate for the gain error. These additional terms increase the frequency range for which the overall transfer function has only limited gain error, without introducing phase errors. The additional feedforward terms repeatedly reduce the tracking error proportional to ε2, ε4, ε6, …, where ε is the ZPETC tracking error. The new feedforward algorithm or new substitution scheme is therefore called “extended bandwidth zero phase error tracking control algorithm” (EBZPETC). Experimental results on a one-link flexible robot compares both methods.

A DSP-based adaptive controller for a smooth positioning system

The implementation of a self-tuning regulator for the positioning of a direct-drive servomotor is described. The servo motor is a permanent magnet DC motor in which no speed reducer is used. The auto-tuning regulator consists of two major loops. The inner loop contains a feedback (PD or PID) regulator with additional feedforward terms. The parameters of the feedforward compensation are adjusted by the outer loop, which contains an online parameter estimator. The estimator is based on a recursive least-squares equation, and the estimated parameters are the load inertia and viscous friction. This self-tuning regulator has been simulated with PC.MATLAB, and the results demonstrate the high performance of the scheme. Experimental results obtained with a small DC motor (Electrocraft E-576) are presented, and these results show good agreement with the digital simulation results. There are two innovative aspects to this work. First, parameter estimation is used to adapt the feedforward compensation terms instead of the gains of the feedback controller, as usually is the case in conventional indirect self-tuning regulators. Secondly, the complete adaptive controller has been implemented using a single-chip digital signal processor (DSP), which results in the reduction of system hardware and cost.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>