In various fields, small actuation systems are required to aid human activities in a narrow space and to realize fine motions. Although some miniature actuators have been developed, the size of the sensors attached to them prevents the miniaturization of the systems. In this paper, a sensorless actuation system with a compact solenoid actuator is proposed. An input signal including the AC and DC components is used. The inductance and position are estimated from the AC component. The DC component is applied in order to drive the solenoid actuator. Simulations and experiments concerning frequency characteristics and simultaneous estimation are conducted to verify the validity of the proposal. From the results of the frequency characteristics, the position and force estimation are accurately achieved up to 10Hz frequency. As a result of the simultaneous estimation, the position and force are simultaneously estimated in real time. This proposal is useful, because a small solenoid actuation system whose position and force are simultaneously estimated in real time without the need for any position and force sensors is realized.
In recent years, there have been several studies on human-machine cooperative systems. These systems involve contact between humans and the environment. Therefore, it is necessary to create a safe system in order to avoid injuring humans and the environment. On the other hand, task realization is also important. From this viewpoint, a force-based compliance controller with a force threshold is proposed in this paper. This controller achieves both task realization and adaptation to the environment. This motion selection is conducted by comparing the force command and a force threshold. In addition, a human-machine cooperative grasping/manipulating system using the proposed controller is developed. This system can conduct a human-machine cooperative motion, which consists of an autonomous motion and a human-assisting motion. The priorities of the autonomous motion and the human-assisting motion are easily designed by changing the force thresholds. The validity of the proposed system is confirmed with experimental results.
The theory of functionally different effective muscles allows for detailed measurement of the muscular strength of limbs by separating mono-articular muscles and bi-articular muscles. The theory represents the output force distribution (OFD) at the end of a limb as a hexagon whose opposite sides are parallel and the same length. Because the OFD is divided into independent muscular strengths, it must be measured accurately. The OFD can be drawn from measurements of five of the six sides. This paper proposes a method to draw the OFD from measurements of all six sides. Because the measurement does not satisfy the characteristics of the OFD, an optimization problem is proposed for minimizing the distance from the original measurement while satisfying the characteristics. These two drawing methods of OFD were applied to the lower limbs of three subjects for three different postures. Because the error of the five-side measurement method is dependent on the estimated side and posture of lower limbs, the estimated side that realizes the smallest error of the OFD for a given posture can be determined from geometrical analysis. The OFD error of the optimization method was about half that of the five-side measurement method.
The key task performed by CNCs is the generation of the time sequence of set points for driving each physical axis of the machine tool during program execution. This interpolation of axes movement must satisfy a number of constraints on axes dynamics (velocity, acceleration, and jerk), and on process outcome (smooth tool movement and precise tracking of the nominal tool path at the desired feed rate). This paper presents an algorithm that aims at solving the axes interpolation problem by exploiting an optimal control problem formulation. Unlike other solutions proposed in the literature, the approach presented here employs an original approach by assuming a predefined path tracking tolerance—to be added to the constraints listed earlier—and calculating the entire trajectory (path and feed rate profile) that satisfies the given constraints. The proposed solution is used for preprocessing a milling part program and redefining the sequence of positioning commands to cope with the solution of the OC problem. The new part program is then executed by a state-of-the-art industrial CNC, and the effectiveness in reducing execution time and axes accelerations is experimentally tested and reported.
In various servo applications, the design of a precise positioning control constitutes a trade-off between the fast transient response and the short settling time for the required accuracy. Mostly, neither ‘universal’ control gains can be found to be equally suitable for both of these objectives. In this study, the design of a cascaded precise positioning control is analyzed in the presence of nonlinear friction. The nonlinear friction strongly impacts the reference settling and can deteriorate the positioning control performance. Two common cascaded structures, and that P-PI and PI-PI, are analyzed by exposing the closed-loop dynamics of the control system with friction in details. It is shown that a more ‘traditional’ integral control part is less useful when compensating for the nonlinear friction at the reference position settling. Furthermore, we show how the settling performance can be improved by applying a feed-forward friction observer. This can be included as a plug-in after designing the surrounding cascaded feedback control and without re-tuning. The proposed control strategy is evaluated experimentally on a standard industrial linear positioning axis, with a relatively high reference speed of +/-500mm/s and narrow positioning error band of +/-0.01mm.
This paper discusses the suppression of torque ripple using multi-level inverters in motor drives. Because it is possible to improve the output waveforms using multi-level inverters, it is also possible to improve the control performance of motors. In this paper, the multi-level inverters achieve precise motion control, and a theoretical distortion index for multi-level motor drives that takes the control sensitivity into account is proposed. By calculating the distortion index, the optimal equivalent carrier frequency that can minimize torque ripple can be obtained. The validity of the proposed method is confirmed by our experimental results.
The performance of a motion control system depends on hardware capabilities, such as sensor resolution and actuator technology. For a certain plant, an amelioration in performance can often be achieved by using better sensors. This however may require a redesign of the system, as the new sensor may not fit into the existing plant. As for the actuators, many motion control systems using stepper motors exist; these stepper motors known for their high torque ripple and limited dynamic performance, especially in the presence of mechanical resonances of the driven load. A possible solution is to replace the actuator with a better one; however, this again may require a costly redesign of the plant. To improve the performance of existing plants with minimal invasive modifications, our group has developed a set of solutions, based on the use of low-cost MEMS inertial sensors, which can be easily placed on the system to be controlled. We will show the manner in which we used such sensors to develop new control strategies, to enhance the performance of existing sensors and actuators, without major modifications to the plant. This will be demonstrated through some examples taken from both laboratory and industrial applications.
Over the past decade, variable gain control has aided both the positioning accuracy (overlay and imaging) and the productivity (throughput) of several motion systems in wafer scanners. In the control of wafer scanners, i.e., the lithographic machinery used to produce chips, nonlinear elements are traditionally used to linearize the feedback loop. In this paper, however, nonlinearity is introduced to de-linearize the feedback loop, which is carried out with the aim to better deal with design trade-offs. For wafer scanners, an overview is given of the most relevant nonlinear control designs, which are based on variable gains. This includes posing a framework not only for stability analysis in which (measured) frequency response functions are key in providing graphical checks, but also for discussing performance via nonlinear Bode diagrams.
The rendering of tactile sensations has recently been attracting attention in the fields of multimedia and communication. This study focuses on thermal sensation, which is one of the important elements for rendering a tactile sensation. Humans feel an object by hand movements such as tracing, grasping, and stroking. In response to such movements, it is necessary to take into account the change in the contacting point on the system. However, most conventional studies on thermal rendering did not take such situations into account in their system design. This study aims to fill this gap by proposing a control method for reproducing thermal conductance with the detection of a single contacting part. In this study, a copper plate attached to a Peltier device is regarded as a thermal system, and the system is modeled with three parts. Heat disturbance observers and heat inflow observers are then allocated to each part, and the thermal conductance at any single part is virtually reproduced. Moreover, a detection method of single contacting part is introduced in the proposed system. Combining the detection method with the controller enables different thermal sensations to be rendered depending on the contacting part.
Recently, the development of motion-copying systems has enabled us to preserve and reproduce motion data including contact motion by humans. A system having multiple degrees of freedom (DOFs) is necessary for precise preservation of human motion. However, the DOF of a permeating device might not be the same as the DOF of the device used at the time of motion saving. The motion data should be reproduced precisely even if the system has a lower DOF than the system of the motion-saved phase. To address this problem, a method to determine redundant motion data on the basis of analysis of human motion is proposed. Hierarchical clustering is used for the analysis of motion data having high similarity. Furthermore, redundant motion data are determined for the maximum eigenvalue and eigenvector obtained from the adjacency matrix in graph theory. Using graph theory, the redundant data can be determined in each motion. The saved motion is reproduced by a lower-DOF system. The validity of the proposed method is confirmed by experiments in which grasping-motion data with four DOFs are reproduced by a 3-DOF device.
The recent rapid growth of the information industry has strongly increased the demand for large-capacity hard disk drives (HDDs). This means that increasing the areal recording density has become an important technical challenge in HDD development. To increase the HDD areal recording density, we have developed emerging technologies for high-accuracy positioning control of magnetic heads in the head-positioning system of HDDs. This paper introduces two examples of emerging technologies for future HDDs: “Vibration Control with Thin-Film-Coil Actuator” and “High-Bandwidth Control with Thermal Actuator”. The former utilizes a film-coil actuator attached to a coil of a voice-coil motor. The latter utilizes a thermal actuator in the magnetic head. These control systems employ triple-stage-actuator systems to improve the control performance of the head-positioning system. The validation results showed that these control systems enable us to achieve high-accuracy positioning control of magnetic heads for future HDDs.
In the last few decades, the increase in the worldwide elderly population and the progress in the treatment of severe and chronic pathologies have led to a growing demand for rehabilitation therapies. Meanwhile, rehabilitation robotics has started to grow and to evolve, in order to develop suitable robotics devices and control strategies to better assist patients during training and to promote rehabilitation processes. In particular, some control strategies are designed to assist patients in completing the desired movements while applying the minimum force necessary. As a result, an “assist-as-needed” behavior can be achieved. A novel nonlinear adaptive compliance controller, that aims to achieve such “assist-as-needed” behavior, has been developed and is presented in this paper. In addition to promote the active participation of patients, the proposed control also provides a tool to estimate and evaluate patient's state and therapeutic improvements. The proposed controller is obtained by appropriately merging a PD (proportional and derivative) control and an adaptive learning control. The latter is driven by the errors made by patients while performing the assigned exercise. As a result, the PD controller parameters are adapted according to different patient injuries and degrees of impairments and may be used to evaluate the improvements during training sessions. The paper presents an overview of the novel control algorithm and some preliminary clinical trials with real patients, demonstrating benefits of the controller.
To achieve force control of an industrial robot, this paper proposes a new force control method based on the spring ratio and the instantaneous state observer. To analyze the behavior of an industrial robot in contact with the environment, this paper analyzes a two-inertia system in contact with the environment. On the basis of the resonance ratio considering the environment, this paper shows that the stability of the resonance ratio control depends on the bandwidth of the torsional torque estimation. To achieve stable resonance ratio control, this paper employs the resonance ratio control with the instantaneous state observer. In addition, a force control system using the I-P force controller and the instantaneous state observer is employed. This paper shows that the resonance ratio of the force control system is determined to be the spring ratio S. The effectiveness of the proposed method is confirmed by a numerical simulation and experiments using the industrial robot arm.
High-precision stages are widely used in the semiconductor and flat panel industries. Because six degrees of freedom have to be controlled in these stages, coupling forces can degrade their control performance and stability. This paper proposes a decoupling control method from the scanning motion x to the pithing motion θy using multiple actuators considering the misalignment of the actuation point, the center of gravity (CoG), and the center of rotation (CoR). The method proposed in this paper consists of three steps: 1) a detailed modeling of the the scanning motion x and the pitching motion θy, 2) a changeable actuation point stage, and 3) an integrated design of mechanism and control. The validity of the proposed method is demonstrated by experiments.
A helical motor, which is composed of helically-shaped stator and mover, has various advantages. However, in order to control this motor, we need two encoders to measure the translational position and rotational angle of the mover. This paper describes a method of estimating the gap displacement, which is the relative position of the mover from the center of the stator slot. If we can estimate the gap displacement accurately, one of two encoders used by present helical motors can be removed. Our proposed method uses the relation between the gap displacement and the inductance, which is derived from the mathematical model of the helical motor. In order to estimate the gap displacement, high-frequency voltage is injected into the armature windings and a current of the same frequency component is extracted. The validity of the method is confirmed by numerical simulations and experiments.
Recent advances in theory, algorithms, and computational power make it possible to solve complex, optimal control problems both for off-line and on-line industrial applications. This paper starts by reviewing the technical details of the solution methods pertaining to three general categories: dynamic programming, indirect methods, and direct methods. With the aid of a demonstration example, the advantages and disadvantages of each method are discussed, along with a brief review of available software. The main result that emerges is the indirect method being numerically competitive with the performance of direct ones based on non-linear programming solvers and interior point algorithms. The second part of the paper introduces an indirect method based on the Pontryagin Minimum Principle (PMP). It also presents a detailed procedure and software tools (named PINS) to formulate the problem, automatically generate the C++ code, and eventually obtain a numerical solution for several optimal control problems of practical relevance. The application of PMP relates to the analytical derivation of necessary conditions for optimality. This aspect—often regarded in the literature as a drawback—is here exploited to build a robust yet efficient numerical method that formally eliminates the controls from the resulting Boundary Value Problem, thus gaining robustness and a high convergence rate. The elimination of the control is obtained either via their explicit formulation function of state and Lagrange multipliers—when possible—or via an iterative numerical solution. The paper closes presenting a minimum time manoeuvre of a car using a fairly complex vehicle model which also includes tyre saturation.
This paper presents advanced torque and current control techniques for Permanent Magnet Synchronous Motors (PMSMs) with a real-time simulator which has a nonlinear motor model. This model is called “Behavior motor model”, and it is developed based on finite element analysis results considering non-linear characteristics that include spatial harmonics and magnetic saturation. The real-time simulator is implemented within a circuit simulator that couples the behavior motor model with switching circuits in real-time, and it can be applied to torque and current control as an advanced controller. In this paper, the torque and current control of a PMSM with the proposed system is presented. Effectiveness of this technique is verified by performing simulations and experiments.
A loosely coupled inductor for interleaved power converters has attracted interest for downsizing magnetic components. Although the loosely coupled inductor is usually designed by adjusting the coupling coefficient in order to meet the design specifications, the coupling coefficient is saturated by the fringing fluxes in the central leg and the external leakage fluxes in the case of the EE or EI magnetic core structures. It is a matter of concern that these fluxes are causes of electromagnetic induced noise and reduction in the downsizing performance. To solve this problem, this paper proposes a short-circuited winding for the coupled inductor in order to reduce the external leakage flux in the windings. The effectiveness of the coupled inductor with a short-circuit ring is evaluated with experimental tests.
Interior type permanent magnet synchronous motors (IPMSMs) have been widely used for various applications because of their high efficiency and high power density. It is well known that the flux weakening control of an IPMSM can extend the operating range by reducing the back-EMF and producing reluctance torque. An IPMSM, however, experiences a large harmonic iron loss in the stator teeth caused by the distorted flux density. In particular, the harmonic eddy current loss significantly increases in the high speed region under flux weakening control. This paper describes a novel motor-drive system that includes an individual multi-phase drive in order to control the current and voltage in each armature winding. The proposed system can be expected to achieve high efficiency driving, a wide operating range, fail-safety and so on. Furthermore, the proposed method gives the distorted flux density distribution a sinusoidal waveform, because the individual control can observe and regulate the instantaneous flux linkage in each stator tooth, although the motor consists of a full-pitch distributed winding. Therefore, the proposed drive can suppress the harmonic iron loss under the flux weakening operation. Simulation and experimental results show that the proposed method can achieve an extensive reduction in the total iron loss in particular in the high speed region.
A major problem in high-voltage full bridge dc-dc converters is the high-voltage surge that occurs across the secondary-side rectifying diodes. In order to solve this problem, a simple solution with a snubber capacitor has been proposed. This technique provides effective surge-voltage suppression. On the other hand, it degrades the control characteristics of this converter. In this paper, the control characteristics are analyzed by the state-space averaging method. On the basis of this analytical result, a novel technique for control characteristics improvement is proposed. Experiments confirm the effectiveness of the proposed technique.