Multi-axis Control Research Articles

Pre-Compensation Strategy for Tracking Error and Contour Error by Using Friction and Cross-Coupled Control

This paper focuses on improving the tracking accuracy for servo systems and increasing the contouring performance of precision machining. The dynamic friction during precision machining is analyzed using the LuGre model. The dynamic and static parameters in the friction model are efficiently and accurately identified using the improved Drosophila swarm algorithm based on cross-mutation. The friction tracking error can be deduced from the friction state space and an expression is derived. To compensate for the tracking error caused by friction, a feedforward compensation control is designed to avoid signal lag in traditional friction controllers. Furthermore, the factors of multi-axis parameter mismatching that impact the machining profile accuracy are analyzed for multi-axis control. An adaptive cross-coupled control-based pre-compensation strategy of contour error is designed to reduce both the tracking error and the contour error. The effectiveness of the proposed method is validated through several experiments, which demonstrate a remarkable improvement in tracking performance and contour accuracy.

The Design and Real-Time Optimization of an EtherCAT Master for Multi-Axis Motion Control

To address the issues of low bandwidth, weak real-time performance, and poor synchronization in traditional fieldbuses for multi-axis motion control, a solution for the implementation of an EtherCAT master based on the IgH EtherCAT Master open-source software framework and an embedded hardware platform is proposed. On a hardware platform centered around the AM64x Sitara processor, a Linux real-time operating system based on the Xenomai real-time kernel is constructed, and the IgH master framework is ported to realize a high-performance EtherCAT master. The configuration process of the EtherCAT bus is detailed, a master application program is developed, and methods for the real-time performance optimization of the master—such as exclusive CPU usage by the master process and the optimization of the network card driver—are proposed. Finally, experiments are conducted on a six-axis servo control platform, with the packet analysis of the periodic EtherCAT data frames sent by the master. The experimental results show that the optimized master, under a high-speed communication cycle of 500 microseconds, maintains maximum jitter within 20 microseconds and average jitter within 1 microsecond, meeting the requirements for high-precision multi-axis motion control.

Active microparticle propulsion pervasively powered by asymmetric AC field electrophoresis

HypothesisSymmetry breaking in an electric field-driven active particle system can be induced by applying a spatially uniform, but temporally non-uniform, alternating current (AC) signal. Regardless of the type of particles exposed to sawtooth AC signals, the unevenly induced polarization of the ionic charge layer leads to a major electrohydrodynamic effect of active propulsion, termed Asymmetric Field Electrophoresis (AFEP). ExperimentsSuspensions containing latex microspheres of three sizes, as well as Janus and metal-coated particles were subjected to sawtooth AC signals of varying voltages, frequencies, and time asymmetries. Particle tracking via microscopy was used to analyze their motility as a function of the key parameters. FindingsThe particles exhibit field-colinear active propulsion, and the temporal reversal of the AC signal results in a reversal of their direction of motion. The experimental velocity data as a function of field strength, frequency, and signal asymmetry are supported by models of asymmetric ionic concentration-polarization. The direction of particle migration exhibits a size-dependent crossover in the low frequency domain. This enables new approaches for simple and efficient on-chip sorting. Combining AFEP with other AC motility mechanisms, such as induced-charge electrophoresis, allows multiaxial control of particle motion and could enable development of novel AC field-driven active microsystems.

An Improved Deviation Coupling Control Method for Speed Synchronization of Multi-Motor Systems

In order to enhance the synchronization of welding robot arms and improve welding quality, this study proposes a fuzzy PID-based improved deviation coupling multi-axis synchronous control method. Firstly, in response to the intricacies inherent in the compensation mechanism of the deviation coupling control structure and the substantial volume of system computation, the integration of average speed and sub-average speed is proposed to optimize the speed compensator. This integration aims to mitigate speed synchronization errors, minimize synchronization adjustment time, and elevate overall system synchronization performance. Moreover, the fuzzy PID algorithm is employed to design the controller to realize the single-motor adaptive control, leading to improvement in both system stability and dynamic response performance. Finally, a simulation model for six-axis synchronization control and an experimental platform were developed. Both simulation and experimental results demonstrate that the improved deviation coupling control method exhibits superior synchronization performance. The proposed multi-axis synchronous control method effectively heightens the synchronous performance of the six-degrees-of-freedom robotic arm.

Conditional adaptive time series compensation and control design for multi-axial real-time hybrid simulation

The structural performance of critical infrastructure during extreme events requires testing to understand the complex dynamics. Shake table testing of buildings to evaluate structural integrity is expensive and requires special facilities that can allow for the construction of large-scale test specimens. An attractive alternative is a cyber-physical testing technique known as Real-Time Hybrid Simulation (RTHS), where a large-scale structure is decomposed into physical and numerical substructures. A transfer system creates the interface between physical and numerical substructures. The challenge occurs when using multiple actuators connected with a coupler (i.e., transfer system) to create translation and rotation at the interface. Tracking control strategies aim to reduce time delay errors to create the desired displacements that account for the complex dynamics. This paper proposes two adaptive control methodologies for multi-axial real-time hybrid simulations that improve capabilities for a higher degree of coupling, boundary, complexity, and noise reduction. One control method integrates the feedback proportional derivative integrator (PID) control with a conditional adaptive time series (CATS) compensation and inverse decoupler. The second proposed control method is based on a coupled Model Predictive Control (MPC) with the CATS compensation. The performance of the proposed methods is evaluated using the virtual multi-axial benchmark control problem consisting of a steel frame as the experimental substructure. The transfer system consists of a coupler that connects two hydraulic actuators generating the translation and rotation acting at the joint. Through sensitivity analysis, parameters were tuned for the decoupler components, CATS compensation, and the control design for PID, LQG, and MPC. Comparative results among different control methods are evaluated based on performance criteria, including critical factors such as reduction in the time delay of bothactuators. The research findings in this paper improve the tracking control systems for the multi-axial RTHS of building structures subjected to earthquake loading. It provides insight into the robustness of the proposed tracking control methods in addressing uncertainty and improves the understanding of multiple output controllers that could be used in future cyber-physical testing of civil infrastructure subjected to natural hazards.

Trajectory optimization of B-splines interpolation based on dynamic error adjustment

In this paper, we propose a motion planning algorithm based on the five-fold B-spline curve for multi-axis motion control systems with non-smooth trajectories, acceleration, plus acceleration over the limit, etc. We construct a smooth transition model for the micro-line segment G3 of the continuous five-fold B-spline curve. A derivation of the corresponding theoretical equation is given. Using the transition model, we propose an optimization method for dynamic adjustment of motion trajectory errors based on the velocity forward planning constraints by smoothing the transition after the trajectory of the micro-line segment for the forward planning velocity. Finally, the design flow for the forward planning of the velocity is given. Finally, the above algorithm is used to perform simulation experiments for the optimization algorithm. Experimental results show that the proposed optimization improves the accuracy of fitting motion trajectories under the constraints of acceleration and deceleration addition, which validates the proposed algorithm.

Hierarchical conducting networks constructed as resistive strain sensors for personal healthcare monitoring and robotic arm control

Flexible strain sensors are strongly demanded in fields of personal healthcare monitoring and human–machine interface, as they can accurately perceive external action to produce electrical output signals. Aiming at the existing problems, such as the balanced sensitivity and sensing range, durability as well as expanded application, this paper proposes a flexible strain sensor prepared based on electrospun thermoplastic polyurethane (TPU) membrane, followed by anchoring mixed conductive materials (carbon black (CB)/carbon nanotubes (CNT)) on its surface via an effective ultrasonic-assisted method. The resulting CB/CNT@TPU strain sensor delivers a desirable integration of sensing performances: good sensitivity, wide work range, fast response speed (<150 ms) and reliable cyclic stability (5000 cycles). Ascribed to the sensing behavior and additional breathability, strain sensor can be applied well to monitor human health, including human joints movements (fingers and wrists) and physiological signals (pulse and laryngeal vibration recognition). Finally, a multiaxial robotic arm control system is constructed, which provides a feasible strategy for intelligent robotic arms.

Design and test of an efficient seedling pick-up device with a combination of air jet ejection and mechanical action

Low degree of transplanting automation will affect production efficiency and planting quality in vegetable cultivation. A new seedling pick-up device was designed and constructed to reduce direct grasping damages to seedlings and improve transplanting efficiency. The pick-up device consists of an air jet loosening device, a flexible pick-up manipulator, a parallel feeding device, and a multi-axis motion control system. Its working principle is to use air jet ejection to assist in loosening of seedling roots from the tray cells, grasp their stems for extracting with the pick-up manipulator, and finally transfer them to the delivery device for feeding into the planting device as needed. The mechanical structure and working parameters of each component were designed, and the control system was constructed according to the working requirements of ejecting, extracting, transferring, and discharging operations. A prototype of the new pick-up device was constructed, and its performance evaluation was conducted using an orthogonal experimental design using cucumber, pepper and caluiflower as test objects. The results showed that the test object, the root lump's moisture content and the loosening way (either as a whole or individual loosening of seedlings) had significant effects on the success ratio in picking up seedlings. Overall, the success in picking up seedlings from the cell was found to be influenced by horticultural and mechanical factors. Under the optimal level group, the maximum success ratio for automatic picking up seedlings was up to 94.49% for pepper while that of cucumber and cauliflower recorded 90.75% and 92.62%, respectively. The seedling pick-up performance was satisfactory for efficient transplanting requirements.

Data-driven robust iterative learning control of linear systems

We propose a data-driven robust iterative learning control (ILC) technique to multi-input-multi-output (MIMO) linear systems. Control of MIMO linear systems, particularly with strong cross-axis coupling, is challenging as modeling of a MIMO system can be complicated, time-consuming, and often requires a trade-off between robustness and performance. As such, limitations exist in current ILC techniques. The aim of this paper is to develop an efficient and easy-to-use data-driven ILC technique to output tracking of MIMO linear systems under random disturbance. Through the proposed technique, the complicated modeling process and the robustness-accuracy trade-off are avoided, and the up-to-now system dynamics is captured by constructing and updating the iteration gain using the input and output data in the last iteration. It is shown that monotonic convergence of the ILC algorithm is guaranteed, and an optimal gain can be obtained to maximize the convergence rate and minimize the residual tracking error. The proposed technique is illustrated through experiments on a three-input three-output piezoelectric actuator system, with comparison to the adaptive multi-axis inversion-based iterative control (A-MAIIC) technique. The experimental results show rapid convergence and improved formance of the proposed technique when the cross-axis coupling is strong.

A three-degree-of-freedom piezoelectric actuator with sandwich structure: Design, modeling, and experiment

With the increased motion requirements of precision drive devices, multi-degree-of-freedom piezoelectric actuators have become key technologies for realizing multi-axis motion control. To expand upon the three-degree-of-freedom piezoelectric stick-slip driving method, a sandwich-type three-degree-of-freedom rotating inertial stick-slip piezoelectric actuator is proposed in this paper. Firstly, the structure and control strategy of the actuator were introduced. Statics analysis of the core component of the piezoelectric vibrator was carried out, and the relationship between the structural parameters and the deformation of the driving foot was obtained. Then the free and forced vibrations of the piezoelectric vibrator are analyzed to determine the dynamic response of the driving foot in the time domain. Finally, a prototype was fabricated and an experimental system was built to test its output characteristics. The test results show that when the driving voltage was 150 V and the frequency was 320 Hz, the proposed actuator achieves maximum speeds and load torques around the x-axis and y-axis of 306 mrad/s and 2 mN m, respectively. When the driving voltage was 150 V and the frequency was 450 Hz, the proposed actuator achieves a maximum speed and load torque around the z-axis of 140 mrad/s and 1.55 mN m, respectively.

Low-consumption stepper motor controller with real-time target position change responsiveness based on field programmable gate array.

In response to the demand for low resource consumption, parallel control, and real-time response to target position changes in precision measurement and manufacturing of multi-axis stepper motor controllers, this paper proposes a field programmable gate array-based method for generating trapezoidal velocity profiles and pulse generation, which can easily keep parallelism and independence during multi-axis control. By avoiding using multiplication and division, this controller not only reduces resource consumption but also enhances the pulse output frequency. To address the real-time responsiveness of the velocity profile generation algorithm to changes in the target position during the control process, the algorithm introduces a novel real-time comparative state transition logic for speed control, which makes it capable of adjusting the acceleration within a single clock cycle, enabling its application in scenarios that require higher levels of real-time performance. Finally, the designed controller is applied to a four-axis positioning system for performance validation.

Research on Multiple-Axis Contour Error Suppression Method Based on Composite Layered Control

With the widespread application of multi-axis machining in the industrial manufacturing, aerospace, and military equipment sectors, the demand for machining ultra-precision components has been steadily increasing. Contour errors directly impact the quality of machined parts. In conventional multi-axis motion control systems based on cross-coupling, it is conventionally assumed that all individual axes are of equal significance during machine processing. However, in practical machining scenarios, diverse machining trajectories and accuracy requirements give rise to distinct control necessities for each axis. This complication leads to challenges in ensuring a consistent single-plane contour, thereby constraining the elevation of the overall contour accuracy. To address this issue, this study proposes a multi-axis contour error suppression method based on composite hierarchical control. The approach advocated in this paper initially ensures the precision of single-axis position control through the development of an advanced S-shaped function-based sliding-mode disturbance observer. Building on this foundation, the three-dimensional spatial contour is segregated into upper and lower layers. Subsequently, dedicated fuzzy PID cross-coupling controllers are devised for each layer. The experimental outcomes substantiate that in comparison to conventional cross-coupling control methods, the method introduced in this study, rooted in composite hierarchical control, not only guarantees the accuracy of single-plane contours but also further enhances the overall contour precision.

Design of a High Precision Multichannel 3D Bioprinter

Three-dimensional (3D) printing technology is expected to solve the organ shortage problem. However, owing to the accuracy limitations, it is difficult for the current bioprinting technology to achieve an accurate control of the spatial position and distribution of a single cell or single component droplet. In this study, to accurately achieve the directional deposition of different cells and biological materials in the spatial position for the construction of large transplantable tissues and organs, a high-precision multichannel 3D bioprinter with submicron-level motion accuracy is designed, and concurrent and synergistic printing methods are proposed. Based on the high-precision motion characteristics of the gantry structure and the requirements of concurrent and synergistic printing, a 3D bioprinting system with a set of 6 channels is designed to achieve six-in-one printing. Based on the Visual C++ environment, a control system software that integrates the programmable multi-axis controller (PMAC) motion, pneumatic, and temperature control subsystems was developed and designed. Finally, based on measurements and experiments, the 3D bioprinter and its control system was verified to fulfil the requirements of multichannel, concurrent, and synergistic printing with submicron-level motion accuracy, significantly shortening the printing time and improving the printing efficiency. This study not only provides an equipment basis for printing complex heterogeneous tissue structures, but also improves the flexibility and functionality of bioprinting, and ultimately makes the construction of complex multicellular tissues or organs possible.

Investigation on active vibration isolation with multi-axis control: Analysis and experiment

Vibration isolators are usually used to attenuate the transmission of vibration of power machinery to the flexible foundation. Since vibration energy is transferred nearly through all the degrees of freedom of an isolator, it is difficult to guarantee sufficient vibration attenuation by the single-axis control at each active isolator. In order to enhance attenuation at each vibration isolator, a novel configuration of active vibration isolation is proposed. Each active isolator is composed of two rubber isolators and one intermediate rigid mass in which eight electromagnetic actuators and six error accelerometers are attached. Six control forces are applied on the intermediate mass to suppress its multi-axis vibration. Moreover, the weakened interaction between isolators allows independent local control at each isolator. A numerical model is established to verify the proposed active vibration isolation and the results show that the transmission of vibration from the source to the flexible hull structure can be nearly cut off when the error vibrations of each intermediate mass are attenuated completely. The effectiveness is also proved by an experimental active vibration isolation system and the proposed isolation configuration with multi-axis control can achieve more significant attenuation of vibration of the hull structure than the commonly used active vibration isolation with only single-axis control.

Multi-axis Current Loop Control Based on FPGA

In this paper, according to the three-loop control structure, FPGA is used as the control chip of the current loop, and the parallel control of the multi-axis motor is realized by hardware nation. The current loop code is written in Verilog HDL language, and the simulation results of each control module are given by the SignalTap simulation tool. The main contents include a parallel sampling of the multichannel encoder, parallel sampling of multichannel current, vector coordinate transformation control (CLARK module, PARK module, IPARK module), PI controller module, etc. The results show that the rich logic resources and powerful parallel processing ability of FPGA enable a single FPGA to control multiple motors at the same time. Compared with the traditional serial execution of the program, this method greatly reduces the whole calculation time of the servo system.

Improved lubrication and wear resistance of gallium-matrix liquid metal containing molybdenum diselenide nanoparticles under heavy load conditions

Owing to their advantages of high load-carrying performance, efficient heat conduction, and low friction coefficient, liquid metal-lubricated spiral groove bearings are critical mechanical components in high-speed mechanical support systems including computed tomography X-ray tubes, machine spindles of multi-axis numerical control machining center, etc. Therefore, it is imperative to understand the tribological mechanisms between liquid metal lubricants and tribo-pairs aiming to improve their functional properties and extend the service life of spiral groove bearings. In this study, molybdenum diselenide (MoSe2) nanoparticles, a friction-reducing lubricant, were blended as an additive into the gallium-based liquid metal (LM, Ga64In24Sn12) using mechanical grinding techniques. Following lubrication with MoSe2/Ga64In24Sn12 LM lubricant, friction and wear characteristics of Si3N4 ceramics ball/AISI 52100 steel disk friction pairs were evaluated by a high-frequency linear reciprocating oscillation friction test machine with ball-on-disc configuration under a normal load ranging from 50 N to 1500 N. Experimental results demonstrate that as the MoSe2 dosage increases, the viscosity of MoSe2/Ga64In24Sn12 LM lubricant gradually rises. Both normal load and MoSe2 nanoparticles content have a notable influence on the lubrication properties of MoSe2/Ga64In24Sn12 LM lubricant at a light (50 N) or a heavy load (1500 N). Of all these LM lubricants, the best lubrication performance and wear resistance (f = 0.10, k = 1.93×10−8 mm3/Nm) was achieved for 5.0 wt% MoSe2/Ga64In24Sn12. The improvement in the lubrication characteristics of MoSe2/Ga64In24Sn12 LM lubricant under heavy load conditions originated from the generation of tribo-films from the Tribo-chemical reactions involving MoSe2 nanoparticles. The thickness of Ga-based tribo-films is 1.50 ± 0.10 μm. This kind of tribo-films is rich in various nanocrystals consisting of orthorhombic α-Fe2O3, monoclinic β-Ga2O3, cubic In2O3, tetragonal rutile SnO2, orthorhombic α-MoO3, and noncrystalline SiO2. Friction-induced lubricating films prevent adhesion wear of frictional pairs at heavy loads and consequently increase the load-bearing characteristics of LM lubricant, thereby improving the lubrication characteristics of LM.

Fixation of pelvic acetabular fractures using 3D-printed fracture plates: a cadaver study

Open reduction and internal fixation of pelvic acetabular fractures are challenging due to the limited surgical exposure from surrounding abdominal tissue. There have been a number of recent trials using metallic 3D-printed pelvic fracture plates to simplify and improve various elements of these fracture fixation surgeries; however, the amount of time and accuracy involved in the design and implantation of customised plates have not been well characterised. This study recorded the amount of time related to the design, manufacture and implantation of six customised fracture plates for five cadaveric pelvic specimens with acetabular fracture, while manufacturing, and surgical accuracy was calculated from computed tomography imaging. Five of the fracture plates were designed within 9.5 h, while the plate for a pelvis with a pre-existing fracture plate took considerably longer (20.2 h). Manufacturing comprised 3D-printing the plates in Ti6Al4V with a sintered laser melting (SLM) 3D-printer and post-processing (heat treatment, smoothing, tapping threads). The manufacturing times varied from 27.0 to 32.5 h, with longer times related to machining a thread for locking-head screws with a multi-axis computer numerical control (CNC) mill. For the surface of the plate in contact with the bone, the root-mean-square errors of the print varied from 0.10 to 0.49 mm. The upper range of these errors was likely the result of plate designs that were relatively long with thin cross-sections, a combination that gives rise to high thermal stresses when using a SLM 3D-printer. A number of approaches were explored to control the trajectories of locking or non-locking head screws including guides, printed threads or hand-taps; however, the plate with CNC-machined threads was clearly the most accurate with screw angulation errors of 2.77° (range 1.05–6.34°). The implanted position of the plates was determined visually; however, the limited surgical exposure and lack of intra-operative fluoroscopy in the laboratory led to high inaccuracies (translational errors of 1.74–13.00 mm). Plate mal-positioning would lead to increased risk of surgical injury due to misplaced screws; hence, it is recommended that technologies that can control plate positioning such as fluoroscopy or alignment guides need to be implemented into customised plate design and implantation workflow. Due to the plate misalignment and the severe nature of some acetabular fractures comprising numerous small bone fragments, the acetabular reduction exceeded the clinical limit of 2 mm for three pelvises. Although our results indicate that customised plates are unsuitable for acetabular fractures comprising six or more fragments, confirmation of this finding with a greater number of specimens is recommended. The times, accuracy and suggested improvements in the current study may be used to guide future workflows aimed at producing customised pelvic fracture plates for greater numbers of patients.

Contour error dynamic analysis and predictive control for multi-axis motion system

Contour tracking accuracy is the key performance index of a multi-axis motion control system. In the previous design of the contour error feedback controller, the independent design idea of the tracking controller and contour controller of each logic axis is often adopted, which is not easy to realize the optimal control of tracking accuracy and contour accuracy. The coupling increment of each controller is inevitable in motion control compensation, which impacts the control system’s stability. Here, we propose a predictive control method for contour error. At first, through the state space model of the feed system, the system dynamics are judged to predict the contour accuracy in the future finite time domain. Besides, the linear quadratic performance index function of contour error is established, and the performance of the control objective is continuously optimized during the limited time domain with the form of rolling optimization. A closed-loop solution is obtained, which is most suitable for the current motion situation and could add to the system to realize the optimal control considering the trajectory contour and the tracking stability of each servo axis in the subsequent few sampling cycles. Finally, utilizing comparative experiments on the three-axis motion control platform, it is verified that the proposed contour predictive control strategy not only effectively reduces the tracking error but also significantly improves the contour accuracy, which is suitable for application in a complex industrial environment.

다자유도 구형 트랙션 전동기의 틸팅 안정성 향상을 위한 구조에 관한 연구

The technology related to the multi-axis spherical motor is a technology that implements a three-dimensional motion, and various countries and research institutes are currently conducting prior research. A motor using a spherical type rotor can dramatically increase the degree of freedom of vehicle steering and robot motion according to the expansion of the tilt angle that determines the movable range of the rotation axis. when a multi-axis control system is used by adding mechanical axes, the size of the motor increases due to additional devices such as complex power conversion devices such as multiple gears and links. To solve this size problem, a spherical motor that does not use a mechanical axis can significantly affect the size. However, if a mechanical axis is not used, a problem in stability, such as shaking, occurs during driving. In this paper, we proposed an auxiliary structure to improve the stability of an inductive spherical by using a permanent magnet instead of a mechanical shaft.

A dynamic multi-axis control allocation scheme for real-time applications

The Dynamic Weighting Control Allocator (DWCA) was introduced by Sadien et al. (Control Engineering Practice, 2019) to solve the allocation problem raised by the yaw control of an on-ground aircraft. It handles the classical tradeoff between virtual reference inputs realisation and control power minimisation. But it also offers three specific features. First, the actuators reach saturation almost simultaneously, which allows an efficient recovery in case of failure. Then, several industrial requirements are taken into account, such as implementation ease, low computational cost and compatibility with certification constraints. And finally, the actuators can be prioritised, the last ones being used sparingly (to avoid overheating, fatigue, maintenance cost…) only when the virtual reference inputs cannot be realised by the first ones only. But despite its attractiveness, the DWCA is limited to the 1-dimensional case, which means that only one degree of freedom can be controlled. This is not sufficient to deal with today's challenges in the aeronautical and automotive fields, for example, where certain control problems are inherently multi-dimensional. With the progressive advent of autonomous cars, new concepts of light hybrid or electric vehicles are, for example, emerging, where both hydraulic and electric actuators are mounted on each wheel to improve the combined management of speed and steering. In this context, this paper introduces a non-trivial generalisation of the DWCA, which retains the various characteristics and advantages of the initial version and can be used in the multi-dimensional case.