Closed-loop Position Control Research Articles

Application of the Indoor Large-length Standard Device

The indoor large-length standard device is a comprehensive standard for traceability of large-length measurement value. It provides that the standard ensures its accuracy and reliability in the effective detection range. In order to ensure the accuracy of the quantity traceability of the large-length measuring instrument, we independently developed the domestic first set of metal linear guide indoor large-length standard device in the southwest. The high-precision metal linear guide cooperating with precision gear rack transmission is used in this device. The main drive system with an independent position closed-loop control is employed to eliminate effectively the influence of a series of problems, including the unstable floating track deformation and loss of friction wheels, difficult maintenance. The device can also hold an accurate and reliable statue throughout the execution, which can also maintain stable acceleration and velocity and has the advantages of no vibration transmission system, automatic operation. The measuring range of the device is (0∼56) m, and the uncertainty of measurement is U=0.14um+10−7L (k=2). Its function is mainly used to carry out the distance meter, laser interferometer, laser tracker, standard steel tape equipment, and terrestrial laser scanner; also the device can also be used for linear displacement sensors, liquid level measuring devices, standard baseline feet, and other non-standard class length equipment calibration.

An Identification-Based Method Improving the Transparency of a Robotic Upper Limb Exoskeleton

SUMMARYOver the past decade, research on human–robot collaboration has grown exponentially, motivated by appealing applications to improve the daily life of patients/operators. A primary requirement in many applications is to implement highly “transparent” control laws to reduce the robot impact on human movement. This impact may be quantified through relevant motor control indices. In this paper, we show that control laws based on careful identification procedures improve transparency compared to classical closed-loop position control laws. A new performance index based on the ratio between electromyographic activity and limb acceleration is also introduced to assess the quality of human exoskeleton interaction.

Application of PLC-Based Stroke Control in Industrial Production

According to the requirements of large-stroke and high-precision positioning control in industrial production, the paper proposes a position closed-loop control system based on belt-type magnetic scale and PLC. The Siemens S7-200 PLC is used to capture the high-speed pulses of the magnetic scale in real time, realize the closed- loop automatic positioning control of the large-stroke position, and apply it to the plastic steel profile sawing centre control system, which solves the traditional stepper motor open loop at a lower cost The accuracy problems caused by the control greatly reduce the threshold of precise positioning control, which has strong practical application value.

Output constrained adaptive neural control for generic hypersonic vehicles suffering from non-affine aerodynamic characteristics and stochastic disturbances

This paper proposes a novel output constrained non-affine control structure by combining backstepping adaptive control with coordinate transformation technique for Generic Hypersonic Vehicles (GHVs) attitude tracking problem suffering from stochastic uncertainties. Firstly, by introducing several specific nonlinear functions, the output constrained control problem is transformed into a stabilization problem of several new variables. Then a novel adaptive control scheme that employee the boundary estimation technique and Nussbaum-type functions to counteract the effect of non-affine aerodynamic characteristics is integrated to address the attitude tracking problem. Meanwhile, two Neural Networks (NNs) are introduced to approximate and compensate the unknown nonlinearities. With the aid of a four-order Lyapunov function, the closed-loop attitude control system is proved to be stochastically stable. As a result, the output tracking errors can be guaranteed to stay in the predefined constraints during the whole control process and finally converge to an acceptable small value. Comparative simulation results are presented to demonstrate the effectiveness and advantages of the proposed control strategy.

Design, tuning and evaluation of a stand-alone nitinol based thermomechanical actuator driver with a closed-loop position control system

Nitinol based actuators can be used in many applications. Many of them are simple logic-type applications where intermediate positions or contractions are not required. For such applications, a simple current limiting circuit with enable function can be used. The biggest problem is when we want to use the actuator for positioning applications. In this case, by simply adjusting the current flowing through the nitinol wire is not going to solve the problem. On one hand, the range where we can adjust the current is very small. On the other hand, the hysteresis and thermal inertia are involved in the process. The last and the worst thing is the ambient temperature that will vary in a random way. As a conclusion, the author’s original research for a dedicated driver with position feedback did the job done, eliminating all the related problems. Finally, the driver was implemented, tuned for the mechanical test jig and evaluated under rough conditions in order to prove that the driver can reject the external disturbances. Furthermore, unlike a traditional on/off driver, the designed driver can prolong the lifetime of the actuator by keeping the actuator current at a safe level even in saturation conditions and, by limiting the maximum rate of change of the temperature, thermal shocks can be avoided.

Fixed-time observer based fault tolerant attitude control for reusable launch vehicle with actuator faults

This paper focuses on the fault tolerant attitude control problem for the reusable launch vehicle (RLV) with model uncertainties, external disturbances and actuator failures/faults during the reentry phase. First, the kinematics and dynamics of the RLV are presented, and the control-oriented mode subject to actuator malfunction is established. Next, a novel fixed-time observer is designed to estimate the compound disturbances, including failure information and uncertainties. Specifically, the proposed observer can guarantee the estimation errors converging to zero within a settling time, which is independent with the system initial conditions. Based on the proposed observer, a finite-time fault tolerant attitude control scheme is developed based on the nonsingular terminal sliding mode technology, which would improve the reentry attitude tracking performance and ensure the finite-time stability of the closed-loop attitude control system even in presence of the actuator faults. Finally, the effectiveness of the proposed method is demonstrated by numerical simulations.

Attitude Determination and Control System Design of Sub-Arcsecond Pointing Spacecraft

Spacecraft pointing accuracies with sub-arcsec to milli-arcsec levels are becoming a norm for the future space missions, which is mainly achieved by employing mission-specific fine guidance sensor (FGS). European Space Agency’s Herschel is the only spacecraft to date that has achieved an accuracy between 0.8 and 0.9 arcsec without employing FGS and by using only precise commercial-off-the-shelf (COTS) sensors. This paper improves upon Herschel’s pointing accuracy by at least [314, 925, 925]% for German Aerospace Center’s future mission named “Infrared Astronomy Satellite Swarm Interferometer (IRASSI).” IRASSI’s closed-loop attitude determination and control system (ADCS) is carefully developed by selecting high-precision COTS sensors and actuators, implementing two optimal attitude estimation algorithms for the sensor data fusion, three robust control algorithms for the attitude control, and a pseudoinverse distribution algorithm for spacecraft momenta allocation among the redundant reaction wheels in three different configurations. Monte Carlo simulations of IRASSI’s ADCS demonstrate that an unprecedented three-axis absolute pointing error of [0.193, 0.078, 0.078] arcsec is achieved. Furthermore, performance comparison between the rate-integrating and rate-gyroscope-based multiplicative extended Kalman filter for this mission is demonstrated. Finally, performance comparison of three attitude controllers establishes their feasibility for this mission.

A hybrid brain-computer interface for closed-loop position control of a robot arm

Brain-Computer interfacing (BCI) has currently added a new dimension in assistive robotics. Existing brain-computer interfaces designed for position control applications suffer from two fundamental limitations. First, most of the existing schemes employ open-loop control, and thus are unable to track positional errors, resulting in failures in taking necessary online corrective actions. There are examples of a few works dealing with closed-loop electroencephalography (EEG)-based position control. These existing closed-loop brain-induced position control schemes employ a fixed order link selection rule, which often creates a bottleneck preventing time-efficient control. Second, the existing brain-induced position controllers are designed to generate a position response like a traditional first-order system, resulting in a large steady-state error. This paper overcomes the above two limitations by keeping provisions for steady-state visual evoked potential (SSVEP) induced link-selection in an arbitrary order as required for efficient control and generating a second-order response of the position-control system with gradually diminishing overshoots/undershoots to reduce steady-state errors. Other than the above, the third innovation is to utilize motor imagery and P300 signals to design the hybrid brain-computer interfacing system for the said application with gradually diminishing error-margin using speed reversal at the zero-crossings of positional errors. Experiments undertaken reveal that the steady-state error is reduced to 0.2%. The paper also provides a thorough analysis of the stability of the closed-loop system performance using the Root Locus technique.

Design and Construction of a Low-Cost Test Bench for Testing Agricultural Spray Nozzles

Droplet size distribution is probably the most important feature of a spray as it affects all aspects of a phytosanitary treatment, i.e., biological, environmental, and safety aspects. This study describes a low-cost laboratory test bench able to analyze agricultural spray nozzles under realistic conditions. The design of the equipment was mainly based on the ISO 5682-1 standard. It has a couple of 3 m long rails, along which the nozzle under test moves while spraying, controlled by a closed-loop position and speed controller. The drops were captured with three Petri dishes containing silicone oil, photographed by means of a digital single-lens reflex (DSLR) camera, and then analyzed with the ImageJ software in order to measure the usual spray parameters: the volumetric diameters, the Sauter mean diameter, and the number mean diameter. Spray trials and tuning of the system parameters were managed by means of a purposely designed user interface running on a Windows 10 PC. Some tests were carried out by using an Albuz ATR80 orange hollow cone nozzle at the working pressures of 0.3, 0.5, 1.0, and 1.5 MPa. The results about spray quality agree with the factory information, and the whole system, even if some aspects still need improvements, has proven reliable.

Study on integrated control for supersonic inlet and turbofan engine model

Considering the supersonic inlet model with normal shock position feedback, the integrated control method of inlet and turbofan engine is studied. The integrated model includes the supersonic inlet model and the component level model of engine. Combining the relationship between the normal shock position and the total pressure recovery coefficient, the supersonic inlet and engine model is constructed. On the basis of this model, the normal shock position closed-loop control simulation is carried out, which shows that the normal shock position matching point could be stabilized near the optimal value while restraining the inlet stream disturbance. Furthermore, based on the H∞ control algorithm, an inlet and engine integrated control is designed to control the installation thrust and turbine pressure ratio with fuel, nozzle throat area, and normal shock position as control variables. The simulation results show that the response time of the integrated control is faster than the independent control. The integrated control has stronger ability to restrain the atmospheric disturbance, which could ensure the stable and reliable operation of the propulsion system.

MILiMAC: Flexible Catheter With Miniaturized Electromagnets as a Small-Footprint System for Microrobotic Tasks

Advancements in medical microrobotics have given rise to an abundance of agents capable of localised interaction with human body in small scales. Nevertheless, clinically-relevant applications of this technology are still limited by the auxiliary infrastructure required for actuation of micro-agents. In this letter, we approach this challenge. Using finite-element analysis, we show that miniaturization of electromagnets can be used to create systems capable of providing magnetic forces adequate for micro-agent steering, while retaining small footprint and power consumption. We use these observations to create MILiMAC (Microrobotic Infrastructure Loaded into Magnetically-Actuated Catheter). MILiMAC is a flexible catheter employing three miniaturized electromagnets to provide localized magnetic actuation at the deeply-seated microsurgery site. We test our approach in a proof-of-concept study deploying MILiMAC inside a test platform to deliver and steer a 600 [\boldsymbol{\mu }m] ferromagnetic microbead. The bead is steered along a set of user-defined trajectories using closed-loop position control. Across all trajectories the best performance metrics are the mean error of 0.41 [mm] and the steady-state error of 0.27 [mm].

Research on variable mass control of series manipulator based on linear active disturbance rejection control

The series manipulator of the demolition robot has different working postures and large self-weight so that the bearing mass of the joint hydraulic cylinder is a variable value, and the change of mass affects the dynamic characteristics of the valve-controlled hydraulic cylinder system. In this paper, an equivalent mass estimation method of the big-arm joint hydraulic cylinder is given, and the natural frequency range of the big-arm hydraulic cylinder-load system is calculated. In order to suppress the influence of the change of the natural frequency and external disturbance of the hydraulic cylinder on the system, the linear active disturbance rejection control is adopted in the closed-loop position control. The simulation results show that the change of load mass of the joint hydraulic cannot be ignored when designing the joint position controller of multi-joint hydraulic manipulator. And the simulation tests prove that the linear active disturbance rejection control strategy keeps good dynamic characteristic and steady-state accuracy for the change of natural frequency and external disturbance of hydraulic cylinder and has good tracking performance for periodic sinusoidal signal. Linear active disturbance rejection control is robust in the variable mass control of the manipulator and meets the requirements of the joint position control of the heavy-duty hydraulic manipulator.

Self-Sensing Electro-Ribbon Actuators

In this letter, we investigate the viability of capacitance self-sensing of electro-ribbon actuators, allowing them to be used simultaneously as an actuator and a sensor. Initially the electro-ribbon actuator was implemented only as a sensor, and it was found that the exponential relationship between capacitance and displacement made the device a poor sensor at distances greater than 2 mm. When used simultaneously as an actuator and a sensor, the `zipping' motion of the electrodes changed this relationship, making it more suitable for position sensing. Capacitance was found to be a good indicator of the amount of zipping experienced by the electrodes. For effective displacement sensing, the mass of the the load being applied to the actuator should be known. Finally, a closed-loop PI position control system using self-sensing feedback was implemented. The results demonstrated that, provided the applied load is known, capacitive self-sensing can be used to control the position of electro-ribbon actuators with reasonable accuracy.

Development of an Improved Rotational Orthosis for Walking With Arm Swing and Active Ankle Control.

Based on interlimb neural coupling, gait robotic systems should produce walking-like movement in both upper and lower limbs for effective walking restoration. Two orthoses were previously designed in our lab to provide passive walking with arm swing. However, an active system for walking with arm swing is desirable to serve as a testbed for investigation of interlimb neural coupling in response to voluntary input. Given the important function of the ankle joint during normal walking, this work aimed to develop an improved rotational orthosis for walking with arm swing, which is called ROWAS II, and especially to develop and evaluate the algorithms for active ankle control. After description of the mechanical structure and control schemes of the overall ROWAS II system, the closed-loop position control and adjustable admittance control algorithms were firstly deduced, then simulated in Matlab/Simulink and finally implemented in the ROWAS II system. Six able-bodied participants were recruited to use the ROWAS II system in passive mode, and then to estimate the active ankle mechanism. It was showed that the closed-loop position control algorithms enabled the ROWAS II system to track the target arm-leg walking movement patterns well in passive mode, with the tracking error of each joint <0.7°. The adjustable admittance control algorithms enabled the participants to voluntarily adjust the ankle movement by exerting various active force. Higher admittance gains enabled the participants to more easily adjust the movement trajectory of the ankle mechanism. The ROWAS II system is technically feasible to produce walking-like movement in the bilateral upper and lower limbs in passive mode, and the ankle mechanism has technical potential to provide various active ankle training during gait rehabilitation. This novel ROWAS II system can serve as a testbed for further investigation of interlimb neural coupling in response to voluntary ankle movement and is technically feasible to provide a new training paradigm of walking with arm swing and active ankle control.

Decoupled nonlinear adaptive control of position and stiffness for pneumatic soft robots

This article addresses the problem of simultaneous and robust closed-loop control of joint stiffness and position, for a class of antagonistically actuated pneumatic soft robots with rigid links and compliant joints. By introducing a first-order dynamic equation for the stiffness variable and using the additional control degree of freedom, embedded in the null space of the pneumatic actuator matrix, an innovative control approach is introduced comprising an adaptive compensator and a dynamic decoupler. The proposed solution builds upon existing adaptive control theory and provides a technique for closing the loop on joint stiffness in pneumatic variable stiffness actuators. Under a very mild assumption involving the inertia and actuator matrices, the solution is able to cope with uncertainties of the model and, when the desired stiffness is constant or slowly varying, also of the pneumatic actuator. Position and stiffness decoupling is achieved by the introduction of a first-order differential equation for an internal state variable of the controller, which takes into account the time derivative of pressure in the stiffness dynamics. A formal proof of the stability of the position and stiffness tracking errors is provided. An appealing property of the approach is that it does not require higher derivatives of position or any derivatives of stiffness. The solution is validated with respect to several use-cases, first in simulation and then via a real pneumatic soft robot with McKibben muscles. A comparison with respect to existing techniques reveals a more robust position and stiffness tracking skill.

A Comparison Study of a Novel Self-Contained Electro-Hydraulic Cylinder versus a Conventional Valve-Controlled Actuator—Part 1: Motion Control

This research paper presents the first part of a comparative analysis of a novel self-contained electro-hydraulic cylinder with passive load-holding capability against a state of the art, valve-controlled actuation system that is typically used in load-carrying applications. The study is carried out on a single-boom crane with focus on the control design and motion performance analysis. First, a model-based design approach is carried out to derive the control parameters for both actuation systems using experimentally validated models. The linear analysis shows that the new drive system has higher gain margin, allowing a considerably more aggressive closed-loop position controller. Several benefits were experimentally confirmed, such as faster rise time, 75% shorter settling time, 61% less overshoot, 66% better position tracking, and reduction of pressure oscillations. The proposed control algorithm is also proven to be robust against load variation providing essentially the same position accuracy. In conclusion, the novel self-contained system is experimentally proven to be a valid alternative to conventional hydraulics for applications where passive load-holding is required.

Two Heuristic PID Tuning for a 4-DOF Robot Arm Control

This paper discusses the development of closed-loop position control for the joint motor of a 4-DOF robot arm. In developing a practical robot, types of input and control algorithm are important elements in the control system. The objectives of this paper are to select an input type that could produce smooth arm’s motion and to apply PID control for the joints. Step and parabolic inputs have been tested as reference trajectory where the former used step signal as the constant value of motor angle while the latter requires calculation using spline interpolation method based on initial and end points of each joint motion given by user. From the latter, a parabolic curve input which consists of point-to-point values calculated for each 1ms sampling time was produced. Meanwhile, the PID position control tuned by trial and error and Ziegler Nichols (Z-N) methods were carried out and measured for comparison in terms of steady state error, overshoot and response time in real-time experiments. It could be observed that step input has caused abrupt movements and vibration to the arm body compared to the parabolic input that moved the arm smoothly to imitate motion like human. Meanwhile, the intuitive values of trial and error method were found to produce not much different with the Z-N tuned gains but comparatively produced small error against the parabolic input reference during the transient state. The result from this study is useful in the future to ensure position accuracy of robot joints when attending object for manipulation.

Real-time multi-threaded reflectometry density profile reconstructions on COMPASS Tokamak

In COMPASS tokamak, the ordinary mode (O-mode) microwave reflectometry diagnostic has been equipped with an upgraded acquisition system and dedicated computational node running the Multi-Threaded Application Real-Time executor (MARTe). This upgrade aims at the deployment of a real-time (RT) plasma position reflectometry (PPR) diagnostic system for future application in closed-loop plasma position control experiments. The system is synchronized with COMPASS MARTe main controller slow cycle, running at 500 μs, and allows a maximum measurement delivery latency of 450 μs. The RT signal processing algorithms, used to reconstruct the O-mode density profiles are the most time consuming step in the control cycle. This is due to the computation of a large number of Fast Fourier Transforms (FFT) over long data arrays, produced for each reflectometry profile measurement. MARTe is a software control framework that allows the integration of external C/C++ libraries and code such as the FFTW3 C library, a highly optimized and performant implementation of the FFT algorithm into the real-time designs. The framework also provides a set of tools to handle the execution of parallel threads, which can be used to expedite the execution of computationally demanding signal processing algorithms. Herein, we present the processing architecture implemented for the multi-threaded reflectometry density profile reconstruction on MARTe framework, using the FTTW3 library. Consistent systematic results have shown that the implemented solution satisfies COMPASS slow control cycle maximum data latency requirement. Moreover, the obtained reduced latency opens the possibility to either increase the number of processed measurements, leading to a more robust plasma position estimate or even to provide multiple position measurements during each control cycle.

Design of Motion Controller for Satellite-Borne Data Transmission Antenna

In order to realize stable, reliable, and high-precision motion control of the satellite-borne data transmission antenna, a FPGA-based motion controller is designed and achieved. The controller receives commands through the RS-422 asynchronous transmission serial port and performs speed planning autonomously. The controller also performs closed-loop position control with the collected resolver angle and controls two antennas synchronously or independently in a subdivision driving method. The controller is firstly designed using the hardware description language VHDL, simulated in ModelSim software. Then, it is connected to the stepping motors through the LMD18200H bridge chip by using aerospace-grade FPGA, controlling the data transmission antennas to regulate the angle, go to zero-position, or stop rotation. The simulation and experiment results show that the design can control the antennas accurately and stably. The accuracy of angle regulation reaches 0.0085° under the condition of 64 subdivisions and 100 : 1 reduction ratio.

Absolute Stability Condition Derivation for Position Closed-Loop System in Hydraulic Automatic Gauge Control

In the metallurgical industry, hydraulic automatic gauge control (HAGC) is a core mechanism for thickness control of plates used in the rolling process. The stability of the HAGC system’s kernel position closed-loop is key to ensuring a process with high precision, speed and reliability. However, the closed-loop position control system is typically nonlinear, and its stability is affected by several factors, making it difficult to analyze instability in the system. This paper describes in detail the functioning of the position closed-loop system. A mathematical model of each component was established using theoretical analysis. An incremental transfer model of the position closed-loop system was also derived by studying the connections between each component. In addition, based on the derived information transfer relationship, a transfer block diagram of disturbance quantity of the system was established. Furthermore, the Popov frequency criterion method was introduced to ascertain its absolute stability. The results indicate that the absolute stability conditions of the position closed-loop system are derived in two situations: when spool displacement is positive or negative. This study lays a theoretical foundation for research on the instability mechanism of an HAGC system.