Design of perturbation signals for the estimation of proprioceptive reflexes

This study aimed to identify the functional contribution of reflexes to human motor control during posture maintenance. Continuous random force disturbances were applied at the hand while the subjects were instructed to minimize the deviation resulting from the force disturbances. The results were analyzed in the frequency domain with frequency response functions (FRFs). Two FRFs were evaluated: 1) the mechanical admittance and 2) the reflexive impedance, expressing the dynamic relation between position and muscle activation (assessed via electromyography, EMG). The reflexive impedance is a direct measure of the proprioceptive reflexes. To record all relevant dynamical characteristics of the arm, wide bandwidth signals were used as force disturbance. Distributing the power of the signal over fewer frequencies within the bandwidth improved the signal-to-noise-ratio SNR of the EMG recordings, facilitating reliable estimation of the reflexive impedance. The coherence indicated that the relation between force disturbance and EMG is linear under the given conditions and improved with the SNR. The method of designing disturbance signals and the estimation of the reflexive impedance are useful for studies aiming to quantify proprioceptive reflexes and to investigate its functionality.     

Simultaneous and nonlinear identification of mechanical and reflexproperties of human elbow joint muscles

The naturally coexisting intrinsic mechanical and reflex properties of the human elbow joint were identified simultaneously using nonlinear, time-delay, continuous-time, and dynamic models. Angular random perturbations of small amplitude and low bandwidth were applied to the joint using a computer-controlled servomotor, while the subject maintained various levels of mean background muscle torque. Joint neuromuscular dynamics were identified from the measured elbow angle and torque. Stretch reflexes were modeled nonlinearly with both dynamic and static reflex gains. A continuous-time system identification method was developed to estimate parameters of the nonlinear models directly from sampled data while retaining realistic physical or physiological interpretations. Results from six subjects showed that dynamic stretch reflex gains, joint stiffness, and viscosity generally increased with mean background muscle torque; and that dynamic stretch reflex gain was higher during muscle stretch than that during muscle shortening. More importantly, the study provided realistic simultaneous estimates of the relative contributions of intrinsic mechanical and reflex actions to net joint torque. In particular, reflexively-mediated stiffness generated a significant portion of the total joint stiffness and the percentage varied systematically with background muscle torque     

惯性稳定平台中的加速度反馈控制技术

陀螺稳定是惯性稳定中的常见装置,由它仅仅构成一个简单的单速度稳定环。控制带宽是影响陀螺稳定的惯性控制系统的一个重要因素。高的控制带宽很难获取,主要是因为系统的非线性影响,比如机械谐振。将一种基于加速度反馈的多环控制结构引入速度控制系统中,提高稳定性能。采用两只线性加速度计测量角速度信号,而不是利用位置和速度信号计算得到。利用 Lyapunov 函数分析了多级稳定回路稳定,以及对摩擦、扰动抑制能力的效果。多级稳定控制环路的误差抑制能力是陀螺反馈系统的能力以及加速度反馈之积。实验验证了理论分析的正确性:相比经典的陀螺稳定系统,扰动抑制能力有较大的改善。     

Feedback regulation of hand grasp opening and contact force duringstimulation of paralyzed muscle

A fixed-parameter, discrete-time, first-order, feedback control system is described for regulating grasp during electrical stimulation of paralyzed muscles of the hand. The stiffness of the grasp (relationship between grasp force and grasp opening) is kept constant by linearly combining force and position feedback signals. Thus, a single continuous command signal can control the size of the grasp opening prior to object acquisition and both grasp force and opening after contact. The controller achieves this change in controlled variables by scaling and summing the force and position feedback signals, rather than by a discrete switch in control strategy. Experimental tests of the control system in quadriplegic subjects show that control can be obtained over conditions ranging from unloaded position regulation to isometric force regulation, as well as in the transition between these conditions. The robustness of the control system was evaluated during force regulation with isometric loads. Step response rise time and overshoot were much more dependent on system gain than on the location of the controller zero. Responses with rise time less than two seconds and overshoot less than 30% were obtained over a gain range up to ten, indicating good robustness to muscle gain reductions such as might be caused by fatigue.     

Positive acceleration,velocity and position feedback based damping control approach for piezo-actuated nanopositioning stages

This paper proposes a new damping control approach with positive acceleration, velocity and position feedback (PAVPF) scheme for piezo-actuated nanopositioning stages to implement high-bandwidth operation. To achieve this objective, the intrinsic hysteresis nonlinearity of the piezoelectric actuator is firstly handled by a feedforward compensator with a modified Prandtl–Ishlinskii model. Afterwards, the PAVPF controller with the pole-placement method is implemented to suppress the lightly damped resonant mode of the hysteresis compensated system. With the PAVPF controller, the poles of the damped system in a third-model can be placed to arbitrary positions with an analytical method. Finally, for accurately tracking a predefined trajectory, a high-gain proportional-integral (PI) controller is designed, which could deal with the disturbance and the unmodeled dynamics. For verifying the proposed PAVPF-based control approach, comparative experiments with positive velocity and position feedback controller and with PI controller are conducted on a piezo-actuated nanopositioning stage. Experimental results demonstrate that the developed control approach with PAVPF controller is effective on damping control and improves the control bandwidth of the conventional PI controller from 111 Hz to 766 Hz, which leads to the significant increase of the tracking speed.     

Galvanic vestibular stimulation elicits consistent head-neck motion in seated subjects

Humans actively stabilize the head-neck system based on vestibular, proprioceptive and visual information. Galvanic vestibular stimulation (GVS) has been used previously to demonstrate the role of vestibular feedback in standing balance. This study explores the effect of GVS on head-neck kinematics and evaluates the approach to investigate the vestibular contribution to head-neck stabilization. GVS was applied to 11 seated subjects using seven different stimuli (single sinusoids and multisines) at amplitudes of 0.5-2 mA and frequencies of 0.4-5.2 Hz using a bilateral bipolar configuration while 3-D head and torso kinematics were recorded using motion capture. System identification techniques were used evaluating coherence and frequency response functions (FRFs). GVS resulted in significant coherence in roll, yaw and lateral translation, consistent with effects of GVS while standing as reported in the literature. The gain of the FRFs varied with frequency and no modulation was observed across the stimulus amplitudes, indicating a linear system response for the stimulations considered. Compared to single sine stimulation, equivalent FRFs were observed during unpredictable multisine stimulation, suggesting the responses during both stimuli to be of a reflexive nature. These results demonstrate the potential of GVS to investigate the vestibular contribution to head-neck stabilization.     

Robust motion controller design for high-accuracy positioningsystems

This paper presents a controller structure for robust high speed and accuracy motion control systems. The overall control system consists of four elements: a friction compensator; a disturbance observer for the velocity loop; a position loop feedback controller; and a feedforward controller acting on the desired output. A parameter estimation technique coupled with friction compensation is used as the first step in the design process. The friction compensator is based on the experimental friction model and it compensates for unmodeled nonlinear friction. Stability of the closed-loop is provided by the feedback controller. The robust feedback controller based on the disturbance observer compensates for external disturbances and plant uncertainties. Precise tracking is achieved by the zero phase error tracking controller. Experimental results are presented to demonstrate performance improvement obtained by each element in the proposed robust control structure     

Analysis and experimental validation of force bandwidth for force control

Controlling robots in contact with the environment is an important problem in industry applications. In the conventional force control, much research has paid attention to develop novel force control systems and implemented force sensors to detect external force. This paper shows that narrow bandwidth of force sensor has a big influence on the force control system. Generally, to solve the instability in force control, the velocity feedback gain is enlarged. The system becomes unstable with small velocity feedback gain, and robot's response becomes slow with large one. Inasmuch as there is a tradeoff between the stability and the response, it is considered that force control by robots is difficult. This paper proposes a force control system with disturbance observer. It is possible to obtain the force information with wide bandwidth by using the disturbance observer. This paper shows that bandwidth of force sensing is very important for contact motion control. By using the wide bandwidth of force sensing, both stability and response are improved. Furthermore, force control is attainable by the construction of the easiest force control architecture. Therefore, the ideal zero-stiffness-force control is attained. The numerical and experimental results show viability of the proposed method.     

Transmission of Force Sensation by Environment Quarrier Based on Multilateral Control

In recent years, realization of a haptic system has been strongly desired in the fields of medical treatment and expert's skill acquisition. The bandwidth of force sensing and friction compensation are very important problems for reproduction of vivid force sensation. In this paper, an environment quarrier is proposed for bilateral teleoperation instead of force sensors. The environment quarrier is a novel force-sensing method that consists of a twin robot system. Two of the same type of robot are required and they are controlled in the same position, velocity, and acceleration by bilateral acceleration control based on a disturbance observer. One robot is in contact motion and the other is unconstrained. The purity of external force is obtained by subtracting the disturbance torque in the unconstrained robot from the constrained one. The environment quarrier can observe the external force with high bandwidth and friction compensation. In this paper, the idea of multilateral control is introduced for attainment of simultaneity. Furthermore, this paper shows the controller design of the multilateral control in the modal space. The experimental results show the viability of the proposed method     

Closed loop low-velocity regulation of hybrid stepping motorsamidst torque disturbances

To regulate the velocity of hybrid stepper motor motion control systems, a control law which exploits the nonlinear dynamics to create an analog positional control in conjunction with a traditional linear control is introduced. This nonlinear approach allows a much coarser position sensor to be used, including position estimates based on back EMF measurements. The form of the control law admits the use of a wide variety of compensators, whereas earlier laws use only velocity damping compensation. Two specific compensators, i.e., velocity damping and integral control are analyzed in detail, then compared to each other and to open loop microstepping control. It is shown that velocity damping allows the design of the eigenvalues of the closed loop system and provides a linear system approach about a specified operating point. Unfortunately, this operating point includes the value of external DC torque (drag) present, so the closed loop dynamics cannot be guaranteed amidst steady state torque fluctuations. Integral feedback (within a PID controller) improves upon velocity damping by not only allowing the design of the closed loop eigenvalues, but also by completely linearizing the system regardless of external DC torque values. Furthermore, the integral feedback produces zero steady state position error (as expected from linear control theory) and significantly decreases the tendency of the motor to lose step. Experimental results validate the analyses     

Stability enhancement of admittance control with acceleration feedback and friction compensation

This paper presents an experimental investigation of a new position control scheme that enhances the stability of admittance control by using: (a) PDD² (proportional, derivative, and second derivative) feedback, (b) dither-based friction compensation and (c) sliding-mode-based noise filter with a variable gain. The PDD² structure and the friction compensation are for expanding the bandwidth of the internal position-controlled subsystem. The sliding-mode-based filter is for the attenuation of noise in the acceleration signal without producing a large phase lag. The variable gain of the filter is for suppressing acceleration-measurement noise at low velocity. The proposed controller is validated by employing a 1-DOF device.     

Vibration suppression in 2- and 3-mass system based on the feedbackof imperfect derivative of the estimated torsional torque

In this paper, a new vibration-suppression control method for 2- and 3-mass system is proposed, which uses imperfect derivative feedback of the estimated torsional torque. This controller consists of three simple elements: the disturbance observer, the imperfect derivative filter, and the feedback gain. By adequately adjusting this feedback gain, the damping factor of original flexible system can be controlled so that the vibration caused by the mechanical resonance can be effectively suppressed. Due to the simplicity of the proposed controller, it can be easily applied to various flexible systems with other regulators. The combination of this method with P&I regulator shows good performances in vibration suppression and disturbance rejection which are shown in both simulations and experiments     

Linear Quadratic Control of a Loaded Agonist?Antagonist Muscle Pair

A discrete-time control law which gives the optimal applied force for a given position trajectory of a linear second-order external load is computed using linear quadratic regulator (LQR) theory. This control law is used with an existing first-order fixed interpulse interval (IPI) muscle force controller to regulate position in a simulated electrically stimulated loaded agonist-antagonist muscle system. Both force and position feedback are required to implement the control strategy. A nonlinear second-order model of a predominantly slow twitch muscle is used in the simulation. The results suggest that good position control of a loaded antagonist-agonist muscle system is possible if a linear model of the external load is appropriate.     

Single-loop output-feedback positioning technique for servo systems via second-order pole-zero cancellation approach

This article presents an advanced order-reduction technique for servo motor positioning applications by explicitly handling the model-plant mismatches caused by parameter and load uncertainties. The resultant output feedback system establishes an observer-based single-loop structure ensuring first-order position dynamics by the second-order pole-zero cancellation mechanism. Two contributions are provided: first, the plant parameter information-free angular velocity and acceleration observer leading to the first-order estimation dynamics by the proposed gain structure; second, the output-feedback positioning law constraining the closed-loop behavior into the first-order dynamics through the observer-based active damping injection causing the second-order pole-zero cancellation, requiring the only nominal plant parameter values. An experimental testbed using QUBE-servo2 confirms the practical effectiveness of the proposed control scheme.     

Observer-Based Ripple Force Compensation for Linear Hybrid Stepping Motor Drives

This paper describes our research on a force ripple compensation and closed-loop position control scheme using linear hybrid stepping motors (LHSMs) with significant thrust vibrations. In order to estimate unobservable force ripple components, we propose the Jacobian linearization observer that guarantees the convergence of state estimates into true states. For the precise control of velocity and position, an input-output feedback linearization controller is derived from a nonlinear position-dependent model of the LHSM based on elaborate reluctance network analysis. In addition, we discuss the separation principle used to separate the observer design from the controller design. Common problems associated with the force ripple, such as the positioning error, mechanical stress, and acoustic noise, are efficiently handled using the proposed active damping control scheme. Experimental results show that the positioning accuracy is significantly improved through a closed-loop control while restraining the thrust ripple.     

Power conditioning systems for a magnetically levitated test vehicle

The power conditioning for a magnetically levitated linear-motor propelled test vehicle is described. Control circuits have been developed to provide a constant air gap for levitation and improved damping of lateral motion due to force disturbances. Transistorized magnet drivers supply the required magnet current. A longitudinal control system with programmed limits on the second and third derivatives of the position command allows both precise position stopping and control of acceleration and jerk. A programmable controller is used for sequencing and monitoring the levitation and propulsion systems.     

Hybrid force-velocity sliding mode control of a prosthetic hand

Four different methods of hand prosthesis control are developed and examined experimentally. Open-loop control is shown to offer the least sensitivity when manipulating objects. Force feedback substantially improves upon open-loop control. However, it is shown that the inclusion of velocity and/or position feedback in a hybrid force-velocity control scheme can further improve the functionality of hand prostheses. Experimental results indicate that the sliding mode controller with force, position, and velocity feedback is less prone to unwanted force overshoot when initially grasping objects than the other controllers.     

A dynamic neuromuscular model for describing the pendulum test ofspasticity

Both dynamic and static thresholds, as well as the gain in the stretch reflex loop, affect the sensitivity of motoneurons to muscle stretch. How the variation in each parameter will influence the mechanical behavior of patients with spasticity is not well understood because of the difficulty in experimentally isolating individual parameters. A neuromuscular dynamic model, based on the pendulum test of spasticity, has been developed to study the specific contribution of individual parameter abnormalities in stretch reflex loops to the observed mechanical abnormalities. The model contains detailed nonlinear dynamics of muscle force generation and stretch reflexes. A computer simulation of the model indicates that the stretch reflex thresholds and the gain have different influences on the leg swing in the pendulum test of spasticity. Individual changes in the static stretch reflex threshold, in the dynamic threshold, or in the gain can not stimulate the whole spectrum of spasticity severity. When simultaneous changes in all three parameters of the stretch reflex loop occur, a small variation of the gain coupled with changes in both static and dynamic thresholds can produce increasing severity of spasticity as the thresholds further decrease. The model is also successful in simulating the effect of posture changes on spasticity