Torque Perturbations Research Articles

A Novel Inertia Identification Method and Its Application in PI Controllers of PMSM Drives

A novel method for inertia determination of a permanent magnet synchronous machine (PMSM) drive is proposed and analyzed, which employs a sinusoidal perturbation torque with a dc offset to drive the PMSM, and the amplitudes of reference sinusoidal torque and sinusoidal speed response are used to compute the system inertia. Compared with other methods, it is robust and fast to obtain the accurate quantity of the inertia. First, the inverter measures the resistances and inductances of a PMSM through a static voltage vector and stand-still frequency response experiments. The parameters are adopted to design the current proportion and integral controller and make both dq-axes currents to perform the first-order response. Second, the results of constant torque experiment employing the current controller are applied to get the friction coefficient and no-load torque. Third, a sinusoidal torque with a dc offset is applied and the response performs a sinusoidal speed with a constant offset. All the above experimental results and other preinstalled parameters are employed to get the system inertia, and the accuracy is verified by another experiment. As compensation for a position signal from a hall sensor, a corresponding method is also proposed to compute the sinusoidal speed response amplitude, which is the key in the procedure.

Reducing the Cogging Torque Effects in Hybrid Stepper Machines by Means of Resonant Controllers

Permanent magnet machines are not free from the interaction between magnets and the stator and rotor slots, which causes an undesired disturbing torque. Such cogging or detent torque is especially larger with salient pole machines, as it is the case of the permanent magnet hybrid stepper machines (PMHSM). Depending on the application requirements, these torque perturbations can be unacceptable and the application of solutions that minimizes the cogging torque effects are mandatory. This paper analyzes the minimization of the cogging torque using resonant controllers. More specifically, this paper details the analysis and design of a speed-adaptive resonant controller, which is not only directly designed in the Z-domain but also considers the current (or torque) inner-loop delay. Pole-zero placement and the disturbance-rejection-frequency response are attained in the design of the speed and position speed-adaptive controllers. Experimental results with two off-the-shelf PMHSMs demonstrate the superior performance of the proposal in both speed and position closed-loop applications for tracking, as well as in disturbance (load impact) rejection tests and against inertia variations. A comparison with a conventional PI is carried out from the design stage to experimental results and the improvement of the proposal is numerically quantified.

Human Joint Impedance Estimation With a New Wearable Device Utilizing Snap-Through Buckling of Closed-Elastica

We propose a new wearable device for human joint impedance estimation. The device utilizes snap-through buckling of closed-elastica to impose perturbations on human joints. Actuation is based on the law of momentum conservation, that is, the proposed device induces an impact on human joint dynamics. The snap-through buckling generates a comparatively high velocity even with small actuators, and thus, the device is significantly lighter in weight and more compact compared to conventional systems. In addition, it can impose a torque perturbation on a human joint within a sufficiently short time period. Because of the perturbation in the short period, the estimation algorithm also developed in this letter enables us to consider the resulting response as an impulse response, and thereby no synchronization between the input–output data is required. We conduct pilot tests to verify the proposed method for ankle joint impedance estimation and discuss the basic performance of this method compared to the related work. We also discuss the viability of using this method for a variety of purposes in human joint impedance estimation.

Control of Pitch Attitude by Abdomen During Forward Flight of Two-Dimensional Butterfly

The objective of this paper is to understand roles of abdominal motion in the pitch stability of flapping flights of butterflies numerically, and a two-dimensional butterfly model has thoracic pitch instability in the periodically flapping flight. Because the obtained uncontrolled flight is unstable, the hierarchical sliding-mode control is used to design the control input for thorax–abdomen joint torque. It is found in the controlled flight that the abdominal motion is sufficient to counteract the thoracic pitch instability and can maintain flapping flights with short recovery time to artificial thoracic torque perturbations. On the other hand, it is also found that the long-term pitch stabilization is not possible only by the abdominal control for all flapping trajectories. However, with the introduction of a wing lead–lag control, the long-term pitch stabilization is obtained. Hence, these observations indicate that the abdomen is used for a short-term control and the wing for a long-term control in stabilizing the pitch of the butterfly’s flight.

Experimental investigation of the sloshing motion of the water free surface in the draft tube of a Francis turbine operating in synchronous condenser mode

Hydropower units may be required to operate in condenser mode to supply reactive power. In this operating mode, the water level in the turbine or pump-turbine is decreased below the runner by closing the guide vanes and injecting pressurized air. While operating in condenser mode the machine experiences power losses due to several air–water interaction phenomena which cause air losses. One of such phenomena is the sloshing motion of the water free surface below the runner in the draft tube cone of a Francis turbine. The objective of the present work is to experimentally investigate the sloshing motion of the water free surface in the draft tube cone of a reduced scale physical model of a Francis turbine operating in condenser mode. Images acquisition and simultaneous pressure fluctuation measurements are performed and an image processing method is developed to investigate amplitude and frequency of the sloshing motion of the free surface. It is found that this motion is excited at the natural frequency of the water volume and corresponds to the azimuthal wavenumber $$m = 1$$ of a rotating gravity wave. The amplitude of the motion is perturbed by wave breaking and it decreases by increasing the densimetric Froude number. The sloshing frequency slightly increases with respect to the natural frequency of the water volume by increasing the densimetric Froude number. Moreover, it results that this resonant phenomenon is not related to the torque perturbation.

Reliability of assessing postural control during seated balancing using a physical human-robot interaction

This study evaluated the within- and between-visit reliability of a seated balance test for quantifying trunk motor control using input–output data. Thirty healthy subjects performed a seated balance test under three conditions: eyes open (EO), eyes closed (EC), and eyes closed with vibration to the lumbar muscles (VIB). Each subject performed three trials of each condition on three different visits. The seated balance test utilized a torque-controlled robotic seat, which together with a sitting subject resulted in a physical human-robot interaction (pHRI) (two degrees-of-freedom with upper and lower body rotations). Subjects balanced the pHRI by controlling trunk rotation in response to pseudorandom torque perturbations applied to the seat in the coronal plane. Performance error was expressed as the root mean square (RMSE) of deviations from the upright position in the time domain and as the mean bandpass signal energy (Emb) in the frequency domain. Intra-class correlation coefficients (ICC) quantified the between-visit reliability of both RMSE and Emb. The empirical transfer function estimates (ETFE) from the perturbation input to each of the two rotational outputs were calculated. Coefficients of multiple correlation (CMC) quantified the within- and between-visit reliability of the averaged ETFE. ICCs of RMSE and Emb for all conditions were ≥0.84. The mean within- and between-visit CMCs were all ≥0.96 for the lower body rotation and ≥0.89 for the upper body rotation. Therefore, our seated balance test consisting of pHRI to assess coronal plane trunk motor control is reliable.

Control and Performance Assessment of Variable Rotor Resistance Based Wind Turbines Regarding the Aerodynamic Power Fluctuations

This paper first deals with the analytical controller design in wind turbines with variable rotor-resistance control. Wind turbines with variable rotor-resistance control, known as limited variable speed wind turbine (LVS-WT), provide a limited variation of the generator speed. In the LVS-WT, the rotor current and consequently the output power can be controlled by varying the rotor resistance. Then modal and small signal analysis of the wind turbine is examined. It is found that for certain value of shaft stiffness, frequency of the mechanical modes coincides with the triple oscillation frequency appeared due to tower shadow effects. This in turn results in resonance phenomena magnifying the fluctuation of the generator power and electromagnetic torque. The paper next evaluates the impact of aerodynamic torque fluctuations on the dynamic response of the LVS-WT. In this way, analytical expressions for the fluctuations of the stator voltage and current, electromagnetic torque, and generator active power are proposed. These fluctuations arise as a consequence of the aerodynamic torque and rotor speed perturbations. The paper also investigates the effects of shaft stiffness, slope of power-slip curve and rotor resistance on the wind turbine response. At the end, results of theoretical analyses are verified by time domain simulations.

On-line attitude perturbation estimation in the earth-orbiting satellite

This paper presents three innovative techniques to estimate external perturbation torques acting on an Earth-orbiting satellite. The proposed techniques overcome the problem of derivative reconstruction from noisy measurement in the estimation procedure of the environmental perturbation torques. For this purpose, in the first technique, known as nonlinear disturbances observer (NDO) technique, derivative term is eliminated in the estimation equations by employing auxiliary variable vector and by selecting appropriate design parameters. In the second proposed approach, the robust exact differentiation via second-order sliding mode theory, known as Levant differentiator, is employed to decrease derivative estimation error in the process of estimation. To improve the performance of the Levant differentiator, the final innovative approach, known as sub-band adaptive robust differentiator (SARD) technique, is proposed in which the derivative term in the estimation process is reconstructed by using Levant differentiator and sub-band adaptive filter theory. In the SARD technique, the original noisy measurement signal is divided into sub-band signals and then, the Levant differentiator is applied to each sub-band signals to construct exact differentiator from noisy measurement. The novel aspect of paper is employing second-order sliding mode theory in the sub-bands frequency to improve the performance of derivative reconstruction in the presence of measurement noise. In this regard, a nonlinear model of the earth-orbiting satellite is simulated using specific navigation disturbances and the results verified the feasibility of the proposed strategies.

Estimating driver time-varying neuromuscular admittance through LPV model and grip force

Humans can rapidly change their low-frequency arm dynamics (i.e., stiffness) to resist forces or give way to them. Quantifying driver’s time-varying arm dynamics is important for the development of steer-by-wire systems and haptic driver support systems. Conventional LTI identification, and even time-varying techniques such as wavelets, fail to capture rapidly-varying low-frequency dynamics. In this study, we propose to estimate driver admittance in real-time, using grip force measurement of the hands on the steering wheel and linear parameter-varying (LPV) modeling techniques. We hypothesized that grip force is strongly correlated to neuromuscular admittance, and can serve as an appropriate scheduling variable for an LPV model. We performed an experiment in which 18 subjects performed a boundary tracking task, and applied torque perturbations to the steering wheel to perform a baseline LTI identification. Six different boundary widths were used to evoke changes in admittance, while their grip force was measured with pressure gloves. A global LPV model is identified by linear interpolation between the local LTI models identified for each boundary width. The estimated stiffness and damping parameters varied proportionally with the grip force. Although small between-subject variations in grip force levels are found, we conclude that grip force can indeed serve as an appropriate scheduling variable for a global LPV model, which is capable of tracking fast-changing admittance changes. Future work focuses on using the LPV model in realistic driving tasks, permitting admittance estimates to be obtained without the need to apply external disturbance torques on the steering wheel.

A study of spacecraft reaction thruster configurations for attitude control system

Reaction thrusters (RTs) are used as an alternative to momentum exchange devices when disturbance torques exceed the control authority of momentum exchange devices. The reaction control system (RCS) can employ some rocket thrusters to provide attitude control during the thrusting or coast phase. Within the control loop, the RCS's target could be either achieving and keeping a certain attitude or controlling the rate of an attitude change. In the coast phase, some tasks such as preacceleration, settling of liquid propellant, damping of structural vibrations, providing a velocity increment to separate stages and payloads, and carrying out orbital and nonorbital maneuvers may be included in its functions. The propulsion perturbation torques, whose size is relatively large, are primarily produced because of the center of mass offset, which itself is produced as a result of static unbalance, transient gas flow phenomenon, and nozzle cant angle misalignments. Nozzle cant angle misalignments are produced because of manufacturing tolerances and pressurizations, which should be compensated by the attitude control system. In addition, rocket thruster plume impingement against surrounding structure or components produces sizable disturbance torques and cross-coupling effects that degrade the dynamic stability and increase the duty cycle that should be corrected by additional thrusters to restore vehicle attitude. During each maneuver, there are always undesirable angular rotations in consequence of some errors and uncertainties in the various components of a spacecraft. The attitude control system has to be capable of compensating these imperfect effects of the mechanical system.

Using the attitude response of aerostable spacecraft to measure thermospheric wind

In situ measurements of the thermospheric wind can be obtained by observing the attitude response of an aerostable spacecraft. In the proposed method, the aerostable spacecraft is left uncontrolled, freely reacting to the aerodynamic torques, and oscillating around its equilibrium attitude. The wind’s magnitude and direction is determined by combining the attitude observations with estimates of the other perturbing torques, atmospheric density, and spacecraft’s aerodynamic properties. The spatial resolution of the measurements is proportional to the natural frequency of the attitude’s oscillation. Spacecraft with high aerodynamic stiffness to inertia ratios operating at low altitudes exhibit higher natural frequencies, making them particularly suited for this method. A one degree-of-freedom case is used to present and illustrate the proposed method as well as to analyze its performance.

Investigation of the Driver’s Arm Viscoelastic Properties During Steering Vehicle Maneuver

Driver's arm viscoelastic properties during steering vehicle maneuver are very important information to understand how the driver regulates his torque and they could be used to design a new driver assistance system. This correspondence paper investigates the driver's arm viscoelastic properties during steering vehicle maneuver. First, a time-varying method for estimating driver's arm viscoelastic properties during driving the vehicle is proposed. Then, this method is implemented experimentally in order to examine the driver's arm viscoelastic properties during driving the vehicle. The estimation method does not require a specific perturbation torque to be applied during driving and could be applied online. The method is based on a time-varying model of the human driver's arm coupled with the electric power-assisted steering system model. Experiments were carried out with five healthy subjects who drove on a hardware-in-the-loop simulator without assistance and with assistance.

Manipulation of visual information affects control strategy during a visuomotor tracking task

Proper understanding of motor control requires insight into the extent and manner in which task performance and control strategy are influenced by various aspects of visual information. We therefore systematically manipulated the visual presentation (i.e., scaling factor and optical flow density) of a visuomotor tracking task without changing the task itself, and investigated the effect on performance, effort, motor control strategy (i.e., anticipatory or corrective steering) and underlying neuromechanical parameters (i.e., intrinsic muscle stiffness and damping, and proprioceptive and visual feedback). Twenty healthy participants controlled the left-right position of a virtual car (by means of wrist rotations in a haptic robot) to track a slightly curved virtual road (presented on a 60” LED screen), while small torque perturbations were applied to the wrist (1.25–20Hz multisine) for quantification of the neuromechanical parameters. This visuomotor tracking task was performed in conditions with low/medium/high scaling factor and low/high optical flow density. Task performance was high in all conditions (tracking accuracy 96.6%–100%); a higher scaling factor was associated with slightly better performance. As expected, participants did adapt their control strategy and the use of proprioceptive and visual feedback in response to changes in the visual presentation. These findings indicate that effects of visual representation on motor behavior should be taken into consideration in designing, interpreting and comparing experiments on motor control in health and disease. In future studies, these insights might be exploited to assess the sensory-motor adaptability in various clinical conditions.

Design and Evaluation of a 2-DOF Instrumented Platform for Estimation of the Ankle Mechanical Impedance in the Sagittal and Frontal Planes

This paper describes the fabrication and initial evaluation of a vibrating platform with two degrees of freedom (DOF) to estimate the human ankle's mechanical impedance in two DOFs; namely dorsiflexion–plantarflexion (DP) and inversion–eversion (IE). The device consists of an actuation and a force plate module. The actuation module generates torque perturbations up to 168 N·m in DP and 26 N·m in IE in the force plate module using Bowden cables. This provides a low-profile system that can be installed in a walkway. The frame of the force plate module rotates in two DOFs, applying torque perturbations to the human ankle in DP and IE. The ankle's rotations are measured using a motion capture camera system. The analytical and numerical approaches for estimation of the ankle's torques, rotations, and impedances are presented. A system validation using a mockup was conducted to verify the system's ability to estimate the impedance of a physical system in two DOFs. The developed system was capable of identifying the mockup's physical properties up to 15 Hz. The mockup's impedance magnitude at 0.9 Hz using a stochastic identification method was shown to be within 1.68% and 0.54% of the mockup's stiffness in DP and IE, respectively.

Estimation of Human Hip and Knee Multi-Joint Dynamics Using the LOPES Gait Trainer

In this study, we present and evaluate a novel method to estimate multi-joint leg impedance, using a robotic gait training device. The method is based on multi-input–multi-output system identification techniques and is designed for continuous torque perturbations at the hip and knee joint simultaneously. Eight elderly subjects (age 67–82) performed relax and position tasks in three different leg orientations. Multi-joint impedance was estimated nonparametrically and was subsequently modeled in terms of inertia and (inter)joint stiffness and damping. The results indicate that all stiffness and damping parameters were significantly higher during the position task compared to the relax task. The majority of the stiffness and damping parameters were not significantly affected by leg orientation. The results also emphasize the importance of considering the visco-elastic coupling between joints when modeling multi-joint dynamics. Measuring joint stiffness with the same device that is used for robotic gait training allows convenient testing of joint properties in conjunction with the robotic gait training protocol. These measures might serve as a good basis for quantitative assessment and follow up of patients with abnormal joint stiffness due to neurological disorders, and may reveal how changes in these joint properties affect their gait function.

Adaptive-gain multivariable super-twisting sliding mode control for reentry RLV with torque perturbation

Adaptive-gain multivariable super-twisting sliding mode control for reentry RLV with torque perturbation

Modelling deep groove ball bearing localized defects inducing instantaneous angular speed variations

Instantaneous angular speed (IAS) analysis has been presented as an advantageous tool for non-stationary machinery surveillance. The physical analysis indicates that the instantaneous rotating speed variations are caused by torque perturbations. In a previous work, a method for introducing angular periodic disturbances in rotating systems dynamic models was presented. This analysis was based on the introduction of a parametric perturbation applied to the system as an external torque. The current work introduces an original formulation to induce tangential forces to the shaft due to the bearing components dynamics by means of a Hertzian contact roller bearing model. The methodology takes into account rotational bearing races and rolling elements degrees of freedom, making the model suitable for non-stationary conditions.

Mechanical Impedance of the Non-loaded Lower Leg with Relaxed Muscles in the Transverse Plane.

This paper describes the protocols and results of the experiments for the estimation of the mechanical impedance of the humans’ lower leg in the External–Internal direction in the transverse plane under non-load bearing condition and with relaxed muscles. The objectives of the estimation of the lower leg’s mechanical impedance are to facilitate the design of passive and active prostheses with mechanical characteristics similar to the humans’ lower leg, and to define a reference that can be compared to the values from the patients suffering from spasticity. The experiments were performed with 10 unimpaired male subjects using a lower extremity rehabilitation robot (Anklebot, Interactive Motion Technologies, Inc.) capable of applying torque perturbations to the foot. The subjects were in a seated position, and the Anklebot recorded the applied torques and the resulting angular movement of the lower leg. In this configuration, the recorded dynamics are due mainly to the rotations of the ankle’s talocrural and the subtalar joints, and any contribution of the tibiofibular joints and knee joint. The dynamic mechanical impedance of the lower leg was estimated in the frequency domain with an average coherence of 0.92 within the frequency range of 0–30 Hz, showing a linear correlation between the displacement and the torques within this frequency range under the conditions of the experiment. The mean magnitude of the stiffness of the lower leg (the impedance magnitude averaged in the range of 0–1 Hz) was determined as 4.9 ± 0.74 Nm/rad. The direct estimation of the quasi-static stiffness of the lower leg results in the mean value of 5.8 ± 0.81 Nm/rad. An analysis of variance shows that the estimated values for the stiffness from the two experiments are not statistically different.

LIMPACT:A Hydraulically Powered Self-Aligning Upper Limb Exoskeleton

The LIMPACT is an exoskeleton developed to be used in identifying the reflex properties of the arm in stroke survivors. Information on joint reflexes helps in designing optimal patient specific therapy programs. The LIMPACT is dynamically transparent by combining a lightweight skeleton with high power to weight ratio actuators. The LIMPACT is supported by a passive weight balancing mechanism to compensate for the weight of the exoskeleton and the human arm. Various self-aligning mechanisms allow the human joint axes to align with the axes of the exoskeleton which ensure safety and short don/doff times. The torque-controlled motors have a maximum torque bandwidth of 97 Hz which is required for fast torque perturbations and smooth zero impedance control. The LIMPACT's weight is reduced five times as gravitational forces are lowered using a model-based gravity compensation algorithm. The impedance controller ensures tracking of a cycloidal joint angle reference. A cycloid with an amplitude of 1.3 rd and a maximum velocity of 6.5 rd/s has a maximum tracking error of only 7%. The LIMPACT fulfills the requirements to be used in future diagnostics measurements for stroke patients.

The dynamics of spanwise vorticity perturbations in homogeneous viscoelastic shear flow

The viscoelastic analogue to the Newtonian Orr amplification mechanism is examined using linear theory. A weak, two-dimensional Gaussian vortex is superposed onto a uniform viscoelastic shear flow. Whilst in the Newtonian solution the spanwise vorticity perturbations are simply advected, the viscoelastic behaviour is markedly different. When the polymer relaxation rate is much slower than the rate of deformation by the shear, the vortex splits into a new pair of co-rotating but counter-propagating vortices. Furthermore, the disturbance exhibits a significant amplification in its spanwise vorticity as it is tilted forward by the shear. Asymptotic solutions for an Oldroyd-B fluid in the limits of high and low elasticity isolate and explain these two effects. The splitting of the vortex is a manifestation of vorticity wave propagation along the tensioned mean-flow streamlines, while the spanwise vorticity growth is driven by the amplification of a polymer torque perturbation. The analysis explicitly demonstrates that the polymer torque amplifies as the disturbance becomes aligned with the shear. This behaviour is opposite to the Orr mechanism for energy amplification in Newtonian flows, and is therefore labelled a ‘reverse-Orr’ mechanism. Numerical evaluations of vortex evolutions using the more realistic FENE-P model, which takes into account the finite extensibility of the polymer chains, show the same qualitative behaviour. However, a new form of stress perturbation is established in regions where the polymer is significantly stretched, and results in an earlier onset of decay.