Waldemar Rączka

Modified Clipped-LQR Method for Semi-Active Vibration Reduction Systems with Hysteresis

Smart materials are being applied more and more widely in semi-active vibration reduction systems. Actuators built with their use are characterized by nonlinearities and hysteretic effects. Their omission in mathematical descriptions may lead to deterioration of the vibration reduction systems. For that reason, it is important to take into account these negative phenomena associated with the actuators at the controller synthesis stage. One method for determining the control laws in semi-active vibration reduction systems that is frequently discussed in academic literature is “Clipped-LQR”. The present paper proposes modification of that method to allow inclusion in the controller synthesis of the hysteretic properties and other nonlinearities of an actuator. The method developed was verified by determining the controller for the semi-active suspension of a machine operator’s seat. A magnetorheological damper was used as an actuator. The dynamic properties of the foam covering of the operator’s seat were included in model. Simulation tests were performed on the vibration reduction system and function of vibration transmissibility was determined. The semi-active vibration reduction system tested was compared to a passive system. The considerations presented herein relate to the semi-active suspension of a machine operator’s seat, and the method presented may be applied to other controlled systems with many degrees of freedom.

Mathematical Model of a Shape Memory Alloy Spring Intended for Vibration Reduction Systems

The article discusses a prototype of a Shape Memory Alloy (SMA) spring intended for controlled vibration reduction systems. The spring has been subject to experiments and the article presents selected static and dynamic characteristics. The experiments were conducted at the Dynamics and Control of Structures Laboratory of the AGH University of Science and Technology. They permitted the formulation of a mathematical model for the SMA spring. The model takes into account the phenomena of energy accumulation and dissipation. The parameters of the spring model have been determined, based on the experimental data. The model takes into account the relationship of stiffness and damping to alloy temperature and the frequency of excitation. It has been demonstrated that the properties of the spring may be altered under controlled conditions. The spring model was then used in simulations. They served as the basis for the determination of the frequency response characteristics, which were then compared to the characteristics of a real spring. The mathematical model developed may be applied in the design of passive, semi-active, and active vibration reduction systems, as well as in the synthesis of adaptive smart vibration reduction systems.

The Control System for a Vibration Exciter

The paper presents the control system for an electro-hydraulic vibration exciter. A vibration exciter is used to carry out kinematic and forced excitations. Such excitations are needed during the static and dynamic examinations of suspension assemblies and their elements, such as damping springs. The most important elements of the exciter are: a hydraulic servo valve, a hydraulic actuator, a control system and a hydraulic supply station. An inductor works in a negative feedback loop. A set point generator and controller are parts of a real-time control system. The authors used a cRIO-9022 industrial computer from National Instruments analogue input and output modules. Apart from supporting I/O modules, the cRIO controller in a real-time system communicates with an FPGA circuit that forms a control loop. The use of an FPGA (Field Programmable Gate Array) allowed for implementing a hardware control loop through a vibration exciter, as well as other control loops used in the research (such as controlling an active executive element). The use of such a system architecture enabled the parallel operation of multiple control threads. The selected structure also allows for flexible selection of the measuring channels, which are essential for system monitoring and data acquisition.

Laboratory Tests of Shape Memory Alloy Wires

In this paper, tests on Shape Memory Alloy (SMA) wires for use in controlled actuators have been discussed. Selected static and dynamic characteristic curves being the result of experiments were presented in the article. Experiments were conducted at the Dynamics and Control of Structures Laboratory of the AGH University of Science and Technology. Laboratory tests of SMA wires used as actuators have been presented in the paper. Actuators made from the wires contract by about 4-5% of their length when heated, like small muscles, and loosen when cooled. SMA wires used as drives are significantly smaller than traditional solutions using motors or electromagnet to execute work. However, these actuators have flaws, such as strongly non-linear hysteresis. These are main problems in designing actuators, which is why SMA wires are often used in the construction of two-state actuators working as on-off actuators. The problems with SMA wires in their applications as drives are their static and dynamic properties, sensitivity to the environment, poor repeatability of production, non-linearity and hysteresis loop. The tested wires were made from a nickel and titanium alloy; this is an alloy which is often used in drive systems.

Robot Control System for Stereotactic Surgery

The article presents a prototype of a robot control system for stereotactic brain surgery. The development of neuroimaging, as well as the implementation of modern devices for neurosurgery, has undoubtedly contributed to significant progress in this field of medicine. Reaching a pathology located deep inside does not pose significant difficulty from the technical point of view; it is the selection of the access route that is the problem - to prevent the occurrence of permanent neurological deficits, such as severe paresis, vision disorders, speech or consciousness disorders. Obviously, knowledge of brain anatomy allows for adopting appropriate surgical tactics; however, some patients with a high-risk access route are not qualified for surgery. Even the hand of the most skilled operator does not guarantee precise removal of the tumour whilst bypassing important neural pathways, hence the necessity of introducing robots, which increase the level of surgical precision. A neural surgeon is able to perform very complicated surgical actions; however, human psychophysical limitations in neurosurgical procedures are becoming increasingly important, as surgeons are more and more frequently willing to perform operations with accuracy higher than 1 mm, but this is virtually impossible without any enhancement. Apart from the natural limitations of precision of the human hand, the surgeons mental and physical condition deteriorates during a several-hour-long surgical procedure, thus lowering the surgeons fitness and precision of movement. High concentration and a frequently unnatural position during the surgery cause additional tiredness. Therefore, numerous research centres and commercial companies are working on surgical robots. It seems that neurosurgery is particularly inclined for the introduction of robots enhancing the precision of a surgeons work.

Active Vehicle Suspension with a Weighted Multitone Optimal Controller: Considerations of Energy Consumption

Vibration reduction is a significant problem in the design and construction of vehicle suspensions (Lozia and Zdanowicz, IOP Conf Ser Mater Sci Eng 148:12014, 2016; Konieczny et al., J Low Freq Noise Vib Act Control 32:81–98, 2013). Passive semi-active and active methods are used in order to reduce vibrations. Considerations related to active and semi-active vibration reduction and the influence of disturbances on such objects can be found in many publications. In the case of these systems, the aim is always to find a compromise between their efficiency and energy consumption. The control law for such systems is usually determined as a solution to the optimisation problem with quadrant quality indicator. Energy limitation is taken into account by selection coefficients of the weighting matrix associated with the control signals vector. The efficiency of vibration reduction is able to be improved in the entire useful frequency range of the system operation but this generally results in an increase in the demand for external energy. An additional problem in the case of vehicle suspensions includes the need for increased vibration reduction for selected frequencies. This is related to the internal vibration frequencies of the driver’s internal organs. The paper presents the synthesis of a weighted multitone optimal controller (WMOC) for an active vibration reduction system. The control signal in this case is determined on the basis of the identified sinusoidal disturbances vector. The vibration transmissibility function and the energetic indicators for the active suspension were determined while taking note of nonlinearities occurring in the actual vehicle. The analysis of energy indicators (e.g. energy, maximum power) is presented, depending on the level of vibration reduction efficiency. The results were compared with analogous linear-quadratic regulator (LQR).

Sliding Mode Controller for Vehicle Body Roll Reduction Using Active Suspension System

In many designs of vehicle suspension anti-roll bars are used for body roll reduction (Cronje and Els, J Terramechanics 47:179–189, 2010; Her et al., IFAC Proc. 2013. https://doi.org/10.3182/20130904-4-JP-2042.00152). Such solutions are used in compact cars (C-segment), mid-size cars (D-segment), mid-size luxury car (E-segment) and even sports cars (S-segment). Antiroll bars are torsion bars connecting both sides of car suspension and they are connected with suspension arms. When car wheels move in vertical direction these torsion bars are twisted. Increasing torsional stiffness of front or rear suspension is the main role of sway bars. The use of these rods reduces tilting of the body while riding at high speed when the tilt of a car body is caused by the centrifugal force acting on the car. This force compress suspension spring with one side of the car, and extends on the other. The torque generated by the torsion bars reduce the difference in springs suspension. It reduces the compression of springs on one side of a car and extension on the other. Reduction of car body tilt improving handling and road holding and therefore improve safety. Antiroll bars also allow for the reduction of lateral vibrations caused by road roughness. During straight driving on equal road surface or under the same excitation of the wheels, the stabilizer doesn’t work. During overcoming high road irregularities, the force acting on one of a wheel is transferred to the other via the bar. Antiroll bars also allow for the reduction of lateral vibrations caused by road inequalities. This causes unwanted oscillations. This effect is more noticeable if angle stiffness of the stabilizer is higher. In order to avoid such phenomena, active anti-roll bars are used. The most commonly used solutions are two-part active disconnect anti-roll bar or just active antiroll bar. In case of a two-part disconnectable anti-roll bar a hydraulic actuator connected to the ends of the roll bars usually is used. In such case, wheels work independently of each other so that the reactions from one wheel are not transferred to the other. If necessary, the stabilizer is connected by means of an actuator and works just like the classic one. In an active stabilizer a hydraulic actuator connected to the ends of the antiroll rods usually is used. Controlling the pressure or flow rates of the oil gives wide possibilities for acting actively on the anti-swaying of the vehicle.

Optimal Controller for Active Vehicle Suspension Disturbed by Sinusoidal Signals

The problem of optimal control of systems disturbed by sinusoidal signals for infinite control time is considered in the paper. The control laws described in [1] is base of a modified mean-square performance index with an infinite control time. The performance index was formulated in such a way that each sinusoidal component corresponds to a separate weight matrix. This allows energy constraints on the control signals to be differentiated based on frequency. An optimal solution to the optimization problem was found. In the paper the problem of the impact of time on the identification of sinusoidal disturbance on vibration isolation system frequency characteristic. The controller was synthesized for slow-active vehicle suspension [2,3]. The model of suspension, synthesis of the controller and implementation of the system was described. The results of simulations of the designed vehicle active suspension system are presented.

Discrete Preisach Model of a Shape Memory Alloy Actuator

Shape Memory Alloy is a material used to designing actuators. These actuators have many advantages. They are light, strong and silent. They are building in laboratory and tested because beside advantages they have disadvantages too. SMA actuators have nonlinear characteristics with hysteresis loop.In the first part of the paper Shape Memory Alloys are shortly described. Next mathematical model was formulated. In the paper the Preisach model was developed. Discrete form of the model was considered and implemented. After parameter identification model was implemented in LabView. Tests of the model were conducted and results were worked. Obtained characteristics of the SMA actuator are shown in the paper. At the end of the paper the conclusions were formulated.