Thorben Hoffstadt

Online identification algorithms for integrated dielectric electroactive polymer sensors and self-sensing concepts

Transducers based on dielectric electroactive polymers (DEAP) use electrostatic pressure to convert electric energy into strain energy or vice versa. Besides this, they are also designed for sensor applications in monitoring the actual stretch state on the basis of the deformation dependent capacitive–resistive behavior of the DEAP. In order to enable an efficient and proper closed loop control operation of these transducers, e.g. in positioning or energy harvesting applications, on the one hand, sensors based on DEAP material can be integrated into the transducers and evaluated externally, and on the other hand, the transducer itself can be used as a sensor, also in terms of self-sensing. For this purpose the characteristic electrical behavior of the transducer has to be evaluated in order to determine the mechanical state. Also, adequate online identification algorithms with sufficient accuracy and dynamics are required, independent from the sensor concept utilized, in order to determine the electrical DEAP parameters in real time. Therefore, in this contribution, algorithms are developed in the frequency domain for identifications of the capacitance as well as the electrode and polymer resistance of a DEAP, which are validated by measurements. These algorithms are designed for self-sensing applications, especially if the power electronics utilized is operated at a constant switching frequency, and parasitic harmonic oscillations are induced besides the desired DC value. These oscillations can be used for the online identification, so an additional superimposed excitation is no longer necessary. For this purpose a dual active bridge (DAB) is introduced to drive the DEAP transducer. The capabilities of the real-time identification algorithm in combination with the DAB are presented in detail and discussed, finally.

Analytical modeling and optimization of DEAP-based multilayer stack-transducers

Transducers based on dielectric electroactive polymers (DEAP) use electrostatic pressure to convert electrical into mechanical energy or vice versa. To scale up the actuation or the energy gain, multilayer transducers like DEAP stack transducers are appropriate. Within this contribution, a model of such a stack transducer is derived and experimentally validated. The model is based on a multi-domain approach to describe the mechanical dynamics and the electrical behavior of the DEAP. Since these two domains influence each other they are coupled afterwards by a novel approach using interchanging power flows. To parametrize this model, tensile and compression tests for different polymer materials were performed under static and transient considerations. The results of these experiments show that the parameters obtained from the tensile test sufficiently describe the compression mode and can therefore be used for the model. Based on this transducer model the overall energy and the different parts of the multi-domain are analytically determined for arbitrary operating points. These expressions for the energies are finally used to optimize well-defined coupling coefficients, by which a maximum part of the electrical input energy is converted to mechanical energy, especially mechanical work.

Optimization of the energy harvesting control for dielectric elastomer generators

Energy harvesting using dielectric elastomers is an upcoming possibility to convert ambient energy into electric energy. Published results for energy harvesting cycles deal with charging and discharging of the polymer during a constant stretch state. However, real applications feature a continuously changing stretch and thus the time frames of the charging- and discharging-intervals have a considerable influence on the amount of harvested energy. This paper presents the calculation of the optimal charging- and discharging-intervals to maximize the energy gain. For this purpose the authors investigate the physical model of a lossy generator to derive the converted energy as a function of the timing of the charging- and discharging-interval. The subsequent optimization results in an energy-optimal harvesting cycle that combines the fundamental harvesting cycles with constant electric field and constant charge. Finally, we present the achievable energy gain of this optimized harvesting cycle as well as control laws to realize the optimized harvesting cycle.

Power electronics concepts for driving EAP actuators

Compared to single layer electroactive polymer (EAP) actuators, stack actuators exhibit advantageous properties such as large displacements at acceptably low operating voltages. Therefore, EAP stack actuators seem to be a promising option for the successful use in commercial applications. For an energy-efficient operation of EAP stack actuators or EAP actuators in general an adequate power electronics with high voltage capability is indispensable. Depending on the specific application, the use of different converter topologies combined with suitable control concepts has to be considered. In this contribution different possible converter concepts for driving EAP stack actuators are presented. For each fundamental converter concept, different converter topologies are proposed. A flyback converter combined with an active discharging circuit is analyzed in detail and suitable control designs are developed. Finally, simulation and experimental results of the prototype flyback converter are presented.

Sensorless BCM control for a bidirectional flyback-converter

Transducers based on dielectric elastomers (DE) represent capacitive loads from an electrical point of view. Thus, for an efficient operation of these transducers tailored power electronics are required, preferable with bidirectional energy flow to improve the energy efficiency. Therefore, in this contribution a bidirectional flyback-converter is modeled in order to design the control. Since the superimposed application-oriented control of the DE transducer requires an accurate inner control of the utilized converter, a model of the flyback-converter feeding capacitive loads is carried out under consideration of losses. Based on this a sensor-less current control is developed that operates the converter in boundary conduction mode enabling the highest possible dynamics. This control can be superimposed e.g. by a voltage control. Finally, the obtained control approaches are experimentally validated by measurements with a realized prototype of the bidirectional flyback-converter.

Automated manufacturing process for DEAP stack-actuators

Dielectric elastomers (DE) are thin polymer films belonging to the class of electroactive polymers (EAP), which are coated with compliant and conductive electrodes on each side. Due to the influence of an electrical field, dielectric elastomers perform a large amount of deformation. In this contribution a manufacturing process of automated fabricated stack-actuators based on dielectric electroactive polymers (DEAP) are presented. First of all the specific design of the considered stack-actuator is explained and afterwards the development, construction and realization of an automated manufacturing process is presented in detail. By applying this automated process, stack-actuators with reproducible and homogeneous properties can be manufactured. Finally, first DEAP actuator modules fabricated by the mentioned process are validated experimentally.

Integrated Sensor Concepts for Dielectric Elastomer Actuators

Actuators based on dielectric electroactive polymers (DEAP) use the electrostatic pressure to convert electric energy into strain energy. Besides this, they are also predestined for sensor applications to monitor the actual stretch state based on the deformation dependent capacitive-resistive behavior of the DEAP. Considering DEAP actuators for positioning applications, like stack- or roll-actuators, the actual position, length or stretch of the actuator is required for a precise control. Thus, integrated sensors made of DEAP can be used to determine the actual stretch state with sufficient accuracy and high dynamics on the one hand. On the other hand the electrical behavior of the DEAP transducer itself can be evaluated for the estimation of the stretch state representing a sensor-less concept. In this paper at first the state of the art of sensor-based and sensor-less concepts for determining the stretch state of DEAP transducers is presented. Afterwards the authors propose novel concepts for DEAP-based sensors integrated into stack- and roll-actuators. These concepts are compared with each other in terms of sensitivity, accuracy, dynamics and integration efforts for the realization. Finally, fundamental concepts, estimation algorithms and different approaches for monitoring the actual stretch state are presented based on the electrical parameters of a lossy DEAP transducer, which are suitable both for sensor-based and sensor-less concepts.Copyright

Adaptive Sliding-Mode Position Control for Dielectric Elastomer Actuators

Multilayer stack transducers made from dielectric elastomers (DEs) generate considerable tensile forces and deformations when they are electrically stimulated. Hence, due to their capacitive behavior, they are energy-efficient substitutes, for example, for conventional electromagnetic drives, and enable various completely new applications. Within this contribution, we present the design of a position control for DE stack actuators electrically fed by a bidirectional flyback converter. Due to the unique property of the flyback converter providing an almost constant feeding power for charging and discharging, the sliding-mode control approach is used for the proposed position control. In a first step, a two-point controller is developed and extended afterwards to a three-point controller with hysteresis to significantly reduce the switching frequency. In order to further improve the control behavior and energy efficiency, an adaptation approach for the inner power converter control is carried out that is used to adapt the hysteresis threshold of the three-point controller. Finally, the experimental validations with a prototypic silicone DE stack actuator and bidirectional flyback converter demonstrate that the proposed adaptive three-point controller combines both high dynamics and accuracy with high efficiency due to a significantly reduced switching frequency.

Comparison of bidirectional power electronics with unidirectional topologies using active discharging circuits for feeding DEAP transducer

To enable a continuous operation of a DEAP transducer, the feeding power electronics must provide the capability to charge and discharge the transducer to enable a continuous voltage adjustment. While in case of energy harvesting applications a bidirectional power electronics is mandatory, for actuator applications also unidirectional power electronics with active discharging circuits can be used. Thus, in this contribution a bidirectional flyback-converter is compared to a unidirectional with different discharging circuits. For this purpose, the design of a resistive and an inductive-resistive discharging circuit is proposed, that are connected in parallel to the DEAP and activated when required. Modulation schemes for both discharging circuits are derived that enable a continuous voltage control. Based on realized prototypes of the investigated topologies the different converters are finally compared to each other.

Model-based control of a dual active bridge for bidirectional feeding of DEAP transducers

Transducers based on dielectric electroactive polymers (DEAP) use the electrostatic pressure to convert electrical into mechanical energy or vice versa. Since the achieved actuation or energy gain depends on the applied electric field, the authors present a model-based voltage control of a DEAP transducer driven by a Dual Active Bridge (DAB) with bidirectional energy flow. Therefore, first of all a model of the electric behavior of the DEAP is derived. Afterwards the DAB is described as an appropriate high voltage DC-DC converter topology to feed DEAPs. Based on the models of these two components, a discrete voltage controller is presented that ensures an operation with high dynamic and accuracy. Finally, the developed controller is validated by measurements feeding the DEAP transducer with the proposed DAB.