Lina Hao

Modeling and adaptive inverse control of hysteresis and creep in ionic polymer–metal composite actuators

Like most smart materials, such as piezoelectric materials and shape memory alloys, ion-exchange polymer–metal composite (IPMC), which is a kind of electroactive polymer material, exhibits the properties of hysteresis and creep. In this paper we explain the hysteresis and creep properties of IPMC, analyze the hysteresis using a discrete Prandtl–lshlinskii model, obtain a creep model of IPMC through modifying the creep model of piezoelectric material and present an inverse model of the hysteresis. For hysteresis and creep properties of IPMC changing with time at different rates, we applied the LMS (least mean square) algorithm to identify the hysteresis parameters online. An offline identification algorithm was used to obtain the creep parameters. An adaptive inverse strategy of control for IPMC actuators was set up on the basis of a superposition model of nonlinear hysteresis and linear creep, and we obtained good simulation and experiment results.

Compensating asymmetric hysteresis for nanorobot motion control

Atomic force microscopy (AFM) is a powerful measurement instrument, which has been widely implemented in various fields. To enhance the maneuverability, an AFM can be modified and its cantilever can be controlled as a robotic end-effector. To precisely control the nanorobots, their scanners inherent hysteresis, which is the main disadvantage, should be compensated sufficiently. Mostly, hysteresis compensators used in AFM control system only employ the symmetric models, which cannot well represent the asymmetric hysteresis phenomena of piezo-scanners. However, under many cases, the smart material based scanners possess asymmetric hysteresis slightly or seriously, which is far more complex than the regular symmetric case; in addition, for commercial or customized AFMs, the scanners are usually controlled without feedback. These drawbacks tackle further improvement of positioning accuracy of AFM systems. To effectively describe and further reduce the hysteresis of general cases, we propose a new type of generalized Prandtl-Ishlinskii (PI) operator based superposition model, named unparallel PI (UPI) model. The flexible edge of the UPI operator can be tilted freely within a defined range, which enables it to capture asymmetric hysteresis efficiently. To cancel the hysteresis effect in an open-loop system, three inverse compensation approaches are proposed, compared, and the corresponding stability is analyzed. Experiments with AFM based nanorobot verified the proposed approaches with convincing performance.

A novel discrete adaptive sliding-mode- like control method for ionic polymer- metal composite manipulators

Ionic polymer‐metal composite (IPMC), also called artificial muscle, is an EAP material which can generate a relatively large deformation with a low driving voltage (generally less than 5 V). Like other EAP materials, IPMC possesses strong nonlinear properties, which can be described as a hybrid of back-relaxation (BR) and hysteresis characteristics, which also vary with water content, environmental temperature and even the usage consumption. Nowadays, many control approaches have been developed to tune the IPMC actuators, among which adaptive methods show a particular striking performance. To deal with IPMCs’ nonlinear problem, this paper represents a robust discrete adaptive inverse (AI) control approach, which employs an on-line identification technique based on the BR operator and Prandtl‐Ishlinskii (PI) hysteresis operator hybrid model estimation method. Here the newly formed control approach is called discrete adaptive sliding-mode-like control (DASMLC) due to the similarity of its design method to that of a sliding mode controller. The weighted least mean squares (WLMS) identification method was employed to estimate the hybrid IPMC model because of its advantage of insensitivity to environmental noise. Experiments with the DASMLC approach and a conventional PID controller were carried out to compare and demonstrate the proposed controller’s better performance. (Some figures may appear in colour only in the online journal)

Modeling and control with hysteresis and creep of ionic polymer-metal composite (IPMC) actuators

Hysteresis and creep hinder the effective use of IPMC in sensors and actuators. This paper proposes a hybrid model that can precisely portray hysteresis and creep in piezoelectric actuators, which is constructed by a Preisach operator with a piecewise uniform density function and creep operator. Then, the corresponding inverse models for both hysteresis and creep are developed. It studies online recursive identification of hysteresis and creep drift. Based on the obtained models, a method for simultaneous compensation of the hysteresis and creep of piezoelectric actuator is applied to the control of system nonlinearities. Simulation and experimental results based on a IPMC actuator are provided to illustrate the proposed approach. The result verified the validity of the model and effectiveness of the controller.

Periodic reference tracking control approach for smart material actuators with complex hysteretic characteristics

Micro/nano positioning technologies have been attractive for decades for their various applications in both industrial and scientific fields. The actuators employed in these technologies are typically smart material actuators, which possess inherent hysteresis that may cause systems behave unexpectedly. Periodic reference tracking capability is fundamental for apparatuses such as scanning probe microscope, which employs smart material actuators to generate periodic scanning motion. However, traditional controller such as PID method cannot guarantee accurate fast periodic scanning motion. To tackle this problem and to conduct practical implementation in digital devices, this paper proposes a novel control method named discrete extended unparallel Prandtl–Ishlinskii model based internal model (d-EUPI-IM) control approach. To tackle modeling uncertainties, the robust d-EUPI-IM control approach is investigated, and the associated sufficient stabilizing conditions are derived. The advantages of the proposed controller are: it is designed and represented in discrete form, thus practical for digital devices implementation; the extended unparallel Prandtl–Ishlinskii model can precisely represent forward/inverse complex hysteretic characteristics, thus can reduce modeling uncertainties and benefits controllers design; in addition, the internal model principle based control module can be utilized as a natural oscillator for tackling periodic references tracking problem. The proposed controller was verified through comparative experiments on a piezoelectric actuator platform, and convincing results have been achieved.

The sliding mode control for different shapes and dimensions of IPMC on resisting its creep characteristics

Ionic polymer metal composite (IPMC) is a novel smart material which has been widely implemented in MEMS, biomimetic mechanical and electrical integrated system and micro operation system. While the IPMC with different shapes and dimensions has been implemented in many different types of biomechanical integrated systems, one of its inherent properties called creep characteristic is difficult to be handled, which limits the further application of different IPMCs in integrated systems. A promising control method called sliding mode control (SMC) is proposed to resist the creep characteristics in this paper. The SMC controller can regulate IPMC actuators with different shapes and dimensions effectively to resist the creep characteristics without changing parameters of the control system. Experiments of four different types of IPMC actuators were conducted on the semi-physical SMC experimental platform. All the experimental results confirm the feasibility of the SMC control approach on regulating the multi-IPMCs with different shapes and dimensions based integrated system.

Active disturbance rejection control for output force creep characteristics of ionic polymer metal composites

Ionic polymer metal composites (IPMCs) are a type of electroactive polymer (EAP) that can be used as both sensors and actuators. An IPMC has enormous potential application in the field of biomimetic robotics, medical devices, and so on. However, an IPMC actuator has a great number of disadvantages, such as creep and time-variation, making it vulnerable to external disturbances. In addition, the complex actuation mechanism makes it difficult to model and the demand of the control algorithm is laborious to implement. In this paper, we obtain a creep model of the IPMC by means of model identification based on the method of creep operator linear superposition. Although the mathematical model is not approximate to the IPMC accurate model, it is accurate enough to be used in MATLAB to prove the control algorithm. A controller based on the active disturbance rejection control (ADRC) method is designed to solve the drawbacks previously given. Because the ADRC controller is separate from the mathematical model of the controlled plant, the control algorithm has the ability to complete disturbance estimation and compensation. Some factors, such as all external disturbances, uncertainty factors, the inaccuracy of the identification model and different kinds of IPMCs, have little effect on controlling the output block force of the IPMC. Furthermore, we use the particle swarm optimization algorithm to adjust ADRC parameters so that the IPMC actuator can approach the desired block force with unknown external disturbances. Simulations and experimental examples validate the effectiveness of the ADRC controller.

Modeling and compensation control of asymmetric hysteresis in a pneumatic artificial muscle

Pneumatic artificial muscle is a novel compliance actuator, and it has many excellent actuator characteristics, such as high power density, safety, and compliance. However, it also has strong nonlinear and asymmetric hysteresis, which makes the accurate trajectory control for a pneumatic artificial muscle very difficult. In this article, the pressure/length hysteresis of a pneumatic artificial muscle was analyzed via an isotonic test. And then, it was described using extended unparallel Prandtl–Ishlinskii model, and the model parameters were identified by an adaptive weight particle swarm optimization with a mutation portion algorithm. For the comparison, the classical Prandtl–Ishlinskii was also considered, and its parameters were identified by least square method. Based on the hysteresis model built by extended unparallel Prandtl–Ishlinskii model, an integral inverse compensator was proposed, and then a proportional–integral–derivative controller with the integral inverse compensator (integral inverse-proportional–integral–derivative) was designed. The simulations and experiments validated that the integral inverse-proportional–integral–derivative controller has good dynamic performance. Compared with conventional proportional–integral–derivative controller without a hysteresis compensator, the control precision of integral inverse-proportional–integral–derivative controller is improved by 43.86%.

Scan range adaptive hysteresis/creep hybrid compensator for AFM based nanomanipulations

Atomic force microscopy (AFM) based nanomanipulations have been successfully applied to various fields such as physics, material science and biomedical studies. In general, the precision of AFM based nanomanipulation has been compromised mainly by hysteresis and creep of the piezo actuator. In this paper, a new approach, named scan range adaptive hysteresis/creep hybrid (SAH) compensator, is proposed to compensate the nonlinear rate-independent hysteresis and linear rate-dependent creep effects of the open-loop AFM based manipulation system. The nonlinear portion of the SAH compensator consists of Prandtl-Ishlinskii (PI) play operators and the linear portion, which serves as an input amplifier, consists of creep operators. The advantage of the SAH compensator is that the hysteresis compensator portion can optimize its parameters to adapt to the manipulation range, which guarantees the same level of relative positioning accuracy in different operation scales. This SAH compensator is easy to implement in a range of scanning probe microscopies (SPMs). Experimental results show that the SAH compensator can compensate hysteresis and creep with higher accuracy than the conventional creep/hysteresis hybrid compensator in different operation scales.

Robotic cell injection force control based on static PVDF sensor and Fuzzy-PID control method

Cell injection procedure is very essential in the field of molecular biology, and the injection force affects the success rates very much. However, conventional methods of manipulating individual biological cell failed to make use of the injection force information. This article is intended to design a static micro-force sensor with a simple structure which employs the piezoelectric material PVDF (polyvinylidene fluoride) film as its sensing element to detect the micro-force during cells injection and to develop a close-loop control method to regulate the whole fore-tracking system. A Fuzzy-PID and an ordinary PD feedback control method are employed separately in this article to regulate the micro-force tracking system which is used to carry out automatic living-cell injection tests. Experimental results with different control methods are achieved. And the Fuzzy-PID and PVDF sensor based force control method is validated.