| Biological experiments have shown that fish can change their natural frequencies to match the driving frequencies by modulating the stiffness of their bodies,then the fish can improve the performance of swimming.Inspired by this principle,scholars have developed many varieties of soft biomimetic variable-stiffness robotic fishes.However,at present,there are still some problems need to solve as follow for the soft biomimetic variable-stiffness robotic fishes: The range of the stiffness variation is less than the real fish.The shape of fish can not be kept when modulating stiffness.The stiffness can not be modulated on-line.In order to solve the above problems,a variable-stiffness soft biomimetic robotic fish with decoupled active stiffness and passive stiffness is presented.In addition,the research works including modulating the total stiffness and the local stiffness of serial-parallel mechanism,stiffness coupling and stiffness decoupling,the design of the mechanically adjustable compliance and controllable equilibrium position actuator(MACCEPA),the large stiffness variation and swimming performance of the biomimetic robotic fish are developed.Firstly,the internal structure of biomimetic fish is analyzed,the structure of the hyper-redundant serial-parallel mechanism and the mechanism of the internal force modulating stiffness are determined.The stiffness model of the hyper redundant series-parallel mechanism corresponding to viscoelastic continual fish is established.It is found that the integral natural frequency of the hyper-redundant serial-parallel mechanism can be changed by modulating the integral stiffness or modulating the local stiffness.Due to the complex structure and the low mechanical efficiency of the integral variable-stiffness mechanism,the simple method of modulating the local stiffness is better.Therefore,on the premise of achieving continuous swimming of the robotic fish,according to the principle of modulating the local stiffness,the soft robotic fish with local variable stiffness by connecting the single redundantly actuated planar rotational parallel mechanisms to the viscoelastic tail is presented.Secondly,the analytical expressions relating stiffness and geometrical parameters of redundantly actuated planar rotational parallel mechanisms(RAPRPM)were formulated.The analytical expressions reveal the mechanism of internal force modulating the stiffness of RAPRPM.The geometrical parameters to maximize the stiffness variation with internal force and to minimize the the dynamic change of stiffness with the dynamic location of the mechanism are obtained.This new approach to variable stiffness design can enable off-line planning of the internal forces to avoid the difficulties of on-line control of the internal force.For RPRPM,the stiffness consists of active stiffness and passive stiffness.The active stiffness is determined by internal forces,the passive stiffness is determined bycompliances of flexible elements and the geometrical parameters of the mechanism.The initial stiffness is determined by the passive stiffness.The more the ratio of the active stiffness to the passive stiffness,the more the stiffness variation.However,due to the physical limits of the deformation and the workspace of the flexible elements,it is difficult to maximize the active stiffness besides to decrease the passive stiffness.The coupling between active stiffness and passive stiffness limits the stiffness variation of the mechanism,the stiffness can not be changed in a large range.In the case of regular structure,the stiffness coupling can't be solved when applying linear flexible elements or non-linear flexible elements.To solve the stiffness coupling,the mechanism to decouple active stiffness from passive stiffness is presented to expand the range of stiffness variation.The active stiffness and the passive stiffness can be regulated by the two RPRPM independently and respectively.The two RPRPM have the respective flexible elements.The flexible elements with more stiffness are applied to increase the stiffness variation for the RPRPM regulating the active stiffness,while the flexible elements with less stiffness are applied to decrease the initial stiffness for the RPRPM regulating the passive stiffness.As a result,the time of stiffness variation is reduced by increasing the ratio of the active stiffness to the passive stiffness.The two mechanisms to regulate the active stiffness are as follows: one is the leg-crossed RPRPM,the other is the mechanically adjustable compliance and controllable equilibrium position actuator(MACCEPA).The stiffness of the variable-stiffness decoupled mechanism based on MACCEPA increases with the increase of the pretension,which more conforms the property of stiffness variation of biological fish.At last,the soft biomimetic robotic fish with large stiffness variation based on MACCEPA is presented,the prototype of biomimetic robotic fish is produced.The fish with variable-stiffness decoupled mechanisms consists of the shell of silicone rubber and the MACCEPA.MACCEPA provides the active stiffness,while the soft shell provides the passive stiffness.Enabling the robotic fish to achieve the continuous swimming of the biological fish,the soft robotic fish with local variable stiffness is formed by connecting variable-stiffness decoupled mechanisms to the tail of flexible silicone rubber.It is found that the body stiffness of the robotic fish varies with the change of pretension.It is proved by the swimming experiment that the robotic fish achieves the maximum swimming speed,the maximum thrust and the maximum oscillation amplitude by modulating the pretension.i.e.,the swimming performance is improved when the body stiffness matches the driving frequency.The optimal matching curve of pretension,driving frequency and performance is obtained.For the robotic fish,the pretension can be modulated in the range of the pretension from 0 to 14 N,the ratio of stiffness variation of the robotic fish varies 16 times,the stiffness can be matched the range of in the range of swinging frequencies from 0.8 Hz to 2.4 Hz.According to theoptimal matching curve,the modulating scheme of optimal swimming is presented.The pretension is matched and modulated automatically to satisfy the desired swimming speed. |
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