Ashish Dutta

SCARA based peg-in-hole assembly using compliant IPMC micro gripper

Robotic assembly is difficult as there always exist position errors between two mating parts. Compliance is added in a selective compliant assembly robot arm (SCARA) in the form of a two ionic polymer metal composite (IPMC) fingers based micro gripper. This micro gripper is integrated at the end effector position of a SCARA robot. Peg-hole interaction is analytically modeled and based on it the force required to correct the lateral and angular errors by IPMC is calculated. A proportional-derivative (PD) controller is designed to actuate the IPMC to get the desired force for correcting the peg position before assembly. Simulations and experiments were carried out by developing an IPMC micro gripper and using it to analyze various cases of peg in hole assembly. The experimental results prove that adding compliance through IPMC helps in peg-in-hole assembly.

Two IPMC Fingers Based Micro Gripper For Handling

This paper presents the behavior of two finger based micro gripper which is made of Ionic Polymer Metal Composite (IPMC), an Electro Active Polymer (EAP). An IPMC shows great potential as high-displacement and light weight actuator. Low mass force generation capability is utilized for micro gripping in micro assembly. IPMC responds to low voltage in the range of 0-3V. The material contains an electrolyte which transport ions in response to an external electric field. IPMC actuation for micro gripping is produced by deflecting material according to bending moment theory. An external electric field generated by suitable RC circuit causes this deflection. It is found that an IPMC actuates from 1–5 seconds. The maximum jaw opening and closing position of micro gripper are found to be 5 mm and 0.5 mm respectively. The effect of tempearture, as observed, shows that the acceptable limit varies from 23.1°C to 30.4°C while an IPMC is in operation. An experimental proto type is developed for evaluation of performance.

New potential field method for rough terrain path planning using genetic algorithm for a 6-wheel rover

Motion planning of rovers in rough terrains involves two parts of finding a safe path from an initial point to a goal point and also satisfying the path constraints (velocity, wheel torques, etc.) of the rover for traversing the path. In this paper, we propose a new motion planning algorithm on rough terrain for a 6 wheel rover with 10 DOF (degrees of freedom), by introducing a gradient function in the conventional potential field method. The new potential field function proposed consists of an attractive force, repulsive force, tangential force and a gradient force. The gradient force is a function of the roll, pitch and yaw angles of the rover at a particular location on the terrain. The roll, pitch and yaw angles are derived from the kinematic model of the rover. This additional force component ensures that the rover does not go over very high gradients and results in a safe path. Weights are assigned to the various components of the potential field function and the weights are optimized using genetic algorithms to get an optimal path that satisfies the path constraints via a cost function. The kinematic model of the rover is also derived that gives the wheel velocity ratio as it traverses different gradients. Quasi static force analysis ensures stability of the rover and prevents wheel slip. In order to compare different paths, four different objective functions are evaluated each considering energy, wheel slip, traction and length of the path. A comparison is also made between the conventional 2D potential field method and the newly proposed 3D potential field method. Simulation and experimental results show the usefulness of the new method for generating paths in rough terrains. Proposed a new potential field method for rough terrain path planning for a rover.A gradient function is introduced in the conventional potential field method.The gradient function depends on the roll, pitch and yaw angles of the rover.Weights of potential field function are optimized by using GA.Results prove that the new method is superior to conventional potential field method.

Vision Based Tactile Sensor Using Transparent Elastic Fingertip for Dexterous Handling

Humans have the ability to sense the weight and friction coefficient of the grasped object with their distributed tactile receptors. The ability makes it possible for humans to prevent from the slippage of manipulated object or collapsing the object. Such dexterous handlings are achieved by feeding back the signals from the receptors to muscle control system through neural networks. Therefore, it may be a key point for establishing dexterous handlings of robots when we try to mimic skilled human functions. For tactile sensing of robots, several methods and sensors have been proposed by using electrical resistance, capacitance, electromagnetic component, piezoelectric component, ultrasonic component, optical component, and strain gauge (Shinoda, 2002), (Lee & Nicholls, 1999). There exist many problems of these sensors to be solved for establishing practical ones. For an example, the sensor which consists of elastic body and strain gauges requires a lot of gauges and the wiring. Moreover, the signal processing is not simple to obtain the values of the contact forces and the friction coefficients (Maeno et al, 1998). On the other hand, optical sensors have been introduced because wiring is not required in the contact part to the object (Ohka et al, 2004), (Ferrier & Brockett, 2000), (Kamiyama et al, 2003). The introduction of optical sensor makes the size small and the wiring simple. However, the sensing of friction coefficient is not considered in those papers. Piezoelectric sensors have a certain potential to solve the problems of size and wiring but there has not been a practical solution yet for measuring friction coefficient. It is required for establishing dexterous handlings of robots to provide a purpose-built sensor for the measurement of friction coefficients between robot hand and the target surfaces. So as to avoid multiple usage of tactile sensors, we have proposed a new design of tactile sensors for multiple measuring of contact information including friction coefficient (Obinata et al, 2005).

Pseudo-rigid Body Modeling of IPMC for a Partially Compliant Four-bar Mechanism for Work Volume Generation

Conventional four-bar crank rocker mechanisms made of rigid links can generate only one path, at the rocker tip, for one revolution of the crank. However, if the rocker length can be actively changed then its tip can generate a work volume. This study describes an application of ionic polymer metal composite (IPMC) as a partially compliant rocker in a four-bar mechanism for work volume generation. First, an experiment is conducted to study the voltage verses bending characteristics of IPMC and based on the experimental data the IPMC is modeled using a pseudo rigid body model. The model is based on the fix-pin support type of cantilever mode and its derivation is explained in detail. The maximum and minimum length of the rocker is controlled by changing the voltage applied to it and this generates a work volume for one revolution of the crank. Simulation results are compared with the experimentally obtained work volume and the differences are found. The proposed mechanism has the potential for application in micro positioning, compliant structures, etc.

Control of an optimal finger exoskeleton based on continuous joint angle estimation from EMG signals

Patients suffering from loss of hand functions caused by stroke and other spinal cord injuries have driven a surge in the development of wearable assistive devices in recent years. In this paper, we present a system made up of a low-profile, optimally designed finger exoskeleton continuously controlled by a users surface electromyographic (sEMG) signals. The mechanical design is based on an optimal four-bar linkage that can model the fingers irregular trajectory due to the fingers varying lengths and changing instantaneous center. The desired joint angle positions are given by the predictive output of an artificial neural network with an EMG-to-Muscle Activation model that parameterizes electromechanical delay (EMD). After confirming good prediction accuracy of multiple finger joint angles we evaluated an index finger exoskeleton by obtaining a subjects EMG signals from the left forearm and using the signal to actuate a finger on the right hand with the exoskeleton. Our results show that our sEMG-based control strategy worked well in controlling the exoskeleton, obtaining the intended positions of the device, and that the subject felt the appropriate motion support from the device.

Visual motor control of a 7dof redundant manipulator using redundancy preserving learning network

This paper deals with the design and implementation of a visual kinematic control scheme for a redundant manipulator. The inverse kinematic map for a redundant manipulator is a one-to-many relation problem; i.e. for each Cartesian position, multiple joint angle vectors are associated. When this inverse kinematic relation is learnt using existing learning schemes, a single inverse kinematic solution is achieved, although the manipulator is redundant. Thus a new redundancy preserving network based on the self-organizing map (SOM) has been proposed to learn the one-to-many relation using sub-clustering in joint angle space. The SOM network resolves redundancy using three criteria, namely lazy arm movement, minimum angle norm and minimum condition number of image Jacobian matrix. The proposed scheme is able to guide the manipulator end-effector towards the desired target within 1-mm positioning accuracy without exceeding physical joint angle limits. A new concept of neighbourhood has been introduced to enable the manipulator to follow any continuous trajectory. The proposed scheme has been implemented on a seven-degree-of-freedom (7DOF) PowerCube robot manipulator successfully with visual position feedback only. The positioning accuracy of the redundant manipulator using the proposed scheme outperforms existing SOM-based algorithms.

An active vibration control strategy for a flexible link using distributed ionic polymer metal composites

Ionic polymer metal composites are a class of electro-active polymers that are gaining importance as smart actuators due to their large bending deflection. The property of generating high strains with low actuation voltage makes ionic polymer metal composites (IPMC) suitable for applications requiring large motion such as in large deflection vibration suppression. In this paper we propose an application of IPMC as an active damper for large deflection vibration control of a flexible link. The modes of vibration for a long flexible link are derived using the modal approach and two IPMC patches are placed to suppress the vibration. A distributed PD controller is designed to suppress the vibration of the flexible link for the desired positioning of the tip. Simulations are first done to demonstrate effective vibration suppression and the results proved that the proposed method can suppress vibration effectively. Experiments are conducted to verify the application of IPMC for active vibration suppression. The performance of the proposed distributed PD controller is also found to be better than a single PD controller loop.

Active Vibration Suppression of a Flexible Link Using Ionic Polymer Metal Composite

Polymers labeled as EAPs (electro-active polymer) have a mechanical response to electrical stimulation and produce an electric change in response to mechanical stimulation. The high strains of ionic polymer metal composite (IPMC) make them attractive as mechanical actuators for applications requiring large motion but little force. This paper describes the results an application of IPMC as active damper for a flexible link. IPMC is studied experimentally to find out material loss factors and damping characteristics. A single link rotary flexible manipulator with IPMC as smart patches was studied for attenuation of vibration actively. Modeling of the flexible rotating beam with IPMC has been done using modal approach to determine the fundamental modes of vibration. The end effector position of link was controlled by generating localized bending by giving input current to the IPMC in a phase opposite to the excitation. The results prove that IPMC can be used as an active damper for suppressing vibration in a flexible link

Impedance control of a robotic gripper for cooperation with humans

Abstract The control system of a robotic dextrous gripper for cooperation with humans should be based on the human control system so that the robot is “human friendly”. In this paper three different impedance control systems for a robotic gripper are evaluated, and the control system which can most effectively represent human characteristics identified. First a task performed by two humans was analyzed and various impedance parameters based on it identified. Three different types of impedance model based controllers were designed for the gripper. A robot–human cooperation task was then simulated to compare the effectiveness of the three control methods. It is concluded that the control method in which the impedance parameters are varied actively with the velocity of motion gives the best performance for a robot–human cooperation task. Finally a robot–human cooperation experiment was performed to demonstrate the practical application of the proposed control method.