Flexibility and speed of response are two key requirements in the design of machinery for high-speed manufacturing operations. These two requirements are often conflicting and their resolution requires considerable ingenuity on the part of the designer. A novel actuator based upon the use of twin electro-rheological (ER) clutches is described together with its modification to control the motion (angular displacement, angular velocity) of a robot manipulator arm. The development of a new experimental facility involving the robot manipulator arm is described. In the basic twin ER clutch facility, the motion of a toothed belt is controlled by manipulating the electric field applied to each ER clutch. The belt, in turn, controls the angular position and velocity of the robot arm. The use of twin clutches allows motion to be imparted in opposite directions without the need for return springs or similar mechanisms. To improve the positional performance an ER brake is added to the robot arm mechanism. The extension to the dynamic model for the ER clutch mechanism to incorporate the robot arm and ER brake is outlined and is validated experimentally. The displacement response of the robot arm is then examined as a trend study using different motor driving speeds. The positional accuracy of the robot arm and its repeatability is then demonstrated.
There is much current interest in the development of smart fluid clutches for use in the design of high-speed machinery. This interest stems from the flexibility, controllability and fast response of such fluids. In this paper the authors outline the modifications to an Electro-rheological clutch mechanism for a robotics application. The clutch mechanism consists of twin ER clutches that are driven in opposite directions. By controlling the electric field applied to each clutch it is possible to cause a toothed belt to move in a desired manner in each direction. This belt motion can then be used to control the motion of a robot arm via a gear train. To improve the positional performance an ER brake is added to the robot arm mechanism. The extension to the dynamic model for the ER clutch mechanism to incorporate the robot arm and ER brake is outlined and is validated experimentally. The displacement response of the robot arm is then examined as a trend study using different motor driving speeds and load inertias. The positional accuracy of the robot arm and its repeatability is then demonstrated over a significant number of reciprocating tests.
There is much current interest in the development of smart fluid clutches for use in the design of high speed machinery. In this paper, the authors employ an ER clutch mechanism in a robotic application. This clutch mechanism consists of twin ER clutches which are driven in opposite directions by two electric motors. By controlling the electric field applied to each clutch, it is possible to control the angular displacement of a robot arm. Before considering control, an established mathematical model is validated. The purpose of this model validation is to help design a control strategy for accurate positioning of the robot arm. Through the use of a Simulink program and a digital controller, both the simulated and experimental angular displacements are compared and shown to be in close agreement. Finally, the displacement response of the ER- driven and DC servo-actuatored robot arm are compared and conclusions are drawn as to the suitability of the ER clutch mechanism as a robotic actuator.
The potential of smart fluids (both electrorheological, and magnetorheological) in damping devices is now well-known. Whilst both types of fluid can suffer from drawbacks such as sedimentation, fluid degradation, and problems with containment or sealing, these issues are not insurmountable and solutions have been engineered such that practical damping devices are now commercially available. However, one drawback is that the free-velocity characteristics of a smart fluid device are inherently non-linear, possessing the general form associated with a Bingham plate. This means that while practical devices have the potential to modify rapidly their behavior, it can be difficult automatically to adjust the device's response.
Ongoing research at the University of Sheffield is currently concerned with the design and construction of magneto- rheological (MR) squeeze-flow vibration damper. Previous work has demonstrated the feasibility of employing such a device as the key component in a controllable vibration isolator. The work also demonstrated the inadequacies of existing mathematical models which do not account for the observed behavior of MR fluids in squeeze flow. In parallel with investigations into the behavior of MR dampers, a collaborative programme between the Universities of Liverpool and Sheffield is also in progress. Here attention is focussed on ER fluids in squeeze-flow and a new test facility has been constructed for use in the development and validation of mathematical models. It is anticipated that this collaborative programme will assist in the development of both ER and MR squeeze-flow models. In this paper, the authors present a summary of progress to date.
It is now well known that smart fluids [electrorheological (ER) and magnetorheological (MR)] can form the basis of controllable vibration damping devices. With both types of fluid, however, the force/velocity characteristic of the resulting damper is significantly non-linear, possessing the general form associated with a Bingham plastic. In a previous paper the authors showed that by using a linear feedback control strategy is it possible to produce the equivalent of a viscous damper with a continuously variable damping coefficient. In the present paper the authors illustrate an extension of the technique, by showing how the shape of the force/velocity characteristic can be controlled through feedback control. This is achieved by using a polynomial function to generate a set point based upon the damper velocity. The response is investigated for polynomial functions of zero, 1st and 2nd order. It is shown how the damper can accurately track higher order polynomial shaping functions, while the zero order function is particularly useful in illustrating the dynamics of the closed-loop system.
A high-speed traversing mechanism using two electro- rheological clutches is described. An application of the traversing mechanism is in winding filaments onto bobbins. The traverse speed is 5 m/s; the required turn round period is 10 to 20 milli-seconds; the traverse length is 250 mm; the turn round position must be controllable and repeatable within +/- 1 mm; and the traverse requires to be controlled to shape the resulting bobbin. These combined criteria of high speed and controllability makes the use of electro-rheological fluids an attractive proposition. The paper considers the optimization of the traversing mechanism; both geometric and fluid parameters are considered. The limiting performance of the mechanism is detailed together with the effects on the precision of the mechanism. The paper also outlines control aspects of the mechanism and uses this to indicate important areas for consideration in the future development of electro- rheological fluids.
It is now well established that magnetorheological (MR) fluids can provide the basis for constructing controllable vibration damping devices. Moreover, the characteristics of MR fluids are generally compatible with industrial requirements and there is enormous scope for commercial exploitation. In this paper the authors describe the design and construction of a vibration isolator which incorporates an MR damper. The damper is unusual in that it operates in the squeeze-flow mode. A quasi-steady model of the MR damper is summarized and then extended to include the vibration isolator dynamics. Model predictions are compared with experimental results. It is shown that by employing the MR damper a wide range of control can be exercised over the transmissibility of the vibration isolator. Numerical experiments are used to show that a feedback control strategy can provide even more control over transmissibility.
Magneto-rheological (MR) fluids are rapidly rising in prominence as a means of producing controllable damping devices for vibration control. MR fluids can be used in various modes of operation in order to provide damping forces. One of the least exploited of these modes is commonly known as squeeze-flow, where large, controllable forces can be generated over relatively small displacement ranges. In this paper the authors describe a recently constructed test facility in which an MR squeeze-flow device is incorporated as the damping element in a vibration isolator.
A high speed traversing/positioning mechanism using two electro-rheological clutches is described. The traversing mechanism can be used to wind filaments onto bobbins. The traverse speed is 5 m/s, the required turn round period is 10 to 20 milli-seconds, the traverse length is 250 mm, the turn around position must be controllable and repeatable within +/- 1 mm and the traverse requires to be controlled to shape the resulting bobbin ends. These combined criteria of high speed and controllability makes the use of electro- rheological fluids a potentially viable solution. A dynamic simulation is available to predict the performance of the mechanism, however, a number of the electro-rheological fluid properties required by this simulation are temperature dependent. The methodology for predicting the thermal equilibrium temperature of the electro-rheological fluid within the high speed traversing mechanism is presented. Heat generation within the electro-rheological fluid, due to the fundamental operating mechanics of the mechanism, shearing of the electro-rheological fluid and the electrical excitation, are combined with the heat transfer from the mechanism to enable the operating temperature of the fluid to be determined. This operating temperature enables the temperature dependent fluid properties to be used in simulating the dynamic performance of the mechanism.
It is widely acknowledged that the inherent non-linearity of smart fluid dampers is inhibiting the development of effective control regimes, and mass-production devices. In an earlier publication, an innovative solution to this problem was presented -- using a simple feedback control strategy to linearize the response. The study used a quasi-steady model of a long-stroke Electrorheological damper, and showed how proportional feedback control could linearize the simulated response. However, this initial research did not consider the dynamics of the damper's behavior, and so the development of a more advanced model has been necessary. In this article, the authors present an extension to this earlier study, using a model of the damper's response that is capable of accurately predicting the dynamic response of the damper. To introduce the topic, the electrorheological long-stroke damper test rig is described, and an overview of the earlier study is given. The advanced model is then derived, and its predictions are compared to experimental data from the test rig. This model is then incorporated into the feedback control simulations, and it is shown how the control strategy is still able to linearize the response in simulations.
A high speed traversing/positioning mechanism using two electro-rheological clutches is described. The traversing mechanism can be used to wind filaments onto bobbins. The traverse speed is 5 m/s, the required turn round period is 10 milli-seconds, the traverse length is 250 mm and the turn round position must be controllable and repeatable within +/- 1 mm. These combined criteria of high speed and controllability makes the use of electro-rheological fluids an attractive proposition. Simulations produced using a dynamic model are compared with experimental results and these validate the simulation techniques. The effect on the performance of various fundamental electro-rheological fluid characteristics, namely electro-rheological shear stress, electron-hydraulic time delays and zero volts viscosity are considered together with the design of the mechanism. This illustrates the need for optimization of such mechanisms to meet the varied and difficult design requirements found in high speed controllable devices. Some practical difficulties in achieving a reliable mechanism are also discussed.
The present state of development of an electrically adjustable linear motion device is reported. Design methodologies are indicated which will, when integrated with the characteristics of the electrorheological fluid engagement means, predict the performances of the traverse. Some proof of the techniques used is given for dynamic, thermal and electrical aspects of operation. Approximate sizing data are outlined. Potential turn round acceleration (approximately 100 g) and precision of position (<EQ 0.5 mm) control duty are highlighted.
A high speed traversing mechanism which utilizes two electro- rheological clutches is described. The traversing mechanism can be used to wind filaments onto bobbins. The traverse speed is 5 m/s, the required turn round period is 10 milliseconds, the traverse length is 250 mm and the turn round position must be electronically controllable and repeatable within plus or minus 1 mm. These combined criteria of high speed and controllability makes the use of electro-rheological fluids an attractive proposition. The dynamic model used to predict the performance of the mechanism is outlined and theoretical performance predictions made for a variety of electro- rheological fluid characteristics. Supporting experimental data is used to illustrate the validity of the dynamic modeling. The effect of various fundamental electro- rheological fluid characteristics, such as electro-shear stress, time delays and viscosity are considered in relation to the requirements for the operation of the high speed mechanism. This study indicates important areas for consideration in the future development of electro-rheological fluids.
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