Evaluation of the Variation in Dynamic Performance and Service Life of a Manipulator after being Gravity Balanced
|關鍵字:||重力平衡;操控性比;加速度衰化率;gravity balance;maneuverability ratio;acceleration radius|
Manipulators have widely been utilized in industrial field to do assembly jobs in production lines. There are many different types of manipulators have been deployed for different applications, but most of them have a common characteristic, and that is the payload of a manipulator is much smaller than its self-weight. This is because a manipulator needs stiff structure to prevent from the excessive deformation resulted from the objects it holds to keep the positioning accuracy. However, the stiff structure results in the increase of the self-weigh and consumes considerable the output of the actuators of the manipulator. This not only increases the energy being consumed but also decreases the dynamic performance of the manipulator. The dynamic performance of a manipulator is usually presented by acceleration radius. Acceleration radius is an index which is used to measure of the acceleration capacity of a manipulator with a certain configuration and at a specific posture. Dynamic performance will be influenced by the configuration, the posture, and the output capacity of the constituent joint actuators of the manipulator under discussion. When it is represented by acceleration radius, it means that the maximum acceleration which the end of a manipulator with certain configuration can achieve in all directions at that specific posture. Conventionally, there are two approaches can be used to increase the dynamic performance of a manipulator, and they are: 1. raising the output limits of the actuators it uses; 2. reducing the weight of the manipulator system. Raising the output limits of the actuators means that more energy needs to be exerted or/and the specification of the actuators needs to be promoted. However, raising the output limits of the actuators would result in cost increase, and exerting more energy will increase the cost and reduce the derating rate. Lowering derating rate usually results in the decline of the designed service life. Reducing the weight of a manipulator system usually can be achieved by using better and stiffer materials or complicated but stiffer structures, or reducing the materials it uses. Using better materials and structure means the increase in the fabrication cost. Reducing the materials in use means the stiffness of the system decreases, and this will result in the deterioration in positioning accuracy which is caused by the increase of the compliance of the system. Based on what is stated above, these two conventional approaches used to promote the dynamic performance of a manipulator are not suitable to be implemented in real cases. In most applications, the output of actuators of a manipulator spends on counterbalancing the gravitational force resulted from the stiff but heavy structure, not on accelerating the object it holds. To redeem this insufficiency, this study utilizes auxiliary mechanisms which is designed based on gravity balance theory to eliminate the influence of the self-weight of a manipulator and the mechanism. However, the auxiliary mechanism can eliminate the influence of self-weight but also changes the configuration of the original manipulator. This change may affect the dynamic performance and the service life of the manipulator. To cope with this issue, this study utilizes maneuverability ratio to evaluate the influence of an auxiliary mechanism on the dynamic performance of a manipulator after being equipped with that mechanism. Besides, this study also utilizes deterioration rate to investigate the deterioration in dynamic performance of a manipulator with the errors resulted from the operation and evaluate the influence on the designed service life. This study provides an effective methodology to evaluate the influence of a gravity balance mechanism on the dynamic performance and the designed service life of a manipulator. With the help of proposed methodology, designers of manipulators can not only have the ability to find out the relationship among the energy efficiency, performance, and designed service life of a manipulator but also have the capability to choose the best design to match the prescribed service conditions based on the results of evaluation.
|Appears in Collections:||Thesis|
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