重力平衡機構對機械手臂動態表現與壽命的影響評估

Abstract

在工業界，機械手臂已被廣泛的應用在生產線上，以增進產能及降低成本。然而目前在工業界所常見的機械手臂有一共同的現象，即設計負載遠低於自重。此起因於機械手臂需具有相當剛性的結構，以避免因外加的負載而導致結構產生過大的應力變形而影響定位精度。然而機械手臂系統剛性的提升導致了自重的增加，自重的增加不但可能會降低該機械手臂動態的表現，也增加了機械手臂的能源消耗量。 機械手臂的動態表現常使用加速度半徑來表示。加速度半徑係用以度量一機械手臂在某特定組成及姿態下的動態表現，而該動態表現可藉由該機械手臂的組成、姿態及致動器的輸出能力來求得。當一機械手臂的動態表現是由加速度半徑代表時，其意指該機械手臂夾爪在該組成及姿態下，於所有方向可達成的最大加速度。 傳統上增進機械手臂動態表現的方法有下述兩種：1.增大所使用致動器的輸出；2.降低結構重量。然而增大所使用致動器的輸出意指需較多的能量輸入或（且）提升所使用致動器的輸出規格。輸出規格的提升往往導致較大的空間損耗與成本的投入或減少其減額比；而輸入較多能量不符合環保與成本節約的原則，且易導致減額比的下降而降低系統可能的壽命。 在降低結構重量部分，一般需使用更高級的材料、較複雜的結構形狀或減少系統剛性的方式達成。然而使用更高級的材料、較複雜的結構形狀往往導致成本的增加；而減少系統剛性將使該機械手臂負載變形增加而降低其定位精度。故，傳統上所習用增進動態表現的兩種方式不但會導致較大的成本或空間損耗，且易使原機械手臂之可用性下降。 在大多數的應用上，致動器的輸出主要消耗在克服機械手臂原始重量，僅有少部分用以加速其所夾持的物件。有鑑於此，本研究探討當應用重力平衡原理-即使用外加機構來消除原機械手臂與外加機構的自重影響，增進能源使用效率與節省使用成本時可能產生的影響。由於外加機構能消雖除機械手臂的自重影響，但也改變了該機械手臂的原始構造，所以可能會影響該機械手臂的動態表現與壽命。為能解決此一問題，本研究利用操控性比來評估外加機構對機械手臂動態表現的影響，並利用加速度衰化率作為機械手臂動態表現受使用所產生的誤差而下降之評估標準，並進而評估該外加機構對原機械手臂的可能壽命之影響。 本研究所提出的方法，可有效的評估該應用重力平衡的外加機構對機械手臂動態表現與可能壽命的影響，使得設計人員得以同時評估機械手臂在能源使用效率、功能表現與可能使用壽命間的關係，以選出最佳符合該使用環境的機械手臂設計。

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.