Kinematic robotic arm provides Dr Sunveer Matadin
INNOVATION FOCUS
of freedom
Industrial robots are commonplace on workshop floors to perform repetitive tasks where a high degree of accuracy and repeatability is required. Existing models are based on one of two architectures: serial or parallel kinematic machines. While each has its advantages, there are also disadvantages associated with using either one of the two designs. Research aimed at optimising machine design and functionality, conducted in the University of Pretoria’s Department of Mechanical and Aeronautical Engineering, has resulted in a novel invention that considers the positive traits and trade-offs of both types. As part of the research conducted to determine the benefits and drawbacks of existing models, a study was made of the two categories of robots presently in use. It was found that serial kinematic machines that consist predominantly of links interconnected with revolute joints are examples of open kinematic chains in which the actuator that activates the subsequent link is carried by the preceding link, although combinations of prismatic and revolute joints are also used for specific applications. This implies that each link carries the cumulative mass of all the links and their respective actuators that follow that link, in addition to the mass of the payload. Actuators therefore have to compensate by being higher powered in order to be put in motion and remain in motion, both in terms of the payload and the physical robot structure. The percentage of power used to actuate the physical robotic structure may be large considering that end effectors may carry low mass payloads such as spray guns. The result is increased inertia and poorer dynamic performance at high velocities where high acceleration is required to reach the constant velocity on a typical trapezoidal velocity profile. It also has an adverse impact on the stiffness of the system and introduces unwanted vibration. This will adversely affect the accuracy of the end effector and repeatability of the system, while leading to higher power consumptions. This is especially true for higher order or degree of freedom (DOF) systems such as the 6 DOF systems that are commonly deployed due to their greater flexibility and operating workspace.
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Serial kinematic machines have the advantage of offering a larger workspace and a larger degree of agility, while also having a low footprint-to-end effector reach ratio (a small footprint to large end effector reach). Parallel kinematic machines, on the other hand, are examples of closed kinematic chains. All the actuators that provide the required degrees of freedom are located on or closest to the base, hence ensuring the lowest centre of mass. This architecture improves dynamic response and performance, especially at high velocities and accelerations. This design also offers reduced vibration and an increase in the system stiffness, pose stability, accuracy and repeatability of the machine. Power consumption and actuator torque requirements are reduced as the actuators do not need to be as heavy and high powered as the serial kinematic design since they do not need to put heavy subsequent links and actuators in motion. Link structures are also lighter than the serial kinematic design as they do not need the increased mechanical strength to carry subsequent actuators and links. The power required from actuators is used to predominantly move the payload or end effector and payload combination. The percentage of power used to actuate the physical robotic structure is therefore low and most of the power capacity is dedicated to the movement of the payload.
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