The packaging industry's most successful rapid transfer robot — the Delta robot — was developed in Switzerland during the early 1980s by Reymond Clavel. Since then, this lightning-quick, spiderlike mechanism has spread to factories around the world, proliferating in packaging, medical, and pharmaceutical applications. Their forte is pick-and-place, where they are often deployed in pods of up to 20 synchronized systems that collectively handle 100 to 2,500 products per minute.
The need for productivity that spawned the development of Delta robots also drives the ongoing efforts to refine them. No matter how well they operate in terms of speed, accuracy, reliability, and uptime, it's never enough. As a result, robot builders are continually looking for ways to improve their designs and optimize components to meet tomorrow's ever more demanding needs. Not surprisingly, those who follow the interdisciplinary (mechatronic) upgrade path are finding success, especially when they focus on the torque-producing elements, servomotors and gears.
The basic geometric concept behind the “Delta” parallel robot design is the use of parallelograms. A parallelogram allows an output link to remain at a fixed point of reference with respect to an input link. By using three such parallelograms, the orientation of the mobile platform is completely restrained, so that the robot offers three translational (and one rotational) degrees of freedom. Input links of the three parallelograms are mounted on rotating levers using revolute joints.
The revolute joints of these rotating levers can be actuated in two different ways, using either rotational ac or dc servomotors or linear actuators. What makes the Delta design ideal for pick-and-place applications is the robot's fourth leg, which is used to transmit rotary motion from the base to an end-effector mounted on the mobile platform.
An early manufacturer of Delta robots, Bosch Sigpack Systems, Minneapolis, originally designed theirs using linear rack-and-pinion actuators. However, this design is less than ideal. Issues include:
High maintenance, due to the exposed rack-and-pinion/linear actuator design
Lengthy assembly times
Heavy robot weight
Poor control and accuracy
To minimize these problems in their newer Delta robots, Sigpack designers replaced the linear rack-and-pinion setup with a planetary-gearbox servo actuator. The motor-gearbox combination includes an integrated lever arm so that all drive components are enclosed and protected against water and dust, and overall envelope and weight are reduced. In fact, the design reduces component count by 75%.
The servo actuator used in the redesigned Sigpack Delta includes a high-precision gearhead, high-pole motor, and feedback integrated into a single package. The power source in the electromechanical solution — the magnets — is moved closer to the load, thereby improving stiffness and resulting in higher dynamics. Because bearings, couplings, shafts, and other rotating parts are eliminated, the actuator's inertia is smaller. In addition, assembly time is reduced, and control is significantly improved.
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|Delta robot||XR33 (old)||XR31 (new)|
|Cycles per min.||150||185|
|Max. acceleration, g||0.315||0.455|
|Cycle time, sec||0.4||0.32|
|Working area, mm||48 × 8.5||48 × 9.2|
|Total weight, lb||242||165|
|Assembly time, hr||6.5||2.5|
Software sizes up designs
Component selection is perhaps the most critical step when it comes to meeting motion system requirements with off-the-shelf solutions. Ideally, all component attributes, including full torque, speed, and force behavior, are simulated and analyzed within a software environment before any designs are finalized.
Today's intelligent servo sizing software also acts as a sort of in-house application engineer. While machine designers focus on system requirements, the software automatically generates optimized gear reducer/motor combinations. By accounting for motor inertia and balancing out the optimized ratio, the software minimizes motor torque (and energy use) necessary for the application at hand. It also estimates design factors such as bearing service life and how much lateral force the flange on the gearbox can withstand.