Over the past few years, you’ve likely seen collaborative robots (cobots) doing everything from stacking pallets in a warehouse to making fries at a fast-food chain. Today’s co-bots are strong enough to handle pay-loads as heavy as 55 lbs with precision and safe enough to operate side by side with human workers. While cobots continually learn to perform more complex tasks, such as configuring applications, the human machine interface (HMI) has become much simpler. Setup can be done in a short of time, and PC-like tablets with touchscreens and intuitive programming make the system easy for employees to learn.
Capitalizing on the benefits cobots, of course, requires choosing the right cobot for the job. This article discusses five key considerations for selecting a cobot for your manufacturing application.
Functionality
Within manufacturing, cobots perform tasks including pick and place, palletizing, assembly, welding, and machine tending. The selection process starts with an understanding of the parameters of the tasks a cobot will perform. Is the cobot’s movement based on a fixed, repetitive motion or a variable movement based on a sensor input? How fast does the movement occur? What is the available space the cobot can operate within? The specific types of motion required will identify the needed number of axes or joints. Is direct interaction with humans required? In a palletizing application, for example, will the cobot unload from the manufacturing cell directly to the pallet or to a staging area? Is there a need to rotate the items to be palletized? The discovered information drives decisions to determine the specific cobot configuration and interfaces required for the intended operators.
Sizing
The important factors needed to design the optimal cobot system include payload size and weight, expected reach, repeatability requirements, and available footprint. The load is extremely important as it affects the entire cobot package, while the weight and reach requirements establish the size and number of joints necessary to properly control the payload.
The motion task and design of the robot sections (waist, shoulder, elbow, wrist, and end-effector) determine the torque and speed requirements of the joint motors as well as the length of the arms between joints. The heavier the payload and the longer the reach, the more torque that is required to achieve the optimal motion. The repeatability of the cobot identifies how close to the target position the robot can achieve each time it is requested to move to that position.
Repeatability is represented by a plus or minus position relative to the target — the smaller that value, the better the repeatability. The available footprint must be considered for the entire reach of the robot. In the prior mentioned palletizing application, the weight and shape of each item to be palletized, plus the weight of the gripper mechanism is needed. The travel distance the picked item must move from the work cell to the pallet, as well as the width of the pallet, will determine the reach of the cobot. Finally, the expected speed of the work cell to deliver items to the pallet station will determine how fast the cobot must pick and place the item.
Cobot designers collect these specifics to analyze the kinematics associated with the motions required. Inverse kinematics analyze specific motion paths and determine the parameters of each variable joint to achieve the position and orientation of the end effector. Calculations review inertial requirements that affect the acceleration requirements of each joint, the torque and velocity parameters associated with the motors at each joint, and the geometry of the motion. The lengths of the elements between joints determine the moment loading on each of the joints as it moves the end effector to the desired position. The basis of these calculations is the Denavit-Hartenberg Parameters that include:
offset along previous z axis (axis of rotation) to common normal (x axis, which is the z of next joint)
angle about previous z axis, from old x axis to new x axis
length of common normal (assuming a revolute joint, the radius about previous z) angle about common normal, from old z axis to new z axis.
These parameters (Figure 1) are typically provided by the cobot manufacturer and will also include specifics on joint and link mass as well as center of gravity location in reference to the base plane coordinates.
Compatibility
While cobots can operate independently from any other system, they are often used within a network of other devices. Many industrial devices utilize common communication protocols that allow them to operate on common networks. The control systems of the cobot must have the right “hooks and handles” to communicate with any PLCs, HMIs, or other electronics that are incorporated in the factory.
IoT is another concept that identifies how the cobot communicates remotely through the internet, possibly with other similar cobots in the factory. In a factory machining area, a cobot might be used to assist the machine operator load material into a machine tool. Ideally, the machine tool would communicate with the cobot to indicate when a new blank was required or when a finished part was ready for removal. The cobot would need to communicate via a common bus network and share the same communication protocol (e.g., EtherCAT, Sercos, Profinet, or similar).
Ease of Use
Since cobots, as designed, work in conjunction with humans in the manufacturing environment, they must be easy to operate by workers of various skill levels. The software solutions designed for cobots have advanced considerably over the past decade.
An interface for a cobot (Figure 2) should be designed to accommodate the human operators with graphical user interfaces that are intuitive and easy to learn.
The purpose of a cobot is to assist an operator, such as a welder, operate a welding machine. The experienced welder will need to operate the cobot without an in-depth understanding of each of its elements. Selection of a cobot that can be pre-programmed with the knowledge of the welding profession will provide an easier transition into the work cell.
Safety Features
Safety is paramount in the selection of any cobot. Since cobots collaborate with humans in a shared space, they must be equipped with many inherent safety functions. A thorough risk assessment is essential to determine if a specific cobot design meets the intended safety needs. Typical safety functions include limits that prevent excessive force, torque, or speed based on sensors integrated into the design. Safe Motion configurations are determined by the outcome of a safety risk assessment that defines the stop modes associated with specific safety events.
Many cobot controllers allow the creation of specific safe zones where pre-determined actions are activated, such as reduced speed, lower torque limits, or other collision avoidance routines. Since cobot applications include humans in close proximity, system designers must run a thorough safety analysis and the appropriate Safe Motion and Safe Machine SIL recommendations must be incorporated. ISO/TS 15066 provides guidelines for design and implementation of a collaborative workspace (Figure 3).
Mobile Cobot in a Palletizing Application
In a palletizing example, the components to consider depend on what palletizing actions are required. Consider the functions the cobot will perform, including how the product will be loaded or unloaded, where the cobot will be positioned, and what sensor techniques will be used for motion and product locating. In this example, the maximum pay-load is as high as 55 lbs and the desired reach is 1.7 m. The package pick-up device will be a lightweight vacuum gripper weighing approximately 6.5 lbs. The cobot will use a touchable PC under a windows operating system.
The software must include specific palletizing application programs with a simplified GUI. The vertical lift system should incorporate a servo actuator and wheels will be added to provide a mobility element to the cobot. A radar sensor with 3 axes of freedom (up, down, left, and right) will be used for range detection for enhanced safety. This application specific collection of requirements begins to cover the functionality, sizing, and ease of use considerations. The compatibility element will tie into the PC operations and how it is networked into the factory operations.
The most important consideration is safety. The safety risk assessment in this example might consider the stop event and safety conditions below.
SIL3, PL e CAT4, including safe torque off (STO), Safe brake control (SBC), and Emergency Stop.
SIL2, PL d CAT3, including Tool Center Point (TCP)/Robot Position Limit, TCP Orientation limit, TCP Speed Limit, TCP Force Limit, Robot Momentum Limit, Robot Power Limit, Collision Detection, Safety I/vO, and Reflex Stop.
All these safety events are tested in accordance with the appropriate IEC or ISO standards, with the results certified by a Machine Safety Specialist. A document that certifies the cobot has met all applicable safety standards is required prior to introducing the cobot into production.
Cobots continue to play a critical role in supplementing the human workforce. Their use in applications enhances production and makes the work environment safer for humans. When designing a cobot into an application, following these key considerations will lead to a successful implementation and a strong return on investment.
This article was written by Stephanie Callaway, Marketing Development Manager, Doosan Robotics Americas (Plano, TX). For more information, visit here .
