The selection of appropriate robots for industrial and automation applications is one of the most crucial and difficult decision-making exercises since manufacturers need to get several attributes and criteria for effective implementation, ensuring increased productivity, accuracy, and precision in operations.
They need to first understand the complex relationships of the relevant qualitative and quantitative attributes before decision-making regarding selecting appropriate robots from the vast spectrum of the robots and their models from various manufacturers available today for the particular application in hand.
The selection criteria are based upon application, maximum reach, payload, number of axes, repeatability, and mounting position. Before the selection, the manufacturer also needs to consider various other conditions such as production demands, manufacturing systems design, and economic impact.
At present, you can find computer software to assist the industry in selecting the right robot based on the multiple attribute decision-making methodology. This rule-based approach has been developed based on expert experience and heuristic rules, considering the selection of user interface, knowledge hierarchy, programming, program validation, testing, and documentation and maintenance.
This post will briefly explain 30 key facilitator level criteria and their relevance in the robot selection process.
Payload capacity (PC): It refers to the robot&#;s specified ability to lift maximum payload at its extreme horizontal/vertical reaches. It is important that the selected robot can handle the payload applied during the working cycle in the given range.
Overload capacity (OC): The robot must handle minor cyclic fluctuations in the end payload. Thus, the robot&#;s overload capacity is an essential consideration for its selection for a job at hand.
Workspace (WS): It is the maximum horizontal and vertical reach of the robot to constitute its envelope. It is essential to ensure that the tooltip&#;s desired area of operations and movements attached to the robot is within the robot envelope.
Accuracy (AC): It specifies the minimum error with which a robot can reach a commanded position. It may vary with speed and position within the working envelope and with payload and hence must be considered selection criteria.
Computational efficiency (CE): It determines the speed of processing and calculations to make the robot&#;s operations and control in real-time. However, there is a tradeoff between the computational efficiency, processing speed, real-time control, and the cost of the algorithm and the controller.
Resolution (RS): The lower and above least counts of robot working parameters must be checked to ensure their applicability in the robot working range.
Manipulability (MA): The skillful and possible movements and orientations of the robot&#;s links/joints give it versatility and increase its working utility. Manipulability thus becomes an important consideration for robot selection.
Power source req. (PR): The power specifications required by the robot, viz. separate earthling for welding robots, uniform voltage for painting robots, must be available at the installation site of the robot. Else the robot may not work optimally.
Joint type (JT): The kinematics details, including the link and joint types, influences many criteria, including payload, workspace, manipulability, etc.
Project Specs (PS): For the project at hand, the robot must meet the application&#;s specifications. Thus, the flexibility in robot operations and its handling increases its utility for a project.
Speed of the robot (SP): The robot&#;s configuration should support high speed with minimum vibrations. However, the speed of the robot varies with its payload and geometric configurations.
Weight of the robot (WT): The robot&#;s overall weight increases the dead weight on the installation site. Hence, if the installation itself is on a mobile platform, viz. a conveyor, etc., it tremendously increases the total system&#;s power requirements. On the other hand, a low weight robot has more vibrations and dynamic effects involved in it. Thus, decision making in selection involves a tradeoff in conflicting characteristics.
Electrical drives (ED): It is imperative to select robots with appropriate drives as per the requirements. Thus, robots in which electric motors are connected to the joints via gears have definitive backlash, whereas the robots in which motors are coupled to the joint directly can take up only small torques and loads. Thus, decision making involves choosing the right kind of electrical drives, which affects the payload capacity, resolution, accuracy, etc.
Programming flexibility and capacity of Controller cards (CC): The selection criteria include interfaces, like touch screens and other peripherals like programming, controller chips, etc.
Hydraulic drives (HD): For stronger and cyclic operational requirements, the hydraulic joints are viable. The selection criteria involve closed-loop, internal air pressurization, etc.
Type of cable and harness (CH): Cable-routing and harness specifications must match the project specifications making them important for consideration during the installation of the robot.
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Singularity (SL): The geometric constraints in the robot linkages manifest themselves so that some locations, despite falling inside the workspace, may require huge computational or input efforts. These singularities must be avoided. Care must be taken at the time of selection of an appropriate robot.
Dextricity (DX): It is a comprehensive parameter that includes geometric constraints on the manipulability, speed, singularity, and other aspects of robot performance. A robot with higher dextricity is more capable of performing intricate jobs and hence becomes a choice of robot selectors.
No. of axis (NA): For some applications, such as simple pick-and-place assembly, the robot may need simple joints and less no. of the axis. However, for more sophisticated applications, such as welding/painting, etc., the robot may require an additional no. of the axis. This is achieved by selecting an appropriate robot architecture suitable for the application.
Servos (SM): The servos selected can be geared or direct drive. Moreover, the gear trains may be epicyclical or direct. Thus, the servos have an impact on the overall performance and repeatability of the robots.
Precision (PS): The robot&#;s repeatability, determined by its precision, ensures appropriate reiteratability in the robot operations.
Reasons for breakdown (RB): As a decision-maker in robot selection, it may be imperative for the selector to know the reasons for a particular robot&#;s breakdown. If the breakdowns are due to machine designs&#; poor workmanship, the repeatability of the robot and its reliability are affected.
Mean Time to Repair (MTTR): In case of a breakdown, the robot must be serviced to bring it in working order as soon as possible. A large MTTR indicates serious breakdowns, low availability of spares, poor service network of the supplier, etc. Hence, it is detrimental to the selection of robots by the decision-makers.
Mean Time Between Failures (MTBF): The mean time between failures provides a reasonable estimation of the robot&#;s total breakdown time. Thus, for reliable performance, the MTBF should be as large as possible.
Supplier service quality/contract (SQ): The company procuring the robot may have contractual agreements with some specific suppliers. This may provide the company with the benefits of economics of scale. Such contracts influence the selection of decision-makers and hence must be given due weightage.
Total cost (CT): The cost of the robot is undoubtedly an essential decisive criterion. However, note that cost alone is not the overall governing criteria, but various aspects of quality and return on investments complement the cost as selection criteria.
Supplier rating (SR): Based on the previous purchases, after-sales services, and performance of their robots, the user companies generally assign ratings to the robot suppliers. These ratings play an important role in the company&#;s new decision-making for new procurements and robot selections.
Availability of spares (SA): Quick availability of spare parts reduces the robots&#; downtime and positively influences the decision of robot selection.
Local network of suppliers (NW): A robot supplier with a local network of dealers and service centers is better equipped to serve the maintenance/training/ robot breakdown issues than a company with supplier dealers based abroad. The robot selection by decision-makers is influenced by considering the supply and service network chain of the robot supplier.
Warranty terms (WT): Warranty, replacement of parts, etc., form the intangible benefits which the robot procurement company seeks to get from the robot supplier. Of-coarse the supplier which provides the robot with better warranty terms has a better influence on decision-makers.
As you embark on the robot selection process, the following 10 items are important to consider before making your final decision.
1. Compact, efficient robot design. A compact robot design with slim arms and a small footprint makes integration easier and saves valuable shopfloor space. In addition, designs with concealed air and electrical lines keep those lines from interfering with other equipment, as well as protect them from wear and damage, which helps reduce overall costs.
2. Robot controller features. Desirable features to look for in robot controllers include compact size and light weight; fast processing speed; modular expandability to accommodate additional peripheral equipment without having to purchase a new controller; ease of integration with a vision system, PLC or other devices; and ease of servicing.
3. Affordable offline programming software. Be sure that the offline programming software being offered does not include expensive, advanced features that are unnecessary for your needs.
4. Low energy consumption. Ask about the robot&#;s energy consumption. Efficiently designed, slim and lightweight robot arms require less power, so their motors draw less electrical current, which can result in significant long-term cost-savings.
5. Safety codes. To protect employees and limit your company&#;s liability, verify that the robot meets or exceeds all current safety codes.
6. Short training. Ask about the length of required training. Unnecessarily long training can result in excessive unproductive employee time and travel costs.
7. Documented mean time between failures. Above all, robots must be reliable, and this can be verified with documentation of a robot&#;s mean time between failures (MTBF) provided by the manufacturer.
8. High maximum allowable moment of inertia. Look for a robot with a high maximum allowable moment of inertia, a measure of how much force it can exert. The higher the maximum allowable moment of inertia, the more easily the robot can lift and move a given payload size, putting less strain on the robot&#;s motors and resulting in a longer working life.
9. Continuous-duty cycle time. When comparing robot cycle times, be sure to ask whether the figures given are for continuous duty, or only shorter bursts of an hour or less. If the latter, the robot will have to operate at a slower speed in normal operation.
10. Experience and reputation of the robot manufacturer. Look for a manufacturer that has established itself as an industry leader and whose robots have stood the test of time. Also look for a manufacturer who understands the importance of product training by offering basic and advanced comprehensive operator, programming and maintenance training for its robots through a variety of classroom sessions, computer-workstation interaction and hands-on robot training.
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