There are 5 major factors that I think about when I am specifying a robot for an application; reach, payload, speed, application type/versatility and lastly, cost.
Reach is one of the most important factors to getting the right size robot for your application. Some applications are as easy as just a simple pick and place and you can measure from the pick position to the place position and verify what kind of a reach you need from your robot.
However, sometimes you need to reach around certain objects, for example, for a machine tending application, you might need to account for the door of a CNC machine. Or the pick/place locations change dynamically. It gets tricky to measure exactly how much of a reach you need in these scenarios. In cases like these, I usually use a model of the robot and manipulate it to see if it can actually move around the cell or its workspace. If the robot has a simulator, that is something to consider because it will make it very easy to see if that particular robot is going to work for your application.
Payload is the second most important factor. If the robot can’t carry the parts you are handling, then it is not going to work.
Payload calculations get trickier than the reach calculations sometimes. It is a common mistake to forget to include the gripper’s weight into consideration when you do these payload calculations. And that’s not even enough. Most robot manufacturers will specify their robot’s payload exactly at the end of it’s last joint. So you need to find a graph in their manual that shows the payload capacity of the robot at a distance from the end of the arm. Finding that graph is definitely harder than calculating the load..
Most of the applications, the robot is servicing another machine or limited by another equipment down the manufacturing line. So the robot’s speed is dictated by the cycle time of those bottlenecks. Let’s say we are in a bottling plant and every bottle takes about 5 seconds to get filled, labeled and inspected. Every 5 seconds, we will be seeing a bottle to be picked up by the robot. Our robot needs to be able to complete its task under 5 seconds and be ready for the next one.
This is where we can start being clever. 5 seconds is not too fast for industrial robots, but for cobots, it definitely stretches their “safe” speeds. Now, we should ask the question, can we handle more than one bottle at a time? If we can handle 6 bottles at a time and do the task that’s required, now our cycle time is about 30 seconds.
It is definitely smart to explore the options of handling more than 1 part to reduce cycle time but we still need to be careful to be under the payload of the robot. Also, usually handling multiple parts is harder than handling only 1.
After making sure of the details and understanding how many parts to handle, the best way to test is to do a real time cycle time test with the specific robot. If the robot manufacturer offers a simulator, that might also be a good way to estimate the cycle time. Speeds that are listed on spec sheets are good for getting an overall feel, but you really should test the robot if your application is speed critical.
4. Application Type and Versatility
It is important to consider all types of robots in the application at first but some application types just require different robots. Doing a very delicate and precise quality inspection, you might need a robot with high precision, or if you need to orient your part, you want to go with an articulated robot rather than a SCARA.
You also need to consider if the specified robot can handle other applications in the future. This is one of the reasons collaborative robots are so popular these days since they are easy to deploy and versatile.
Cost is obviously a very strong factor in the decision making process. The key parts to justifying the cost of the robot is to understand the cost reduction that it will introduce and the return on investment of it. Obviously that return on investment will take a shorter amount of time if the price of the robot is cheaper, however, you should get the robot that is suitable for the job and also checks out the other factors listed.
These are 5 major factors I personally consider when I am picking a robot. In some scenarios, one factor might be more important than others, but everything starts with understanding the application requirements. Please reach out to Gibson Engineering if you have an application and we can choose the robot that fits your needs together!