How to select a robotic arm for particular application

SELECTING A ROBOT ARM

When exploring robot arms as the core device of an automation machine, several questions will arise:

1.  What kinds or types of robots are there?
2.  What are their speeds?
3.  What are their cycle times?
4.  What are their payloads?
5.  What are their costs?
6.  How intelligent are they?
7.  How are they different from automation?

We will explore the first question in the next section. The next four questions can be related to the consumer comparing automobiles and asking the car salesperson for information, but there are hidden issues, just as with cars.

 The payload question seems simple enough. Either a robot can lift X pounds or it cannot, but one will find that the robot manufacturer will list a maximum payload that corresponds to the robot being able to operate under effective control of its electronics, 24 hours a day for 7 days a week. A robot listed for 10 lbs sounds like a weakling compared to the average human.

One might compare it to a child. However, this rating is for continuous operation. If one misused the robot, it might lift 50 or 100 lbs before something broke.

This is why for safety reasons, a novice thinking a robot is only as strong is as a child draws a false sense of security.

In an uncontrollable motion, the robot night be able to swing a 50 lb weight at a tremendous speed and either knock one out cold or perhaps even kill.

The payload question is also coupled to the lack of a gripper out of the shipping crate. If one decides upon a robot with a 10 lbs payload, and the required gripper is 5 lbs, one only has 5 lbs of capacity to lift something! So it is important to scope out the tasks, and the probable robot gripper, before selecting the right robot.

As robots evolved over the 1970s, 1980s, and 1990s, their speed and cycle times were advertised similar to a car going from 0 to 60 mph. Faster speeds and quicker cycle times seemed an obvious goal, and in many operations this is basically true.

 It assumes a cycle where the object is grabbed and lifted 1 in., moved 12 in., and placed 1 in. below. The robot and gripper must also return to the starting grabbing location to complete the cycle. The cycle time for many years hovered around 1 sec. Then some SCARA manufacturers and others were able to
reach 0.5 sec and less.

Now if this were a reduction of time for 0–60 mph, race car enthusiasts would be drooling. It has its merits for automation also, but there are other issues to consider. If one uses high school physics, one will determine that the 0.5 sec time and given displacement conditions produce accelerations of close to 10 times gravity.

Other issues are:

•  Can the gripper close and open almost instantaneously?
•  If it does close instantaneously, will the reaction forces on the gripped part be OK?
•  Would the part be misaligned if the gripper closes this fast?
•  If a suction cup is used, will a vacuum be achieved in time?
• Will the part slip during the 12 in. motion and become a projectile?

Many of these issues cannot be answered without some experimental testing

on the specific parts one has to assemble. Many robot vendors have customers

testing robots at various locations around the country just for these purposes. One

may not get all of the answers to 100% satisfaction, but many of the risk factors

can be removed.

The next two questions on robot costs and intelligence are somewhat linked. Usually robots right out of the shipping crate are not well outfitted as mentioned before, but one needs to explore if the candidate robot has the appropriate controller processing capabilities if one does add sensors and external devices.

Most robot controllers today have these features, but they range from limited to powerful. One needs to discuss the details of a pending automation project with robot vendors, well beyond payload and cycle time. Nothing is more frustrating than someone giving a project team a 15-year-old robot that seems to be able to perform all of the required motions without trouble, yet the controller does not have the capacity to handle the types and flows of sensory information. As for how a robot differs from automation, one needs to look at the components found in most robots:

• Structural members and bearings;
• Electrical motors;
• Optional gearboxes (depending on motor type);
• Position encoders;
• Wire cabling for motor power and feedback from sensors;
• A computer/controller;
• Amplifiers to boost computer level output signals to high amperage power signals.

All of these components are most likely to be found in automation. It is just that as an automation designer, one would be selecting these individually from catalogs, not simply selecting the assembled robot as one item. And the robot’s software is usually far more versatile than what one uses in automation. So this is where robots get their advantage in reprogrammability and ready availability.