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Choosing the Right Robot

The following write-up offers a basic checklist that may be helpful in creating a decision matrix for use when choosing the type of robot to use for your application.  When it comes to choosing an appropriate robot, there are several factors to consider.  Begin by asking yourself some of the following basic questions.

What is the Application being considered?

Narrow your search based on the application.  Below is a list of common robot applications:

• Laser, Plasma, Arc, spot welding
• Cutting, Heating
• Dispensing
• Machine loading
• Material handling
• Packaging
• Part holding, positioning for a production or assembly fit
• Part transfer
• Press, injection mold, die cast tending
• Palletizing
• Clean room
• Cutting
• Drilling
• Material removal
• Painting
• Thermal spray
• Bonding-sealing
• Coating
• Deburring
• Polishing
• Assembly, pick & place, vision-guided

What types of robots are available?

• Scara – Fancy pick & place.  Very fast and programmable.
• 6-Axis – Typically slower than the Scara the 6-Axis has significant flexibility and versatility.  This type of robot is by far the most common.  It allows for part orientation and significant flexibility within performance parameters.
• Gantry – Typically pick & place.  Lack flexibility but can be fast and easy to program.
• Spider – These are pick & place unit on steroids.  Typically lightning-quick and integrated with vision systems they are primarily used on small parts that are difficult to feed.  This technology continues to improve in capacity and use cases.
• Collaborative – Oh yes, your management has heard that these robots are all the rage and want you to implement them for bragging rights.  The reality is that they are typically slow and limited in “safe” applications.  The speed is slow to minimize inertia and allowing for a quick stop.  They allow for more limited guarding (light curtains) but most still require guarding to be used effectively.  They are getting better but practical applications are still a challenge to find.  I am sure the collaborative robot salespeople would challenge my view and experience but it is all I have to offer!

What is the Payload being moved?

The weight of the application will be one of two critical factors in the robot selection process.  To calculate the capacity required simply estimate the weight of the part to be lifted or manipulated and the weight of the end of arm tooling (EOAT).  The capacity of the robot selected should exceed the combined weight of the expected payload by 20%.

What is the inertia?

The power required to move the load is important to understand and calculate when selecting the appropriate robot for the application.  There are many calculators available to help with the calculation but don’t miss this second critical factor in determining which robot to integrate.

What positional accuracy is required for the job? How precisely does the application or function require for positioning?

• Does the application require 1”, .125” or .005” positional accuracy?
• What repeatability is required – how repeatable is the positioning of the robot?

What is the required reach of the application?

In general, it is not wise to operate the robot fully extended.  Each manufacturer will have recommendations based on payload considerations.  Beware that robot manufacturers will typically advertise ideal max capacity scenarios but the reality is that pushing the limits can compromise the life of the robot, accuracy, and speed.  The inertia calculator will take the length and payload into consideration for an accurate calculation.

What is the speed required to make cycle time?

Speed is a key consideration because acceleration and deceleration factor into the inertia calculations and will affect the accuracy of the application.  Speed will also affect the life of the robot.  In many cases, the cycle time requirements will dictate that an additional robot needs to be added.  The addition of a robot will require additional calculation because of potential idle time as one robot waits for the other to clear.

Does the application need guidance?

Is the robot vision-guided?  This can get into an entirely different discussion, but consider how the parts will come to the robot, or if orientation is required in gripping or placement.  Vision guidance is very common in spider robot applications in which parts are picked and placed quickly.  Other applications will trigger the robot to orient the gripper before picking up the part to achieve the desired orientation for placement.  Welding applications often use seam tracking.  Seam tracking can be tricky technology to implement and maintained.  Many companies offer a turnkey solution for seam welding that will be simpler to integrate.

Thinking outside of the Box

You can expand a robot’s work envelope by augmenting its reach.  Models are available with extended reaches and extensions can be added to standard robots (this has a major impact on the inertia calculation).  Far easier than changing a work envelope’s scope is changing its orientation.  Install the robot in different locations (wall, shelf, floor, or ceiling) to reduce the work envelope or improve utilization.

Is 6 or 7 axis required?  What are the tooling or functions, angles, access that the application requires?

Scara versus 6-axis Scara robots are known for their speed and accuracy.  They also seem to last forever!  The work envelop is typically smaller for Scara’s and essentially they are programmable pick and place units.  6-axis robots have much greater flexibility.  The flexibility comes at a price and ease of use, although nowadays the software and pendant used for programming are typically user-friendly.  6-axis allows for orienting the part and easily accommodates workspace elevation changes.

What type of operational environment will the robot work in?

• Evaluate operating temperature range extreme heat or cold, dusty (what is the particulate), radiation, wet, caustic environment, vibration.

• Does the robot have the proper sealing and protective means to survive the intended environment?

• Environmental factors can affect the accuracy, life, and dependability.  Be assured to closely match the robot duty to the application.

• Customer recommendations/references – Ask around and talk with your machine builder or integrator on which robotic company or provider they might recommend.

• Service and support track record – How responsive are the national and local service centers?  What typically fails on the robots being considered?  Check references if you have no experience.

Additional considerations:

1. Orientation – Some robots work upside down and some don’t.  Some can be “wall mounted” and some can’t.  Understand the movements and best-fit orientation for the application.

2. Controller Architecture – Some robots are easier to program than others from what I have come to understand.  The guys that program robots regularly give me the rundown on the simplest and the most difficult.

3. Safety – For safety considerations, the user should visit the RIA at www.robotics.org to learn the risks of using a robot and the required guarding.

 

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