Monday, January 11, 2016

Robots

This post is a very brief overview of  robots used in manufacturing. It talks about what is it that makes a machine a robot, what differentiates the various types of robots, different ways robots can move, and three types of power sources for robots.

The Term Robot

Karl Capek (pronounced "Saypeck") coined the term robot in 1920. He was a Czech playwright who wrote R.U.R. which stands for Rosumovi UniverzálnĂ­ Roboti (Rossum’s Universal Robots).

What Makes a Robot a Robot? 

What differentiates a robot from say a crane? How is it different than a 3D printer or a CNC router?
There are three key capabilities which make a robot a robot:
  1. A robot needs at least 4 degrees of freedom. 
  2. A robot needs to be Reprogrammable.
  3. A robot needs to be Multi-functional.

Classification

Industrial robots can be classified into six categories based on the following characteristics:
  • Degrees of Freedom
  • Arm Geometry
  • Power Source
  • Types of Motion
  • Path Control
  • Intelligence

Degrees of Freedom

The number of movable motions in a robot define its degrees of freedom. In articulated robots such as those in the Fab Lab have at least 6 degrees of freedom. These joints, or axes, are broken into two categories. The three joints nearest the base of the manipulator are called the positioning axes. The three closest to the tool are called the orientation axes. Robots can have larger degrees of freedom by having external axes, for instance the entire robot can be mounted on a sled which moves along a track. This would be a seventh degree of freedom.

Arm Geometry

The arm geometry, that is the configuration and type of joints used, determines the shape of the work envelope.

Arm Geometry Overview Video

Rectangular (Cartesian)

The work envelope is a box. All three axes are linear.

Cylindrical

The work envelope is a cylinder. Axis 1 is rotary. Other axes are linear.

SCARA

This is a variation of a cylindrical work envelope robot. SCARA is an acronym for Selective Compliance Articulated Robot Arm. Joints 1 and 2 of this type are rotary and in the same plane. Joint 3 is linear.  These are often called Pick and Place robots.

SCARA Robot Video

Spherical

This type of arm geometry produces a ball shaped work envelope.
Axes 1 and 2 are rotary. Axis 3 is linear.

Articulated

This type of arm geometry, which is what we use in the Fab Lab is also referred to as Jointed Spherical.


Articulated Robot Video - ABB Robots Katana Fight
Inside a Kuka Robot

Power Source

The three most common method of powering robots are air pressure (pneumatic), fluid pressure (hydraulics) and electricity. The main characteristics of each of these methods is listed below: 

Pneumatic

  • Weakest
  • Fastest
  • Clean
  • Inexpensive
  • Low Tech
  • Open loop (non-servo)
  • Stop-to-stop for path control
  • Uses hard-stops determine program locations
  • Loud - referred to as "bang bang" robots

Hydraulic

  • Most powerful (greatest payload)
  • Messy to repair
  • Closed loop (servo)
  • More flexible than pneumatic
  • Mid-range in noise
  • Oil used can contaminate paints
  • Most expensive (have to buy both hydrallic and electronic systems)
  • Most costly to repair (have to fix both hydraulic and electronic systems)

Electric

  • Most popular
  • Clean
  • Quiet
  • Closed loop (servo motors)
  • Most flexible
  • Can use sealed motors for painting