09. Serial Manipulator Applications

Serial Manipulator Applications

Here we take a slightly more in depth look at the principal types of manipulators by considering their pros, cons, and typical applications. We also discuss an important property known as the **workspace ** (sometimes called work volume or work envelope).

Workspace

The workspace is the set of all points reachable by the end effector and is a primary design constraint when selecting a manipulator for a task. The workspace can be divided into two regions: the reachable workspace, i.e., what is implied by the simpler term workspace, and the dextrous workspace. The dextrous workspace is the set of all points reachable by the end effector with an arbitrary orientation. The dextrous workspace is a subset of the reachable workspace.

In many cases, e.g., machining or painting, the tool tip must interact with the environment in a particular configuration in order to have the desired result so ensuring the task lies completely within the manipulator’s dextrous workspace is essential. Unfortunately, it can be quite difficult to precisely define the boundary of the dextrous workspace.

Note: in the descriptions that follow, the joint types of each manipulator are indicated in parentheses after the name, where "P" indicates a prismatic joint, and "R" indicates a revolute.

Cartesian Manipulator (PPP)

The first three joints of a Cartesian manipulator are prismatic joints with mutually orthogonal axes of translation.

Pros

  • Can have very high positional accuracy

  • Large payloads (gantry)

  • Simplest control strategy since there are no rotational movements

  • Very stiff structure

Cons:

  • All the fixtures and associated equipment must lie within its workspace

  • Requires large operating volume

Typical Applications:

  • Palletizing

  • Heavy assembly operations (e.g., cars and airplane fuselage)

Cylindrical Manipulator (RPP)

As the name suggests, the joints of a cylindrical manipulator are the cylindrical coordinates of the wrist center relative to the base.

Pros:

  • Large, easy to visualize working envelope

  • Relatively inexpensive for their size and payload

Cons:

  • Low average speed

  • Less repeatable than SCARA

Typical Applications:

  • Depends on the size, small versions used for precision assembly, larger ones for material handling, machine loading/unloading

Anthropomorphic Manipulator (RRR)

Anthropomorphic (sometimes called articulated) manipulators provide a relatively large workspace with a compact design. The first revolute joint has a vertical axis of rotation and can be thought of as mimicking a human’s ability to rotate at the waist. The other two revolute joints have axes of rotation that are perpendicular to the "waist" and mimic a one DoF “shoulder” and a one DoF “elbow”.

Pros:

  • Large workspace

  • Compact design

Cons:

  • Positional accuracy and repeatability is not as good as some other designs

Typical Applications:

  • Welding, spray painting, deburring, material handling

SCARA (RRP)

The SCARA, or Selectively Compliant Assembly Robot Arm, was invented by Professor Hiroshi Makino of Yamanashi University (Japan) in the early 1980s. SCARA robots typically employ a single revolute wrist with the axis of rotation parallel to the other two revolute joints. Since the base link typically houses the actuators for the first two joints, the actuators can be very large and the moving links relatively light. Thus, very high angular speeds are obtainable with this design. The arm is very stiff in the vertical (z-axis), but relatively compliant in the x-y plane, which makes it ideal for tasks such as inserting pegs or other fasteners into holes.

Pros:

  • Fast

  • Compact structure

Cons:

  • Operations requiring large vertical motions

Typical Applications:

  • Precision, high-speed, light assembly within a planar environment

Spherical (RRP)

Like the cylindrical manipulator, the spherical manipulator’s wrist center can also be described as a well-known coordinate system. Probably the best known version of this kinematic type is Stanford’s Scheinman arm, invented by Victor Scheinman in 1969.

It was adapted by manufacturers to become the leading robot in assembling and spot-welding products, ranging from fuel pumps and windshield wipers for automobiles to inkjet cartridges for printers.

Pros:

  • Large working envelope

Cons:

  • Complex coordinates more difficult to visualize, control, and program

  • Low accuracy

  • Relatively slow

Typical Applications:

  • Material handling

  • Spot welding

One other type that we do not cover in this course but you should at least be aware of is called a parallel manipulator. Parallel manipulators have many variants but they are characterized by all having at least one closed kinematic chain. The dynamics and control strategies can be quite a bit more complex than serial manipulators, but in general they have more precise movements due to their structural rigidity.