Robot Primer 8: Robot Types
In this post, I will take a quick look at industrial robot types.Â I know there are more robot types (such as cylindrical and polar) and variations in each type, but these are the most common.Â The manufacturers’ web sites and other resources such as books and system integrators have more opinions about when to use what robot type.
Robot terminology can vary between manufacturers.Â I will use this post to define my terminology.
A Few Words About Coordinates and Planes
Since I will be using reference axes, the picture above shows my reference system.Â The X axis is toward the viewer, the Y axis is left to right, and the Z axis is down to up.Â The XY plane, highlight, is formed by X and Y axes.
Note that while three positions (such as X, Y, Z position in rectangular coordinates) uniquely define a point in space, it takes more to define the position and orientation of a real object in space.Â Robot controllers often borrow from aviation and use yaw (nose left/right), pitch (nose up/down), and roll (rotation about the principal axis) to define the robot end effector’s orientation.
The end effector is the tooling attached to the end of the robot’s arm.Â End effectors consist of whatever is needed to get the job done, such as vacuum cups, grippers, cameras, glue dispensers, and welding equipment.
The articulated robot is constructed from a series of interconnected rotary joints or axes, typically 4 to 6 in total.Â It’s biggest advantage is flexibility; a 6 axis model should have full 6 DOF (degrees of freedom), and thus can approach a given XYZ point with any desired yaw, pitch, and roll (within the robot’s mechanical limitations).Â All those joints make it easy for the robot to reach around obstacles.Â A five axis articulated robot will still be more flexible in its moves than a 4-axis SCARA.
If you want to visualize this flexibility, consider performing automatic screwing at several locations on the surface of a sphere.Â The robot’s screwdriver needs to be perpendicular to the sphere’s surface at each location: a 6-axis articulated robot can do this, but a 4-axis SCARA can’t.
Like SCARA robots, articulated robots have a large work area and a small base.Â Very large articulated robots are available.Â They are often slower than SCARA or delta robots and less rigid in the Z axis than SCARA robots.
I number the robot axes by starting with the joint closest to the base as Axis 1 and work out from there.Â The picture above shows my numbering.
The SCARA robot has all three rotary axes in the XY plane, with only 1 axis that can move up and down.Â This configuration gives the robot more rigidity, or less compliance, along the Z axis, and thus the name: Selective Compliance Assembly(or Articulated) Robot Arm.
Like articulated robots, SCARA robots have a large work area and a small base.
Since the SCARA robot is fast and rigid, they are often used for assembly (especially when downwards force is required), pick and place, dispensing, and palletizing.
The picture shows my axis numbering.Â The quill combines Axis 3 (Z, up/down) and Axis 4 (rotation around axis 2).Â The quill is typically hollow to allow passing through cabling (pneumatic and electrical) to the end effector.
Cartesian robots are made from a combination of linear stages, typically stacked.Â Using linear stages offers a potentially wide range of possible characteristics, including:
- Very heavy load capability, especially with gantry (parallel) stages.
- Very high acceleration and velocity, for exampleÂ with linear motor based stages
- Very high precision, for example with stages using air bearings and flexures
- And more with options such as piezo motor stages and belt-driven stages.
If a cartesian robot makes sense, then there are several possible approaches.Â I’ll use Adept’s lineup as an example, since I’m familiar with it and they offer cartesian robots:
- Use an Adept robot controller with an Adept cartesian robot.Â This is by far the easiest approach, with very little integration work: basically plug the robot into the controller, and go.
- Use an Adept robot controller with Adept servo drives and motors and third party stages.Â This will take considerably more integration, including defining the kinematics.
- Use an Adept robot controller with standard servo amps and motors and third part stages.Â Again, this will approach requires much more integration, including defining the kinematics, but provides the most flexibility.
- Or if the application doesn’t require the extra capabilities of a robot controller, use an appropriate motion controller.Â For example, the ACS SpiiPlus is oriented towards precision motion (think semiconductor) while the Schneider LMC-20 is targeted towards packaging.Â This willÂ require substantially more integration work than using an integrated robot and controller package.
Given the variety of possible cartesian robots, it’s hard to give definite comparisons.Â In general, they are going to take up more area (larger base) than a SCARA or articulated robot, and will not have the flexibility of the 6-axis articulated robot.
The delta robot is a parallel robot with various arms joined together at the end effector.Â Typically it has 3 degrees of freedom (XYZ), although some models have an additional rotary axis.
The delta’s strong points are speed and stiffness: since the arms are very light, it can accelerate and move very quickly.Â The multiple connected arms add stiffness, but they also reduce the work envelope.
The delta robot is typically mounted above the work area.
Delta robots are popular for packaging and other high speed pick and place type operations.