Skip to content

Joint limits and directions

Joint numbering and positive directions

Joints are numbered from the base up. Joints 1, 2 and 3 are also called the base, shoulder and elbow joints, and joints 4, 5 and 6 wrist 1, wrist 2 and wrist 3, the names the robot model and ROS use. Each picture shows one joint turned 30° in its positive direction, in colour, over the pose in grey. The arrow at the joint points the same way, and the commander software's left arrows jog in that direction.

Each joint turned 30 degrees in its positive direction, in colour, over the pose in grey Each joint turned 30 degrees in its positive direction, in colour, over the pose in grey
Figure 3.1: Each joint turned 30 degrees in its positive direction, in colour, over the pose in grey.
Joint Also called Positive direction
J1 base counter-clockwise, seen from above
J2 shoulder leans the upper arm forward, the way the forearm points in the standby pose
J3 elbow raises the forearm
J4 wrist 1 clockwise, seen from the tool
J5 wrist 2 tips the tool up, in the standby pose
J6 wrist 3 clockwise, seen from the tool

Standby and after homing

The standby pose is the pose of the Denavit-Hartenberg table and the kinematics sheets, and the reference for the limits below: the upper arm vertical, the forearm horizontal, the arm pointing away from the electronics case at the back of the base. After homing the robot stands in the same pose turned 90° about joint 1, counter-clockwise seen from above: the firmware sets joint 1 to +90° when it finishes homing.

Pose Joint 1 Joint 2 Joint 3 Joint 4 Joint 5 Joint 6
Standby 0° −90° 180° 0° 0° 180°
After homing 90° −90° 180° 0° 0° 180°

The Python API calls the after-homing pose its standby (STANDBY_ANGLES_DEG), and a later home() moves there; the commander software's standby is the standby pose above.

The standby pose, from the front and from the side, with the joints marked The standby pose, from the front and from the side, with the joints marked
Figure 3.2: The standby pose, from the front and from the side, with the joints marked.

Limits

The joint limits of PAROL6's model in robot_descriptions (version 2.15.1), in degrees, in the angles the software shows (the standby pose above).

Joint Negative limit Standby Positive limit Range
J1 −97.40° 0° +97.40° 194.8°
J2 −146.15° −90° −32.70° 113.4°
J3 105.52° 180° 294.59° 189.1°
J4 −114.59° 0° +114.59° 229.2°
J5 −120.32° 0° +120.32° 240.6°
J6 none 180° none no limit with the bare flange; with a tool, its cables and tubes set it
Each joint at both its limits, in its colour, over the standby pose in grey; the arc spans the range, in the angles the software shows. Joint 1 is seen from above, with the after-homing position marked; joint 4 is seen from the tool, with joint 5 at 90 degrees so that the tool's swing shows; joint 6 has no limit Each joint at both its limits, in its colour, over the standby pose in grey; the arc spans the range, in the angles the software shows. Joint 1 is seen from above, with the after-homing position marked; joint 4 is seen from the tool, with joint 5 at 90 degrees so that the tool's swing shows; joint 6 has no limit
Figure 3.3: Each joint at both its limits, in its colour, over the standby pose in grey; the arc spans the range, in the angles the software shows. Joint 1 is seen from above, with the after-homing position marked; joint 4 is seen from the tool, with joint 5 at 90 degrees so that the tool's swing shows; joint 6 has no limit.

The firmware and the software still use the earlier limits

Firmware 1, the commander software and the Python API (0.9.1) enforce the limits of the earlier model until they are updated: J1 ±123.05°, J2 −145.01° to −3.38°, J3 107.87° to 287.87°, J4 ±105.47°, J5 ±90°, J6 0° to 360°. A target between the two is accepted by them; keep to the table above. Before homing they apply these limits to a position count that starts at 0 at power-up, so joint 2 cannot be jogged in its positive direction, nor joints 3 and 6 in their negative one, until the robot is homed (Jogging).

Joints 1 to 5 are stopped at both ends by printed blocker parts, the mechanical stops a little past the limits above. The limit switch or sensor that homes a joint sits near its blocker, so that it triggers just before the joint reaches it. Joint 6 has no blocker: with the bare flange nothing stops it, and with a tool only the tool's cables and tubes do.

Grippers and bases change the limits

A gripper changes where joint 5 can go without colliding with the forearm, and a riser under the base changes what joint 2 can reach. The software limits do not know about either: check the real clearances with a slow jog before you run a program near a limit.

The limits are not a safety function

The limits only stop the software from commanding a joint further. Inside them the arm can still hit a person, the table or itself, no safety-rated part checks them, and a wrong homing shifts them with it. They protect the robot's cables and parts, not the people near it (Safety).

Never more than one turn, except joint 6

No joint may ever be turned more than one full turn, by hand or by software: the cables and pneumatic tubes that pass through the joints wrap and tear. The one exception is joint 6: with the bare flange it can turn without limit; with a gripper or a tool, the tool's cables and tubes wrap as it turns, and they set how far it may go (Moving the robot by hand).

Comments

Found a mistake on this page? Edit it on GitHub · Report it · Get help · This page as Markdown · llms.txt · The whole manual as PDF