Page 152 - Welding Robots Technology, System Issues, and Applications

Robotic Welding: System Issues 139

4.7 Semi-autonomous Manufacturing Systems
Actual manufacturing systems need to have high degrees of autonomy requiring
less operator intervention to improve agility and efficiency. In fact, if the
manufacturing systems are designed to operate remotely using simple commands
and parameterization, then the tasks necessary to adjust the setup to manufacture a
different product model may be fully automatic. This procedure should lead to big
improvements in terms of productivity, along with considerable gains in terms of
agility and flexibility. In the particular case of robotic welding, the following are
examples of things that can be done without operator intervention:

1. Change the welding parameters: current, voltage and speed.
2. Change the way the welding power source is controlled (Figure 5.2).
3. Change the welding sequence just by adjusting the welding points, the
trajectories between them and the definition on what are the welding and
the approach/escape trajectories.

This can easily be implemented by properly designing the services offered by the
robot controller application software, having them working based on the remote
parameterization. Furthermore, to include the robotic welding cell in a
manufacturing line, an “automatic mode” service must be designed with the
objective of having the system responding to sensor signals and to PLC
commanding signals. In fact, depending on the application, a few sensors are
needed to inform about the necessary conditions to execute the task, which
includes information like: “work-piece in place”, “power source on-line”, “welding
gun clean”, “cell not violated”, etc. It is also very important to interface with the
PLC managing the manufacturing line, because the coordinating signals that make
the manufacturing sequence occur are generated by the software running on the
PLC (usually specified using GRAFCET).

To demonstrate how this can be done, let’s consider the simple welding process
presented in Section 4.6. The services discussed there are to be used in “local or
manual mode” to adjust points, simulate the procedure, etc. They shouldn’t be used
in “automatic mode”, where the robot needs to react to sensor information and to
coordinating signals coming from the PLC managing the manufacturing line.
Figure 4.18 represents a possible manufacturing line where the simple welding
example was inserted. The system is composed by a conveyor belt that carries the
pieces to weld, a centering and holding pneumatic device that holds the pieces
during the welding procedure and a collection of position sensors, responsible for
detecting the piece to stop the conveyor and to verify if the work-piece is in
position after being trapped by the pneumatic cylinder. Only after the previous
tasks are completed is the robot commanded to perform the welding operation.
When the operation is done, the robot should signal the controlling PLC to free the
welded piece, re-start the conveyor motion and wait for another one to weld.