Friday, March 4, 2011

The macro Steps of Grafcet Programming

The program that runs on the computer sends data referring to the operations which are to be executed in the memory buffer area. The terminal program is also used for displaying and controlling some of the machine parameters. The operator can adjust those parameters so the machine can function properly.

The PLC program is organized into sections. There is a dedicated section for determination of the malfunctions of the machine, a section for managing the working modes, a section the safety locks of the outputs and a section dedicated for the sequential processing Grafcet. The application PLC allows the graphical programming of the Grafcet.

Some PLC’s do not have this facility, the Grafcet section is programmed with specific instructions. Due to its flexibility Grafcet is widely used for programming PLC’s commanding robots, manipulators, flexible manufacturing systems.

The Grafcet section is organized in two pages and contains three independent charts. The sequential process was decomposed into elementary sequences. A hierarchical approach was used for some elements for allowing the decomposing of some sequences into more little entities. For each work mode there are steps which are active when that mode is inactive.

The homing of the axes is accomplished in the macro-step M2. This macro-step is active only when the operator requires this procedure and the conditions of the machine allow this operation. This macro step consists in the following steps:
• IN- step used for enable the speed drives. The condition transition to the following steps is done when the PLC receives the confirmation signals from the speed drives.
• 3 and 1 – in these steps the homing procedure is enabled for each axis. The dedicated module of the PLC commands the movement of each axis to a extreme position where the homing switches are activated by the corresponding cam. The homing procedure is completed when for each axis the Z signal of the incremental encoders is detected.
• 2- this step is used for the movement of the axis in the loading zone. The operator can use the pinchers for holding a piece.
• OUT – the output step, no actions. After the homing of macro-step is deactivated ad step 9 is activated.

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