Friday, March 4, 2011

Development of a Grafcet Chart

The machine has with two movement axes for positioning. The work piece is held by a set of hydraulic pinchers. Each axis is powered by a brushless Servo Motor equipped with encoder and tachometer. Each motor is controlled using a speed drive. The machine is equipped with a tool holder turret designed to hold five punches and one marking tool arranged at equal intervals of 60°. Similar with the tool holder turret there is a die holding turret. For each punch mounted in the tool holder turret, a matching die must be mounted in the die turret. The machining consists in the hydraulic punching.

The command system of the machine consists in a PLC produced by Schneider Electric. The sequential processing was programmed using Grafcet language. The PLC structure is modular. For the positioning task, the PLC is equipped with a dedicated movement module that controls the speed drives and acquires the signal from each incremental encoder.

The human machine interface of the machine is represented by an industrial PC and a set of conventional elements (push buttons, lamps and controllers). The industrial PC is used for running a program developed under the Visual C++ framework. Such architecture is detailed in [9]. The computer program uses an OPC data server to communicate with the PLC using a dedicated protocol. The program that runs on the computer sends to the PLC information about the working mode to be selected or the operations to be made and reads from the PLC data to be displayed to the operator and other information about the state of the machine. The terminal program is also used for displaying the state of the Grafcet chart. The service personal can determine the cause of a malfunction by visualizing the chart.

The machine can function in one of these modes: homing, manual and automatic. The homing mode allows finding of the measurement origin point via a specific procedure. If the homing procedure was not completed the movement of the axes can not be made, so the manual or automatic mode can not be selected.

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