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The first part is to design those kinds of standard circuits for each step,the next task is to link the standard circuits and the last part is to connect the control elements that receive signals from sensors, switches and the previous movements,and give the air or electricity to the supply lines of each step. The standard circuits are drawn for pneumatic and electro-pneumatic system.It is possible to see the relations with the previous and the next steps.
3.the method applied inside the controller
The result of the method presented before is a sequence of movements of the actuator that is well defined by stepas .It means that each change on the position of the actuators is a new state of the system and the transition between states is calld step.
The standard circuit described before helps the designer to define the states of the systems and to define the condition to each change between the states. In the end of the design,the system is defined by a sequence that never chances and states that have the inputs and the outputs well defined.The inputs are the condition for the transition and the outputs are the result of the transition.
All the configuration of those steps stays inside of the microcontroller and is executed the same way it was designed. The sequence of strings are programmed inside the controller with 5 bytes;each string has the configuration of one step of the process There are two bytes for the inputs,one byte for the outputs and two more for the other configurations and auxiliary functions of the step.After programming,this sequence of strings is saved inside of a non-volatil Memory of the microcontroller,so they can be read and executed.
The controller task is not to work in the same way as a conventional PLC,but the purpose of it is to be an example of a versatile controller that is design for an specific area.A conventional PLC process the control of the system using a cycle where it makes an image of the inputs,execute all the conditions defined by the configuration programmed inside,and then update the state of the outputs. This controller works in a different way,where it read the configuration of the step,wait the condition of inputs to be satisfied,then update the state or the outputs and after that jump to the next step and start the process again.
It can generate some limitations,as the fact that this controller cannot execute, inside the program,movements that must be repeated for sometime,but this problem can be solved with some external logic components.Another limitation is that the controller cannot be applied on systems that have no sequence.These limitations are a characteristic of the system that must be analyzed for each application.
4.Characteristics of the controller
The controller is based on the MICROCHIP microcontroller PICF877[6,7]with 40 pins,and it has all the resources neened for this project.it has enough pins for all the comonents,serial communication implemented in circuit,EEPROM memory to save all the configuration of the system and tne sequence of steps.For the execution of the main program,it offers comlete resources as timers and interruptions.
The list of resources of the controller was created to explore all the capacity of the microcontroller to make it as complete as possible.during the step,the program chooses how to use the resources reading the configuration string of the step.this string has two bytes for digital inputs,one used as a mask and the other one used as a value expeced.one byte is used to configure the output or time-out.the EEPROM memory inside is 256 bytes length that is enough to save the string of the steps,with this characteristic it is possoble to save between 48 steps(table1).
The controller has also a display and some buttons that are used with an interactive menu to program the sequence of steps and other configurations.
4.1. Interaction components
For the real application the conteoller must have some elements to interact with the final user and to offer a complete monitoring of the system resources that are available to the designer while creating the logic contrl of the pneumatic system.