When the speed at which a continuous task is scanned changes, the frequency of how often the calculations inside the PID instruction is executed also changes. The ‘tuning’ of parameters for a PID loop is in fact the changing of variables for different calculations. This will have an adverse effect on PID loops. The speed at which this occurs may gradually decrease over time, as the project expands or may even drastically increase when the controller is upgraded to a faster controller. Once the task has finished executing, the whole process will be repeated. It will sequentially scan through all the assigned programs, as scheduled by the programmer. A continuous task is one that runs continually. This indicates that ‘MainTask’ is a continuous task. In the folder, a circular arrow can be seen. ‘MainTask’ is automatically created, hosting the ‘MainProgram’. When a new project is created in RSLogix 5000, the following basic configuration under the tasks folder is created automatically. This is a topic that needs brief attention before the actual programming occurs. Output %: The control variable’s value or controller output value (OP) is displayed here.Process Variable: The current value of the Analog input is displayed here.Setpoint: The desired value (SP), to where the process must be controlled is written to PID.SP and is displayed here.Control Variable: In this field, the Analog output tag is used where the output from the controller must write to the final control element.Process Variable: In this field, the Analog input tag is used to input the measured value (PV) that must be controlled.Example: LevelControl.SP to use the Setpoint member of the data type.
After this tag has been created, all members of the Data type may be accessed by using the tag together with a suffix.
The problem exists that there are multiple forms of human error that may occur, like:
After some experience, he may realize that the valve must be open to a certain point to maintain the desired level, with a constant outflow. When the tank level decreases below the desired level, he will increase the inflow by opening the valve and when the tank level increases to more than the desired level, he will close the valve. The means he has to his disposal to carry out this task is by opening and closing this valve. The instruction was given to him to maintain a desired water level in the tank. An operator visually monitors the level in the tank.
RSLOGIX 500 PID EXAMPLE MANUAL
This is an example of Manual Level control.