Panasonic PLC Filling Control -Application Note-

October 03, 2011
By William A Nieves- Technical Support Supervisor

PLC Filling Control

In this application, a valve must be open for some period of time, and close exactly when the tank is full- not before or after- to avoid waste (Figure No.1). This requirement implies accuracy through the filling process. A classic solution is to use a PID algorithm. However, the valve’s mechanical lifetime could be compromised when the PID keeps the valve opening and closing frequently.

Panasonic Technical Support presents two options that can be used to omit the PID program. The valve could be controlled by a PLC using an analog or digital signal. In the case of an analog signal, the PLC could use a voltage level to keep the valve full open  , close (0 V) or in between . For the digital approach, it is possible to estimate the total filling time and mass flow rate, knowing the tank volume and using a sensor with one PLC discrete input.

1. Analog Control 

Figure No.2 shows the valve’s states profile. The PLC will use a voltage level as a control signal.

The voltage  is the output of a Digital to Analog converter (e.g. PLC application cassette AFPX-DA2 –Figure No. 3- ). The implementation of these formulas in a Panasonic PLC is showed next (e.g. structure text -FPWINPRO PLC programming software).

2. Digital Control

A level Sensor could be used to estimate the mass flow rate, which is critical to calculate the time when the valve will be close. Figure No. 5 shows a section of the tank during the filling process.

At   the filling process will start, and the valve is open. The Sensor Level A will provide a control signal to a timer used to measure the process time at Sensor Level A (  –This is the variable level_A_time in the PLC program showed bellow-). This time can be used to calculate the actual flow rate since you know the tank volume at this time:

With the flow rate and the tank volume   you can estimate the total filling time  :

Finally, you can set a timer that will turn off the valve (valve close), the set value of this timer is:

PLC program implementation:


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