Control Configuration

The ultimate in process control, the WaterFeature8 can give you eight points of analog data to make process control decisions with help from a Programmable Logic Controller (PLC).

The diagram below shows a WaterFeature8 deployment connected to a PLC.  The PLC makes decisions based on a ladder logic program, and it outputs control commands to compatible hardware.

The basic control process is a constantly iterative loop.

First, the sensors read the water quality parameters and send raw information to the EZO circuits on WaterFeature8.

Next, the WaterFeature8 interfaces with the EZO circuits, processes the values, displays the readings on the local LCD, and outputs the properly scaled values via 4-20 mA current loop to the PLC.

The PLC is constantly receiving the current signals and cycling the ladder logic program to make real-time decisions.  For example, the PLC might process the ladder logic program 500 times per second, and every cycle it performs a Proportional-Integral-Derivative (PID) calculation to determine an action.

 

Finally, the PID program in the PLC outputs a signal to control a driver.  For example, if the dissolved oxygen in the tank drops below 2.0 mg/L, the PID control will speed up the Variable Frequency Drive (VFD) that powers the blower motor.  As the DO rises back up to 2.0 mg/L, the PID control will slow the VFD back down.  In this manner the PID program and control loop continually maintain a user specified value of dissolved oxygen in the tank.

Then the process repeats itself in a continual loop.

This type of control scheme can be used to dose a chemical to maintain a target pH, open or close a valve to maintain a target flow rate, or trigger a heat exchanger to maintain the temperature in the tank.

In addition to the simple and complex systems, this type of  control configuration requires two new components:

  1. Programmable Logic Controller for processing analog signals, running a ladder logic program, and outputting control signals.

  2. Compatible driver hardware that accepts the PLC output signals and controls a device; this might be an actuated valve, a peristaltic pump, or a variable frequency drive.

Continue below for more description of these devices.

1.  Programmable Logic Controller (PLC)

The PLC analog input terminals are wired to the analog output terminals of the WaterFeature8 board.

A ladder logic program is written to do any number of control based activities.

The PLC output terminals send a control signal to a driver to control some aspect of the water treatment process.

DIY Control # 2 PLC.png
DIY Control # 3 Driver.png

2.  Driver Hardware

The driver is some sort of actuating device.  It might be a modulating valve, a dosing pump, a variable frequency drive, or a direct-drive pump, among many other types of devices.

The driver can also be a discretely controlled device such as an open-close valve, an indicator light, or a local alarm.

What can you do with 8?

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© 2019 by Integrated Treatment Systems, LLC.