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FX-8500-1 Single

Loop Controller

"Cruise Control"

Foxcroft Equipment and Service Co., Inc.

2101 Creek Road

Glen Moore, PA  19343  USA

610-942-2888

FAX 610-942-2769

email sales@foxcroft.com

"Cruise Control"

Let's start by comparing a typical system to something everyone already familiar with... a car's cruise control.  In a typical car cruise control system, there are many components that make up the entire system as a whole, but we will only focus on those that compare to a typical chlorine control system.  Let's start with the care itself, moving along at a given speed.  The speed is what we are interested in controlling automatically.  This is our process.  In our chlorine system, this would be the chlorine residual in the water being treated.  To control the speed of the car, we first need to know how fast it is going, at any given second.  We use the speedometer reading as a signal input to the cruise control computer.  You may not be aware that there is one in your car, but there is.  We select the desired speed in the computer with a push-button on or near the steering wheel.  The output signal from the cruise control computer connects to a servo motor (or a vacuum positioner), which controls the accelerator, and thus the speed of the engine and transmission, and finally the speed of the car.  This is know as the "closed-loop" or "feedback" method of process control.  There are "open-loop" or "feed-forward" methods used in other types of process control, but

we will confine our discussion to the "closed-loop method.

Chlorine Residual Control

In our chlorine residual control system, much of the same functionality of the car cruise control system is implemented, using different equipment specific to our process.  We already know that the process we are interested in controlling is the chlorine residual level in the water.  We read the chlorine residual level, or the "speed" of our process, with a chlorine analyzer.  The signal from the analyzer is connected to a single-loop controller, where we select our desired chlorine residual level.  This would compare to the cruise control computer and speed selector push-button in the car.  The output signal from the controller connects to a chlorine gas pacing valve or liquid metering pump, which varies the amount of chlorine going to the water being treated, and thus the chlorine residual.  The chlorine residual is then picked up again by the chlorine residual analyzer, which closes the control loop.  This would be the servo motor and accelerator in the car, which varies the amount of gasoline going to the engine, and eventually the speed of the car. An important thing to  note here is that the analyzer must be installed downstream of the chlorine injection point to form the closed loop necessary for the system to work properly.  Some of the other components in our chlorine control system do not directly relate to the car cruise control example, but they either already inherit, or not needed  in the car.  With basic understanding of the chlorine control system, the importance of the additional will be more apparent.