Hach EC1000, Instruction Manual

Page: 49 / 128

49
SECTION 4 PID CONTROL
4.1 pH Controller Basics
A simple pH control loop is shown in Figure 6 on page 24. This same configuration and
procedure can be applied to ORP applications. There are endless variations to this basic
loop, but the main elements of the loop are shown. The loop operates as follows: The pH
meter measures the value of the pH in the effluent, and, if the pH is different from the
setpoint, the controller actuates the reagent pump (or valve) that adds reagent to a mixing
tank. The added reagent adjusts the value of pH.
The physical layout of the loop, the sizing of the pump (valve), type of mixing tank, and
location of the pH electrodes all have a major effect on the ultimate performance of the
loop, after the controller is tuned for optimal performance. The largest single performance
factor is the delay time around the loop. This includes the response time of the
electrode/meter, time required to deliver the reagent to the process water, time required for
the reagent to mix with and react with the process water, and the time required to deliver
the completely mixed water to the electrode. If the delay times are too long or the mixing
is not complete, the control will be poor regardless of how well the controller is tuned.
The Process pH/ORP Meter uses a PID control algorithm. PID stands for proportional,
integral (reset), derivative (rate) control. Each of the instrument settings (in the
CONTROL menu), along with their affects on the control loop, are described below.
4.2 Instrument Settings
4.2.1 Set Point
This is the value of pH (mV) that is desired.
4.2.2 Dead Band
This is the window around the set point where the controller takes no action (any input in
this window is considered to be at the set point).
4.2.3 Preset
This is the value of the proportional component of the controller when the input is at the
set point.
Example 1—
In an application where the pH of the process water is always above the set point, only an
acid feed is required. In this case, the preset is typically set to 0%. When the pH of the
water is at or below the set point, the controller output is at zero and there is no chemical
feed (this assumes the feed pump is set up to deliver no output for a zero input and
maximum output for a full scale input). When the pH of the water goes above the set point,
the controller output goes up, increasing the chemical feed, which drives the pH of the
water down.
Example 2—
In an application where the pH of the process water can be either above or below the set
point, both an acid and caustic feed is required.
4.3 Gain
Refer to Figure 8 for a diagram of the controller output response (proportional only) to a
step change on the input. The gain setting determines the instrument’s sensitivity to an
error (the difference between the set point and the input pH). If the gain is doubled, then
the output signal doubles for a given error signal. For a proportional only controller
(integral and derivative settings off), the steady state error decreases with higher gains, but,