Walchem's WPH Controller offers software algorithms for time-proportional control. |
First, a quick review of the first two common methods. Proportional analog is the most common control signal. This signal is often sent to a modulating control valve (positioner), variable speed pump (centrifugal or diaphragm metering), or to a variety of mechanical control options such as speed of a belt drive, etc. The premise is straightforward – the signal value from 4-20 mA creates a direct, proportional output response (0-100%) of the control device.
In basic proportional only control, the signal value is calculated from a linear relationship based on the variance from set-point with a maximum output set at a fixed maximum variance. A common example is in pH control where the output to an acid metering pump will vary from 4-20 mA as the pH increases from 8-10 pH. At 8 pH or less, 4 mA is sent to achieve 0% output of the pump. As the pH increases to 10 pH, the output will increase linearly, until at 10 or greater pH, 20 mA will be sent to the pump to achieve 100% output.
In more sophisticated control scenarios, the algorithm to determine the output value may incorporate Integral and possibly Derivative calculations (PI, or PID Control) to enable a more predictive output based on the speed of the process response to the output. Even with PI or PID control, the actual output signal remains a 4-20mA resulting in a proportional 0-100% response of the control device.
Proportional Pulse control is used specifically for proportional control of solenoid driven metering pumps. A controller varies a pulse output based on deviation from setpoint, similar to Proportional analog control. Using our same acid feed control example as above, the controller will send 0 pulses per minute to the metering pump at pH 8 or less, increasing the pulse/minute output proportional until at pH 10 or greater, the maximum pulse output will be generated. The metering pump will stroke one time for each pulse it receives, enabling the increase in pH to cause an increase in the volume output of the metering pump.
Time-Proportional Control
Time-Proportional control is a less widely used method for achieving proportional control, and has the advantage that it uses a lower cost on/off control device such as a solenoid valve, or fixed output pump. By proportioning the on-time vs. off-time of the control device within a fixed time period (sample period), a proportional response is achieved. The off-time portions of control provide an additional benefit by enabling better mixing of the process to occur, or time for reactions to take place.
The parameters used to program a time-proportional output include the sample period, the set-point, the proportional band, and the control direction. The set-point is the desired pH of the system; while the control direction determines whether the output will increase above the set point (often called a High Set-Point) or below the set-point (Low Set-Point).
The sample period should be set to approximately 1½ times the amount of time that it takes for the system to react to the chemical addition. This can be determined by making a manual addition of chemical and timing how long it takes for the process to react. Setting the sample period too low will result in a second addition being made before the first is detected and will cause set point overshoot. Setting the sample period too high will delay the next addition and can prevent the set point being reached.
Finally, the Proportional Band is the deviation from set point that will result in a 100% on time of the output. Returning to our acid feed pump example for pH control, we could set a sample period of 10 minutes, a High Set-Point of 8.0, and a Proportional Band of 2 pH (from 8 to 10). At 8 pH or below, the output would be off 100% of the time. At pH 10.0 or above, the output would be on for 100% of the next 10 minute sample period. At 9 pH, the output would be on for 5 minutes, then off for 5 minutes. The percentage of on-time of the 10 minute sample period increases proportionally as the process moves away from the setpoint. The actual on-time can be calculated as follows: