Process Control Strategies: Cascade, Ratio, and Feedback Control
Process Control Strategies
Cascade Control
A cascade control system utilizes two process measurements, two controllers, and a single final control element. This architecture enhances disturbance rejection by employing a secondary loop to control the coolant temperature, thereby improving the primary loop’s control over the product stream temperature.
Ratio Control
A ratio controller maintains a specified ratio between two streams, typically used in blending applications. It adjusts the flow rate of a dependent stream based on the flow rate of an independent stream. This ensures consistent blending and provides safety benefits by dynamically adjusting fuel flow based on air flow changes.
Heat Exchanger Inverse Response
Heat exchangers can exhibit inverse response due to changes in flow rate. An increase in the warm oil flow initially raises the exit temperature due to faster flow but eventually lowers it as the cooler mixed liquid passes through the exchanger.
Process Identification and Testing
Step Test
Step tests involve stepping the controller output and observing the process response. While useful for fitting FOPDT models, they can disrupt normal operation.
Pulse Test
Pulse tests are two rapid step tests that minimize process disruption but only provide data on one side of the steady state.
Doublet Test
Doublet tests consist of two pulse tests in opposite directions, offering data both above and below the steady state while minimizing off-spec production.
PRBS Test
PRBS (Pseudo-Random Binary Sequence) tests use random pulses to generate dynamic process data with minimal deviation from the steady state. However, designing optimal PRBS experiments can be challenging.
Self-Regulating Processes
Self-regulating processes naturally reach a steady state when manipulated and disturbance variables are constant. Examples include gravity-drained tanks, heat exchangers, and distillation columns.
Feedback Control and PID Controllers
Controller Action
The sign of the controller gain (Kc) determines the controller action. Positive Kc requires reverse acting control, while negative Kc necessitates direct acting control.
P, PI, and PID Controllers
P-Only controllers are simple but can lead to offset. PI controllers eliminate offset using integral action but may increase oscillations. PID controllers add derivative action to reduce oscillations but introduce tuning complexity and sensitivity to noise.
Choosing the appropriate controller type and tuning its parameters is crucial for achieving optimal process performance.