All feedforward control systems can be reduced to a common form that enables a better understanding and recognition that leads to the best performance and the best interface for the operator. For “smart controls” to be fully appreciated and utilized, the operator needs to know what is going on and how to participate. Here we show how to make the advantages of feedforward control more achievable and recognizable.
99% of the feedforward control systems for PID control in process industry actually involve ratio control where the numerators and devisors of the ratio are flow, energy, power, or speed. Consider the following examples:
- Blend composition control - additive/feed (flow/flow) ratio
- Column temperature control - distillate/feed, reflux/distillate, reflux/feed, steam/feed, and bottoms/feed (flow/flow) ratio
- Combustion temperature control - air/fuel (flow/flow) ratio
- Drum level control - feedwater/steam (flow/flow) ratio
- Extruder quality control - extruder/mixer (power/power) ratio
- Heat exchanger temperature control - coolant/feed (flow/flow) ratio
- Neutralizer pH control - reagent/feed (flow/flow) ratio
- Reactor reaction rate control - catalyst/reactant (speed/flow) ratio
- Reactor composition control - reactant/reactant (flow/flow) ratio
- Sheet, web, and film line machine direction (MD) gage control - roller/pump (speed/speed) ratio
- Slaker conductivity control - lime/liquor (speed/flow) ratio
- Spin line fiber diameter gage control - winder/pump (speed/speed) ratio
We have a desired ratio setpoint (e.g. feedforward gain) at a given process operating point based on process and operational knowledge. The primary process controller (e.g., composition, level, pH, or temperature) corrects this ratio that is then multiplied by the devisor (feedforward signal) and sent as the setpoint to a secondary controller (e.g., flow, speed, or power).
Besides understanding that fundamentally we are essentially dealing with ratio control corrected by a primary process controller, we need to give the operator an interface to foster recognition and participation. The operator must be able to see the actual ratio (corrected ratio) and be able to set the desired ratio. Almost always the operator must be given the ability to put the primary controller in manual and operate on ratio control by simply setting the desired ratio. This capability is critical during startup of many unit operations (e.g., distillation columns) and for composition control when an analyzer is providing extraneous values and during subsequent analyzer servicing, replacement, or recalibration. There must be a bumpless transition between manual ratio control and corrected ratio control. Process engineers also appreciate the interface because they know that most process variables important for process performance are a function of a ratio of an input to manipulated flow or speed. The scaling for the feedback correction must be readily observable and adjustable. I can’t emphasize enough how important this setup and interface is in terms of longevity and performance of the feedforward control system.
The desired ratio setpoint can be computed from a material or energy balance as detailed in the online White Paper "First Principle Process Relationships" and explored for different setpoints and conditions from a plot of the controlled variable (e.g. composition, conductivity, pH, temperature, or gage) vs. ratio of manipulated variable to the independent variable (e.g. feed) but is most often simply based on operating experience.
Plots are based on an assumed composition, pressure, temperature, and/or quality revealing why we need feedback correction by a primary controller. Here are some examples of assumptions:
- For concentration and pH control, the flow/flow ratio is valid if the changes in the composition of both the manipulated and feed flow are negligible.
- For column and reactor temperature control, the flow/flow ratio is valid if the changes in the composition and temperature of both the manipulated and feed flow are negligible.
- For reactor reaction rate control, the speed/flow is valid if changes in catalyst quality and void fraction and reactant composition are negligible.
- For heat exchanger control, the flow/flow ratio is valid if changes in temperatures of coolant and feed flow are negligible.
- For reactor temperature control, the flow/flow ratio is valid if changes in temperatures of coolant and feed flow are negligible.
- For slaker conductivity (effective alkali) control, the speed/flow ratio is valid if changes in lime quality and void fraction and liquor composition are negligible.
- For spin or sheet line gage control, the speed/speed ratio is valid only if changes in the pump pressure and the polymer melt quality are negligible.
The correction by the feedback controller for plug flow systems, sheets, extruders, and spin-line is best done by the controller providing the ratio (feedforward multiplier) because the input flow or speed multiplication compensates for the inverse relationship between the process gain and the input flow or speed. For well mixed volumes (crystallizers, evaporators, columns, neutralizers, and reactors), the decrease in feed flow increases the residence time and primary time constant which offsets the increase in process gain in terms of primary controller tuning. Here a bias correction of the input after multiplication by the desired ratio (feedforward summer) is best. The feedforward summer is also easier to scale and corrects for measurement drift and offset. A bias correction has a long history of being a robust correction of predictions of Model Predictive Controls, Neural Networks, and Inferential Measurements. The ratio setpoint can be optimized by simply adding an integral-only valve position controller whose controlled variable is the current bias correction, whose setpoint is a zero bias and whose output is the desired ratio.
For dynamic compensation of the feedforward signal, the input flow, speed, or power is simply passed through a dead time and lead-lag block. The objective is for the effect of the manipulated variable to arrive in the process at the same point and the same time as the feedforward input. If the input feedforward and manipulated variable enter the process at the same point (e.g. blend and reaction unit operations), a setpoint of a speed or flow controller is used for the feedforward input. The controllers for the feedforward input (e.g., leader) and the manipulated variable (e.g., follower) can be tuned for the same closed loop time constant. If pressure disturbances are a consideration, the controllers can be each tuned for the fastest response and a filter applied to each setpoint so that the changes in flow are synchronized. Unfortunately, this strategy often does not work well for neutralization because reagent flow can have a much longer injection delay than the effluent flow (injection delay that is dip tube volume divided by injected flow is incredibly large for conventional dip tube size and length and an extremely small reagent flow). Consequently the effluent flow setpoint must be delayed and the setpoint before the delay used as the feedforward for the pH controller.
For a glimpse of the enormous potential opportunity, see the March-April 2011 Intech feature article "Feedforward control enables flexible, sustainable manufacturing"