Ratio control is used to ensure that two or more process variables such as material flows are kept at the same ratio even if they are changing in value. In industrial control, examples of ratio control that come to mind are:
- Burner air/fuel ratio.
- Mixing and blending two liquids.
- Injecting modifiers, pigments, etc., into resins before molding or extrusion.
- Adjusting heat input in proportion to material flow.
Proportional mixer, where a controlled stream of blown air entrains the gas and carries it to the burner. The gas at the point of entrainment has been reduced to atmospheric pressure by a zero governor.
Cross-connected, where the combustion air pressure is piped across to operate the gas valve.
- Linked valves, where the temperature controller drives a single actuator that moves both air and fuel valves in unison.
In the face of varying pressures, nonlinearity of valve flow coefficients, etc., none of the above can guarantee accurate tracking of the streams. Some energy losses due to excess air or excess fuel have traditionally been tolerated in exchange for lower priced, simpler control equipment. You should review this cost tradeoff arithmetic as energy prices change.
Where the mass-flow true ratio system is used for fuel/air mixing, the system is as in the liquid blending system described below.
Mixing and Blending Two Liquids. Figure 1 shows two streams, wild flow (F1) and controlled flow (F2), that converge to make the blended flow. (The wild flow may be controlled somewhere upstream and is not necessarily wild). The controller's job is to drive F2 to a value that assumes the chosen preset ratio to F1.
Adding Modifiers to Plastics. A variety of additives find their way into the virgin polymer stream to aid the extrusion or molding process and to meet the specifications of the final product. They usually come in powder or granular form, so they are not amenable to liquid blending techniques. Gravimetric (loss of weight) feeders or hoppers, which deliver simultaneously the additives and polymer, are usually the most consistent and accurate.
The feeders' weights are continuously monitored and they need to be refilled periodically depending on the feed-rate and size of the hopper. Refilling is achieved as required from a refill hopper directly above the feeder. Although the feeder maintains a continuous feed into the extruder, it must be refilled periodically and uses computer control to take account of the continuous weight-loss feed to the extruder during a refill (whereas the feeder is actually gaining weight).
Competing claims are made for the system of augers delivering additives directly into the extruder between the main hopper and the barrel. This volumetric delivery depends on the bulk density being consistent or taken into account. Bulk density can vary with size and shape of pellets and powder grains.
Matching Power to Material Flow. Figure 2 shows an open-loop control system that keeps a fixed ratio between fluid flow and heater power delivered to the fluid. Its performance depends on linearity of the flow transmitter signal and a linear delivery of output power from the controller in obeying this signal. The fluid stream is the wild flow here -- it can do what it likes and power will respond. Given a stable inlet temperature, this arrangement will keep the fluid exit temperature reasonably constant. It can therefore be used as the feedforward component of a temperature control loop, relieving the temperature controller of the job of chasing wild temperature swings.