Which type of control is used when a specific blend of two different materials is necessary?

Ratio control is specifically designed for applications where a precise blend of two or more materials is necessary. This method involves maintaining a consistent ratio between different inputs to achieve the desired mixture. In the context of blending materials, the control system continuously monitors the flow rates of each material and adjusts them in real-time to maintain the set ratio, ensuring that the final product meets quality standards.

This type of control is particularly important in industries like chemical processing, food production, or any application where the quality and characteristics of the final product depend heavily on the correct proportions of its components.

Cascade control, on the other hand, involves two levels of control—where a primary controller sets a set point for a secondary controller, but it is not specifically aimed at blending ratios. Feedback control primarily adjusts output based on feedback from the process, which is more about correcting errors rather than maintaining specific ratios. Feed forward control anticipates the effects of changes in input variables and adjusts the output preemptively, but it doesn't focus on maintaining specific blending ratios.

Therefore, ratio control stands out as the correct choice for scenarios where blending two different materials needs to be tightly controlled for consistent results.