11.1 Introduction
This section will discuss actuators and regulators and their use for the control of
gas flow, liquid flow, and pressure control. In many processes this involves the control
of many thousands of cubic meters of a liquid or the control of large forces,
as would be the case in a steel rolling mill from low-level analog, digital, or pneumatic
signals. Temperature can also normally be controlled by regulating gas
and/or liquid flow. Control loops can be local self-regulating loops under pneumatic,
hydraulic, or electrical control, or the loops can be processor controlled with additional
position feedback loops. Electrical signals from a controller are low-level
signals that require the use of relays for power control or amplification and powerswitching
devices, and possibly opto-isolators for isolation. These power control
devices are normally at the point of use so that electrically controlled actuators
and motors can be supplied directly from the power lines.