Generally, to understand these types of control, you must start with the elements of control. These elements are considered the control loop. One of those elements is the actuator. Below, the differences between On/Off, Floating, Two Position and Modulating Control in respect to actuator control are explained.
ON/OFF control is the basic type of control in a control loop. There are two variations with ON/OFF control. In many cases, ON/OFF control is the standard terminology used, but it can also be referred to as FLOATING control. Floating is typically called THREE POSITION control (open/stop/close or on/stop/off). Two digital outputs are used (DO-1 opens and DO-2 closes the actuator and if there is no signal, the actuator stops and maintains its present position.
A less common ON/OFF control is where there is only one digital output (DO-1). This is referred to as TWO POSITION control. This type of control is used with failsafe actuators. For example, when DO-1 is disabled, the actuator closes and when DO-1 is enabled the actuator opens. The digital output (DO) must stay energized or de-energized to maintain the actuator in its desired state.
Modulating control provides a higher degree of positioning control. In modulating/proportional control, the output to the actuator varies continuously and is not limited to being fully open or fully closed. Popular modulating control signals include 4-20mA and 2-10V.