In process control, the term "mode" is used in two contexts:
Control modes. These are the proportional, integral, and derivative modes.
Operational modes. These are manual and automatic modes.
As illustrated in Figure above, the significance of the automatic/manual selection is as follows
Manual. The value for the controller output is specified by the process operator. The PID calculations are not performed. This mode is sometimes called "open loop" as the loop is broken at the auto/manual switch within the controller.
Automatic (or just "auto" ). The value for the controller output is the result of the PID calculations being performed for the purpose of automatic control. This mode is sometimes called "closed loop".
On switching the mode from manual to automatic, a smooth transition called "bumpless transfer" is desired; specifically, the value of the controller output the instant after the switch is to be the same as the value of the controller output just prior to the switch. This approach requires that the PID control equation be properly initialized or "balanced".
