Many controllers have the facility to ramp their effective setpoint towards the final target value at a predefined rate. When the setpoint reaches the top of the ramp, a “soak period” begins where the setpoint is maintained at this value.
A deviation alarm is often used with this feature to check that the process is closely following the ramp.
Ramping protects a process from rapid changes in the setpoint and the resulting thermal shock as the controller tries to force the process variable to follow. This is especially useful if there is a power-cut, because it guides the rise back to the target setpoint when power is restored.
For example, if you set the ramp rate to 600°/hr and the setpoint to 400°C, and the current temperature is 100°C at power-on, the effective setpoint starts a 100° and rises towards 400° at 600°C per hour. A similar process occurs when switching back to automatic mode from manual control.
The exact implementation of setpoint ramping varies with the controller model. Some implement a ramp whenever the setpoint value is adjusted, others only do so when the active setpoint is changed (e.g. from local setpoint 1 to local setpoint 2). Also some models have an adjustable time for the soak, after which the control outputs are disabled. Others have an indefinite soak following the ramp.