Controller and control method

A control method of a control system comprising:

• a first controller executing control calculation needed for process control for a plant and performing the process control by reflecting the executed control calculation in the process control;
• a second controller forming a redundant controller with the first controller, and
• an external device connected to the second controller via a network and executing control calculation of the second controller,
• wherein the second controller does not have any control calculation function which executes the control calculation needed for the process control for the plant in the second controller, the control method including:
• switching, by the first controller, states of the first controller between a plurality of states including an active state where a result of control calculation is reflected in process control and a standby state where the result of the control calculation is not reflected in the process control;
• and
• switching, by the second controller, states of the second controller between the plurality of states including the active state and the standby state, and communicating with the external device,
• wherein
• the switching further comprises switching, by the second controller, the states of the second controller to the active state by obtaining the result of the control calculation executed by the external device and reflecting the obtained result of the control calculation in the process control,
• wherein the control method further includes performing by the second controller the process control only by (i) receiving the control calculation from the external device and (ii) reflecting the received control calculation in the process control,
• wherein the plural states include a stopped state where the result of the control calculation is not obtained, and
• the second controller comprises a redundancy management unit configured to switch the second controller between the standby state and the stopped state at any time when the first controller is in the active state,
• the control method further comprises:
• by the redundancy management unit of the second controller, detecting, an abnormality of the first controller, and sending an abnormality notification to the external device to cause the external device to stand by for processing,
• determining whether the second controller is able to be switched to the active state to shift control from the first controller to the second controller,
• when the answer is affirmative, switching the second controller to the active state, and
• notifying the external device of control resumption to cause the external device to resume the control calculation.