The Control Period parameter is used in the PID control method, you can set or adjust the time between ON cycles of output. A shorter control period provides better control performance, but in the case of Relay outputs, shorter control period is not recommended because of the life time limitation at 100,000 times for Relay outputs. So, we will recommend setting the control period to 20 seconds as factory default to protect the Relay. Compared with Relay output, voltage outputs control allows shorter control period (factory default: 4 seconds) to reach accurate control performance, but cost will increase due to a SSR is needed for voltage outputs.