The connection and differences between manual and automatic adjustment
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I study polymers and have no knowledge of DCS. I would like to know the relationship and differences between manual and automatic control adjustments, as well as when it is appropriate to use manual control and when to use automatic control
During stable operation, when the parameters change smoothly, use automatic mode. If there are large fluctuations and you want to stabilize them as quickly as possible, switch to manual mode for a while, but it’s still best to use automatic mode in most cases!
For example, suppose a temperature control valve is set to manual mode: once you set the opening degree of the valve, it remains at that level unless you change the setting. In automatic mode, you specify a temperature value, and the valve automatically adjusts its opening degree to maintain that specified temperature
Take the steam pneumatic valve as an example. When the steam pressure varies significantly, manual adjustment is used; the large rate of change achieved through manual control allows it to keep up with the fluctuations in steam pressure. When the steam pressure is more stable, automatic adjustment takes over – once a pressure value is set, the valve will maintain that value. If the steam pressure increases, the automatic control adjusts the valve to close it further, thereby keeping the pressure at the set level; if the steam pressure decreases, the valve is automatically adjusted to open wider. Automatic mode is used in situations where conditions are relatively stable or the degree of variation is small, as the rate of change in automatic mode is limited and does not allow for significant adjustments
Manual operation: This involves adjusting the opening degree of the control valve in order to regulate it. Automatic mode: This involves setting a parameter value, after which the control valve automatically opens or closes to a certain degree based on that parameter, thereby achieving regulation of the set value. It doesn’t matter what the original poster studies; read more books on instruments and you’ll understand.
The most fundamental difference lies in the contrast between the human brain and a computer; of course, computers still use mechanical mechanisms for adjustment, operating based on parameters designed by humans. They compare the detected input values with the preset values set by the user, and then output the appropriate adjustment amount to the actuator.
I believe there is no need to be too precise regarding whether control valves should be operated in manual or automatic mode. When a plant first starts up, fluctuations are usually significant. However, with too many control valves, it becomes impossible to manage them all. A few key control valves, such as those related to reaction temperature, must be controlled manually to ensure stability. For others, such as those related to pressure and liquid level, automatic operation can be used; but when doing so, it is important to have the instrumentation staff adjust the valve’s proportional, integral, and derivative settings (these are the parameters that determine the valve’s sensitivity, and the instrumentation staff know how to handle them), so that the valves can make the most effective adjustments in response to changes in the process conditions.
When production is stable, switch to automatic mode； When production fluctuates greatly (such as starting up, shutting down, increasing production capacity, reducing production capacity, or unstable supply of utility services), switch to manual mode.
Manual adjustment is no different from ordinary valves; the only difference is that it can be controlled remotely； In automatic control, it is essential to properly adjust the P, I, and D settings of the valve; otherwise, issues such as large fluctuations will occur.
The difference between manual and automatic control is that with manual control, you provide a pneumatic signal manually, and the on-site control will perform only one action; whereas in automatic mode, the pneumatic signal is continuous based on the values you set, ensuring smooth operation when in automatic mode； Switch to manual mode when production fluctuations are large
Manual operation is used in special situations, such as when starting the vehicle, when parking, or in case of process abnormalities. It usually operates in automatic mode; this is the main function of DCS.
Use manual mode during abnormal conditions such as startup and shutdown, changes in production volume or product types, and troubleshooting.
One should look into professional books on automatic control.
It’s not just the difference between the human brain and a computer; the main reason is that computers offer higher precision and stability in control, and they don’t slack off. :)