Level Control Experiment
Proportional Only Control Answers
You should have noticed two main features of proportional only control
- Increasing the proportional band decreased the amount of oscillation in the process variable (ie increased the "damping" in the system)
- As proportional band was increased, so the process was controlled at a point increasingly offset from the original setpoint.
The effect of increasing proportional band is summarised in more detail below :
- At very low proportional bands the level in the tank tended to oscillate greatly. In response to this, the valve position increasingly oscillates and would tend towards on-off control when the valve oscillates between the fully open and fully closed position.
- At low proportional bands, the process was prone to oscillate before returning to a steady state and this oscillation in process variable tended to be away (offset) from the setpoint.
- At very high proportional bands, the process was quickly returned to a steady state and with very little oscillation but at a point significantly different from the original setpoint.
The actual significance of the proportional band can be understood in relation to the range of the measurement sensor (here the level sensor) and the range of operation of the control valve. A proportional band of 100 indicates that the control valve will move over 100% of its range for a 100% measurement sensing range.
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