|
|
USEFULL
2009-12-29
P-control, P-band
P-control stands for proportional control; that is to say, a change at the sensor bears a certain relation to change in actuator. The magnitude of the movement of the actuator is determined by the amplification F. A small amplification results in a small movement for a given change, while a large amplification results in a large movement for the same change. However, the amplification F is not usually used in terms of comfort: instead, we refer to a P-band. The P-band is equal to 1/F (%). The P-band can also be expressed as the temperature change required for the actuator to move from the closed to the open position.
The P-band value is then specified in °C. One example of P-controllers is automatic thermostat valves fitted to radiators. When the temperature in the room drops, the valve opens to the corresponding extent. These valves usually have a P-band of 2°C; that is to say, a change in the room temperature of 2°C is required for the valve to open fully, which means that the temperature in the room will vary within these 2°C. This is know as P devation. It should then be possible for the P-band to be reduced in order to achieve a more even temperature, but the system would then become unstable; that is to say, the valve would start to open and close continuously, with a fluctuating temperature (increasing and decreasing) as a result.
| Large amplification means a small P-band |  |
| Small amplification means a large P-band |  |
P-controllers are sometimes used for the ventilation purposes in order to maintain a constant temperature, i.e. supply air temperature. The P devation then results in an undersirable temperature variation. If no P devation is required, it is possible instead to use a controller containing an integrator so that PIcontrolling is achieved.
Ever yours
NIT
|
|
