# Floating action

Floating control is so called because the final control element floats in a fixed position as long as the value of the controlled variables lies between two chosen limits. When the value of the controlled variable reaches the upper of these limits, the final control element is actuated to open, say, at a constant rate. Suppose that the value of the controlled variable then starts to fall in response to this movement of the final control element. When it falls back to the value of the upper limit, movement of the final control element is stopped and it stays in its new position, partly open. It remains in this position until the controlled condition again reaches a value equal to one of the limits. If the load alters and the controlled variable starts to climb again, then, when the upper limit is again reached, the final control element will again start to open and will continue to open until either it reaches its maximum position or until the controlled variable falls again to a value equal to the upper limit, when movement will cease. If the load change is such that the controlled variable drops in value to below the lower limit, say, then the final control element will start to close, and so on. Thus, the final control element is energised to move in a direction which depends on the deviation; a positive deviation gives movement of the element in one direction and a negative deviation causes movement in a reverse direction. There is a dead or floating band between the two limits which determines the sign of the deviation.

Figure 13.6 illustrates an example of floating control. A room is ventilated by a system as shown. The capacity of the LTHW heater battery is regulated by means of a motorised two-port valve, Rl, controlled by floating action from a thermostat, Cl, located in the supply air duct. There is negligible lag between Cl and Rl. It is assumed that the variation in the state of the outside air is predictable and so the response rate of the controller can be properly chosen.

When the load is steady, the valve is in a fixed position, indicated by the line AB in Figure 13.6(6), provided that the supply air temperature lies between the upper and lower limits, as indicated by the line AA’, in Figure 13.6(c). Under this steady-state condition, the capacity of the heater battery matches the load. If the outside air temperature increases, but the valve position remains unchanged, the supply air temperature starts to rise, as shown by the line A’B in Figure 13.6(c). When the air temperature in the supply duct attains a value equal to the upper limit (point B), the valve starts to close (point B in Figure 13.6(6)). The rate of closure remains fixed, having been previously established during the initial process of setting up the controls, the battery output is continuously decreased, and the supply air temperature, after some overshoot, starts to fall. When the temperature reaches the upper limit (point C in Figure 13.6(c)), the movement of the valve stops (point C in Figure 13.6(6)). Suppose that this new valve position (line CD in Figure 13.6(6)) gives a heater battery output which is less than the load. The supply air temperature will continue to drop, until the lower limit is reached at the point D in Figure 13.6(c). This will then make the valve start to open (line DE, Figure 13.6(6)) and, after some undershoot, the

 R1

 (a)

 Supply air-«— ■ C1 © •*— Outside air

Conditioned room

 Time

 Y Overshoot I Positive deviation B/ C (valve closing) Upper Limit
 Time

(c)

Fig. 13.6 Floating action.

Supply air temperature will start to climb, passing the value equivalent to the point E, Figure 13.6(c), and stopping the opening movement of the valve. The battery may now have an output that will permit the supply air temperature to be maintained at some value between the upper and lower limits. If this is the case, then the air temperature will climb but will level off, along a curve EE’, the valve position remaining constant. The line EF in Figure 13.6(6) will then indicate the fixed valve position that gives a steady value of the supply air temperature, corresponding to the line E’F in Figure 13.6(c).

This example is intended only to describe floating action. It is not intended as a recommendation that floating control be applied to control ventilating systems. In fact, for plenum systems in particular, the form of control is unsuitable; apart from the question of lag, load changes are so variable that although the speed at which the valve is arranged to
open may be quite satisfactory during commissioning, it may prove most unsatisfactory for the other rates of load change which will prevail at other times.

The slopes of the lines BC and DE are an indication of the rate at which the valve closes and opens. With single-speed floating action this rate is determined once and for all when the control system is set up. One method whereby a choice of speed is arranged is to permit the valve to open intermittently with short adjustable periods of alternate movement and non-movement. If the timing device is set to give short periods of movement and long periods of non-movement, the slope of the lines BC and DE will be small.

Sophistication may be introduced to floating control, the speed at which the valve opens and closes being varied according to the deviation or to the rate of change of deviation, in the same way that proportional control may be modified, as described in section 13.11. If the dead band is tightened, the control tends to become less stable, degenerating towards two-position.

Posted in Engineering Fifth Edition