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Design and testing of a electrovalve for a BCD

The BCD (Buoyancy Control Device) is an accessory used for scuba diving that by varying its volume allows its user to adjust your buoyancy in water by acting directly on the buoyancy that it receives. The adjustment mechanism is simple: the BCD is inflatable and then acting on the internal air pressure it is possible to change the volume and thus the resultant buoyancy. Since the thrust realized depends on the volume of the same BCD, to maintain a constant attitude is indispensable adapt the inflation pressure, in order to compensate the external pressure variations due to the depth that tends to press the BCD reducing its volume. In particular, the hydrostatic pressure is proportional to the depth, with an increase of about 0.1 bar per meter. The use of the BCD and the buoyancy regulation require great caution, especially for an inexperienced subaqueous; in fact, if the BCD is swelled excessively or the subaqueous goes back quickly without worrying about to reduce the inflation pressure may occur the so-called pallonamento: due to the decreased external pressure, the air inside the BCD may expand thereby increasing the volume and therefore the thrust which consequently tends to further accelerate the ascent towards the surface with all the risks that a too rapid decompression of the sub can cause.

To allow the operator a rapid deflation of the BCD, even in "potentially critical conditions" this is provided with a series of quick exhaust valves, manually operable. Although there is no complete standardization in terms of dimensions and mechanical interfaces of the device, the design concepts are substantially always adopted the same. Consequently the discharge valve, normally consists of the following elements:

1) disc with rubber seal: ensures the seal effect of the BCD;

2) preload spring: ensures the maintenance of the seal until the maximum pressure;

3) cord with knob: allows the subaqueous to pull out the disc, winning the preload of the spring in order to cause the discharge of the BCD.

4) valve body: separable usually into two parts assembled by means of threaded connection.

MDM Team and the University of Florence evaluated the possibility to integrate within a generic commercial exhaust valve an Antipallonamento device able to intervene by opening the discharge valve of the BCD in the event of too rapid ascent. This activity was carried out for Applicon SRL (www.applicon.it) under the SARIS project. The regulator scheme is relatively simple: the external pressure is proportional to the depth consequently the variation of pressure with respect to time is a rough estimate of ascent velocity, assuming known and constant the density of water ρ and the gravity acceleration g:

 

Therefore by evaluating the derivative of the ascent velocity if this exceeds a suitably calibrated threshold value, the controller acts on the valve controlling the opening. To make the system more secure you could incorporate within the device at least one MEMS triaxial or capacitive accelerometer, the use of the triaxial device is essential because the orientation of the device relative to the vertical is not necessarily assigned. Once evaluated the acceleration and eliminated the steady contribution due to gravity it can be achieved by integrating an estimate however approximate of the velocity. Really it is a redundant measurement and affected by errors, for which the adoption of an estimation filter is more convenient (for example a Kalman filter), which provides where the estimator filter implements the Data Fusion necessary for a correct estimate of the ascent velocity.

 

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