Miscellaneous

Control Components

Torsionmeters

Angular Displacement

Torsion meters are used for the measurement of power transferred through a propulsion shaft.

Principle


A torque of value T is applied to a shaft of fixed length L and radius r. An angle of twist θ is generated and is dependent of the modulus of torsional rigidity G and given by


T/r = Gθ/L


The modulus of rigidity, the radius and the length of the shaft are all fixed thus the torque on the shaft is proportional to the angle of twist



Typical system


Two AC generators are mounted so that they are driven by the main shaft and area at set distance apart L. A sinusoidal waveform is produced. One of the generators is adjusted so that at minimum torque the generated waveforms are 180' out of phase. The outputs from the two generators are then added and the resultant voltage is used as the measurement of torque


As the torque is applied to the shaft so the twist causes the waveforms to shift in phase. When the two waveforms are now added an output ac current is produced which may be amplified and rectified to give an output voltage proportional to the torque applied to the shaft.


Another method of achieving this is to replace the generators wit sensors and toothed ring.

Power Calculation

Power is a product of the Torque and revs of the shaft, one of the generator outputs is used to measure the shaft rev/s and a calculation performed

Magnetic Stress Sensitivity

This type measures magnetic fields in the shaft surface, the distortion of these fields gives and indication of the torque. The principle behind this is that in some ferromagnetic materials reluctance ( magnetic resistance) is less along the plane of stress than across it.

In the torductor three rings are fitted around the shaft each ring having four electromagnetic poles. The centre ring acts as a transformer primary with the two outer secondary rings having their poles arranged 45' apart to the primary poles but in line with each other. The poles are held in a frame so that there is no contact between the poles and the shaft which have a gap of about 3 mm between them.


An alternating current is fed to the centre ring thus generating a magnetic field. This educes an emf in the outer two rings. The outer two ring coils are connected in series in such away that at zero stress the emf generated in each ring is opposite and equal in value giving an output of zero volts.


When torque is applied to the shaft the stress lines are distorted to to the axial. The distortion of this field affects the emf induced in the coils increasing on one side and reducing on the other. Thus a resultant emf of a few milivolts is available at the output. The size of the output is proportional to the stress applied.


Diaphragm operated control valve


Hand jack

May be used either for local manual control in the event of signal failure. Or it may be used to prevent or limit opening of the valve

Valve glands

For accurate valve positioning it is important to keep friction to a minimum. The largest source of friction is the valve gland. For steam valves using asbestos type packing a suitable lubricant must be applied. A better solution takes to form of a pack of several v seals made from teflon. Fins may be cast or machined into the gland housing for cooling

Fail safe- air to open/close

The control valve may be so designed to fail in either a full open or full closed direction. In addition the pneumatic signal may open or close the valve


Butterfly valve


Consists of a short cylinder generally rubber lined both for corrosion protection and to act as a seal for the disc. The disc forms the closing device and is locked to a spindle. The spindle gland generally takes the form of an o-ring or v-seals.


The torque acting on this disc is caused by the dynamics of fluid flow across it. When the valve is partially open the flow across the top half is much smoother than the bottom. This results in a torque acting on the driving spindle


The torque rises from zero to a peak when the valve is in mid position and falls again to zero . This non-linearity makes for difficult accurate positioning. To overcome this it is normal to fit oversized actuators. The flow through the valve is also very non linear rising quickly after initial opening and near full flow being achieved before the valve is 50% open


Valve Positioners

Valve positioners are used on controlling valves where accurate and rapid control is required without error or hysterises.


Advantages



Assembly

A valve positioner consists of a very high gain amplifier- this may be pneumatic, electropneumatic etc, and a feed back link which detects the actual position of the valve.


The required movement is for the valve to close. The input pressure from the controller to the bellows falls. The flapper moves away from the nozzle and the pressure after the orifice falls. The pressure to the diaphragm falls and the valve begins to close. The feed back arm moves up rotating the cam clockwise. This raises the beam increasing back pressure in the nozzle until equilibrium is again achieved.


The change over cock allows the signal from the controller to be placed directly on the diaphragm



Force balanced Valve Positioner


Motion balanced Valve Positioner