The Motion-Balance System used in Pneumatic Instruments ~ Ourengineeringlabs

The Motion-Balance System used in Pneumatic Instruments

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The motion - balance system is used in pneumatic instruments to produce a proportional output of 3 - 15 psi when a corresponding input signal is applied to the instrument. A typical motion-balance system in a pneumatic instrument is shown below:

The above diagram shows a motion-balance system in which the input motion is applied to point P on the lever PQ. The opposite end (Q) of this lever is pivoted to a second lever QRS which in turn has point S pivoted in a lever positioned by movement of the feedback bellows.
At the center (R) of the lever QS there is a stem on which one end of the lever RTU is supported while it is pivoted at point U and has a flapper nozzle sensor located at point T. A horizontal displacement which causes P to move to the left is transmitted via Q to R, and as a result the flapper at T moves off the nozzle so that the back pressure falls. This change is amplified by the pneumatic relay so that the pressure in the feedback bellows falls and the lever carrying the pivot S moves down until equilibrium is re-established. The output pressure is then proportional to the original displacement. By changing the inclination of the lever RTU sensitivity or gain of the system may be changed.
The motion-balance principle has been applied successfully in pneumatic temperature measuring devices while the force-balance principle is common pneumatic differential pressure transmitters, pressure, flow and level instruments.
Motion-balance principle:
Virtually all pneumatic instruments used for measuring process parameters such as pressure, flow, level and temperature use a flapper/nozzle sensor and pneumatic relay . In real pneumatic instruments, there are two basic schemes for utilizing the flapper/nozzle/pneumatic relay system, namely, the motion-balance and the force-balance systems.

As shown above, when an input pressure signal, is applied to a bellows which is opposed by a similar bellows for the reference signal, the differential force applied to the lever supported on an adjustable pivot is opposed by a spring/bellows combination. Adjacent to the bellows is a flapper/ nozzle sensor.
During operation, if the input signal exceeds the reference signal, the resultant force causes the force bar to rotate clockwise about the adjustable pivot so that the flapper moves closer to the nozzle, with the result that the pressure in the output bellows increases until equilibrium of the force bar is re-established. The change in output pressure is then proportional to the change in the measurement signal and a standard 3 - 15 psi signal is generated.
Practically all pneumatic measuring systems depend on a primary element such as an orifice plate, Bourdon tube, etc., to convert the physical parameter to be measured into either a force or a displacement which, in turn, can be sensed by some form of flapper/nozzle sensor or used directly to operate a mechanism such as an indicator, a recorder pen, or a switch. The measurements most widely used in the process industries are temperature, pressure, flow, level, and density

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