The motion - balance system is used in pneumatic instruments to produce a proportional output of 3 - 15 psig 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.