Bimba PCS - Manual (Page 4)

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General Overview
The Bimba Pneumatic Control System (PCS) is a closed loop electronic controller with
pneumatic valves that can move the rod or shaft of a pneumatic position feedback
actuator and hold it in any desired position with accuracy and force. The PCS system is
designed for series PFCN, PFCNL, PFC, and PFCL cylinders for linear motion and the
Position Feedback Pneu-Turn (PTF) for rotary motion. The PFC should be ordered with
the -L option, low friction seals, for the best positioning accuracy and smoothest
operation. The standard PTF includes low friction seals.
The standard PCS accepts a 0 to 10 VDC or 4 to 20mA analog command signal, jumper
selectable (refer to the illustration on page 11). The command signal defines the position
the rod must move to. The rod stops when the feedback voltage from the probe is equal to
the command signal.
For example, if the application has a stroke of 10 inches and zero and span adjustments
are set for zero to ten inches as well, then a 1 volt change in the command voltage is
equal to a 1 inch movement. Similarly, a change in command signal of 0.005 of a volt
equals a position change of 0.005 of an inch for the same 10 inch stroke application. If
the application has a stroke of 5 inches, a change of 1 volt in the command signal
represents a half inch movement. For rotary applications, the convention is similar. If the
application has a rotation of 180 degrees, then a 1 volt change in the command signal is
equal to 18 degrees of rotation.
The system utilizes the feedback from the actuator to close the control loop. The control
loop compares the 0-10 VDC or 4-20mA input command signal to the feedback signal
from the actuator. The difference between the command and feedback is referred to as the
error term. When the error term is zero, all valves close, trapping air on both sides of the
actuator piston. (The error term is considered to be zero when it is within the deadband
range. The deadband is defined in the Glossary of Terms section.) This holds the rod at
its commanded position. If a force attempts to move the rod or shaft out of the
commanded position, the system will react by increasing the restoring force to maintain
position. Likewise, if the command signal changes, the system will respond to make the
feedback equal the command signal.
The actual accuracy/repeatability of the movements will depend on many
factors, including signal noise, load, velocity, supply pressure, supply
voltage, and application friction. Refer to the Application Sizing charts for
detailed information regarding sizing and suggestions for your application.