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Overview / Design Principle:
This instrument is a recorder which utilizes a potentiometer-type contolling mechanism which provides automatic temperature programming by use of a pre-cut plastic cam. It governs proportional positioning motors which operate modulating valves, damper systems, or electric multi-state systems where extremely close sensitive or straigh-line control is required. The instrument automatically positions any of a wide variety of standard motor operators to provide precise temperature control without the sawtooth line characteristics of conventional on-off control. CSA listed.
Operation:
Control point of the RFCP is determined by a cam-follower which rides the edge of a revolving pre-cut cam and postiions the potentiometer in accordance with the program for which the cam is shaped. Cam is configured and fabricated by end user.
The pen arm, which moves up or down scale in response to the expansion and contraction in the thermal sensing element, also slides the contact finger along the potentiometer coil within the modulating range. In essence, the coil forms half of a Wheatstone bridge circtui, while the other half of the bridge is formed by a potentiometer of similarly electric characteristics built into the proportioning motor and driven by the motor shaft. A detector relay, either incorporated into the proportioning motor or operating as a separate unti, detects any imbalance in the Wheatstone bridge caused by a change in control temperature and drives the proportioning motor in the direction necessary to regain a balanced bridged circuit.
The shaft fo teh proportioning motro is connected through linkage to the device which controls the process fuel flow or temperature variable. When the contact finger is located at the low end of the potentiometer coil, the motor drives the device to the fully open position. When the contact finger at the high end of the coil, the moteor moves the divide to the fully-closed position.
Resistance of the potentiometer coil to the Model RFCP should match that of the coil in the positioning motor.
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