RELIANCE 57552-4 logic card

(4 to 20) mA Position Feedback Devices There are a variety of position feedback devices available having a (4 to 20) mA current output. When the SPC is configured for a (4 to 20) mA transducer, an external 18 V (dc) power supply will be required to power the transducer. The output of the transducer need not equal exactly 4 mA and 20 mA at the ends of the actuator stroke, because the SPC will be calibrated to the transducer. The transducer current does not have to increase with position, for example, the output could be 20 mA at minimum position and 4 mA at maximum position.

Position Feedback Devices

 The SPC accepts position transducers with dc voltage outputs. The exciter output can be configured to provide 18 V (dc) at the exciter output for powering transducers that require an external power source.The output of the transducer need not equal exactly 0 V (dc) and 10 V (dc) at the ends of the actuator stroke, because the SPC will be calibrated to the transducer. In fact, it is desirable for the output of the transducer to not reach 0 V (dc) to facilitate checking the transducer health via the Feedback 1 Range Fault or Feedback 2 Range Fault The transducer voltage does not have to increase with position, for example, the output could be 8 V (dc) at minimum position and 2 V (dc) at maximum position.

RVDT and LVDT Information

 (Figures 4-27 through 4-30) LVDTs (Linear Variable Differential Transformers) and RVDTs (Rotary Variable Differential Transformers) are transformers whose output voltage is linearly proportional to the position of their movable core. An LVDT measures linear position and an RVDT measures angular position. The SPC provides excitation for the primary winding of the device. The amplitude and frequency of the exciter are configurable.

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