| 摘要 |
<p>A position transmitter (8) includes a d c power supply (10) having a positive terminal connected to an input line (12) and a negative terminal connected, through a system controller (14) having a variable load. to an output line (16). A voltage divider including a potentiometer (30! is connected across the input and output lines (12. 16). A wiper (281 of the potentiometer (30) is mechanically connected to a movable member (24) of an actuator device (22) so that a variable voltage is applied between the input and output lines (12, 16). The wiper (28) is electrically connected to a negative input of a zero adjust amplifier (50) which has an adjustable positive input. The movable member (24) is moved to its zero position which also moves the wiper (28) to a zero position. At the zero position, the zero adjust amplifier (50) is adjusted to apply a 4 mA current to the output line (16). This is done by applying the output of the zero adjust amplifier (50) through a series of amplifiers to the base of a transistor (60), which has an emitter and collector connected across the input and output lines (12, 16) for converting the voltage signal into a current signal. A span adjust amplifier (54) has a negative input connected to the output of the zero adjust amplifier (50) and has an adjustable feedback (56) from its output to its negative input. The movable member (24) is moved to its maximum position, thus also moving the wiper (28) to a maximum position. The span adjust amplifier (54) is then controlled to apply a 20 mA current signal to the output line (16). As the movable member (24) moves between its minimum and maximum position, a 4-20 mA signal which corresponds to the position of the movable member is applied to the output of the position transmitter.</p> |
