Emerson Process Management 9110-00A, Series 9110, CI-9110 Guard Input, Current Output

The feedback pot connects across a regulated +5 Vdc source. The voltage at the feedback pot’s contact corresponds to the mechanical position of the pressure regulator. This reference voltage is applied to the Raise and Lower Comparators.

If an increase in signal level occurs at the command input, the Raise Comparator will become unbalanced since this signal will exceed the voltage present at the center arm of the pot. The Lower Comparator will be unaffected since the input signal will be of opposite polarity compared to the feedback pot voltage. The CPU senses the increased output at the Raise Comparator and outputs a signal that drives the stepper motor in an upward direction. The upward driving action will continue until the voltage at the pot’s center arm becomes equal to the new signal level. Once the comparator is balanced, the CPU will turn off the stepper motor and the pressure regulator will provide a corresponding pressure out- put level. Input changes that reduce the output value will have the opposite effect.

The comparators are biased so that they include a certain amount of hysteresis. This bias keeps the stepper motor in a stable, deenergized zone that is less affected by noise bursts that may appear on the command input line.

The stepper motor is actuated in incremental steps. The pulse signals received from the stepper driver will increment the motor one step at a time. The motor requires ap- proximately 22,000 steps for full rotation.

1.2.2 Guard Input

An optional signal can be applied to the guard input to Enable or Inhibit the command Sig- nal. The guard circuit consists of a non-polarized, opto-isolated circuit whose output is applied to the CPU via a logic circuit.

Analog Regulator models include a fault detector circuit that is activated through jumper selection. The fault detector monitors the analog input value for a value that is less than 0% of scale. This condition, which occurs when the AI command input signal fails, will stop the stepper motor and maintain the last output value prior to the failure.

Two types of signal failure modes are provided by jumper selection as shown at the left of Figure 1-2. Position "A" of the jumper provides fail-hold mode, while position "B" provides fail-hold zero. Should the AI signal fail (signal < 0% of scale), fail-zero mode will drive the output slowly to zero, while fail-hold mode will retain its last output value.

Mechanical limiting is also provided by adjustable tangs located on the main drive gear of the stepper motor. When the input signal is of a value that causes the stepper to drive against a limit, a mechanical clutch on the drive shaft will slip. After making several revolutions, the software will time out and cut off power to the motor. Power will be re- applied when the signal level returns to the normal operating range.

1.2.3 Current Output

A 4-20 mA current output is obtained via an amplifier/converter circuit. The input to this circuit is the feedback pot whose center arm adjusts the current output to track the valve position.

Introduction 1-4 CI-5450