Emerson Process Management Bristol Series 9110 - Manual (Page 14)

Introduction 1-4 CI-5450
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.