Micromod MOD: 1726F Output holder for MOD 30ML - Manual (Page 7)

Using The Output Holder With MOD 30ML
Output Holder Purpose
3
OUTPUT HOLDER PURPOSE
GENERAL
For safe operation some processes require that valve position be maintained above a
closed position even when a controller might be removed for service or replacement. The
Output Holders task is to identify a controller output fault and to maintain the last good
value to the field control device for safe operation. When the controllers output signal to
the output holder is restored and again valid the output holder must return control of the
field device to the controller. Under normal operation i.e.; the controllers output is good,
the controllers output is passed through the output holder to the field control device
unaffected.
From the example above, it can be understood that if an output holder has taken control
of a field device signal due to a controller being removed, when the controller is replaced
there could be a “bump” to the control valve if the controllers output is not first brought
equal to or “synchronized” to that of the output holder. To prevent this bump from
happening the controller must first be aware of the output holders output value and make
its output equal to it before the controller can turn its output on.
The MOD 30ML Output Holder Compound strategy demonstrated in this document is
designed to identify that an output holder exists in the controllers output circuit and how to
synchronize its output value to that of the output holders output prior to turning the
controllers output on.
The 1726F Output Holder card can be used on as many as three MOD 30ML control
loops.
1726F THEORY OF OPERATION
Figure 1 shows a block diagram of the 1726F Output Holder operation.
The controllers output is fed into the output holder as a 4-20 milliamp signal where it is
converted to a 1-5 Vdc value by the I/V Converter circuit. This signal is then fed to the
Analog MUX, Input Monitor and Input Conditioner circuits.
The Input Monitor circuit compares the input signal to a low limit of -8%. The result of this
block is a “Run/Hold” selection signal which is used by the Analog MUX circuit to select
the controllers output or hold value as the value to be sent to the field.
The Up/Down circuit compares the controllers output as conditioned by the Input
Conditioner circuit to that of the hold value. Its resultant signal causes the Up/Down
Counter to either increment or decrement its count based on whether the controllers input
is higher or lower than that of the hold value. The goal is maintain the hold value equal to
that of the controller output during normal operation.
The Clock Control circuit gates the Clock signal through to the clock input of the Up/Down
Counter. When the Analog MUX selects the hold value as the value to be sent to the field
then the Clock Control circuit blocks the clock pulses from reaching the Up/Down
Counter, thereby causing the hold value to become steady. While the controllers output