FC 23 - M/A Station (Ratio)

Ratio is one of three station functions that provides an interface between a controller module and the following interface units: the digital control station (DCS), operator interface station (OIS), management command system (MCS), analog control station (SAC), and process control unit (PCU).

 

A ratio station provides the same functions as a basic station in normal mode, but differs from the cascade station in its method of set point generation while in the ratio mode. The wild variable <S2> multiplied by a ratio adjustment factor (ratio index) determines the set point output while in ratio mode. The initial ratio index value is calculated by the station to maintain the current set point output value when the station is placed into the ratio mode. While in the ratio mode, the ratio index value is displayed as follows.

 

For hard stations (i.e., IISAC01 and NDCS03), the ratio index is displayed on the alphanumeric display. Actual set point is displayed on the set point bar graph.

 

For consoles, the ratio index is displayed in the control prompt/field and identified with the label TRGT RAT on the control prompt when the user requests to change it. The actual set point (as calculated by multiplying the ratio index by the wild variable) is displayed on the set point value/indicator.

 

Refer to function code 80, Figure 80-2, control station element.

 

The ratio index value can be adjusted (ramped up or down) by the operator to obtain the desired set point output. Specifications S2 and S14 select the input value that the set point tracks in the manual basic mode. In the basic (non-ratio) mode, the set point signal is equal to the value set by the operator.

 

NOTE: The maximum ratio index is ten and the minimum practical ratio index is 0.05.

 

Other options include a control output override <S5> and a manual interlock <S6>.

 

The control output override forces the control output value to a particular state (e.g., when the maintenance of process safety requires immediate action).

 

The manual interlock may be used to hold the station in manual mode. This allows conditional logic to be used for preventing automatic operations.

 

 

 

Outputs:

Blk

Type

Description

N

R

Control output (CO) in percent (0% to 100%)

N+1

R

Set point in engineering units

N+2

B

Mode:

0 = manual

1 = automatic

N+3

B

Level:

0 = local

1 = computer

N+4

B

Station Mode:

0 = basic

1 = ratio

N+5

B

Computer backup indicates computer time-out:

0 = no

1 = yes

 

 

Specifications:

Spec

Tune

Default

Type

Range

Description

S1

N

5

I

0 - 255

Block address of process variable

S2

N

5

I

0 - 255

Block address of wild variable

S3

N

5

I

0 - 255

Block address of auto signal or input

S4

N

5

I

0 - 255

Block address of control output track signal (TR)

S5

N

0

I

0 - 255

Block address of control output track switch (TS):

0 = no (normal)

1 = yes (track)

S6

N

0

I

0 - 255

Block address of manual interlock:

0 = no

1 = yes (manual)

S7

Y1

100.000

R

Full

Process variable high alarm point in engineering units

S8

Y1

0.000

R

Full

Process variable low alarm point in engineering units

S9

Y1

4.0 E6

R

Full

Process variable set point deviation alarm point in

engineering units

S10

N

100.000

R

Full

Signal span of set point and process variable

S11

N

0.000

R

Full

Zero value of process variable

S12

N

0.000

R

Full

Zero value of set point

S13

N

0

I

0 - 255

Engineering unit identifier (*for console use only)

S14

Y

0

I

0 - 255

Set point track option:

0 = no

1 = process variable <S1>

2 = <S2>/manual only

3 = <S2>/manual and auto

S15

N

0

I

0 - 255

Computer time-out option:

0 = computer auto/manual mode unchanged

1 = computer manual

2 = computer auto

3 = computer cascade/ratio

4 = local auto/manual mode unchanged

5 = local manual

6 = local auto

7 = local cascade/ratio

S16

N

0

I

0 - 255

Control station address:

>15 = no station

 

NOTE:

1.   S7, S8 and S9 are tunable but not adaptable.

 

 

 

23.1 Explanation

 

Specifications

Spec #

Name

Description

S1

PV

Block number whose output is the process variable. This

input drives the digital station process variable indicator (in

engineering units).

S2

WILD

Block address of external reference input. A multiple of the

output of this block is the set point signal when the station is

in ratio mode.

S3

COAUTO

Block number whose output value is the control output when

the station is in auto mode (usually the output of a PID block).

S4

COTRCK

Block number whose output is the control output when the

station is tracking (control output override mode) and is the

initial value for the control output on start-up.

S5

COFLG

Block  number  whose  output  value  determines  whether  the control output is to be tracked. If the output of the block is:

0 = no tracking (normal)

1 = track <S4>

S6

MILOCK

Block number whose output value determines whether the

station locks in manual mode.

0 = no

1 = yes

 

S7

PVH

Engineering unit value of process variable at which a high

alarm will be generated and displayed.

S8

PVL

Engineering unit value of process variable at which a low

alarm will be generated and displayed.

S9

PVDEV

Engineering unit value of allowed deviation between process

variable and set point. A high deviation alarm generates when

the process variable is greater than the set point and the

value of the difference between the two is greater than or

equal to this specification. A low deviation alarm is generated

when the process variable is less than the set point and the

value of the difference between the two is greater than or

equal to this specification.

S10

EUSPAN

Value corresponding to engineering unit span for process

variable and set point.

S11

EUPVZ

Value corresponding to engineering unit zero for process variable.

S12

EUSPZ

Value corresponding to engineering unit zero for set point.

S13

EUID

Engineering units type.

S14

TRACKSP

Value that determines which input the set point will track in

manual mode. Selecting three causes set point tracking in

both manual and auto modes.

0 = no tracking

1 = track PV <S1> (manual mode)

2 = track WILD <S2> (manual mode)

3 = track <S2> always (auto and manual)

S15

COMPDEF

Value that determines the system default mode if the computer

times out while the loop is under computer control.

0 = computer auto/manual mode unchanged

1 = computer manual

2 = computer auto

3 = computer cascade/ratio (unused in function code 21)

4 = local auto/manual mode unchanged

5 = local manual

6 = local auto

7 = local cascade/ratio (unused in function code 21)

S16

DIGSTAAD

Value of the hardware digital station address that the station

function block is representing. A number greater than 15

indicates no hardware station. When using a loop command

module, address 15 indicates a second displayed station.

When using a DCS or SAC station, valid addresses are zero

through seven.

 

NOTE: Specifications S7, S8 and S9 are tunable but not adaptable (function code 24).

 

 

The ratio station function code can control a control station in both basic and ratio modes. When a ratio station is in the basic mode, it functions like a basic station (function code 21).

 

The set point of a control station in ratio mode is the product of the multiplication of the ratio index by the wild variable <S2>.

 

When a control station in ratio mode is displayed on a SAC/ DCS hard station, the value shown in the alphanumeric display is the ratio index, not the set point. The bar graph display on the hard station displays the set point. The SET UP and SET DOWN pushbuttons on the hard station can be used to ramp the ratio index value up or down.

 

When a control station is displayed on a console, it is displayed using the standard control station element as shown in Figure 80-2 in function code 80. For control stations in ratio mode, the value shown on the control/prompt field is the ratio index and is identified by the label TRGT RAT. The moving set point value/indicator field displays the calculated set point value. The ratio index value may be changed via the console by first depressing the SET key and then either ramping the ratio index up or down using the ramp keys or directly setting the ratio index by entering a numeric value in response to the TRGT RAT control/prompt field.

 

 

23.2  Station output example:

 

A standard station is assigned to block number 30. The system then assigns the next five block numbers as listed in Table 23-1. If there is a need to assign more block numbers after the station block, six must be added to the station block number to obtain the next available block number.

 

Table 23-1 - Station Output Description

 

Block Number

Station Output

30

CO (Control Output)

31

SP (Set Point)

32

A (Mode)

33

C (Level)

34

B (Station Mode)

35

CF (Computer Backup)

 

 

In this manner, each output of a station block has a unique reference number.

 

23.3  Station Operating States

 

The station function block acts as an interface from a controller module to the digital control station (DCS), analog control station (SAC), operator interface station (OIS), or a computer through a network interface unit (NIU). The operating state of the station block is its level (local/computer), mode (manual/auto) and the station mode (basic/cascade/ratio). The operating mode  determines which outputs may be adjusted from the operator interface devices. The modification of the operating state and the block output is determined by the block input and messages received from operator interface stations.

 

23.4  Control Output Tracking

 

When the control output track state <S5> equals one, it does not affect the flag output of the block but will override the current control output with the value in the block specified by <S4>. The actual operating state is saved, and is restored when the track flag input goes to zero.

 

In the local manual and computer manual states, the control output is adjusted from the interface device unless a connected DCS or SAC indicates that the analog output for the control is bypassed. In this case, the control signal comes from the DCS or SAC bypass and the station will track the DCS or SAC control output signal.

 

While the station is under local manual control, the output is a percentage from zero to 100 percent. If the input to the station is a calculation function, the input value must be scaled to percent, and the output of the station scaled to the required units, external to the station.

 

When the manual interlock state <S6> equals one, the station trips to the manual mode.

 

 

23.5  Set Point Tracking

 

The set point may be adjusted from the operator interface device only if set point tracking is not selected (S14 equals zero). If one of the tracking options is selected, the set point will be determined by the option chosen.

 

23.6  Examples

 

S2

Number of the block output the set point tracks as determined

by the set point track option (S14).

 

 

S14

Set point track option:

0 = no tracking. Set point can be varied using local manual control.

1 = track PV <S1> when station is in manual. Provides bumpless transfer when the station returns to automatic mode; set point cannot be varied manually.

2 = track value in block selected in <S2> in manual mode only. Set point may be changed when the station returns to automatic mode. A bump is possible when transferred.

3 = track value in block selected in <S2> always (auto and manual) no direct control of set point from station.

 

 

 

 

23.7  Deviation Alarms

 

A high deviation alarm is generated when the process variable is greater than the set point, and the absolute value of the difference between the two is greater than or equal to S9.  A low deviation alarm generates when the process variable is less than the set point, and the absolute value of the difference between the two is greater than or equal to S9. These two alarm conditions report to the loop.

 

Figure 23-1 illustrates a manual/auto ratio station with output tracking. The wild variable <S2> is a process input.

 

NOTES:

  1. While the station is under local manual control, the output is zero to 100 percent. Therefore, if the input to the station is a calculation function, the input value must be scaled to percent and the output of the station scaled to the required units, external to the station. The control output scale can be driven by the station output or a feedback signal. This applies to the first two stations connected to a given termination unit. Stations connected by daisy chain do not have this option.

  2. The station can only be transferred to computer operation when a network interface unit and computer are involved in the transaction. Depressing the computer pushbutton on the station will not provide a usable output.

  3. When the station is in ratio mode, the set point signal is equal to the wild variable <S2> multiplied by the ratio set point. In automatic mode, the set point signal is equal to the set point output (N+1).