FC 4 - Pulse Positioner

The pulse positioner (PULPOS) function code compares two analog input signals and produces output pulses that are proportional in time duration to the difference between these two analog signals. Both inputs are expressed as a percentage of the total range or span of the process parameter. Any difference is converted to a timed forward or reverse boolean output. The time duration of the boolean signal is proportional to the percent error and the specified stroke time. There are provisions for specifying the error dead band and the cycle time.

 

The PULPOS function code uses two consecutive block addresses. For correct operation, block addresses N and N+1 should be sent directly to two digital output blocks in the same I/O module. This is accomplished by using any digital output (such as function code 79, 83, 225, etc.) for the Harmony controllers.

 

NOTE: Outputs N and N+1 of function code 4 must be in the same I/O group of function code 83 (digital output

group).

 

Outputs:

Blk

Type

Description

N

B

Output value of timed raise (forward pulse)

N

R

Output value of timed loser (reverse pulse)

 

 

Specifications:

Spec

Tune

Default

Type

Range

Description

S1

N

5

I

Note 1

Block address of set point signal

S2

N

5

I

Note 1

Block address of feedback signal

S3

Y

0.000

R

Full

Forware stroke rate (% / sec)

S4

Y

0.000

R

Full

Reverse stroke rate (% / sec)

S5

Y

0.000

R

Full

Deadband (%) - absolute deadband

S6

Y

0.000

R

Full

Cycle time (secs)

 

NOTES:

1.  Maximum values are:  9,998 for the BRC-100, IMMFP11/12 31,998 for the HAC

 

 

4.1 Explanation

The output of the pulse position function is a series of pulses having pulse durations proportional to the difference between the desired set point value input signal <S1> and the actual value feedback signal <S2>. These two signals are expressed in percent of total range or span of the process parameter. <S1> and <S2> are typically scaled to the same engineering unit to obtain proper operation.

 

This function block produces two outputs. Output N generates a signal when the measured process value is less than the desired output. Output N+1 generates a signal when the measured process value is greater than the desired value. The assigned block number (N) references the first output which is a forward or increase output. The next consecutive block number (N+1) references the second output which is a reverse or decrease output. If this block drives a digital I/O module, both outputs must be directed to the same group on the I/O module and must use consecutive I/O module outputs.

 

The difference between the desired value signal <S1> and the actual value (feedback) signal <S2> is called the error signal. Specification S5 is the deadband, i.e., the amount of error that is allowed before a correction is necessary. If <S2> is less than <S1> by an amount greater than the deadband, there will be a forward output. To determine the forward output pulse widths use the calculation:

 

 

 

NOTES:

  1. Forward and reverse output pulse durations are computed to the nearest ten-millisecond minimum for Harmony controllers.

  2. If output blocks N and N+1 are not directly connected to a digital output, the forward and reverse output pulse durations are set to module segment time.

 

 

4.1.1 Cycle Time

Cycle time (S6) sets the time between calculations, or how often this function is calculated. It delays the processing of the block.

 

4.1.2 Stroke Rate

The stroke rates (S3 and S4) are entered in units of percent per one second. The stroke rate sets the length of time that the forward or reverse signal remains high for each percent of error (when error is greater than deadband). If S3 is set to ten percent per second and the deadband is set to two percent, then the forward output will be held high one second for every ten percent error, or until the next cycle, whichever comes first. If there is 90 percent error above the deadband when S3 is ten percent per second, then the forward output remains high for nine seconds or until the cycle ends. The minimum pulse duration is ten milliseconds and the smallest incremental pulse length possible is ten milliseconds.

 

 

 

4.2 Applications

Figure 4-1 shows how the pulse position function code can control a pulse type valve positioner. The PULPOS function block is internally automatic, but there are no provisions for operator intervention. Using the PID (function code 19) and the M/A (function code 80) control loop allows selecting a desired set point for operation in the auto mode. The control loop then regulates the set point signal to the PULPOS function block to maintain a constant valve position based on the valve position feedback.

 

 

In manual mode, the PULPOS function block set point is directly selected via manipulation of the control output value. In either manual or auto mode, the PULPOS function block controls the field device based on the specification settings and the relationship exhibited between <S1> and <S2>. The PULPOS function block pulses a raise or lower output signal to adjust for errors.

 

NOTES:

  1. The outputs of the PULPOS must go to the same device definition function code when used in BRC-100 or HAC controllers.

  2. The control stations should be configured to display the position feedback on the output bar graph.

  3. For BRC-100 and HAC controllers with firmware earlier than D0, a maximum of two pulse positioner function blocks can be configured in each Harmony I/O block (i.e., DIO-400, CIO-100).

 

 

Figure 4-2 demonstrates how the pulse positioner function code can control a pulse type valve positioner utilizing a Harmony CIO-100 block. The following conditions must be met for this configuration:

 

 

NOTES:

  1. The pulse positioner function block, its associated digital output function blocks (FC225), and the device definition function block (FC221) must all reside in the same segment control block.

  2. The pulse positioner function block must be connected directly to a pair of digital output function blocks (FC225).

  3. The pulse positioner function block and its associated pair of digital output function blocks must be assigned to the same device definition function block (FC221).