FC 18 - PID Error Input

This function provides proportional, integral and derivative actions on an error signal developed from the process variable (PV) and set point (SP) inputs. The block has three inputs and one output. Besides the error input, there are track reference and track switch input signals. If the track switch <S4> is a zero, the output follows the track reference signal <S3>. This provides for smooth control transfers from manual to automatic mode. The parameters for this function block include an overall gain constant (S5), a proportional constant (S6), an integral constant (S7) and a derivative gain constant (S8).

 

Outputs:

Blk

Type

Description

N

R

Output is PID signal in engineering units (EU)

 

 

Specifications:

Spec

Tune

Default

Type

Range

Description

S1

N

5

I

Note 1

Block address of error signal

S2

N

1

I

0 or 1

Reserved

S3

N

5

I

Note 1

Block address of track reference signal

S4

N

1

I

Note 1

Block address of track switch signal:

0 = track

1 = release

S5

Y

1.000

R

Full

(K) gain multiplier

S6

Y

1.000

R

Full

(Kp) proportional constant

S7

Y

0.000

R

0 - 9.2E18

(Ki) integral constant (1/min)

S8

Y

0.000

R

Full

(Kd) derivative constant (min)

S9

Y

105.000

R

Full

High output limit

S10

Y

-5.000

R

Full

Low output limit

 

NOTES:

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

 

18.1 Explanation

This function operates on the input error signal according to the equation:

 

 

The purpose of the gain multiplier (S5) is to convert or scale the output. Typically it is used to convert the output to percent for input to a station or output to a field device. Figure 18-1 shows an example.

 

 

To calculate S5 in engineering units, use the equation:

 

 

To calculate S5 in percent, use the equation:

 

 

If the signal range is zero to 100 percent, the possible range of the error signal is -100 to +100 percent. Use the following equation to determine the gain multiplier S5:

 

For example, if the range of an error signal is zero to 50 cubic feet per second, then determine S5 as follows:

 

NOTE: When a negative output is anticipated, the low limit (S10) must be adjusted in a negative direction to encompass the limits of the output signal range.

 

18.2 Applications

Figure 18-2 illustrates a PID error input block used to calibrate a demand value with an error value. By using function code

18 versus function code 19, the error input to S1 can be reported to a console.