FC 19 - PID (PV and SP)

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

 

 

Outputs:

Blk

Type

Description

N

R

Output Value of Function

 

 

Specifications:

Spec

Tune

Default

Type

Range

Description

S1

N

5

I

Note 1

Block address of process variable input

S2

N

5

I

Note 1

Block address of set point

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

S11

Y

0

B

0 or 1

Set point change:

0 = normal

1 = integral only (Ki not equal 0)

S12

Y

0

B

0 or 1

Controller action on error:

0 = reverse acting on error

1 = direct acting on error

 

NOTES:

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

 

19.1 Explanation

This function supports two modes: direct and reverse. The function is in the direct mode when S12 is a logic 1 and in the reverse mode when S12 is a logic 0.

 

Direct Mode

(S4 = 1 and S12 = 1)

 

The set point <S2> subtracts from the process variable input <S1>.

 

Reverse Mode

(S4 = 1 and S12 = 0)

 

The process variable input <S1> is subtracted from the set point <S2>.

Refer to Figure 19-1 for an illustration of the PID algorithm for function code 19.

 

S5

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 19-2 shows an example.  To calculate S5 in engineering units, use the equation:

 

 

To calculate S5 in percent, use the equation:

 

For example, with the function in the direct mode, the range of the process variable signal is zero to 200 cubic feet per second and the maximum control output is 100 percent.

 

The gain multiplier (S5) is determined as follows:

 

 

 

Controller Start-Up  (Block 10 – start-up in progress = 1)

PID output = <S3>

 

Manual Mode (S4 = 0)

PID output = <S3>

 

 

High and Low Output Limits (S9 and S10)

Specifications S9 and S10 set the limits of the output block value (N). The default values of S9 and S10 provide an output range of -5.000 to +105.000. When a negative output is anticipated, the low output limit (S10) must be adjusted in a negative direction to encompass the limits of the output signal range.

 

Set Point Change (S11)

Set point modifier. This specification defines the action taken on a set point change. A normal setting results in a jump in the control output due to the proportional contribution created by a set point change. When set to integral only on set point change, the proportional and derivative contributions of the error are not applied with set point changes. This action eliminates the jump in the control output and results in an integral only action on a change in set point.

 

0 = normal

1 = integral only on set point change

 

 

19.2 Examples

 

Figure 19-1 illustrates the PID algorithm for function code 19. Figure 19-2 shows how the PID block is typically used with a station in a control loop.

 

Notes for Figure 19-1

Note 1 - Bias

The bias term is either equal to the value of the combined proportional plus integral term calculated when Ki was last set to

a value greater than zero (normal reset), or it is equal to the value of the track reference (external reset or tracking).

 

 

 

Note 2 - Normal Reset, Auto Selected External Reset, and External Reset

When Ki is less than zero (proportional plus integral), the internal value of the integral term is determined based on the PID reset mode specified in S5 of function code 82.

 

When S5 equals zero (normal reset), the PID calculates the value of the internal integral by summing the proportional term with the previous value of the internal integral.

 

When S5 equals two (external reset), the PID sets the value of the internal integral to the value of the current track reference.

 

 

When S5 equals one (auto selected external reset), the PID sets the value of the internal integral to the value of the current track reference only if the value of the PID output from the previous execution period does not match the current track reference value. Otherwise, the PID calculates the internal integral value as if it were set for normal reset. The auto select external reset mode only applies when Ki is less than zero. When Ki is greater than zero (proportional only), the auto selection is disabled.

 

Note 3 - Range Limiting

The track reference, the proportional plus integral term, and the output are all range limited based on the high and low limits specified in S9 and S10 respectively. The high and low range limits for the derivative term are:

 

Derivative high limit = output high limit - output low limit

Derivative low limit = output low limit - output high limit