FC 58 - Time Delay (Analog)

The time delay function code provides a pure delay on an analog signal. It can be used to create fixed or variable time delays, or model systems that represent dynamic time delays.

 

 

Outputs:

Blk

Type

Description

N

R

Time delayed function of input

 

 

Specifications:

Spec

Tune

Default

Type

Range

Description

S1

N

5

I

Note 1

Block address of input (X)

S2

N

6

I

Note 1

Block address of rate input (R, in units/sec)

S3

N

1

I

Note 1

Block address of track switch signal:

0 = track

1 = release

S4

N

0.000

R

Full

Length of queue (L, in units)

S5

N

1

I

0 - 255

Number of intervals (N)

 

NOTES:

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

 

 

58.1 Explanation

 

58.1.1 Specifications

 

S1 – <X>

Block address of the input.

 

S2 – <R>

Block address of rate input in units per second.

 

S3 – <T>

Block address of track switch signal.

 

S4 – L (Length of queue)

Length of the queue in units. The queue is the number of units over which the time delay is effective.

 

S5 – N (Number of intervals)

Number of times, from one to 190, that the input is to be sampled. Determine N by dividing the time delay (TD) by the desired sampling frequency.

 

 

 

 

58.2 Applications

 

58.2.1 Fi8xed Time Delay

 

For a fixed time delay, the rate input, <S2>, is constant. The time delay between output and input varies only with S4. It is directly proportional to S4. For example, simulate the time delay for flow through a pipe. Assume a required time delay of two minutes with input sampling desired every five seconds. Select the default value of 1.0 (found in fixed block six) for <S2> since rate is constant for fixed delays.

 

<S2>

=

Rate in units per second = 1.0

 

S4

=

Length of the queue in units

 

S5

=

Number of intervals

 

Time

Delay

=

2 minutes = 120 seconds

TD

=

120 Seconds

 

TD

=

  S4  

<S2>

 

120

=

S4

1.0

 

S4

=

120 units = Length of queue

 

For input sampling every five seconds:

 

N    =      TD  

   5 Sec

 

= 120 sec

     5 sec

 

= 24 intervals

 

 

Figure 58-1 is an illustration of this example.

 

 

 

 

58.2.2 Variable Time Delay

 

Variable time delays may be dynamically adjusted by changing the value of <S2>. Using a function code 9 block, as illustrated in Figure 58-2, the two fixed input rates can be switched. In the fixed time delay example, when <S2> equals 1.0, the time delay, S4/<S2> equals 120 seconds. By changing <S2> to 2.0, the time delay becomes 60 seconds, and the timing interval, TD/N equals 2.5 seconds. Changing the rate input <S2> while holding all other parameters constant changes the timing interval. Faster rates produce more frequent input sampling, and slower rates produce less frequent input sampling for the same number of intervals.

 

 

 

58.2.3 System Modeling

 

The analog time delay block may be used to model a physical system that represents a dynamic time delay. For example, an oil pipeline may have a measurement device at a different location than the indicator/controller. With this function code, a measurement can be taken. This function code delays sending the value to the controller until the element of oil reaches the controller. Specification 4 may be specified in feet, <S2> in feet per second, and N to establish the needed resolution.

 

If S4 = 100 feet and <S2> varies from ten feet per second to 20 feet per second, then TD will vary between ten seconds and five seconds.

 

If sampling is required every 0.5 seconds to achieve the needed resolution, then:

 

N = longest time delay

              .5 sec

N = 10 sec

       0.5 sec

 

N = 20 intervals

 

The longest time delay can present a worst case scenario. No matter what the time delay, the input will be sampled 20 times over that period to insure adequate resolution.

For the shortest time delay, the sampling intervals will be:

 

     5 sec         = 0.25 second

20 intervals

 

 

In most cases, the delay in a process consists of more than a pure time delay (deadtime). There is usually an additional time lag that may be a first, second, or higher order lag. In general, the process responds to a second order lag response. This can be simulated accurately by using a time delay and a first order lag. If necessary, another first order lag function block can be added. Figure 58-3 shows a graphic representation of a function and a simulated response. Figure 58-4 illustrates the configuration required to simulate the response shown in Figure 58-3.