7/27/2019 2.Direct_Digital_Control.pptx

1/28

Direct Digital Control

7/27/2019 2.Direct_Digital_Control.pptx

2/28

Introduction

Figure: Supervisory Computer Control29-09-2013 2

7/27/2019 2.Direct_Digital_Control.pptx

3/28

DDC structure

29-09-2013 3

7/27/2019 2.Direct_Digital_Control.pptx

4/28

Features

Interfaces directly with the process for data

acquisition and control purpose.

it has necessary hard ware (opto-isolator,

signal conditioner, ADC) for interfacing directly

with the process.

It should have memory for performing ALU

29-09-2013 4

7/27/2019 2.Direct_Digital_Control.pptx

5/28

Microprocessor performs the following tasks:

Reads

Determines

Outputs

29-09-2013 5

7/27/2019 2.Direct_Digital_Control.pptx

6/28

DDC Software

The two algorithms used for programming

control loops are:

Position Algorithm

Velocity Algorithm

29-09-2013 6

7/27/2019 2.Direct_Digital_Control.pptx

7/28

Position algorithm

29-09-2013 7

7/27/2019 2.Direct_Digital_Control.pptx

8/28

29-09-2013 8

7/27/2019 2.Direct_Digital_Control.pptx

9/28

29-09-2013 9

7/27/2019 2.Direct_Digital_Control.pptx

10/28

29-09-2013 10

7/27/2019 2.Direct_Digital_Control.pptx

11/28

29-09-2013 11

7/27/2019 2.Direct_Digital_Control.pptx

12/28

Flow chart forPID control

29-09-2013 12

7/27/2019 2.Direct_Digital_Control.pptx

13/28

Draw backs

Lack of bumpless transfer from manual to

auto switching

Reset windup due to integral saturation in test

mode.

These are not present in velocity algorithm.

29-09-2013 13

7/27/2019 2.Direct_Digital_Control.pptx

14/28

Velocity Algorithm

In number of control loops, the final control element is stepper motor or

stepper motor driven valve. In such cases, the requirement at the

computer output will be a pulse train specifying the change in valve

position.

Thus output of position algorithm cannot be used, since it give the new

position of the valve, in absolute term.

In velocity algorithm, the computer calculates the required change in valve

position.

The output is digital pulse train which can be directly used in case valve is

stepper motor driven. In case of other valves, stepper motor combined with slide wire

arrangement as shown in figure can be used.

The same function can be performed by an integrating amplifier.

29-09-2013 14

7/27/2019 2.Direct_Digital_Control.pptx

15/28

Velocity Algorithm

29-09-2013 15

7/27/2019 2.Direct_Digital_Control.pptx

16/28

29-09-2013 16

7/27/2019 2.Direct_Digital_Control.pptx

17/28

29-09-2013 17

7/27/2019 2.Direct_Digital_Control.pptx

18/28

Controller drift

The velocity algorithm should always include

the integral term otherwise it will give rise to

controller drift.

To explain this we substitute the following

equation in velocity algorithm

en

= S - Vn

29-09-2013 18

7/27/2019 2.Direct_Digital_Control.pptx

19/28

We get,

From above equation it is clear that if integral

term is not there it will lead to controller drift

and give rise to oscillations.

29-09-2013 19

7/27/2019 2.Direct_Digital_Control.pptx

20/28

Let us consider the velocity algorithm with

only proportional and integral terms:

29-09-2013 20

7/27/2019 2.Direct_Digital_Control.pptx

21/28

29-09-2013 21

7/27/2019 2.Direct_Digital_Control.pptx

22/28

Integral overshoot

The above modification solves one problem

but creates another problem integral

oscillations

When proportional term is assigned the

integral term sign the set point will be reached

at a faster rate where by integral overshoot

occurs.

29-09-2013 22

7/27/2019 2.Direct_Digital_Control.pptx

23/28

Case of oscillation in velocity algorithm

29-09-2013 23

7/27/2019 2.Direct_Digital_Control.pptx

24/28

Effect of setpoint band in velocity

algorithm

29-09-2013 24

7/27/2019 2.Direct_Digital_Control.pptx

25/28

29-09-2013 25

7/27/2019 2.Direct_Digital_Control.pptx

26/28

Velocity algorithm flow chart

29-09-2013 26

7/27/2019 2.Direct_Digital_Control.pptx

27/28

Position Vs Velocity Algorithms

Reference point

Reset wind up

Auto manual switching

29-09-2013 27

7/27/2019 2.Direct_Digital_Control.pptx

28/28

Microcomputer based DDC

29-09-2013 28