introduction - rmd engineering college · power generation, transmission and distribution is cover...

Introduction

Direct Digital Control (DDC) Feedback control system where the controller action is attained numerically by a y y yprogrammable digital device

disturbances

Control Algorithm

output Actuator Processreference output

g

I/O

inputTransducer

Microprocessor (μp)

The overall system is an hybrid system or a sampled data system: digital part (discrete

A.Cenedese Introduction to Digital Control

The overall system is an hybrid system or a sampled data system: digital part (discrete time: controller)+ analogic part (continuous time: process, actuator, tranducer)

1

Direct Digital Control

disturbances

Control Algorithm


g

I/O

inputTransducer


A.Cenedese Introduction to Digital Control 2

Control Algorithm

Clock Reference Feedback i l Elaboration Output to W itinterrupt reading signal

acquisition (control algorithm)p

actuator Wait

The interrupt is provided by a Real Time Clock RTC.


The interrupt is provided by a Real Time Clock RTC. The reference is already available in digital format or is computed in real time or is

an analogical signal acquired by the transducer

Control at discrete times regularly spaced every T (sampling time) given by the RTCThe control algo output signal is piecewise constantTh t ll t i th “di it d i ” ( k ith b ) hil i t i lThe controller acts in the “digit domain” (works with numbers) while input signals

to the plant (actuator commands) and output signals from the transducer(measurements) are usually analog signals.

h h f d f bl b h d l d h l


There is therefore need for a suitable interface between the digital and the analog parts of the system.

3

Direct Digital Control

disturbances

Control Algorithm


g

I/O

inputTransducer



Input interface (to the controller) – 1

ADC - A/D Analog to Digital Converter:VFS

It provides the sampling of the analogsignal from the transducer and the conversion into a sequence of bits

ADCbitV

Uniform quantization Nonuniform quantizationUniform quantization Nonuniform quantization


Input interface (to the controller) – 2

GiUniform quantization:

Given:Number of bits n ( number of levels) Full scale value FS ( maximum value managed by the device)Analog signal VQuantization step (quantum) q: n2

FSq =

Quantization (nonlinear operation):

q1nVq1nifnqQ[V]V ⎟⎞

⎜⎛ +<<⎟

⎞⎜⎛=→

ADC Mathematical model:

q2

n V q2

-n if nqQ[V]V ⎟⎠

⎜⎝

+<<⎟⎠

⎜⎝

=→

ADC Mathematical model:

Qn bitsV


T6

Output interface (from the controller)

DAC - D/A Digital to Analog Converter:It converts a binary digit into the analog signalVFS It converts a binary digit into the analog signal commanding the actuator (voltage or current, proportional in value to the input signal value).DAC V

bit

VFS

bitThe converter also interpolates the signal:ZOH - Zero Order Hold o holder

output

tinput

NNote: the output interface problem is of immediate solution when the actuator is a digitalactuator, that is a system that “accepts number as its own inputs” (e.g. step motor: the


input is the number of rotation steps to advance – and not the rotation angle…)

7

Direct Digital Control scheme

DDC scheme is therefore:

Rif.

-ControlAlgo H0

ActuatorProcess

TTransducer

Q

Note: reference is a digital signal (already acquired/in memory)

DDC raises two main issues related to sampling and quantization:1. Error correction happens only at discrete times ( sampling)2. Nonlinearities are present in the system ( quantization)


Oil & Gas Instrumentation Engineering Computer Control (SCADA)

Lecturer: Yousef M.A Page | 4

1. Manual Control Before 1920

Figure 1 shows a simple scheme for a manual control which the Operator manipulated

the wheel of manual valve to control (flow/level/temperature) by looking at mounted

gauges. The operator was the controller and used visual feedback, control and

information was fully distributed.

2. Automatic Control (1920 to 1950)

Control panels with pneumatic controllers were introduced on a process unit basis as

shown above in Figure 2. Several operators regularly monitored and manipulated the

plant from the control panel. Control and information was centralized on a unit level

basis.

Figure 1: Manual Control System Figure 2: Automatic Control System



3. Automatic Control (1950 to 1970)

Figure 3 shows a high density panels with electronic instruments were introduced.

Operators had a complete picture of the process from a central control panel and could

manipulate the entire process with very little physical movement.

Control and information was totally centralized.

4. Distributed Control Systems 1975

Figure 4 shows three elements CRT-based man machine interface, microprocessor

based distributed controllers and data highway resulted in distributed system .

Control functions are distributed and information centralized.

5. Computer Controlled System Configurations

Direct Control

Figure 3: Automatic Control Figure 4: Distributed Control



The main characteristics of a direct control system are:

The analog controller is replaced by the computer

Control modes such as PID control, neural network and fuzzy logic can easily be implemented via programming.

If the computer fails, the plant is out of control.

6. Supervisory Computer Control

Actuator Plant 1

Measurement

Multiplexer

ADC

Computer

DAC

Multiplexer

-

Figure 5: Direct Control block

diagram

C1



The main characteristics of a supervisory control system are:

The control loops are still analog in nature.

The computer monitors measurements and updates inputs.

The computer use appropriate software to optimize inputs for the best overall plants operation

If the computer fails, the analog loops will maintain the operation of the plants.

7. Supervisory Control and Data Acquisition (SCADA)

SCADA is a system and technology for remote monitoring and control, which allows

the operator to:

Actuator Plant 1

Measurement

Multiplexer

ADC

Computer

DAC

Multiplexer

-

R1 + Controller

Figure 6: Supervisory Control block

diagram

C1

b1

https://en.wikipedia.org/wiki/Remote_monitoring_and_control



Get data and gather information from the processes.

Exchanging data from the plant floor to a supervisory computer.

Displaying and supervise process with some geographical scattered

Data logging, data display, trending, downloading of recipes.

Also, SCADA is not a full control system, but rather focuses on the supervisory level.

SCADA working to collecting data and information via distributed devices like

(remote terminal unit) RTU,PLC, transferring data back to the central unit, carrying out

any necessary analysis and control and then displaying that information on a number of

operator screens or displays. Finally, send control actions to the process depending on

the information.

7.1 Applications and Uses of SCADA

Now days, Modern SCADA are used extensively in many industrials around the world

for remote control of sectors and substations. The following are some examples of the

usage of SCADA systems:

The independent PLCs and RTUs in a SCADA system perform I/O control signals on field devices while being supervised by a SCADA/HMI software package on a master computer.

Figure 7: SCADA System.



Electric Energy

Power generation, transmission and distribution is cover very wide areas, so SCADA

systems are used to detect and monitor any change in loads ,currents flow and line

voltages, and control the sectors by opening and closing the switches.

Oil and Gas

Transmissions, distributions, productions, Pipelines, Wells, refinery, pumps are

distributed in very large Areas, which

need to get data and information for fluid

pressure and measurements.

Water and sewage treatment

SCADA systems used to monitor and

control the distributed equipment used like filters and valves. Besides, SCAD A used to

regulate water flow, reservoir levels, pipe pressure and other factors.

Buildings, facilities and environments

Facility managers use SCADA to control HVAC, refrigeration units, lighting and

entry systems.

Industrial Processes

SCADA systems are used in Industrial and manufacturing to manage processes

and robots like (Car, Food manufacturing, Chemical process).

Nuclear Processes

Figure 8: oil and gas site.

Figure 9: water treatment.



Transportation

7.2 SCADA System Benefits

Besides SCADA can monitor and control any kind of equipment, SCADA used to

automate more complex processes. Further SCADA has many benefits as follows:

Save time and money.

Saves energy.

Increases productivity and profitability.

Less physical movements for workers.

Wear and tear on equipment can be reduced by continuously monitoring levels.

Manage a real-time system trouble.

Fast Troubleshooting and Maintain.

Cost effective for power systems.

Reliable.

Expansion capability

7.3 Popular SCADA Vendors and Brands

N.o Brand Name Software Name Country

1 Siemens Wincc Germany

2 Schneider Citect Scada France

3 Rockwell RS View USA

4 Wonderware In Touch USA

5 Honeywell Experion

6 Ci technology Citect Australia



7.4 Architecture of SCADA

SCADA is highly complex system includes the following components: master terminal

unit (MTU) or host computers, operating equipment, PLCs, instruments, remote

terminal unit, intelligent electronic device IED, and Actuators. These components were

distributed in terms of layers or levels. The basic SCADA architecture system is

represented as shown in Figure 10.

Level 0: is the Input/ Output Level which distributed at one level including all type

of sensors and actuators.

Level 1: is the Process and Field Level which is gather field values, I/O – analog,

digital, measuring and other commands, such as RTUs, Valves IEDs, and PLCs.

Level 2: is the Control Level which includes the control devices like PLC, PC.

Level 3: is the Supervisory Level with dynamic changing states and real time control.

Level 4: world Connection using Internet and various new technologies related to mobile /

cell phone operations.

Amount

of Data

Reaction

Time

Figure 10: SCADA Architecture.

L0

L1

L2

L3

Data Flow



7.5 Advantages/ Disadvantages of SCADA

The advantages of the SCADA system are:

• The computer can record and store a very large amount of data

• The data can be displayed in any way the user requires

• Thousands of sensors over a wide area can be connected to the system

• The operator can incorporate real data simulations into the system

• Many types of data can be collected from the RTUs

• The data can be viewed from anywhere, not just on site

• scalability of the hardware

The disadvantages are:

• The system is more complicated.

• Different operating skills are required, such as system analysts and programmer

• With thousands of sensors there is still a lot of wire to deal with

• The operator can see only as far as the PLC

8. Elements of SCADA

SCADA system is used to supervise, control and monitor data from subsystems, by using

physical cables or remote access. All these procedures are done by using some

components and elements. SCADA systems components as shown in Figure 11 are:

Master Terminal Unit ( MTU ) “ Host Computer, Server”

Remote Telemetry/Terminal Unit ( RTU)

Programmable Logic Controller ( PLC )

Human-machine Interface (HMI)

Sensors and actuators

Communication

MTU is The Master Terminal Unit ( heart of a SCADA system) operated by an operator

who has the authority to access the system by input devices as a computer keyboard or

pointing devices as touchscreen. The MTU can monitor and control the field devices even

when the operator in not present by means scheduler programed.



MTU has other auxiliary devices considered to be part of the MTU such as (printers,

backup memories and modems, HMI, Historical Data logging).

The MTU initiates virtually all communication with remote sites and interfaces with an

operator. Data from remote field devices (pumps, valves, alarms, etc.) is sent to the MTU

to be processed, stored and/or sent to other systems. MTU CAN BE

Web server

Data logging, Analyzing data

Serve the clients through a firewall

Real-time decision maker

Figure 11: SCADA Elements [6,9].



8.2 RTU

Remote Terminal Unit (RTU) is a microprocessor-based device with very good radio

interfacing. RTUs are deployed in the field as an interface between the I/O Devices and

SCADA system. RTUs are used to transmit telemetry data from the sensors to the

master unit and receive the control signal from the master station to control the output

devices. RTU can do next:

Gather information from field devices (analog values, digital values, alarming).

Save information in memory unit and exchange with MTU.

Open/close valves, turns switches, change set points).

8.2.1 RTU Features

Intelligent to control a process and multiple processes

Data logging and alarm handling

Can control IEDs (Intelligent Electronic Device)

Expandable

Slave/Master device

RTU RTU

MTU

RTU

Figure 12: RTU in SCADA.



8.2.2 RTU Architecture

Figure 13 shows the block diagram of the RTUs hardware which includes (power

supply, backup battery, CPU, Memory, analog/digital outputs, radio module,

communication ports). Small RTUs generally have less than 10 to 20 A/D signals;

medium sized RTUs have 100 digital and 30 to 40 analog inputs.

RTU has two or three communication ports (RS232, RS422 and RS485) or Ethernet

link. A watchdog timer provides a check that the RTU program is executing regularly.

The RTU program regularly resets the watchdog timer and if this is not done within a

certain time-out period the watchdog timer flags an error.

Figure 13: RTU Architecture [10].



8.2.3 RTU Types

There are two types of RTU

1- Single board RTU: is compact device and contains all input output (normally has

fixed I/O) in the same board.

2- Modular RTU: which has separate modules for (CPU, I/O) and can added more

modules.

The RTU’s are expected to read data from field devices FD’s in a plant, and to

transmit data to the MTU. Likewise, the RTU’s expected to receive control

instructions from the MTU to control the plant as shown in Figure 15. This two-way

digital communication between RTUs and MTU is carried out on the so-called MTU-

RTU communication sub-system, which constitutes the third component of our

SCADA system. The RTU-FD communication may be analog (in old systems) or

digital (in modern systems) of SCADA system.

Figure 14: RTU Block Diagram [9].

RTU

MTU

Figure 15: MTU-RTU-FD communications.

D: Data

C: Control Sign.



The communication between MTU-RTU is two-way (duplex) digital

communication.

The communication between (non-smart) field devices - RTU is in one

direction only (simplex), from non-smart sensors to the RTU. Similarly, the

information to the unintelligent actuators has to come from RTU.

The set points received by the RTU from the MTU are always digital in nature,

because RTU-MTU communication is always digital.

Each RTU has a unique address.

Cost of laying cables (copper or optical fibre) would be very high.

Simplex: One way communication

Data in a simplex mode is always one way. Simplex mode is not often used because it is not possible to

send back error or control signals to the transmit end.

Half Duplex: Two way communication, but

only one way at a time

A half-duplex mode used to send and receive, but not at

the same time. it is possible to perform error detection and

request the sender to retransmit information.

Full Duplex: Two way simultaneous

communication

Data in duplex mode travel in both directions

simultaneously. Figure 16 shows the tree types of channels’ mode.

8.3 SCADA versus DCS

SCADA system and distributed control system (DCS), both, are suitable for the control

of large distributed industrial processes, a comparison between the two would be in

order. The table below shows the comparison between them

Feature DCS SCADA

1 Data transmission One way Two way

2 Type of control Full or elaborate control Limited or simple control

3 Extent of intervention by

operator Limited intervention Regular intervention

Figure 16: Communication Modes.

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