Digital control systems provide more flexible and precise control over industrial processes compared to previous pneumatic and analog electronic implementations. A digital control system (DCS) collects data through local control units and uses a data highway and general purpose computer to implement advanced control algorithms across multiple control loops and processes in a programmable way.

1. Shaheem TM
S7 | 24

2. • Collection of hardware and instrumentation
necessary for implementing control systems
• Provide the infrastructure (platform) for implementing
advanced control algorithms

3. • Pneumatic Implementation:
• Transm issio n: the signals transmitted pneumatically are slow
responding and susceptible to interference.
• Calculatio n: Mechanical computation devices must be
relatively simple and tend to wear out quickly.

4. • Electron analog implementation:
• Transm issio n: analog signals are susceptible to noise, and
signal quality degrades over long transmission line.
• Calculatio n: the type of computations possible with electronic
analog devices is still limited.

5. • Digital Implementation:
• Transm issio n: Digital signals are far less sensitive to noise.
• Calculatio n: The computational devices are digital computers.

6.  Digital computers are more flexible because they
are programmable and no limitation to the
complexity of the computations it can carry out.
 Digital systems are more precise.
 Digital system cost less to install and maintain
 Digital data in electronic files can be printed out,
displayed on color terminals, stored in highly
compressed form.

7. PROCESS
Final
control
element
Data
acquisition
Main
Computer
Display
1. PC Control:
• Good for small
processes such as
laboratory prototype
or pilot plants, where
the number of control
loops is relatively
small

8. 2. Programmable Logic Controllers:
• specialized for non-continuous systems such as batch
processes.
• It can be used when interlocks are required; e.g., a flow
control loop cannot be actuated unless a pump has been
turned on.
• During startup or shutdown of continuous processes.

9. Operator
Control
Panel
Main
Control
Computer
Operator
Control
Panel
Archival
Data
Storage
Supervisory (host)
Computer
PROCESS
Local
Computer
Local
Computer
Local
Computer
Local Display Local Display
Data highway
To other Processes To other Processes
Local data acquisition and
control computers
3. DCS
•Most
comprehensive

10. • Local Control Unit: This unit can handle 8 to 16
individual PID loops.
• Data Acquisition Unit: Digital (discrete) and analog I/O
can be handle.
• Batch Sequencing Unit: This unit controls a timing
counters, arbitrary function generators, and internal
logic.
• Local Display: This device provides analog display
stations, and video display for readout.
• BulkMemory Unit: This unit is used to store and recall
process data.

11. • General Purpose Computer: This unit is programmed by a
customer or third party to perform optimization, advance
control, expert system, etc
• Central OperatorDisplay: This unit typically contain
several consoles for operator communication with the
system, and multiple video color graphics display units
• Data Highway : A serial digital data transmission link
connecting all other components in the system. It allow for
redundant data highway to reduce the risk of data loss
• Local area Network(LAN)

12.  Access a large amount of current information from the data
highway.
 Monitoring trends of past process conditions.
 Readily install new on-line measurements together with local
computers.
 Alternate quickly among standard control strategies and
readjust controller parameters in software.
 A sight full engineer can use the flexibility of the framework to
implement his latest controller design ideas on the host
computer.

13. Digital DCS systems are more flexible. Control
algorithms can be changed and control configuration
can be modified without having rewiring the system.

14. • (A/D) Transducers convert analog signals to digital
signals. (Sensor Computer)
• (D/A) Transducers convert digital signals to analog
signals. (Computer Valve)

15. • Accuracy depends on resolution.
• Resolution depends on number of bits:
Resolution = signal range × 1/(2m
-1)
m = number of bits used by the digitizer (A/D) to represent
the analog data

16. Advanced control topology
DCS
Workstation
Process
Two-ways
Data Bus
Online: Real-time data
PI interface
PI interface: C++ or Fortran
PI
system