sampled data system

1. COMPUTER CONTROL
OF PROCESS
UNIT – I (SAMPLED DATA CONTROL
SYSTEMS)

2. Supervisory Control
 Supervisory control is a system which is
controlling whole plant or more than one
field devices through a master system.
 There will many types of field devices in
a plant, each of the devices will have
controllers to control the control action.
 We can’t install a signal controller to
control the whole plant, it’s logic less. But
can connect all the controller to a single
master device. So a human can verify
the process variable and set the set point
to each controller distinctly.

3.  A natural extension of a computer data logging
system involves computer feedback on the
process through automatic adjustment of loop set
points.
 As various loads in a process change, it is often
advantageous to alter set points in certain loops
to increase efficiency as to maintain the
operation within certain precalculated limits.
 The choice of set point is a function of many
other parameters in the process. In fact, a
decision to alter one set point may necessitate
the alteration of many other loop set points as
interactive effects are taken into account.
 Given the number of loops, interactions, and
calculations required in such decisions, it is more
natural and expedient to let a computer perform
these operations under program control.

4. Example on Supervisory
control

5.  Here, a computer “supervises” the furnace’s
temperature by communicating setpoint values to
the temperature indicating controller (TIC) over a
digital network interface such as Ethernet.
 From the temperature controller’s perspective,
this is a remote setpoint signal, as opposed to a
local setpoint value which would be set by a
human operator at the controller faceplate.
 Since the heat-treatment of metals requires
particular temperature ranges and rates of
change over time, this control system relieves
the human operator of having to manually adjust
setpoint values again and again during heat-
treatment cycles.
 Instead, the computer sets different setpoint at
different stages according to the needs.

6. Process controllers configured for supervisory
setpoint control typically have three operating
modes:
 Manual mode
 Automatic mode with local SP
 Automatic mode with remote SP

7. Supervisory Control Structure
 Now we consider the control structure of
supervisory control as

8.  The master computer or supervisor makes the
setpoint and gives to the controllers. Controllers
adjust the PV with respect to the setpoint using
PID controller.
 This forms a two-layer process control system:
the “base” or “regulatory” layer of control (PID
loop controllers) and the “high” or “supervisory”
level of control (the powerful computer with the
mathematical process models).
 A single cable can’t do the setpoint changes at
once or much frequently, so a series of digital
data lines are implemented to transfer setpoint
from supervisor to controller.
 But it may also carry process variable information
from those controllers back to the supervisory
computer so it has data for its optimization
algorithms to operate on.

9. Basic functions of Supervisory
control
The basic functions performed by
supervisory controller in a plant are:
Channel Scanning
Conversion into Engineering Units
Data Processing

10. (i) Channel Scanning:
 There are many ways in which
microprocessor can address the various
channels and need the data.
Polling:
 In polling technique, each interrupting
channel will have an interrupt or status flag
which is checked by the processor one by
one.
 The channel selection may be sequential or
in any particular order decided by the
designer.
 It is also possible to assign priority to some
channels over others, i.e., some channels
can be scanned more frequently than others.

11.  Channel scanning and reading of data
requires, the following actions to be
taken:
Sending channel address to
multiplexer
Sending start convert pulse to ADC.
Reading the digital data.
 For reading the digital data at ADC
output, the end of conversion signal of
ADC chip can be read by processor
and when it is ‘ON’ the digital data can
be read.

12. Channel Scan Array:
 The scan array contains the
addresses of the channels in the order
in which they should be addressed.

13.  Interrupt Scanning:
 Another method of scanning is interrupt
scanning. It sends interrupt request signal to
processor when the analog signal from
transducer is not within High and Low limits
boundary set by Analog High and Analog Low
Signals.
 This is also called scanning by Exception.
When any parameter exceeds the limits than
the limits checking circuit would send
interrupt request to microprocessor which in
turn would monitor all parameters till the
 parameter values come back within pre -
specified limits.

14. (ii) Conversion to Engineering Units:
 The data read from the output of ADC
should be converted to the equivalent
engineering units before any analysis
is done as the data is sent for display
or printing.
 For an 8-bit ADC working in unipolar
mode the output ranges between 0
and 255. Conversion is done by the
following parameters.
 The basic voltage and current range of
transmitter is 0-5 V (or) 4-20 mA range

15. (iii) Data Processing:
 The data read from the ADC output for various
channels is processed by the microprocessor to
carry out limit checking and performance analysis.
 For limit checking the highest and lowest limits for
each channel are stored in array. The limit array
unit is shown

16.  The limit array simplifies the limit checking
routine.
 Through this, the facility to dynamically change
the limits for any channel may also be provided,
on the lines similar to scan array.
 In addition to limit checking, the system
performance may also be analysed and report
could be generated for the manager level.
 This report will enable the manager to visualise
the problems in the system and to take decisions
regarding system modification or alternate
operational strategy to increase the system
performance.
 The analysis may include histogram generation,
standard deviation calculation, plotting one
parameter with respect to another and so on.

17. Advantages of Supervisory
control
 More productive, remote access
 Sustainability
 Interoperatability
 Increases efficiency of process control
 Easy maintenance

18. Disadvantages
 External interference
 Need trained personnel
 High investment needed