TECHNICAL FIELD OF THE INVENTION
The present invention generally relates to field of ‘Automation’ for deriving pulse width
modulated output from PID function block based on Programmable logic controller.
More particularly, the invention provides a method of deriving the pulse width
modulated output from linear PID function block in programmable logic controller
using split range control.
BACKGROUND OF THE INVENTION
In the past, PLC’s were generally used for sequential logic controls replacing relays
essentially involving digital input / output signals. However, with the advancement in
hardware and software capabilities PLC’s augmented its domain in the field of analog
process control. Complex Proportional Integral Derivative (PID) controls are feasible in
PLC in the form of function blocks much similar to that of a Distributed Control
System (DCS).
Proportional Integral Derivative (PID) control is immanent constituent of regulatory
process control, where based on deviation (error) between control parameter (process
value) and set point, controller adjusts its output to drive the actuating element
aiming to reduce the error to minimum.
A basic type of close loop control system is depicted in Fig 1.
The controller output ‘m(t)’ after proportional, integral and derivative action on error ‘e’
is derived from the following equation 1. The values of constants Kp, Ti and Td are
dependent on process and are adjusted for best performance of controller.
…..1
The controller output ‘m (t)’ is generally linear viz. 0 – 100%, which is generally used
for controlling linear elements like pneumatic actuators. However this linear controller
output cannot actuate electro-mechanical actuators, which needs electrical pulses for
 


 


= + ∫ + dt
de t
e d T
T
m t K e t d
t
i t
p
( )
( ) )
1
( ) ( )
0
t t
3
its operation. The PLC’s are generally capable of delivering 0 – 100% linear output
from PID control blocks, which can be easily used for driving pneumatic actuators by
first converting 0 – 100% to 4 – 20 mA (milli-ampere) and then to 3 – 15 psi (pounds
per square inch). However, linear 0 – 100 % output cannot drive electro-mechanical
actuators.
Pulse output from PLC based PID function blocks are generally generated by
comparing linear percentage output from PID blocks with actual opening / closing
percentage of actuating valve. An open or close pulse is generated based on the
deviation between the above mentioned two signals. Partial application of split range
control is also used in some places for deriving pulse output from PLC based PID
function block
Derivation of pulse output by comparison between controller output and actual valve
position depends heavily on the accuracy of valve position signal. Any mismatch
between actual valve opening / closing percentage and transmitted signal to PLC may
introduce serious error in Controller Output, which may deteriorate control accuracy
and plant performance. Partial application of split range control is also not precise as
its response time is extremely slow to address any deviation between actual process
parameter and set value. Poor control accuracy adversely affects the plant
performance. This has been elaborated in the subsequent sections.
OBJECT OF THE INVENTION
A primary object of the present invention is to overcome the disadvantages/drawbacks
of the known art.
Another object of the present invention is to provide a novel method of deriving a pulse
width modulated output from linear output type PID control function blocks of PLC
using split range control.
Another object of the present invention is to provide overlapping of control bands in
split range control to generate instant smooth pulses for raise as well as Lower signal
generation.
4
Another object of the invention is to provide a novel method which is very fast in
response
Yet another object of the present invention is to provide a novel method which is
effectively used for driving electro-mechanical actuators from analog type PID control
blocks used in PLC.
These and other advantages of the present invention will become readily apparent
from the following detailed description read in conjunction with the accompanying
drawings.
SUMMMARY OF THE INVENTION
The following presents a simplified summary of the invention in order to provide a
basic understanding of some aspects of the invention. This summary is not an
extensive overview of the present invention. It is not intended to identify the
key/critical elements of the invention or to delineate the scope of the invention. Its
sole purpose is to present some concept of the invention in a simplified form as a
prelude to a more detailed description of the invention presented later.
In one aspect the present invention provides a novel computer program implemented
method of derivation of the pulse width modulated output from linear output type PID
control function blocks of PLC using split range control.
In this method four function blocks are connected in cascade. The cascaded function
blocks are split range control module, pulse width modulation module followed by two
Boolean “AND” operation.
Split range control module splits the 0-100% control output from PID block in two
parts specifically in two ranges by overlapping of valve opening and valve closing.
Valve closing lies between 20-90% and valve opening lies between 20-100%.
The next function block is pulse width modulation which converts the percentage
output from overlapped split range function block to pulses of varying width.
5
Simultaneous open and close signal is restricted by performing Boolean AND with
error signal. Second phase of Boolean AND is done to avoid simultaneous triggering of
both open and close signals
In other aspect of the present invention is to provide very fast response and being
effectively used for driving electro-mechanical actuators from analog type PID control
blocks generally used in PLC. The fast control action accentuates accurate process
control.
Other aspects, advantages, and salient features of the invention will become apparent
to those skilled in the art from the following detailed description, which, taken in
conjunction with the annexed drawings, discloses exemplary embodiments of the
invention.
BRIEF DESCRPTION OF THE ACCOMPANYING DRAWINGS
The following drawings are illustrative of particular examples for enabling methods of
the present invention, are descriptive of some of the methods, and are not intended to
limit the scope of the invention. The drawings are not to scale (unless so stated) and
are intended for use in conjunction with the explanations in the following detailed
description.
Figure.1 illustrates schematic of close loop process control
Figure.2 illustrates general split range control
Figure.3 illustrates modified split range control
Figure.4 illustrates cascaded function blocks
Persons skilled in the art will appreciate that elements in the figures are illustrated for
simplicity and clarity and may have not been drawn to scale. For example, the
dimensions of some of the elements in the figure may be exaggerated relative to other
elements to help to improve understanding of various exemplary embodiments of the
present disclosure.
6
Throughout the drawings, it should be noted that like reference numbers are used to
depict the same or similar elements, features, and structures.
DETAIL DESCRPTION OF THE INVENTION
Accordingly present invention provides a novel method of derivation of pulse width
modulated output from linear output type PID control function blocks of PLC using
split range control.
This method is based on cascaded operation of four function blocks. The cascaded
function blocks are split range control module, pulse width modulation module
followed by two Boolean “AND” operation.
Split Range Control Module splits the 0-100 % control output from PID block in two
parts specifically in two ranges. In general type split range breaks control output or
control variable (CV) in two ranges 0-50% and 50-100% as shown in Fig 2.
The 0-50% range is used in case of negative error i.e. process parameter is greater
than set parameter. This range is called cooling range implying control valve should
close. The 50-100% range is used in case of positive error i.e. process parameter (PV)
is less than set parameter (SP). This range is called heating range implying control
valve should open. The percentage opening or closing of control valve depends on “OP”
as calculated from equation (2) and (3).
It is apparent from above that in case of 0% CV, OP1 = 100%, OP2 = 0 i.e. valve is fully
closed, while in case of 100% CV, OP1 = 0, OP2 = 100% i.e. valve is fully open. The
above implies that in case of negative error (PV > SP), valve will close and in case of
positive error (PV < SP) valve will open. The percentage opening or closing depends on
equation (2) and (3).
7
General split range control suffers from a major drawback that if error sign is reversed
i.e if process variable has exceeded set point or vice versa, it will wait till the PID
output CV crosses 50%. It means that valve movement from opening to closing or vice
versa will take place only when CV crosses 50%. This makes control action very
sluggish leading to large error value, which is not desirable. This has been indicated in
drawbacks of present art
To overcome the aforesaid drawback, a novel technique has been worked out by
overlapping the heat (valve opening) and cool (valve closing) zones. As indicated in Fig
3 below, Cool zone lies between 20 to 90% while heat zone lies between 20 to 100%.
Based on equation (2) and (3) above, opening and closing action (OP1 and OP2) at
various output level (CV) of PID can be calculated as shown in following table 1.
Table 1 : Results of Modified Split Range Control
CV
(%)
OP1 (Closing
%)
OP2 (Opening
%)
Action
20 100 0 Closing : 100 %
Opening : 0%
40 71.5 25 Closing : 71.5 %
Opening : 25 %
50 57 37.5 Closing : 57 %
Opening : 37.5 %
60 43 50 Closing : 43 %
Opening : 50 %
80 14 75 Closing : 14 %
Opening : 75 %
8
It is evident from Table1 that as CV increases closing action reduces and opening
action increases. However at a time over entire CV range, there are both opening as
well as closing actions. But the control valve will either open or close and
simultaneous opening & closing is not feasible. This problem of both closing and
opening signal at a time is resolved by performing logical AND of Split Range output
with error signal explained in subsequent paragraphs. But the overlapped Split Range
action immediately generates opening / closing signals based on magnitude and sign
of error value.
The next function block is pulse width modulation which converts the percentage
output from overlapped split range function block to pulses of varying width. The
pulse duration is determined by following equation:
Where, PWM is output cycle time of
modulator, OP is output of split range control and TC is total cycle time. Hence if TC =
50 sec. then output pulse width for 60% OP will be 30 sec. So there will be pulses of
varying widths both for opening and closing action. But as soon as error (SP-PV) value
changes the system immediately generates open and close command. The cascaded
function block is depicted in Fig 4.
As explained above, simultaneous open and close signal is restricted by performing
Boolean AND with error signal. In case of positive error i.e. SP > PV, open command
line will be logically true (i.e. output will be open command) and in case of negative
error i.e. SP < PV, close command line will be logically true (i.e. output will be close
command).
Both the output from first stage of Boolean AND is further crossed and again second
phase of Boolean AND is done to avoid simultaneous triggering of both open and close
signals.
The above schematic is very fast in response and being effectively used for driving
electro-mechanical actuators from analog type PID control blocks generally used in
PLC. The fast control action accentuates accurate process control.
The innovation is best implemented in all latest generation Programmable Logic
Controllers (PLC) and Programmable Automation Controllers (PAC). Due to the advent
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of powerful processors and enhanced software development features, the current
generation PLC’s or PAC’s are bestowed with cascaded function block programming.
The present method can be best applied in any latest PLC or PAC with feature of
function block programming.
The method as described in the present embodiment is applicable for on-line control of
Sinter Machine Parameters in Steel Plants using latest PLC models, for heating control
of Bell Annealing Furnaces implemented through appropriate PLC models fetching
very high control accuracy. The method as described in the present embodiment is
also applicable for any process control application accomplished through latest PLC
with electromechanical actuator as final control element. This method will render a
very fast process control solution, where plant is controlled by open / close command
in electro-mechanical actuators.
The methodology and techniques described with respect to the exemplary
embodiments can be performed using a machine or other computing device within
which a set of instructions, when executed, may cause the machine to perform any
one or more of the methodologies discussed above. In some embodiments, the
machine operates as a standalone device. In some embodiments, the machine may be
connected (e.g., using a network) to other machines. In a networked deployment, the
machine may operate in the capacity of a server or a client user machine in a serverclient
user network environment, or as a peer machine in a peer-to-peer (or
distributed) network environment. The machine may comprise a server computer, a
client user computer, a personal computer (PC), a tablet PC, a laptop computer, a
desktop computer, a control system, a Programmable Logic Controller (PLC), a
Programmable Automation Controller (PAC), a network router, switch or bridge, or
any machine capable of executing a set of instructions (sequential or otherwise) that
specify actions to be taken by that machine. Further, while a single machine is
illustrated, the term "machine" shall also be taken to include any collection of
machines that individually or jointly execute a set (or multiple sets) of instructions to
perform any one or more of the methodologies discussed herein.
The machine may include a processor (e.g., a central processing unit (CPU), a graphics
processing unit (GPU, or both), a main memory and a static memory, which
communicate with each other via a bus. The machine may further include a video
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display unit (e.g., a liquid crystal display (LCD), a flat panel, a solid state display, or a
cathode ray tube (CRT)). The machine may include an input device (e.g., a keyboard)
or touch-sensitive screen, a cursor control device (e.g., a mouse), a disk drive unit, a
signal generation device (e.g., a speaker or remote control) and a network interface
device. The machine may include input device (e.g., an analog or digital input card)
and output device (e.g., an analog or digital output card).
The disk drive unit may include a machine-readable medium on which is stored one or
more sets of instructions (e.g., software) embodying any one or more of the
methodologies or functions described herein, including those methods illustrated
above. The instructions may also reside, completely or at least partially, within the
main memory, the static memory, and/or within the processor during execution
thereof by the machine. The main memory and the processor also may constitute
machine-readable media.
Dedicated hardware implementations including, but not limited to, application specific
integrated circuits, programmable logic arrays and other hardware devices can
likewise be constructed to implement the methods described herein. Applications that
may include the apparatus and systems of various embodiments broadly include a
variety of electronic and computer systems. Some embodiments implement functions
in two or more specific interconnected hardware modules or devices with related
control and data signals communicated between and through the modules, or as
portions of an application-specific integrated circuit. Thus, the example system is
applicable to software, firmware, and hardware implementations.
In accordance with various embodiments of the present disclosure, the methods
described herein are intended for operation as software programs running on a
computer processor or a processor of Programmable Logic (PLC) Controller or a
processor of Programmable Automation Controller (PAC). Furthermore, software
implementations can include, but not limited to, distributed processing or
component/object distributed processing, parallel processing, or virtual machine
processing can also be constructed to implement the methods described herein.
11
The present disclosure contemplates a non transitory machine readable medium
containing instructions, or that which receives and executes instructions from a
propagated signal so that a device connected to a network environment can send or
receive voice, video or data, and to communicate over the network using the
instructions. The instructions may further be transmitted or received over a network
via the network interface device.
While the non transitory machine-readable medium can be a single medium, the term
"machine-readable medium" should be taken to include a single medium or multiple
media (e.g., a centralized or distributed database, and/or associated caches and
servers) that store the one or more sets of instructions. The term "machine-readable
medium" shall also be taken to include any medium that is capable of storing,
encoding or carrying a set of instructions for execution by the machine and that cause
the machine to perform any one or more of the methodologies of the present
disclosure.
The term "machine-readable medium" shall accordingly be taken to include, but not
be limited to: tangible media; solid-state memories such as a memory card or other
package that houses one or more read-only (non-volatile) memories, random access
memories, or other re-writable (volatile) memories; magneto-optical or optical medium
such as a disk or tape; non-transitory mediums or other self-contained information
archive or set of archives is considered a distribution medium equivalent to a tangible
storage medium. Accordingly, the disclosure is considered to include any one or more
of a machine-readable medium or a distribution medium, as listed herein and
including art-recognized equivalents and successor media, in which the software
implementations herein are stored.
The illustrations of arrangements described herein are intended to provide a general
understanding of the structure of various embodiments, and they are not intended to
serve as a complete description of all the elements and features of apparatus and
systems that might make use of the structures described herein. Many other
arrangements will be apparent to those of skill in the art upon reviewing the above
12
description. Other arrangements may be utilized and derived therefrom, such that
structural and logical substitutions and changes may be made without departing from
the scope of this disclosure. Figures are also merely representational and may not be
drawn to scale. Certain proportions thereof may be exaggerated, while others may be
minimized. Accordingly, the specification and drawings are to be regarded in an
illustrative rather than a restrictive sense.
Thus, although specific arrangements have been illustrated and described herein, it
should be appreciated that any arrangement calculated to achieve the same purpose
may be substituted for the specific arrangement shown. This disclosure is intended to
cover any and all adaptations or variations of various embodiments and arrangements
of the invention. Combinations of the above arrangements, and other arrangements
not specifically described herein, will be apparent to those of skill in the art upon
reviewing the above description. Therefore, it is intended that the disclosure not be
limited to the particular arrangement(s) disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include all embodiments and
arrangements falling within the scope of the appended claims.
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WE CLAIM:
1. A computer or Programmable Logic Controller (PLC) or Programmable
Automation Controller (PAC) program implemented method of deriving Pulse
Width Modulated Output from PLC based linear PID function block by using
current generation PLC having advanced function block programming feature,
said method comprising:
(a) Converting of 0-100% controlled linear variable output from PID into heat
and cool zones of different ranges;
(b) Overlapping said zones to generate instant smooth pulses for heat and cool
signal generation;
(c) Generation of pulse of variable width; and
(d) Filtering simultaneous triggering of both Open / Close commands by
performing two stage Boolean AND w.r.t. process error signal.
2. The method as claimed in claim 1 wherein generation of open and close
command corresponds to the change in error (SP-PV) value.
3. The method as claimed in claim 1 wherein open command line is logically true
when positive error occurred.
4. The method as claimed in claim 1 wherein Close command line is logically true
when negative error occurred.
5. The method as claimed in claim 1 wherein opening and closing signals are
generated based on magnitude and sign of error value.
6. The method as claimed in claim 1 wherein converting of output from PID into
heat and cool zones is done by the split range control module
7. The method as claimed in claim 1 wherein Pulse of variable width is generated
using PWM module
8. The method as claimed in claim 1 wherein cool zone lies between 20-90%.
9. The method as claimed in claim 1 wherein heat zone lies between 20-100%
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10. The method as claimed in claim 1 the said method is effectively used for driving
electro-mechanical actuators.
11. The method as claimed in any preceding claims comprising determination of
pulse duration by means of
Wherein PWM is output cycle time of modulator, OP is output of split range
control and TC is total cycle time.