The Human Response to a Digital Control System

locketparachuteElectronics - Devices

Nov 15, 2013 (3 years and 11 months ago)

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The Human Response to a Digital Control
System


BrianNetz, SeniorElectrical Engineer.



Abstract



A

brief
description

of how
human interaction is a necessity for the
continual operation of digital control
systems

is presented. This document
contains a d
etailed description of some
of the components involved with the
digital control system. A strong
emphasis is placed on the integration of
all of these components.


I.

INTRODUCTION


Genesis 1:28a “God blessed them and said to them,
‘Be fruitful and increas
e in number; fill the earth
and subdue it’
.”

This passage can easily be taken out of context
without giving serious thought to the implications
that this means. The responsibility that this passage
bears is mind
-
numbing! Engineers are more
directly affe
cted by this command than many other
professions and as Christians we are called to learn,
understand, and improve on the things that God has
given us.
This document provides a brief summary
of how digital controls systems are incorporated
into human cont
rol systems to improve response
times and to subdue the desired process.


II.
THE DIGITAL CONTROL SYSTEM



Digital
control systems feature a number of
different electrical
components

that are integrated
into one or multiple systems. Digital control
syst
ems are used in any number of different
applications. They are used to control robot
ic

arms,
airplane flaps, robotic cranes, and conveyor
systems to name a few.
Digital control systems are
increasing in complexity and size. They are very
versatile rangi
ng from watches to
gigantic robots.
The main design of a digital design system is to
have several different components
feeding digital

or analog

signals to a processor, the processor in
turn takes the signals
,

uses some sort of logic on the
input signals,

and then the processor turns on or off
certain outputs determined by the logic in the
processor.

This method of always turning on or off
different components without feed back and
without gain control is known as the “bang bang”
approach to control syst
ems.


Since digital control systems are so varied in
nature and design, a specific design will be used for
this paper
.
Allen
-
Bradley is one of the leading
manufactures of control system components and
they pr
oduce several families of PLC’s that will b
e
discussed later in this paper.


A. PLC



The PLC is known as the
programmable logic
controller

and is the “brain” of the entire control
system. PLC’s are relatively new to the market but
they
have already made a substantial impact.

They
were first m
ade in the late 60’s to eliminate the
massive amounts of relays used to make entire
system
s

operate. They were also more reliable as it
was a software based thing, not a mechanical
device which
would

wear out over time. Naturally
the first ones were not
all that fast, and certainly did
not have many features. They could also not store
very many lines of code. The past 40 years has
brought many changes to the PLC. They are now
faster, smaller, have more memory, and can now be
linked together. The latte
r has only recently been
an achievement as early on, they did not have any
standards and hardly anything had the same
network protocol

[1]
.


PLC
’s

work by programming code, ladder logic,
visual block
s
, or many numerous other types of
languages

into the

PLC so that it only triggers the
right output under the conditions you set for it.

The
ladder logic is very comparable to IF…THEN
statements.

See
Fig
.

5

for an example of the PLC
code.

This eliminates the need for a tremendous
amount of relays, which also cuts down on cost,
labor for wiring, and the chance that it gets wired
incorrectly. It can easily be seen how much the
PLC has added to the manufacturing market.


The logic that is in most controllers today
uses

all
of the c
ommon ones: AND, NOT, OR, NOR,
NAND, etc. They also have many advanced logic
statements that vary the voltage of the output based
proportionally on a separate input that the controller
is getting fed. The “digital relays”
,pictured in
Fig
.

3
,
that are used in the controllers are called
“examine if closed” and “examine if open.” These
two pieces are commonly used for the logic and
eliminate the mess of having hundreds to thousands
of relays and the mess of wiring th
em all up
together.


As seen in Fig. 6, the output is highlighted.
When using this specific code, if the relay is
allowing current through it, the piece of code is
highlighted. On the other hand, if no current is
being allowed the relay, there is no co
lor around
that specific piece of code. This makes it
incredibly easy for troubleshooting for engineers.
Often digital control systems have PC’s hooked up
to the PLC that enables them to “watch” what the
system is doing. This enables shorter down times
on manufacturing lines, as well as an easier, user
-
friendly interface.


Allen Bradley created many different families of
their manufacturing processors. Their first large
sales processor was the PLC
-
5. The following is
the brief history of the Allen Br
adley’s PLC
-
5.


In the early 1970’s Allen Bradley revolutionized
the automotive industry. The PLC
-
5 made its debut
in 1985 which lead to the conformation of
control
engineering around the world. In 1990 the PLC
-
5’s
input/output cards were improved and
became
standardized. The communication modes for the
PLC
-
5 were greatly improved in 1993 by adding
Ethernet capabilities and then broadening it further
in 1995.

By 2000, Allen Bradley had sold over
400,000 PLC
-
5’s and 9million input/output cards.
The l
atest change to the PLC
-
5 was made in 2001
by adding internet capabilities [
2
]. The PLC
-
5 was
replaced by several of the newer families such as
the PLC
-
500 series and the PLC
-
5000 series talked
about below.


ControlLogix processor modules are available i
n a
range of memory. Logix5555 processors (1756
-
L55Mxx) have 750K, 1.5M, 3.5M, or 7.5M bytes
of user memory. You can replace the memory sub
-
module on a Logix5555

processor to change the
memory size
.


The ControlLogix556x processor (1756
-
L61,
L62,
-
L63) h
as 2, 4, or 8M bytes of fixed memory
plus a built
-
in socket for the addition of
CompactFlash nonvolatile memory. This high
-
performance processor has an advanced CPU with
high clock speed and built
-
in floating
-
point math
coprocessor for faster execution of
discrete logic as
well as motion control, function block diagram, and
real data type math. This improved performance
means you can run more motion axes at lower
coarse update rates. You can also run more
demanding logic in
the same processor with
motion.



The ControlLogix processor can perform motion
planning functions and generate motion commands
that are sent to a motion control module. Motion
control modules are available with analog or digital
SERCOS interfaces. You choose one of the various
motion co
ntrol modules to interface with servo
drives or hydraulic servo valves. High
-
level
command execution is performed on the processor

[3]
.


B.
Input/Output Cards




Input/Output cards, commonly referred to as I/O
cards, are the veins and arteries of the di
gital
control system. These cards are an easy way to
deal with all of the inputs and outputs that the PLC
requires. The cards can hold up to sixteen different
inputs or outputs, depending on the card, and have
to be configured correctly in order to talk

to the
PLC. Once correctly configured, the cards are
connected via wires to the desired places. This
makes the PLC a very versatile piece of technology
as the number of inputs and outputs can vary for
many different applications.


C. R
ELAYS




To

understand control systems is to understand
relays (see
Fig
.

1
). Relays have been around for a
long time and at one point in time they were the
primary way for logic control. Robot systems used
to be made solely of relays. In o
rder for there to be
enough relays to control an advanced robotic
system, there would usually have to be several
panels filled with relays that were five to six times
larger than the robot itself. This meant that control
systems would take up a lot of pla
nt floor and were
not very practical for small manufacturing firms.
All relays must be hardwired which took a lot of
time for the electricians or engineers to hook up
(see
Fig
.

2
). Also this made troubleshooting nearly
impossible
. If there were eight wires to hook up on
each relay, and there are hundreds to thousands of
relays, then there are millions of different
possibilities to make a mistake. Relays were also
expensive, which meant that buying several
hundred to several tho
usand was not a light
investment. Just like all mechanical devices, relays
break down over time, which made upkeep
expensive on systems that utilized many relays.


Fig
.

1
: A relay



Fig
.

2
: Wiring diagra
m for a relay



Although relays have their downside, they are
necessary for the modern control system. Relays
come in many different sizes and shapes, and have
many different characteristics; however, many of
the relays can be modeled using programming
software that is put on the PLC (see
Fig
.

3
). This
cuts down on the initial cost, as well as the cost for
replacing them, as there are fewer relays needed.



Fig
.

3
: Example PLC code for relays



I)
Re
lay’s operation:



Relays act a lot like switches. They can either
turn on or off a circuit determined by how that relay
is hooked up. A relay consists of a coil that
switches the current state of other circuits when
current goes through the coil. The
re are two types
of states that the circuits connected to the relay can
be in; normally open and normally closed.
Normally open refers to the state that when the coil
is closed by having current go through it, the circuit
that is connected to the relay al
so closes. This can
be thought of as a normal light switch; when the
light switch is turned on, the circuit is closed and
the light goes on. Normally closed state is the
compliment of normally open, and it works exactly
opposite of normally open. When t
he coil of the
relay is closed, then the normally closed circuit
opens. This is like a not gate. When the coil circuit
is closed then the normally closed circuit is open,
turning it off.


This principle of having one circuit turn on and
off other circ
uits is the underlying notion of the
PLC. Instead of using actual relays, the PLC uses
virtual relays and any other logic function to turn
on and off the desired outputs. This saves the time
and money of wiring up many physical relays.
Physical relays a
re still used mainly for turning on
circuits that use different voltages than the output
voltage that the output cards use for the PLC.


E. Sensors



Sensors are some of the major inputs to the PLC.
They are the eyes and the ears of the digital control
system. They come in many different kinds, from
photoeyes to lasers that calculate distance. The
sensors can send many different types of inputs to
the PLC as well such as: numbers, fault messages,
distances, or simply on or off signals.



I) Photoeye
:



Photoeyes are a common, easy way to solve
detection problems. They are not as precise as
Normally Open

Normally
Closed

some means of measuring, but they are cheaper and
effective for general use applications. The
photoeye has three main parts to it, the emitter,
collector, and
relay. The emitter produces a beam
of light which is then reflected off of something and
interpreted by the photo
-
collector. Depending on
the type of photoeye, the relay is either tripped
when the collector does not see the beam or when it
sees the light
. When the relay is tripped, the
photoeye sends a signal to be interpreted by the
PLC. There are two main kinds of photo eyes as
described below, but the principle remains the
same. The photo detects whether there is
something right in front of it.



a:
Reflective
p
hotoeye:





Reflective photoeyes work by emitting a beam
that is reflected back to it. The choice of reflective
material greatly influences the distance the
photoeye will work. There are three different types
of reflective material to u
se which consists of: a
mirror, reflector, or anything that really does not
reflect enough. In
Fig
.

4

the three different types of
reflective material are shown with the general
results produced by using each type
of material.
The mirror can reflect a greater distance, but since
it only reflects one small beam back, it is not
practical as it would be too easy to miss the
reflection. The non
-
reflective material is also a bad
decision as the beam will not make it ba
ck to the
photoeye. A reflector of some sort would be the
best decision as it emits a nearly as strong as the
mirror signal back to the photoeye, but disperses
the signal at various angles to give a better signal
back to photoeye. This is the most practi
cal
method as most of the environments that the
reflector will be in will be experiencing many
different vibrations and collisions in a
manufacturing setting. When the signal is bounced
back and the photoeye recognizes it, the relay
inside is tripped. Th
is means that the photoeye is
sending a signal to the PLC that is saying that the
photoeye is clear and nothing is blocking its field of
vision. The general range of a reflective photoeye
is in the magnitude of six to eight feet, depending
on how good of
sensor and reflector is being used.




Fig
.

4
: Different reflective qualities




b:
Proximity photoeye
:



The proximity photoeye works a lot like a
reflective photoeye except it does not require a
reflective surface to register

if there is something in
front of it. The photoeye emits its beam and if
there is a change in intensity it figures there is
something in front of it. The range of the proximity
photoeye is a lot shorter than the reflective
photoeye, but they work out re
ally well in situations
where mounting reflectors is nearly impossible.



II)
Light curtain:



Light curtains act a lot like photoeyes (see
Fig
.

5
).
An easy way to think about light curtains is to
envision an a
rray of photoeyes very close together
in a straight line. Light curtains act the same way
as photoeyes as they emit a small beam of light and
register the reflection back to it. If any one the
little beams of light is broken, the curtain is tripped
and i
t sends a signal to the PLC. Light curtains are
often used as safety instruments as they are placed
in front of an area that should not be entered during
normal operations of the machine.



Fig
.

5
: Graphical representation of the

operation of
a light curtain



III)
Proximity Sensor:



Some p
roximity sensors actually use ultrasonic
waves to register if anything is in front of the
sensor. Generally proximity sensors have a very
short range, magnitude of several millimeters.
Th
ese are generally used for more precise shorter
detection
. For example, a photoeye might be used
to see if there is a fridge sitting on a bed of
conveyors, while a proximity sensor might be used
to see if there is a
part

in the welding
grips
, ready to
be
welded.



F
.
Dislay




A great tool for digital control systems is their
ability to create unique and easy
-
to
-
understand
displays. Two of most common software packages
for monitoring a digital control system are RSView
and PanelView. These two program
s hook directly
to the PLC and can output anything you want it to.
Although lots of time is often required to get
everything communicating properly and the designs
made for the screens, it can show a very accurate
portrayal of the system as well as have c
apabilities
for human interfacing.


A distinct advantage of the digital control system
is its ability to output to a screen or a series of
screens. This often ups the initial cost of buying
the displays, but looking further into it, it saves a
lot of t
ime

for the mechanics. If the system faults
out, display screens can (if they are programmed
correctly) show the mechanic what is wrong with
the system, saving time and labor cost for the
mechanic to investigate the problem.


G. An example, tying it altog
ether



The following section gives examples of code
that Allen
-
Bradley uses for their ladder logic, some
block diagrams, and simulink models to give a
more in
-
depth look at how the PLC interprets
signals and functions as a whole.


The control system u
sed in this example consists
of a laser distance input (a distance to an object is
calculated by using a laser that is reflected back to
the
receiver

to calculate the distance) and an output
to turn a motor on. The goal of this design is when
the distance

to a certain object is less than five feet,
a different motor will turn on. The ladder logic
code is show in
Fig.
6
.

A Simulink block diagram
model is shown in
Fig.
7

of t
he exact same control
scheme.



Fig.
6
: Ladder logic code for example.


Fig.
7
: Simulink block diagram.


In Fig. 7 a ramp input is used to simulate the
input distance of the laser. The ramp input start
s at
ten and linearly decreases to zero at a rate of 1 foot
per second. The IF statement is just like the one
seen in the ladder logic and operates the same way.
For the Simulink model ties the output to ground if
the input signal is greater than five an
d ties the
output to high if the signal goes below five. This is
how PLC’s work. It is a “bang bang” approach; the
output is either tied high or tied low depending on
the logic of the controller.


When the model in
Fig.
7
is simulated, the
input

is
seen in
Fig.
8
, while the output to the motor is
shown in
Fig.

9
.

This shows how the PLC works.
The output remains constant until it rea
ches five
feet. At that point it is triggered high even after the
distance keeps going down.



Fig.
8
:

The ramp input to the Simulink model
shown in
Fig.
7
.



Fig.

9
: Output sign
al.

III.
THE PHYSICAL CONTROL SYSTEM



Without the physical control system the digital
control system is useless. The digital control
system might be able to tell you if it encountered a
problem, but could never fix itself. That is where
the human int
eraction fits into the control system.

If a motor faults out, an alarm of some nature can
be set off by the digital control system, but the
motor will remain broken until somebody fixes it.

At this point the digital control system is at the
mercy of huma
n control

and fixing. The human
reaction acts as a feedback loop, as there is no
transfer function for a PLC.

Fig.

10

illustrates a
digital control system with human interaction. The
simulation is similar to the
one earlier in this
document. The input is a laser measurement,
except this time, when the distance is less than five,
an alarm is triggered, and will not shut off until
somebody clears the fault and resets the system.
This is represented by having a neg
ative high signal
for when the operator clears the fault, and no signal
when the system has faulted, but nobody is doing
anything about it. The manual switch labeled
below is exactly that, a manual switch. The system
will remain faulted, until a manual s
witch has been
switched by the user.





Fig.

10
: Digital control system with human
interaction added in.


Fig
.

11
: Input signal from laser
.


Fig.

12
: The output warning signa
l goes high when
the distance is less than five feet.



Fig.

13
: The operator clears the output signal
around 250 seconds.



Figures 11
-
13

show the outputs on the given
simulation. The input distance from the laser is
once again

a linearly depreciating line. The
warning signal is tripped and set high when the
distance is less than five feet. Once the warning
signal is tripped, the operator responds and resets
the system after 240 seconds. This is an arbitrary
number. For some

instances the response time for a
given digital system fault could be in the order of
days, but if it is production sensitive, the problem is
fixed as soon as possible, no matter w
hat it takes.
Fig. 14

shows the output response if nothing is
done when th
e alarm signal is tripped high. The
manual switch never is flipped, and the system is
never restored.



Fig.

14
: Output signal when nothing is done about
the warning.


I
V.
References:


http://www.plcs.net/history2.htm

copyright ©

1996, 1997, 1998, 1999 by Phil Melore


[1]
P. Melore, “PLC History,” Your Personal PLC
Trainer
. Available:

< http://www.plcs.net/history2.htm>


[2] “History of the PLC
-
5,” Allen
-
Bradley.
Available:

<
http://www.ab.com/plclogic/plc5/evolution.html
>


[3] “Co
ntrolLogix Processor,” Allen
-
Bradley.
Available:

<
http://www.ab.com/en/epub/catalogs/12762/21813
76/2416247/
360807/1837516
>
















VI. Biography



Brian Netz

(M’ 2004)

was born in
Grand Rapids, Michigan, on April 3, 1983. He
remained in the Grand Rapids area for the first four
years of his life, and then his family mov
ed to Iowa
where he spent the next fifteen years of his life. He
attended Calvin College and got his Bachelor of
Science and Engineering Degree with a
concentration in electrical engineering. While he
was in high school, he worked for DeVries Electric,
a
n electrical contractor. He also received an
internship at Beta Integrated Concepts his junior
year at Calvin College where he worked with
digital control systems until he graduated from
college in 2005