Starting and ignition

Starting and ignition

rotation of the starter ceases. A typical air starting
system is shown in fig. 11-7.
12. A combustor starter is sometimes fitted to an
engine incorporating an air starter and is used to
supply power to the starter when an external supply
of air is not available. The starter unit has a small
combustion chamber into which high pressure air,
from an aircraft-mounted storage bottle, and fuel,
from the engine fuel system, are introduced. Control
valves regulate the air supply which pressurizes a
fuel accumulator to give sufficient fuel pressure for
atomization and also activates the continuous
ignition system. The fuel/air mixture is ignited in the
combustion chamber and the resultant gas is
directed onto the turbine of the air starter. An
electrical circuit is provided to shut off the air supply
which in turn terminates the fuel and ignition systems
on completion of the starting cycle.
13. Some turbo-jet engines are not fitted with starter
motors, but use air impingement onto the turbine
blades as a means of rotating the engine. The air is
obtained from an external source, or from an engine
that is running, and is directed through non-return
valves and nozzles onto the turbine blades. A typical
method of air impingement starting is shown in fig.
11-8.
Gas turbine
14. A gas turbine starter is used for some jet
engines and is completely self-contained. It has its
own fuel and ignition system, starting system (usually
electric or hydraulic) and self-contained oil system.
This type of starter is economical to operate and
provides a high power output for a comparatively low
weight.
15. The starter consists of a small, compact gas
turbine engine, usually featuring a turbine-driven
centrifugal compressor, a reverse flow combustion
system and a mechanically independent |free-power
turbine. The free-power turbine is connected to the
main engine via a two-stage epicyclic reduction gear,
automatic clutch and output shaft. A typical gas
turbine starter is shown in fig. 11-9.
16. On initiation of the starting cycle, the gas turbine
starter is rotated by its own starter motor until it
reaches self-sustaining speed, when the starting and
ignition systems are automatically switched off.
Acceleration then continues up to a controlled speed
of approximately 60,000 r.p.m. At the same time as
the gas turbine starter engine is accelerating, the
exhaust gas is being directed, via nozzle guide
vanes, onto the free-power turbine to provide the
drive to the main engine. Once the main engine
reaches self-sustaining speed, a cut-out switch
Starting and ignition

A triple-breech cartridge starter.

A triple-breech cartridge starter.
Cartridge
7. Cartridge starting is sometimes used on military
engines and provides a quick independent method of
starting. The starter motor is basically a small
impulse-type turbine that is driven by high velocity
gases from a burning cartridge. The power output of
the turbine is passed through a reduction gear and
an automatic disconnect mechanism to rotate the
engine. An electrically fired detonator initiates the
burning of the cartridge charge. As a cordite charge
provides the power supply for this type of starter, the
size of the charge required may well limit the use of
the cartridge starters. A triple-breech starter is
illustrated in fig. 11-4.
Iso-propyl-nitrate
8. This type of starter provides a high power output
and gives rapid starting characteristics. It has a
turbine that transmits power through a reduction gear
to the engine. In this instance, the turbine is rotated
by high pressure gases resulting from the
combustion of iso-propyl-nitrate. This fuel is sprayed
into a combustion chamber, which forms part of the
starter, where it is electrically ignited by a highenergy
ignition system. A pump supplies the fuel to
the combustion chamber from a storage tank and an
air pump scavenges the starter combustion chamber
of fumes before each start. Operation of the fuel and
air pumps, ignition systems, and cycle cancellation,
is electrically controlled by relays and time switches.
An iso-propyl-nitrate starting system is shown in fig.
11-5.
Air
9. Air starting is used on most commercial and
some military jet engines. It has many advantages
over other starting systems, and is comparatively
light, simple and economical to operate.
10. An air starter motor transmits power through a
reduction gear and clutch to the starter output shaft
which is connected to the engine. A typical air starter
motor is shown in fig. 11-6.
11. The starter turbine is rotated by air taken from
an external ground supply, an auxiliary power unit
(A.P.U.) or as a cross-feed from a running engine.
The air supply to the starter is controlled by an electrically
operated control and pressure reducing valve
that is opened when an engine start is selected and
is automatically closed at a predetermined starter
speed. The clutch also automatically disengages as
the engine accelerates up to idling r.p.m. and the
Starting and ignition
124

SAFETY RULES SUMMARY

SAFETY RULES SUMMARY
A set of safety rules was developed by Jim Rowell (http://www.mrplc.com,
"Industrial Control Safety; or How to Scare the Bejesus Out of Me"). These are summarized
below.
Grounding and Fuses
• Always ground power supplies and transformers.
• Ground all metal enclosures, casings, etc.
• All ground connections should be made with dedicated wires that are
exposed so that their presence is obvious.
• Use fuses for all AC power lines, but not on the neutrals or grounds.
• If ground fault interrupts are used they should respond faster than the control
system.
Hot vs. Neutral Wiring
• Use PNP wiring schemes for systems, especially for inputs that can initiate
actions.
• Loads should be wired so that the ground/neutral is always connected,
and the power is switched.
• Sourcing and sinking are often confused, so check the diagrams or look
for PNP/NPN markings.
AC / DC
• Use lower voltages when possible, preferably below 50V.
• For distant switches and sensors use DC.
Devices
• Use properly rated isolation transformers and power supplies for control
systems. Beware autotransformers.
• Use Positive or Force-Guided Relays and contacts can fail safely and prevent
operation in the event of a failure.
• Some ’relay replacement’ devices do not adequately isolate the inputs and
output and should not be used in safety critical applications.
Starts

INTRODUCTION

INTRODUCTION
If you know how to program in any high level language, such as Basic or C, you
will be comfortable with Structured Text (ST) programming. ST programming is part of
the IEC 61131 standard. An example program is shown in Figure 19.1. The program is
called main and is defined between the statements PROGRAM and END_PROGRAM.
Every program begins with statements the define the variables. In this case the variable i is
defined to be an integer. The program follows the variable declarations. This program
counts from 0 to 10 with a loop. When the example program starts the value of integer
memory i will be set to zero. The REPEAT and END_REPEAT statements define the loop.
The UNTIL statement defines when the loop must end. A line is present to increment the
value of i for each loop.

THE ALLEN-BRADLEY VERSION

THE ALLEN-BRADLEY VERSION
Allen Bradley only supports IL programming on the Micrologix 1000, and does
not plan to support it in the future. Examples of the equivalent ladder logic and IL programs
are shown in Figure 18.4 and Figure 18.5. The programs in Figure 18.4 show different
variations when there is only a single output. Multiple IL programs are given where
available. When looking at these examples recall the stack concept. When a LD or LDN
instruction is encountered it will put a value on the top of the stack. The ANB and ORB
instructions will remove the top two values from the stack, and replace them with a single
value that is the result of an Boolean operation. The AND and OR functions take one value
off the top of the stack, perform a Boolean operation and put the result on the top of the
stack. The equivalent programs (to the right) are shorter and will run faster.

Figure 18.5 shows the IL programs that are generated when there are multiple outputs.
This often requires that the stack be used to preserve values that would be lost nor-

mally using the MPS, MPP and MRD functions. The MPS instruction will store the current
value of the top of the stack. Consider the first example with two outputs, the value of A is
loaded on the stack with LD A. The instruction ST X examines the top of the stack, but
does not remove the value, so it is still available for ST Y. In the third example the value of
the top of the stack would not be correct when the second output rung was examined. So,
when the output branch occurs the value at the top of the stack is copied using MPS, and
pushed on the top of the stack. The copy is then ANDed with B and used to set X. After
this the value at the top is pulled off with the MPP instruction, leaving the value at the top
what is was before the first output rung. The last example shows multiple output rungs.
Before the first rung the value is copied on the stack using MPS. Before the last rung the
value at the top of the stack is discarded with the MPP instruction. But, the two center
instructions use MRD to copy the right value to the top of the stack - it could be replaced
with MPP then MPS.

INTRODUCTION

INTRODUCTION
In previous decades (and now) PLC manufacturers favored “proprietary” or
“closed” designs. This gave them control over the technology and customers. Essentially,
a proprietary architecture kept some of the details of a system secret. This tended to limit
customer choices and options. It was quite common to spend great sums of money to
install a control system, and then be unable to perform some simple task because the manufacturer
did not sell that type of solution. In these situations customers often had two
choices; wait for the next release of the hardware/software and hope for a solution, or pay
exorbitant fees to have custom work done by the manufacturer.
“Open” systems have been around for decades, but only recently has their value
been recognized. The most significant step occurred in 1981 when IBM broke from it’s
corporate tradition and released a personal computer that could use hardware and software
from other companies. Since that time IBM lost control of it’s child, but it has now
adopted the open system philosophy as a core business strategy. All of the details of an
open system are available for users and developers to use and modify. This has produced
very stable, flexible and inexpensive solutions. Controls manufacturers are also moving
toward open systems. One such effort involves Devicenet, which is discussed in a later
chapter.
A troubling trend that you should be aware of is that many manufacturers are mislabeling
closed and semi-closed systems as open. An easy acid test for this type of system
is the question “does the system allow me to choose alternate suppliers for all of the components?”
If even one component can only be purchased from a single source, the system
is not open. When you have a choice you should avoid “not-so-open” solutions.

ASSIGNMENT PROBLEMS

ASSIGNMENT PROBLEMS
2. Using 3 different methods write a program that will continuously cycle a pattern of 12 lights
connected to a PLC output card. The pattern should have one out of every three lights set. The
light patterns should appear to move endlessly in one direction.
3. Look at the manuals for the status memory in your PLC.
a) Describe how to run program 7 when a divide by zero error occurs.
b) Write the ladder logic needed to clear a PLC fault.
c) Describe how to set up a timed interrupt to run program 5 every 2 seconds.
4. Write a program that will run once every 5 seconds and calculate the average of the numbers
from F8:0 to F8:19, and store the result in F8:20. It will also determine the median and store it
in F8:21.
5.Write a program for SPC (Statistical Process Control) that will run once every 20 minutes using
timed interrupts. When the program runs it will calculate the average of the data values in
memory locations F8:0 to F8:39 (Note: these values are written into the PLC memory by
another PLC using DH+). The program will also find the range of the values by subtracting the
maximum from the minimum value. The average will be compared to upper (F8:50) and lower
(F8:51) limits. The range will also be compared to upper (F8:52) and lower (F8:53) limits. If
the average, or range values are outside the limits, the process will stop, and an ‘out of control’
light will be turned on. The process will use start and stop buttons, and when running it will set
memory bit B3:0/0.
6. Develop a ladder logic program to control a light display outside a theater. The display consists
of a row of 8 lights. When a patron walks past an optical sensor the lights will turn on in
sequence, moving in the same direction. Initially all lights are off. Once triggered the lights
turn on sequentially until all eight lights are on 1.6 seconds latter. After a delay of another 0.4
seconds the lights start to turn off until all are off, again moving in the same direction as the
patron. The effect is a moving light pattern that follows the patron as they walk into the theater.
7. Write the ladder logic diagram that would be required to execute the following data manipulation
for a preventative maintenance program.
i) Keep track of the number of times a motor was started with toggle switch #1.
ii) After 2000 motor starts turn on an indicator light on the operator panel.
iii) Provide the capability to change the number of motor starts being tracked, prior
to triggering of the indicator light. HINT: This capability will only require the
change of a value in a compare statement rather than the addition of new lines
of logic.
iv) Keep track of the number of minutes that the motor has run.
v) After 9000 minutes of operation turn the motor off automatically and also turn
on an indicator light on the operator panel.
8. Parts arrive at an oven on a conveyor belt and pass a barcode scanner. When the barcode scanner
reads a valid barcode it outputs the numeric code as 32 bits to I:001 and I:002 and sets