Technical Review
(Issue 3, 2005)
A
Condensed Guide to Automation
Control System Specification, Design
and Installation
Part 1: System Identification and Safety
by Tom Elavsky,
AutomationDirect
_____________________________________________________________________________________
If
you have not been directly involved in the world of factory
automation, data acquisition, process instrumentation or electrical
controls in general, then the above words and acronyms may be
somewhat overwhelming. But these words, and many others, are
part of the language that's used in the industrial automation
world. (For "A Guide to Common Automation Terms" refer
to
http://support.automationdirect.com/docs/glossary.html.)
The following is a series, with continuation in future issues
of Automation Notebook, to act as a general guide to the specification,
design and installation of automated control systems. The information
and references are presented in a logical order that will take
you from the skills required to recognize an operation or process
suited for automating, to tips on setting up a program, to maintaining
the control system. Whether you are an expert or a novice at
electrical control devices and systems, the information presented
should give you a check list to use in the steps to implementing
an automated control system.
Electrical control systems are used on everything from simple
pump controls to car washes, to complex chemical processing
plants. Automation of machine tools, material handling/conveyor
systems, mixing processes, assembly machines, metal processing,
textile processing and more has increased productivity and reliability
in all areas of manufacturing, utilities and material processing.
You may have come to realize that an operation or process used
to produce your end product is very laborious, time consuming,
and produces inconsistent results. You may have also visualized
ways that would allow you to automate the operation. Automating
the process will reduce the amount of manual labor, improve
throughput and produce consistent results. You may have the
skills to develop the mechanical means and select the appropriate
equipment to make this happen, and although you have a basic
understanding of electrical control devices, you may not have
the experience to put it all together. Your first option may
be to enlist the help of a qualified System Integrator. If you
do decide to use a System Integrator, it would be beneficial
to understand as much as possible about automation control system
devices and their terminology so that your communications with
the System Integrator go faster and more smoothly.
In most cases, special expertise is required to design and install
industrial automation control systems. Persons without such
expertise or guidance should not design and install automation
control systems because they can fail and cause serious injury
to personnel or damage to equipment. The information provided
in this series of articles is provided "as is" without
a guarantee of any kind. We do not guarantee that the information
is suitable for your particular application, nor do we assume
any responsibility for its use in your application.
It is our intent to produce this series of articles into a usable
guide, with additional information, including a typical "real
world" application that can be followed from concept to
completion. It is not our intent for the guide to cover every
possible topic dealing with automation control systems or to
even suggest that the topics being covered are fully detailed.
Instead, the topics are aimed at giving the reader a good starting
reference for automated control systems. We will be posting
the full version of the guide on our Technical Support Web page
as a downloadable document in the near future.
In this issue, we will cover the topics of Safety and Identifying
an operation or process that could benefit from automation.
In upcoming issues we will cover control device specification,
control system design and construction, control system installation,
and finally control system maintenance. The topics will be broken
down as follows:
Part 1 - SAFETY and IDENTIFICATION
Part 2 - SPECIFICATION
Part 3 - DESIGN and BUILD
Part 4 - INSTALL and MAINTAIN
Safety:
The first and most important item to consider before attempting
an automated control system, or even a simple on/off control
for a pump, is safety, both for personnel who may be working
with or near the automated equipment, as well as to prevent
damage to the equipment.
To minimize the risk of potential safety problems, you should
follow all applicable local, state and national codes that regulate
the installation and operation of your control system, along
with the equipment or process it is designed to control. These
codes vary by area and usually change over time. It will be
your responsibility to determine which codes should be followed
and to verify that the equipment, installation, and operation
is in compliance with the latest revision of these codes.
Most
likely your control system will be dealing with electrical energy,
so your first goal will be to eliminate the risk of fire and
electrical shock to personnel. The top organizations that provide
applicable standards and codes are listed below, but even before
you get to this area of safety, it would be wise to educate
yourself as much as possible about electricity and electrical
equipment in general. A good understanding of basic electricity,
including DC and AC theory and practice, Ohm's Law, etc. will
go a long way in helping you understand the various codes and
standards. There are many good publications and articles on
the subject of basic electricity and some local technical colleges
offer courses covering subjects dealing with basic electricity.
Some even offer courses in Programmable Logic Controllers (PLCs),
which can be very useful when dealing with automated control
systems. Also, many Web sites offer free tutorials covering
basic electricity and PLCs. It would be beneficial to have some
understanding of electronic devices, such as the operation of
a transistor and other solid state devices, as well as understanding
of the use and operation of electrical test and measurement
instruments, such as voltmeters, current loop meters, clamp-on
amp meters, etc.
At a minimum, you should follow all applicable sections of the
National Fire Protection Association (NFPA) fire code, and the
codes of the National Electrical Manufacturer's Association
(NEMA). There may also be local regulatory or government offices
that can help determine which codes and standards are necessary
for the safe installation and operation of electrical control
equipment and systems.
Please keep in mind that if the automated control system you
are developing needs to be accepted in the international market,
the National Electrical Code (NEC), as a publication of NFPA,
is being harmonized with the International Electrotechnical
Commission (IEC) (Web site: www.iec.ch/)
and the European Hazardous Location Ratings. For more information,
check the Instrument Society of America's (ISA) Web site at
www.isa.org. Additional resources
on the subject can be found at www.ul.com/hazloc/
Another area of safety that needs to be considered for automated
control systems is lockout/tagout procedures as specified by
Occupational Safety and Health Administration (OSHA). "Lockout/tagout"
refers to specific practices and procedures to safeguard operators
and maintenance personnel from the unexpected energization or
startup of machinery and equipment, or the release of hazardous
energy during service or maintenance activities. In order to
have your control system make use of a lockout/tagout procedure,
the design should include the ability to shut off, neutralize,
or isolate any energy source, such as the main electrical feed,
but also any pneumatic, hydraulic or mechanical energy storage
device. The means to do this should be considered in the initial
design of the automated control system. Additional information
can be found on OSHA's Web site at:
http://www.osha.gov/SLTC/controlhazardousenergy/
There are many reasons why the electrical devices that you will
use in the design of your automated control system should be
listed, approved or registered with a testing laboratory. One
reason is to ensure that the device meets standards that will
prevent failure that could lead to catastrophic results. Another
reason might be for insurance or compliance purposes. One of
the most specified and premier safety testing laboratories is
Underwriters Laboratories (UL). The most applicable area of
interest for control systems is UL's Standard for Safety 508A.
If your control system panel requires being built to UL508A,
then you will need to contract directly with UL to become a
UL508A panel builder or use an existing UL508A panel builder.
Additional information can be found at: http://www.ul.com/controlequipment/devices.html
Compliance to UL508A for AutomationDirect products can be found
on our Web site at: http://support.automationdirect.com/compliance.html
The following are other safety points to consider in the design
of your automated control system:
- Emergency Stop - The control system must provide a quick manual
method of disconnecting all system power to the machinery, equipment
or process. The disconnect device or switch must be clearly
labeled "Emergency Stop". After an Emergency shutdown
or any other type of power interruption, there may be requirements
that must be met before the control system or PLC control program
can be restarted. For example, there may be specific register
values in the PLC memory that must be established (or maintained
from the state prior to the shutdown) before operations can
resume. There may also be mechanical positions of equipment
that have to be moved or jogged to the proper position.
- Accidental Powering of Outputs - Do not rely on the automation
control system alone to provide a safe operating environment.
You should use external electromechanical devices, such as relays
or limit switches, that are independent of any electronic controlling
device, such as a solid state relay or a PLC output module,
to provide protection for any part of the system that may cause
personal injury or damage. These devices should be installed
in a manner that prevents any machine operations from occurring
unexpectedly. For example, if the machine has a jammed part,
the controlling system or PLC program can turn off the motor
rotating a saw blade. However, since the operator must open
a guard to remove the part, you should also include a bypass
switch that disconnects all system power any time the guard
is opened.
- Orderly Equipment Shutdown - Whether using a control system
designed around relays and timers or a PLC, an orderly system
shutdown sequence should be included in your design. If a fault
is detected, then any mechanical motion, valve position, etc.,
needs to be returned to its fail-safe position and the equipment/process
stopped. These types of problems are usually things such as
jammed parts, broken cutting tools, bin full, etc. that do not
pose a risk of personal injury or equipment damage. If a detected
problem would result in risk of personal injury or equipment
damage, then use other means to deal with it, such as applying
a brake to rotating equipment to stop it before personnel can
come in contact with it.
- Grounding - To prevent electrical shock, incorporate good
grounding practices in the design, construction and installation
of your system. Use protective devices for faulted conductors
to prevent fire, and also realize that good grounding practices
can reduce electromagnetic and radiated noise interference to
sensitive electronic devices.
- Control Power Distribution - Develop a power distribution
scheme in the control system circuitry, according to code, that
ensures all circuits are protected with fusing, circuit breakers
or other interrupting means coordinated such that only the faulted
circuit will be opened (de-energized), allowing other powered
equipment and devices to continue to operate.
- Unauthorized Access - Make sure all enclosures and cabinets
that have energized circuits are secured to prevent unauthorized
personnel from gaining access without the proper tool, key or
other authorized means.
- Finger-Safe and Dead Fronts - Another safety area to consider
is the use of devices that have finger-safe terminal connections,
which are surrounded by insulated guarding. The use of protective
guards over live circuits should also be considered, even on
control panels that have limited access, so it is safer for
maintenance electricians and authorized personnel to troubleshoot
or make adjustments to electrical control devices. Dead fronts
should be used on control system enclosures where the operator
needs to make adjustments to devices, such as selector switches,
thumbwheels, potentiometers, etc., and the controls need to
be inside the enclosure so as to protect them from outside weather
conditions. The dead front is normally an interior door that
is mounted in front of the main control panel. The outside enclosure
door may still require key entry by the operator, but the dead
front interior door with adjustable devices is interlocked so
that it requires a switch to open it, disconnecting power to
the electrical devices mounted on the main control panel.
- Closed-loop Control - It is your responsibility in any type
of closed-loop control system to ensure that if the feedback
signal is lost, the system shuts down so as not to cause injury
to personnel or damage to the equipment.
Identifying Process for Automation:
The first step in configuring an automated control system is
to identify what can be automated. You need to have a good understanding
of basic electricity and safety. It is also important that you
have an understanding of basic hydraulics, pneumatics, mechanical
operating mechanisms, electronics, control sequences, etc. and
a solid knowledge of the operation or process that you are going
to automate.
You should understand how to control motion and movement, regulate
the flow of fluids, dispense granular materials, orient parts,
sense product in position, detect when an operation is complete,
etc. As a simple example, let's say we have a conveyor that
moves our product from point A to point B. The conveyor is powered
by a 3-phase AC motor, which is turned off and on by a manually
controlled motor starter and includes, for fire protection,
both short circuit and overload protection. The system requires
an operator standing at the motor starter to watch as the product
reaches the entrance to the conveyor, and to turn the conveyor
on to move the product. The operator must also turn the conveyor
off once the product has reached the discharge end.
To automate the conveyor, we will need to replace the manually
controlled motor starter with an electrically controlled motor
starter, including short circuit and overload protection. We
will need to size the motor starter to work with the existing
conveyor motor. (Refer to our Web site at:
http://www.automationdirect.com/static/specs/fujimcselection.pdf
for information on specifying and sizing motor starters.)
We will also need to identify where to locate sensors such as
limit switches, photoelectric sensors, proximity sensors, etc.
that will indicate when an operation is completed. This is required
so our control system knows when to proceed to the next step
in our operation. As an example, we usually need a limit switch
to detect when a cylinder is fully extended, as in the case
when the cylinder is used to push our product onto a conveyor.
The cylinder "fully extended" signal is used to de-energize
the solenoid valve that provided the air pressure to the pneumatic
cylinder. We also need a limit switch to indicate when the cylinder
has fully retracted, and provide a signal to the start/stop
control of the conveyor that the product push cylinder is out
of the way for the next product. Another application for a sensor
is to indicate when the product has reached the conveyor. The
sensor can be a limit switch with a roller arm that comes in
contact with the product or a photoelectric sensor that can
detect the product by using an infrared beam of light. The photoelectric
approach may be the better choice because the position of the
product on the conveyor belt may vary. (Refer to our Web site
at: http://www.automationdirect.com/static/specs/peselection.pdf
for information on selecting photoelectric sensors.)
We would continue with this analysis, looking at each piece
of equipment or component in our system, and select a device
that could control or sense it. Some examples include an electrical
solenoid valve to control water used to wash residue from a
product, or a pneumatic valve to control air pressure to a cylinder
operating a gate that diverts product on a conveyor, or energizing
a control relay to signal that a product is in position on a
scale.
In some instances we may need to vary the speed, rate or position
of our controlling device, such as varying the speed of a conveyor,
changing the amount a valve opens to control a flow rate, or
remotely changing the setpoint level for a tank. This could
be accomplished by using an analog output signal. An analog
output signal is a varying signal that corresponds to the real
value we have determined and calibrated into the device. For
example, a 0 to 10 VDC signal could represent a conveyor speed
of 0 to 500 feet per minute. An analog signal to the speed controlling
device for the conveyor motor of 5 VDC would result in a conveyor
speed of 250 feet per minute.
Identifying devices to control motion, flow, events, etc. and
sensing completion is basically identifying the I/O (inputs
and outputs) of our control system. Once these devices are identified,
they can be used as the field devices in a PLC-based system,
or they can be “hard-wired” for simpler applications.
You will also want to determine if your automated control system
will benefit from the use of an operator interface, also referred
to as a Human Machine Interface (HMI). If your process requires
making changes to setpoint values, process time, flow rates,
etc., then the use of an HMI is the best way to proceed. In
these situations, you will most likely need a PLC that can easily
communicate with the HMI device.
If your application requires keeping data records for reference,
traceability, history, trending, meeting regulations, etc.,
then you should look at using a control system that would fall
into the category of a "Supervisory Control And Data Acquisition"
(SCADA) system. Most of these control systems would be comprised
of PLC-type I/O that interface to a PC with appropriate software.
Watch for Part 2 on System Specification in our next issue.
References:
For information on "PLC Logic and Principles" by Doug
Bell of InterConnecting Automation, PLC training books, and
training through technical schools and
organizations visit: http://support.automationdirect.com/docs/training.pdf
For Web sites with free tutorials that cover basic electricity
principles visit any of the following:
http://www.thelearningpit.com/elec/bas/theory/etb-menu.html
or
http://www.kilowattclassroom.com/Index.htm
or
http://www.mrplc.com/ or http://www.plcs.net/
You may also want to visit the AutomationDirect Customer Forums
at: http://forum1.automationdirect.com/cgi-bin/Ultimate.cgi
Information for the National Electrical Manufacturer's Association
(NEMA) can be found at their Web site at: http://www.nema.org.
NEMA is also being harmonized with the International Electrotechnical
Commission (IEC) (Web site: www.iec.ch/)
and other European standards. Additional information can be
found at Global Engineering Documents' Web site at: www.global.ihs.com.
Global Engineering Documents is also the source for obtaining
NEMA, IEC and CE documents.
Further information for the National Fire Protection Association
(NFPA) can be found at their Web site at http://www.nfpa.org/.
Some of the more useful publications are the National Electrical
Code (NEC), publication NFPA 70 and also as a good reference
refer to the Electrical Standard for Industrial Machinery, publication
NFPA 79.
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