09-10-2012, 03:53 PM
Logic Control Systems
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To begin the discussion of industrial logic control systems, consider the simple pneumatic
system shown in Figure LC-1. The pneumatic cylinder moves in a linear dimension until it
reaches the limit switch at the extended end. The cylinder is controlled with a simple two
position, four-way solenoid valve as shown. The solenoid valve shown is activated by an
electrical current passing through the solenoid coil. This type of simple ON/OFF
programming has traditionally been done by relay control systems.
Figure LC-3 shows the most common components of ladder logic diagrams. Input elements
include limit switches, momentary contact push-buttons, pressure switches, manual switches,
and relay contacts. Typical outputs include solenoid coils, control relay coils, pilot lights,
and annunciators (or horns). Note that each of the inputs is available in both normally open
(NO) and normally closed (NC) configurations. This distinction is easily explained by
observing the limit switch configurations. A normally closed limit switch will carry current
if it is not activated (the "normal" state). If a normally closed limit switch is pressed, then it
no longer will carry current. A normally open limit switch is the opposite - it will not carry
current inactivated, it must be pressed to allow current to flow through it.
Programmable Controllers
One of the disadvantages of the relay logic systems of the previous section is the difficult
nature of the "programming." The program logic is "hard-wired" by the interconnection of
the relays, limit switches, timers, counters, etc. Changing the task performed by the simple
system of Figure LC-1 requires physically moving the wires from the relays and limit
switches and placing them in the desired new configuration. For circuits with only three or
four components this is not difficult. However, systems containing ten to several hundred
individual components are not uncommon in industrial automation systems.
The programmable logic controller (PLC) was developed in the early 60's to overcome the
deficiencies of relay logic systems. Programmable logic is implemented using a
microcomputer instead of the hard-wired logic of the conventional hard-wired relay system.
The major advantage of PLC's (frequently referred to as just programmable controllers or
PC's) is that the programming can be done in ladder logic, just like relay logic systems.
Electricians and technicians can readily adapt to this familiar type of programming. A
computer language like BASIC or Pascal might be too intimidating and is not required to
implement straightforward machine logic.
Logic Control Circuit Design
Designing new logic control circuits from “scratch” can be a daunting task. Oftentimes a
designer can reuse logical blocks from previous successful designs. Unfortunately, this is
not always possible, and can sometimes lead to unforseen interactions between various parts
of the logic system. Some broad general guidelines (which are often violated!) for designing
logic systems are given below:
1. Dedicate control relays for specific functions (such as starting the system, activating a
solenoid, etc.) and use as many as are necessary. Control relays are essentially “free”
once a programmable controller has been purchased, so don’t be miserly!
2. Control relays almost always use a holding circuit, so design in terms of both a “turn
ON” and a “turn OFF” rung with an “OR” connection between them. Note that some
circuits will require multiple rungs for turning ON or OFF, which must be connected
through the OR structure.
3. Normally open (N.O.) components are usually used to activate the “turn ON” rung.
4. Normally closed (N.C.) components are usually used to activate the “turn OFF” rung.
5. Be absolutely certain that any holding circuit formed will be actively turned off by your
system. Do not depend on a power shutdown to release and holding circuits.
6. Provide safety interlocks either on the “turn ON” rung before the control relay or on the
associated solenoid activation rung, depending on the type of interlock required.
7. Use the master control relay (MCR) concept for long-term effects, such as turning the
entire system on, or starting a long sequence of actions.
Machine Safety
Jamming occurs when a cylinder is unable to complete a desired stroke. Pressure switches
can be installed in the lines leading to the cylinder to indicate excessive pressure levels.
Figure LC-7 shows a circuit with simple "jamming" protection. The cylinder normally
retracts when it reaches the far limit switch, without a large increase in the system pressure.
If a "jam" occurs, motion of the cylinder will stop and the pressure will rapidly increase. The
pressure switch PS-9 will be activated which opens the normally closed contacts CR-9B.
Solenoid SOL-A is prevented from being re-activated until the "jam" is cleared and the
release button PB-9 is pressed.
Operator Safety
The remaining safety topic for design consideration is operator safety. System designers
must anticipate problems that might occur with their products and provide a safely operating
system. At a minimum a designer should consider the following:
1) warning/operating signals,
2) emergency/"panic" stop, and
3) manual two-hand or "deadman" switch operation.
Warning and operating lights can easily be provided for a system by adding additional rungs
to the ladder diagram. A large flashing (or strobe) light is often used to indicate that the
system is powered and operating. Additional warning lights can be provided during critical
portions of the operating cycle, such as rapid cylinder advances