Document Sample
PLC Powered By Docstoc
					Programmable Logic Controllers

       A Brief Overview
                        Programmable Logic Controllers

Most of control theory deals with a specific subset of control problems. Specifically,
servocontrollers or regulators in which the control action is smoothly variable over a
certain range and the controlled variable is measured in a continuous manner. In a large
portion of industrial situations, the controlled variable is not continuously monitored and
the control action is not smoothly variable. For instance, many machines monitor the
position of various elements by the placement of so-called 'limit switches' on the
machine. These devices are simple electrical switches that close (or open) an electrical
circuit when a moving element actuates the switch. The switches can be wired to allow
the machine to take appropriate action (e.g. stop the moving element) when the switch is

This simple concept: using on/off sensing elements to control on/off devices, forms the
basis of a large class of control problems called sequential control. And the devices used
to implement sequential control in a robust and flexible manner are called Programmable
Logic Controllers (PLC's).

Ladder Diagrams:

The 'language' of PLC's is best understood by examining the historical perspective of
their development. The figure below shows a simple electrical circuit to control a light
with a single switch.



                   Figure 1: Simple single switch control of a light bulb

Let's introduce the symbols most often associated with programmable controllers. First,
the switch is represented more schematically, usually as just a pair of contacts, as shown
           Figure 2: Schematic representation of a normally open (NO) switch

Most switches in our common experience are meant to remain open, non-conducting, in their
un-actuated state. Figure 2 above indicates such a switch. On the other hand, there may be
reasons to have switches which are closed in their un-actuated state, and open in once they are
actuated. Such switches are called Normally Closed (NC) and the symbol is shown below.

           Figure 3: Schematic representation of a normally closed (NC) switch

In applications, both of these switches can be used to control a wide variety of loads:
light bulbs, motors, buzzers, etc. The details of the load are unimportant to the logic of
the control. Therefore, all loads can be represented by a generic circle, as shown below:

                        Figure 4: Schematic representation of a load

Finally, the power supply is not explicitly represented in these diagrams but assumed to
be common, in the background. A vertical line appears along the left-hand side of the
page, representing the positive terminal of the power supply, while a similar line on the
right hand side represents the return, or negative side. This way, the electrical power can
be 'seen' to move left-to-right across the page. The simple circuit shown in Figure 1 can
now be re-drawn as shown below:

                  N.O. Switch                                  Light

                 Figure 5: New Schematic of simple light control circuit

In this manner, a circuit using switches and loads of arbitrary complexity can be drawn
by inserting any elements between the two 'leads' of the power supply. The diagram will
simply 'grow' down the page. This type of representation is called a ladder diagram and a
single complete circuit is called a rung.

Logic Operations in Ladder Diagrams:

Of course, if our objective was to control our loads with single switches, then there would
be no need for a graphical language like ladder diagrams. The next step in complexity is
the representation of logic operations in ladder diagrams. Suppose we have an application
for which we want the load application only if two switches, switch A and switch B are
activated. This is an instance of the logical AND operator. The load is activated if both A
AND B are on. The rung in the ladder diagram would look like this:
                         A          B                                  C

                          Figure 6: Ladder rung for C = A AND B

Similarly, if we wish the load C to be activated if either A OR B are activated, the
following diagram would apply:

                  A                                                        C


                             Figure 7: Ladder rung for C = A OR B.

Now let's consider a simple application. Back in the 1970's, automotive manufacturers
implemented feature called 'seat belt interlock' in which the car could not be started if a person
was sitting In the front seat and the seat belt was not fastened. Let's consider the ladder logic
implementation of this system. First, we assume that there is always a driver seated behind the
wheel. That would imply that the driver side seat belt must always be fastened to allow the car to
start. Let's represent the driver side seat belt detection switch as D, a normally open switch which
is closed when the seat belt is buckled. The circuit must be constructed such that D is closed
before the engine ignition system, E, gets power. On the passenger side, it's a bit more
complicated. We can only require that the passenger side seat belt is buckled if there is a
passenger seated. Let's consider two more switches, PS, a normally closed switch which opens
when a passenger is seated and PB, a normally open switch which detects whether or not the
passenger seat belt is buckled. So, we allow power to the engine ignition system if the driver side
belt (D) is on AND there is no one in the passenger seat (PS closed) OR the driver side belt (D) is
on AND the passenger side belt (PB) is buckled. This combination of AND's and OR's can be
represented quite succinctly as shown below:

                  PS                           D                            E


          Figure 8: Ladder diagram implementation of seat belt interlock system.
Implementation of Ladder Diagrams:

The ladder diagrams described in the previous section make up a formal graphical
language to represent logical relationships between on/off inputs (e.g. switches) and
on/off loads (e.g. lights, motors, buzzers, valves.) While it is possible to implement the
logical operations represented in ladder diagrams as electrical circuits, they are most
often used as a 'programming language' for a class of devices known as Programmable
Logic Controllers (PLC's). PLC's have many advantages when compared to hardwired
implementation of ladder diagrams. Perhaps the most notable of which is that their
programs can be easily modified and edited where as small changes in a hardwired
implementation can be difficult and tedious to carry out.

PLC's are industrial-grade microprocessor systems consisting of a Central Processing
Unit and various input and output modules that can be interfaced directly to the machine's
circuitry. Typically, there is also interface circuitry between the CPU and a traditional
personal computer. This interface is typically an RS-232 serial link which allows for
ladder logic programming in the PC environment. PLC programs are then 'compiled' in
the PC and downloaded through the serial line to the PLC. This arrangement also allows
for off-line mass storage of PLC programs (on the PC).
Lab setup for ME 410


We will be using an Allen-Bradley Programmable Logic Controller, model 1762-
L24BWA, one of their MocroLogixTM 1200 Series controllers. This controller has 14
inputs and 10 outputs. The PLC is wired to machine that was originally used to place
bar-code labels on printed circuit boards for a local company.


To program , debug and operation the PLC, we are using the commercial programming
package for PLC’s: RSLogix 500. RSLogix 500 is a very powerful package with a large
number of features, most of which we will not be using in this class (so don’t get
overwhelmed by the complicated-looking windows you’ll see).


We have very little documentation on the machine itself, part of your task will be to
figure out much of its operation. Both the PLC and the programming package have
several manuals. The best place to get started is the Getting Results Guide for the
RSLogix package. For more information on the PLC itself, refer to the AB Micrologix
12000 Programmable Controller Installation Instructions (small white booklet) and the
AB MicroLogix 1200 Programmable Controllers Bulletin 1762 User’s Manual (Larger
format, but thin, red stripe across top).

PLC Wiring:

Over the summer, we wired up the PLC to the machine so that you will have access to
most, but not all of the machines motors and switches. The table below indicates how the
machine is wired.
PLC Inputs

  Input       Label          Description
    0         X-Axis Limit   Optical limit switch that detects the furthest limit
                             of safe motion for X-axis
    1         X-Axis A       Detects markers on X-axis
    2         PCB Detect     Looks for presence of an object on the conveyor
    3         Label Adv PB   Pushbutton on control box: “Label Advance”
    4         X-Axis D       Detects markers on X-axis
    5         Y-Axis Limit   Optical limit switch that detects furthest limit of
                             safe travel for Y-axis (top carriage)
    6         Y-Axis A       Detects markers on Y-axis
    7         X-Axis B       Detects markers on X-axis
    8         Exit eye       Detects objects at end of conveyor
    9         Reset PB       Pushbutton on control box: Reset
   10         Sstop PB       Pushbutton on control box: Soft Stop
   11         Bypass PB      Pushbutton on control box: Bypass
   12         X-Axis C       Detects markers on X-axis
   13                        Not Used

PLC Outputs

 Output       Label          Description
   0          Conv Mtr       Turns rubber belt conveyor on and off (uni-
    1         Disp Mtr       Motor running rubber wheel on left-hand side of
    2         X Mtr          Motor running the lower carriage that traverses
                             from front to back of machine
    3         X Mtr Sl       When active, runs the x-axis motor at lower speed
    4         X Mt Dr        Controls direction of x-axis motor
    5         Y Mtr          Motor running upper carriage that traverses left to
                             right of machine
    6         Y Mtr Sl       When active, runs y-axis at lower speed
    7         Y Mtr Dr       Controls direction of y-axis motor