PLC Programmer Logic Control

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					                            A Low-Cost Programmable Logic Control (PLC)
                      Trainer for Use in a University Agricultural Electricity Course

                                                Submitted by:

                                    Aaron Dickinson, Graduate Student
                                       Donald M. Johnson, Professor
                              Department of Agricultural & Extension Education
                                           205 Agriculture Bldg.
                                          Fayetteville, AR 72701

                                            voice: 479-575-2035
                                            FAX: 479-575-2610

Statement of Primary Audience: This article is primarily intended for college and university faculty teaching
courses in agricultural electricity or control systems.

Programmable logic controllers (PLCs) are widely used in agricultural production and processing. Thus,
agricultural systems management students should receive hands-on experience with these devices as part of
their undergraduate education. However, in many cases, the cost of providing class-quantity PLCs for student
laboratory use is prohibitive. This article describes a low-cost PLC trainer developed for use in a university-
level agricultural electricity course. A sample laboratory activity used with the PLC trainer is also described.
End-of-course student comments indicated that use of the trainer had stimulated student interest in learning
more about programmable logic controllers.


        A programmable logic controller (PLC) is an industrial computer that receives inputs from switches

and sensors, evaluates these inputs in accordance with programmed instructions, and controls output devices

based on the results of this evaluation (Bern and Olsen, 2002). According to Gustafson and Morgan (2004),

PLCs “have replaced hard-wired relay-based control systems in most industries because of their compact size,

ruggedness, and, most importantly, their ability to be reprogrammed” (p. 277). Reprogramming a PLC allows

changes to be made in the functional operation of a machine system without major physical changes in the

control or output system components or wiring (Cox, 2001). Thus, labor, equipment, and downtime costs are


        The use of PLCs and similar devices in the agricultural industry is widespread and growing. Example

applications include food processing (Rywotycki, 2003), building environmental control (Gates, Overhults,

and Turner, 1992), grain drying (Cox, 1997), aquacultural production (Kolkovski, Curnow, and King, 2004),

and tractor and machinery systems (Goering, Stone, Smith, and Turnquist, 2003).

        Schumacher, Ess, Strickland, and King (2002) determined that electricity and electronics was

perceived by university coordinators as being the most important area of study for agricultural systems

management students. Given the prevalence of PLCs in the agricultural industry, undergraduate agricultural

systems management students should receive hands-on experience with these devices as a component of these

electricity and electronics courses.

        Agricultural systems management professionals have written extensively concerning effective methods
for teaching electrical concepts and skills (Harner and Slocombe, 1989; Schumacher and Currence, 1989;

Johnson, 2002). While specific approaches differ, all authors agree that effective instructional programs in

electricity must provide students with active, hands-on learning experiences. These experiences are necessary

for both concept and skill development (Johnson, 2002).

        The cost of commercially available PLC trainers (from $1600 to $5000 and more per unit) (AIT

Training & Technology, Farmersburg, IN; TEC Trainers, Jordan, MN) makes it difficult for agricultural

systems management programs to provide class quantity trainers for student laboratory study. Thus, a need

existed to develop a low-cost PLC trainer that would allow programs with limited budgets to provide multiple

units for student use.


        The purpose of this project was to develop a functional PLC trainer, suitable for use in a university

agricultural electricity course, which was less expensive than commercially available PLC trainers. The

following criteria were established to guide the development of the trainer:

        •   The trainer must cost less than $500 per unit to produce

        •   The trainer must be safe for students to use

        •   The trainer must allow for programming the PLC via computer

        •   The trainer must allow programming the PLC using relay ladder logic (RLL)

        •   The trainer must include typical PLC functions such as logic, counting, timing, latching, pulse, etc.

        •   The trainer must accept a minimum of four digital (on-off; high-low) PLC inputs

        •   The trainer must allow two or more outputs

Materials and Methods

        Development of the PLC trainer began with the selection of the PLC unit. After evaluating various

manufacturers and models, the Allen-Bradley 1760-L12AWA-NC Pico™ Controller (Rockwell Automation,

Milwaukee, WI) was selected. This PLC has eight analog inputs (120 or 240V AC) and four opto-isolated relay

outputs (120 V AC), rated at 8 amperes resistive or 3 amperes inductive (Rockwell Automation, 2001). The

cost for the Pico™ Controller was $173.

       The Pico™ Controller can be programmed using the built-in touch pad and display, or via a serial port

connected computer running PicoSoft™ (v. 3.0) software, available as a no-cost download. The cost for the

serial programming cable was $72.

       PicoSoft™ is a relay ladder logic programming language with three display options (Device, DIN IEC,

and ANSI CSA). The software incorporates two features that are especially helpful to students. First, the

software allows the user to visually simulate operation of the program, allowing students to test and de-bug

programs before downloading them to the device. Second, when the computer is connected to the PLC, the

“circuit diagram status display” option enables the on-screen ladder diagram to mimic the operation of the PLC,

showing the status of all rungs of the ladder diagram.

       In addition to the Pico™ Controller, several other items were required in order to construct the trainer

(Table 1). These included a 120 V AC, 3-prong power cord; a SPST switch, box, and cover; 1- and 6-ampere

cube fuses with DIN-rail mounted finger safe fuse holders; five power distribution blocks; three, 4-terminal and

two, 8-terminal barrier strips; and one 8-terminal copper equipment grounding bar. These components were

surface mounted on a piece of 24-in x 24-in x ¾-in plywood and were wired with AWG 12 gauge THHN

stranded conductors.

        For safety, finger-safe components were used to minimize the potential for accidental contact with

energized components. Also, electricity is supplied to the trainer through a GFCI-protected outlet. The total

cost for the PLC trainer (Figure 1) was approximately $365.

                                                                                     Control Device(s)

                                                          Eq. Grd             Ungrounded                   Ungrounded

                                                                                                  L1 L2 1 2 3 4 5 6 7 8
                                  1A                                                                      Inputs          Pico
                                  6A                                                                   1    2 3      4

    120 V AC

                                                                    Eq. Grd           Grounded             Ungrounded
                                       Terminal Blocks

                                                                                 Load Device(s)
                                       Figure 1. Wiring diagram for PLC trainer.

        As shown in Figure 2, input (control) devices (such as pushbutton switches, thermostats, limit switches,

proximity sensors, etc.) are wired between the barrier strip connected to the 1-ampere fused ungrounded

terminal block and the barrier strip connected to the input terminals of the Pico™ Controller. The input device

is addressed based on the number of the Pico™ Controller input terminal to which it is connected. For example,

when developing the relay ladder logic program, a thermostat connected to input terminal one would be

addressed as “I1.” An equipment grounding conductor connects the equipment grounding bar and the control

device’s equipment grounding screw.

                                   Figure 2. Programmable logic control trainer.

        Output (load) devices are simulated with 120 V AC lamps. The ungrounded conductor from the lamp

fixture connects to one terminal of the barrier strip connected to the Pico™ Controller’s output terminals, and

takes its address from the terminal number. For example, an output device connected to output terminal 2

would be addressed as “Q2.” The grounded conductor and equipment grounding conductor for each output

connects to the appropriate load barrier strip.

Example Lab Activity

        The PLC trainer allows students to connect devices and write relay ladder logic programs to solve a

variety of process control problems. An example problem requires students to control an auger and feed grinder

system. A normally-open (NO), momentary-contact pushbutton is to start both the feed grinder (immediately)

and then the auger (after a 30- second delay), allowing the grinder to reach full speed before the auger delivers

grain. A normally-closed (NC), momentary contact pushbutton is to stop the auger (immediately) and the

grinder (after a 30-second delay). Each student group is provided with a pushbutton start-stop station (contains

NO and NC pushbuttons in single device), and two 120 V AC lamp fixtures to simulate the auger and grinder

motors. The students must correctly connect the devices to the PLC trainer and write a relay ladder logic

program (Figure 3) to cause the loads to operate as specified.

                           Figure 3. PicoSoft™ screenshot showing simulation of grinder and auger
                           control circuit. (Note: The “auger” has been stopped and the “grinder” is in
                           the 30-second “off” delay period).

Summary and Conclusions

           The objective of this project was to develop a low-cost PLC trainer that could be successfully integrated

into a university agricultural electricity course. Seven specific criteria were established to guide development of

the trainer. As developed, the PLC trainer met all of these criteria. The total cost was approximately $365 per


           One prototype trainer was built and successfully incorporated into a university agricultural electricity

course in fall 2004. Student reactions were quite positive. On written course evaluations, several students

indicated that the PLC labs were among the most interesting and useful labs in the entire course. Students

expressed a desire to, “learn more about programmable controllers, “and to “spend more time on PLCs.” Based

on the success of the trainer, four additional trainers have been constructed for use in the course.


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