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ET472 – Integrated Control Systems
Laboratory Report
Lab 1: PLC Control of an Alarm System
Lab Partners: John Doe, Jane Smith
January 20, 2008
ABSTRACT:
The purpose of lab exercise 1 was to develop a control program for a basic alarm system. Program development was carried out on January 19, 2007 in the Engineering Technology automation laboratory in Belk 171. The MicroWin software package was used to graphically create a ladder logic control program. This program was then transferred to a Siemens S7-224 programmable controller for simulation. Upon verification, the program was downloaded to a working laboratory alarm station for testing. In the test procedure, the system was first enabled through an arming switch. The system allowed a 5-second delay to elapse before an attached motion detector was enabled. When movement was detected by the motion detector, both visual and audible alarms were activated. Alarms continued until the system was deactivated and reset by a disarming switch.Testing proved that the control program successfully met the objectives of the lab exercise.
SYSTEM DIAGRAM:
Figure 1: Alarm system control flow diagram
TERMS AND DEFINITIONS:
Ladder LogicA graphical representation of a PLC control program which resembles a ladder and uses contacts and coils to define a control sequence.
Scan CycleAn interval of time requiredfor a PLC to observe system inputs, evaluate its ladder diagram, drive system outputs and conduct system diagnostics one time.
InputsPLC contacts representing real-world stimulus contacts (such as sensors or switches) to a ladder diagram.
Outputs“On” or “Off” conditions driven by the PLC to control real-world devices based on the evaluation of a ladder diagram.
LatchA condition used in a ladder diagram used to control a system element in a given state during some phase of system operation.
TON TimerA timing device in which there is a controlled delay from the time it is activated until its output is driven to a true state.
AddressingA numerical representation of a storage location in PLC memory.
Siemens 224 PLCA Siemens-brand programmable logic controller powered by 120 VAC or 24 VDC and having a maximum of 14 DC inputs and 10 relay outputs.
MicroWin SoftwareA Siemens software tool used in the programming and simulation of Siemens PLCs.
REQUIRED SYSTEM OPERATION:
The alarm system is activated when an ARM switch is depressed. The ARM switch will initiate a timer which allows a period of 5 seconds to elapse before a motion detector is enabled. This delay allows the operator to exit the area. If at any time thereafter, the motion detector then senses movement, both a strobe light and an audible siren are activated. They system will continue to activate both the strobe and siren until a DISARM switch is depressed. The system is reset by the DISARM switch and is then ready to be re-armed.
SYMBOLS CHART:
Table 1: Symbols Chart
Label / Address / DescriptionArm / I0.0 / Switch input which arms the system
Disarm / I0.1 / Disarm switch which resets the system
Timer_Enable / Q0.0 / Output of the latch which holds the Arm/Disarm condition and enables the timer
Timer / T33 / Five-second delay timer which activates the motion detector
Motion_Detector / I0.2 / Output of the motion detector which activates the siren and strobe light
Strobe / Q0.1 / Output activated by detected motion and enables the strobe light
Siren / Q0.2 / Output activated by detected motion and enable the siren
SEQUENCE CONTROL CHART (SCC):
Table 2: Sequence Control Chart
Operation / Controlled by / ConstraintsSystem armed / Arm (I0.0) / Arm switch is depressed
Timer enabled / Timer_Enable (Q0.0) / System is armed
Motion detector activated / Timer (T33) / Timer has reached its preset time (PT)
Strobe light activated / Strobe (Q0.1) / T33, Motion detected
Siren activated / Siren (Q0.2) / T33, Motion detected
System reset / Disarm (I0.1) / None
EXPLANATION OF OPERATION:
The ladder logic diagram for this lab is given in the appendix. A network-by-network description of the control program is shown below.
Network 1:The alarm system is armed by anormally open pushbutton Arm switch (I0.0). When the Arm switch is depressed, the programmed contact closes and energizes a coil which enables the timer (Timer_Enable, Q0.0). The parallel branch of this network provides a latch to retain the state of the timer enabling coil and keep the system “on” after it has been armed. The system will remain armed until the normally closed Disarm switch (I0.1) is depressed.
Network 2:The Timer (T33) is enabled by the latched output of network 1 (Timer_Enable, Q0.0). The timer has a preset value of +500 (x 10 ms time base = 5 seconds). Once this 5-second delay elapses, the output of the Timer becomes true.
Network 3:Once the Timer in network 2 becomes true, the normally closed motion detector input (I0.2) will remain closed until motion is detected. Once motion is detected, the strobe light (Q0.1) will be activated. The strobe light is wired internally to remain active until the Disarm switch (I0.1) is depressed.
Network 4:Once the Timer in network 2 becomes true, the normally closed motion detector input (I0.2) will remain closed until motion is detected. Once motion is detected, the siren (Q0.2) will be activated. The siren is wired internally to remain active until the Disarm switch (I0.1) is depressed.
PROBLEMS / RECCOMENDATIONS:
Problems were encountered during the lab due to an incomplete understanding of the latching construct needed to hold the system in an armed state after the Arming switch was depressed. The function of the normally closed contact also produced some confusion when stating true and false conditions. These problems tended to resolve themselves as experience with PLC programming increased. Further confusion also centered on the fact that the motion detector was internally wired to retain its internal “detect” state until the Disarming switch was depressed regardless of any cessation in detected motion. A recommendation of this behavior should be considered a premise of the lab exercise.
ATTACHMENTS:
[Cite and attach control program]
[Cite and attach any other supporting documents]
[Cite and attach your signed program verification sheet]