EE599 Spring 2001
PLC Homework 2


1.  Consider an alarm system with inputs x1 and x2 and outputs z1 and z2:
	x1: overtemperature switch
	x2: alarm shutoff switch
	z1: alarm signal on (output)
	z2: red light on signal (output)

Initially, the alarm and the red light are off.  When the overtemperature switch 
becomes high, the alarm is turned on.  If the operator pushes the shutoff switch, 
the alarm turns off (and stays off) but the red light turns on (and stays on).  
The system returns to the initial state (light and alarm off) whenever the 
overtemperature switch becomes low again. 

 a.  Draw a state diagram for this system showing z1 and z2 for each state. 
 On the arcs between each state, indicate the change in input signals that 
 trigger the transition between the states. 

 b.  Using the cascade method discussed in class, implement the state diagram 
 in ladder logic.  (Note this will _not_ be a "smallest" implementation.)

 c.  Can you find a smaller control program consisting of only two rungs? 



2.  A piston has a limit switch x2 at its retracted position, and another limit 
switch x3 at its extended position.  When a part comes before a sensor (when x1 
becomes high), a piston should be driven from its retracted position (x2=1,x3=0) 
to its extended position (x2=0,x3=1), and then back to its retracted position 
where it remains until x2 transitions again from low to high.  The piston goes 
forward with actuator signal z1=1, and retracts automatically by a spring when z1=0.

 a.  Draw a state diagram for this system showing z1 for each state.  On the 
 arcs between each state, indicate the change in input signals that trigger the 
 transition between the states. 

 b.  Using the cascade method discussed in class, implement the state diagram 
 in ladder logic.  

  

3.   Suppose that you are controlling a paint spray system.  When parts come 
down a conveyor, they pass in front of a detector (input x1).  When they pass 
in front of the detector, the solenoid for the paint spray should be activated 
(output y1) for two seconds.  (Note that the detector signal will remain high 
for longer than two seconds.  Develop a ladder program for this control. 



4.  When a start switch (x0) is pushed and held down, a motor is supposed to 
rotate three times.  There is a limit switch (x1) that is adjacent to the wheel 
and is activated when the wheel is at angle 0 degrees.  The motor is driven by 
a signal z1.  Write a ladder program using a counter to implement this.  
(Note that the start switch should be released to reset the counter.)