Need Help Looking For A Non PLC Ladder Diagram For:

Thread Starter

Jure George

Joined Dec 28, 2008
I am New to this field of study and this was an Aissgnment from our instructor;

We have just recentently been introduced to ladder diagrams that make use of Control relays and contacts. Our assignment is to create a ladder diagram for a Pnuematic system that controls a drill press. There are as follows; Ram A - loads blank into position. Ram B - Locks blank into position. Ram C - Drives Drill down into Blank.

The materials we have access to in our Lab to see the Ladder Diagram work are:
3 Single Ended Double Acting Cylinders
3 Spring return, single solenoid 5/2 valves
Throttle valve/muffler combo valves( speed control )

We the Sudent MUST use :
5 Machine Tool relays
One Eletromagnetic Pneumatic Energiztion Timer
120 Volt Supply
6 Reed switches mounted on the cylinders( 1 NO Contact each )
2 Mushroom Cap Palm buttons ( 2 NO contacts Each )

From the basic drawing given to us: 3 Air cylinders have spring returns on each and SV-A RS1 ,RS2
SV-C RS5, RS6 The RS are the reed switches that are at the start and end of each piston travel.

Also we need to show in the ladder diagram, 2 start positions so that the operator has to have both hands on the starters for the system to work. Also a anti tie down circuit so that the operator can`t cheet and tape one start button down, We have to use a Time delay with a On Delay Timer. The operator MUST keep both hands on the start buttons and the operator has .5 seconds between both start buttons being pressed for the system to start. Finally after one cyle is done the operator MUST restart both start buttons for the next cycle. IF one button is released The system must be disabled. Until reset by the operator.

Can anyone help with a Ladder Diagram showing All the Ladder Rungs and Wire Numbers ? We have not started with PLC this is the befor PLC came into the industry. I need this Ladder Diagram To Have the Sequence : A+,B+,C+,C-,A-,B-. Since the air cylinders have return springs for the shafts I think that means less parts needed in the ladder diagram ? From my limited knowledge so far, we have, Contacts NO,NC, Control Relays, Timer -On Delay etc. I also Need a Timing Diagram shaowing each part signal.

I hope someone out there can help me with this.

Thanks in advance, George


Joined Feb 24, 2006
The PLC is actually a discrete time simulation of the actual relay ladder logic. There is a strong connection between boolean algebra and ladder logic and you can "think" in either domain and convert to the other.

To think in ladder logic, consider the relay coils as outputs and draw them next to the right hand leg of the ladder. The coils have a mechanical feedback to the contacts. When a coil is "energized", ie AC current from the left hand leg of the ladder flows through a rung to the right hand leg of the ladder, then NO contacts will close and NC contacts will open.

If a coil requires multiple conditions to "energize" it will look like contacts wired in series from the left hand leg of the ladder to the left hand side of the coil. If a coil can be "energized" by alternate conditions then these will be parallel connections starting at the left leg of the ladder and ending at the left side of the coil.

As an example think about the logic involved in procucing a "sealing contact". When you push a NO mushroom head button (PB1), a coil (CR1) is energized. When you push a NC mushroom head button(PB2), the coil (CR1) is deenergized. A NO contact from CR1 is placed in parallel with PB1. This contact will "seal" the "energized" condition of CR1 after PB1 is released, until PB2 is depressed.

In a boolean expression I could write something like:
Rich (BB code):
In the following --> means "implies", and <- means "becomes"
CR1 = (PB1 | CR1(NO)) & PB2
PB1 = 0 ; it is normally open
PB2 = 1 ; it is normally closed
CR1(NO) = 0 ; this contact is normally open
CR1 = "de-energized"
PB1 = 1 --> CR1 <- "energized" --> CR1(NO) = 1
PB1 = 0 ; PB1 is released
PB2 = 1 ; nobody has touched PB2
CR1(NO) = 1
CR1 = "energized"
PB2 = 0 ; --> CR1 = "de-energized" --> CR1(NO) = 0
And: we have returned to our initial condition of CR1 being "de-energized.
PB2 can now do what ever it wants to do but the state of CR1 is no 
longer affected by what PB2 does, until something happens with PB1 again.
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