Ladder Logic, PLC Basics

[DMB5mil]

Can anyone recommend good reference(s) for understanding the basics of ladder logic design? I don't want to delve into theory. Basically I need to get up to speed on how to "walk through" a typical ladder schematic, and how to design a basic ladder schematic using relay coils, relay N.O. & N.C. contacts, HOA switches, and all the other goodies needed for applications such as motor control circuits, lighting control circuits, etc. Are there design guides out there to steer me in the right direction?

Thanks,

Brian

 

[wilheldp_PE]

Can anyone recommend good reference(s) for understanding the basics of ladder logic design? I don't want to delve into theory. Basically I need to get up to speed on how to "walk through" a typical ladder schematic, and how to design a basic ladder schematic using relay coils, relay N.O. & N.C. contacts, HOA switches, and all the other goodies needed for applications such as motor control circuits, lighting control circuits, etc. Are there design guides out there to steer me in the right direction?
Thanks,

Brian

I don't know of any specific reference manuals, but if you have any questions about that type of stuff, I do it on a daily basis so I can probably answer them.

I hope you are only looking at this stuff for the ECC afternoon, because I know that it's not on the general section.

 

[DMB5mil]

Can anyone recommend good reference(s) for understanding the basics of ladder logic design? I don't want to delve into theory. Basically I need to get up to speed on how to "walk through" a typical ladder schematic, and how to design a basic ladder schematic using relay coils, relay N.O. & N.C. contacts, HOA switches, and all the other goodies needed for applications such as motor control circuits, lighting control circuits, etc. Are there design guides out there to steer me in the right direction?
Thanks,

Brian

I'm not looking for it for the exam, but rather for my understanding at work. Sometimes I've got to run through a contractor submittal that includes ladder logic drawings and I often get lost or confused with what the system is actually doing functionally.

 

[Art]

the best thing you can do is take an Allen-Bradley training course...

they are very good and have great reference material...

talk to your local A-B PLC vendor, they offer them frequently

here's a basic book $10 http://www.plc-training.com/

 

[Twofrogs]

A book that I have used in the past as a reference is Electrical Control for Machines by Kenneth Rexford. It is really basic with its presentation of relay logic/ladder diagrams, though my edition is from 1992, so a more current version may be more helpful.

 

[chaosiscash]

the best thing you can do is take an Allen-Bradley training course...

Unless, of course, the first PLCs you ever work on are Hitachi or Omron, then it will just confuse the hell out of you.

 

[clemente]

theres a good book called "Programmable Controllers Theory and Implementation"

provided by Industrial Text. Its pretty good and you can find everything pretty easily.

If you can get the training from Allen Bradley its a good idea

 

[Sschell]

I use the koyo (automation direct) PLCs, there are courses offered for their programming software (I cant remember the name of the company that does the training, but Doug Bell is the instructor.) I would highly recommend the beginner class, three days and you will be able to trouble shoot and write simple programs.

 

[FutureCSE]

Go HERE for a free downloadable PLC book. It's a bit rough around the edges, but I found it to be very helpful.

 

[bfj]

HI,

thanks for the offer to answer a question:

I do it on a daily basis so I can probably answer them.

When controlling motion of a slow moving (less than 50 cm/sec), track guided car, with a mass that might crush a finger, but not a hand. How should the sensors that detect objects in the way be wired, and how should the logic be diagrammed? Say there were 4 sensors in a position on the car, such that if something were on the two tracks, a sensor would toggle when it felt the object in it's way.

Should it be diagramed like follows? (note the white spaces in front of the R1, R2, R3, etc. are removed when I post the comment, so they don't line up over the normally closed sensors.)

| R1 R2 R3 R4

|---|\|---|\|---|\|---|\|----[MotorController]-----|

such that the four N/C sensors are wired in series, and logic on the Motor Controller is set such that it will tell the motor to stop should any one of the sensor open?

Or is it more appropriate to desribe these sensors as switches (which is what they really are!)

| S1 S2 S3 S4

|---|\|---|\|---|\|---|\|----[EStop input for MotorController]-----|

or should these be diagramed such as

| x1 x2 x3 x4

|---|\|---|\|---|\|---|\|----[Estop input for MotorController]-----|

If describing the logic only is it necessary at all to include any information about the voltage levels that the Estop input for the motoro controller requires, or should this information go in a seperate schematic?

thanks,

 

[wilheldp_PE]

As for naming conventions, that is usually left up to the programmer/diagramer, but sometimes it is specced. I usually used LS if it was a limit switch, or PE if it was a photoeye.

I think you should wire the sensors in series with the E-stop circuit since sometimes the Motor Controller (soft start, vfd, etc.) will sometimes let the motor come to a controlled stop (i.e. the motor will coast to a stop instead of potentially damaging itself on a hard stop). The E-stop circuit will likely trigger a hard stop, which can mean either immediately setting a VFD to 0 Hz, cutting power to the drive, or cutting power and setting the brake on the motor. This "hard stop" is what you want for any safety devices on a machine.

That being said, either of your final 2 schematics would work, but you might want to check the spec to see if they call for a certain naming convention for these switches.