Am I doing this correctly? Please help!!

Hello,

I am extremely new to the world of industrial controls and PLC's. I am taking a class on beginning PLC, but I'm having a hard time. The instructor's only assistance is generally 'read the book' which I've done many times. However, I can read something 1000 times over and still not be quite sure about what I'm doing. I learn best by actually seeing how something works. I'm not asking for answers, I just want to know if what I'm working on is correct. The project assigned is as follows:

Install an anti tie down circuit. The only way the machine can start is two foot switches must be made and two palm switches must be made at the same time before the machine will index. The materials required are 2 Start Buttons, 2 timers, 2 foot switches, 1 relay.

What I have drawn out is correct in my mind, but I'm often wrong. Again, I'm not looking for someone to do this for me, I just want to know if I'm on the right track. Here is my schematic-

(Start)(Start)
----II----II---------------(TIMER T1)5seconds

(Foot) (Foot)
----II----II---------------(TIMER T2)5seconds

(T1) (T2)
----II----II---------------(C1) Relay
' '
'----II----'
(C1)

(C1)
----II--------------------(Y0)Motor start

This is how I interpret the instructions, am I correct? If not, please explain why, it would be a tremendous help!

Thanks in advance!

Where does 5 seconds come from - don't see it in the assignment.
What does timer T1 do - delay on for 5 seconds, pulse for 5 seconds or what ?
Also, what do you mean by "index" ?

Greetings dandelo,



what type of PLC are you using? ... and if T1 and T2 refer to the timers’ Done bits, then start rethinking your approach ... question: do you really want to “wait” until the timers get “done” before the next step happens? ... do your timer’s also support a “Timer Timing” bit? ... for example: T4:0/TT in the Allen-Bradley platform would be a valid address ... if your timers do NOT support the “timer timing” format, then start by asking yourself how you could develop the same type of function with your existing hardware ... hint: combine whatever “contacts” you DO have available to come up with a “timer timing” signal ...



if you need more help than that, feel free to post again ...



and welcome to the forum ...

Dandelo,

I think your timers are the format generally used by Siemens and Automation Direct, where the Timer bit is ON when the timer value is reached. They are definitely not the Allen-Bradley type. If so, then your circuit works as follows:

(1) Both palm swithes must be closed for 5 seconds before machine will index, AND
(2) Both foot swithces must be closed for 5 seconds before machine will index.
(3) After C1 is energized, it is okay for the 4 switches to be released; the machine keeps on working.

Accoring to my interpretation (which has been questioned on this forum many times!) the instructions mention "machine can start" and do not say "machine can run", in which case you are correct.

Now that you have got it started how are you going to stop it.
Or I guess thats not part of the assignment.

SimonGoldsworthy-
There is no specified time for the timers. The way I was going to do the assignment was that once both push buttons and both foot switches were held for 5 seconds, T1 and T2 would be true (would close) turning on C1. C1 would seal in T1 and T2 allowing you to release the 4 start buttons.

Ron Beaufort-
To my knowledge, I think I think I do need to wait for the timers before the next step. This is a theortical project, inorder to keep injuries down at a plant.

do your timer’s also support a “Timer Timing” bit? ... for example: T4:0/TT in the Allen-Bradley platform would be a valid address ... I'm really not sure what you're talking about here, sorry! As I said previously, I'm brand new to this and was just thrown head first into the projects with no instruction whatsoever. I'm really not sure what type of PLC I'm using, sorry.

Mickey said:

Now that you have got it started how are you going to stop it.
Or I guess thats not part of the assignment.

Click to expand...

Although it is not mentioned in the assignment, I thought I could put a normally closed (XIC) contact in the same rung as the T1 and T2 series contacts. Once the normally closed stop is pressed, that should break the continuity to C1 output 'killing' the motor, correct?

Yep

My guess is that an index is a timed run of the machine, not a start that latches on.

Dandelo,

No, that will NOT stop the motor, if I am seeing the second C1 contact as a seal-in contact. Once C1 locks in, stopping T1 and T2 will not stop the motor. A better place to put the stop contact is in series with the Motor Output. That will stop it, even if some novice programmer comes in later and modifies your timer logic. It is best, where possible, to locate logic close to where it is used, not indirectly in a remote rung.

As there are no stop buttons mentioned they cannot be part of the solution. Also as there are two footswitches and two palm switches (I pity the poor operator on this one), then even though this is a theoretical assignment, it would be ridiculous to latch into run when you get off the footswitches etc.

SimonGoldsworthy said:

As there are no stop buttons mentioned they cannot be part of the solution. Also as there are two footswitches and two palm switches (I pity the poor operator on this one), then even though this is a theoretical assignment, it would be ridiculous to latch into run when you get off the footswitches etc.

Click to expand...

I agree, as it is not requested in the materials needed, I will not include the stop.

If there are any other thoughts/comments, I would love to hear them. I will be asking MANY questions on this board over the next few weeks (perhaps several a day), and all I ask is that you have patience with me! I appreciate all the assistance!

I hate to be the one to throw a monkey wrench into the works, but "anti tie down" generally means that the two palm buttons must be closed within a fraction of a second of each other and both must make the transition from opened to closed within that tome frame.

The idea behind it is to require that both of an operator's hands are on the buttons before the cycle can start. The anti tiedown feature prevents an operator from jamming one of the buttons closed. Note that in the real world you should be using two-hand anti tiedown devices specifically engineered for the purpose rather than doing it in PLC logic.

What dandelo has posted looks more like a "Start delay" which is typically used to ring a warning bell or flash a light before allowing the machine to start.

Let's assume that an index is a timed run (for say 2 seconds). That uses up one of the timers. The only use I can think of for the second timer is that once the index has finished, you have to wait (say 3 seconds) before you can do another index - that's the second timer used up. Anyone any better suggestions ?

Steve Bailey - that's not a monkey wrench - I think you've put this thread on track. A quick search on Google gives the following for anti tie-down

Provides a maintained output signal when both
buttons are pressed within 0.3 seconds and held.