Conveyor Belts & Timers

[leitmotif]

Yes I know what you mean

But thank you all so much. I feel much better,and a lot clearer about things... Cheers!

I think if you have this in the shop and you do not have to let production have it and you can test run and find your mistakes all is well and good.

To determine if motor is actually running I would use current sensing. You can monitor 3 phases or just one to save bucks. THis will tell you motor is actually running AND you get a rough idea of whether or not it is actaully doing work. This is commonly done to detect broken belts on fans. The advantage here is that an aux contact says motor is running but there is no way to detect motor is actually doing any work ie making fan rotate.

This will also let you detect a jam or overloaded conveyer.


Dan Bentler

 

[ruby.n]

Hi Dan,

Thank you so much for that. I am now using the phase current (from an E3 Plus Intelligent Device), and comparing it with a preset value. If the phase current is less than the preset value, I activate the Conveyor not healthy alarm.

And about checking if the conveyor is running or not, can I use the following logic:

The underspeed (Pulsing signal from conveyor shaft-proxy switch) NO contact triggers a TOFF timer with a Preset value of 10 sec (say). The next rung is a NC timer.DN signal driving a Conveyor Healthy coil ???? So if the timer ever finishes the 10 sec, something's gone wrong...?

 

[TheStarr]

I am a Graduate...

Ive never seen an Electrical drawing before!

Is it just me or do these statements not go together?

Not to be rude but...what are you a 'graduate' of?

It seems strange that you would be put into the position!

 

[TConnolly]

The underspeed (Pulsing signal from conveyor shaft-proxy switch) NO contact triggers a TOFF timer with a Preset value of 10 sec (say). The next rung is a NC timer.DN signal driving a Conveyor Healthy coil ???? So if the timer ever finishes the 10 sec, something's gone wrong...?

Thats a good start but what if your shaft happens to be stopped right where the target is directly under the prox swich so that the prox switch remains on? Without the input switching OFF the TOF will never begin timing and you will miss the conveyor stopped signal. By using a one-shot which remains true for a single scan only to trigger the TOF then you can avoid that problem.

  PROX             ONESHOT

---] [---[OSR]------( )---



 ONESHOT

---] [------------[TOF]----

 

[PeterW]

Thats a good start but what if your shaft happens to be stopped right where the target is directly under the prox swich so that the prox switch remains on? Without the input switching OFF the TOF will never begin timing and you will miss the conveyor stopped signal. By using a one-shot which remains true for a single scan only to trigger the TOF then you can avoid that problem.

  PROX             ONESHOT

---] [---[OSR]------( )---



 ONESHOT

---] [------------[TOF]----

I'd put a NC of the output in parallel with the oneshot, so its healthy when stopped via the control logic.

 

[TheStarr]

I use a prox switch on the gear and three rungs.

Prox
---] [---------------[TON (timer 1)]---

Prox
---]/[---------------[TON (timer 2)]---

Timer 1.DN Timer 2.DN
---]/[-----------]/[---------------(Chain is running)----

I usually set the timers at 1 sec.

Simple, easy to troubleshoot, and always works.

 

[ruby.n]

I actually used the method that TheStarr suggested, but they would not accept this method for some reason. I don't know why.
So thats why I changed it to the TOFF method.

In the method that Peter suggested, with the One-Shot: What triggers the OSR?

 

[ruby.n]

Sorry I am an Electronic Graduate, hence the Electrical diagrams were a handful to understand at first.

 

[bernie_carlton]

When I was first shown a PLC relay ladder logic diagram I asked, "what are all those capacitors for?"

 

[jstolaruk]

In the method that Peter suggested, with the One-Shot: What triggers the OSR?

A true result of the rung contents preceding the OSR.

 

[PLCMentor]

When I was first shown a PLC relay ladder logic diagram I asked, "what are all those capacitors for?"

Bernie,
That's great! Definitely an electronics view of the world. I'm afraid with my warped sense of humor I may have had to string you along a bit with that response. Of course you probably just got a blank look for a few seconds while the person showing you worked that one out.

 

[todster]

You could also take it a step further if not already. If the downstream conveyor is faulted or off then this conveyor should also be off. If this conveyor is faulted or off then the upstream feed should shut down.
A simple jam timer on a photo eye in combination with the aux contact would be enough to indicate that the conveyor isn't running and set a fault which would display on the HMI as a fault or a jam.
If the motor is getting power but not running,overloaded, or lost a phase the thermal should kick anyways. Regardless of the technique used, someone would still have to show up to see why the line isn't running. Broken chain/belt = jam
A photo eye on the feeder discharge and a photo eye on the charge end would also give the capability to go "GREEN" (For the extra points) with jam detection/faulted. If no product on charge conveyor in 5 minutes, idle conveyor downstream. When product detected, start conveyor- watch for jam or fault. Don't forget your jam reset timer and P.B. as well as fault clear P.B and idiot lights for both.
Your jam timer should be based on FPM and longest expected product length + a margin. The jam reset should be long enough to force them to actually look for a jam before allowing a reset. Illuminated P.B's will help the operators in locating the fault/jam.

 

[Bobbias]

Sorry I am an Electronic Graduate, hence the Electrical diagrams were a handful to understand at first.

Yes, unfortunately, your going to encounter a LOT of ladder logic diagrams if you keep working with PLCs. It's tempting to think of things in the computer programming terms, if you know them, but it's much better to look at ladder logic as a representation of boolean logic.

For instance, 2 things in parallel is a boolean OR. 2 things in series in boolean AND. -|/|- is boolean NOT. If you can translate ladder logic to boolean logic, you can very quickly and easily dissect those ladder logic diagrams in a way that is likely a lot more familliar to an electronics graduate.

 

[ruby.n]

Hi All,

Thanks to your help I have finally finished writing the code!