On/Off Control Circuit Question

[pprendeville]

I’ve got a few queries about the operation of the 3-step control circuit. Have an exam on it this week and would like to clarify a few points as we’ll be asked a few questions on it. I’ll give an explanation of what the circuit demonstrates first.
We want to maintain a constant pressure in a system. In this case, we want a constant pressure of 3 bar, this is set-point 1. To achieve this we are using an ON/OFF controller made by Festo and a pressure sensor. We use a hysteresis of 0.4 and finally we’ve got a 2nd setpoint of 3.5 bar which is like a high pressure limit. If pressure goes above this value, the reservoir will have to exhaust. 1V corresponds to 1 bar of pressure so to get three bar, we need to set S A1 (which you can see in the picture where the selector switch is to) to 3V and S A2 to 3.5V. H A1 is set to 0.4V and there is no hysteresis for the second setpoint, 3.5V.
So that’s they’re the setup values that need to be entered. Next up is the pneumatic circuit. I understand the principles of operation of the pneumatic circuit and can set it up fine. It’s the electrical circuit that I’m getting stuck on. The contacts on relay KA1 are NC. When the reservoir reaches 3 bar pressure, i.e. the pressure sensor sends a 3V signal, the NC contacts of KA1 open and solenoid Y1 deenergises. On the pneumatic circuit this sends the 5/3 way valve back to the mid position. So this is where I get a bit stuck. When KA1 is deenergised, does the contact of KA1 that’s wired in series with solenoid Y2 open also. I think this is an electrical interlock so both Y1 and Y2 can’t operate at the same time. If the pressure goes to 2.79 bar, Y1 will be energised again to top up the reservoir. Will fill it to 3.21 bar (taking into account hysteresis of 0.4V).
Perhaps someone can just clarify the interlocking technique for this particular circuit and I should be good to go then.

 

[xraytek]

So this is where I get a bit stuck. When KA1 is deenergised, does the contact of KA1 that’s wired in series with solenoid Y2 open also.

The contacts are 'normally closed' which means that when KA1 is energised the contacts open.
When KA1 is de-energised the contacts remain closed

 

[jpmiff]

Hi looking at your control circuit,

this is how see the control cicuit working, (may not be correct )

A1 is normally closed,(only opens when pressure is 3 bar), KA1 is energised so nc contact in series with Y2 is open.

A2 is normally closed,(only opens when pressure is 3.5 bar), KA2 is energised so nc contact in series with Y1 is open.

valve in mid position.

pressure reaches 3 bar A1 opens KA1 de energises nc contact in series with Y2 closes, valve opens to Y2.
Y2 opens & shuts depending on working pressure of 3 bar.

if pressure reaches 3.5 bar both Y1 & Y2 will be energised

they dont appear to be interlocked electrically!

hope this helps

 

[pprendeville]

If KA1 was NO would they be electricall interlocked?

The way I see it is that it needs to be needs to be in order for the circuit to be interlocked coz at the moment relay KA1 is energised keeping the closed contact in series with Y2 closed. So both solenoids can operate at present.

this is how see the control cicuit working, (may not be correct )

A1 is normally closed,(only opens when pressure is 3 bar), KA1 is energised so nc contact in series with Y2 is open. At the beginning (when pneumatic air supply is switched on) KA1 is already energised so the set of contacts on KA1 in series with Y2 remain closed. So as the pressure is building up the 3 bar can't solenoid Y2 be energised also?

A2 is normally closed,(only opens when pressure is 3.5 bar), KA2 is energised so nc contact in series with Y1 is open. A2 is N/O (only closes when pressure reaches 3.5 bar). When Y2 is energised it relay KA2 opens so Y1 can't energise. To me it looks like it's only half electrically interlocked.

valve in mid position.

pressure reaches 3 bar A1 opens KA1 de energises nc contact in series with Y2 closes, valve opens to Y2.
Y2 opens & shuts depending on working pressure of 3 bar.

if pressure reaches 3.5 bar both Y1 & Y2 will be energised. Y2 should only energise when pressure reaches 3.5 bar and it will exhaust the reservoir

they dont appear to be interlocked electrically!.......

hope this helps[/QUOTE]

 

[jpmiff]

Hi,

just had another look at the circuits, did not see that A2 is normally open, sorry if this caused any confusion

A1 is working pressure at 3 bar

A2 is safety exhaust system at 3.5 bar.

The purpose of interlocking is to inhibit one function operating while another function is operating. The safety exhaust system must always operate at 3.5 bar and overide all other functions and make system safe.

If you remove the NC contact KA1 which operates Y2 and leave the existing circuit as is,this will electrically interlock Y1 so it cannot operate when Y2 is energised.

A1 will operate according to pressure and operate Y1, if the pressure reached 3.5 bar A2 will operate and energise Y2 and KA2, at the same moment Y1 will be de energised by the normally closed KA2 contact opening(electrically interlocked).

When the pressure drops below 3.5 bar A2 will drop out allowing Y1 to operate.
this cykle will be continuall until the system is turned off.

Is this what your trying to acheive?

 

[pprendeville]

Hi,

just had another look at the circuits, did not see that A2 is normally open, sorry if this caused any confusion

A1 is working pressure at 3 bar

A2 is safety exhaust system at 3.5 bar.

The purpose of interlocking is to inhibit one function operating while another function is operating. The safety exhaust system must always operate at 3.5 bar and overide all other functions and make system safe.

If you remove the NC contact KA1 which operates Y2 and leave the existing circuit as is,this will electrically interlock Y1 so it cannot operate when Y2 is energised.

A1 will operate according to pressure and operate Y1, if the pressure reached 3.5 bar A2 will operate and energise Y2 and KA2, at the same moment Y1 will be de energised by the normally closed KA2 contact opening(electrically interlocked).

When the pressure drops below 3.5 bar A2 will drop out allowing Y1 to operate.
this cykle will be continuall until the system is turned off.

Is this what your trying to acheive?

Yup, that's exactly what I'm trying to acheive. The wording of that is bang on to be honest. That particular question didn't come up in the exam but it's good to fully understand it as I'm sure I'll come across it in the future. Ta.

 

[thepurple1]

Instead of using an hysteresis , If you can change the parmeters in the controller, I would go for a PID loop witha gain of about 0.25 and an integeral of about 2-3mins. A loop is what we use for air handling unit pressure control and also pump pressure control. As for the settings you have to play around with them.

 

[pprendeville]

We're just moving on to PID controllers in the course next week so we'll be learning about excessive and insufficient gains and the like of that. ta.