'Air Pressure Low' - Interact 308 TNC2500B

Hi,

I've just started getting 'Air Pressure Low' warning when pressing Tool Unclamp or running M6 tool change on a working machine.

Manual tool change functions as expected and I can swap tools. M6 won't run through of course. Air pressure reads normal at 80psi at machine regulator. Pressure switch functions and closes at 40psi. 24v supply okay.

I've tried looping through the pressure switch (in case air pressure drops during tool change) to maintain 24v signal and traced through to X27 input - all okay. Tool Clamped/Unclamped microswitches and signals traced through to X22, all okay.

What am I missing?

Thanks.

Mechanical pressure switches do go bad. If you put a jumper wire across the switch and the problem disappears, it's the switch.

Thanks, but as I said in my post above, I've already tried that.

Quote:

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Originally Posted by Rickytoo

I've tried looping through the pressure switch (in case air pressure drops during tool change) to maintain 24v signal and traced through to X27 input - all okay.

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"All okay" meaning the problem was gone when you did that? Then either the switch is intermittent, or the pressure indeed drops below 40psi at the point where the switch is mounted.

Meaning the problem still exists. Effectively, I bypassed the pressure switch so the control sees 24v regardless.

As I'm bypassing the pressure switch, I'm wondering how the control still considers the air pressure to be low. Is there a timing parameter that works in conjunction with the tool clamp microswitches?

I am not familiar with your controller, does it allow monitoring input/output signals in real time? If so, I would watch the pressure switch input status while the clamp is open. Something could be going on with the controller itself.

Also, is there more than one pressure switch in the machine?

It can display a table of PLC inputs, the logic states of which are dynamically displayed. For example, pressing Tool Unclamp changes a number of logic states on the display, but I have no information as to what these logic stated correspond to.

Just the one pressure switch on the incoming line.

You could try manually triggering the pressure switch and see which PLC input changes value. Then check if that input also toggles during tool change.

I found PLC inputs:
128 air pressure switch
7 tool unclamp microswitch
8 tool clamp microswitch
9 tool change button
Opening the air supply to the machine closes the air pressure switch at 40psi and the input 128 changes state to logic 1. Thereafter it is unaffected by doing a manual tool change.

I found PLC output:
5 drives relay no.9 - unclamp air solenoid, which also coincides with 'air pressure low' displayed.

Using an oscilloscope, I measured the transition time between the clamp/unclamp micro switches and found switching take approximately 175ms when pressing tool unclamp. PLC programmed duration for timers 4110.x are in units of 24ms according to my parameter list, giving 7.3 units transition time. Thinking that if the performance of the unclamp piston assembly had deteriorated, it might be slower than when new, I looked for timer parameter values of less than 7.3 units. Parameters 12, 14, 16 are 5 units or less, Sadly, increasing each of these in turn to 15 still didn't stop the low air pressure warning when pressing the manual tool change button.

OK. So the PLC turns on the unclamp solenoid, then waits for the unclamp microswitch to activate. If the microswitch signal is not received in time, the PLC assumes that the air pressure is too low to operate the clamp. Did I get this right?

(This is one thing I dislike about most CNC controls: misleading error messages. It would make much more sense to say something like "Tool unclamping failed" in this situation, instead of "Air pressure low".)

When you looked at the microswitch signal on the scope, did you see any bouncing? How about the other microswitch ("clamp"), does it operate properly?

What if you disconnect the solenoid (so the tool never unclamps), does the error change to a different one?

Yes, that's the theory I'm working on anyway. I have since found a parameter list (online, in Spanish) for a later control that demonstrates this principal is used.

It would make more sense to me too. It's just a response to input/output and what the PLC sees as a fault. Although low air pressure would give rise to the displayed message, on this occasion I don't believe it has anything to do with it.

Nothing abnormal for a mechanical switch I feel. All the switches listed above show clean and steady change of state on the PLC input page when performing a manual tool change.

Measuring the timing between the microswitches I realise is not what I should be measuring. It should be between the tool unclamp button and the unclamp microswitch outputs I think. Doing this increases the time taken to 250ms or 10.87 units in the timing parameters. Although there are a couple of parameters of 10 units, changing these to 15 didn't change the outcome, it still displays low air pressure when pressing the tool change button.

Does anyone have a list of what the timing parameters 4110.x correspond to? Of the possible 47, mine only has 27 with a value not equal to zero and 5,6 and 7 are tied up with lube timings.

Maybe this Tech Manual will help. It's for TNC2500B control

Thank you kindly. I have all the available Heidenhain manuals for this control I believe, the technical manual being well thumbed at the moment :-)

Having observed changing states in the tables, I've got useful output from trace-buffer. Only the first part of a manual tool change is shown here, but it completes as expected.

Input 128 Air pressure switch
Input 7 Unclamp microswitch
Input 8 Clamp microswitch
Input 9 Tool change button
Output 5 Relay 9 - air solenoid
Timer 26 - on for as long as tool change button pressed
Timer 74 - on for 1725ms or 1.75s
Marker 2942 - 'Air Pressure Low' warning - stays on until cancelled with CE

I used I9 as a trigger. I'm not sure of the significance of the timers.

From the attached image of the display, you can see that the air pressure low warning comes on immediately with pressing the tool change button and that the air pressure switch remains logic high, or on, all the time. You can see the microswitches switching, with 8*23=184ms being the ram operating time.

Although the tail-end of the tool change sequence is not shown, it performs as expected.

So, apart from the low air pressure message, toolchange appears to work correctly.

I've just about run out of ideas as to what might be causing the air pressure warning...

Bridgeport have a habit of 'grouping up' error messages. Check the level of your lube oil reservoir, I have a feeling that it's level switch is in series with the air pressure switch

I've just checked today. The reservoir has sufficient oil and the switch is closed. In the past it has reported low oil correctly.

However, I connected a switch on flying leads in place of the level switch so I could interact with it whilst observing the CRT display. Setting the switch to 'oil level low' or 'sufficient oil' makes no difference to the issue with the low air pressure warning. Furthermore, running PLC Program Trace, I see input 6 change state when operating my temporary switch, marker 3286 changes state also. Interestingly, when the switch is set to 'low oil level', it takes approximately 135seconds for the low oil message to show, yet I don't see a timer running for that delay.

What does the manual say for min pressure? On my Haas it’s 100 lbs when we installed a air dryer we had to up the pressure for when it discharges lowering the air pressure.

It's not specified, currently the switch operates at about 40psi and the pressure is set to 80psi. However, the control only sees an open or closed switch. As I've described, I've bypassed the switch for testing purposes, so the error message is not caused by low air pressure.