Electronic Explanation Needed Please

Discussion in 'The Projects Forum' started by GARYN, Sep 25, 2009.

  1. GARYN

    Thread Starter Member

    Sep 4, 2009
    47
    0
    Hello,
    I am an Mech/Elec Engineer in a food production factory, i was today asked to fix a machine which at first seemed a simple fault- a 5 port, double acting solenoid pneumatic valve was not changing although the solenoids was getting 24VDC.
    We have no info on this particular machine and i have never worked on it before but there is a computer hard drive in the panel, Balder drive/interface circuit board and servo amplifiers.
    Basically after a lot of messing about including changing solenoid/valve the valve would not change unless i connected it to the workshop bench power supply (24v). The interface card has outputs to feed various solenoids and the ones i needed had 24v on them when on but would not provide enougth power to move the spool in the pneumatic valve.(the outputs did not drag down when the load was connected)
    The only way i got the machine to run in the end was to run 2 leads direct to solenoid pins from the outputs and do away with the caps which contain LED indicators. I had already tried new leads/caps which did not help and these are the same parts that have always been on the machine.
    My electronics is limited but if anyone can explain whats happening it would be appreciated. I thought if we had the correct voltage weather its from a bench power suppply or card it should work as the solenoids are only low power.
    I have experiened before tranistor outputs playing up but usually the output voltage drops when the load is connected?
    I am not sure if this card has relay or transistor outputs but can both cause this fault i.e. we have the voltage but cant supply the current.
    Is it possible the card is failing and just by bypassing the extra load of the caps there is just enougth to energise valve.
    I am sure there is an electronic explanation for this, thanks.......:confused:
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    If you measure the voltage near the output of the board and you get 24V then a wire (wires going to the valve) is not good (cut).
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Sounds to me like the driver circuit in the card is failing (becoming high resistance) whether it's a relay, transistor or MOSFET that's driving it.

    Relay contacts are very tough, but eventually burn, and their resistance increases. More power is dissipated across the contacts and less across the load (the solenoid in your 5-way pneumatic port.) Eventually, there is just not enough power to cause the solenoid to engage. Removing the LED from the circuit is but a temporary reprieve; the problem will shortly recur.

    Note that this type of failure is not limited to relays; it can also happen if the solder joints or screws holding transistors/MOSFETs in place loosen or crack with age.

    When you were measuring the output with your multimeter or DMM, the impedance of the multimeter is extremely high in comparison to the load of the solenoid. You really needed to check the control boards' output when it was under a load similar to that of the solenoid.
     
  4. GARYN

    Thread Starter Member

    Sep 4, 2009
    47
    0
    Thanks for the above respones, their is no broken cable as i checked the voltage at the solenoid and i tried new cables.
    The second reply from Sergeant Wookie is more likely but what does not make sense is that i checked the output under load from the solenoid with my Multimeter and it did not drag down and stayed at 24V but would not pull solenoid in.
    If the driver output was faulty which as suggested is most likely would the voltage not drop when checked under load?????

    Thanks
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    If you are getting 24VDC across the valve's solenoid under load then the solenoid is not good.

    Are you sure the output is DC and not AC?
     
  6. GARYN

    Thread Starter Member

    Sep 4, 2009
    47
    0
    Hello its defo a 24VDC output and 24VDC solenoid and i have tried connecting direct to the output but would not operate valve, this was the same with a new solenoid but both are fine with bench power supplies. These are the original machine components so are compatible.....the thing that gets me is the supply on the solenoid is saying 24v and not dragging down but it will not move valve...surly if the output was faulty the voltage wouls drop under load......or is there another electronic explanation for this?????
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Check if the solenoid draws current and its value.

    Maybe the solenoid is fine but the air pressure is too high for the valve to operate.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Those 5-port valves have spools in them moved by the solenoid that direct air pressure and atmospheric venting from one end of the pneumatic cylinder to the other.

    The spools can get contaminated if the air line filter gets clogged or accumulates excessive moisture, causing higher resistance to movement by the solenoid. Also, they eventually wear out. Frequently changing the air filters and draining moisture traps will help extend the life of the valves, but they all wear out eventually.

    If you were measuring 24VDC at the solenoid when the driver board was attempting to operate it and air pressure was applied, the spool valve is likely contaminated or just plain worn out.

    Purge the air pressure line prior to replacing the valve to ensure there is no moisture or debris waiting to contaminate a new valve.
     
  9. lostcowboy

    New Member

    Sep 20, 2009
    2
    0
    Hi GARYN, a lot of power supplies besides regulating voltage, also limit current. That means if your 24 volt power supply, has more than one load and one of the loads has had a decreased in resistance that load would get most of the current, and your solenoids would not get enough current to operate. Also it may mean that all you have to do is adjust the current limiting in the power supply.

    I would also tell your boss that you need both a operating manual, and a diagnostic manual for this piece of equipment so you can keep it up and running correctly.

    It is my guess that this equipment originally came with a factory maintenance contract, that is why you don't have the proper manuals. After your company found out the equipment was reliable, they let the contract expire. So here you are trying to fix it with out manuals.
     
  10. swcash

    New Member

    Sep 26, 2009
    4
    0
    When you are energizing the solenoid on the bench, are you applying the same pressure to the ports as when installed? Will your bench power supply energize the valve when it is installed and pressurized in the equipment?
    You need to measure the electrical load in each situation and match it to the rating listed on the valve.
    Squido W Cash
     
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