Sub VI triggering problem

[GeorgeR]

Hi Tom

Thanks for your reply. Since I last posted we've got the valve working nicely (have changed the vi around, not a master-slave' anymore) and have added quite a bit. So now we have the valve being triggered only when the corresponding sensor is broken. We've now added some more things....we want to have 3 'feeders' (3 sensors, 3 valves......and next 2 motors turning swivel arms on 2 of the feeders which will be the next step in programming). The problem is adding the third feeder in to the system.

This isn't a programming issue but we are having a problem with grounding in the setup. All feeders need to have a 12v and 5v in addition to the harware D I/O. 2 of the feeders have a circuit board with the 12v supplying the 5v signal so this only requires a 12v and ground (and the harware ground with this). This works fine, but unfortunately the third feeder has an older circuit board where the 12v and 5v are supplied seperately (from a duel power supply). This means that if the harware ground is connected with the 12v ground, the componant (sensor) on the third flower which has the 5v seperate supply dosn't seem to be in the circuit properly and when tested in Measurement and Automation explorer, shows up as being high all the time. We probably should just get the new circuit board for the third feeder but not sure how much of an option this is at the moment so just wondering if there's a way round this.  We can't really test any further programming untill the third feeder is working correctly so this has become a bit of a problem. As you may have guessed, all things electrical are not my strong point  and I don't want to ruin all the equipment by connecting up the wrong things!

Sorry if this is a bit of a ramble

Thanks
George

[johnsold]

George,

It sounds as though the 5 V supply on the third unit may not be grounded properly. Does the dual power supply have separate grounds for the 5 V and 12 V outputs? What is the ground of the 5 V supply connected to? Do you have a voltmeter available to check the power supply voltages?

Lynn

[GeorgeR]

Hi Lynn

The dual power supply does have seperate grounds for the 12v and 5v outputs. The ground of the 5v and 5v ground are both connected to the third feeder. We have checked the power supply voltages with an oscilloscope and they seems to be fine which is why we wern't sure exactly what the problem could be.

Thanks for any help
George

[johnsold]

George,

Try measuring the voltage at each of the grounds with your oscilloscope. Connect the scope ground to the ground on one of the units which is working correctly. Then probe all of the other ground points around the third unit with the scope probe. If you find one which does not measure zero volts (with a small amount of noise), then that is likely where you have something connected incorrectly. It could be a broken wire or a loose connection - something which may not be apparent on a visual examination. Broken ground connections can be subtle and tricky to locate at times.

Another way to check the grounds is to turn off all the power and wait until all the capacitors in the power supplies have discharged. Then connect an ohmmeter to one of the ground points and check the resistance to all the other points which should be grounded. Any point showing more than a fraction of an ohm is likely not connected properly. If your loads are low impedance, there may be parallel paths which make this technique less useful.

Lynn

[GeorgeR]

Hi Lynn, thanks for your advice, i tried checking all the grounds and they seemed to be ok so now I have a seperate power suppply for the 5v and then the 12v for all 3 feeders from another supply. I have a ground from the harware going into each power supply and now all the feeders are working fine.