FieldPoint suddendly looses connection, but only for some seconds

[Oliver S.]

Hi,

I have a problem with LabVIEW and/or FieldPoint modules. I use the ethernet controller FP-1600, the analog input module FP-AI-110 and the digital output module FP-DO-400.

The controller is connected with the PC via crossover cable.

I have written a VI, which should control an engine with the digital output module and measure some parameters with the analog input module.

For the engine control I use the following cycle:

1. 20 sec load (digital out: channel 0 = true; channel 1 = true)
2. 5 sec no-load (digital out: channel 0 = true; channel 1 = false)
3. 2 sec power-off (digital out: channel 0 = false; channel 1 = false)

The VI set automatically the output channels.

With the analog input module I measure continuously (every second) temperature, motor current und the speed.

For the most part the VI works without any problems.

But sometimes the PC looses the connection with the FieldPoint modules. This failure occurs only in the change from “load” to “no-load”. In the other changes I didn’t see this failure yet. During this failure the motor parameters shown on the frontpanel don’t refresh. After 3 – 5 seconds the failure is gone and the VI works fine again.

What could cause such a problem? And what could I do to solve it? Thanks.

[Wayne.C]

Oliver,

It sounds like your test setup is generating some electrical noise.  What kind of circuitry are the two digital outputs connected to?

[Oliver S.]

I think you had the right idea.

The digital outputs are connected to relays. The first relay switches an electric motor (on/off). The second relay switches a magnetic brake (on/off). This brake is technically comparable with an eddy current brake.

I suppose the high inductivity in the switching moment could cause the described failure.

Now I will try to use a recovery diode to minimize the high inductivity.

Do you think that could be the cause of error?

[Wayne.C]

Oliver,

Any time you switch inductive loads, electrical noise/transients will be generated.  What varies from system to system is what kind of problems/damage the noise will cause.  My personal approach is to do everything I can to reduce this noise.  I highly recommend a snubber of some sort across all relay/contactor coils.  The snubber needs to be physically close to the coil.  Flyback diodes work well for DC coils and there are several RC snubbers available that work well for AC coils.  Some people also use MOV's but my personal testing using a chattering relay setup and an EFT generator has shown the RC snubbers to be more effective that the MOV's.  You may need to also consider some sort of protection across the DC supply to the FP module itself.  I have seen mention in the forums of using a TVSS across the power to FP modules as well.

Your description of the problem would seem to indicate that it happens when a DC relay is de-energized.  Flyback diode should fix this.

[Oliver S.]

Thank you for your detailed answer. A flyback diode fixed the problem.