NI USB-6008 Power/Grounding problem

[HrvojeD]

Hi,

we have constructed laboratory experiment used for level measurement. Idea is to educate students about level measurement.

We have two tanks. Bottom one is used as water container, and upper one is used as tank where we measure level. We use pump for getting water from bottom to upper tank, and valves to control water direction. You can find process scheme lower in message.

For level measurement we use Siemens sensors: capacitive, radar, ultrasonic, submersive probe (hydrostatic pressure) and weighing system. All of this instruments use 24 VDC power input (3000 mA). Their output signals are connected to NI USB-6008. Solenoid valves are controlled using digital signals from USB-6008.

There are two situations that happen.

First situation happens when I connect USB-6008 to my laptop. Radar, submersive probe and weighing sensors are working correctly and measurement signals are accurate. Capacitive and ultrasonic sensors are not working correct. Their signals are not as they should be and also there are screens onboard these instruments, which are not showing anything. From this I conclude that there may be a problem with powering up these instruments. Digital control signals for valves are working correctly.

Second situation happens when I connect USB-6008 to PC connected to 220 VAC. In this situation none of measurement signals is correct and digital signals sent to valves are not working, but all sensors are powered up ( onboard screens are working and there is characteristic sound coming from ulstrasonic sensor). From this I conclude that in this situation there is something wrong with my DAQ.

From everything written I believe there is some problem with electrical connections. Either there is not enough power from power source (24 VDC, 3000 mA) or there is some problem with grounding.

Below I attached process schematics.

I look forward to reading new ideas about this problem.

Regards,

Hrvoje Dorić

[Duffy2007 · Accepted solution]

Hello HrvojeD,

one idea,what you can try:

Is the GND or minus of the 24V connected to earth ground (PE), if not make a connection. Also connect the USB-6008 GND lines to this earth ground. The USB 6008 get a ground reference over the USB cable and this could be different, if you are using a laptop or a PC. The USB 6008 has no glavanic isolation and this can result in some situations, in different measurement results, because of different groundings.

[MarijnBlokland · Accepted solution]

I would think that you need to hook up M24- to pin 16 (GND) of the NI-device. All currents from the sensors towards the 250E resistors now either have a return path through the USB ground and system GND... or they find a return path through the other sensors if they have a low signal.

Off topic, is it wise to name different relay Y01 and Y1?

[Henrik_Volkers]

Same with me: You need a connection (current path) from the 24V supply GND/neg  (M24-) to the GND of the 6008.

KUDO for a well documented question!