runtime error with USB 6009

[Susmit]

I am using an USB6009 along with LABVIEW 7.0 and NI-DAQmx Base software to control the temperature of a hot chamber. I use a Pt100 temperature sensor and the analog inputs of the USB-6009 to measure the temperature and round this off to the nearest integer. I compare this with the dial setting on the front-panel and set a digital output channel to high if the measured temperature less than the dial setting, otherwise the digital output is set to zero. This digital output turnson a 12V relay through an optocoupler (MCT-2E) which in turn drives a 230V relay. The 3-phase 'NO' contacts of this relay turns on the heating elements (5KW!). The entire vi block is within a while loop.

This works fine for some time, but around the set point, when there is a lot of relay-chatter, the system crashes with an error message. Sometimes it is an I/O error (on the USB link, I presume), sometimes it is a VISA error on the NI-DAQmx Base 'Clear Task' block.

Any suggestions on how to get around this problem? I am running the VI from the front panel and not building an executable just yet.

The round-off to nearest integer of the temperature reading adds hysterisis and reduces the chatter. This reduces the frequency of the error but the program crashes nevertheless, though less frequently.

All wires from the Pt100 sensor are shielded with the shield grounded. The relay outputs are opto-isolated, and I am sinking eactly 8.4mA (rated at 8.5mA) from the digital output line.

I am also using the USB cable that came along with my USB-DAQ-6009 (is this shielded?).

Thanks.

Susmit

[Sean_C.]

Hello Susmit,

Can you provide the exact error codes and maybe some screen shots so that I might have a better idea of what is happening?

Please look at page 16 of the 6008/6009 user manual at http://www.ni.com/pdf/manuals/371303a.pdf . Did you follow this procedure to select the appropriate value for an external pull up resistor. This external resistor is needed to source up to 8.5 mA.

Please let me know if you have any more questions.

Regards,
Sean C.

[Susmit]

Dear Sean,

Thank you for your reply. I got the factory management to replace the 3-phase mechanical relays with Solid State Relays (SSR). That seems to have solved the problem. So it was electrical noise generated by the relay contacts. This has enabled me to do-away with the 12V Relays on my interface board, though they were not to blame. Yes, I am following the directions on page 16 of the user manual. The port sinks current thru the LED inside the opto coupler MCT-2E which is pulled up to +5V by a // resistor network of 680 ohms and 10k. Since the resistance value seemed to be critical for the proper operation of the MCT-2E, I have now put a 74ls245 octal buffer to augment the drive capability and replaced this with a 470 ohm pull-up.

The exact error message I was getting is

"Error 1073807346 occurred at an unidentifed location. Possible reasons VISA:(Hex 0xBFFF000E) the given session or object reference is invalid"

With automatic error handling being ON this highlights: DAQmxbase Clear Task.vi

I found that I can only isolate the digatal signal lines and the Vcc using the opto couplers. (The interface board gets 12V power from an independent adapter and a 3-terminal 5V regulator on-board.) But the grounds remain common as the Pt100 Analog inputs are refenced to ground (RSE mode), but gets their sense currents from the external adapter. Even if this were to be used in differential mode, I would still not be able to isolate the Analog Outputs which I have used to control the rpm of two induction motors (5HP each) using an inverter drive. (The ON/OFF of the motors are controlled through the DIO lines pulled up to 24V thru opto-isolators as above.)

I find the USB-DAQ-6008/6009 to be a very low-cost, very useful solution. I would request NI to develop a totally isolated version with external power adapter for Industrial use. Even the USB lines could probably be an optical cable with an interface at the computer end. This would also permit very long cables. (I am planning to use 3rd party USB extenders for this.)

Thanks again.

Best regards,

Ssmit Sen