Bouncing Signals

[jpbedard]

I have the Compaq DAQ with the NI 9211 & NI 9205 modules.  I’ve written a program that acquires 1 thermocouple reading, 2 voltage readings from load cells and 1 mV reading from an LVDT.
 
Each of these signals is being displayed on the front panel using the waveform graph VI (See Figure 1).  I am taking the 1 output wire from the DAQ assistant and using the split signal VI to display each channel signal individually into waveform graphs (See Figure 2).  I have set up custom scales for each channel with no offset.

The problem is that the signals are not static.

For Example, with a constant load of 10 lbs on the load cells, the graphs will indicate 10 lbs then fluctuate + or – 2 lbs.  With the LVDT, it will be at position 0.0000” but will increase up to 0.0050” and anything in between.  Same story with the thermocouple – constant fluctuation at a constant temperature.

They only way I can get the front panel to display a static value is using the mean VI and numeric indicators.  See Figures 3 & 4. 

Since I’ve used the mean VI to “average” the signals, I cannot get them to display in the waveform graph.  In other words, I have inserted a mean VI between the signal wire of the Split Signal VI (one for each channel) and the individual Waveform graphs.  The only way I have succeeded in displaying the “averaged” reading is to use a numeric indicator in the place of the waveform graph. 

My concern is that the mean VI is still outputting false readings since it will output a 3 lbs load seeing as it averages between the 0 and 6 lbs load from the bouncing signal.
 
I have 3 questions:
 
1. How can I get rid and/or prevent these bouncing signals without the use of the mean VI?
2. Why can I not see a waveform in the graphs once I’ve used the mean VI?
3. How do I get the graph to display a nice horizontal line that moves along the X axisas the data changes?  As of right now, the graph is static and only displays the current values.

Figure 1

Figure 2

Figure 3

Figure 4

Thanks

JP

jean-pierre.bedard@nrc-cnrc.gc.ca

Message Edited by jpbedard on 04-16-2007 01:52 PM

[Logan_K]

Could you please explain in more detail which signals are connected to which modules? Also, for any signal connected to the 9205 we will need to know the input range that you have set for that channel using the DAQ assistant?

For the Load Cell - I assume you are using it with an external excitation voltage? How accurate is your excitation voltage, and what is the output level? Do you have any more details on your load cell, such as it's sensitivity?

For the 9211 - how many degrees is the signal bouncing?

Regards,
Logan

[jpbedard]

The Bale Load and Friction Force are both load cells with a 0-100 lbs range.  They are being exicted externally by a 10VDC Sola Power supply.  Both load cells are wired into the 9205 Module.  Although not show in my block diagram, all 4 signal wires are being used with the mean VI.

Load Cell Details:

Sensitivity: 3.130mV/V

Input Resistance: 380 Ohms

Output Resistance: 350 Ohms

The LVDT is being powered by a 5VDC sola power supply and is wired into the 9205 module.

The type T thermocouple is wired into the 9211 module.  The thermocouple is boucing about + or - 1 degree C.  I understand that this is most likely in tolerance and for that reason, I used the mean.VI  I simply cannot get it to display on the graph after its been "averaged" let alone get it to display dynamically.

Input Ranges:

Thermocouple:

Max: 100

Min: 0

Scaled Units : deg C

Type T

CJC - Built in

Load Cells:

Max: 100 The min and max are the values expected after custom scaling.

Min: 0

Scaled Units : lbs

Terminal Configuration : Differential

Custom Scaling in MAX

Pre-scaled

Max:0.0313V

Min:0V

Scaled

Max:100lbs

Min:0lbs

LVDT:

Max: 0.092 " The min and max are the values expected after custom scaling.

Min: 0

Scaled Units : inches

Terminal Configuration : Differential

Custom Scaling in MAX

Pre-scaled

Max:-0.301V

Min:-2.813V

Scaled

Max: 0"

Min:0.092"

Hopefully this answers all your questions,

Thanks for your help,

JP

[jpbedard]

Also, I forgot to mention that the power supplies are adjustable and I have set them up to exactly the required voltage.  I believe this is reason I cannot find the accuracy.

JP

[jpbedard]

Another piece of information : all signal wires are shielded.

I just ran the program again and the load cell for the friction force goes from 0 to 50lbs with a constant 10lbs load !

Even with the mean VI, the bale load displays anything from 10-13 lbs with a constant 10 lbs load.