speeding the Keithley 6485 readings with LabView

[Zeyad]

Hi All,

I'm new with LabView and with current measurements. I'm using a Keithley 6485 Pico-Amp to recored the current changes in microchannels. I downloaded the instrument drivers from the NI website and I tried to control the Pico-Amp current readings with LabView 7.1 but the rate of readings from the Pico Amp was slow ( a reading every 0.5 s). I tried to change a lot of parameters and I found the main problem causing such slow measurements is the read VI for the Pico-Amp was very slow in the while loop.

is there any way to recored fast continuous measurements from the Pico-Amp using the read VI in a while loop with LabView 7.1?.  or if there is another to do fast recordings form the Pico-Amp with LabView? 

I appreciate your help and suggestions.

thanks

[Brian_R]

The Read.vi function has a timeout associated with it that you can shorten.  However, your data may not transfer completely.  My guess is the instrument has a fair amount of delay that is causing slow loop rates.  Are you able to control it by other means with faster response time?

[Dennis_Knutson]

You might also want to look at using the multi-point read functions. I could not find a driver on NI's site, only a link to Keithley's IVI driver. I'm assuming this is the one you are using. There are a couple of shipping examples. There is a Binary Read that uses a Fetch Multi-Point and one called Buffer Read that uses a Read Multi-Point. A fetch operation is usually much faster than a read.

[F._Schubert]

I have no experience with that one, but with other models you could improve speed a lot when you did not send the 'MEASUREMENT' command, but a ´'CONFIGURE' and afterwards (in a loop) the 'READ' commands (measurement is configure+read), having a single instrument connected, the capture-fetch does not improve  much. (You normaly can look into the LV-drivers what command they use).
The other options is the NPLC-setting (number of power lines cycles), that is often used in DMMs, check that value (if NPLC is 20 and PLC is 40Hz = 0.5 sec).
If none of these works, post some code.

Felix

[Zeyad]

thanks a lot guys for your suggestions and comments.

at the present time I'm using the analog output of the Pico -Amp to recored the readings via a Daq since I couldn't control it form the drivers. So,  I only control the Pico Amp from the front panel.


Dennis, thanks for your suggestions and it is true that fetch multi-point is faster but it is still not fast enough since I want reading at a rate of 40-50 Hz. if you know of a way to increase the readings rate to the frequency I need it will be helpful.


F. Schubert, thanks for comments. I don't know how to change the NPLC since when I change the value in the VI an error message appears and Labview terminates. the NPLC setting is 1 and PLC 60 Hz.


I attached a sample of VI I'm using to find the readings from the Pico-Amp and I would appreciate any suggestions to improve the readings rate. also how can I change the NPLC value of the device?

Thanks

Message Edited by Zeyad on 09-13-2007 11:51 AM

[Brian_R]

What is the LabVIEW error message?

[rodmanhust]

Hi zeyad,

were you able to get a faster reading rate afterwards? I know it's been a long time but I'm encountering the same problem right now with 6485. Could you update with a new vi file? 

[Eric_B.]

rodmanhust,

Did you try any of the other suggestions above?  Did you have problems with those?

[Zeyad]

Hi rodmanhust, 

I gave up on trying to speed up the picoameter with lab view. I reached the conclusion that there were limitations in the speed of the device and the Labview. Keep in mind that I’m using R232.  Finally I decided to record the analog signal with a BNC concoction and I dumped the idea of automating my experiment.   

[rodmanhust]

Zeyad and Eric,

thanks for your comments.

I'm also using a RS232 connection for KE6485, and with the Fetch function I used (as in the attached vi file) I could still only get fastest sampling rate at about 0.5s. With Read function I get ~0.7s, so the fetch is only slightly better (only one device connected, so no need for Multi Point, right?) . However, in the vi file if I place the "Initiate" function ("help" says it has to be used before Fetch function) outside the while loop the sampling rate is much faster, reaching down to ~0.01 sec, the problem is that it only reads a static signal. I think KE6485 is capable of generating fast sampling, because if I switch "rate" to fast on the front panel, it shows a really fast changing current. Maybe RS232 is the bottleneck? I'm not quite sure what's a BNC, but did it help Zeyad? 

In my setup, frequency is 60HZ, and NPLC is 1. When I tried to change NPLC and run, program freezes w/o any data reading.  so right now I really don't know how to improve sampling rate. Even ~0.2 sec would be a huge help. I wonder if you guys could take a look at the vi file for me and let me know if there is anything I can do. thanks!