how can I measure a current continually after switching it to a DMM?

[小木虫]

Whether a current could be measured continually after switching it to a DMM? How to write the scan list? 

I tried many times and failed. 

Thanks advance

小木虫

[chericks1]

Hi 小木虫,

By default, a scan is Break Before Make, meaning previous entries are disconnected before the next entries are connected.  To scan current, a Make Before Break scan mode is often desired (next entries are connected before previous entries are disconnected).  Unfortunately, this scan mode is not supported.  

A No Action scan mode is available.  With No Action, the scan list syntax specifies when to connect and when to disconnect.  You should be able to program a scan list to do something similar to the following:

/PXI1Slot3/r0->c0;

/PXI1Slot3/r0->c1 && ~/PXI1Slot3/r0->c0;

The tilda (~) means disconnect.

Note: The switch will not send a trigger if a disconnect action is immediately before a semicolon.  To send a trigger, include a dummy entry before the semicolon.  For example:

/PXI1Slot3/r0->c0;

/PXI1Slot3/r0->c1 && ~/PXI1Slot3/r0->c0 && /PXI1Slot3/r3->c100;

Hope this helps!

Chad Erickson

Switch Product Support Engineer

NI - USA

[小木虫]

Hi, Chad Erickson,

"PXI1Slot3/r0->c0; /PXI1Slot3/r0->c1 && ~/PXI1Slot3/r0->c0;" . I am sorry I could not understand this and the second scan list (I used no action mode in my test also). Which two rows the DMM were connected? Only c1 was connected to r0 in this scan list，how could a current measurement be finished? 

      For example:  Firstly, the DMM are connected to r2 and r3. c0 and c1 are two channels with current.  Scan list:  c0->r2&&c1->r3;  At this time, we could only get one current. How could we measure variation of current with time?

Best regards

小木虫

帖子被小木虫在 02-19-2009 07:53 PM

时编辑过了

[chericks1]

Hi 小木虫,

Ignore my first reply.  I misunderstood your question and provided an example of how to create a "make before break" scan list entry.  You do not need this for your application.  I think "break before make" will suffice.

I now understand that you want to make multiple current measurements after creating a switch connection.  The scan list entry you provided: r2->c0 & r3->c1; will create the connection required for the DMM to measure the current flowing between c0 and c1.  However, the DMM will send a trigger back to the switch after making its first measurement.  

One way to make multiple measurements is to create multiple connection entries in the scan list.  Each scan list entry triggers the DMM to make one measurement.  I don't like this idea much, as it will actuate the relay many times (once for each measurement). 

Is hardware-timed operation necessary?  If you used a switch software trigger, you could make the scan list connection (r2->c0 & r3->c1; ) and then begin a DMM multi-point acquisition.  Once you've collected all the required current measurement samples, you could send another switch software trigger to continue with the next scan list entry. 

Will this work?

Chad Erickson

Switch Product Support Engineer

NI - USA

Message Edited by Chad PSE on 02-20-2009 01:51 PM