"Synchronous" Multimeter Measurements

[Alex.Pinion]

As the title may imply, I would like to use a digital multimeter to measure several lines at once for my LabWindows application.  I want to measure 8 or so signals at once and run threads in LabWindows that monitors each signal.  I have come across some products that basically require one multimeter and a switch interface that allows for several lines to be connected to the multimeter.  Then, the switch interface will multiplex to each line according.  

I am curious if anyone has performed a similar task in their LabWindows career?  Is there anything that can perform this multiplex switching, but at such a rate that you could measure each line in an "almost synchronously", asynchronously fashion, and still have LabWindows process the measurements individually? If so, what are some of the instruments you used for this kind of task?

Thanks,

Alex

[RobertoBozzolo]

I don't know why you are thinking to multimeter+switch design instead of a multichannel data acquisition board: the latter can offer a higher acquisition rate and a lower time difference between measurements, at the expense of possibly have to design some signal conditioning circuitry. It's still a design that you should consider in my opinion comparing it to your DMM+Switch hipothesis in terms of efficiency and speed, if the "synchronous" in the title means as close as possible as I seem to understand.

Having said this, a possible architecture to think of is some sort of producer-consumer paradigm: a thread that continuously acquires measures and makes them available to other threads that process individual measurements as needed.

With so few details on your scenario I'm afraid I cannot offer more than a proof-of-concept.

[Alex.Pinion]

I am wanting to measure currents and voltages from multiple lines (8 to 15 lines) at the exact same point in time.  If at any point one of these voltages or currents drop too low or spike too high, I want my program to know exactly which line did so and what value etc., all in LabWindows.

Are there specific multichannel data acquisition boards that you would recommend?  Also, the producer-consumer paradigm idea may be the exact implementation I need; I will look into figuring out how to make the measured data in one thread, available to other threads.

[Alex.Pinion]

I should also mention that the voltages range from +/-32V and the currents vary from 200mA to 4.5A.

[RobertoBozzolo]

The "same time" concept needs to be discussed a bit but it seems to me that it rules out the DMM approach due to intrinsic slowness of such devices.

The high end solution for this problem are data acquisition devices that guarantee real simultanous sampling capabilities: NI offers some devices for simultaneous sampling but be prepared to 4-digit prices.

Having said this, a true contemporaneity may not be needed, as far as a few milli- or microseconds time difference can satisfy your requirements. If this is your situation, then the scenario for DAQ devices is considerably much larger: spanning from on-board up to wireless devices, from dedicated- to general purpose boards and from low-resolution up to 24-bits devices you have a very wide range of devices among which to choose your best option. This is the starting point for DAQ devices selection: by making your decisions in the menu on the right you can refine your search and choose the correct daq board for you.

[Alex.Pinion]

I believe a few microsecond "asychronous" readings would suffice, as long as they can be processed in LabWindows in a "real-time" fashion.  I like the DAQ devices you have provided; however, I noticed that these devices only measure up to +/- 21.5 mA.  Is that correct or am I misinterpreting something?  I will need to measure components varying anywhere from 200mA to around 4.5A.  Are there NI multichannel DAQ's that have that capability?  Thanks for the responses, I appreciate them a lot.

[RobertoBozzolo · Accepted solution]

Current measurement is a difficult beast to bring under control, especially if mixed with other type of signals and with a high count of channels to acquire.

While I cannot pretend to know all DAQ boards Ni is producing, when I have to acquire mixed signals I normally go for a general purpose daq board and some external signal conditioning circuitry, that could also take care of signal isolation in case you also need it. Such conditioning circuitry may span from simple shunt resitors to isolated Hall-effect transducers, depending on the situation.

When you want to start a system design first of all you must list your requirements both from the signal side and from the acquisition side. From the signal side you need to consider for example the number of signals, need for isolation, need for conditioning, bandwidth... From the acquisition side you must first of all decide the DAQ board type: whether to put a card inside the PC or connect to an external one via USB, Ethernet, wireless... Next you can look into existing devices and try choosing an appropriate one depending on number of channels, speed, resolution... not an easy task!

Normally it is a good thing to contact a local distributor that can share his experience with you and guide you in the design process.

[RobertoBozzolo]

"Real time" is another concept that needs to be discussed the same as "simutaneous acquisition".

If you need a true, deterministic real time system then you need to use a dedicated hardware and use the CVI Real-Time module, which is an addition to the CVI environment. This page holds several resources that can help you in understanding what the module offers and whether you need such a solution.

On the other hand, if you can deal with some unpredictable latency in your system, then you can go on with a Windows system handling some data acquisition hardware.

In this case again the correct solution depends on your scenario.

[Alex.Pinion]

I think and "hope" that a little latency will not be an issue, depending on how much it actually is.  My overall procedure will have several things going on and being measured/processed, but nothing is time critical from an operational standpoint.  I am doing exactly as you said; identifying all of the requirements from a signal standpoint and identifying more and more instrumentation that may be most suitable for my application.  Thank you for your help!

[SteveD123]

A couple of points:

1. What is the shortest duration spike that you have to detect?

2. When looking at any multiplexed data acquisition, pay close attention to the settling time required when switching between channels. This will set the time lag between the first and last samples. It will be made much worse if you have to do any range switching, especially between the voltage inputs and the current inputs.