Duty Cycle Control on NI ELVIS Function Generator?

[TBennettcc]

I am a student at Florida Gulf Coast University in southwest Florida, USA.

I am working with the NI ELVIS. It appears to be the first version, not the NI ELVIS II.

I am working on a simple system, trying to write a pulse width modulation VI from within LabVIEW.

I cannot find any way to modify the duty cycle of the function generator of ELVIS.

I am very inexperienced when it comes to any NI software. I'm not sure of the interactions between the software.

In some sample VIs, I have seen (sub-VIs?) from NI-DAQmx. I only have DAQ installed on this computer. Is DAQmx required?

Is there another way to do this?

Thanks for your time.

[Jason_D]

TBennett,

I do not know what version of LV you have but I have attached a VI from earlier version of Labview that should do the trick.  If not I have included a screen shot of the block diagram.  You will want to use a DAQmx channel property node to change the duty cycle.  Yes you will want to have DAQmx for this and many other examples.  You can do it without but I HIGHLY recommend it as it will make your life much easier.

Message Edited by Jason_D on 02-20-2009 02:08 PM

Message Edited by Jason_D on 02-20-2009 02:10 PM

[TBennettcc]

Thank you so much for your reply, Jason.

In addition to more information, I also have more questions...

The version of LabVIEW I'm running I believe to be either 8.2 or 8.2.1, not sure if there's a difference there.

NI-DAQ is installed on the machine, not DAQmx. Will I be able to 'upgrade' to DAQmx (or uninstall NI-DAQ and install DAQmx) without messing anything up? I am not sure how each NI suite / module interacts with one another, and from what I've seen so far, there is only a certain gap where certain versions will interact with each other.

Will I need to do anything with the Measurement and Automation Explorer?

Is there anything else that should be installed?

Is there any information that can be had regarding the proper use of DAQmx, and how it interacts with the equipment? Is it fairly straightforward?

That's all I can think of for the moment. Thank you so much for your time.

[Jason_D]

TBennettcc,

DAQmx is an independent "program" and can be installed alongside Traditional DAQ, I have both on my machine here. 

Measurement&Automation may be automatically updated with DAQmx but you don't need to do anything. 

I can't think of anything else you will need.

There is more information out there than you can imagine.  I believe it is fairly straightforward, but I have been told the opposite.  With experience it will become easier.  Two pieces of advice: First always start with an example program if you can (there are a ton both made by us and by other customers).  Second, start your quest for knowledge here, but if I could only show you one document it would probably be this one.

[TBennettcc]

Sorry to bother you again.

I am using the VI you gave me. I am now using LabVIEW 8.6 with NI-DAQmx.

I am setting up the input as follows:

I am using Dev1/ctr0, which corresponds with the Counter_0 output on my NI-ELVIS board

Frequency is set to 100 kHz, aka 1,000,000 Hz.

Duty Cycle is set to 0.99, or 99%.

Idle State is set to low, I'm not sure what this is, or what it's supposed to be set to.

The amplitude is 2.5 volts, with a DC offset of +2.5 volts, so that the pulse generation goes between 0 and +5 volts.

I am using a very small DC motor. When I connect the motor to a benchtop power supply, it tells me that the motor will spin from a minimum of 0.5 volts, and it is drawing about 0.07 amps. I can crank it all the way up to 6 volts, with a power draw of about 0.17 amps. If I hold the motor still while voltage is applied, the current is 0.23 amps at 0.5 volts, and 2 amps at 6 volts.

When i measure the voltage across ctr0_out to the NI-ELVIS' onboard DC power supply ground, the voltage reads +5.

However, when I connect this DC motor's + terminal to ctr0_out, and the - terminal to the NI-ELVIS' onboard DC power supply ground, the voltage drops to about 69 mVDC.

I am stumped. Any ideas?

[Jason_D]

TBennettcc,

I assume you mean 100Khz or 1MHz probably just a typo and will not affect the system.

Why are we running at 99% duty cycle does it work at less than this?

How much current does the motor need to run?  What do you mean by "hold the motor still"?

You mention that you are measureing the V before and after connecting the motor what is the current?

[TBennettcc]

What is the difference between the frequencies I typed out and the ones you typed out?

I just tried a 99% duty cycle to see what would be about the maximum volts and amps I could get the board to produce.

When I hook the motor up to a variable DC power supply, it starts to run at about 0.5 volts with a current draw of about 70 milliamps (0.07 amps).

When i said 'hold the motor still', I meant I would grab a hold of the motor axle to stop it from spinning while it was still being powered.

Using the VI you sent me, and measuring the current from ctr0_out to ground, the current is approximately 30.144 mA DC with no motor in the circuit, and approximately 29.747 mA DC with the motor in the circuit, with a frequency of 1000.0 Hz, a duty cycle of 99.0%, and an idle state of low. I am guessing I need close to double that to get my motor even close to starting.

Thank you so much for your help so far. It is much appreciated.

[Jason_D]

TBennettcc,

100KHz = 100,000Hz not 1,000,000 (that is 1MHz)

Is your duty cycle questions related to the motor questions?

According to the specifications on the ELVIS found here we do not have the amperage to power this motor and the arbitrary function generator will only power 25mA so our final option is the variable power supply.

[FriedrichX]

Hello,

We're working on a similar project.... However we need to generate 3 to 4 pulses with different frequencies and synchronize them together. This VI is using counters for sending out pulses and the ELVIS only has two so that we can only generate two at the moment.. Is there anyway to get over this problem?

thanks,

Friedrich

[Patricia B]

Hello,

Unfortunately, there are only two counters on this device, so the max the number of pulses that can be generated is two.

Regards,