PWM control through USB 6008/09

[nikhilvastarey]

Hello all

I have already read a thread regarding the topic but i am not satisfied with the replies.

I have discovered few programs that can run PWM in LabView, but they all are for DAQmx. So, will those programs run through USB 6008? and i also read that the output will be a sort of slow response output?

I am aiming to power a micro-pump rated at 12VDC. So, i want to run the program in LabView, use the USB 6008 to connect to a the DC motor. I know that some amplification will be required because the output of the USB 6008 is rated as 0-5V.

But, my question is can i successfully control the speed of the DC micro-pump ( motor) using the USB 6008??

Regards

Nikhil

Many more kudos to give 🙂 

[Hillman]

Hi Nikhil,

Good Morning and I hope your well today. 

I would strongly recommend keeping your question to one thread - what aspect of your replies where you not satisfied with, and could you please send/post me a link?

The USB 6008 is support by DAQmx and DAQmxBase. 

The issues you will face is with the Hardware. The USB 6008 isn't suitable for motion controll for these reasons:

1) Software timed - PWM will depend on how fast your PC: this is not ideal. 

2) Maximum update rate of 150Hz - this is how often the output can be changed - this will give you have jittery control of your motor - not ideal.  

Counters can be used to generate PWM outputs. They are hardware-timed, making them very fast and precise. Because they are a limited resource they are not always an option. If your application does not have those requirements (your cycle time is in the seconds range and you do not mind some milliseconds of delay) you can use a digital output with software timing control for your PWM generation. However, if your controlling a motor then your cycle time is NOT in the seconds range. 

I would consider a USB DAQ 6210 for such output applications like pulses, pulse train with dynamic updates, frequency division, equivalent time sampling.

Please let me know if you have any questions,

[nikhilvastarey]

Hello Hillman

I am sorry.

http://forums.ni.com/ni/board/message?board.id=170&thread.id=286121&view=by_date_ascending&page=1

Actually, i just need to control the voltage of the micro-pump. When the voltage to the micro-pump (which is in fact a DC motor) is varied, the pressure (which is the output of the micro-pump) changes.

So, this got me thinking, do i really need PWM control for the micro-pump (DC motor)?

Can i use the USB 6068 as a sort of voltage generator from 0 to 5V? and i suppose that can be controlled through LabView?!!

And what will the resolution of the output analog voltage be? it is given that the DAC is 12-bit. will the resolution be 5V/(2^12) ???

And yeah my friend,

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[Hillman]

Thanks for the reply.

If you just need to output voltages(not PWM) then you can use the Analog outputs of the 6008,

Analog Output
Analog outputs........................................2
Output resolution ....................................12 bits
Maximum update rate .............................150 Hz, software-timed
Output range ...........................................0 to +5 V
Output impedance...................................50 Ω
Output current drive................................5 mA
Power-on state ........................................0 V
Slew rate .................................................1 V/μs
Short circuit current ................................50 mA
Absolute accuracy (no load) ...................7 mV typical,
36.4 mV maximum at full scale

So yes you could output a voltage between 0 and 5 Volts. You could only change the voltage level every 10ms or so. 

In answer to what your device (pump) requires I don't know. 

"So, this got me thinking, do i really need PWM control for the micro-pump (DC motor)?"

Have you got a specification datasheet (website) of the pump? - I dont mind taking a look. 

Have you looked in the NI Example Finder to VIs that generate PWM and AnalogOutput (AO?). In the example finder (help>>Find Example) it gives you information on what devices can use the examples - simply look for the ones which say usb6008. 

[Coq_Rouge]

Hi Nikhil

In order to control the motor you need to boost up the current output. I can give you a simple solution but you must have some electronics skills. Or get help from some who has.   

[nikhilvastarey]

Hello Hillman

The below is the link for the Micro-pump that i am using.

The current requirements are also specified. the max current being 380mA.

But when i set it up, i find that it provides 8 psi @ 12VDC @ current of about 330mA. ( which is way less than what they have specified in the data sheet). 

http://www.hargravesfluidics.com/pdf/BTC-IIS/ST_D771-11_C.1F28G1.1F28N1.B12VDC_Rev_A.pdf

Please do check it out and let me know.

Thank you for the reply and your time 🙂

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[nikhilvastarey]

Hello t06afre

As you have rightly mentioned, the micro-pump needs a high amount of current ie about 330mA ( compared to the 5mA that is provided by the USB 6008).

I first thought of using the LM317 to have some voltage regulation but i found that having a resistive voltage regulator will be a pain when compared to using a PWM.

Later, (as i have mentioned) i realized that a PWM technique is not required as i just have to vary the voltage. So, was thinking of this direct approach.

Hence, i thought that i can simply use an opamp ( with variable resistance @feedback, for variable gain)

But will that shoot up the current to the required amount. Please do have a look at the datasheet @the link given above.

Trying to keep the circuit as simple as possible.

Thank you for your reply 🙂

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[johnsold]

Nikhil,

You can use the LM317 as an amplifier.  Feed the analog output from the 6008 to the adjust terminal.  The output from the LM317 will follow that voltage with 1.25 volt offset (LM317 output is 1.25 volts more positive than the adjust voltage).  You get the advantage of the built in current limiting of the LM317. This can handle the pump's inrush current.   You may need to put a resistor to ground from the adjust terminal to handle the case of the power being applied to the LM317 but the 6008 being off.   Consider what the pump should do under those conditions.  

Lynn 

[Coq_Rouge]

Your basic motor control circuit may look something like this. It is quite simple 1 OPamp 1 Transistor, some resistors and capacitors. Remember to put the transistor on cooling finn. Then the motor is started give it 12V. This will give a smooth start. The motor voltage is regulated so it will keep constant. Feel free to ask me questions.

Good Luck 

[nikhilvastarey]

 Hello t06afre

First of all, WOAH. now tat was a fast one!!!

Thanks a lot for the patience. and the circuit. Will I be asking too much if i ask you to explain the circuit. just a bit!! 

The use of R6,R7,C7,C8,C3,C4?

And also, what must be the gain of Q1?

C7 and C3 are variable capacitance. So, should i use some sort of variable capacitance box or can i choose some random capacitance value.

The above words makes me look like a layman. well, i am one 🙂

Many more Kudos to give 🙂