Stepper motor and USB 6008

[ramkis]

Hi i am new user to labview .. 

I saw many post in ni forum , it states there is big problem in USB 6008 digital output because it is software timed...

That is it can only generate 150Hz...

1. The 150Hz is for each individual port or it is divided by number of ports used?

2. If I use bipolar stepper motor of 1.8 degree step angle what is the maximum speed I can  get....?

[Raviteja_NI]

Hi Ramkis,

Software timing means, every time the program runs an iteration, you can set the state of the DAQ device. In this case you want to set the state of the Digital O/P pins. Also, you can control a Digital output port as a single bundle. As you need only 4 pins to control the stepper motor, you can connect all the pins to a single port (say DO0, DO1, DO2, DO3) of the USB 6008 device (You need to have the motor driver IC in between of course). You can write a set of 8 booleans to this port at once. Hence you can write one byte to the port at one time and keep writing data at about a rate of 100 - 200 Hz. This will result in the generation of 100 to 200 steps per second (maximum).

Regards

Raviteja Chivukula

[ramkis]

thank u raviteja

the ic i required is for timing the pulses or to amplify the digital output to run motor?

i think u r in india ....can u send further details in my mail id ramkumars1990@gmail.com

[Raviteja_NI]

Hi Ramkumar,

The IC is a power amplifier IC. the timing can be taken care of by the USB 6008.

Regards

Raviteja Chivukula

[svijay]

but before in forum they mentioned we can get more than 75Hz from usb 6008, because it is software timed

what is the maximum Rpm i can get from usb 6008 with 1.8degree step angle stepper motor

[Raviteja_NI]

Hi Vijay,

I am not aware of where the 75 Hz or the 150 Hz are mentioned. But software timed Digital O/P means that every time the loop iterates, the Digital O/P can be updated. And everytime the Digital O/P is updated, the stepper motor moves by one step = 1.8 degrees. The loop rate can depend on what is the processing being done in the loop, the background processes running in the computer and a lot of other things and hence it cannot be said for sure as to how much the maximum RPM possible is. If the Digital output loop iterates at about 100Hz, it means you can give 100 steps per second. On a 1.8 degree/step motor this corresponds to 1/2 rotations per second. This 100Hz, can reach a maximum of 300Hz also sometimes and then, the speed of the stepper motor can reach about 1.5 rotation per second.

Regards,

Raviteja Chivukula

[svijay]

thank u raviteja

now i planned to built one stepper motor controller and i just going to triger the forward and reverse switch using the digital output.. 

is it possible to control the potentiometer  using labview (i.e ) is it possible to make potentiometer function virtually so that i can control the potentiometer in controller  from labview?

[johnsold]

Raviteja,

The 150 Hz/75 Hz most likely came from one of my posts in another thread.  I had recalled seeing the 150 Hz number in the USB-6008 specifications.  When I went back to verify, I found that AO was specified at a maximum of 150 updates per second.  Further, the specification indicates that both AO and DIO are software timed.  The DIO update rate is not specified. It SHOULD be, even if it includes a qualification about variations in OS performances.

If you have followed the Forums much, you will have seen that questions about the performance of the USB-6008/6009 are quite common.  Given that these devices are popular becuase of their low cost, and that they are likely often selected and used by inexperienced persons because of the low cost, NI should make an extra effort to clarify the performance limitations in the documentation. Simply stating that AO and DIO are software timed may not mean anything to the inexperienced user.  This leads to disappointment in NI's products - not because the product does not perform as it was designed to do, but because the naive user may have unrealistic expectations.  Please include realistic numbers, even if only typical, in the documentation of these devices and in the descriptive literature on NI's product web pages.

Lynn

[Raviteja_NI]

Hi Lynn,

We will look into what can be done in this regard. Thank you very much for your feedback.

Regards