sampling time

[SK0480]

Hello

Iam building a position control, and Iam modeling it as a digital control system using Z transform, now Iam using an NI Elvis board with a built-in DAQ, I need to know--now to represent my DAC(Digtial to Analog Converter), Iam using a zero order hold... I need to know what should I select as sampling time in the zero order hold for my DAC.

any help would be appreciated..

Thanks

SK

[James_M.]

Hello SK,

Could you give more information on what you're trying to do? Which version of Elvis are you using?

If you're building a position controller, are you likely to be generating signals anywhere near the max rate of 2.8 MS/s for Elvis II?  Irrespective, the 'sampling time' you are looking for is dependent on the rate you are trying to write at.

All the best,

James M.

Applications Engineer

National Instruments

[SK0480]

Ok what iam doing is trying to control unipolar stepper using labview, I am using darlington transistor as a current amplifier; Iam sending a pulse train through DAQ board to move the stepper motor. 

So obviously Labview is controller and the stepper is what iam trying to control.... Iam using NI Elvis DAQ board..

I know that the DAQ board has ADC and DAC...so this whole is a digital control system, so if Iam sending a pulse train to the DAQ, then DAC of the DAQ is going to send this pulse to my stepper.  Iam modeling this entire control system in Z transform, because the S domain is only for continuous time signal, and Z domain is for discrete time signal..

I know that I'd have to use the Zero order hold to represent the DAC(digital to analog converter), zero order hold is used a mathematical model of the signal reconstruction done by the DAC....so what iam trying to find out is the sampling time interval--which is the time interval that DAC holds and then produces a pieicwise equivalent of the continuous signal. Attached is the document which shows what Iam saying.

Thanks

[James_M.]

The sample time is a user-defined parameter - it is a function of the sampling frequency at which you're trying to write out data. The timing resolution of the ELVIS II is 50 ns, while the maximum update rate is 2 MS/s (i.e. a 'sample time' of 500 ns - 10x slower than the actual timing resolution). The slew rate is 20 V/us, meaning the device can go from min to max output (or vice-versa) with only one intervening step - this is particularly relevant, given that you will be outputting a pulse train. Also, the value of modeling the DAC is dependent on the voltage resolution that your device is sensitive to. At full-scale, the code-width of the 16-bit DAC is approximately 300 uV (i.e. size of one LSB voltage step).

ELVIS II Specifications (p. 4):

http://www.ni.com/pdf/products/us/cat_nielvisii_plus.pdf

James M.

Applications Engineer

National Instruments