National Instruments PXIe-4302, Руководство пользователя

Страница: 21 / 31

2-14 | ni.com
Chapter 2 Using the Module
Figure 2-11. Transfer Time and Application Time Relationship
Note
For control applications, it is important to consider the group delay of the data
being acquired and analyzed when calculating the control system bandwidth.
Regardless of the sample rate, the bandwidth of the system is as follows:
2 kHz Control Loop Rate Calculation Example
Figure 2-12 represents a typical control system in which you have a process to control, a DAQ
device to do the acquisitions and generate stimulus, and a PXIe controller or computer to do the
data processing and determine the proper AO value and returning it to the process.
Figure 2-12. Typical Control System
To successfully close a 2 kHz control loop, make sure that the time between the time the AI
sample is acquired and the time the AO stimulus is generated is <500
μ s. Refer to Figure 2-13.
Figure 2-13. Input and Output of a Control System with Bandwidth
≥
2 kHz
Tr a n s fer
Next Sa mple Filter Gro u p Del a y
Applic a tion Time
20
μ
s *
120
μ
s **
N
Sa mple
Clock
Sa mple
Clock
Ac qu i s ition R a te Period
W a it Time (Idle)
*Tr a n s fer time m a y v a ry depending on s y s tem.
**120
μ
s i s the a pproxim a te gro u p del a y of the 2 kHz filter for inp u t fre qu encie s < 1 kHz
(p assba nd of the filter).
bandwidth
1
TransferTime ApplicationTime GroupDelay
+ +
------------------------------------------------------------------------------------------------------------------------------- =
Control Loop Time, 500
μ
s
PXIe Controller or Comp u ter Proce ss
DAQ Device
AI Me asu rement
AO S tim u l us
Tr a n s fer + Applic a tion < 500
μ
s
2 kHz Inp u t
2 kHz O u tp u t
Time
Sa mple 0 . . . Sa mple 1
Sa mple 0 . . . Sa mple 1