After you choose a proper dynamic system model, you must acquire the input and output
data from the Direct Current (DC) servomotor. You then preprocess and use the data to
estimate unknown coefficients of the dynamic system model. You can use an encoder or a
DC tachometer to measure the shaft speed of the DC servomotor.
Set up the following experimental circuit to excite the DC servomotor and acquire response
signals with an encoder:
Figure 154. Experimental Circuit Setup for DC Servomotor
Excitation and Response Signal Acquisition
Set up the following experimental circuit to excite the DC servomotor and acquire response
signals with a DC tachometer:
Figure 155. Experimental Circuit Setup for DC Servomotor
Excitation and Tachometer Signal Acquisition
Both circuits use an NI Data Acquisition (DAQ) card to generate digital stimulus signals. You
also can use a Reconfigurable I/O (RIO), CompactRIO, or Single-Board RIO device to
generate the stimulus signals and acquire data. In these two circuits, a D/A output
channel of the NI-DAQ card converts the stimulus signals to analog voltage signals and
sends these signals to an amplifier. An A/D input on the NI-DAQ card acquires the
original analog signals, converts the signals to digital signals, and records these
digital signals for identification. The amplifier sends the enhanced analog signals to
drive the DC servomotor. If you use an encoder to measure the shaft speed of the DC
servomotor, the encoder generates pulses when the DC servomotor rotates. The encoder
sends the pulses to a digital input channel of the NI-DAQ card. A counter in the NI-DAQ
card counts the number of pulses and computes the rotational speed of the DC servomotor.
If you use a DC tachometer to measure the shaft speed of the DC servomotor, the DC
tachometer generates back-EMF voltage signals when the DC servomotor rotates. The DC
tachometer sends the voltage signals to another A/D channel of the NI-DAQ card. The
NI-DAQ card calculates the rotational speed by using the voltage signals.
Note Ensure the NI-DAQ card
acquires stimulus voltage signals and rotational speed synchronously. Ensure you
acquire and save synchronous stimulus signals and response signals with constant
time steps.
You can use the following types of stimulus signals:
Step signals—Use step stimulus signals for the low frequency band of a linear dynamic system.
This type of signal is easy to generate but has a low Signal-to-Noise Ratio
(SNR) in the high frequency bands. To generate a step signal, create an array
that begins with a series of zeros and then shifts to a series of ones, such as
"0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1". An element in this array
represents a sampling point. Ensure the array contains enough elements according
to the sampling rate of the device you use. You then use the Build Waveform
(Analog Waveform) function to convert this array to a waveform signal.
Swept sine signals—Use swept sine stimulus signals for the full frequency bands of a linear dynamic
system. This type of stimulus signal has a high SNR in the full frequency bands
of a linear dynamic system. However, if the response signals contain non-linear
distortion, this type of stimulus signal cannot eliminate the distortion and
leads to an inaccurate dynamic system model. Use the Chirp
Pattern VI to generate an array containing a swept sine pattern.
You then use the Build Waveform (Analog Waveform) function to convert this array
to a waveform signal.
Random signals—Use random stimulus signals for the full frequency band of dynamic systems. This
type of stimulus signal has a high SNR in the full frequency band. You must
repeat exciting the dynamic system and acquiring data for multiple times to
cover the full range of the dynamic system and eliminate any non-linear
distortion in the response signals. Use the Periodic Random
Noise VI to generate an array of periodic random noise. You then
use the Build Waveform (Analog Waveform) function to convert this array to a
waveform signal.
In this example, you can use any of these three types of stimulus signals to excite the DC
servomotor.
Alternately, if you do not have the required hardware or a DC servomotor, use the SI
Data Samples VI to generate the input and output data. This tutorial uses
the SI Data Samples VI to complete the following parts. If you also
use this VI to complete the following parts, you can get identical results as the
results in this tutorial.
Complete the following steps to generate the input and output data by using the SI Data
Samples VI:
Create a blank VI in LabVIEW.
Add the SI Data Samples VI to the block diagram of the VI you created. This
VI generates single-input single-output waveform signals by default.
Right-click the data input of this VI and select
Create»Constant from the shortcut menu to create a
constant for this input.
Ensure the value of this constant is Motor.
You can save this VI and use this VI to complete the following parts of this tutorial.
