Measuring Control
System Stability
One of the most fundamental measurement requirements for control systems is
measuring loop stability. The Agilent 35670A provides both the traditional
swept sine measurements (Option 1D2) and broad band measurement stimulus
like chirp and random waveforms for faster production testing. In addition, the
internal math capabilities of the Agilent 35670A allow you to easily convert
closed loop measurements to open loop responses and correct for phase
inversions and frequency response errors found in test fixturing.
A measurement method with minimal overhead is to find the open loop
frequency response function using the reference input summing junction. That
is, to inject the test stimulus into r and measure points b and e in the figure 5-4.
Math is not required. However, if your system is a high-gain (80 dB) analog
system this method may not work since e may be so small that it approaches
the noise floor of the analyzer.
Figure 5-4.
Block Diagram for Open Loop Frequency Responce
Agilent 35670A
Measuring Control Systems Operator's Guide
5-12
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