How should I tune the PI200 slow integrator?

The SLOW INT switch sets the slow-integrator frequency; it is not a modulation-frequency control. The selected value is also not, by itself, the closed-loop bandwidth: the latter depends on the complete slow-actuator transfer function and loop gain.

Tune the fast path first, with SLOW INT disabled. Once the fast loop is stable, verify the SLOW feedback polarity and enable the slow integrator at its lowest setting. Increase the setting while observing ERROR, FAST OUT, and SLOW OUT, allowing the TEC or piezo to settle after each change.

Because the slow-actuator transfer function is system-dependent, the final setting must be determined experimentally. This procedure makes the selection a bounded stability-margin adjustment rather than blind trial and error.

The slow path should transfer the low-frequency correction to the large-range actuator and keep the mean FAST OUT voltage away from saturation. Select a setting that provides the required drift rejection while retaining adequate damping and actuator headroom. Low-frequency peaking, ringing, or hunting indicates insufficient stability margin; reduce the setting by one or more positions rather than operating immediately below the onset of oscillation.

The available settings are:

• PI200: 800 mHz, 2.4 Hz, 8 Hz, 24 Hz, 80 Hz, 240 Hz, or 800 Hz.
• PI200-T: 8 mHz, 24 mHz, 80 mHz, 240 mHz, 800 mHz, 2.4 Hz, or 8 Hz.

See the PI200 user guide for the frequency-response controls and typical FAST/SLOW actuator connections. Disabling an integrator does not disconnect its output.