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Servos

Servos. Servos are everywhere. System to be controlled. Error signal. Sensor. Correction. Diff amp. Integrator. Transducer. Reference. Elements of servo. System -- to be controlled Sensor or detector -- measure quantity to be controlled Reference -- desired system output

cytheria-zenon
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Servos

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  1. Servos Servos are everywhere

  2. System to be controlled Error signal Sensor Correction Diff amp Integrator Transducer Reference Elements of servo • System -- to be controlled • Sensor or detector -- measure quantity to be controlled • Reference -- desired system output • Diff amp -- compare sensed signal to reference • Error signal -- produced by diff amp • = voltage proportional to system error • Integrator -- integrates error to give correction signal • Correction signal -- will be used to modify system and correct output • Tranducer -- converts correction voltage into action to control system

  3. System to be controlled Laser beam Error signal Laser AOM Photodiode & preamp Correction Diff amp Integrator RF signal RF driver Stable voltage Example: Stabilize laser intensity • System is laser source plus AOM to deflect some (most) of laser power • Sensor is photodiode w/ preamp • converts optical power to voltage (Volts/Watt opt) • Reference is stable voltage • can be ac voltage -- ex: quiet amplitude modulation is desired • Transducer is RF driver to AOM • converts voltage into RF power (Watts RF /Volt) • System converts RF power to optical power (Watts opt / Watts RF) • Overall units of sensor, transducer, system cancel

  4. Noise amplitude Desired servo gain 1/f noise 1/f noise log(Vnoise) White noise Unity gain 0 0 log(f ) 0.1 1 10 100 1kHz log(f ) 0.1 1 10 100 1kHz Servos and noise Frequency dependence of noise • Low frequency ~ 1 / f • example: temperature (0.1 Hz) , pressure (1 Hz), acoustics (10 -- 100 Hz) • High frequency ~ constant = white noise • example: shot noise, Johnson noise, spontaneous emission noise • Servo must compensate noise spectrum • integrator • servo bandwidth out into white noise region log(gain)

  5. Gain response Max gain Single pole 6 dB log(Vout/Vin) Unity gain 0 log(f ) Double pole 12 dB Phase response log(f ) 0 degrees Phase shift -90 degrees -180 degrees Servo stability • Real servo will have second pole eventually • 12 dB/octave rolloff • converts negative feedback to positive feedback • causes oscillations, noise, instability • Must have closed loop unity gain point before second pole • Real servo also has finite dc gain -- like op amp

  6. Single pole 6 dB Double pole 12 dB log(gain) Unity gain 0 log(f ) Closed loop response • Closed-loop gain and phase depend on all parts of servo • Closed loop gain is product of all gains • Includes sensor and transducer • Recall product of gain units must cancel • Closed loop phase is sum of all phases • Chain is as strong as weakest link Shunted integrator Integrator w/lead Effective “servo” =

  7. Acoustic noise Electronic noise Spatial filter Laser Laser intensity noise • Laser intensity noise • due to mode competition, power supply noise, resonator acoustics • can also pass laser beam through spatial filter, fiber, etc before using • May also want controlled modulation • ultra-quiet intensity modulation

  8. AOM crystal/glass Refractive index variations due to sound waves Sound absorber (suppress reflections) Aperture Deflected beam Input laser beam Undeflected beam Sound transducer ex: LiNbO3 RF signal ~ 1 Watt 40 MHz AOM = acousto optic modulator • AOM = acousto optic modulator (or deflector) • RF signal converted to sound waves in crystal • Use fast piezo-electric transducer like Li NbO3 • Sound waves are collimated to form grating • Bragg scatter from grating gives deflected beam • can separate from original • Problem with AOM -- weak link • Sound takes time to travel from transducer to laser beam • Time delay: tD = l / v -- acts like multi-pole rolloff • (phase shift increases with frequency) l

  9. Photo-diode amplifier Cs Shunt capacitor - Rf light + Gain response If Unshunted Vbias Vout Shunted Ipd log(Vout/Iin) Rpd Ipd Cpd log(f ) Photo-diode equiv. circuit Photodiode with pre-amplifier • Photodiode like current source but with capacitor • Input capacitor causes op amp gain to diverge at high freq. • Amplifies high freq noise • Oscillation • Solution: • Shunt capacitor in feedback

  10. Servo design procedure • Measure gain and phase response of photodiode/preamp • use LED driven with signal generator • Measure response of AOM • use photodiode (PD) -- correct for PD response • Design integrator to compensate for first pole • Gain control to set max gain w/o oscillation ?? • variable resistor at integrator input • Sign of correction signal • each op amp reverses sign • overall servo must be negative feedback • sign change in transducers unknown until measured • may have to add op amp • Use undeflected beam from AOM ? • More laser power

  11. Oscilloscope System to be controlled Laser beam Laser AOM Photodiode & preamp Correction Diff amp Integrator Error signal RF signal RF driver Stable voltage Summing Add error Measure servo performance • Insert noise using transducer • Summing junction • extra op amp ?? • Measure frequency response, phase delay • both error and correction signals

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