Axis 670 Manual de Instruções descarregar pdf (Página 16)

used as another source of velocity information, hence damping. The advantage of this form of

damping is its inherent low noise. Its bandwidth can be set high without degrading its signal to

noise ratio. The overall bandwidth of the system can be extended much ftuther than with

position differentiation alone. The current integrator is considered the high frequency damping

source and is adjusted with

R59.

The summing amplifier algebraically sums all four of

these

signals to obtain a composite signal

that is sent to the output stage. During startup, a FET, Ql is tumed on slowly so that the servo

has time to stabilize in a contiolled manner. Also, when an error condition is sensed, an analog

switch, U7, is shorted across this amplifier, thereby opening the loop and shorting the signal

being sent to the output stage to ground. During mirror alignment the gain of the summing

amplifier is reduced by two orders of magnitude v^ch drastically lowers the loop gain. This

allows the user to align the minor manually without the servo going unstable. This is explained

further in the mirror alignment section in appendix 6.0.

3.8.

Output Amplifier

3.8.1 Overview

The output stage uses a power op-amp to supply the large currents used to create torque in the

motor coil. A current feedback loop is tied around this output amplifier allowing the summing

amp to contiol the current in the scanner directiy. Thus changes in cable length, coil resistance,

contact resistance, back EMF voltages, etc. do not affect the summing amplifier's ability to

control the torque produced in the scanner. This produces a very stable and repeatable system

response with time and temperature.

Current flowing through the motor coil is detected by a low resistance current shunt, R52 and

differentially detected by the current monitor U4D. The current monitor signal is then used to

close the current feedback loop around the output op-amp U5. It is also used for the "coil

temperature calculator" to monitor coil heating and by the current integrator to obtain a velocity

signal. The current monitor signal is monitored on TP3. Use TP2 or W7.2 for the ground

reference. The current monitor gain varies depending on which scanner is being driven. Check

the scanner data sheet for the maximum rms current

the

scanner can maintain.

The bandwidth ofthe output stage is mainly contiolled with a lead in the current feedback loop.

The series/parallel combination of R50 and C8 and R49 in the cmrent monitor's feedback path

provides this lead. This RC combination is set so that the output current waveform is adequately

damped when a square wave is fed in. Secondary bandwidth limiting is provided via the "noise

gain" compensation technique with

R40.

For large mirror loads, a notch filter module is used to

stabilize large inertia systems that tend to "sing" or resonate at the system's torsional resonant

frequency. See section 6.2 for more information regarding Notch Filters.

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