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3.7.1.

Class 1

Our class 1 servo consists of the following circuits: position differentiator, position amplifier,

error integrator, current integrator, and summing amplifier. The ttansfer fimction can be

characterized by the following differential equation:

V(t) = Al • d rVpos(t)) + A2 • Vpos(t) + A3 • jError(t)dt + A4 • Jl(t)dt

where:

V(t) = output of summing amplifier

Al - A4 = coefficients that are adjusted with the tuning pots R25, R28, R31, and R59

respectively.

Error(t) = the error signal generated as the difference between the position signal, Vp, and the

output ofthe slew rate limiter (command signal).

I(t) = the current

flowing

through

the

motor coil.

The position differentiator takes the first derivative of the position signal to yield angular

velocity. This velocity signal is one of two sources of damping for the servo. Its -3db bandwidth

is set relatively low This is to prevent high

frequency

noise, present on any differentiated signal,

from entering the summing amplifier. Its contribution to the servo is to provide damping at low

frequency and is contiolled by

R25.

The position amplifier uses the position signal to generate an "electrical spring". Its contribution

is contiolled with

R28.

The error integrator compares the actual position to the commanded position and integrates the

difference over time. This signal will allow the scanner to overcome any slight spring or friction

yielding a zero steady-state ertor. Extiemely repeatable positioning is thus obtained. The

ultimate accuracy

then contiolled by the repeatability ofthe position detector contained within

the scanner. The contribution of this amplifier to the servo response is contiolled by

R31.

The current integrator produces a signal proportional to the integral of the cmrent flowing

through the rotor. Since the current flowing through the rotor is proportional to the torque

produced, it is also proportional to the angular acceleration. Thus, the integral of ciurent can be

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 fiirther than with

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

source and is adjusted with

R59.

The current integrator is high-passed into the summing amp by

way of

R107,

R60, and C9.

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