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exceed the capability of the power supply or the board's output amplifier, noise and even

instability can result. By controlling the maximum slew rate of the input signal, the system's

output amplifier can be kept from saturating. This is always advantageous for accurate

positioning of the scanner. For some applications, fast large-angle positioning is not needed.

For those applications, the slew rate limiter can be used to slow the maximum angular speed

attained for these large

moves,

thus decreasing the amount of wobble and jitter associated.

The slew rate limit is contiolled by

R78.

Refer to section 6.0 for adjusting the slew rate limiter.

••Note: During start up, the output of the slew rate limiter is grounded by the action of an

analog switch, U15B, for about 3 seconds. This allows the servo time to stabilize (about 1

second).

**Caution: Do not adjust R78 without complete understanding of the appropriate section of

6.0. Damage to the scanner could result.

3.7. Tuning Section

The tuning section can be configured two ways depending on the user's needs. If extieme

positioning repeatability is required, the servo board is set up as a class 1 servo. If fast

positioning is of paramoimt importance, the servo board has class 0 capabilities.

The class of

servo

is determined by how many error integrators are in the servo loop. The error

integrator of a class

servo makes the system settle to a very high degree of

accuracy.

Even as

friction or other torque disturbances try to affect repeatability, the integrator will eventually take

out all error. This is done at a slight speed penalty. Most applications' requirements are met

very well using the class

servo.

Class 0 servos are slightiy faster and more stable than class 1 servos. However, the tradeoff is that

any finite friction causes a

vWndow

of non-repeatability to form around the commanded position.

The error is equal to:

Ertor = FD/KS where. Error = difference between actual and

commanded position in radians

= the disturbance torque in dyne-cm

Ks = the servos stiffness in dyne-cm/radian

Certain applications do allow for the use of the class 0 configuration, along Avith its inherent

non-repeatability. It is described below after

the

class

section.

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