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3.6.4. Command Input Scale Factor Calculation

The Command Input scale factor is defmed as the nimiber of volts required at the input of the

servo board to cause the scanner's output shaft to rotate one mechanical degree. For some

applications, fine control of this scale factor is critical. Also, since the input voltage is limited to

+/-10V, this also sets the maximum contiollable angle or Input Range ofthe system. This Input

Range is also referred to as the "fieldsize" ofthe system.

The 670 Single Axis Mirtor Positioning System is normally set up for the maximum allowable

scan angle for the application unless otherwise specified by the customer. The maximum

fieldsize for all Cambridge Technology scanners is +/-20° mechanical. As the Conmiand Input

scale factor increases, the inherent fieldsize ofthe system decreases. Thus,

0.5V/°

system will

yield

+/-20°,

1.0V/°

will yield

+/-10°,

2.0V/° will yield

+/-5°,

etc.

Note: It is possible to set the scale factor less than 0.5V/°, however it will allow the input signal

to attempt to drive the scanner further than 20°. This will cause the system to sense the

overposition and shut the servo down. Ensure that no matter what the servo's input scale factor

is set to, the input signal stays within the bounds that keep the scanner within its normal +/-20°

range.

The Command Input scale factor is coirtrolled by the following factors:

1.) Position Output Scale Factor - always set to

0.5V/°

mechanical (unless otherwise stated)

2.) Error integrator summing resistor

ratio,

R29/R30 - usually set to 1:1

3.) Slew rate limiter voltage gain, R89/R83 - usually set to

1.074:1

4.) Command Input scale adjustment,

R51

& R82 - adjustable from ~0.8:1 to 1:1

**Note: The R51 and R82 combination allows the user to make small adjustments to the

Command Input scale factor. For gross changes use the equations shown below to determine the

value of

R30.

For small changes, use R51. For minimum drift from R51, set the pot to the

maximum CW position. Then use the procedure in section 6.0 to measure the input scale factor

obtained.

The following equation describes the interaction ofthe above factors:

Command Input Scale Factor = Position Output Scale Factor x (R30/R29) x Slew Rate Limiter

Voltage Gain x Command Input Scale Adjustment

For

example:

Let

Position Ou^ut Scale Factor = 0.5

V/*'

mechanical

Slew Rate Limiter Voltage Gain =

1.074:1

Command Input Scale Adjustment = 0.9311:1

R29=10Kohm

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