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Two diodes 10 and 12 in parallel, with reverse polarity, are wired in series with a servo tach-feedback voltage divider to provide high tach gain at low speeds.
English (United States)
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Electrical Low Speed Servo Damping
Two diodes 10 and 12 in parallel, with reverse polarity, are wired in series
with a servo tach-feedback voltage divider to provide high tach gain at low
Due to low efficiency of tach generators at low speeds, the only damping to
provide stability at very low speeds is provided by friction. If this friction is
provided by oil or grease, it will change with temperature and age, making tuning
of precision servo systems very difficult.
If the lubricant is changed so that it is light and will not
change with temperature and age, it may not provide sufficient
damping. In the following equation, lubrication and feedback terms
are identified. It should be noted that they are identical except
(AD)+(T/dD/over(dt)) + KG = S/dD/ over (dt) + d/2/D over dt/2/.
D = error distance
A = error gain
T = tach gain
K = offset
G = electrical system overall gain
S = friction factor due to lubricant
M = moving mass.
These terms provide total system stability, maximum speed and damping.
Tach usually degrades when a system reaches position, leaving lubrication to
provide damping. Diodes shown increase "T" at low speeds, increasing electrical
damping. The diodes are chosen with "turn-on" characteristics that match servo
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