Vinten Radamec HS-2010MED Pan & Tilt Head - Manual (Page 66)

HS-2010MED Maintenance And Repair
4-16 User Manual
employed). The inputs to this board are the position demand voltages from the 4
Channel Board and the potentiometer feedback signals from the lens. The output
is the current drive signal to the lens. The zoom and focus sections of the board
are identical.
The voltages from the lens follow pots are compared to the demand signals in
U3 and U4 to determine the error signal. The board attempts to produce a motor
velocity that is proportional to this error. The error signal is compared to the
velocity signal from the tachometer. The error is then amplified, shifted in volt-
age to be symmetrical about a level that is half of the supply voltage, and fed to
the TIP102/107 emitter-follower current amplifiers, which in turn drive the
zoom and focus motors. A delay circuit slows the turn on of the servo circuit to
prevent power up transients which could force the lens into its end stops.
Two DIP switches (SW1) and four pots (R1, R2, R3, R4) are calibrated during
installation to ensure that the 10V range of demand voltage from the 4 Channel
Board causes full end to end motion of the lens. The DIP switches set the gain of
the follow pot buffer amplifier (U3 and U4) to X1 or X2. If the X1 setting does
not produce full range motion of the lens, choose the X2 setting. See Chapter 3
for the calibration procedure. These switches and pots are accessed by removing
a small cover in rear cover.
A resistor carrier adjacent to the pots contains 8 fixed resistors that set the speed
and gain characteristics of the lens servo. Several carriers have been optimized
by Vinten for popular lens types, and it is important that you have the correct
one installed for your lens. Contact Vinten for more information. Four of the
resistors are in the focus servo and four in the zoom servo. Typically, the two
servos have the same resistor values. Two resistors in each servo (RZI1/RZI2
and RFI1/RFI2) set the maximum current to the lens motors to provide adequate
power for fast response, without supplying over-current that could shorten
motor life. Vinten selects these resistor values to match the lens motor specifica-
tions. The third resistor in each servo (RZT and RFT) sets the tachometer gain.
The value affects the traverse speed of the servo but is primarily used to prevent
overshoot. The value is chosen at the factory by providing a step voltage to the
demand input to the board. With the optimum value, the servo should move
responsively to the new position, without overshooting the target. This will pre-
vent the lens from slamming into the end stops and causing wear or damage.
The fourth resistor in each servo (RZS and RFS) sets the loop gain or stiffness.
A stiffer servo has more gain and produces greater repeatability. However, if the
gain is too high, the servo will start to buzz or oscillate or overshoot. Ideally, the
gain should be as high as possible, but short of the point of oscillation.
Native Lens Control Board
The Native Lens Control Board allows use of the motors and feedback potenti-
ometers already provided in many servo controlled lenses and saves the cost and
complexity of adding outboard hardware.
One function of the Native Board is to eliminate problems caused by ground
loops between the lens and dolly which might cause the lens to jump or twitch