Overview
2.4.2.4 Local Control
Local (Maintenance) control of the antenna drives is facilitated through a set of
switches on the PMCU in the drive cabinet. A MAINT/REMOTE switch located on
the Relay PCB allows the operator to select between ACU control and local drive
cabinet control. With the select switch set to the MAINT position, ACU control is
disabled; however, all status reporting remains fully operational at the ACU.
2.4.2.5 Drive Cabinet Overcurrent Protection
As described in Section 2.4.2.1, "intelligent" electronic overcurrent protection is
provided for AZ and EL drive motors by the variable frequency inverters. In
addition, there are several other protection devices integral to the drive cabinet.
The inverter inputs are individually protected by circuit breakers, offering short-
circuit protection in the event of a drive inverter catastrophic failure. The +24
VDC logic power supply has a line input circuit breaker for short circuit protection.
In three-axis and four-axis systems, the POL motor circuit(s) is individually
protected by a circuit breaker. A main input power circuit breaker is also provided,
which serves as an internal disconnect for the entire cabinet.
2.4.2.6 AZ and EL Drive Motors
Three-phase induction gearmotor assemblies are utilized for actuation of the AZ
and EL axes. The motors are sized based upon deadweight, frictional, and wind-
loading requirements, as well as the required axis velocities. The standard motors
can be connected for either 208 or 380 - 415 VAC three-phase input, based upon
the line voltage available to the drive cabinet. The motors have sealed, permanent,
synthetic grease lubricated bearings and the gearboxes are lubricated with
synthetic gear oil, minimizing maintenance requirements.
2.4.2.7 Absolute Position Transducers
Angular position feedback is provided by absolute position transducers (resolvers)
for each axis. The standard configuration includes size 11 single-speed, brushless
resolvers which, combined with the position encoding circuitry in the ACU, yield an
accuracy of 0.02 degrees, root mean square (RMS). The resolver reference voltage
for the standard devices is 4.6 V RMS, at 2500 Hz.
Encoding system options include electrically wound two-speed resolvers or
absolute optical encoders to provide increased resolution and accuracy. With the
two-speed option, an overall control system accuracy of 0.01-degree peak error is
achieved. In this configuration, dual monolithic resolver-to-digital conversion IC's
are used in the ACU with bit rotation techniques incorporated to significantly
increase binary resolution. Various optical encoder configurations allow for
resolution and accuracy levels commensurate with the most demanding system
applications.
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