Research Concepts RC2500 - Manual (Page 109)

102 RC2500 Antenna Controller Appendix H Polarization Sensing with a Potentiometer
Research Concepts, Inc. • 5420 Martindale Road • Shawnee, Kansas • 66218-9680 • USA www.researchconcepts.com
Appendix H – Polarization Sensing with a Potentiometer
General Description
The RC2500 v1.07 software release supports a potentiometer based polarization position sensor.
Versions of the software that support this feature can be identified by examining the five character string
that immediately follows the software version number on the controller’s startup screen. The five
character will consist of lower case letters and the hyphen character (‘-‘). The fifth character in that string
will be ‘p‘ for versions of the software that support a potentiometer based polarization sensor or a hyphen
for versions of the software that are configured for the usual resolver based sensor.
The polarization potentiometer is interfaced to the controller via a small printed circuit board that attaches
to the controller’s AGC port. The AGC port is normally used to input analog signal strength information
for implementing a step track based inclined orbit satellite tracking algorithm. When the software that
supports a polarization potentiometer is installed the controller’s AGC1 input is connected to the
polarization sense potentiometer. Note that the controller’s AGC2 input is still supported when the
controller is configured for use with a polarization potentiometer. The cables that interface to the
polarization potentiometer and the AGC2 signal source terminate on a connector on the circuit board that
is attached to the controller’s AGC port.
The printed circuit board that is used as the interface between the AGC port and the polarization sensor
is RCI p/n FC-2_5KPLP2. The Layout and Schematic of this PCB is shown at the end of this section.
When used with FC-2_5KPLP2, the RC2500 AGC1 channel is disabled and wired for a direct pass-
through of the 0 to 5V polarization pot signal. Referring to schematic of the RC2500 digital board
(appendix G), pots P3 and P4 were removed. Resistor R16 was replaced with a 20K resistor. The wiper
of pot P4 was jumpered to the CW terminal of P4. The wiper of pot P3 was jumpered to the CCW
terminal of P4. An additional 20K resistor was added to ground from pin 3 of U17:B.
The Polarization potentiometer connections are made to the terminal strip on the 2_5KPLP2 PCB. The
pot terminal closest in resistance to the pot wiper when the Pol axis is fully CCW should be connected to
the “Ref” terminal. The wiper of the pot should be connected to the “Signal” terminal. And the remaining
terminal of the pot should be connected to the return terminal.
Software that supports the potentiometer based polarization sensor includes an additional CONFIG mode
item to allow for the display of polarization position in an angular format. The CONFIG mode item is
designated Pol Display Scale Factor and it specifies the number of degrees of polarization movement per
for each one volt change in the polarization position sense voltage.
POL DISPLAY SCALE FACTOR: 20 CONFIG
DEG/VOLT, 5 – 500 ENT,BKSP,SCRLL ^v
To sense the polarization position the controller applies a 5 volt DC reference voltage to the
potentiometer via the interface board’s Ref and RTN terminals. As the potentiometer shaft is rotated the
voltage on the pot’s wiper terminal (connected to the interface board’s Sig terminal) varies between the
voltages applied to the pot’s other two terminals. The controller’s microprocessor reads this voltage as a
numeric value between 0 and 1023 (1023 = 2 10 -1).
The controller’s displayed polarization position is calculated according to the following formula …
Pol_Position = ((5 * Pol_Cnt * Scale_Factor) / 1024) + Offset (Equation 1)
Where …
Pol_Position is the polarization angle in degrees.tenths_of_degrees
Pol_Cnt is the potentiometer wiper voltage converted to a numeric value 0 to 1023 for a 0.0 to 5.0
volt input.
Scale_Factor is the Pol Display Scale Factor CONFIG mode item described above.
Offset is the Pol Display Offset CONFIG mode item described in the RC2500 User Manual.