Appendix F - Controlling Antennas Powered By Ac Or Large Dc Motors - RESEARCH CONCEPTS RC2000C Manual

94 RC2000C Az/El Tracking Antenna Controller Appendix F AC or Large DC Motors

Appendix F - Controlling Antennas Powered by AC or Large DC Motors

The RC1000A, RC2000A, and RC2000C antenna controllers are designed to control satellite antennas
powered by 36-volt DC motors that use pulse type position sensors. The RC1000A is designed for use
with single axis polar mount antennas. The maximum output current from the RC1000A is 5 amps. The
RC1000A has a built-in interface for a Chaparral type Polarotor. The RC2000A and RC2000C are dual
axis antenna controllers designed for use with elevation over azimuth or azimuth over elevation type
mounts as well as polar mounts with a motorized elevation (latitude angle) or motorized declination
adjustment. The RC2000C has all of the features of the RC2000A as well as support for tracking
inclined orbit satellites via step track and memory track algorithms. The RC2000A and RC2000C share
a common motherboard. In the text that follows the term RC2000 will be used to refer to both versions.
The maximum actuator drive current available from the RC2000 is 8 amps. The RC2000 has a built-in
interface for a Chaparral type Polarotor. There is an optional interface (designated RC2KPOL) for a 24
volt DC polarization motor which uses a potentiometer for feedback. With this option the output current
available to the polarization motor is approximately 200 milliamps. This option was designed to be used
with Seavey motorized feeds.
The two issues which must be addressed to successfully interface one of these controllers to a large
antenna is the application of power to the antenna motors and sensing the antenna's position. This
paper describes a technique to use the antenna controller's +/- 36-volt output voltage to activate AC
and higher voltage DC motors as well as the selection and placement of pulse type position sensors
that are compatible with these antenna controllers. This paper also includes a description of the
products available from Research Concepts, Inc. (RCI) which implement the motor control scheme
outlined in this paper and a sensor design example.
Controlling the Motors
Please refer to figure 1. The azimuth motor drive output of the RC2000 on the AZ1 and AZ2 terminals
will be +/- 36 volts. When azimuth ccw movement is specified, AZ1 will have the higher voltage, and
when azimuth cw movement is specified, AZ2 will have the higher voltage. (Note that the sense of
antenna azimuth movement is as seen by an observer located above the antenna). In a similar fashion,
when down movement is specified, EL1 will be at the higher voltage, and when upward movement is
specified, EL2 will be at the higher potential. When azimuth ccw movement is specified, current will
flow out of the AZ1 terminal of the RC2000, through the dropping RESISTOR, through STEERING
DIODE D1, through the AZ CCW RELAY COIL, through the AZ CCW LIMIT SWITCH, through the AZ
CW LIMIT SWITCH, and back into the AZ2 terminal of the RC2000. Current flowing through the AZ
CCW RELAY COIL will activate the relay and close the AZ CCW CONTACT CLOSURE. This will
configure the POWER CONTACTOR to move the antenna in the azimuth ccw direction. When azimuth
ccw current flows STEERING DIODE D2 keeps current from flowing through the azimuth CW RELAY
COIL.
When the antenna is within the azimuth CCW limit, the AZ CCW LIMIT SWITCH is closed. When the
azimuth ccw limit is reached, the AZ CCW LIMIT SWITCH will open. When the AZ CCW LIMIT
SWITCH is open, STEERING DIODE D3 will keep azimuth ccw current from flowing, but will allow
azimuth cw current to flow to move the antenna out of the ccw limit. Azimuth cw movement is
accomplished in a similar fashion. Note that limit switches are not required. The RC1000A and
RC2000 maintain logical limits based on the position count. Limit switches are pretty cheap insurance,
however.
The purpose of the dropping resistor is to match the output voltage of the RC1000A or RC2000 (36
volts) to the voltage necessary to activate the relay coils. A common relay coil voltage is 24 volts. If the
relays have 24 volt coils, the dropping resistor should be selected so that at the relay's rated coil current
the voltage drop across the resistor will be 12 volts. If 36 volt relays are used the dropping resistor is
not needed.
A number of products are available from Research Concepts which implement the motor control
scheme outline above. See the section below entitled Motor Control Interface Products for a description
of these products.
Note that the scheme shown in Figure 1 does not support slow speed movement. With 36-volt motors,
the RC1000 and RC2000 vary the motor speed by rapidly switching the 36-volt antenna drive signals off
and on - which gives an average voltage of less than 36 volts. If this signal is applied to the circuit of
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