Antenna positioner control system

An antenna positioner control system and related method is disclosed. The antenna positioner control system includes a housing and a hub mounted within a housing. A support plate is rotatably mounted on the hub. An antenna is pivotally mounted on the support plate. At least one elevation drive servomotor is mounted on the support plate and interconnects the antenna for pivoting the antenna a predetermined angle and adjusting elevation of the antenna. At least one azimuth drive servomotor is mounted on the support plate and interconnects the antenna for rotating the support plate relative to the hub a predetermined arcuate distance for adjusting azimuth of the antenna. An antenna control unit is operatively connected to the elevation drive servomotor and azimuth drive servomotor and includes an elevation control circuit and azimuth control circuit. Each of the control circuits include a position feedback control loop, a resolver positioned within the position feedback control loop, a rate feedback control loop, a tachometer positioned within the rate feedback control loop, and a motor feedback control loop.

FIELD OF THE INVENTION
 This invention is related to antenna positioners, and more particularly,
 this invention is related to an antenna positioner control system.
 BACKGROUND OF THE INVENTION
 Direct broadcast satellite (DBS) signals are often transmitted to aircraft
 and other moving vehicles. These transmitted signals are often KU-band
 television signals that are transmitted to commercial aircraft, trains and
 other moving vehicles, and are typically UHF and VHF band signals, which
 can be received on small antennas, such as the common 18" disks placed on
 the sides of houses. The antenna can also be formed as a phased array
 antenna, and designed as a flat plate, as is known to those skilled in the
 art. Many different types of housings and positioners have been designed
 to point the antenna's main beam at the desired direct broadcast satellite
 while an aircraft maintains various commercial cruise flight dynamics.
 These dynamics include a role of 5.degree./second and
 5.degree./second.sup.2 ; a pitch of 5.degree./second and
 3.degree./second.sup.2 ; and a yaw of 5.degree./second and
 5.degree./second.sup.2.
 One current method has been to use a mechanical device with an in-line jack
 screw actuator for elevation and a direct drive azimuth. In most types of
 controls, an antenna controller receives position commands and directs
 movement of various motors. However, these type of requirements are not
 adequate because with a mechanical system, the slew rate is slow and
 motors often overheat in maintaining positions. Also, the controller does
 not include a rate feed forward, which is desirable. Also, many prior art
 antenna positioners have mechanical designs that allow control over
 azimuth and elevation, but the motors and drive mechanics have excessive
 backlash. Also, many prior art designs do not fit into low profile
 housings that are adapted for mobile applications, such as mounting on the
 fuselage of an aircraft.
 U.S. Pat. No. 5,025,262 to Abdelrazik et al. discloses a pedestal with a
 helical element antenna that is mechanically steered with reference to an
 azimuth axis and elevation axis. A mechanical steering system includes a
 supporting frame having an azimuth member and an elevation member that is
 integral with the azimuth member. It includes a longitudinal axis
 displaced from the azimuth axis.
 U.S. Pat. Nos. 5,689,276 and 5,420,598 to Uematsu et al. disclose an
 antenna housing for a satellite antenna device, which mounts on a moving
 body and includes an automatic tracking mechanism. An elevation motor is
 fixed to a rotary base. A series of pulleys and shafts act as a driving
 mechanism. A rack has teeth formed along a circle about the rotating axis
 in elevation direction of the antenna unit A. The teeth of the rack mesh
 with the pinion gear to be driven circumferentially by the driving torque
 transmitted to a pinion gear. Thus, the antenna unit is driven for
 rotation in the elevation direction. An azimuth motor is fixed on the
 rotary base. Through a sufficient pulley mechanism, the driving torque of
 the azimuth motor is transmitted to the pinion, which meshes with teeth of
 a belt such that the driving torque of the azimuth motor is transmitted
 through the pulleys.
 U.S. Pat. No. 5,153,485 to Yamada et al. discloses a high gain antenna that
 is mounted on board an automobile for reception of satellite broadcasting.
 The system uses a beam antenna in the form of a flat plate that is secured
 to an antenna bracket. A turntable has a disk-shaped spur gear that
 includes a gear around its lateral side. Turntables are rotatably mounted
 on a stationary base by a bearing. Reduction gearing in a motor is mounted
 on the support plate and secured to a stationary plate base. The beam
 antenna can be moved in both azimuth and elevation.
 Many of these systems suffer some of the drawbacks noted above.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide an antenna
 positioner control system that allows adequate control over elevation and
 azimuth with a rate forward.
 It is still another object of the present invention to provide an antenna
 positioner control system that allows adequate control over elevation and
 azimuth with adequate command signaling and control.
 In accordance with the present invention, an antenna positioner control
 system includes a housing and a hub mounted within the housing. A support
 plate is rotatably mounted on the hub. An antenna is pivotally mounted on
 the support plate. At least one elevation drive servomotor is mounted on
 the support plate and interconnects the antenna for pivoting the antenna a
 predetermined angle and adjusting elevation of the antenna. At least one
 azimuth drive servomotor is mounted on the support plate and interconnects
 the antenna for rotating the support plate relative to the hub a
 predetermined arcuate distance for adjusting azimuth of the antenna.
 An antenna control unit is operatively connected to the elevation drive
 servomotor and the azimuth drive servomotor. The antenna control unit
 includes an elevation control circuit operatively connected to the
 elevation drive servomotor for adjusting elevation and an azimuth control
 circuit operatively connected to the azimuth drive servomotor for
 adjusting the azimuth angle of the antenna. Each of the control circuits
 includes a position feedback control loop and a resolver positioned within
 each position feedback control loop. Each control circuit also includes a
 rate feedback control loop and a tachometer positioned within the rate
 feedback control loop. Also included is a motor feedback control loop
 within each circuit.
 In one aspect of the present invention, a current compensator is positioned
 within the motor feedback control loop. A position compensator is also
 positioned within a position feedback control loop. A tachometer
 compensator can be positioned within the rate feedback control loop.
 In still another aspect of the present invention, an antenna subsystem
 controller is operatively connected to the antenna control unit. The
 antenna subsystem controller further comprises a circuit for generating
 azimuth and elevation pointing commands to the antenna control unit. The
 antenna can include a phased array antenna. An antenna support shaft can
 be mounted on the antenna such that rotation of the support shaft pivots
 the antenna and adjusts elevation. The elevation servomotor can be
 operatively connected to the support shaft. The elevation drive servomotor
 can include an output shaft and a drive mechanism operatively
 interconnecting the output shaft and drive shaft.
 In still another aspect of the present invention, the antenna control unit
 includes a circuit for generating a rate feed forward command to each of
 the azimuth drive and elevation drive servomotors corresponding to an
 anticipated velocity position.
 A method aspect of the present invention is also disclosed. The method
 controls azimuth and elevation of an antenna and comprises the step of
 providing a hub mounted within a housing, a support plate rotatably
 mounted on the hub. The antenna is pivotally mounted on the support plate.
 The method comprises the step of generating an azimuth pointing command
 and elevation pointing command within respective azimuth and elevation
 control circuits to respective azimuth and elevation drive servomotors.
 The respective azimuth and elevation drive servomotors are driven through
 respective azimuth and elevation current acceleration loops. The azimuth
 and elevation voltage commands are generated to the respective current
 acceleration loops through respective tachometer rate loops that are
 closed about respective azimuth and elevation tachometers. The respective
 azimuth and elevation velocity commands are generated to the respective
 tachometer rate loops through respective azimuth and elevation position
 loops.
 In still another aspect of the present invention, the method includes the
 step of closing the respective azimuth and elevation position loops about
 the tachometer rate loops through the use of resolvers. The method also
 includes the step of generating a rate feed forward command to increase
 the responsiveness of the respective circuits by bypassing a lower
 bandwidth position loop and injecting a command directly into a higher
 bandwidth tachometer rate loop.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The antenna controller of the present invention is advantageous because the
 antenna fits within a low profile housing and can point the antenna's main
 beam at a chosen direct broadcast satellite, while an aircraft maintains
 typical commercial cruise flight dynamics. The antenna positioner allows
 control of the positioner on a moving platform and has anti-backlash
 capability through its efficient mechanical design. The positioner can be
 used with a dish, flat array or phased array antenna.
 As shown in FIG. 1, the antenna positioner of the present invention is
 illustrated at 20, and shown mounted on the underside of an aircraft 22. A
 direct broadcast satellite (DBS) 24 initially receives signals from a TV
 station 26 and its satellite dish 28. The antenna positioner 20 adjusts
 its azimuth and elevation to point the antenna beam and receive KU-band
 television signals, which are then processed and forwarded throughout the
 aircraft for display over an aircraft television terminal 30 as shown in
 the drawing.
 The antenna positioner 20 includes a housing 32 as shown in FIG. 2. The
 housing 32 is preferably annular configured and has a diameter at least
 twice the height of the housing as shown in FIG. 2. The housing 32 can be
 formed from many different materials as known to those skilled in the art,
 including a resin plastic that is preformed or premolded, metal, or fiber
 impregnated substances, such as an epoxy. The housing 32 should be strong
 to withstand shock and excessive mechanical forces. When an antenna that
 is designed to receive KU-band signals is used with the housing, a typical
 diameter of the housing 32 can be about 34 inches. This type of annular
 design is only one example of a housing 32 that can be used in the present
 invention and other designs can be used as suggested by those skilled in
 the art. However, the annular design is advantageous because it is well
 adapted to mobile applications and for breaking wind with its aerodynamic,
 annular design.
 As shown in FIGS. 2, 4 and 5, a control hub 34 is mounted within the
 housing. The hub 34 includes a generally cylindrical spindle 36 forming
 the central portion of the central hub. The hub 34 is substantially
 annular configured and includes an outer peripheral wall 38 spaced from
 the spindle axis. The wall 36 includes an inner bearing race 40 (FIG. 7).
 As shown in FIG. 7, the hub 34 is shaped somewhat as a dish with the
 central spindle axis and the outer upstanding wall 38 that forms a part of
 the inner bearing race 40. As shown in FIG. 5, the spindle axis 36 forms
 the central point of the housing diameter within the annular configured
 housing 32.
 A substantially planar configured support plate 34 is rotatably mounted on
 the central hub within the annular configured housing 32. As shown in
 FIGS. 2 and 5, the support plate 42 is formed similar to a truncated
 triangular configured design and formed as a plate with a central opening
 44 that is received over the annular configured central hub 34. The
 central opening 44 has an inner wall 46 forming an annular configured
 support mount, having an outer bearing race 48 that cooperates with the
 inner bearing race 40 formed on the annular configured central hub 34.
 Ball bearings 50 are positioned with the ball bearing channel formed by
 the races 40, 48. The ball bearings 50 can be kaydon type C KA series
 bearings having a starting torque of 70 inch-ounces at -50.degree. F. with
 factory "cut" grease. The running torque is about 70"-ounces. The races
 40, 48 can also be formed by bonding a metallic race to the edges of the
 support plate and central hub. Although one illustrated design has been
 described, other designs could be used as suggested by those skilled in
 the art. The support plate 42 with this type of race and ball bearing
 assembly is easily moveable relative to the central hub 34.
 A ring gear 52 is positioned on the central hub 34. An azimuth drive
 mechanism 54 is mounted on the support plate 42 and engages the ring gear
 52 to drive same, and thus rotate the support plate 42 a predetermined
 arcuate distance. As illustrated in the figures, the azimuth drive
 mechanism, in one preferred aspect of the invention, is designed as two
 servomotors 56, 58, each having an output shaft 56a, 58a and pinion gear
 56b, 58b mounted thereon, which engage the ring gear 52 for rotating the
 support plate 42 relative to the central hub 34 and housing 32 a
 predetermined arcuate distance on the central hub 34 for adjusting azimuth
 of the antenna. The two servomotors 56, 58 are advantageous because
 backlash is minimized when two servomotors are used to adjust azimuth. The
 ring gear 52 and pinion gears 56, 58 in one aspect of the present
 invention establish about a 16:1 gear reduction ratio. Although many
 different types of servomotors can be used, the typical azimuth drive
 mechanism that has been found acceptable uses two DC brushed motors that
 are torque-biased to mitigate backlash. It has been found advantageous to
 use Kollmorgen N9M4T ServoDisk motors. The gear heads can be fabricated by
 techniques known to those skilled in the art and can have a 6.5:1
 structural reduction ratio.
 As illustrated in FIGS. 2 and 5, the longer end of the support plate 42
 forming the hypotenuse 42a has two edge cutouts 42b on which are
 positioned antenna mounts 60 forming hinges to support an antenna 62,
 which in one preferred aspect, is formed as a flat panel plate and phased
 array antenna having a plurality of individual antenna elements 62a. The
 antenna 62 in the illustrated aspect of the invention is rectangular
 configured. However, different antenna configurations can be used as known
 to those skilled in the art.
 As illustrated, the antenna 62 is substantially elongate and rectangular
 configured and pivotally mounted on the support plate 42. It extends
 across a substantial portion of the housing 32 defined by a chord having a
 length about the diameter of the housing. Support tabs 64 extend from the
 rear side of the antenna 62 and form the pivot connection with the mounts
 60 that are positioned on the cutouts 42b.
 An elevation drive mechanism 66 is mounted on the support plate 42 and
 interconnects the antenna 62 for pivoting the antenna a predetermined
 angle and adjusting elevation of the antenna 62. As illustrated in FIG. 2,
 the elevation drive mechanism 66 includes a servomotor 68 having an output
 shaft 68a. A drive mechanism 70 interconnects the shaft 68a, and connects
 to a shaft 72 that extends along the rear side of the antenna. The shaft
 72 couples to the pivoting hinge of the antenna at the intersection of the
 antenna mount 60 and support tab 64. The drive mechanism 70 forms a
 pull/pull drive design to minimize backlash. In one illustrated aspect of
 the invention, the pull/pull drive is formed by thick cables 74 that
 interconnect a pull/pull tab 76, similar to a pulley type of design
 arrangement. Thus, the elevation servomotor 68 is exactly controlled and
 the preferred amount of arcuate output shaft rotation allows exact
 elevation movement of the antenna. The elevation drive mechanism can be
 formed from a single DC brushed motor, such as a Kollmorgen accurex
 S6M4H/86060, with a backlash free gear head having a 60:1 reduction ratio.
 A structural reduction ratio of 2:1 has been found acceptable.
 To minimize backlash by reducing component weight, the various components,
 such as the support plate 42, can be formed from a lightweight material,
 such as a honeycomb structure, typically formed as an expanded plastic.
 Other materials could include lightweight metals and other materials known
 to those skilled in the art.
 The present invention is also advantageous because it allows adequate
 antenna positioner control using a controller 80 mounted on the support
 plate, such as on its rear end 42c opposite the hypotenuse 42a. The
 controller 80 is operatively connected to the elevation drive mechanism
 and azimuth drive mechanism, and controls the azimuth and elevation drive
 mechanisms and adjusts elevation and azimuth.
 The controller 80 includes an antenna control unit 82 that is operatively
 connected to the elevation drive servomotor 68 and azimuth drive
 servomotors 56, 58 (FIGS. 8-10). As shown in FIG. 8, the antenna control
 unit 82 includes an elevation control circuit operatively connected to the
 elevation drive servomotor for adjusting elevation. Elevation pointing
 commands are generated by an Antenna Control System (ACS) and into the
 circuit having a position compensator 86, tachometer compensator 88 and
 current compensator 90 and then to the elevation drive servomotor 68. As
 illustrated, the elevation control circuit includes a position feedback
 control loop 92, which allows position feedback of antenna movement. This
 loop 92 extends to an input before the position compensator 86 into a
 mixer/summer 94 where the pointing command originally is input. A resolver
 96 is positioned within the position feedback control loop 92. The
 resolver 96 can be a Computer Conversion Corporation, RN0-11HB, size 11
 with an input voltage of 8.5 volts and 1,000 HZ. Although this is only one
 type of resolver, other resolvers can be used as known to those skilled in
 the art.
 As illustrated, a rate feedback control loop 100 extends from the elevation
 servomotor 68 to a mixer/summer 102 that is positioned after the position
 compensator 86 and before the tachometer compensator 88. A rate feed
 forward command 103 generated by the Antenna Control System 84 is received
 into the mixer/summer 102. A tachometer 104 is positioned within the rate
 feedback control loop 100. A motor feedback control loop 106 extends from
 the motor 68 to a mixer/summer 108 positioned between the tachometer
 compensator 88 and current compensator 90. The motor feedback control loop
 106 also acts as a current or acceleration loop, and can also be referred
 to by this term.
 As shown in FIG. 9, the azimuth control circuit includes similar
 components, such as a position compensator, tachometer compensator and
 current compensator and the mixer/summers, which are given the same
 reference numeral except with the addition of the prime notation a. Second
 elements are given the reference numeral the same as the first, except the
 addition of a letter a. One key difference is that two azimuth servomotors
 are used and referred to as motor 1 and motor 2. Thus, there is a second
 motor feedback control loop 106a and a second tachometer 104a positioned
 within the rate feedback control loop. Additionally, the summer/mixer 108
 includes a torque bias input. Also, a second motor feedback control loop
 106a is included, and includes a second current compensator 90a and
 mixer/summer 110 that receives inputs from mixer/summer 108.
 FIG. 10 illustrates another block diagram of the antenna control unit 82 of
 the present invention, which includes the control circuits as described
 above. The antenna control unit 82 includes four main modules that connect
 into a bus 112, such as a PC/104 bus. A first CPU module 114 is formed as
 a real time device and typically could include at least two RS-422 serial
 ports for receiving the azimuth and elevation position commands. An analog
 input/output module 116 is also formed as a real time device. A
 digital-to-analog module 118 is also formed as a real time device. A
 resolver-to-digital module (R/D) 120 can be formed, such as by a Computer
 Conversion Corporation's PC 104-AMAM-3WRHB circuit. This
 resolver-to-digital module 120 provides resolver excitation, such as 8.5
 volts at 1,000 HZ.
 The modules can be enclosed by a ruggedized box with a power supply. One
 example is a Kinetic Computer Corporation RCC-104. The antenna control
 unit 82 receives pointing commands via the RS-422 serial interface and
 commands the elevation and azimuth drive amplifiers 122. These drive
 amplifiers 122 power the azimuth servomotors 56, 58 and elevation
 servomotor 68 and the requisite tachometers.
 FIGS. 11 and 12 illustrate more detailed block diagrams of the antenna
 control unit 82, including the elevation control circuit (FIG. 11) and the
 azimuth control circuit (FIG. 12). The block diagrams illustrate the
 various digital/analog converters 124 and illustrate the rate feed forward
 command to the respective mixer/summer 94, 94'. Similar elements are given
 similar reference numerals with prime notation as noted before. Additional
 mixer/summers are given reference numeral 123. Appropriate switches 126,
 126' and analog/digital converters 128, 128' are illustrated. Low pass
 filter 125 is positioned between the tachometer compensator and the
 current compensator. The tachometer for each of the elevation and azimuth
 control circuits in the rate feedback control loop also includes an
 anti-aliasing filter and limiter 130, 130'. Each resolver 96, 96' also
 inputs to the resolver/digital module 120, with the reference, which also
 includes a feedback loop 132, 132'. The anti-aliasing filters and limiters
 input into analog-to-digital converters and multiplexer differentiators
 134, 134' as part of the rate feedback control loop.
 In operation, the positioners are slaved to pointing commands. Each
 pointing command can be in pedestal coordinates as an elevation or an
 azimuth, angle. The motor feedback control loops 106, 106', 106a' will
 typically act as a current or acceleration loop, and have a
 transconductance amplifier driving the respective servomotor. A current
 loop bandwidth should be at a minimum of about 1.0 KHZ, as typified by a
 drive amplifier specification as required by those skilled in the art. In
 both elevation and azimuth axes, the rate feedback control loop 100, 100'
 is closed about the tachometer 104, 104', 104a' and provides voltage
 commands to the motor feedback control loop also acting as a motor current
 feedback loop. This type of loop should be implemented as a type 1 loop.
 The position compensator 86, 86' provides velocity commands to the rate
 feedback control loop 100, 100'. The position feedback control loop 92,
 92' is closed about the rate feedback control loop 100, 100' by the
 resolver 96, 96'. The position feedback control loop 92, 92' can be
 implemented as either a type 1 loop or a type 2 loop. The rate feed
 forward command generated by the Antenna Control System 84 increases the
 responsiveness of the system by bypassing the lower bandwidth position
 feedback control loop 92, 92' and injecting a command directly into the
 higher bandwidth rate feedback control loop 100, 100'. A baud rate between
 the antenna control system 82 and the antenna control unit 82 can be
 specified as about 9.2 Kbaud. The antenna control system 84 also provides
 pointing commands to the antenna control unit 82.
 This patent application is related to commonly assigned, co-pending patent
 application entitled "LOW PROFILE ANTENNA POSITIONER FOR ADJUSTING
 ELEVATION AND AZIMUTH" filed on the same date of the present application
 by the same inventors.
 Many modifications and other embodiments of the invention will come to the
 mind of one skilled in the art having the benefit of the teachings
 presented in the foregoing descriptions and the associated drawings.
 Therefore, it is to be understood that the invention is not to be limited
 to the specific embodiments disclosed, and that the modifications and
 embodiments are intended to be included within the scope of the dependent
 claims.