CNCTRK Article in pdf form - Freeman KI4SBL
Contents
1. as needed to configure the parameters For more information see the user manual at http sourceforge net projects gpredict files Gpredict 1 3 gpredict user manual 1 3 pdf download Open the Antenna Control from the pull down menu small icon located at the top right of the main GPredict screen Initiate control of the APS by clicking on the Engage button The APS can also be manually jogged using the up down arrows on the numerical position display The Track and Engage buttons will remove manual control and place the APS under control of the GPredict program as needed for satellite tracking A screenshot of of the Antenna Control window during active tracking of satellite AO 27 1s shown in Figure 6 Field Alignment of Coordinate System To align the system in the field a tripod mount and sunny day is assumed Start the LinuxCNC program the tracking program Predict and proceed as follows Gpredict Rotator Control Amateur MIEI Azimuth Elevation A A A A A A A A A A Atlanta 1 7 3 84 24 52 Read 0 00 Read 0 00 Target settings AO 27 si Track Device Engage Az 173 86 Cycle 1000 g msec El 24 597 AT 10 41 Tolerance 0 01 g deg Figure 6 GPredict Antenna Control During Tracking of AO 27 1 Level the Azimuth axis 2 Initiate Sun tracking in Predict this steers the Azimuth and Elevation axes to the correct angle 3 Unl2. DC powered operation and weighing about eight 8 5 pounds is suitable for use on a sturdy camera tripod This article has described the CNCTRK system the prototype APS integration of the unit with available tracking software and has touched on the design approach and possible enhancements of the system Questions suggestions and inquiries re garding the CNCTRK system are welcomed by the author Additional information may be found at http ki4sbl dodropin org CNCTRK Biographical information for Bob Freeman KI4SBL Bob is a career microwave antenna engineer with several years experience in antenna design and measurement Interests include microwave engineering amateur radio hobby CNC RVing tree climbing and other outdoor activities Bob and wife Cathy KI4SBK currently reside in Cumming GA or somewhere along a scenic highway in the USA
3. added to the unit to increase survivability in the outdoor environment Kits and Assemblies Supply of APS kits is a possibility depending on the level of interest Mechanical assembly and wiring of the APS is required but the motor driver electronics are furnished as fully assembled modules Kits would include the following items e CNC Aluminum parts and components for two gearboxes AZ EL e Configuration Adapter The AMSAT Journal May June 2015 www amsat org e Stepper motors and drivers e Assembly instructions in electronic form CDROM or DVD with Linux CNC distribution e CDROM or Flash Drive with setup file and cnctrk c Note that since custom machined parts are used there may be significant lead times for the APS kit components All RF equipment and cables needed to complete a ground station are to be provided by the user Summary and Conclusion A new low cost approach to antenna positioning for satellite tracking has been described The approach employs existing open source motion control software in the form of the LinuxCNC to handle the hardware layer mechanical control A new APS has been designed built and tested Further the prototype units are integrated with open source satellite tracking programs thus enabling tracking of LEO satellites The design intent of the system is to enable satellite tracking using small to medium Arrow and Elk sized antennas The system provides portable
4. CNCTRK A LinuxCNC Based Satellite Tracking System his paper presents a new approach to antenna positioner control and introduces a new Azimuth AZ and Elevation EL positioner designed for use in satellite tracking The prototype positioning units have been integrated for use with the well known open source satellite tracking programs Predict and GPredict to enable tracking of LEO satellites A complete ground station with tracking system called CNCTRK is shown in Figure 1 The block diagram shown in Figure 2 illustrates the simplicity of the CNCTRK system The initial design intent of the overall system is to enable satellite tracking using small to medium sized Arrow and Elk like antennas The system provides portable 12VDC operation and is camera tripod mountable This paper describes the prototype AZ EL Antenna Positioning System APS the motion control approach integration of the APS unit with tracking software and touches on design considerations for the system Prototype Hardware A custom low cost gearbox has been designed for use in the new APS The basis of the gearbox is a rigid CNC machined aluminum housing Starting with the housing the assembly is built up using heavy duty ball bearings worm gear drive and 1 2 steel shafting Stepper motors are used to drive the system as they are economical and provide simple repeatable performance Motor drivers are integrated into the syste
5. atellite then press Enter The designator for the desired satellite A to X is found in PREDICT s Track screen T You may also enter a Y or Z to track the Sun or Moon respectively Once the designator A to Z is entered the APS will swing into position and the system will begin to track the target with position updates every second The commanded Azimuth and Elevation values are displayed on the terminal as long as the unit 1s tracking To stop the process enter Ctrl C in the same terminal window or tracking will also stop if Predict is removed from track mode using q or ESC Tracking using GPredict Setup of GPredict is done in the normal fashion as described in the GPredict documentation Follow the instructions in the manual to setup radios if needed Gpredict employs the hamlib library for control of multiple radios and rotators A new positioner rotator interface has been introduced for inclusion into hamlib for the CNCTRK system The software is not yet included in the current release of hamlib so the sources must be compiled using the provided patch to the current release presently hamlib 1 2 15 3 tar gz Compiling Hamlib with CNCTRK Patch Download the current release of hamlib from the above web site then unpack using tar xzf hamlib 1 2 15 3 tar gz Copy the provided patch file to the hamlib e PREDICT Tracking SAUDISAT 1C SO 50 PREDICT Real Time Satellite Tracking Track
6. components 1 a laptop or other computer with printer port and cable 2 a DC power supply and 3 the APS Power for the APS may be provided from any available DC source in the range of 12 to 24 VDC including battery power Flexibility and Future Enhancement Opportunities The CNCTRK system provides a suitable platform for its intended purpose of tracking amateur radio satellites Enhancements to the system are always possible as are alternative uses for the system Some ideas along these lines include 15 deg s default setting 0 007 deg at 1 16 microstep Weigh Table 1 System Characteristics The AMSAT Journal May June 2015 www amsat org Home Limit Switches The LinuxCNC program can easily accommodate the use of limit switches wired to the 25 pin I O connector This enhancement provides repeatable position location from one session to the next The aluminum housing provides ample area for addition of this and other custom features Raspberry Pi The LinuxCNC program has been demonstrated to run on the Raspberry Pi RP1 for an example see http www youtube com watch v Kz02FxYAvC4 A Raspberry Pi based system has been successfully implemented using Predict An additional circuit board is employed to generate step pulses the PCB is an open source design and may be found at http github com kinsamanka PICnc V2 The PRi solution requires much less power than the laptop system and no changes to the APS
7. eal time software See the following site for more information regarding the latency test http wiki linuxcnc org cgi bin wiki pl Latency Test For reference the laptop used in the development system exhibited latency measured at less than 25 micro seconds LinuxCNC Setup Setting up the LinuxCNC the first time is done by selecting the program from the File Machine View CORE Manual Control F3 MDI F5 S ZTE Home all _Touch of Feed Override Jog Speed Max Velocity 100 16 in min 72 in min 3 text EMC2 5 AXIS l AXIS splash g code Not intended for actual milling 2 To run this code anyway you might have to Touch Off the Z axis 3 depending on your setup As if you had some material in your mill 4 Hint jog the Z axis down a bit then touch off 5 Also press the Toggle Skip Lines with to see that part If the program is too big or small for your machine change the scale 3 font usr share fonts truetype freefont FreeSerifBoldItalic ttf ESTOP No tool Position Relative Actual Figure 4 LinuxCNC running AXIS GUI Applications Menu Select a generic stepper configuration to start and have the program make a Desktop icon or Shortcut for the system The specific information needed for control of the CNCTRK system is achieved using an ini file An initialization file is provided with the CNCTRK system for this purpose
8. hardware are needed Use of the RP1 with CNCTRK may also serve as an avenue of interest for young operators that are already familiar with programming the platform Modular Approach The modular characteristic enables two or more axes to be assembled as needed These units may be configured as a mast mounted system polarization rotators azimuth table for antenna range measurements and so on Or how about a Roll Over Elevation Over Azimuth configuration for AZ EL and Polarization control The LinuxCNC will control at least four axes using a single parallel port Solar Tracking The system can be made to track the Sun This is already enabled when using the Predict program interface cnctrk c and is a feature that is quite useful for system alignment in the field An alternate application might include positioning of photo voltaic solar panels for maximum energy collection Education Educational opportunities exist through the exploration and use of this project In addition to the diverse offerings that come with amateur radio and satellite communications there is much to learn in the area of mechanical systems motor and motion control and the concepts of CNC machinery The CNCTRK ground station approach may be beneficial to the student by providing relatively low cost hardware to enable the exploration of the concepts and practices of CNC and mechanical systems Performance The mechanical per formance of t
9. he basic gearbox may be improved through the addition of a custom worm gear set A gear set with finer pitch could achieve lower backlash improved self braking through higher gear ratios and overall improved position accuracy Note that slower speeds will result with higher gear ratios assuming the control computer is held constant Dual Antenna Mounting The length of the Elevation axis shaft is made purposely long and extends on both sides of the APS This enables one to adapt shaft extensions on the APS unit to provide separation distance between antennas that may be mounted on both sides of the APS For example one might mount a 2m crossed Yagi Uda on one side and a 70 cm beam on the opposite side Counterweight A counterbalance may be employed for antennas that require greater torque than is readily available with existing motors and gearing More Power The motor driver circuits employed in the prototype system were adjusted to approximately 1 0 A per driver at 12 VDC The motor driver circuits provided with the system are based on the Toshiba TB6600 and are actually capable of driving up to 4 5 A at 24 VDC While use of the drivers at these levels has not been tested by the author it does offer the potential for much greater delivered torque from the APS with the existing motors Weatherproofing The initial APS design is meant for portable operation and is not weatherproof Additional parts or features could be
10. ing SAUDISA On Fri 30Jani5 19 02 58 Not Enabled Figure 5 PREDICT in single satellite mode directory change to the directory and apply the patch using patch pl lt hamlib 1 2 15 3 cnctrk patch Compile the sources as usual using config make sudo make install To get started power the APS and start the LinuxCNC program press F1 F2 Home All and verify the system is operational via the cursor keys To verify the APS interface is working through hamlib use the function rotctl to start an interactive session From the command line proceed as follows rotctl m 1501 the hamlib model number for the APS is 1501 P enter Azimuth and Elevation data desired When finished press Ctrl C to stop the rotctl interactive program In order to use the APS with GPredict the rotctl program must be run in the operating system as a daemon open a terminal and use the following command to run the daemon process in the background rotctld m 1501 amp 4 AA The AMSAT Journal gt May June 2015 www amsat org KT At this point the LinuxCNC and hamlib processes are ready to accept position commands from the tracking software GPredict Setup Again setup of GPredict is done in the normal fashion Follow the instructions in the GPredict manual to setup the rotator and position limits under the Edit gt gt Prefe rences gt gt Interfaces gt gt Rotators pull down menu Select either Add or Edit
11. it need only be copied to the generic stepper configuration directory and referenced by the desktop icon in order for the system to start Axis GUI The initialization file will direct the LinuxCNC to start the Axis GUI an example is shown in Figure 4 above The Axis GUI is also required for the CNCTRK system to function Once the GUI has started Press F1 Press F2 and Press the Home All button on the interface these actions take the machine out of ESTOP turns on the servos and homes the axes respectively With power applied to the APS the cursor keys will provide manual control of the positioner At this point you will now have a CNC based antenna positioner With the APS working the satellite tracking program s can now be configured for use with the system Satellite Tracking Options Initial development of the CNCTRK system employed the satellite tracking program PREDICT See http www qsl net kd2bd predict html The AMSAT Journal May June 2015 www amsat org This approach employs the use of a small piece of C code cnctrk c to query the Predict program for Azimuth and Elevation values and then send these to the positioner controller Use of CNCTRK with Predict is the easiest system to set up and less resource intensive The Predict program does not provide a radio interface to enable computer control of the user s radio so the CNCTRK concept was extended for use with GPREDICT for this purp
12. m Modular Assembly There is only one gearbox design for the APS In order to achieve dual axis operation two gearbox units are ganged together with a 90 degree angular offset using a Configuration Adapter For the initial prototypes the axes are configured as an Elevation over Azimuth positioner as shown in Figure 3 on the right Bob Freeman KI4SBL uwaveguy gmail com Eres a g mh eer 5 Puy PC running LinuxCNC Predict GPredict Antenna Positioning system APS Battery 12 24 VDC Figure 2 CNCTRK System Block Diagram Figure 3 APS in EL Over AZ Configuration The AMSAT Journal May June 2015 www amsat org Motion Control Concept Motion control for CNCTRK is achieved without building a new rotor interface circuit Instead the motion planning and execution is provided via the open source LinuxCNC software linuxcnc org running on the host computer i e the same computer running Predict or GPredict LinuxCNC is used worldwide for control of big iron computer numeric controlled CNC machinery in both professional and home shop environments NOTE Installation of the LinuxCNC system is a great way to re purpose your old Windows machine LinuxCNC previously known as the Enhanced Machine Controller was originally developed by National Institute of Standards and Technology NIST The program has been revised enhanced and maintained through community effort A wealth of information i
13. ock the tripod mount and rotate about the vertical axis until the Sun is in alignment with the antenna beam Once aligned a dual band antenna will produce a shadow that resembles an X or cross on the ground 4 The system is now aligned lock the tripod vertical axis in place If the Sun is obscured then use of a compass or other means may be employed to align the Azimuth axis Prototype Hardware Characteristics As mentioned previously a custom gearbox is made from a rigid CNC machined aluminum housing heavy duty ball bear ings worm gear drive and 1 2 steel shaft ing used throughout Stepper motors and generic motor drivers accepting Step and Direction signals from the LinuxCNC computer provide the drive for the system Finally the range of Azimuth axis rotation is made greater than 360 degrees through the use of a wrapped power control cable Mechanical and electrical characteristics of the prototype system have been measured The drive settings for the motor drivers were set to conservative values The rotation velocity is a function of the computer used with the system the values listed are for reference only A summary of measured characteristics data from one of the prototype units 1s provided in Table 1 Cost Considerations Several factors were considered in the APS design to reduce cost of the units some of these are listed below 1 To take advantage of the economies of scale the positioner empl
14. ose See http gpredict oz9aec net This required a new rotator back end be written for hamlib so the GPredict system could be set up in the normal fashion See http sourceforge net projects hamlib Descriptions of the setup and use of each of these two tracking approaches are provided as follows Tracking using Predict The Predict program is a relatively small application it is the easiest to set up and is preferred for cases where manual radio control is used Predict provides a text mode display for tracking either a Single satellite or Multiple satellites among other things For use with the CNCTRK system the program is started in server mode from the command line using predict s Select option M for multiple satellite ame ech ty tracking display mode highlighted entries are within view or select option T to display tracking data for a single satellite Once in track mode the predict server is available for use by client programs such as the CNCTRK system A screenshot of Predict in single satellite T mode is shown below for reference Similar as before power the APS and start the LinuxCNC program press F1 F2 Home All and verify the system is operational via the cursor keys A technique for aligning the system with earth coordinates is provided later Open a terminal window and enter following command to initiate satellite tracking cnctrk A Z designator for s
15. oys a modular design and parts reuse The gearbox design including the drive system is re used for each axis in the system Connecting parts are used in multiple applications where possible 2 Worm gears used in the system are commodity gear kits they will provide long service life and are inexpensive to replace The position accuracy achieved using the worm drive system is entirely sufficient for Amateur satellite operation where antenna 3dB beamwidths are in excess of 20 degrees 3 Stepper motors are used to drive the assembly These are widely available and can be purchased with a range of characteristics for torque voltage and current A NEMA 23 standard motor size is used in the CNCTRK system NOTE The first prototype unit used stepper motors from old printers with excellent results It should be noted that the LinuxCNC is capable of controlling a wide range of motor controls including servos see the linuxcnc org Wiki for a list of known working motor interfaces 4 The free open source LinuxCNC program provides the motion control logic for the CNCTRK system This is a flexible and stable real time platform with proven performance Use of LinuxCNC reduces tracking systemcost by eliminating the need for a separate track box assembly and associated cabling or other proprietary interface to commercial antenna rotators 5 The CNCTRK system shown in the previous block diagram employs only three major
16. s available on the project web site linuxcnc org Key features of the LinuxCNC program employed for the CNCTRK project are 1 Motion Planning and Control the system provides soft start stop of the positioner using profiled velocity and acceleration simultaneous in all axes Configuration options are available for user customization 2 Flexible Machine Interface to Motor Controls several different types of motor drives are supported including steppers and servos 3 Graphical User Interface GUI provides both interactive and programmed position control LinuxCNC Installation Instructions for installing the LinuxCNC are available on the linuxcnc org web site http linuxcnc org The web site also contains a Wiki to aid users with installation Basically the latest linuxcne iso image is downloaded and burned to DVD or USB flash drive The media is inserted into the candidate computer for CNCTRK a computer with a parallel port is required and the system is booted Following the instructions for installation will result in a bootable LinuxCNC system Thanks to the developers at linuxcnc org the process can be just that simple IMPORTANT NOTE Live versions of the LinuxCNC are available that allow testing the candidate computer before installing the operating system Further it is very important to test the candidate machine s latency before installation to verify the computer is able to adequately run the r
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