RazTrak Digital Tracking System

Under Construction This page describes the construction of the RazTrak Digital az-el tracking system sold by N8CQ Gary Abercrombie. For info you can reach Gary on MoonNet e-mail reflector. He does not have a website. The system uses digital MAB25 encoders to provide tracking angle data to a custom interface called RazTrak. That board provides display and control via a Raspberry Pi-3 controller and 7-inch LCD touch screen. Data is all 3.3v logic. MABMPU boards use 5v which provides data communication between MAP25 absolute digital encoders and the RazTrak board via RJ12 6-conductor cable. To simplify adding the tracking system to an existing az-el system, the MPU9250 inclinometer is used to provide digital elevation data without need of mechanical pedulum methods with a rotary encoder. Azimuth would normally require a chain-drive sprocket gear attached to mast to drive the MAB25 shaft. That would require removing the antennas array to add the az-drive gear. Instead Im using an A/D converter too take the analog signal from the rotator potentiometer and digitize the 0-3.3v dc voltage coresponding to 0-360 degree rotation of the antenna array. Normally, the Ham-IV or T2X rotators provide 13.5vdc to the 500-ohm potentiometer inside the rotator. That drives an analog meter to provide angle information. The rotator motor drives with 27-30vac with 2.25 ampere load. The rotator brake-release solenoid draws 5 amp. Note that the system is configured with A/B switching to connect both 2m and 6m az-el systems to the RazTrak system. The RazTrak has internal software to provide Moon tracking. The rotators move too rapidly for auto- track to function well, so I purchased a DROK(tm) PWM (pulse width modulator) controller from Amazon.com. The pwm slows the motor by "chopping" part of every ac cycle to lower average voltage. But torque is not affected as full voltage is maintained when not cut off. Roughly 0 to 90 degrees of every cycle of ac voltage is "chopped" depending on setting of the speed control potentiometer. See diagram below: One problem I encountered was that the pwm is made for standard ac mains voltage of either 120 or 220 vac. Lowering the input to 30vac causes the PWM to cease operating so I follow the 120vac pwm with a standard 120/25.6v transformer. Actual ouput at 2.25 amp is 28.5 vac. There is some distortion of the ac waveform in the 28.5v transformer probably a result of switched clipping of the normal sine wave. I've noted no affect on the rotator operatiom by that. I can slow the rotator to approx. 1/3 normal speed. I should point out a particular issue using (CDE now HyGain marketed by MFJ) rotators. The wiper of the position pot is common with the ac return line which is at physical ground. Using an A/D converter with ac present causes problems. I measure roughly 3 vac leakage. The system ground of the RazTrak has to be isolated from station ground (rotator ground) or the A/D input gets grounded. I figured that an ac filter would provide ac isolation. I used a surplus PS choke of 4 Henry (est.) with several bypass capacitors on A/D side of the choke. An 8 uF electrolytic cap reduces this to 0.3v which may have noise at other frequencies. A 2200 uF electrolytic reduced this to zero. I added a 0.05 uF disc ceramic to guard against RF effecting the A/D. I see no jitter in the digital azimuth readout with 30vac brake-release voltage or 28.5vac motor drive voltage applied. A concern has been voiced by some that high power RF may affect the MABMPU sensors. I used a Aluminum diecast box to enclose the MABMPU/MPU9250 inclinometer mounted on the antenna crossboom inside the near-field of 1500w on 2m, with no RF affects seen. The inclinometer communicates via Cat-5e shielded 6-conductor cable. The shield is grounded at the antenna end, only (and isolated from RazTrak ground). I added two clamp-on ferrites at the inclinometer end of the cable. Still in testing mode and have not attempted auto-track (some tracking angle offsets need to be programmed).

  • Schematic Diagram
  • Photos:
(to be provided)
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