Maybe someone can dig the archives better than me, but I remember there was an elegant solution years ago.
Part of a thread can be found here:
http://www.amsat.org/amsat/archive/amsat-bb/200101/msg00697.html
73, Joe kk0sd
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Zach Leffke Sent: Wednesday, August 07, 2013 8:58 AM To: 'AMSAT BB' Subject: [amsat-bb] Non-mechanical Azimuth/Elevation Feedback Advice
Hello Everyone,
I recently came into possession of a Pelco PT170-24P tracking pan tilt pedestal designed to support large security cameras. My intent with this new acquisition is to repurpose it as a "low cost" (got it on Ebay for ~$75 + S&H) alternative for an antenna tracking pedestal for amateur satellites. It uses 24VAC induction motors to move the azimuth and elevation assemblies, pretty much just like the G5500s that I'm sure so many of us are familiar with. It definitely cannot support the same amount of weight as the G5500, but I'm looking to construct a small, portable satellite ground station node and this thing is plenty beefy enough to handle a couple of Arrow style antennas. Here is the problem, it provides absolutely no feedback.
My question to the group is does anyone know of a non-mechanical method for getting relatively accurate feedback for azimuth and elevation. I'm looking for an all electronic means that I can mount somewhere outside of the actual pedestal assembly (like perhaps on the cross-boom) that will be able to provide measurement of the az/el (or pan/tilt, or yaw/pitch, whatever you want to call it) position. I'm using an arduino microcontroller for the tracking controller. Originally I intended to find a way to mount potentiometers in inside the unit and simply use the ADCs on the arduino to read the position feedback voltage from the pots, however, there is barely enough space to mount an elevation feedback pot inside the unit, and there is virtually no space for an azimuth feedback pot. Hence I'm looking for a non-mechanical method.
My first thoughts for the elevation feedback was to use the old potentiometer plus nice heavy weight method mounted out on the boom. This idea doesn't appeal to me very much as other factors can now affect the position feedback (such as high winds). I then thought of something along the lines of an accelerometer. I also tossed around the idea of a 2-axis gyro for both Az/El. My issue is I have limited experience working with these types of sensors, and was hoping to get advice from everyone in this group. I know for example that the gyro will provide rate of motion around an axis and thus I have to integrate over time to get the actual position. This becomes cumbersome because now I have to keep track of time in the Arduino while executing movement commands (certainly do-able, just more complicated than reading an ADC voltage). Additionally, I believe these devices suffer from drift and require frequent calibration (although there may be a scheme of starting from a known position, say at one of the limit switch contact points, for each pass that might work). I also toyed with the idea of an electric compass for azimuth feedback, but I'm worried about distortion of the magnetic field near the pedestal due to the AC induction motors or when the antennas are radiating. In theory the motors are housed inside the metal pedestal enclosure and thus are shielded from the outside world, but I can just see it now, nice steady feedback when the pedestal is stopped and as soon as I execute a motion command the azimuth feedback starts dancing all over the place. Since the motion stop command is based on achieving the target position, system instability is sure to occur. Even if I solve the AC motor EMI problem, I still worry that when transmitting the fields could potentially be distorted if near the antenna (remember my goal is a compact design) and taint the position feedback.
Any ideas from the group would be greatly appreciated. I'm looking for a "sparkfun" type solution here and if anyone has experience working with accelerometers, gyros, electric compasses, etc. I would love your advice on which might be the way to go for the position feedback. If you think I've hit on a good idea above and should go with it please let me know. Again I'm using an Arduino, so analog voltage feedback, I2C, SPI, and UART serial are all on the table for communicating with the sensors to get the feedback info.
Thanks in advance!
Sincerely,
Zach, KJ4QLP
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