We have tried multiple kinds at VT, with varying amounts of success for the quadrature encoder feedback.
For the motor cabling, we use standard unshielded, multiconductor rotator cable. I don’t have the specific specs, but it was something like typical 14AWG or 16 AWG wires in an 8 conductor bundle. If memory serves I believe we got some spools from The RF Connection (therfc.com….highly recommend them for all things cable related, particularly coax). The unused conductors we break out in a junction box under the rotator to perform other functions (like pol switch selection and/or LNA power, PTT control, etc.).
So far, running the motor signals in an unshielded cable, along with other signals for the overall antenna stack (not the rotator feedback) hasn’t been an issue. PWM is used during ramp up/down, on fairly high current carrying lines, so we were a little worried about self-induced EMI, but it turned out to not be an issue. we have not noticed any correlation between motor motion and the feedback EMI issues (we spent countless hours testing this, and sometimes the EMI issues were so bad the controller would ‘count pulses’ and increment/decrement the feedback position even when the motors weren’t active). This has also not caused any noticeable degradation in the RF systems (the internal LNA power conditioning appears to suppress any EMI on the supply line very well….go Advanced Receiver Research! Please start building mast mount, bypassable, LNAs again…..).
The original installation used the manufacturer recommended shielded cable for each feedback pair (a dedicated shielded pair for az, a completely separate one for el, and a completely separate one for the 15V supply to the rotator feedback electronics (over the MIC-8 feedback connector). This worked reliably without a single glitch for over a few years now on 1 of our 4 rotators (our 3m dish system). The other 3 systems exhibited all kinds of intermittent issues. We played all kinds of games with noise suppression capacitors, grounding the shields on one end or the other (leaving the other end floating) or grounding both ends, all with minimal effect. Not sure what makes the one system so special (my ‘golden child’) as nearly everything about it is identical to the other systems, they are so similar they are only one or two digits away from each other in the manufacturer serial numbers…..
We then moved to a 6 pair multiconductor shielded cable, not twisted pair (again can’t remember the specifics, I think we ordered it on mouser/digikey). This performed just about as good as the individual feedback twisted pairs (not well….but sometimes OK). I’ve also used simple shielded Cat5 (Ethernet) cable with some success for the feeback lines. The trick is getting the twisted pair combinations correct for each axis of rotation. This really only worked reliably for shorter runs (maybe 50 ft or less) and I’ve only relied on it for the ‘deployable’ systems (I wouldn’t recommend this for long runs or permanent installs).
The most reliable feedback setup we’ve achieved so far was the use of custom designed differential encoders/decoders. These encoders convert each of the quadrature signals ( two per axis) into a pair of differential signals at the tower just under the rotator (so 8 signals total, 2 per quadrature signal, one ‘positive’ one ‘negative’). Each differential signal is sent over a shielded cat5 cable (rated for water and UV protection, obtained from DX Engineering) with each of the twisted pairs assigned to each differential pair (we use standard RJ45 connectors on the boards / cables). In the shack, next to the control box, we convert the differential signal back to a quadrature signal with the differential decoder board for injection into the feedback port of the MD-01 (which expects the quadrature signal). The 15V supply signal for this board and for the feedback circuitry of the rotator is routed over the general ‘bundles’ that are also used for the motor supply lines (and LNA power, ptt, pol switch, etc.).
The differential encoder/decoder technique has nearly 100% removed our EMI issues. Super clean, perfect square waves, with almost 0 ripple on the output of the decoder (As measured on an oscope between the differential decoder and the MD-01 feeback port)……I wish they would directly include it in the rotator and controller feedback circuitry and highly recommend it for anyone out there cooking up new designs (like the ORI openRotor project). We are certainly not the first to use this technique (we got the idea from hams that have already done it, we just made our own boards). This has NOT removed all of our feedback issues, but has removed the feedback EMI issues. The remaining issues we’ve isolated to the rotator itself and we’re still limping along trying to figure it out, seems to be related to the hall effect sensor itself or maybe the tiny magnet coupled to the motor shaft….not sure……then COVID-19 hit…….no more troubleshooting for a while…..
Hope this helps! -Zach, KJ4QLP -- Research Associate Aerospace & Ocean Systems Lab Ted & Karyn Hume Center for National Security & Technology Virginia Polytechnic Institute & State University Work Phone: 540-231-4174 Cell Phone: 540-808-6305
From: Ryan Butler [email protected] Sent: Friday, June 5, 2020 9:28 PM To: Leffke, Zachary [email protected] Cc: [email protected] Subject: Re: [amsat-bb] Alpha Spid Rotator
What's everyone using for rotor cable to feed the Alpha Spid?
Ryan, NF0T
On Wed, Jun 3, 2020 at 1:45 PM Leffke, Zachary via AMSAT-BB <[email protected]mailto:[email protected]> wrote: Hi Dave, We use a lot of the RAS/HR and Big-RAS/HRs at VT in our ground station and on other projects. My biggest recommendation is if you don't absolutely need the resolution, don't get the high resolution variants (0.2 deg resolution). It uses a quadrature encoder feedback mechanism that is highly susceptible to EMI corruption leading to 'dropped counts' and a general slew of problems related to calibration and reliable pointing knowledge. We have constantly battled this problem since day 1 of the VTGS and still don't have reliable resolution for the problem (but enough band-aid solutions to keep limping along and periodic trips out to the tracking station to recalibrate). The smaller Ras/HRs seem to generally do better on the feedback front (still high resolution with the quadrature feedback) for some reason, but we typically use those in 'deployable' scenarios and the feedback cabling is much shorter (maybe 20ft or so)....so not necessarily an apples to apples comparison with the perm anently installed Big-RAS/HRs that have longer feedback cables (roughly 100-200 ft depending on which antenna stack we're talking about) and generaly seem to be more prone to issues. I also personally own a Ras/HR (smaller one) with the high res feedback and a 2.4m solid dish mounted on it. It can handle the load just fine with proper counterweighting and so far I haven't had any feedback issues (roughly 50ft feedback cable)....knock on wood.
The 'non high resolution' (low resolution?...its still accurate to about 1 deg) variants use plain ol' potentiometer feedback. I don't have direct experience with them, but have reports from some colleagues that they haven't had any serious or repetitive issues with pointing calibration (some radio astronomers in Physics dept are using one with a 2m dish). (I think that one uses a different controller than the MD-01 that we use, but I think the MD-01 or MD-02 could handle either type of feedback).
Other 'warnings' include realizing they have no mechanical stops in azimuth (they do in elevation, roughly at 180 deg limits). That said, it isn't really a problem as most controllers work reliably with 'software limits' and the MD-01s at least have timeouts that help if things 'go wonky' with feedback. There are some cases though that result in the perfect storm of corrupted feedback that goes undetected by the MD-01 that could lead to a bad azimuth situation; in our case we monitor antenna motion on a camera, so are able to mitigate that scenario with human intervention (and our custom control software looks for 'impossible speeds' as reported by the MD-01 positions that further helps mitigate that when the feedback goes awry).
Mechanically, they have done very well for us with the double-worm gear design, imperceptible gear slip (if any). I've heard of folks having mechanical failures, but I don't know the details, and that hasn't been my experience. As an example, at the height of 'performance' I was able to reliably track multiple S-Band downlinks with a 3m mesh dish antenna with no discernible negative effects due to point errors or 'jiggle' in the system (part of that is the ramp up, also part the counterweighting). In the 5 or so years we've been running them, we haven't had any major mechanical malfunctions (knock on wood), and part of that is probably due to periodic maintenance (re-greasing and such), which I've done at least once (can't remember exactly, maybe twice in that 5 year window). Part of that is also the fact that we tend to use the yagi stacks with a lighter load a lot more than the larger dish systems, I might be singing a different tune if the dishes were used daily.
The MD-01 controller (and MD-02s as well for non-rackmount variants) does have some nice ramp-up/ramp-down features, particularly useful for larger systems (like our 4.5m dish). I do wish the API documentation for the MD-01 command interface was a little better...but we've been able to work through it (for example, they changed the protocol a bit in a relatively recent firmware update that broke compatibility with our software and presumably things like rotctl in hamlib, but I think that has all been fixed/updated). The API issue might not be a problem for most folks not interested in writing their own software, or if you are using a different controller. The overall documentation situation is a little 'meh' in general as there have been a lot of versions and such released and they are on different websites run by different folks.....but at least there is documentation at all and a little reading and experimentation is usually enough to get what you need, and the basics such as th e wiring diagram hasn't really changed over the years.
I can't directly speak to the SatPC32 question because we don't use it. Other than that firmware update hiccup, I'd be willing to bet it works fine.
Bottom Line: When they work, they seem to work well, at least in my experience. If I were going to buy one today for personal use, I'd get one with the potentiometer feedback if the intended use is for something like a few Yagis and if I were planning a more permanent installation. If I were going to buy something for VT/work with a better funding source than my 'hobby budget' I'd probably look for something better, like M2 products (pretty significant jump in price point though).
I hope this helps. -Zach, KJ4QLP
-- Research Associate Aerospace & Ocean Systems Lab Ted & Karyn Hume Center for National Security & Technology Virginia Polytechnic Institute & State University Work Phone: 540-231-4174 Cell Phone: 540-808-6305
-----Original Message----- From: AMSAT-BB <[email protected]mailto:[email protected]> On Behalf Of Dave via AMSAT-BB Sent: Wednesday, June 3, 2020 1:20 PM To: AMSAT BB <[email protected]mailto:[email protected]> Subject: [amsat-bb] Alpha Spid Rotator
Is anyone using an Alpha Spid az/el rotor? If so how do you like it? Would you recommend it over the Yaesu's and are there any issues using it with SatPC32?
Thanks,
Dave N2OA _______________________________________________ Sent via [email protected]mailto:[email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb _______________________________________________ Sent via [email protected]mailto:[email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: https://www.amsat.org/mailman/listinfo/amsat-bb