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Adult supervision and advice needed for my hardware design — Parallax Forums

Adult supervision and advice needed for my hardware design

w4fejw4fej Posts: 264
edited 2014-07-14 08:32 in General Discussion
Here is the situation.

I have another ham radio mobile HF (high Frequency) antenna that I have built and used for many years. (This will be version 3.0) The antenna is remotely tuned using a gear head motor (Parallax 28819) with built in encoder. (I want the encoder so I can repeat the tuned position of the antenna and recall that position from the Prop and return to that position later) The antenna is mounted in the right rear corner of my pickup. The motor will be controlled by an HB-25 motor controller (29144) driven by a Prop board in the cab.

My question is this.. The antenna radiates in the area of 100 watts PEP and the motor and encoder are actually INSIDE the antenna. (The antenna is a 2" diameter aluminum tube). The controller board will be inside the cab some 10-15 cable feet away. I know I can run the motor power wires from the HB-25 without issue but that still leaves the encoder outputs needing to SAFELY get back to the Prop board 10-15 feet away.

I have several Prop boards and I'm not necessarily opposed to mounting a second board and the HB-25 near the antenna and communicating between the two boards. (Xbee maybe? Would the Xbees hate the RF environment?)

I'm looking for some ideas for a way to have my cake and eat it too.

How would the adults handle this situation??

Mike B.

Comments

  • PropGuy2PropGuy2 Posts: 360
    edited 2014-07-06 19:19
    Also a Ham, so here is my take on the circuit. Anytime the RF is induced into the Prop signal lines you have a problem right off. Take a look at some of the "screwdriver" mobile antennas. I know some of the hams that worked on these for quite awhile before they got the antenna reliable enough for an actual product. For all that work, I would buy a MFJ antenna matching box with a SWR bridge, and be done with it
  • w4fejw4fej Posts: 264
    edited 2014-07-07 03:36
    PropGuy2 wrote: »
    Also a Ham, so here is my take on the circuit. Anytime the RF is induced into the Prop signal lines you have a problem right off. Take a look at some of the "screwdriver" mobile antennas. I know some of the hams that worked on these for quite awhile before they got the antenna reliable enough for an actual product. For all that work, I would buy a MFJ antenna matching box with a SWR bridge, and be done with it

    Ahhhh, this is a Screwdriver antenna, my 3rd, actually my fourth. and they all work just fine, better than any other design out there for mobile use. I am building this one to overcome a mechanical issue. (Tree vs antenna !!). I have designed and built (breadboard so far) a 4 radio audio control system using a touch screen and decided to also incorporate the screwdriver control into the same box..

    Thanks anyway. 73 - Mike W4FEJ (Extra class, ex K7FQ, W6FFC, WA6URR, WV6URR)
  • kwinnkwinn Posts: 8,697
    edited 2014-07-07 06:19
    You could try using a one or two optoisolator circuit to isolate the RF from the encoder signals, and if that is not enough you can always go to a fiber optic connection.
  • Sir GawainSir Gawain Posts: 32
    edited 2014-07-07 23:36
    I wonder using a boosted infrared --> a super power remote control.
    That is if you are using it to tune, etc...not for audio bandwidth.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2014-07-08 00:33
    Well, you have already gone through several redesigns, so I guess you understand that everything is speculative until you actually build and test your idea. Trial and error are just part of the perfection process.

    100Watts of RF the antenna is quite a bit to handle when transmitting. The higher you go, the more measures you have to take to prevent the RF from interferring with the design.

    I think that fiber optic is an excellent way to go, but I am not sure how easy it is to get RS232/RS422 to fiber optic modules. You would have to have two micro-controllers -- one at each end. Maybe a tiny one at the antenna end.

    Boosted IR? I suspect that IR remote control works best indoors with less ambient IR background to deal with. There may be some IR remote receivers that are taylored for better blocking of ambient IR, but getting the right one might prove a challenge.

    I am sure you know that long communication and power wires just become transmitting and receiving antennas. While the antenna itself uses coax to shield, all the wires under the hood are working against you.

    Digikey has fiber optic products, but they are not cheap. I am not sure anyone here deals with such hostile RF shielding.

    It just might be easier to use a stepper motor than a conventional geared motor and encoder around all that RF.

    RS422 and twisted pair in a shielded cable might be a simpler way to deal with the RF.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2014-07-08 09:30
    I did some additional poking around.

    I don't think fiber optic is cost effective. Digikey has devices that are $40USD and up. There is a wonderful store out of Hong Kong www.fiberstore.com and they seem geared toward much more serious data transmission. The cheapest you might get a pair of RS485 modems for is about $100USD.

    All this seems to point at RS485 with shielded twisted pair being the most likely sane choice. All the parts might cost you $10 USD, and there are even optically isolated RS485 devices it that is a real need.

    I also wonder how your are going to power a microcontroller inside the antenna. It might be wise to use a battery to fully isolate the supply from RFI creeping in on the wires providing power.
  • w4fejw4fej Posts: 264
    edited 2014-07-12 09:32
    "I also wonder how your are going to power a microcontroller inside the antenna"

    The Prop board won't be inside the antenna but rather in the cab of the truck. The Parallax motor/encoder will be inside the tube.

    Look like 485 might be the way to go...
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2014-07-12 10:58
    RS485 certainly will allow an inexpensive trial. And the transceivers do come is such low power that a small battery inside the antenna's housing would eliminate that potential noise source. But the rotary encoder is going to have to be converted to serial to work with an RS485 set-up. Thus I think a microcontroller doing the conversion is going to have to go inside the antenna housing... unless you have a rotary encoder that is already outputting a serial digital for you.
  • w4fejw4fej Posts: 264
    edited 2014-07-13 11:53
    Loopy Byteloose wrote: »
    RS485 certainly will allow an inexpensive trial. And the transceivers do come is such low power that a small battery inside the antenna's housing would eliminate that potential noise source. But the rotary encoder is going to have to be converted to serial to work with an RS485 set-up. Thus I think a microcontroller doing the conversion is going to have to go inside the antenna housing... unless you have a rotary encoder that is already outputting a serial digital for you.

    I have mocked up the motor (28819) on my Prop bread board and ran the quad decoder from the OBEX and it does exactly what I want unmodified. I assume the encoder is from US Digital and they recommend this device as a standard receiver for their encoders: 26C32.

    I have been looking on Digikey for some 3 twisted pair, 16-18 gauge shielded wire so I can mock it up and expose the whole shooting match to the RF it will have to live in..

    Mike B.
  • pmrobertpmrobert Posts: 673
    edited 2014-07-13 12:11
    Check your Home Depot, Lowes or whatever for plenum rated intercom wire. HD had it a little more than a year ago. I use exactly what you described for engine ECU purposes and it has worked very well and at a good price. It's on their giant vertical conveyor belt deal that has a bunch of different bulk cabling, wire and such.
  • w4fejw4fej Posts: 264
    edited 2014-07-13 12:23
    pmrobert wrote: »
    Check your Home Depot, Lowes or whatever for plenum rated intercom wire. HD had it a little more than a year ago. I use exactly what you described for engine ECU purposes and it has worked very well and at a good price. It's on their giant vertical conveyor belt deal that has a bunch of different bulk cabling, wire and such.

    Thanks for the heads up, I'll check with them....

    Mike B.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2014-07-14 08:32
    I actually did some thinking and investigation of the quadrature signals provided by the motor.
    It seems you just might use two RS422 (Not RS485) lines and two twisted pairs directly connected to their outputs... thus eliminating the need for a microprocessor inside the antenna housing doing a serial conversion.

    After all, if you really think about the quadrature signals, they too are serial.

    If this works well, it is just that much less complicated a build. And I think a tiny battery pack inside the antenna housing would power the two RS485 transmitters for days or weeks...at as little as 30ma operating load. These units can be very low power. Some offer excellent EMI protection features. So it would be a big win for you.

    Some details.. RS485 is half-duplex transcievers. RS422 uses driver and reciever devices.
    You can get a Dual RS422 driver for inside the antenna and a dual RS422 receiver for at the Propeller.

    TI has all sort of configurations -- single, dual, and quad. 5V but no 3.3V http://www.ti.com/lsds/ti/interface/rs-422-products.page
    Maxim is another source. with isolated, ESD protected, and low-power choices. http://para.maximintegrated.com/search.mvp?fam=rs485&299=1000&tree=protect

    RS485 Transcievers can be hard-wired to operate at RS422 drivers or RS422 receivers if you desire certain attributes found in not other package.

    Some come is DIP and others in SMDs.
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