What am I missing?

Discussion in 'Wireless & RF Design' started by thebruce, Apr 29, 2019.

  1. thebruce

    Thread Starter New Member

    Dec 10, 2016
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    Hello All

    Lurker post here. I spent a CPA/Planning career enabling my woodworking. With no EE/RF background, I got interested in RF wood drying after I retired. I decided to dig in and learn enough about the process to at least ask reasonable questions in the event I might figure out how to build/assemble an RF vacuum kiln for my personal use. I enjoyed the physics/math overview of parallel plate (near field) heating and the comparative topology papers that led me to a concentration on smps Class D half bridge MOSFET amplifiers as a potential solution. Along the way I identified a vendor (IXYS/DEI) who produced apparently serviceable reference designs, technical notes and evaluation boards replete with gate drive and power supplies.

    In edging closer to the goal, I engaged automated/manual network matching strategies/methods for the capacitive loads I was likely to encounter (think real and imaginary components) as well as the particularities of shielding/Faraday cages for ~m3 batches between roughly 3.5 m (¼ wavelength) x .75m (wide) x .4m (height/separation) aluminum electrode plates in vacuum chambers.

    Now for the first time, I am a bit confused as to what’s next. When I began reading on this there were a few companies offering evaluation boards but now it appears the industry is moving away from me and I can’t seem to find where they have landed. My board of choice is now listed as obsolete/end of life cycle. There are other indicators that, when combined, lead me to look for the more actively advanced 3-7 kW RF generators/systems. I don’t want to look backward for this endeavor.

    Anyone have any insight on the industry’s movement?

    And I apologize for the lengthy somewhat incohesive post.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    That sounds like a big diversion that will pull you away from actual woodworking. Have you looked around for a used kiln or maybe some of the key components that would help you put one together? I get the avoidance of spending money, but you have to choose your battles.

    A high-power RF project is a bad place to start for a beginner in electronics. I'm not saying you couldn't do it, just that there will be a lot of hurdles along the way. For instance it's hard to imagine success on an RF project without an oscilloscope in your tool set. Like any tool, you'll have to learn to use it properly. Are you going to want to build your own oscilloscope, too? Are you prepared to learn all the safety factors you'll need to address to meet local code for such a device?

    I'm not trying to discourage you at all, just offering another path to consider. The buy decision may be the cheaper and more efficient choice compared to build.
     
  3. thebruce

    Thread Starter New Member

    Dec 10, 2016
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    I appreciate your sentiment in general. Clearly I will seek professional design/build guidance. As I said, I needed to get the point of intelligent questions first. At this point I am wondering why littlefuse (sp?) would let this reference design and evaluation board deprecate. Presumably in favor of something better. Being still firmly in the throes of feasibility, when my first choice of evaluation boards goes dark it makes me question the direction of my thinking. Roughly the plan has been to avoid reinventing the wheel by assembling components. It is questionable to me if there is any money to be saved by this method. If you disagree, please make the case. We appear to agree on this.

    I first noticed a glut of used systems from etching and sputtering applications, which, I wrote off as efficiency obsolescence. My initial response was to evaluate a few of the available systems. I quickly concluded the soft costs required to set up new or old equipment dwarfed the savings (not to mention the ongoing cost of matching highly variable loads in the absense of automated matching). Such that it has become common in certain forest products industry (FPI) end use cases (early adopters) to keep the matching function in-house as it complements the proprietary nature of kiln schedules (and other FPI conventions) because each location becomes one-off for a variety of technical reasons (not necessarily electronic).

    Additionally, penetrating capital intensive industries as stodgy as FPI can only be accomplished by proving the technology in the field given that the capital cost of the equipment (read vacuum chamber, rolling stock, cost of funds in inventory and length if the inventory cycle (to name a few) quite literally dwarfs the RF equipment capital cost. While I am convinced such improvements are the way of the future on the grounds of efficiency alone, today's adoptions might be forced to the fore by the need to reduce carbon footprint or improve phytosanitation.

    I readily admit I looked at building the boards, etc when I first started. Unfortunately, even though I’ve had trouble finding available professional help, I could not make it make sense given the safety protocols alone. However that does not stop me from striving to advance my knowledge. I’ve been reading/studying on this for over two years now and I wonder what I am missing.
     
  4. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    You are missing the idea that wet wood makes a poor dielectric. It might sound good in theory and even work to some degree for certain instances.

    But I do believe it would be difficult to control even heating thru out the stack.

    Perhaps a multi/sweeping-frequency strategy might help. Perhaps research could find a even heating frequency.

    If you don't want to stop, go small and prototype. Maybe a large microwave oven size.

    Frequent monitoring/measurement of the dielectric of workpiece might be necessary for heating rate control.

    Perhaps even a grid of plates, to see if dielectric changes occur differently thru-out work piece.

    Many things to check out before any serious money.
     
  5. thebruce

    Thread Starter New Member

    Dec 10, 2016
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  6. wayneh

    Expert

    Sep 9, 2010
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    I’m no expert in the field but what little reading I saw, there’s no question of the efficacy of the technology. The TS wants to build one but is struggling to make a plan and get started on it. He’s not asking for a critique of the technology.
     
  7. thebruce

    Thread Starter New Member

    Dec 10, 2016
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    Thanks BR-549,
    Good suggestions all!
    Actually there has been a lot of research money poured into this...most of which I covered with an exhaustive literature survey.
    The idea has been under slow but active development since the late 1940's.
    Yes, wood is a highly variable dielectric however controlling the vapor barrier and other boundary conditions has been quite successfully proven...as well as multiple plates and sweeping frequencies.
    Generally speaking lower frequencies penetrate better and multiple plates have the tendency to increase arcing/burning much above the 10kW per cubic meter power level. Naturally certain species also tend to "blemish" or crater more than others. All of your suggestions being part of the reasoning behind reduction of pressure to reduce the vapor point (boiling point) of entrained moisture. NO wood really likes that much heat.
    In truth a cubic meter is a pretty small prototype being that 25 m3 chamber with 50kW would be a common size for commercial units.
    I'm thinking much more modestly and moving carefully.
    It could be that the system I selected is deprecating for business rather than technical reasons...which wold make more sense to me and restore my confidence a bit.
    Thanks Again
     
  8. thebruce

    Thread Starter New Member

    Dec 10, 2016
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    Quite right Wayneh...I simply wouldn't have been so blunt. But it does cut to the chase.
    I'm wondering now if littlefuse has business issues. Things are slowing down a bit and it could just be a leading indicator.
    My real fear is that I remain out of the electronics industry loop and I am missing some great technical advance right before I pull the trigger on my prototype. I want to prototype the right thing...one time.
     
  9. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    I can understand the enormous draw for such a process. And with that understanding.....I would venture to guess the tech is not proven yet.

    I back that up with the suspicion that the lumber industry spends a lot of time and labor for drying in kilns.

    And it seems very competitive.......so any proven tech to reduce cost at a decent rate should be implemented quickly.

    A problem that will be richly rewarded. An operational reliable device won't be easy or simple.

    A modulating mechanical or acoustic inducement might help the process.

    Has all that research provided any "signal" preference? One would need somewhere to start.
     
  10. Yaakov

    Well-Known Member

    Jan 27, 2019
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    Am I right in my surmise that the potential attraction of RF heating is the ability to heat the center of the lumber rather than the surface, speeding up the process by avoiding the constant recreation of a barrier of dirty wood on the outside?

    It would seem that unless the method either speeds up the process substantially, or improves the outcomes (or some combination of both), more ordinary methods of heat and air circulation would simply make more business sense. Big RF amps are costly compared to big resistors.
     
  11. thebruce

    Thread Starter New Member

    Dec 10, 2016
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    ding, ding, ding
    Quite right again BR 549!
    Broadly true (in my opinion):
    - Essentially no wood is ready for use until it has been "conditioned" (aka...dried).
    - wood drying consumes in the neighborhood of 12% of a large industry (MANY billions annually).
    - fueled by scrap/by-product burning - deeply entrenched, capital intensive, long term assets would likely replace through attrition.
    - technical complexity is a major deterrent
    - trust/belief is a major problem (like: "Daddy didn't do that - it was good enough for him."
    - nothing happens quickly...but if you have an environmental bone in your body this could reduce carbon footprint drastically.

    The only mechanical inducement I have planned at this point is the 1/4 wavelength dimension to enable matching so I may not understand that question. HAven't really gotten my confidence up to waveguide design just yet.

    I've been studying mostly 13.56 ISM if that's what you mean by signal. Lingo illiterate here.

    Thanks for the reply. I've been hungry for discussion on this for a while.
     
  12. thebruce

    Thread Starter New Member

    Dec 10, 2016
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    Sorry for the delay...suddenly seems busy this morning.
    Yes Yaakov...volumetric heating/drying is the holy grail. rapid expansion/contraction cause internal stresses which result in defects (structural, aesthetic and dimensional).
    Drying is seen as a multi-step process...depending on species, time of/since harvest, intended use, etc.
    Many of the early attempts to prove applications wee specific to use (meaning that higher dollar uses were being proven first because defect reduction makes new methods proof more clear/obvious). There is research data going back generations.
    Please explain your reference to resistors.
    I am convinced from my experience/reading (I am 4th generation FPI) that the traditional haste makes waste methods can be overcome if we attribute part of the time savings back into process improvement. This means if we slow down a little bit we can recapture a LOT of what is considered acceptable process waste - from front to back.
    I'm also pretty sure existing research has been proven for many species given our changing ecological priorities,
    Thanks for your reply.
     
  13. Yaakov

    Well-Known Member

    Jan 27, 2019
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    Electric heaters are simply big resistors and are many orders of magnitude simpler than high powered radio frequency sources.

    While using waste wood to fire kilns is convenient for the industry, even if there was a move to electrically powered kilns, I would expect it to take the form of resistive heating elements. The downside of this being the heat is applied from the outside, which, of course makes drying the inside through the now dry outside problematic. It must require quite careful control to prevent warping and winding in dimensioned lumber being dried this way.

    Of course, even after drying, wood intended to become furniture should live in the house for several months so it becomes normalized to the conditions indoors. On the other hand, who has space for that?
     
  14. thebruce

    Thread Starter New Member

    Dec 10, 2016
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    I see...thanks for that. Since the early 20th century, the use of dry steam to heat and move moisture mass through wood fibre has been essentially perfected given accepted limitations. Clearly, electrical heating coils are more heavily used as time passes due to convenience and cost, not excluding that wood fired boilers now have a pretty heavy environmental regulatory overhead. I probably don't have to run on about the cost of steam/pressure installations, refractory costs, or centrailzation of physical plants (and the cost of relocation).

    However if we are talking carbon footprint, what we would really be talking about is the rate of release as many of the by-products end up landfill...the volume of waste products generated from the forest floor to the output of the sawmill is difficult to comprehend. I'm sure there are published data on this that one could use the develop metrics on biomass percentage of standing forest products to dimension lumber. I'm just too lazy to route that out given that my practical experience has taught me that without by-products action no FPI plan is complete from a cost of net profit standpoint. Important point being that if you attempt to landfill biomass it commonly ends up burning any way. I can't tell you how many perma fires I know of going around FPI installations. Pine bark being one of the most famous sources...very low btu content...long term fires are common primarily because they are nearly impossible to put out.

    So...getting back to my original question...no big recent breakthroughs or regulatory changes that might cause me to change directions? The little I have read on FCC compliance didn't really leave me with warm and fuzzy confidence. Regulation by complaint can be a cruel master, especially for a DIY guy like myself because nearly everything meaningful is multidisciplinary these days.

    And while we're talking about this I have a long list of questions I would like to air off-list if anyone is interested. I've been going at this alone since I signed up for this forum. I'm looking at trying to prototype this season. In my world that means beginning with a tight faraday enclosure in an effort to proceed safely.

    replies appreciated...solutions revered
     
  15. Yaakov

    Well-Known Member

    Jan 27, 2019
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    In general, producing high powered radio energy is heavily regulated as you have found. That said, there are applications that are permitted with bands allocated to them. Even in those cases, you can't radiate enough energy to be detected very far from where you are using it—and this is not trivial. You need not only your vacuum but a Faraday cage as well to contain the radiated energy.

    If you were looking to produce a commercial product, it would require FCC type acceptance, which includes destructive testing and so it very expensive. To build one for yourself, you would have to ensure that you didn't interfere with any licensed service. The potential fines are prodigious, and there is no defense to be mounted against it if they choose to fine you.

    As you've probably already determined. your device will be an intentional radiator. This means you will have to be very careful not to exceed the limits for radiated energy. The FCC doesn't care how much you generate that is contained but they are very concerned about what you don't contain. Have you read this part of the regulations?

    upload_2019-4-30_12-38-50.png
     
  16. thebruce

    Thread Starter New Member

    Dec 10, 2016
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    Thanks for that Yaakov. I am aware of this generally but not that particular matrix.

    Generally, this has been a feasibility study up until now. My budgetary pIan was based on made skin depth calculations for aluminum and a welded 16g aluminum sheet metal total enclosure employing the required grounding recommendations. I realize this is was truly a country boy approach however given that my projected field strength will not be a lot greater than a commercial microwave I was fairly comfortable I wasn't too far off for budget assessment.
    My plan was to dry in an enclosed trailer (aluminum toy hauler). The trailer skin would also function as a secondary line of defense with some minor mods. Electronics would be located in the trailer nose and on the wall of the interior shielding cage so cables etc would be behind 2 levels of shielding. I cannot claim a working knowledge of the harmonic spectrum I will likely encounter but tight is tight. I have seen drawings and details of sheet laps and openings I am confident I can fabricate. There is a lot of information out there from the FCC...they want you to get it right. I believe it was the FCC who published a handbook on RF safety that required a safety mechanism (override) on the entrance doors to the chamber that positively prevent inadvertent activation.
    I did go so far as to calculate that I would be able to cook a pig in this thing in amazing time...something on the order of 30 minutes by temperature ramp projections. That was enough to get my attention.
    I'll put in a few minutes looking for the link that FCC publication to share with the group if anyone is interested.

    thebruce
     
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