Hi Jeff, all
Sorry for the delay in replying, I ate a bunch of nuts a week ago Sunday and got a case of diverticulitis. Note to self, if your going to eat a bunch of nuts watching the Bears loose a football game, follow it with a bunch of roughage.
I know I have written sort of a history of Servodrive, but I can’t find one to copy / paste.
I can’t promise I am 100% yet but I will explain the mechanics and history.
I wanted to make loudspeakers since I was a kid and had tried it a number of times.
http://www.livesoundint.com/archives/2003/april/water.phpIn spite of a somewhat Blues Brothers like outlook on “The band, man”, I had given up on the band and my dream of making loudspeakers for a living and had resigned myself to a life going back to electronic repairs with audio as a hobby.
At one of these jobs in 1979, a co-worker told me about a position at Intersonics and the idea of making experimental space hardware sounded a lot better than fixing the mechanical Klienschmidt Teletype machines and electronic paper punch / readers I was servicing.
Intersonics turned out to have 6 people at that time, so it wasn’t a big company.
Also, the premise, that high intensity sound could position a sample inside a high temperature furnace while in zero G, had yet to be proven.
My first task was to build a control panel for a sounding rocket experiment, yet I was fascinated with these weird “St. Clair” sound sources. They had a water-cooled stationary voice coil (inductively coupled to an aluminum bar resonator) and water-cooled field coils. Perhaps most boggling was a 400lb, 2000 Watt CML amplifier that used 12 little transmitting tubes and sounded like a turbine when you powered it up.
The president of the company, Roy Whymark, was an English Acoustician from WWII, he had worked at Mullard Labs and his team also developed a sonar transducer that was important (some BQS something or other). He had a fiery temper that made one think carefully before speaking, a very sharp but open mind and lots of funny stories from his past.
Anyway, he liked hifi and we slowly hit it off. I eventually was able to by pass the irritating “protection” in his Quad ESS speakers and that helped a bunch too.
Being self taught and having Roy and Dr. Rey (who was a physicist who taught at Notre Dame and worked at Fermi labs) to ask questions was wonderful, I would wave my arms and scribble, they would explain the physics I was talking about.
They allowed me to try new things in addition to electronics and eventually I wore many hats.
Perhaps my proudest moment there was getting my first patent or some years later when it dawned on me that I was building space flight hardware, was Principal investigator on an SBIR contract, was considering astronaut training and was directing a couple of Scientist’s and engineer’s efforts.
At that point I stopped feeling bad about my 5 years in high school and understood what Roy’s encouraging words meant about “anyone can have a good idea”.
He was the first successful person I had met who looked only at results and didn’t think that much about academia.
Anyway, I came up with a practical new levitation source and built hardware around it. On the way home from the numerous flights to MSFC Huntsville, I was paging through a surplus catalogue looking at DC servomotors.
I asked Charlie (Dr. Rey) about what a 10ms mechanical time constant meant for a motor.
The explanation sounded to me like they could respond fast enough to produce sound AND I remembered seeing two of the same motors at Harrison Supply, a local junk and surplus store my Dad started taking me to when I was little.
I fooled around with the motor, made sort of a crank throw to drive a flat foam disk and it actually worked.
After blowing up my receiver at home, I brought prototype number three into Intersonics and demonstrated it to Roy and Charlie.
That same day, Roy said if I wanted to pursue it there casually, I could as long as it didn’t interfere with my job and only cost floor space and lights. Later, when it looked like it might fly as a real product, he allowed me to start the Speaker division.
The Servodrive patent, yes one of the prototypes was a square cube as shown.
http://www.google.com/patents?id=hsw1AAAAEBAJ&pg=PP1& ;lpg=PP1&dq=%22commutated+voice+coil%22+danley&sourc e=bl&ots=gPFxym9kiM&sig=pw_oQCP-V7LMR9eiCVaF5HAip6A& amp;hl=en&sa=X&oi=book_result&resnum=3&ct=re sult#PPA1,M1
Motor description and picture;
http://www.pacsci.com/support/low_inertia_pmdc/lowinertiaove rview.html
http://www.pacsci.com/products/low_inertia_pmdc/index.htmlIn the 17 years following that we had proven container-less processing works we added material sciences division and flew several sounding rocket and two shuttle flight payloads. The company grew to about 45 people at its peak, and the Servodrive speaker division grew as well.
It was truly a sad day when the Shuttle blew up, we were all in Arlene’s office watching it; it took some time before anyone was able to speak.
With the space station work on hold, shuttle flights on hold indefinitely, came the beginning of the end for Intersonics and so to the unique atmosphere that brought about the Servodrive speakers.
I think Greg Bottimer summed his experience up a year ago at an AES meeting when he said “The best job you’ve ever had isn’t supposed to be your first job”.
Howzat work?
Was probably the most frequent question asked at trade shows.
A Voice coil is a DC motor, It has a limited linear motion capacity however.
Also an indicator of a motors strength is a figure of merit found by BL^2 / Rdc or BL/Sq root of Rdc which gives Newton’s of force per Watt of dissipation.
The polarity of the current governs which direction the force is produced.
It seemed clear to me, that while a long VC allowed more excursion, more of the wire was outside the gap and only functioned as a heater and series inductor.
The idea I had was to “commutate” the voice coil so that power only flowered through the coil where the magnetic field was. At the time is was on that airplane flight thinking about servomotors, I had no way to get speaker parts modified to try this.
Later, I did get a chance to try it and it does work.
Commutated Voice coil, the idea that actually lead to the Servodrive woofer.
http://www.google.com/patents?id=NTc-AAAAEBAJ&printsec=a bstract&zoom=4&dq=thomas+danley#PPA1,M1
Once you understand what this does, the Rotary DC servomotor is a mechanical arrangement, which allows a recirculation of the conductor wires, resulting in an actuator that can turn either direction indefinitely without changes in operation, while confining the current to the region where the work is done.
From the first prototype it was “simply” a matter of figuring out how to do something with it (which took several years).
Eventually the Servodrives were refined and in spite of a not having full support internally at Intersonics, they became fairly popular on large tours or for special effects.
Part of this improvement came from a better understanding and ability to model things. At one point I had a mathematician named Dan Riordan who was assigned to help me with acoustic modeling and I challenged him to write a MathCAD program that could model an arbitrary acoustic passage with a thermal gradient. Once that was done (no small task) we could sort of model what happened in the furnaces.
Of course, one could kill the thermal gradient part altogether, substitute a real horn geometry and driver and viola, a predicted frequency response on a real horn.
Being able to model a horn’s response before building was HUGE I thought.
At an AES convention I met and became friends with Dave Martin and Richard Long, a couple of speaker enthusiasts. Dave had his company in England while Richard made giant disco horns in NY. They were fascinated with the Servodrive system and I was as interested in their approaches to Horn design. They were as different as night and day, both had strong ideas about what was needed and Richard was one of those guys who had a calculator in his head. Anyway, sadly Dave and Richard are both gone now but if I were going to thank anyone for helping me see deeper into the horn loaded subwoofer, it was these guys and Dan’s programming.
I concluded that as opposed to lore of the day, the proper driver usually was pretty different from the norm.
One needed a heavy strong driver with a compression ratio, not a lightweight, slightly loaded system as was normal then. Nowadays most bass horns use that heavy strong driver approach.
The first “big” Servodrive sale thus far that was a huge boost for the company was in conjunction with Clair Bros for one of the early Michael Jackson Tours. The Servo’s proved to be much more reliable than the dual 18”s they were using while producing more sound with fewer boxes.
They were eventually used on a bunch of big tours, like U-2, Def Leopard, Garth Brooks etc, and hundreds were used at several Disney venues.
Intersonics eventually started getting a lot of oddball acoustics jobs too.
We built servodrive based sonic boom simulators for BBN and a larger valve speaker as well for the NASA space plane research.
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/1994002 8995_1994028995.pdf
Even a weird measurement trip with a TEF machine;
http://www.livesoundint.com/archives/2000/julyaug/pyramid/py ramid.php
Which was a result of this movie showing acoustic levitation back then.
http://www.youtube.com/watch?v=bYCWIg30Vw8Part of what made the Servodrives work was its motor, it lent itself to forced air cooling and it was possible to nearly eliminate power compression.
While most all of the Servodrives past the TPL-2 had this, it was also possible to apply this to a conventional driver with similar results.
http://www.google.com/patents?id=VDU8AAAAEBAJ&dq=thomas+ j+danley
Anyway, the thing with the motor was that it was developed as a computer tape drive capstan. It was an exotic motor that could accelerate from standing still to thousands of RPM in a few degrees of rotation.
Having an extraordinarily low series L, this meant that when presented with a clipped signal, the force produced followed the current while its velocity followed Voltage.
This proved to be hard on the mechanical parts and it became necessary to add a series L to decouple high frequencies. Keep in mind that the motors went up to 4-5KHz on their own.
In the case of the SDL-5, there was an additional issue in that a secondary belt was used to provide a better mechanical advantage. The belt supplier had been switching over to a new yellowish “wonder fiber” and at some point quietly switched over to the Kevlar based core material.
This might have been great for something’s but compared to the original polyester based cores these had a problem with stripping off the teeth when the amp clipped too much.
This was a heartbreaker for some in the field but just as much for the speaker guys at Intersonics who faced a lot of scrutiny and where we scrambled to figure out what made them die.
The solution was found in going back to a one step drive and using a simpler lighter “snail hell” cabinet like the older TPL-3. This was the BT-7, the most powerful and successful of the Servodrives.
I mentioned the motor was developed as a capstan direct drive motor, as this primary application ‘went away”, the motors became much more expensive. They also had four big bars of alnico magnet weighing pounds (as in pounds of $$) which held no “gosh” factor.
In the 20 odd years it took to be able to make conventional drivers with similar parameters, the Servodrive motors became ridiculously expensive.
One can get a feel where the BT-7 driver is when you consider that the LAB sub cabinet, having the same exterior size and low cutoff, has a very similar horn.
The lab Sub 12’s (heavy, strong 12” VC drivers) are suitable for that horn while the Servodrive was enough more that 2X15’s were about right, given its strength.
Anyway, at this point I have not hung up the idea of making a motor driven sub again; I simply don’t have a design problem at the moment that can be solved this way.
Also, radiators have there own problems, and it was possible to use a rotary system for that too. Actually there are several driver approaches we might try in time but it is much easier / logical to find ways to use conventional components in ways that stop some of the existing issues before tooling up drivers there are no stock parts for.
I have to confess I do have a thing for rotary systems as they follow a different set of rules so far as the distribution of mass.
It is highly likely you will see an unspecified “rotary’ driver from us in time.
For those who haven’t nodded or have reached terminal boredom, there are some “olden day” pictures of some Intersonics space stuff at our Web site and some of the stuff I worked on.
http://www.danleysoundlabs.com/beyond%20danley.html http://www.google.com/patents?lr=&q=%22thomas+j+danley%2 2&sa=N&start=10
John Haliburton (Intersonics alumni) has a picture of the first sonic boom Valve speaker prototype posted. The system was eventually shown on “Beyond 2000” as the “speakers from hell”
http://www.flickr.com/photos/15041669@N00/428896499/Well that ended up running long, but I left out a bunch of stuff, if that helps.
Funny thinking back, I did end up making speakers after all, well you never know I guess.
Best,
Tom Danley
Iain i can't think of what Germans or prior art you refer to.