Gears "breadboard" or how to prototype geared mechanics ? (building a clock)

[jimon]

I'm trying to build a prototype of clock with smooth sweeping hand (inspired by http://wiki.epfl.ch/polymac/documents/hpm-en.pdf).

Industry "standard" approach to this is just to tick second hand N times per second (8 or 16). And only Seiko Spring Drive provides true smooth sweeping hand (they do similar approach as in paper above)

So in result we get this :


On other hand Seiko Spring Drive :


My current plan is : MCU -> drivers -> DC/AC motor -> reduction gear -> flywheel (with RPM sensor) -> reduction gears -> second hand -> reduction gears -> minute hand -> reduction gears -> hour hand. Instead of main spring I want to use a motor and control it RPM with MCU (with RPM feedback of flywheel). As you can see there are a lot of reduction going on + flywheel, also not sure about DC or AC motor, I want to control RPM very very smoothly and precisely. I don't care about scale now, it's fine to do it as wall clock or even "big" wall clock Also I don't care about sound characteristics right now (motor and gears will be loud, and this is fine).

How to approach this ? I just want to build a prototype fast and check out results (how smooth second hand will be).
I see one way is to cut backplane with CNC and put bearings in it, get gears somewhere and just assemble everything. But it's not really agile way. Is there something similar to breadboard but for mechanics ?

[Alex Eisenhut]

fischertechnik?

[SeanB]

I have not seen anything like that, but a "smooth second hand" clock is about $2 in the Fonk kong shop.

They do eat AA cells though, lasting about a month per cell. I got annoyed with replacing the cell every month, so modified it with an old red LED from a PC case, a 1000uf 16V capacitor ( not bulged yet) off the power supply of said case, and a selected on test resistor ( 3k3 worked out fine) off the power supply board to drive the mechanism with 1V2 or so.

The first iteration used 560R and is fed with 12V from a UPS supply, but the voltage across the LED was then 1V9, and the clock gained time. Tried the 3k3 in series with the supply and got the 1V2, and now it keeps good time. The UPS is there to keep the cordless phone running when Eishkom does load shedding, so it only has to do 2 hours at most, but using 2 used 7Ah batteries it probably will do 2 days without power. The whole lot is in an old GRP box, and uses 2 old burglar alarm charger boards, one doing the battery float charging unmodified, and the other modified so it provides a 7V supply from the 723 regulator on board to power the phone base station, replacing the original 6V9 SMPS wall wart. Only issue is it now floats at -48VDC from the phone line, so do not touch the wiring to the clock.

[edpalmer42]

There are professional level 'Meccano' sets, but I don't know where I've seen them.  I do remember that they were very expensive.

Another part of that puzzle is "get gears from somewhere".  That part I can help with.  Check out http://www.wmberg.com/.  Amazing selection of gears, shafts, bearings, couplings, etc.

Ed

[langwadt]

LEGO

[jimon]

Fischertechnik - doesn't really have "generic" gears set
Meccano - not sure about gears there
LEGO - sounds interesting not sure about flywheel though

Another part of that puzzle is "get gears from somewhere".  That part I can help with.  Check out http://www.wmberg.com/.  Amazing selection of gears, shafts, bearings, couplings, etc.

They cost insane for prototyping, min order 200$, each gear is like 60-180$ (or I just haven't found cheap gears there ?)

I have not seen anything like that, but a "smooth second hand" clock is about $2 in the Fonk kong shop.

Chances are that it's "8 tick per second" clock. I reversed one of them :

And signals from coil :

[pmbrunelle]

I believe the standard approach is to use a synchronous AC motor for a smooth-moving clock.

[edpalmer42]

Another part of that puzzle is "get gears from somewhere".  That part I can help with.  Check out http://www.wmberg.com/.  Amazing selection of gears, shafts, bearings, couplings, etc.

They cost insane for prototyping, min order 200$, each gear is like 60-180$ (or I just haven't found cheap gears there ?)

To be honest, I didn't look at the prices since I don't have an account with them.  I'm just amazed at all the items that are available in quantities of ONE.   

Ed

[VK5RC]

Have you seen tamiya gear sets?  Reasonable quality plastic gear but a big range.

[miguelvp]

sketchup gears? have not use the plugin but I recall it does exist

So I don't know if it's a good modeling/simulating tool.

[rob77]

i can suggest Merkur - it's like Mechano ( a clone ? ) manufactured since the 20' in Czech republic (almost a century) - just choose the right set with many gears. and/or buy expansion sets with gears only. - the stuff is good enough in precision but the prices are "toy level"

http://www.merkurtoys.cz/en/

[amyk]

I believe the standard approach is to use a synchronous AC motor for a smooth-moving clock.

Those just "tick" at the much higher mains frequency.

If you want a really smooth clock, perhaps a homopolar motor might work...

[German_EE]

I saw a similar problem to yours a few years ago when I reviewed the mechanics on a photo-typesetter. Their solution was as follows:

1) The stepper motor driving the assembly was four-phase, similar to the stepper motors that you used to see in floppy disk drives with two common connections and four coil pins.

2) The four phases were driven using sine signals rather than square and each sinewave had (from memory) 1024 steps.

The end result was a stepper motor that had very fine control down to small fractions of a degree, close enough to typeset at 2560 dots per inch.

[rob77]

actually fine micro-stepping with a stepper would be a solution as German_EE suggested.

a cheap stepper is 200 steps per revolution => 1,8 degree , add a microstepping controller which does 32 microsteps per step and you'll end up with 6400 steps per revolution. it's slightly more than 100 steps per second when driving the hand directly... and of course you can gear it down even further.