Electromagnetic pendulum clock motor

[DickB]

This is a clock motor for an upcoming clock project, a variation on my Naked Clock. I'm not the first to do this, but I did need to test the concept of making parts with the CarveWright and get started on my new clock design. This is a test mule.

In most clocks with pendulums, the pendulum regulates the clock but does not drive the mechanism. Electromagnetic pendulums do both. Since the pendulum is driving the clock, the usual escapement mechanism is replaced by a pawl and ratchet wheel or the like. The pendulum has a small rare earth magnet at the bottom and a fixed coil in the base, typically hidden. As the pendulum swings toward the coil, the magnet induces a voltage, first a negative pulse and then, as it moves away from center, a positive one. An electronic circuit detects the positive voltage and then applies current into the coil, producing a repelling magnetic force. This keeps the pendulum moving.

There are several two-transistor drive circuits floating around, but they are generally finicky. I came up with one using op amps that triggers reliably and allows the drive pulse width to be tuned. The LED is just for show. I have intentionally left the ratchet wheel and parts a little rough, and applied some friction to the wheel, to make sure that the pendulum is capable of driving a clock mechanism even if it is not perfectly tuned. I'm running the pendulum from a 3 volt power supply, but intend to use a couple of D cells or the like in the finished clock.


It's important to note that the period of the pendulum is set by the length and mass of the pendulum, not the electronic circuit. So the final pendulum will have an adjusting wheel or the like. However, I plan to experiment with a microcontroller circuit to see if I can "influence" the pendulum enough to keep perfect time. This pendulum's period is one second, so the 60-tooth ratchet wheel turns once per minute. It could be used to drive a second hand, and basically replace the 1 RPM timing motor in my Naked Clock.


http://youtu.be/URD0mpSv3eQ (http://youtu.be/URD0mpSv3eQ)

[Digitalwoodshop]

VERY Impressive.... And the Clock too.... The O'Scope.... Sorry.... I was a Test Equipment Calibrator at Sony for 2 years... I have a fondness for Test Equipment.... Excellent Clock !!!!!

AL

[dbfletcher]

Really cool! I love this kinda stuff. I just started playing with the Arduino platform earlier this year. I really wish I just had more time to play with all the stuff that interests me.

[b.sumner47]

You lost me on this one. I'll wait for the results.


Capt Barry

[Jeff_Birt]

Very neat stuff. I like that fact that it has some hi-tech juxtaposed with low-tech.

[gapdev]

Sign me up!

Kenny

[DickB]

I've done some more research and made some improvements to this clock motor. The biggest change was to add a ramp drive mechanism like those of John Pickron. Under the original setup, the pendulum directly drove the pawl. The problem is that the span of pawl travel would vary with the length of swing arc of the pendulum. As batteries wear down, for example, the pendulum arc length gets less, and the pawl could miss the next ratchet tooth. This could be compensated by more complex drive electronics. But the mechanical solution is IMHO more interesting to see operate. Adding a ramp with a circular curve towards the end of travel limits the length of pawl travel. With simple adjusting mechanism, this length can be easily tuned to the distance between teeth on the ratchet wheel when the clock is built.

I also put more aggressive teeth on the ratchet to prevent the pawl from slipping out. With this setup, the motor is working very reliably. You can see I am also starting to convert the simple, even ugly parts into more elegant forms that will be used in the finished clock.

Next step is to fine-tune the length of the pendulum, make it more elegant, hide the magnet, and provide for weight (and therefore period) adjustment.


http://youtu.be/YAx1ixQPEOU

[DickB]

You lost me on this one. I'll wait for the results.


Capt Barry
Does this help?

49115


http://youtu.be/1tBzq-7MUJU

[b.sumner47]

Really beautiful. Time is right to the point.

Capt Barry

[Digitalwoodshop]

Excellent Project !!!!

AL

[DickB]

Designed and cut this clutch assembly, to allow the time to be set by moving the minute hand clockwise. A light spring or a weighted lever should hold the clutch while operating.

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[DickB]

Well, I had one of those d'uh moments about 10 seconds after I installed that clutch. I made it with 12 teeth for no reason except the size of the thing seemed right, then realized that with it I could only set the time in 5-minute intervals. Back to the drawing board. The new clutch works well. I had intended to set the time by manually moving the minute hand, but it's easier to grab the driven part of the clutch and turn it like a knob. I may make this more knob-like in the future.

I tweaked the electronics and got the pulse width down sufficiently that I believe it will run for a year on D cells. I only have AA cells and battery holder, so am using them for now. The clock makes a little more noise than I would like. I already made one small modification to quiet it and I have a couple more ideas. I'm running tests to ensure that the pendulum length is sufficient to be able to make the proper speed adjustments. The lower rod will be replaced with brass threaded rod and a couple knurled nuts to facilitate this. Other than that, this prototype is close to finished.

I have to say myself it is cool to see this running with no visible means of motivation. The owner of a consignment shop I sell in came over, and I showed him the clock - he had not seen or heard of it before. He really liked it. I asked him if he knew what kept it going, and he said that he had no idea.

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[SharonB]

Dick... Wooden gear clocks have always fascinated me and I have enjoyed following your progress as you develop such an interesting clock.

[Jeff_Birt]

Very cool! One of the neatest things I have seen done on the CarveWright!

[DickB]

Not much of a mystery if you've been following this thread. I switched to a better and quieter pawl drive mechanism. The clock is running within one minute per day, and with the pendulum weight adjusting mechanism I can now easily tweak that for better accuracy. I will need to run the clock for a few weeks to better determine accuracy. I did make one or two small improvements making it a bit easier to build, which I'm testing out on my second clock build underway now, but other than that the design is finalized. I believe the design is quite robust and forgiving for beginners, because the pendulum drive appears to have enough power to overcome less-than-perfect parts and assembly.

49606


http://youtu.be/0hvj574hrd4

[gregsolano]

I am really looking forward to building this clock someday! Great Job.

[SharonB]

Dick... does this mean that the project will some day show up in the pattern store?

[DickB]

Dick... does this mean that the project will some day show up in the pattern store?
I'm headed in that direction.

I'm also working on supplying a kit of hardware parts. Although the circuit board is not that difficult to build for an electronics hobbyist, many CarveWright owners are not familiar with electronics. So I think it would be wise to have a kit with a built and tested circuit board, battery holder, and coil available. Likewise, a number of specialty parts - miniature ball bearings, threaded brass rod, brass knurled nuts, rare earth magnet, etc. - are needed. These can all be obtained through internet suppliers, but they are somewhat expensive in small quantities due to shipping. So again, having a kit of these parts available from a single source would make it more convenient and less expensive to build this project.

[dltccf]

I'm headed in that direction.

I'm also working on supplying a kit of hardware parts. Although the circuit board is not that difficult to build for an electronics hobbyist, many CarveWright owners are not familiar with electronics. So I think it would be wise to have a kit with a built and tested circuit board, battery holder, and coil available. Likewise, a number of specialty parts - miniature ball bearings, threaded brass rod, brass knurled nuts, rare earth magnet, etc. - are needed. These can all be obtained through internet suppliers, but they are somewhat expensive in small quantities due to shipping. So again, having a kit of these parts available from a single source would make it more convenient and less expensive to build this project.

Excellent, even though I have experience building this type of electronics I would still prefer a kit due to the very reason you just mentioned. If you need a beta tester for the project when it is nearing readiness I would like to volunteer.

dave

[DickB]

I just got my first batch of circuit boards in this morning. I built one up to check out, and it's running my second clock. I'm able to finish up photos for the build instructions. I hope to have this project available within a few weeks.

49928

[CarverJerry]

This is a cool project. Hope to build it some day. Have any idea what the kit will cost?

[DickB]

Not yet. It will depend in part on how many component parts I am willing to order in advance, to keep shipping costs in check. But I would definitely like to keep it affordable.

[DickB]

While working on build photos and instructions for this project, I've also been experimenting and working on alternative drive electronics. I've learned some things about this type of pendulum and drive in the process, with a very promising development.

If a pendulum's angular displacement is small, the pendulum's period is dependent primarily on the effective length of the pendulum. Most of the wood gear clocks and plans that you see use long, two-second period pendulums with low angular displacement. So this rule largely applies. The pendulum that I (and others typically) use is a shorter, 1-second period with relatively high angular displacement in order to drive the clock works.

For a free pendulum, as the angle of displacement increases, the period of the pendulum also increases. However, for a driven pendulum, at least one driven as I am doing, the pendulum period decreases as the angle increases. I've been varying the pulse with of current applied to the driving coil, and measuring the period to fractions of a second using a microcontroller. I found that can alter the period of the pendulum sufficiently to speed up or slow down the clock without changing the pendulum.

If you look at my first post, you’ll see that I intended to run this clock under control of a microcontroller all along, to regulate the pendulum for accuracy. At the time, I wasn't sure how to do this or if it were possible at all. Turns out it appears to be relatively straightforward: Kick it harder and it moves faster.

The microcontroller is normally in a low-power sleep mode. When the pendulum swings by, it wakes up, and sends a current pulse through the coil of a fixed time period, say 15 mS. Using a watch crystal to run the microcontroller, a very accurate time base is available. By timing each swing of the pendulum, we can determine if the clock is running fast or slow. Then we adjust the pulse width accordingly. This should keep the clock running with very good accuracy.

I’m actually well into the implementation of this design, and hope to have it working very shortly. I think the wait will be worth it. A drawback of many of these wooden gear clocks is that they can be finicky to get running accurately. This approach should make it very easy to get an accurate clock without much if any manual tuning. I have not come across another example of use of a microcontroller to regulate pendulum period like this – I think this is a unique approach.

[DickB]

I have a clock running for several days now with the new microcontroller drive and it is working very well. No pendulum adjustment is needed; just start it running and the controller compensates. This greatly simplifies tuning - there isn't any, really - and makes for an accurate clock - which is the whole point of a clock in the first place.

This breadboard circuit is actually running the clock. The cable at left was connected to a programming/debug device and is no longer being used. The actual circuit is not that complex. I have circuit boards on order.

50205

[DickB]

Even though I have been working almost every day on some part of this project, it seems like it is taking forever to reach the finish line. Patience is a virtue which I don't easily practice.

I've built three clocks now and have run them for several weeks, making observations and minor tweaks to the design to make it easier to build a reliable clock. You should be able to spot one change by comparing the clocks in the photo. I got lucky with the first application of the microcontroller drive circuit, in that no pendulum adjustment was needed. The pendulum just happened to be set about where it needed to be. So actually, in most cases some adjustment is necessary. But I've added an LED indicator that helps quickly set the pendulum in the ballpark, and the circuit then does a good job of keeping time and letting you know if further adjustment is needed. I don't think the batteries will last a full year as I initially expected - I think 6 months is going to be the best I can do with this clock.

I've made numerous refinements to the software. The latest lets you determine if the magnet and coil are of the correct polarity, which is important for proper operation.

Getting close now.

50800

[gregsolano]

Still looking forward to making one!

[CarverJerry]

You and me both Greg.

[jaustin]

I also can't wait to make one.

[dltccf]

still watching this thread with interest. I will be early in line to get the kit when you make it available.

[b.sumner47]

I would like one as well. Great Work.


Capt Barry

[henry1]

Here also would like one , can't wait

[henry1]

Here is one I built a few years ago , its a train clock it has a computor board also and sound at every hr all different train sounds, all done by scroll saw , wish I would of had the machine then , here is a pic

[cestout]

Henry,
That Depot Clock is wild! You need to work it out on the Carvewright.
Clint

[gapdev]

Damn Henry.

Turn that into a project and I'll be first in line to buy it!

Kenny

[DickB]

Mystery Clock (https://store.carvewright.com/product.php?productid=23467&cat=0&page=1) in the PatternDepot.