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Can a clock pendulum be slowed down by adding a resistor to the PSB?

Discussion in 'General Electronics Discussion' started by R92024, Aug 8, 2015.

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  1. R92024

    R92024

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    Aug 8, 2015
    I'm looking for advice, please. I’m a manufacturer, not an engineer, and not very technically minded, sorry!

    I manufacture and sell wall clocks that have a swinging pendulum. The pendulum mechanisms are made overseas and designed for the pendulum to swing approximately 60 times per minute.

    I would like to slow down that swing to approximately 30 or even 20 swings per minute.

    Hopefully attached are photos of the pendulum mechanism (which fits on the back of the wall clock), showing the PCB, and capacitor.

    Could you advise, please, on what's involved in slowing down the pendulum swing? Is it a simple case of installing a resistor, or different size capacitor? (The existing capacitor is 47uf16v).

    I have 1000 of them that need to run slower, instead of at the 60 per minute.

    Many thanks,

    Richard IMG_0664.jpg IMG_0665.jpg IMG_0664.jpg IMG_0665.jpg
     

    Attached Files:

    Last edited: Aug 8, 2015
  2. duke37

    duke37

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    I see no pendulum.
    A pendulum frequency is set by the length and the gravity field. To make it run slower, you can make it longer or move it to the moon.

    The pictures you show appear to be a battery supplied impulse generator to maintain the pendulum, replacing the energy lost in friction. This will not be able to alter the frequency.
     
  3. duke37

    duke37

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    You may be able to devise some kind of pendulum suspension which gives an apparent higher pivot point. This type of thing is used in vehicles so that they tip over less on corners. You need to find a tame engineer:).
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    It looks like the pendulum weight is missing from your photographs. Presumably this weight attaches to the cut-out on the plastic swinging arm. You need to extend this downward to increase the period of the pendulum. There is no electronic solution.
     
  5. Alec_t

    Alec_t

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    I agree with duke 37 (post #2). That circuit does not control frequency; only whether or not the pendulum oscillation keeps going.
     
  6. R92024

    R92024

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    Aug 8, 2015
    Many thanks for the replies.

    Yes, a pendulum rod and bob hangs from the plastic hook (as per the attached photo).

    When I attach a pendulum of standard weight and length (one that is purchased specifically for this pendulum mechanism), it swings at around 60 swings per minute. The pendulum mechanism is obviously manufactured to work like this ( i.e. one second per swing).

    Without the pendulum hanging from the hook, it swings at 95 swings per minute. Something must be deciding this speed. If we can figure that out, surely we can slow down the swing.

    If I add weight and/or length to the pendulum, yes, it slows down the swing, but only slightly (say 5%). But to slow it down appreciably, the pendulum weight has to be substantially more, and it also looks odd, as the swing arc becomes less. The aesthetics of a longer pendulum don’t look right.

    As I see it, there are 4 components that could possibly have a bearing on the speed of swing:
    1. The Printed Circuit Board.
    2. The wire coil beneath it.
    3. The capacitor.
    4. The magnet
    I have experimented with the magnet (using a smaller magnet), but that just reduces the swing arc.

    Any suggestions?
    pendulum and movement.jpg
    Many thanks,

    Richard
     
  7. davenn

    davenn Moderator

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    Im trying to make sense of that comment ??

    so if not hanging from hook what was it hanging from ?
     
    Martaine2005 likes this.
  8. davenn

    davenn Moderator

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    looking at this pic .....

    IMG_0666a.jpg

    the actual pivot point is under that top cover with 2 large screw holes

    the guys who first answered you didn't know that because of the lack of info ... they like me probably assumed that the electromagnet was sitting under the pendulum weight and NOT up near the top of the pendulum
    There will be a lump of iron in that small casing that in the pic is sitting immediately below the electromagnet coil

    so now we see more info, yes, I can see how the pulses from the electromagnet will affect the pendulum period
    whether the electronics can be adjusted to significantly slow the pendulum, Im not sure


    Dave
     
  9. duke37

    duke37

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    The period of a pendulum is 2*pi*sqrt(L/g), weight does not come into it other than moving the center of mass thus affecting the effective length (L)

    To halve the frequency, you will need to make the pendulum four times as long. It is no coincidence that all long case clocks have the same length of pendulum for a one second tick or two second period.

    If you can bear working in SI units, use L in metres and g=9.81m/s/s

    Big Ben has a massive pendulm weight and is adjusted by adding coins. If these are placed above the centre of mass, the pendulum is slightly shortened and the frequency goes up.
     
  10. R92024

    R92024

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    Aug 8, 2015
    Thank you very much for your replies.
    My apologies for not explaining myself better.
    Here are some more photos: (there is nothing but plastic under the small cover at the top of the pendulum mechanism. Just the pivot point).
    IMG_0676.jpg IMG_0675.jpg
    I hear you, Duke, re. the longer pendulum, but that's not an option here.

    Davenn: With the battery in the pendulum mechanism, but without me hooking on the pendulum, that's when the arm of the pendulum goes back and forth like crazy, at 95 swings per minute. So whatever it is that sets that speed of 95 swings per minute, that's what I'm after here. Because by knowing that, it must be able to be slowed down.

    Thanks, guys! :)
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What is doing that is the effective shortening of the pendulum by moving the centre of mass.

    Then you have several options:

    1) manually drive the pendulum with enough force to overcome its natural frequency (and no, the original circuit will not be able to do that)
    2) change the laws of physics
    3) change the gravitational vector

    (1) is impractical.

    Attempting (2) will give us nothing but amusement.

    (3) is possible, but also likely impractical -- possibly even more impractical than (1).

    It might be useful for you to indicate *why* you want to do this -- what are you trying to achieve? Is it just for visual effect? It is potentially a solution to attach some clear perspex to the end of the arm to effectively lengthen the movement without it being overly obvious.

    You have shown us the pivot point. As has been suggested, you could remove this, make another arm, and place its pivot point further up inside the clock. This will make the arm longer without making it hang lower. This will lengthen the period, but will also change the arc through which the pendulum bob moves.
     
    duke37 likes this.
  12. R92024

    R92024

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    Aug 8, 2015
    Changing the laws of physics... yes, I could give you even more enjoyment taking on that challenge! :)

    Re. the "why"... yes, visual effect. Here's my website: www.nowclock.com
    I created this quirky little product a couple of years ago, even got a design patent on the clock face, and I sell them online and through Amazon.
    Slowing the pendulum swing will make it much more in keeping with the concept of the "clock." Slow down, take a breath, be mindful, now :)

    Funny you mention the clear perspex... I've just experimented with that this afternoon:





    IMG_0677.jpg

    IMG_0679.jpg

    I superglued a magnet on the end of the perspex. But it doesn't slow down the swing.

    If (sorry, when) this problem is solved, whatever alteration will have to be performed on a large number of pendulum mechanisms, not just the one. And because I'm selling these online, the appearance has to look professional. I know, it's a challenge...

    Many thanks,
    Richard
     
  13. davenn

    davenn Moderator

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    the magnet isn't going to have any effect out there, so not surprised it didnt have any effect
    not sure what you were expecting to happen ?
     
  14. R92024

    R92024

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    Aug 8, 2015
    The magnet was for weight, sorry I should've made that clear.
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    remember that to double the period you need to quadruple the length. Also remember that the "length" is a measure from the pivot point to the centre of mass.

    so what you need is a very light weight arm so that the apparent length does not have to be huge to achieve the desired effective length, not to mention the load carrying capability of the pivot.

    I would advise you to try it out with a piece of string connected to a compact weight to try out the effect of length.
     
  16. The Electrician

    The Electrician

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    Can you post a picture showing the inside of the clock body, with the pendulum mechanism in place? Do the various interchangeable pendulums weigh the same?
     
  17. duke37

    duke37

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    You could use a minature electric motor and gearbox to crank the pendulum forwards and backwards. It would use much more power than needed to maintain a pendulum at its natural frequency.
     
    Martaine2005 likes this.
  18. Alec_t

    Alec_t

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    One way to increase a pendulum's period without changing the pendulum length is to make the plane of rotation non-vertical (Google "horizontal pendulum"). Unfortunately the mechanism you show doesn't allow that principle to be applied without drastic modification :(.
     
    duke37 likes this.
  19. Martaine2005

    Martaine2005

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    I think that gravity needs to be overcome here. And as Duke suggests, only can be done mechanically.
    Even if you was to put a 20 turn variable resistor in series with the coil, it would speed up or slow down but by a tiny amount.
    As has already been explained in the above posts, the coil keeps the motion by repelling the magnet. But gravity brings it back down. This is called simple harmonic motion. That's the bit you cannot change.
    The LC circuit (inductor / capacitor) is a Tank circuit. This can be changed only by the size of the inductor and capacitor.
    But that is impractical too as kinetic energy and gravity will still play their natural part.

    I think you need to look into changing all 1000 units for a mechanical alternative. Sorry!!

    Martin
     
  20. R92024

    R92024

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    Aug 8, 2015
    Thank you all very much for your input and suggestions.
    Unfortunately I cannot use a mechanical pendulum mechanism... I think I'm stuck with these cheap Chinese mechanisms as I have committed - paid the deposit - to having the clock frames manufactured, and the manufacturer intends to use these plastic pendulum mechanisms. (By the way, I've tried and tried to get the clocks made in the US, but the cost is prohibitive, so if I want to sell my product at a price people will pay, unfortunately I have to go overseas).

    I found it helped a little by fixing the pendulum mechanism higher on the back of the clock (not in the center, as I had before) allowing the pendulum rod to be longer, without having to show too much under the clock. So thank you for those suggestions!

    I've tried to upload a couple more photos, but I'm being told there's a technical issue.

    I'll try later...

    Thanks, guys!
    Richard
     
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