Precision pendulum techniques

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Michael’s red curve is the ‘Path of Centre of Roller’ … not the ’Shape of Pallet End’

… We were ‘reverse engineering’ the diagram, to work out how to mill the shape using a cutter having the same 7/32” diameter as the roller.

MichaelG.

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MichaelG.

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Edit: [for the avoidance of doubt] __ the ‘Diameter of Impulse Roller’ is shown as 7/32”

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Edited By Michael Gilligan on 13/02/2023 03:11:28

This is probably the best video i have seen of the Synchronome Switch in action:

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Nice video! Brian Mumford used to have a time lapse one showing it in slow motion on his website but it has gone.

The problem of profiling the pallet is that 7/32 cutters aren't a standard size so a path has to be worked out to use a smaller one.

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Could be worth grabbing one of these: **LINK**

https://www.chronos.ltd.uk/product/hss-endmill-7-32-diameter/

Edit: or plain-shank carbide from Zoro :

https://www.zoro.co.uk/shop/cutting-tools/carbide-end-mills-regular-series/7-32inch-4fl-standard-carbide-end-mill/p/ZT1151543X

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…or re-scaling the components slightly, and using 5.5mm 

MichaelG.

Edited By Michael Gilligan on 13/02/2023 09:03:12

That makes sense

I must admit I didn't bother to look if a 7/32 mill was available, as I preferred to use a size I already had (4mm IIRC). I could have changed the roller diameter but I was using the original gravity arm and roller.

The circular arc I was referring to was the arc accross the pallet face. Probably my bad description. If the pallet is CNC turned rather than milled you get a solid of rotation so that any inaccuracy in the angle its fixed at still presents a radial slope of your choice to the roller which has to lie in the plane of oscillation. At least that what I read.

regards Martin

Would a barrel shaped roller have the same effect, but easier to make?

Edited By duncan webster on 13/02/2023 12:31:34

I suppose a barrel shaped roller would still have to have the max diameter in the plane of the swing. It may well be easier to adjust which could be a major benefit.
For those requiring the raised cosine shape I suppose the other option is to CNC a form tool with the correct profile and use that to turn the ramp if you don’t have access to a CNC lathe.

regards Martin

I thought that this thread was started in order to discuss the techniques needed to achieve a precision pendulum.

Unfortunately, like many of the threads, this one has moved on to discussing where one might obtain a 7/32 milling cutter.

I think the reason the impulse pallet needs to be the shape is and how this shape contributes to pendulum precision would be more informative.

CS

Being a simple old-fashioned soul, and the original being drawn in thirtyseconds, I had presumed the use of an 8x template and a pantograph machine.

MichaelG.

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Matters fairly well-covered in the two books by FH-J

MichaelG.

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P.S. You mentioned, Clive, that you were going to the Frodsham workshop …

Do you have any pearls of wisdom to share with those of us less fortunate ?

Edited By Michael Gilligan on 13/02/2023 17:18:35

From my reading of the book(s), though Shortt suggested the "raised cosine" design to FH-J after thinking it up whilst listening to Frank presenting at the IEE (in 1910?), it was described as only a theoretical idea. Actually there's no particular reason why a raised cosine should be better except that it starts and ends very gently and builds up in the middle then "fades away". But the actual shape hardly matters for a high Q pendulum. I'd decided to make my pallet by profiling anyway so needed some shape, and having read the book I saw it as an interesting challenge to adopt Shortt's.

Michael

I did indeed go to the Frodsham workshop and spoke to the guys there. What I found out was that Burgess B completion was a team effort which also included inputs from Jonathan Betts and Martin Burgess himself. Unfortunately, David, the guy who did much of the work is away at the moment taking care of his elderly parents but I do hope to speak to him soon. After Burgess B was completed David made a minimalist "clock" with a fused quartz pendulum rod, grasshopper escapement and mechanical remontoire driving the escape wheel. This was built to explore the accuracy of a pendulum operating at a high amplitude. After initial testing of the clock using a weight driven arrangement to power the remontoire they decided to use an electrically wound remontoire to power the mechanical remontoire ie have two cascaded remontoires. I designed the electronics to control the electrical winding.

On installing the new winding system the clock would occasionally stop and naturally they blamed the "new" electronics. I retested its function but could find no reason for it being the cause of a stoppage. In the end I just needed to look in the right place more carefully and found the issue was caused by the mechanical remontoire jamming.

Sadly the project is on hold at the moment but the intention is to fully instrument it to measure environmental conditions, pendulum timing with optical sensors and a laser range finder for monitoring pendulum amplitude.

I'll post more info when I have it.

CS

Thanks for the notes, Clive

MichaelG.

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Philosophical thought for the day:

An accurate pendulum is generally more useful if it keeps going.

The final drive to the escape wheel was through a large wheel onto a trundle type pinion made up of two crossed out discs with 1mm trundle pins. On these were mounted 1mm bore x 3 mm OD ball races to act as a Harrison style anti friction wheel. There were gaps either side of the ball race and the discs which wasn't a problem providing the wheel ran true. However due to wear, handling or accident the wheel didn't run true and a engaging tooth would slip sideways off the race into the gap and jam. My solution was simply to use two races per pin to fill the gap between the discs. Swiss cheese effect in a bent tooth finding a race not exactly in the centre of a hunting tooth configuration.

CS

This post is overflow from my experimental pendulum thread where Joe Noci recommended using a ublox GPS module as a high accuracy signal source. Many GPS modules, not all, provide a one pulse per second output that can be used to calibrate a pendulum. Many of the ublox range go a step further by allowing the user to set the pulse output to any period. This is very handy, for example I need an accurate 10MHz signal, and other frequencies.

For hobby work it's convenient to avoid sub-miniature soldering by buying GPS module already mounted on a breakout board. Mine is a Uputronics uBLOX MAX-M8Q Breakout for Active Antennas - 5V from Pi-Hut, because I shall mainly use it with 5V Arduinos. The 3.3V version is better for microcomputers that run at that voltage. Level converters are cheap.  If the 5V version is bought, the EN pin should be jumpered to 5V.

Photo shows the GPS module connected to an Arduino Nano programmed to convert TTL serial output to USB: a converter cable does the same job. The Arduino program is simple:

When the Arduino is plugged into a computer it registers as a serial device that can be read by any suitable program.

Change frequency, I downloaded u-center 22.07 from ublox. Installed OK on my Windows 10 laptop but not my Windows 10 workstation. I understand the problem, MSCVR202.DLL is missing, but haven't managed to fix it yet.

u-center is intimidating! However, frequency is changed by selecting 'Configure', then 'CFG', then 'TP'. This opens a dialogue allowing frequency to be changed. It's a bit quirky: tick 'active', then change frequencyPeriod AND frequencyPeriodLock. Press the send button. It may be necessary to toggle the active tick box to get the change to take effect. Apologies if the configure instructions are misleading, I experimented briefly and may be doing it wrong!

Manuals:

M8 Receiver Description including Protocol Specification

MAX M8 Series

Will write a program to simplify frequency changing so watch this space.

Dave

Edited By SillyOldDuffer on 17/02/2023 17:53:32

Dave, is u-centre a "suitable program" or does one need to have a terminal program running too please?

Edited By John Haine on 18/02/2023 20:28:26

You only need U-Centerand a serial or usb to serial to the GPS, is all. If your GPS is one with Flash, you can write the setup to it and it will be sticky and come up like that on power up.

Thanks!