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Member postings for Andrew Cattell

Here is a list of all the postings Andrew Cattell has made in our forums. Click on a thread name to jump to the thread.

Thread: Recoil escapement - very variable time keeping
09/12/2021 22:01:12

I put a temporary pendulum onto a clock with a recoil escapement. The upper half of the pendulum rod, down to just below the crutch, is standard and rigid. Below this point the lower part of the rod and bob unhooks for moving the clock around. My temporary lower pendulum is not a tight fit onto the hook on the upper rod allowing the lower pendulum to swing through a different angle to the upper part. The lower part is actually a key fob with a short length chain and has a trolley token for the bob. In use the clock gains 3 mins a day for the first few days after winding and then evens out for a day and then for the remainder of the run it loses progressively more each day until I wind and reset it. It runs for over 10 days if left to.

When I make a more permanent lower pendulum with a snug fit onto the hook, is it likely that the time keeping will become less variable over the week/10 day between winding? I have two other similar clocks, both keep much better time than this one.


Thread: striking clock fly adjustment
04/12/2021 20:49:32

When I serviced the clock I stripped it down to individual parts except for not taking the two springs out of their barrels. I must invest in a spring winder. All of the pieces went into an ultrasonic bath (white spirit) before carefully drying and cleaning with a cloth. The pivot holes in the plates were pegged through. I wiped the old sticky oil from the accessible parts of the springs and barrels and then partially tensioned the springs and dropped a couple of drops of oil into the coils. I then reassembled the whole thing, oiling the pinions and pivots as I went. On winding the spring tensions were about as expected, I have several similar sized clocks in regular use and think I would have noticed if the winding up had been easier. The clock in question strikes significantly slower than all of the others I run, it sounds as if it is in need of a service! it is possible that it has always struck more slowly but it does sound a bit "walking wounded".

I still need to sort out a proper pendulum, the temporary one is still keeping the clock running, albeit losing a minute a day.


01/12/2021 22:43:31

Thanks MG, I had seen that thread before and was bewildered by the range of opinions from the fly should not slip, may slip on starting, may slip on stopping, can be used to regulate the strike speed, etc. I still need to make a replacement spring for the fly as, without one fitted, if the fly is free enough to slip when the arbour rotates it can slid along the arbour and foul other parts of the mechanism. The spring lies between two holes in the blades of the fly across a grove in the arbour allowing rotation slippage but not axial movement. As the clock is of limited historical value it might be a simpler job just to use some tin snips to trim the diameter of the fly down a little to reduce the air drag and keep it gripping the arbour.


01/12/2021 21:21:12

I recently brought an 8 day striking mantlepiece clock into use. While stripping and cleaning it I found that the fly in the striking train was loose on its arbour. A witness mark on the face of the fly and a groove in the arbour show where there should be a spring to enable the fly paddles to grip the arbour but there is no sign of it. As a quick fix to get allow the clock to be test run I bent the fly paddles just enough to make them bind on the arbour and set it all running. It ran for 10 days. Unfortunately, when it strikes it takes an age between bongs. I want to speed up the striking but I don't know how to make a suitable spring for the fly paddles, what material to make one from or how to end up with the correct tension when it is fitted.

Anyone? Thanks.


Thread: Music on TV Programmes.
23/11/2021 20:21:00

We found the sound quality from our latest TV to be quite poor so I hooked up some much better speakers and excellent amplifiers with a bit of grunt to the sound output sockets on the back of the freeview box. Now we find that all sorts of stuff in the background that we shouldn't be receiving can be heard. Traffic rumbles, passing trains, passing aircraft, slamming doors even on studio recorded speech programme material.

As for stuff you are supposed to be hearing being a nuisance, why does a news programme need background music?

I don't have an issue with regional accents but I switch off if anyone says fink (think), fort (thought), anovver (another), free (three) etc. Does nobody correct youngsters speech these days?


Thread: EVs and the heater on a cold morning !
23/11/2021 19:44:15

I understand the technology limitations regarding not being able to use regen braking if the battery is full but.........

how can the battery be full once the car has disconnected from charging and got up to speed? Unless you start from a point of significant altitude and drive down hill the energy from regen will be less than you used getting up to the speed at the start of braking. The recoverable energy will be less than than that used getting going until perpetual motion machines get perfected, and that won't be any time soon!


Thread: A Novel Free Pendulum Clock
15/11/2021 12:49:48

I just had another watch of the video in the OP of this thread. The first thing that struck me is the size of the pendulum swing. My understanding is that a large swing is asking for trouble with circular error which may or may not be significant with a tightly controlled amplitude. It does, however increase the amount of energy the system is losing to air resistance. In my clock the pendulum is 60cm long (3/4 second) and the swing is controlled to plus or minus the closest distance that 2 slotted opto devices can be mounted, approx 6mm between BDC and peak. This gives an angle of swing of less than 1 degree each way from the centre.


15/11/2021 11:32:47

Thanks for the suggestion Tony J,

There might be some merit in mounting the whole clock on gimbals even though it goes against the grain, robust and solid "feels" like the right approach to gaining stability. Having an assembly balanced by gravity and then impulsing one part of it against the other seems to be asking for another set of motion variables to interfere with the system.

I have thought about mounting the opto sensors all on a static pendulum pivoted on the same axis as the moving pendulum and damped to stop it swinging in sympathy. What slowed me down at that point was dealing with the tension in the wires causing the static pendulum to hang off centre. I then came up with a complicated arrangement which put the sensors on a fixed bracket but used mirrors on the static pendulum to get the light beams all across the path of the obscuring pin. Beam divergence reared its ugly head and then laser diodes looked promising...............

Option paralysis and life have prevented any further work..


14/11/2021 22:50:10

Other improvements I want to make.

At the moment the optos LEDs are strobed by the PIC micro to reduce power consumption and increase service life. They are only strobing for a few milliseconds before during and after the time when there is predicted to be a pin passing through. The strobing causes there to be a few 10s of microseconds jitter in the detection time of breaking of the beam when seen by the PIC. This might have some effect on the accuracy of the extremity position on boarderline cases of detect/not detect. It also affects the accuracy of the BDC impulse. Both of these can be made better by using some hard wired logic gates between the opto and the PIC, putting the PIC in control of when the optos are on and when the impulse needs to be on but letting the logic gates actually drive the impulse.

As others have noticed the slotted opts are affected by ambient light levels. I have done my best to shield them but as the pendulum assembly still has no case around it more can be done. What I am not sure about is whether the optos are showing any sign of sensitivity change due to aging. Thinking about how to detect/compensate for this.

I also want to investigate if the performance of the opto devices changes enough with temperature to give a measureable change in the pendulum accuracy.
The radio receiver I use to pick up the MSF time signals suffers from significant jitter, I think this is caused by the action of its built in AGC (automatic gain control) struggling to accurately lock to the transmissions. The MSF signal is fairly weak and at only 60kHz is badly affected by much of the electronic equipment operating within the house. TVs and computer monitors used to have display scan rates of this order and no doubt other stuff around also does. I have an oven controlled crystal oscillator salvaged from some mobile phone network equipment which I want to set up and trim to lock to the MSF signal, using a feedback signal derived from averaging the error between the divided crystal output and MSF pulses. If the averaging is over enough pulses the crystal should end up giving me a much more precise time pulse on a second by second basis than the jittery MSF signal does.

After all of the work I did (and have yet to do) on my clock I came across a design that has taken the subject to a whole new level. See the work of Edward T. Hall and the Littlemore Clock, your favourite search engine will find it.


14/11/2021 22:49:41

I have yet to work around the issues of the house walls tilting with the seasons, to implement temperature and pressure compensation but the clock continues to run. I will change the software so that it monitors the time between consecutive passes of the centre opto breaks to allow detection of the out of beat-ness. I have enough spare pins on the PIC controller to allow it to drive a small stepper motor which will be used to "motor" a carriage with the two opto devices along a path in parallel with the direction of the tilt of the walls. This is not exactly in parallel with the swing direction of the pendulum so needs a bit of thought. When the house moves the pin on the pendulum gets dangerously close to one side of the slot in the extremity opto due to the movement hence the need to move the correction carriage on a non parallel track. The impulse coil has enough space around the pin that it will not drag due to the house tilt. This big air gap was a conscious design feature to minimise pneumatic drag as the pin enters and leaves the hole.

The PIC controller is supplied from a wall wart psu and has a rechargeable battery backup which I built in at the beginning using some old NiCad cells found in my junk box. This battery has maintained the clock through a number of mains power outages, some lasting many hours. It also sends an impulse to an old factory repeater clock every 30 seconds so the time is on display, the display clock has its own battery which has also survived the power cuts so it has only needed attention twice a year when the clocks change.


14/11/2021 22:49:04

So far, so good. After a couple of weeks I stopped taking the readings every hour (it plays havoc with your sleep quality) but continued to record the readings once per day when we are at home. I noticed after a while that the time keeping was beginning to drift off. While investigating the errors I found that the pendulum had gone out of beat, the centre opto was not at the lowest point in the pendulum swing and the extremity opto, being the same distance from the centre opto, was controlling the amplitude to a lower constant size. I reset the position of the optos to restore the beat. Some months later I found that the timekeeping was slipping off the other way and sure enough the pendulum was out of beat the other way and the amplitude of swing had increased. It turns out that the whole house moves on a cyclic basis with the seasons, probably due to it being built on very high clay content ground, the moisture content of the clay changes through the year as the rainfall varies. Unbelievable. Just when you think you have removed or allowed for all of the things that might affect your clock something else comes to surprise you.


14/11/2021 22:47:46

I went down this path some 20 years ago with a constant amplitude pendulum controlled by optical interrupters. I used two slotted opto devices, one to detect the extremity of swing at one side and a second one to detect the bottom dead centre. The interrupter obscurer is a single steel pin, just 1mm diameter poking down from the centre of the bottom of the pendulum shaft. The impulse is delivered by a hollow electro magnet coil mounted to one side of the pendulum shaft attracting a steel pin fixed to the side of the pendulum shaft just above the bob. The whole lot was masterminded by a small PIC microcontroller. The thinking is that if the interuptor pin fails to break the light beam at the extremity the impulse magnet will be energised for the next time that the centre opto is obscurred by the pin thus giving an accurately centred "pull" to the pendulum. The whole assembly of pendulum suspension, optos and impulse coil is mounted in the corner of a room, the suspension bracket is a short length of 1inch X 1inch angle iron bolted diagonally across the corner using 8mm masonry bolts. After setting it running the pendulum was brought to time by adding a few clothes pegs to the shaft above the impulse pin. The swing of the pendulum was compared with a signal from the NPL MSF time reference in Cumbria using a two beam storage oscilloscope. The impulse was made very small so that it would do 8 or nine impulses to to restore the amplitude which was then maintained for a few swings without impulse. This seemed to be better than giving a big pulse and missing many to my mind. Once I got it all trimmed I set about recording its operation, taking readings of time shift compared to 1 second tick from the MSF, the barometric pressure and the room temperature every hour. I did this for 14 days! I wanted to take enough readings to assess the effects of air pressure and temperature with a view to making compensation devices to be added to the pendulum later. By taking readings on the whole minute the MSF one minute pulse (easily itdentifable) and can be compared to the 30 second impulse sent to the display clock mechanism drifts of up to 2 seconds fast or slow can be taken with confidence that the right pulses are being compared. My post was deemed to be too long so more follows in next posting!


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