Magnetic Permeability of Thai Black Steel

I am planning to make an electromagnetic mandrel, some yokes (shaped pole pieces) and a pendulum bob from something called black steel in Thailand. This steel is available in large sheets of varying thicknesses and can be plasma cut for pennies.

The idea is to use an electromagnet to impulse a steel pendulum bob with zero escapement error and the magnetic forces essentially tangential to the arc of the pendulum swing thus keeping g at 9.81 m/s/s.

My modelling suggests that the location of the centre of impulse from the electromagnet (needed to take the escapement error to zero) is a function of the magnetic permeability of the steel used, hence the query.

Assuming black steel isn't just slang for hot rolled steel, do you have any information on the composition of this steel?

I would have thought you could measure (or at least assess) the magnetic permeability using a magnet suspended at various known distances above the centre of mass with the steel placed on a non magnetic scale, reading off the reduction in weight?

I would suggest that the parameter you are most interested in is remenance rather than permiability. (Google magnetic hysteresis if you don't know what I'm talking about.

regards Martin

Edited By Martin Kyte on 13/07/2017 09:17:56

Edited By Martin Kyte on 13/07/2017 09:18:39

Agreed, you don't want the steel to remain magnetic when the current if off. You need what used to be called "soft iron" . The best source would probably be from a scrap transformer.

Russell

When I made the coils for my synchronome I was fortunate to have some pieces of Swedish Iron from work but in that case the armature is actually in contact with the pole pieces so it is of greater importance. For a 'non contact sport' such as your magnetic impulse pendulum remenance is not actually going to stop it working so mild steel should be good enough.

regards Martin

If there is any residual magnetism in the solenoid won't that affect the steel pendulum as it passes? It would be useful if we could see a sketch of the arrangement.

Russell

It's going to be near constant so the effect on period will likely be minimal. It will likely have an effect but could be ignored.

It would be interesting to see the planned arrangement. Especially as Collin is attempting to make something with 'zero escapement error'. Presumable the first thing is to trigger the impulse optically followed by a good constant energy discharge for the drive current, temperature control for the solonoid. Add to this the variation of earths magnetic flux so really the pendulum needs to be impulsed in both directions which would provide the same overall energy but bias the force away from centre. The pendulum itself would need to be magnetic that is probably going to be magnetised to some extent too so apart from the impulse is going to be affected by variations in earths magnetic field. Not surprising that the Shortt synchronome system used solonoid only for resetting the gravity arm and let that do the impulsing

Just a few musings really.

regards Martin

Thanks all.

Hot rolled steel is probably a good description. I can take it from there.

Hysterisis or remanence is no big deal at pulsing rates of 1 cps or so I believe. In fact a bit of remanence is highly desirable so I can use an induced voltage on the return stroke to monitor the amplitude of the swing and provide the timing for the next power swing.

Cheers, Colin

Martin,

The following chart shows how the force in Newtons picks up as the pendulum closes the air gap between the two pole pieces. (The x axis shows the distance between the pole pieces in cms.)

Integrating the area under the curve shows that a balance point exists ( y = 0) with equal impulse before and afterwards this point on the x axis. This balance point should correspond to the bottom of the pendulum swing if the escapement error is to be zero.

The electromagnet should deliver around 1/3 of the energy in the pendulum with a 12V power drive. This is probably too strong but it will allow the voltage to be lowered or the excitation time shortened,

Colin

Yes, much too strong when you consider that the Q-factor of a seconds pendulum is about 10,000. So 1/3 of the energy in the pendulum would only be required every 3,000 or so swings.

Russell

and you really want to keep the amplitude of the swings constant so impulsing every 3000 swings is not really on.

Martin

Russel,

I plan to drive a counting wheel from the pendulum so I guess the Q will come down a lot. Extracting energy without introducing a pseudo escapement error could be fun but I'm looking closely at the techniques used in MORT.

Martin,

I can always drop the drive voltage or add a resistance in series with the coil to reduce the impulse. But if the electromagnet pulse is too small it can't easily be boosted.

As I don't want to build mutliple iterations of the clock I plan on giving myself a very generous overkill on the impulse.

And the good news is with the latest iteration of the magnetic circuit and a few bugs fixed in the spread sheet the balance point is a lot less sensitive to orders of magnitude changed in mu r.

Still, at the moment it looks like I have a 1kg copper coil that's over a kilometre long so I may try some shorter versions and only increase the coil size if I can't maintain the oscillations.

Cheers all.

I made a 'free pendulum' clock with a 3/4 sec pendulum. I used the coils out of 2 off 24vAC relays, but running off I think 12v DC (it's a long time ago!) with the coilds in series. There is also a series resistor. Admittedly mine doesn't drive the count wheel, but I reckon yours is well into overkill. Mine senses mid position of the pendulum with a slotted opto switch, a second opto detects max swing, so if the travel is up to max it misses a pulse on the maintaining solenoid. Of cvourse it uses a processor to do the logic, is that cheating? Timekeeping is purely down to the pendulum, but as it is steel it is hardly precision. The master plan was to put a heater in the case and keep the temperature constant, but my wood work skills were not up to making a cabinet which didn't have big gaps round the door.

I took the view that the slight escapement error doesn't matter as it is the same on every swing (apart from when it misses a pulse).

Duncan,

Given that a quartz clock is just about cheap enough to give away in a box of cornflakes, I think any rules you care to make for a pendulum clock are valid. For myself, I am aiming for a couple of NE555 timer delays and some LEDs to check the timing. If possible I want to dispense with optical swing sensors and use the induced voltage from the remnant magnetism to control both the amplitude of swing and timing of the pulsing.

I too was thinking about using waste heat to stabilise the clock temperature, bimetallic powered louvres perhaps. But that still leaves the barometric pressure compensator unless I partially evacuate or pressurise the cabinet.

I'm going to have to start cutting metal soon!

Colin