Member postings for John Haine

In an SME journal I read that mechanical engineers make guns, civil engineers make targets, chemical engineers make stuff that goes into shells to make them go bang!

Thanks for the observations. In a mechanical clock the impulse has to be applied near the top because that's near the mechanism. This is slightly true of the Synchronome too though the dial is remote. A problem with Synchronomes is that the impulse is too far above the COP so it pushes the top of the rod sideways against the suspension spring and that causes a spurious lateral oscillation which can be seen in detailed timekeeping measurements. The same things happens in my version, in some ways it's even worse because the COP is probably nearly coincident with the CoG because the carbon fibre is so light. This could be a problem with the new one, we'll see when I start measuring it. Of course if any such vibrations are the same each time it doesn't matter.

Fedchenko's clock on the other hand was impulsed magnetically just below the bob, and that was the most accurate pendulum clock ever made!

As I pointed out before, Tesla cars use induction motors.

Answering in a slightly different order...

• When a bar magnet with dipole moment K is "immersed" in a uniform field B it generates a torque KB around an axis mutually orthogonal to the field and the dipole axis. In this case the torque axis is perpendicular to the plane of swing. If you think of the magnet somehow being mounted actually at the suspension point, the sideways force on the bob would be KB/L if L is the pendulum length. If the magnet and coils were placed at the bottom of the pendulum, the sideways force on the pivot will also be KB/L, and for equilibrium the opposite force must act on the bob. So the force on the bob turns out to be the same wherever the magnet is - I thought that must be true and a mathematician friend proved it.
  
  However if the coils and magnet are at the bottom, the gap has to be larger and the coils correspondingly bigger to accommodate the swing amplitude and possibly the bob. And if the bob is iron it has to be kept away from the coils. Putting the "motor" at the top minimises the coil size needed, allows you to mount the coils on the pendulum suspension bracket, and hence minimises the number of places where the pendulum "touches" the backboard. The only significant bit of steel at the top of the pendulum is the trunnion to accommodate back-to-front movement, which is non-magnetic stainless. The bracket is fabricated from aluminium, the suspension springs are beryllium copper, and miscellaneous screws etc are again non-magnetic stainless or brass. There is probably a small eddy-current reaction from the conductive bits but this is only transient at the start and end of the impulse.
  
• The HH configuration creates a symmetrical field which is fairly uniform in the gap, and in particular can be calculated from a well known equation. There is an excellent website that gives lots of information on magnets and has a calculator to give K for bar magnets of any given size and material. This means that the drive current needed for a given pendulum can be calculated.
  
• The optos I use are made by Sharp and have integral Schmitt triggers. Type is GP1A57HRJ00F and available from various retailers. I recommend you also buy the breakout board as the pinout is tricky. Note that though the devices have an integral pullup it's not very strong and I add a 470R to make sure I get a fast edge at the right level. I've done some measurements on these to gauge their repeatability and reported them on here somewhere - well into the submicron level. Robert Atkinson responded to confirm that he had made similar observations. He also suggested using a slotted vane rather than a pin to minimise ambient light sensitivity, and Doug Bateman's clock does the same (see HJ some years back) and Doug didn't observe any light sensitivity either. A previous clock I made using the same device showed strange errors at the same time every spring afternoon, which turned out to be when the low sun was shining on the case through the kitchen window! So I had to hide the bob and sensor in the dark.
  
  What swung it for me though is that the opto output directly drives the "enable" input on the L298 driver which is active high and matches the opto output when not interrupted. The L298 has separate inputs to decide which was to drive and these are "anded" with enable internally and are driven by the processor. Saves an inverter chip which I didn't have in stock!

Sorry to go on a bit, I hope this helps.

**LINK**

I used a backplate casting, which I think came from RDG.

Now relisted at the same price!

There's a wide selection of used units on eBay. The ones we used at work were often Farnell, not sure if they ever did one that could do 10A. Other names are Thurlby, Thandar. Most of the test equipment suppliers have a range, you can pay a lot of you want to!

I have a 20v 5A one that came from Maplin, which is switched mode. Depending on what you want do use it for it may be better to look at a linear supply - the SMPS one I have is not awfully keen on pulsed loads. A couple of features that are nice to have: multiple outputs, a good accurate current meter (the one in mine is a sick joke), and some preselectable o/p voltages, say 5 / 12 / 24v as well as variable.

"...if you do this you don't get the increase in torque that you would from mechanical gearing. This is why Variable Speed DC motors (which are actually three phase AC motors) stall so easily when you run them slowly. Any three phase motor will do the same if you turn the frequency down and then push it. "

See this website: http://thinkvert.com/variable-frequency-drive-affects-on-torque-horsepower/

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As the frequency is reduced below design speed the VFD can drive the motor to generate constant torque down to low speeds. Imagine a motor which is clamped to the rotor can't turn and the stator is driven with current at the slip frequency so that it produced the same field in the air gap as if the motor is running at its maximum torque point. The stator will see exactly the same magnetic conditions and must generate the same torque. The speed of course decreases and therefore the power, but for many jobs torque is what's wanted at low speed - for example tapping.

Most of the cheap machine tools use permanent magnet DC motors and thyristor controls (or sometimes MOSFETs). These behave differently. Some DC drives use brushless motors, which I believe generally use permanent magnet rotors. The field windings are 3 phase to generate rotating magnetic fields to drag the rotor round. The torque depends on the stator field and the physical angle between this and the rotor magnets and is independent of speed.

**LINK**

Neat, Duncan. My dial is so big because I profiled out the gears on the cnc, and the smallest cutter I can use is 1mm (speed limitation). That means the smallest module is around the same size for cycloidal teeth (bigger for involute). Large wheels have 75 and 80 teeth so correspondingly big.

My version, base carved out of a backplate casting. I concur absolutely about being able to set tool offsets - this is on a CNC converted S7. Not as elegant as yours. I have been meaning to organise a mounting for a drill chuck, excellent.

I'm not sure what you mean by symmetrical? If I recall correctly if you reverse the supply to both armature and field windings the motor should run the same way. Maybe this is a permanent magnet field motor despite the plate. Yes I suggest a 20A PSU like Stueee. One hopes that the controller has an internal flywheel diode across its output but you could always add one externally.

At start the motor is essentially stalled. Start current will be much higher than 6.8 amps, and since you say it's having trouble starting it clearly is stalled. Since your controller is rated at 20A max You probably need to have at least 20A peak current available from the power supply. Your drill/driver probably has lithium batteries and could apply much more current on startup. Hopefully that controller is designed to cope with starting currents, and will have appropriate flywheel

From the plate the motor has DC excitation, i.e. wound field. How are you connecting the field windings, in parallel with the armature or separately to the 24v supply? It should be the latter. The extra current 6.8 - 5 = 1.8A is presumably the field current. How do you reverse the motor?

Thanks Duncan.

Do you have a seconds hand? I've got a couple of those 48 step motors and one could make the pendulum beat at 1.25s but I thought it wouldn't look right having the seconds hand moving in bigger steps and the pendulum would need to be 50% longer. Or am I missing something?

Thanks for that Dave!

Obviously a lot more to do. A case for one thing. The power supply arrangements aren't ideal, the L298 supply voltage is derived from a variable boost converter (bottom left of board) driven from 5V As the coil current needs to be ~300mA, and ~18V supply is needed to drive this through the 50 ohm coils, the booster draws over an amp from the supply! I think it would be better to have a 12v supply which would also be easy to backup with a small SLA battery; and use a cheap buck converter to supply the 5V.

Most importantly of course the clock needs to be measured so I need to reassemble my piPET system to do this.

Speaking of which there is no rating nut or other way to adjust the native pendulum period. The design period is 0.5s ("seconds beating" ) and according to the scope the actual period is about 10ms faster than that. Rating will be done by the processor so that the dial is withing a second or better of actual time. I'm also hoping to use the sort of technique that Dave is applying to compensate for temperature and pressure, though as the density of the bob is 2.5x a steel bob of the same size, or 1.5x a lead one, the baro sensitivity should be correspondingly lower.

This photo shows the (regrettable) breadboard layout. The scope trace shows the ticks from the centre sensor on the yellow trace, while the blue shows the amplitude control pulse. When the amplitude of a swing reaches the level set by the spacing between the sensors, this pulse tells the processor to inhibit impulsing for the next 2 swings. It also incidentally "reminds" the processor which side the pendulum is swinging to make sure the impulses are applied in the right direction .

At the top right of the board is a TI DRV8834 stepper motor driver on a breakout board. These drivers are wonderful as they can operate down to ~3v supply though here it's running on 5v. This drives a stepper that drives the dial.

Driving the dial is a processor's busiest task. The stepper drives the seconds (centre) arbor - now steppers generally do 200 or an even multiple thereof steps per rev, so 60 seconds won't divide into a whole number of steps. I use 4x microstepping, or 800 steps/rev. 800/60 = 13.33..., so actually the processor generates a sequence of 13,13,and 14 steps per pendulum tick. You can just see the slight stuttering of the seconds hand but I'm hoping that in operation it will be unnoticeable.  Both optos generate interrupts.  The timing interrupt just resets the routine that generates the dial pulses; and the amplitude one just sets a counter that counts down from 3 (set by the interrrupt) to zero each swing.  Impulsing is only enabled when this counter is zero.  I wrote and debugged the software using another Arduino to generate "pseudo pendulum" pulses, to my considerable surprise when it was loaded on the target and connected to the hardware it just ran.

Edited By John Haine on 22/01/2023 12:22:38

By the way the bottom end of the bob was machined off square so there's a spherical aluminium fairing araldited to it to improve the aerodynamics.

At the top of the rod there are a pair of "Helmholtz Coils" mounted on Corian brackets screwed to the suspension bracket. The coils themselves are 200 turns each of 0.15mm wire, diameter of the coil is 40mm and spacing 20mm. The rod is carbon fibre and mounted inside it so its centre is (I hope) on the axis of the coils is a 13x3mm Neodymium magnet. The coils and magnet form a sort of "moving magnet galvanometer" - when a current is passed through the coils the magnet tries to line itself up with the axial field and in doing so exerts a torque on the rod.

Though there is an Arduino to control the clock, the actual pulses from the centre opto are applied directly in hardware to the coil driver - this is the ancient but useful L298 full-bridge driver. The processor tells the driver which way to drive the coil, alternating on alternate swings.

This is another photo of the sensors with the vane rather than the pin.

Having recently been making better progress with the new clock project I though that I'd post some details in a new thread so as not to divert Dave's.

At the moment the pendulum and sensors are assembled on a bit of laminated birch plank screwed and bolted to the bench - not ideal but fine for initial testing. Sorry the photo is on its side, could a kind moderator please rotate it?

The bob is of tungsten alloy, made for a different purpose and adapted. It had a recess at the top end into which is loctited a steel plug for the rod attachment. I didn't fancy drilling the material so this will be a source of temperature error. Underneath the bob are the opto sensors, one on the centreline for impulsing, the other offset for amplitude control. The Sharp optos are mounted on a small block of Corian running on stainless steel rails, sprung against a micrometer for adjusting the zero position. In the photo there's a pin to interrupt the bean but this has been replaced with a brass vane with a 10mm wide slot in line with the pendulum axis, which inverts the logic level of the optos. So when the pendulum swings through zero the opto generates a +5v pulse about 160ms long.

Mod edit rotated photo.

Edited By SillyOldDuffer on 22/01/2023 12:35:32

Well I think you got yourself a bargain there Hopper!

Provided that it's actually round and not like the length I bought some years back. The centreless grinding machine had given it a distinct "lobed" shape like the rotor of a Wankel engine. Measured at 1" diameter but as I was trying to make an air bearing...