A digital governor for model engines

[MJM460]

Hi Zephyrin, thank you for the compliment, I am glad you enjoyed the video.  It has indeed been quite satisfying to find out what could be done in this field.  Not enough for an industrial machine, but certainly illustrates the principle.

A scale butterfly valve with a disk attached to the operating shaft, as has been achieved in several of the builds on this forum, is not easy when the steam passage is only 3.5 diameter.  Certainly a task requiring a high degree of skill.  And a real assembly problem.  I avoided the problem by drilling and reaming for a 5 mm diameter shaft through the valve body, and machined a small section in the way of the steam passage to make the “disk”.  I left the leading and trailing edge of the machined portion about 1 mm wide, and this section of the surface of the 5 mm rod, when turned to be against the reamed hole, was enough of a seal to stop the engine.  It is also easier to make a gland on a 5 mm shaft than a much smaller one.  It would not pass a pressure test as a stop valve, but it is quite satisfactory as a governor valve.  The body is just a square block at this stage, it really requires a bit more work on the outside profile to make it a bit less chunky, but it has proved the concept.  More work to make the outside cosmetic form than to make the working internals.

MJM460

[steam guy willy]

Hi MJM , just thought that if you had a Venturi arrangement as per an injector would that give you more pressure in the steam chest/cylinders ?? or would you get wire drawing ?? Just an idea !!

[MJM460]

Hi Willy,

I am not quite sure how you are thinking of arranging the Venturi and where you foresee the wire drawing.

In principle, I feel that the best way to get more pressure in the cylinder is to fire the boiler a bit harder so it operates at a higher pressure, then make sure you eliminate unintended losses by having sufficient pipe diameter between the boiler and engine, good steam passages within the cylinder, and a valve and port configuration that operates to open quickly. 

Then some fine tuning of the lead so the valve opens at the optimum time.

Finally, make sure you have good clear exhaust passages, valve opening and timing to minimise the exhaust pressure, as there is no point boosting in inlet pressure if it is working against an elevated pressure on the exhaust side of the piston.  It is difference in pressure between the steam side and exhaust side of the piston that does the work.

I hope that covers what you mean,

MJM460

[steam guy willy]

Hi MJM..I was thinking that as an injector can force water into the boiler using the steam pressure from the boiler could the same physics be used to increase the pressure that is available  else where ?? and I dint think I quite understand wire drawing ?!.

Also thinking about governors...will a scale governor have exactly the same effect as the full size component ? thinking about the relative weights as  with my 18th scale engine that weighs about 1 kilogram ..the full size engine is rather more than 18 KG's ! The Governors on most models are always made to scale, however the engines are rarely put to work ?? This question came about by somebody talking about scale models ...So the idea of a Digital governor seems to be a really good and practical idea !!
Sorry this reply is a bit late..
Willy

[MJM460]

Hi Willy, your model scale at 1/18 means that the mass of the full size engine is 18 x 18 x 18 = 5832 times the mass of the model assuming it is exact scale in every detail, and the model is made from the same material as the full size.  Consequently the governor flyweights are (1/18)^3 or 1/5832 of the mass of the weights on the full size engine.  The centrifugal force on the weights is
m x r x w^2
where r is the radius of the centre of gravity of the centre of mass of the weights from the rotating shaft, and w is the angular velocity of the shaft in radians per second.

The force lifting the collar at the lower end of the arms is roughly equal to the magnitude of the centrifugal force when the arms are at about 45 degrees.  So the force available to move the governor valve has to be further reduced by the scale factor again for that reduction in radius.  Hence the force available to move the governor valve is 1/5830 x 1/18 = 1/104976, say 1/100,000.  Then the governor has to be able to exert that force through the required distance to operate the valve.  (I hope that I have the maths right this time!)

You can see that the maths is against you when it comes to designing a governor valve which has the steam pressure forces exactly balanced, and with sufficiently low friction in the stem seal for the governor to be able to operate it.  And that is without considering how the system has to be set up to get the required speed with the springs or weights that are used to balance the system at the required speed.

I don’t know if this is indeed possible, but I am sure that I don’t have the necessary skill, and I believe that many of the scale governors do not actually operate the governor valve to control the engine at a constant speed.  I am always interested to see news of models where the governors do work as intended.

Many builders of hit and miss engines do seem able to get them working despite the scale factors, but they do operate in an on-off manner, and have advantages in the mechanism used.  But that is the background to my experimenting with a digital version.

With a steam engine, it is not so much a matter of whether the engines are loaded or not, as the idea is to get a constant speed, however, what is important is the amount of governor valve movement that is required to cover the range from minimum load to full load.  Certainly the digital approach overcomes the scale difficulties, though it introduces a requirement for a power supply, but that is a whole other discussion.  Certainly my simple approach seems to be working well, so works as a proof of concept using readily available education level components.  And it can certainly be implemented in more sophisticated ways by those with the necessary expertise.

For models with a vertical shaft, I think it would be practical to extend the governor shaft down through the base board in an unobtrusive manner.  Then the speed pickup could be appropriately concealed under the baseboard, together with the governor valve when the engine is intended only to be operated on air.

Whether it is needed on a steam engine depends on the nature of the load and what effect is desired.  While digital governors are used on modern engines, but application to a slow speed steam engine is probably not representative of any full size practice.

No worries about the timing, just ask your questions when they arise, they are always worth thinking about.  We are back in lockdown and I had just ordered a new computer so have been busy setting that up, as well as rearranging our lives to avoid the need to leave home.  Everything takes more time and thought.  Also rebuilding the governor in a proper case for better protection.

It’s 23:00 here, so way past bed time, so wire drawing tomorrow, unless others would like to join the discussion.

MJM460

[steam guy willy]

Hi MJM... thanks so much for the explanation .. I have been thinking about this for years  but the reply of ...You cannot scale nature    has seamed to be the stock answer !!   There seams to have been quite a lot of research on full size governors to make them more efficient  and many do not seem to have in built adjustment facilities , apart from different sized pulleys... so thamks again and it is  02.30  here in Blighty so am off to bed  !!

Willy

[MJM460]

Hi Willy, I am another who is not content with off hand statements such as not being able to scale nature, I put it down to obfuscation.  The obvious return is, “Why not?”

In reality, there are limits, but there are also many aspects which can be scaled.  In fields such as fluid mechanics, there are areas for which the requirements for similarity cannot be achieved.  For example the air resistance when an object moves through air requires a viscosity condition which is difficult, if not impossible to achieve with real fluids.  Dimensional analysis reveals groups of properties which are dimensionless, which allows us to predict behaviour of a scaled object from model studies.  Groups of properties such as Reynolds number used in aerodynamics and Froude number used in naval architecture are well known.  To study an aeroplane wing using a model, the model test conditions are selected and the Reynolds number for the model test calculated.  Then the results apply to a full size wing at the same Reynolds number.  Similarly, for a ship model, the Froude number is calculated.  The wave making will be similar in the full size ship operating at the same Froude number.  Unfortunately a different speed is necessary to sturdy the surface friction resistance, so wave making and skin friction have to be studied at different speeds through the water.  However if you look up Froude number, you will find there are several different definitions, used in different fields.  But we are getting away from governors.  Perhaps these questions would be better in the thermodynamics thread.

Wire drawing is a little closer to the topic in that we might see it in a governor valve when the boiler pressure is high but the governor valve is nearly closed for low load running.  Again l have seen different definitions, and I am not sure if there is one “real” definition, or if it is just a term used colloquially in different situations.  I came across the term early in my career, in reference to the very commonly used gate valves used in the thousands in any hydrocarbon processing plant.  They are intended to be either open or closed but if the operator cracks the valve open just a little bit, to throttle the flow, the fluid flows through the small gap at high velocity, and particularly if the fluid is wet, such as steam that has been stationary in the closed pipe, the condensate droplets actually cut grooves into the valve seating, even when it is special hard alloy.  Others may have different definitions, it will be interesting to see if others reply.

You were also asking about using a Venturi like those used in injectors, in other applications.

Certainly I have used steam injectors to take air out of a steam condenser operating at deep vacuum, to increase the pressure above atmospheric pressure so it can be vented to atmosphere.  Any model or full size engine with a condenser needs an air pump, you have seen many of those.  The problem in a large installation is that when the pressure is very low, while the air volume is low, it is mixed with a large volume of water vapour.  An injector can handle those large volumes, but the pressure increase is not as spectacular as the locomotive steam injector.  It is not unusual to require two stages to get to atmospheric pressure.  I am sure there are other applications, but I don’t have much knowledge of them.  Energy is used in the process, and in such processes there are always losses.  Energy wise, pumping a liquid to higher pressure is more efficiently done with a pump than an injector.

I am up to the initial checking of the revised governor build, and if all goes well, I should have it working again in a day or two.  It is on one board in a protective case.  I will have some photos when it is ready.

I hope those explanations helped a little to clear some of the blur.

MJM460

[Zephyrin]

We cannot scale nature, but like a lot of modellers, i'm working hard it this direction !
It is not probably a true scale governor, as my engine is not a real model, but it is clearly a functional one.
By pushing manually on the steam valve lever controlled by the governor, hence removing its regulating action, the speed raises from 250 rpm to 360 rpm, far too fast for this engine, as the unbalance leads to unpleasant jolts.
this action on the control lever requires a surprising force on the finger tip...it is funny to do !
the bronze balls have a dia of 16 mm on a 33 mm rod; the speed of the governor is x 1.7 that of the crankshaft.
the movement of the governor allows 90° of rotation of a rotary valve similar to the carb of a RC glow engine. there are some water drops under the valve as the gland of the valve is only lightly screwed for ease of movement...there is room for improvement.

the way to set the speed is obviously limited, the valve being almost fully close all the time...as the steam pressure is above 1.0 bar.

<a href="https://www.youtube.com/watch?v=EEKs0Qr4OZY" target="_blank">http://www.youtube.com/watch?v=EEKs0Qr4OZY</a>

[MJM460]

Hi Zephyrin,

That is outstanding.  A real pleasure to see a small governor working well.  Truly excellent workmanship, and I imagine quite a bit of skilful fiddling with spring rates and so on to make it work so well.

That quarter turn governor valve sounds exactly like the one I call a butterfly valve, though no doubt different in construction from what I have done, and obviously a good way to get a low friction valve operation, even if the stem seal leaks a little.

Thanks so much for posting that video.

MJM460

[steam guy willy]

Hi Zephyrin , good to see the governor working and a nice model too... the governor seems to bee revolving really fast and that maybe why it operates so well...most beam engines are quite slow moving in comparison !?
 Thanks also MJM for the comments about actual practice as well as theory ...

Willy

[Zephyrin]

Yes, most beam engines are slow runner, but often gears were put to increase the speed of the governor...
this little thing always attracts a lot of discussion in steam meetings, but alas I don't see the date of the next one coming, alas...damn times!

in my engine, the governor is simply a speed limiting device, very useful, but not a real governor regulating speed on a broad range as the digital governor does...I learned that, thanks to MJM and his exciting thread.

[MJM460]

Hi Willy, when the governor is scaled down in dimensions it is perhaps best described as a miniature governor.  It generates the force and movement to operate the valve as determined by the same laws of physics as the full size model, rather than a “scale force”. 

With your 1/18 scale model, the force available is very small compared with the full size machine.

Zephyrin’s model shows what can be achieved with a governor of model size.  I don’t know how that compares with the governor proportions and operating speed on your model.  Note that he says it is surprising how much force it took with his finger to change the speed.  It is possible that the full size governor has way more force available than needed.  To get an idea of what your model could do, it might be worth calculating the forces based on your dimensions.  You could use the lever dimensions to see what movement is available at the valve, but it is probably better to continue the build as you are doing, and see what the governor will do. 

I expect that weight on the governor spindle of your model, being subject to the same scale factors, might need some help with an adjustable spring somewhere on the lever system.  But well worth seeing what you can do.  Use the theory to help you understand what is going on in practice and decide what to do from there.   It will all depend on the mass of the weights length of the arms and speed of the governor spindle.

For my tiny models, only 12 mm bore, a scale governor would have very small weights, and I can’t see my being able to make it work.  But if your weights will be 16 mm or so diameter, Zephyrin has shown what can be done.

Hi Zephyrin, those limitations you mention with your model are very much the limitations of any governor of that type.  First it is a simple proportional controller which gives “constant speed”.  It is set up for the speed at a load, and when the load changes, the engine speed changes and the governor will adjust the valve to restore the set speed.  However, it will not quite get there, there will always be an error proportional to the load change.  The springs must be adjusted to restore the original set speed.  Alternatively, the springs can be adjusted, with the load constant and the engine speed will alter giving a new set speed. 

Modern flyweight governors use the governor to adjust a hydraulic valve which controls a hydraulic system to operate the valve, so overcoming some of the limitations of the simple mechanical arrangement.  I suppose it would be possible to design a steam cylinder with the valve operated by the governor to do something similar, though Jadge made some interesting comments on stability, very early in this thread, which could become a problem, if you get the tuning wrong.  But what you have achieved with the simple mechanical governor is outstanding in my view.

The electronic governor certainly provides opportunities with the wider speed range and so on, just as adding computer control adds to machining capability, and even sewing machines which can now sew forwards, backwards and even sideways, to make alphabets and even pictures.  To me, the interesting thing about my project is in seeing how much can be achieved with a very minimal, readily accessible microprocessor.  But it is very simple to implement, and does not require the skill you have put into that traditional design.  The possibilities with a better microprocessor with decent maths handling are indeed endless.

The progress is a bit slow at the moment, between changing to a new computer, and rebuilding the governor in a more suitable case, I have made too many changes at once, with the usual undesirable result.  Still hoping to have it all running again in a day or two.  There is not a lot to go wrong, but it’s difficult to see which bit is not behaving, as the first step in problem solving.

MJM460

[steam guy willy]

Hi MJM, Thanks for the info ,the governor on the Bressingham engine is a PORTER type...this has quite small balls and a very large weight centrally... I suppose this is acting like a spring, but with a constant action throughout the travel.... a few pics.. will the action of the governor behave differently with steam /air and what % difference will there be ?? the engine is still far off all the finishing touches !!

Willy

[MJM460]

Hi Willy,

I am not familiar with the different variations of governor design and what makes the Porter type different, unless it is that weight.  A constant force resisting the lifting of the collar instead of the more often seen spring, which provides a force which varies with the compression of the spring.  It would take some heavy maths to determine the precise effect of that weight on the governor performance, perhaps a similar effect as some pre-load on the spring to get the governor action in the right range for the desired speed.

Those balls certainly look small on such a large engine, but a governor only needs enough force to operate the valve , and once the forces are adequate, a bigger engine does not necessarily need the governor weights to be bigger.  But when you then scale them down to model size, they may not be enough to provide the force necessary to operate the model size valve.  The difference between a big governor, and a small one, the forces determined by the physics, are not necessarily  enough for the small one.  On your model, the yokes on the upper ends of the lower arms, will contribute a significant proportion of the effective flyweight mass.

Your model has reached the stage where you can spin the governor shaft with your fingers and get an idea of what force you get.  Make an adapter for the shaft and a Meccano rig to keep it spinning if you want to explore it further.  It will be interesting to see if it behaves in a similar manner to when you spun the full size one.

With the valve, the maths gets pretty heavy to calculate the difference between air and steam, and I have never really conquered flow through an orifice with different gases, but if you remember, earlier in this thread I described trying out the governor on steam, and while the valve position at a given speed may have been a little different from the same speed on air, I did not particularly notice it, there is definitely more difference with a change in pressure.  I don’t believe you would have to modify the valve for one motive fluid or the other.  Similarly, I have described before an experience running a modern small turbine on air as a commissioning test.  The turbine worked quite satisfactorily, as did the governor and governor valve.  As it was driving the oil pump for a large compressor, it was definitely fully loaded for the test. 

The model governor really looks the part beside the full size one, it’s going well.

MJM460

[MJM460]

Well, we have drifted around a bit while I have been rebuilding my digital governor.

To recap a little, I had the governor working to my satisfaction, but it needed work in two areas to make it reliable for the longer term.  And there was an idea in the back of my mind to try and add the equivalent of the Integral action of the classic PID control.  (No need for the derivative function as has already been mentioned.)

I started out thinking that I could complete the project on a single small standard project circuit board which is available as part of the Picaxe system.  I hoped to just put it in a plastic enclosure under the engine platform for protection from oil and moisture.

The board turned out to be more compact than I could manage at my skill level, and I let the smoke out of some components.  The simple solution was a second board, a small piece of Veroboard to hold the power supply components.  Then, the limits of the processor came apparent, and to deal with timing conflicts in a chip which has only one timer, I needed to add a second chip, on yet another board in order to control the servo which moves the governor valve.  And of course all the flying leads necessary to connect the three boards and other components.  And I wanted to add a potentiometer to allow tuning of the proportional band during operation. 

There was no longer any possibility of fitting the project into a protective box that would fit under the engine platform.  I decided to rebuild it on a larger board, and put it in an enclosure which would also accomodate the display, along with the potentiometers for speed set point and proportional band.

And that is about where I was up to when I last updated my progress.

The reason for the long silence has is best explained as a series of misadventures, compounded by trying to make too many changes at one go.  Some of the maths was very clumsy, so I rewrote that part of the program.  We went back into lockdown, so I couldn’t easily just buy some components I needed.  I made a change to a resistor value that I thought was inconsequential, and promptly forgot that I had even done it.  At least I managed to buy a suitable enclosure in the last few hours before the lockdown.

The circuit was soon rebuilt on a single board, carefully checked over then powered up and the program loaded onto the chip.  It looked like it worked, but had had some serious glitches. The speed readings were all over the place, and in general, nothing like the project I had previously had operating.

Thus began a frustrating couple of weeks of testing, re-soldering, replacing components, checking and rechecking the program, but nothing worked.  I suspect if there were any soldering problems, they were fixed in the first day or to.  I found a couple of program errors I had introduced during the program rewriting.  I imagine it was like trying to coax a reluctant engine into running.  Nothing seemed to be obviously wrong, nothing I did made any difference.

Then something reminded me of that resistor value I had changed.  It  was the load resistor in the collector of the photo transistor.  And I started checking more carefully the output signal from that photo interrupter device.  It was working, but not giving the clear low signal the processor needed as it had previously done. 

Eventually it transpired that the current limiting resistor in the LED part of the photo interrupter was also originally not close to the optimum value.   And the original two resistors were in the right proportions to work as required, despite not being optimum, but when I changed one, .........

As soon as I changed the second resistor, it all worked.  So simple if I had looked in the right place in the first place.  The perils of being an amateur.  An error that someone more knowledgeable in electronics would not have made.

So, I have now been able to add that potentiometer for the proportional band tuning, and even included a little routine that functions much as the usual integral function.  And I even have  a second board, with the correct resistors installed for a future project.

In operation, if the proportional band is too high, the governor hunts a bit, as is expected in this condition.  Reduce the proportional band a little and it quickly stabilises.

However, once it gets to the point where the correction required is less than one unit of the governor movement, about 1% of the range, it was either too fast or too slow, and oscillated each side of the set point, rather than the more classical constant small error.  The range was small, but I wanted to do a little better.  The processor is limited integer maths, so is a little clunky.  The final governor movement is determined by a division by 256, and with integer maths the remainder is just lost.  I devised a routine to accumulate this error, and every few cycles of the program, add one to the governor movement.  This seemed to do the trick, and the speed is now held within a few rpm, which I consider quite satisfactory for a reciprocating engine.  The integral factor in a PID controller is based on integrating the error over time, and adding a bit to the output on a time basis.  My processor did not have an extra timer function available, but my routine accumulates the error from each cycle of the program, so a similar function, though the time is not constant, as the time for the program loop to execute is determined largely by the engine speed, and the timeout takes to read the incoming pulses from the pickup.

Not much action for a movie to show the completed project, but a couple of still shots.  The displayed speed is real, the electronic flash has totally stopped the engine motion in the second photo.

I still need a knob for the second potentiometer, and an extra plug to tidy up the wiring between the Governor and the engine platform.  That will happen when this lockdown is lifted enough to allow a visit to the shop.  I may even be able to find a smaller enclosure that will still accommodate everything.  But for now I am calling it complete.  It is a very suitable substitute for a traditional flyweight governor for a small model, and provides a good adjustable constant speed for engine load testing, steam consumption and so on.  It’s been a good learning exercise, and quite enjoyable.  But it’s time I started a project requiring a bit more swarf.

I am quite happy to provide circuits and program details if anyone would like to build a similar project.  It should also be quite possible to achieve the same result on Arduino or whatever other processor you are familiar with.

Thanks for all the interest and to everyone just looking in.

MJM460