A digital governor for model engines

[MJM460]

Thanks Dave.  That is an interesting survey of the many different systems that have been tried over the years.  I guess some are to avoid patents, while others are genuine attempts at improvements, and I assume some work better than others in any given application.

Continuing with Governor valve and stop valve -

While I am getting time in the shop, I decided to get on with the governor valve to control the steam.  It will be needed before I can do any meaningful testing.  I have also found in in the course of my boiler testing that I really need a stop valve.  In concept, both functions could be incorporated in one valve.  However, the stop valve need to be one that can provide positive closure, easily achieved with a screw down type, but that is not so convenient for servo operation.  On the other hand, the governor valve does not really have to shut pressure tight, but should give a suitable opening characteristic, should operate without too much friction, which the servo has to overcome.  It should also be carefully designed so the pressure forces are balanced, again to minimise the load on the servo. 

Of course there are very powerful servos available, especially if I spend a bit more money, but this is proof of concept, and I will,try with a little servo that I have, and only go for a bigger one if really necessary. 

I decided the simple solution is to use separate valves.  This is also in line with industrial practice where it is normal to have the isolation valve and governor valve as separate items.

Finally, I have to redo the steam piping from the boiler to the engine to include the two valves.

I have previously always made a screwed nut and tail connection for my steam lines.  But as you can imagine, considering the industry I worked in, I wanted to try flanged connections.  I started some sketches for the stop valve, but of course, the boiler already has a connection for a nut and tail connection.  I vacillated for  a while, but after measuring carefully the dimensions between the boiler and engine connections, I decided to compromise and make a screwed connection to mount the stop valve on the boiler, but use a flanged outlet on the valve, to take the governor valve.  Not a really elegant solution, but I wanted to not overhang the edges of the baseboard, and did not want condensate traps in the piping if they could be avoided.  It took me quite a while to reach a solution that met all the constraints, before I could start to make the stop valve.

I took a lot of inspiration from the stop valve on one of Willy’s builds, thank you Willy.  I particularly liked the screw down, globe form of his valve with the outside screw and yoke, with a rising stem, the favoured design in the petroleum industry.  The particular feature of this design is, as the name implies, the outside screw, which means the screw thread is out side the body, so not subject to corrosion from what ever product is in the piping.  This means the nut, which causes the stem to rise, needs to be supported outside the valve body, also clear of corrosive fluids.  The industrial valves also have a non rising hand wheel, which means a non-rotating stem.  I decided that was a step too far, so outside screw and yoke, with a rising hand wheel it is.  I also chose a screwed in gland with a few strands of Teflon impregnated packing for the stem seal.  So many parts for just one valve, yet there were thousands in a reasonable size hydrocarbon processing plant.

 The trickiest parts are probably the small screw threads and the slender stem threaded M3, with M2.5 thread on the end for the nut which secures the hand wheel.  The flanged outlet is on the side of the body silver soldered in.  The inlet connection has a tail screwed into the end of the body.

It is basically a simple turning job with a little milling.  I have taken lots of photos and I hope they will show some of my setups and methods.  I am still a beginner in this area so suggestions of better ways for next time are always welcome.

The first photo shows the main parts ready for some holes to be drilled and tapped, and silver soldering the outlet. This will make it clear where I am going.  I left the body as a full cylinder to make it easier to hold for the remaining operations and trimmed it down a bit later.

Then some progress shots of the individual parts.  The yoke and columns, business end of the stem, threading the stem M3, tapping the threads in the bonnet for the columns, making the handwheel, and the completed valve.

Next time, the governor valve.

Thank you to all for looking in.

MJM460

[Brian Rupnow]

Crueby---the exhaust valve was "disabled" and held open. That way the crankshaft could revolve as many times as it wanted and not come up on compression. And since the valve was held open during the miss cycles, there was no vacuum created to draw fuel into the carburetor.---So--during all of the "miss" cycles when that engine was coasting, it didn't use any fuel either.

[crueby]

Crueby---the exhaust valve was "disabled" and held open. That way the crankshaft could revolve as many times as it wanted and not come up on compression. And since the valve was held open during the miss cycles, there was no vacuum created to draw fuel into the carburetor.---So--during all of the "miss" cycles when that engine was coasting, it didn't use any fuel either.

Makes sense, thanks!

[gary.a.ayres]

Valve looking good, and some nice machining pics.

 

[steam guy willy]

Hi MJM , the stop valve I used was a copy from full-size practice and it gives a more realistic look !! I hadn't actually thought about the corrosion aspect .?!! Also when I was helping with the Beeleigh engine I was playing with the disconnected governor and was really surprised when I gave it a push round ... it just seemed to take on a life of its own and the balls just seemed to take off and rise and whiz round  on their own  and didn't seem to have any resistance anywhere ..quite strange  but  quite interesting ?!!! I also felt quite privileged to be able to investigate and explore all the aspects of this 1830's  prototype !!

Willy

[MJM460]

Thanks Gary, it’s good to have you following along.  I’m glad you like the pictures.  I tried to include some steps that might interest others, but not too many of simple operations.  Overall this part of the process is not demanding on precision or techniques we see in other threads and I don’t want to bore people, (just the brass block.)

Hi Willy, your experiments with that governor illustrate one of the most fundamental laws of physics, conservation of angular momentum.  I hope you took a video, or preferably several, that can be edited into a visual display beside the engine.

We have all been taught about the skater, twirling on the point of one skate, tall and straight with her arms over her head, then bending down as she spins, extending her arms out to the side and the other leg out in front, slowing down as she goes down which seems right, but then, without any visible extra push, speeding up as she stands up again.

I don’t think I could simple do the down and up motion, especially on one leg, let alone balancing on one toe and spinning like that.  It meant very little to me at the time, the texts books all came from abroad, but we had never have ice or snow, and even house hold refrigerators in those days were limited in the number of ice blocks they could make.  At least the ice man had stopped coming to deliver a block of ice for our ice box, which was our only cold food storage before we got our first fridge.  My total knowledge of skating involved a little Dutch girl and her wooden skates. 

I can almost hear Brian laughing at this stage, enjoy it Brian, now I have enjoyed living in your country, I understand that it is appropriate to laugh.

That governor is an even better example, though which of us even new the word back then, or had ever seen one, let alone had the opportunity to play as you have done.  Play is such a good learning technique. 

OK, so why is it so important?

Conservation of linear momentum, and conservation of angular momentum, which could also be described as spin, are two of the two most fundamental laws of physics.  They hold when even conservation of energy falls down.  The linear one is the basis of the mathematics behind Newton’s law about bodies continuing in a state of uniform motion in a straight line unless acted upon by an external force.

The angular momentum law is very similar mathematically, and explains the spinning top, gyroscope and most importantly that poor skater, condemned forever to keep on spinning, to illustrate this law to generations of young children.  And it explains your observations on that uncoupled governor.

So what is going on?  You did not elaborate on what you meant by “coming to a life of its own”.  So let’s see if I can tell you what it did.

That governor with its heavy metal balls has a moment of inertia, just like the flywheel, but unlike the flywheel it’s geometry is not fixed.  When you give it a push to start it spinning, the balls are free to move in response to the centrifugal forces.  So they fly outwards, and due to the levers and connection points, they rise.  But that outward movement changes the geometry, and geometry is fundamental to the magnitude of that moment of inertia.  Remember, the moment of inertia is the sum of the contribution of each little bit of the mass of the system multiplied by the square of the distance from the axis of rotation. (Mathematically I = m x r^2)

So when the balls start to fly out, r increases thus increasing r^2 even more and greatly increasing the moment of inertia (I). 

Now similar to that body moving in a straight line, the angular momentum version is that a body which is spinning continues to spin unless acted on by an external torque.

I hope that you are still with me, ‘cos here’s the rub.  If we allocate the symbol w to the rate of spin in radians per second, then the angular momentum equals I x w^2 or w squared.  The two equations together mean that angular momentum is proportional to w^4, so very sensitive to the distance of those balls from the axis.. Conventionally the symbol used for rotational speed is the Greek lower case version of omega, but I find that difficult to insert on an iPad, so please forgive me for using the vaguely similar looking w.  Read it as omega anyway.

Then we have angular momentum (L) equals I x w.

Putting the two together gives us L = m x r^2 x w for a mass m at radius r.

(Thank you to jadge for pointing out the error in the original text of this post, it is much appreciated.)

Easy, I hope, for fixed geometry like the flywheel.  A larger diameter or more mass in the rim each have a similar effect.  But what happens when the geometry of the spinning body changes.  Well, with that obviously well made governor the bearings are very low friction, and it is disconnected from the engine, so let’s ignore that very small friction torque, along with air resistance, which will eventually slow it all down. 

You give it a push to start it spinning so the balls fly out, then no further push, so no external torque from that source. So when the balls fly out, I increases, Angular momentum is conserved, so if r increases, w, the rate of angular rotation, has to reduce.  The whole thing slows down, centrifugal forces reduce, and the balls start moving down.  And guess what, as they move down, I reduces so it all speeds up again. 

The balls on their levers form another rotational system, though it’s range of spin is obviously limited.  I would guess the balls for a variety of reasons, don’t quickly find a steady spot, so they move up and down a bit, and the speed increases and decreases with the movement. 

I don’t know how much that last bit happens, friction in the pins might quickly damp it out, but I am guessing that you were surprised at how long the governor kept spinning after your initial push, and after initially rising, the balls only gradually drop down as friction inevitably applies that external retarding torque.

Does that about describe it?  I would love to see the video if you get an opportunity.  Physics lessons in every playground!

Thanks for following along.

MJM460

Oh, and I also now know that it actually easier to balance while spinning than it would be if not spinning.  But there is no escaping the strength it needs to do such a deep squat and then stand up again, all on one leg!  I am in awe of that.

[steam guy willy]

Hi MJM..Well I didn't realise there was all this going on !! and of course there's always the maths to support the principals I'm afraid that the engine is now all back together so I can't play with it anymore !! I was quite surprised that the governor behaved the way it did and this is why I mentioned it ...also the Ice skater analogy was interesting ..and I suppose a Flypress is also similar which is why you can do what it does with the minimum of input ? ie just pull the lever... so with a flypress you can do a lot of work with the minimum of energy input.?? try pushing out a washer with just your thumb !!! I suppose there is also similar maths associated with this ?  Thanks for all this and as usual you have gone into so much detail . The energy it takes me to just ask a simple question is multiplied enormously by your succinct reply !!!   

willy

[Zephyrin]

Great thread to follow, quite interesting topic.
nice to see your built.
may I add a link to a video of a working regulator I build for a large steam engine a few  years ago, although the speed of the engine is regulated by the steam supply, their action is perceptible, from 255 to 350 rpm, and funny to play with ! 


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

[MJM460]

Hi Willy, I am not sure what you mean by a fly press in this context.  Essentially a fundamental law of thermodynamics says that you can’t get more work out than you put in.

Certainly you can amplify force with levers or hydraulic pressure, usually involving more force over less distance or vice versa. 

But in the context of angular momentum, the energy you put into that governor by giving it a push is stored as angular momentum in the spin of the governors.  That momentum gradually gets reduced to zero over quite a long time.  It takes a long time to change the angular momentum because the the friction torque is very small.  Let’s say 50 seconds for easy maths.

But what if you stop the fly balls very quickly, say by swinging a large brick in the path of one of the balls?  The brick will be pushed in the direction the balls are travelling, and conservation of momentum means the sum of the momentum of the brick plus the balls will be the same before and after the collision.  Assume the brick crumbles a bit, absorbing the impact of the collision.  But think about the force of the collision.

The balls will stop very quickly. The momentum of the balls changes to about zero in a very short time.  To make the maths easy let’s assume about 0.05 seconds, or 1/1000 th of the time that friction took to stop the pendulum.  The maths says that the torque equals the rate of change of angular momentum, so the torque would be 1000 times the size of the friction force.  But the change of momentum is about the same.

So you can stop the governor or a flywheel or what ever over a long time with a small torque, or over a short time with a large torque.  That torque can be very high indeed, and cause quite an impact with the brick.  I believe there are some tools that operate on this principle, but others will be able to describe them better than I can, as I have not had the opportunity to try them.

I wonder if that is what you mean?

Hi Zephyrin, that is a great model.  Thank you for posting it.  Videos of a working governor are always interesting to see and particularly relevant in a thread like this one.

You will have a good idea of what force is available after playing with the lever like that.  Quite a challenge to make a steam valve that operates with such a small force.  It looks like you have made a pretty good effort.

MJM460

[MJM460]

The governor valve

The point of the governor is to control the speed of a steam engine, and that requires a steam throttle valve to be operated by the servo which provides the output from the electronics. 

The governor valve has some specific design requirements.  First it should involve minimum friction to operate.  The most difficult aspect of this is the seal or packing where the stem exits the body.  I designed the valve body with an adjustable gland and space tor two or three turns of some Teflon impregnated pump packing I had in my miscellaneous materials drawer.  I hope that I can find a point where it is tight enough to seal, but not tight enough to cause too much friction.  Next is the more general friction where the necessary moving parts slide or rotate in the fixed valve body.  The big plus of a separate governor valve is that it does not really need to shut off tight, or even totally stop the engine, the stop valve will do that.  So long as it reduces the speed of the unloaded engine to the slowest required speed, and can vary the opening from there to fully open in the range of the servo, the fit is not too critical.

The governor valves made by Willy and J.L were butterfly valves, which operate in less than a quarter turn, by turning a thin disk so that it varies between fully blocking the flow path and minimum flow resistance when it is side on to the flow.  I decided to go with that style of valve, but my steam passage is only 3.5 mm diameter, and I do not fancy my chances of making a shaft and disk and riveting the two together in that space.  Hat’s off to both of them for achieving that feat.

Then I realised that the shaft did not have to be limited to being small enough to not blocking the flow.  Why not make it 5 mm diameter, and machine a slot in the right place to form a disk, which does not have to be round, by the form of the solid bit between the slots.  The 5 mm was selected because I have a suitable rod of silver steel, and a 5 mm reamer.  That seemed practical, so I set the shaft horizontal and machined one side, then rotated the rod a little and took another light cut, leaving the solid bit thick at the centre, but feathering out towards the outside diameter of the rod.  Then rotated the rod 180 degrees, and did the same on the other.  It is easier to picture than describe, so I have included a picture which I hope shows it clearly. 

With the disk sorted, I needed to complete the shaft design, which needs to take into account the pressure forces on the shaft, which are tending to force it out of the valve body.  Now I only have a hand reamer for that 5 mm hole, so I needed to go right through the body.  That required a plug each end.  One way of achieving pressure balance is to leave both ends protruding and make a gland for each end.  Then there is atmospheric pressure on each end of the shaft, and no net axial force, but two gland packings to cause friction and/ or leakage. 

I decided on a blind plug on one end, and to turn down the shaft at the other end to 3 mm diameter.  The smaller end will have atmospheric pressure against one end which is lower than the steam pressure on the centre 3 mm diameter at the other, but the out of balance force is less than half of what is on the other.  It’s less complex, one gland plus a blank plug instead of two, so seemed like a good compromise for the axial forces and friction.  Reducing the diameter to 3 mm through the gland also means the 5 mm part of the shaft is constrained in the body, so cannot be accidentally blown out by steam pressure, with the attendant undesirable consequences.

Another issue with the governor design is that the steam forces on the moving parts should be minimal, so they do not require undue extra force from the serve to move the stem in response to the control signal.  The butterfly valve is essentially balanced in this regard and the steam flow does not tend to turn the stem one way or the other throughout its range.

The final, even perhaps the most important requirement, is the flow characteristic of the valve throughout its range.  That is, how does the flow resistance vary with the movement of the servo.    The butterfly valve is highly non-linear, the first few degrees of opening have a much greater effect on the flow resistance than the last few degrees.  Also I have a rotary servo giving about 170 degrees of rotation, while the butterfly disk requires less than 90 degrees resulting in further non-linearity. 

A plug In that respect, a plug valve is better than a butterfly valve, as the shape of the plug can be varied to give any desired characteristic.  However, it has a strong axial force due to full upstream pressure in one side of the plug, and the lower downstream pressure on the other.  The ideal solution is the “double beat” design of valve which effectively involves two plugs placed so the axial unbalanced forces cancel out.  However I feel at this stage, that would be beyond me to make(another vital design consideration).  I decided to persist with the butterfly valve.  I will do some testing of the transfer function, and see if I can perhaps make some mathematical correction if indeed it is required in practice.  At the end of the day, theory always needs testing. 

I sketched up all the essential elements of the design and drew a simple rectangular body outline which fitted with some brass bar I had on my stock shelf.  A square bar is easy to hold for all the internal machining operations, and I could do some fancy machining of the outline later, or not, as the mood takes me.

I marked one end to be the top and scribed the centre lines at that end and along one side, and the centre of the cross hole for the steam path.  I make a practice of planning the operations so that I do all the necessary concentric cuts with one setting.  It is beyond me to remove it and set it up again absolutely on the same centre.  So I cut off the required length of bar, and because the sequence does not really require turning, I set it up on the rotary table and machined the side of the bar including the raised gasket face around the steam passage.  It probably would have been better to just face the whole side flat and go for a flat face flange, but hey, I worked in the petroleum industry, and this was not a cast flange!

Then I set up square in the vice, for the top end.  I milled the end flat and square, and drilled the centre bore 4.9 mm in preparation for the reamer. I had decided on a bolted gland using the shaft bore as the packing outside diameter, So I followed the drill through with the reamer to true up the hole and give a nice sliding fit for the shaft.  With that all done, I could turn the block around, centre as well as I could on the 5 mm bore, face and tap the bottom end for the blind plug.  Five mm is the tapping size for a standard M6 x 1 thread, so it was a matter of depth stops, otherwise known as nuts, screwed onto the tap. You can see this set up in the photo.

That is all the critical parts of the valve complete.  The bottom plug and the top “bonnet and gland are simple turning and threading operations, along with drilling the bolt holes and a bit of shaping for the flange of the gland.  Similarly the hub for the operating lever is simple turning, cross drilling and tapping for the set screw, so doesn’t need further description here.  All that remains is the obligatory selection of pictures, showing some of my set ups and techniques.  The last one shows it bolted to the stop valve.

To me the interesting part of this build is the conceptual design process required to ensure everything functions as required.  Nothing at all difficult about the machining, once the design is decided.  This is very different from some of the amazing work being done by other members of this forum, especially that governor Zephyrin has just posted.

I would still like to remove some of the excess metal from the block forming the valve body, purely for cosmetic purposes.  But I decided to see if it all works first.  The functional chunky block shape is starting to grow on me, so it might not happen.

Thank you all for looking in,

MJM460

[jadge]

If we allocate the symbol w to the rate of spin in radians per second, then the angular momentum equals I x w^2 or w squared.  The two equations together mean that angular momentum is proportional to w^4, so very sensitive to the distance of those balls from the axis.

I agree that moment of inertia is mass times radius squared:

I=mr²

But I thought angular momentum was moment of inertia times angular velocity:

L=Iw

Substituting for I gives:

L=mwr²

For the life of me I don't see how to get a w⁴ term?

Andrew

[steam guy willy]

Hi MJM , just wondering about different valve " innards" ? are there different configurations that interrupt the steam flow ?? would a butterfly valve interrupt the flow rather than a modern ball valve ?? and is there turbulence with different types of valves and does it have a detrimental effect on the governor action/function ?

willy

[MJM460]

Hi Andrew, You are quite right.  I am so glad that you came in so quickly on that one.  I normally check each formula before posting, but I did slip up badly there.

Later today, I will go back and add the correction so that it is not inadvertently picked up by anyone who does not see your post.

Thanks again,

MJM460

Willy, I will also come back later on your questions.

Edit:  The correction has now been posted, and carefully checked this time.  In the process, I also found a commonly used symbol for angular momentum, L, which I just could not recall at the time.  That should have been the clue to check my maths!

 I even managed to pick up a new skill in achieving the strikeout.  I just would prefer not to need it too often, but I do want errors picked up and corrected.

MJM460

[MJM460]

Hi Willy, the valve characteristic is as fundamental to the governor operation as any of the other moving parts.

I had not thought much about it until Jadge’s fascinating post about his investigation of the stability of his Pickering governor.  He gave me the terminology I needed to understand a bit more clearly what I am attempting.

The key to it is that the valve characteristic can give extra amplification to the governor motion, which can compensate for the limited lever movement.

Think about that stop valve of yours.  The end that plugs the passage through is about a 90 degree cone.  It needs a certain amount of lift to be effectively open.

If the bottom of that plug was flat, it would be full open on a very small lift.  On the other hand, if it was extended to be a finely tapered needle, it would have to lift a long way.

The governor valve has to have a smooth progression between open and closed, and achieve the whole range within the range of movement of the actuator, whether a servo like I will use, or the lever operated by the collar which lifts as the flyweights rise on you machines.  With sufficiently accurate manufacturing that shape of the needle can even be modified by being given a curved form instead of a straight taper.  The form of that curve can modify the flow characteristic as the valve opens.

Ideally we need to know the transfer function of not just the valve, but the whole system from the controller output to the change in power output of the engine.  Now that Jadge has pointed out the importance, I will do a little test program, once the electronics are complete and working, to both test the electronics and prove that I can drive a servo, alter a set point using the potentiometer.  Clearly it is possible, based on the available commands for the processor, but I have to prove my maths on something simple before I add in the feedback loop needed for control.  Of course this means I will need a display, so I know what signal I am giving to the servo and what engine rpm results.  Then I can draw a graph of the characteristics.   Ideally I would repeat this with a fully loaded engine, but operation at a controlled speed will be an adequate demonstration of the governor feasibility, until I make a suitable load.  Big advantage of programmable electronics is that it’s a program change, no wiring changes needed, to change from the transfer function test to the governor program.

I hope that makes sense.

Your second question about friction for the steam flow.  Extra flow resistance is not a problem in any throttle valve application.  In fact, the whole idea is to vary the flow to control engine speed, which is achieved by varying the friction through the valve.  The energy is not lost, it is an adiabatic process, (remember that term from the thermodynamics thread?). It is retained as heat, though the amount of energy that can be converted to useful work is reduced due to the lower pressure.  But again, that is ok as the engine produces less power anyway at the lower speed, so less work to be done.  For efficiency sake, throttling is not desirable, but there are sometimes other reasons for generating steam at higher pressure then reducing it to control engine speed.

A butterfly valve restricts the flow when the disk is turned so it nearly blocks the pipe, a ball valve would also do so, but would be beyond me to make.  I did consider drilling a hole in the shaft instead of machining something like the disk, which would do the same thing as a ball valve and it would not be too difficult to make another stem with a cross hole to experiment.

MJM460

[derekwarner]

Hullo Willy.....

Ball valves, be they HP or LP, are designed to be a means of fluid isolation or direction [and again be they L port, I port or T port]...either open. directed or closed.....this is true for any fluid

Incorrect installation as a form of metering device is fraught with long term danger.....wire drawing of the Teflon/POM seats.......inability to maintain a fixed setting and the final loss of absolute sealing...or leakage 

Derek