A digital governor for model engines

[MJM460]

There have been some wonderful governor builds as part of recent and current build logs.   I am continually amazed at the workmanship, even of those which do not actually control the throttle plate.  And the ones that do are just fabulous.

Not feeling that I am up to such a task yet, I decided to try a different approach.  To see if I could come up with a digital governor to bring the state of the art, or at least the understanding, up to the present day technology.

The flyball governors we have seen on this forum were of course invented quite early, but those not familiar with modern factory machines might not realise that they are behind the works of  modern governors, and there are many of them on high speed machines even today.  Not in the recognisable form of steel balls whirling around in plain sight, but a miniature, high speed version buried deep inside a boring looking, cast steel or aluminium casing.  About the only obvious change from the original is the incorporation of hydraulics to operate the throttle valve instead of a direct governor powered lever system.  So the fly balls (not even spherical on the last ones I saw) operate a shuttle valve in the hydraulic system, which in turn operates a larger piston, and a hydraulic pump provides the necessary pressure to operate the valve quite precisely.

At the end of the day, despite the different external appearance, deep inside they were the same analogue device as the original flyweight type.  A force usually described as centrifugal force, which is proportional to the square of the rotational speed, causes the weights to fly outwards, lifting a collar which moves a lever in a proportional manner to operate a tiny shuttle valve which in turn drives a piston to cause the required movement of the throttle plate with what ever force is needed to operate the high pressure steam throttle valve.  I have a vague memory showing through the mists of time, of a Dynamics lecturer over fifty years ago, talking about governor power, and how much work can done by the action of the flyweights.  I have often wondered how much that scales down on a miniature, where the masses are so small, but the governor still has to overcome friction of the valve stem seal plus any out of balance force on the throttle valve stem.

However, the most recent machines I installed prior to my retirement had digital governors.  They were sufficiently new at the time that electronic devices were not accepted as independent over-speed trip devices, and a much less consistent and less reliable traditional mechanical device had to be installed as well, to back up the much more accurate and reliable electronic governor.  Not even additional totally redundant electronic over speed trip systems were not accepted.  (I believe this anomaly has since been eliminated and a good triple redundant electronic is acceptable these days.)

As part of preparing the training program for the plant operators, I wanted to work out and explain how these digital governors might work, not only for myself, but as part of the training program I had to prepare for the operators who, when construction of the plant was complete, might meet a digital governor for the first time.  You might guess that I am never happy with the “black box” description.  This was some 20+ years ago, but I did work out a scheme that might be what the governor implements inside.  I  recall that I used pictures out of a Richard Scarry children’s book, with a Nicky Bunny holding a pendulum to measure the speed of a water wheel, and a lever to adjust the gate of the water sluice at the top of the wheel.  I wonder how many of the kids who read that book realise just how clever those pictures were.  We wore those books out reading them to our kids.  And it turns out that we can now make a very good digital governor with low cost, readily available components, that is suitable for our model engines.  I just had to try it.  I hope forum members will find it interesting.  It will not replace the fascination of the fly balls whizzing around for display purposes, but for functionality, I have high hopes.  I envisage it all hidden in the base with a shaft from a servo unobtrusively “helping” to move the throttle valve.

A governor is designed to provide "closed loop" control with negative feedback.  Closed loop means the effect of any control action must be fed back to change the input, thus again affecting the required control action.  Negative feed back means that when a deviation (in speed for a governor), the control action must be to change the speed in the opposite direction to the speed change.  A governor, on detecting the speed to be too high, must move the throttle plate to cause a reduction in speed, and vice versa.  I have drawn a schematic to illustrate this in the first attachment.

On a flyweight governor, the weights or balls move out (and up) when speed increases, and a  linkage must be set up to close the throttle plate when the balls move out.  And the whole thing is analogue, a continuous effect with no discrete values.  Note that at no stage does the actual speed need to be measured and displayed, though a scale could be put at an appropriate point on the linkage and calibrated to read speed.

Normally a governor has a adjustable spring to resist the throttle lever movement, so changes the force necessary to move the throttle.  This is used to adjust the speed at which the engine operates when the force developed by the weights just balances the spring force.

I decided as proof of concept, to just emulate that basic blind governor with a simple speed adjustment.  I did make one change however.  Obviously with the flyweight governor, when the engine is stopped, the weights fall to the lowest position, with the valve fully open (to increase the engine speed).  Hence the engine is usually started on a separate manual throttle valve and carefully brought up to speed until the governor takes over by starting to close the governor valve.  It is not really practical to do anything else with the mechanical linkage.  However, with a digital governor, the logic can be altered depending on whether the engine is running or not, so I have made the logic set the valve at minimum running position if the engine is stationary, and then opens up slowly to increase the engine speed to the set speed once it detects that the engine is running. 

Enough introduction, and a post that is long enough already, so next time, let’s make a start on the actual design.

Thanks to everyone who has hung in there to get this far,

MJM460

[Admiral_dk]

(I believe this anomaly has since been eliminated and a good triple redundant electronic is acceptable these days.)

I should certainly think so .... If not, you would not have all the airplanes from the last 30-40 years or modern cars and motorcycles that are "Fly-by-Wire" (the Wire being electric) ....

Your latest project should give the bit between the ears some challenge - unless you made a number off those before.

Per

[MJM460]

Hi Admiral, thanks for looking in.  In that comment, I was referring specifically to an over speed trip, separate from the speed control, which has obviously been in widespread use for quite a long time.  Couplings can break (though a very rare event) when they are used at maximum power 24/7/365, and a turbine rated at several MW, suddenly relieved of its load, accelerates to destruction very quickly.  The machines I am familiar with, have a separate over speed trip which is supposed to operate to protect the machine in that admittedly rare event as an exploded rotor is not a pretty sight.

It is also written in the context of my experience in the oil industry, which is very conservative on those things, and most of the standards require everything to be well proven in operation before anything new is accepted.  Electronic governors were acceptable, but more common was a Woodward fly weight governor with hydraulic actuators for the steam valve.  The set point for these governors was normally managed by electronic instruments.  But there was a requirement for a totally independent mechanical over speed trip device as a last resort protection.  Independent triple redundant over speed devices, they even had separate toothed wheels, were available and much more reliable than the mechanical trip, and they were installed.  But the industry was reluctant to say that separate mechanical device could be left off, so a steam turbine often had a governor, triple redundant electronic over speed trip and deep in the shaft, a mechanical trip bolt.  Basically all intended to reduce the probability of requiring a shutdown for maintenance before the intended period, often three years or more, but they don’t have to be light enough to fly.

The aircraft industry has very different requirements but at the end of the day, I am sure that they are well focussed on avoiding a plane falling out of the sky.  But I believe they do push things to the limit so they can fly at all, and then to maximise the payload of passengers and freight.  Then they use very intensive maintenance to avoid premature breakdowns.  I am not familiar with the details of their specifications, but I suspect that they don’t normally shut down the engine suddenly without warning, to prevent an engine failure, mid ocean, especially on a single engine aircraft.

I hope that gives my comment a clearer context.  Perhaps I should have avoided the diversion and just talked about the governor.

By the way, I notice there is a word or two missing from the section you quoted, I will edit that after I post this reply.

Yes there will be quite a lot of head scratching before this post is complete.  I have not done it before, definitely a “proof of concept” project.  More on that as we go.

Thank you for commenting.

MJM460

[Don1966]

I worked in the oil field for 55 years As a Tech and Marine Electrician and quit familiar with electronic speed control. I have thought of electronic governors for our engines also and glad to see your taken the initiative to start a thread on it. You have covered a lot of requirements needed. The loop also needs stability control, time control, droop control and speed control. by speed control meaning over speed shutdown or missing pulse ( loss of mag pickup) shutdown.  It being inverse or direct proportional control. It needs an ability to know when your ready to start the engine to produce an Adjustable output for starting and Time ramp to final speed without overspeed.
One of the main issues is to have the output power to drive the device that will control the engine speed with. It will need to be a proportional valve that throttles to outPut signal by PWM or Proportional current control.
My two cents worth. Looking forward to more information as you progress.

Regards Don

[MJM460]

Hi Don, thank you for looking in.  It will be great to have your experienced eye looking over my shoulder as I progress on this one, and it will be most interesting to see how much I can achieve.  That is a great check list of desirable functions to try and include, thank you.

My plan is to start with the most basic functions of a flyweight governor, which is only proportional control, and has none of the extra fault protections, and add features one by one until the little chip can no longer manage it.   And there are more powerful chips in the range if the little one runs out of grunt.  I don’t really know where the limit will be.

My plan is to use a standard radio control servo operating a butterfly type throttle valve in the steam line.  Amazingly, these servos have standardised the wiring and control protocol across the industry, so apart from some using a different connector on the end of the input wire they are basically interchangeable, and the Picaxe has no problem driving them cleanly with a very simple to use command.  There is a very wide range available, ranging from ultra miniature used for rudders on small flying models to quite powerful ones used in robotics.  I am expecting that my throttle valve will seal well while only requiring a servo at the smaller end.

Here’s hoping for big things ahead.

MJM460

[steam guy willy]

Hi MJM. an interesting concept and project...When I was in the Army as a control equipment technician working on the Tanks gyro controlled gun stability system they had a negative  feedback speed control Cct coupled to the rotating azimuth and elevation motors. these were small DC generators...and...one of the favourite tricks of the tutors were to connect the wires in reverse !!! this had the effect of the turret revolving at an alarming rate when the grip switch was operated  !!! really scary .. Also on the Beeleigh Mill engine the governor had a separate handle that disengaged it from the butterfly valve main cock...On one of my engines I attached a magnet to the flywheel and this triggered a bicycle speedometer to give an indication of speed !!>Willy

[crueby]

Very interesting project MJM, going to be following along. There are some of the guys in the rc sub community doing contrllers for pitch and depth control with very small processors.


And Willy, that picture of the handwheel includes some official bailing wire, looks like!

[Roger B]

I will be following this with interest      I started some experiments to design the governor for future engine builds here:

http://www.modelenginemaker.com/index.php/topic,8690.msg192197.html#msg192197

http://www.modelenginemaker.com/index.php/topic,5545.msg162158.html#msg162158

This is a completely enclosed design which worked reasonably in the trials but I have some other problems with the engine design to resolve before I can go any further.

When I started my petrol injection trials Chuck Fellows was interested in using an Arduino to control the relationship between the throttle opening and injection pump stroke.

[gary.a.ayres]

Electronics is a dark art as far as I'm concerned, and here I feel like a Barbarian standing before the gates of Rome!

I have no doubt that it's at least as unforgiving as mechanical engineering is (and maybe more so) if you don't get it right.

Fascinating project, though, which I'm sure will be very rewarding as it progresses.

Very interesting exposition on the development of governors too.

gary

[MJM460]

Hi Willy, magnets and a Hall effect sensor like some forum members use for electronic ignition systems can also be used for speed measurement with these processors, and you can program the internal maths to read out rpm directly.

Disconnecting the governor is I guess an early approach to bringing the engine up to speed slowly as the governor starts calling for the valve full open.  They have a tendency to not close quickly enough to a more reasonable opening as the speed gets near running speed, so over speed is a real possibility.

Hi Chris, the possibilities for these processors are indeed limitless, it’s all about selecting the sensors, and there are so many available.  Accelerometers, gyros and gps are all available in miniature and at quite reasonable cost.  Once you can drive a servo, the processor can control an amazing range of things.  Arduino uses a very similar processor, but a different system for programming and controlling the chip.  It is also quite suitable for this type of project.

Hi Roger, thanks for reminding me of those threads.  I was obviously following them at the time but they escaped my mind when I was looking for them.  Certainly a great idea for a neat totally enclosed governor.  I will definitely keep an eye out for your further experiments with those.

Hi Gary, I must admit that I also like the mechanical aspects as it is usually so easy to see what is going on, and intuitively predict how they might respond.  It is not the same with electronics, especially with integrated circuits used in these microprocessors.  If the circuit does not do what you expect, it is quite difficult to see what is going on.  But you are not alone in finding electronics a bit of a black art and I will be trying not to leave anyone behind while still keeping it interesting for the experts among us.  Don’t hesitate to ask a question if something seems to obscure.

It is great to have you all on board.  Thank you so much for responding.  You are inspiring me to push on and stay the course.  I hope we can all enjoy the ride.

MJM460

[deltatango]

Hi MJM,

This is a great project, thanks for sharing it and taking us along for the ride.

I've built a couple of things using the Arduino and you've just given me the idea for another!

David

[steam guy willy]

Hi MJM, with the cycle speedo you can type in the speed by using the diameter of the wheel to show MPH. and possibly change that to read RPM  I shall fiddle about with it to see if that works !! Thanks for the explanation of the governor handle....  good to see the interest here...

willy

[MJM460]

Thanks Willy and Delta Tango.  Your interest is always very encouraging.

David, I don’t see why it can’t be done with an Arduino, especially if you are more familiar with that system.  It may even be easier to manage the calculations.  I will be delighted if this thread inspires you to give it a go.  Preferably after you finish that engine, we are all anxious to see that proceeding, and don’t want to interrupt your progress.

MJM460

[MJM460]

More Detailed Design Thoughts

The simple schematic in the previous post showed the essential elements of an engine control loop with feed back.

Of course, once this much is working, I have a few ideas to make even better use of the processing capability of these tiny chips.  But l had better start simple.

We know that a digital governor has to have a speed pickup, a calculation of an appropriate output response and a control valve to change the speed, all connected to give the appropriate negative feedback.  It also needs a means to adjust a set point speed for the governor to control to.

With a flyweight governor, the speed set point is adjusted by the initial compression of a spring which resists the throttle movement as the force from the fly weights increases.  For an electronic governor, I can emulate that spring with an analogue input to the processor, using a potentiometer, and scaling that input to adjust the set point speed.  The controller includes an easy to use analogue to digital converter on an input pin, which is connected to the slider terminal on a simple potentiometer.

The speed pickup is achieved by a toothed wheel that interrupts a light beam, which is detected by a photo transistor.  The necessary LED and photo transistor come prepackaged in a quite cheap package ready to connect up with just 3 wires.  You can see it in the attached photo, along with my “toothed” wheel.  (Oops, I forgot to include a miniature penny!). That had to be made before any electronics could do much.  (I will include more detail on how I made it next time, but the post was getting too long and I wanted to include a picture.)

Now the mention of microprocessors might cause visions of complex programming languages and equipment, and you might feel it it not for you.  But a company in the UK has developed a solution for educational purposes in schools.  They load a programme on to a quite powerful standard chip as firmware, and provide for free download of a very user friendly programme editor.  The chip then requires a simple network of a few resistors on the circuit and a special cord which plugs into a USB port on your computer, and you load your programme with the chip still in place in your circuit.  No relationship to the company, but I have found it really great, and even used it to start my young grandson on electronics projects.  Look up Picaxe on your usual search engine.  It is being overtaken by Arduino, but I find it easier to understand and well worth looking at for a tentative toe in the water.  I am using an 08M2 which is an 8 pin chip.  You can see it in the second attachment of my first post, which shows the completed controller board.  The connections and plugs include the programming jack, but I saved some space by using flying leads for some connections.  I am sure this would be frowned on by many, but it will serve for a proof of concept.  Delta Tango has mentioned that he might try something with Arduino, a different system, but I am sure equally suitable.  There are pluses and minuses to both.  Others with the necessary knowledge might go for using the manufacturers base chip, and program in machine language or their preferred higher level language.

Finally, the output actuator can be a simple radio control servo.  The microprocessor can output the appropriate signal to control servo position.  The servos are quite cheap these days, and are used by hobbyists in robotics as well as radio control.  They are available with more than adequate output torque to control a throttle valve in the steam line.  I do have to make a throttle valve for the servo to operate, another item I have not previously tried.

So there we have the background to this build.  It is not all electronics, in the next few posts I will outline the design and making of the mechanical parts which are vital to the governor function.  I hope you find it interesting. 

Next time, some swarf as I make that wheel, shown finished below, the governor valve, and while I am at it, I really need a stop valve on the boiler, so I will make it as well, and only modify the piping once.

Thank you to everyone looking in and especially to those who have commented.

MJM460

[CHP]

nice project you have their. I personally don't think you need a microcontroller
to do that. I think that using two optocoupler (are what ever) as in put to a comparator circuit.
Any change in gain would lower the voltage to your coil and disable it till the output
match the input. The potentiometer that control your gain could also be used to control
engine RPM. 
My 2 cents lolll