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Tachometers
2

Tachometers

Tachometers

(OP)
Hi,
People don't often post about solved problems, but here I am. I'm hoping this will be a fun post for the group. I think have solved a problem and struggling to figure out why I had to spend 7 years working on it when it was so obvious at the end. Several years ago, I was trying to solve a problem that had daunted me for years, came up with a solution, and moved on. Since then I've looked back at what I did and wondered "why hasn't anybody ever thought of that before?"

I can't find any patents for the device, and I can't find any articles in journals (IEEE) that I have searched.
Once I settled on the idea, it seemed "obvious" to me which breaks one of the principles of the patent - but then how come nobody does it...?
I'm not really interested in getting a patent - the rigamarole, the cost, and the futility of trying to defend it are all disincentives. But I really do think I should publish about it - if this truly is a unique idea. It would be nice to have a useful article to my credit. It's an idea I would like to share, but I want to find out if it's really a new idea, first. So I'd like to formulate a little test and see if anybody else finds this "obvious" with a few clues and conditions. If I just told you, then any smarty pants can come on and say "yes it's obvious" and publish it for themselves.

I promise to tell the rest of my story but only if there's interest, or if the answer is so obvious that somebody guesses right away. I'm a mechanical engineer with an interest in all engineering disciplines, dabbling in many others, so I could have missed an obvious solution or the one I came up with is indeed obvious and electrical engineers do it all the time (for some reason they don't talk about it?).



First I have a confession to make: I built a wind turbine. (No apologies to those who are offended) I built it out of personal interest but I have been approached by a person who wants to buy one just like it from me. Never considered this a commercial product before, but if I am going to look at it that way, then I need to work on some of the control and safety aspects.

Its design is fairly elaborate but its construction is simple. It has had good performance for many years, but the obvious missing piece of information is how fast it's turning. I needed a tachometer.

Options that I considered: encoder, hall sensors, tach generator, frequency measurements on AC powerline.

Location of the turbine is at the top of the tower, which is stating the obvious. I point it out because that makes adding little finnicky gadgets difficult. To support some types of tachometer, signal wires going up the tower are needed. Some even need their own power supply to work. Sure, I can add little signal wires but it won't be robust. To make it robust would be costly. Slip rings, custom cables, etc. This wind turbine has run well for almost 10 years without a flaw and I would hate to have a thing like a tachometer in it that forces maintenance yearly. Despite the reluctance, I did actually install a hall sensor into it (two for redundancy) and started bench-testing but I got really frustrated with the difficulty of re-designing the mount of the generator itself to accommodate either a slip-ring assembly that would somehow pass small-signal currents without noise of their own.

I resolved to try to build a tachometer that relies only on the AC on the powerlines. However, the turbine is a customized 3-phase generator and the output is rectified to charge batteries. Off-grid stuff. The rotor blades are direct-drive on the generator, with a variable speed. The speed varies from 0 to 150 RPM (charge current cut-in) to about 400 to 500 RPM where the tail's furling mechanism folds everything out of the wind for protection. The AC coming from the turbine is definitely not a sine wave. It's more of a clipped trapezoid with harmonics and noise. The variable speed and DC rectifier switching puts spikes in that vary in intensity at different speeds. There are also flat spots crossing zero that you can see on an oscilloscope. It's a mess. This does not affect the power output of the turbine. Remember, the load is a DC rectifier, so the noise and spikes are inconsequential to power conversion.

I built the obvious thing with a LM2917 IC and tested several variations but they all failed. The 2917 got its input from one of the three AC lines from the generator to the rectifiers. It would pick up the harmonics at certain speeds, the baseline at others. I tried filtering but that was very tricky to get right. I thought I was close but then made a change in the the generator wiring and found myself back at the beginning. I tried an optoisolator and fed the pulses to a microcontroller and got exactly the same thing. The microcontroller could be programmed with some extra code to "guess" the bad data but always discarded huge ranges of records.

When I solved it, I was able to dispense with any filtering at all. I still used a microcontroller, so I was able to build a more elaborate data-logger around it, so I now have current, voltage temperature and a number of other things measured simultaneously. Every second if I want a mountain of data. Sampling and averaging is easy in the microcontroller programs.

So now that I've described my failures trying to solve it, before I had my "great idea", what would you do? Faced with the need for a tachometer, how would you get a good solid signal proportional to the speed of a machine like this?

www.sparweb.ca

RE: Tachometers

My first try would be a small transformer as a filter and look at the frequency at the secondary.
You may also read the voltage but the voltage will not be proportional to the speed once the charging cuts in.
Once you are into charging you may be able to get countable pulses from a series current shunt.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Tachometers

(OP)
Agreed. There was a transformer as part of the filtering, but somehow it only worked when the generator was in Star. After rewiring the generator in Delta the transformer passed the noise. There's no reason I can understand for that, except if harmonics in Delta increased and the transformer passed some of them. All of the signals were tapped off of 1 AC line at a time (didn't matter which one) so it left me puzzled. This was long before I got an oscilloscope.

The nature of my question is "what if" you can't use a LM2917 or equivalent IC. Then what?

www.sparweb.ca

RE: Tachometers

I'd have to see an actual waveform and how noisy it is, but... the easiest method would simply be to use a zero-crossing detector on a waveform that's suitably DC shifted. Even a very noisy waveform could be filtered with a cap/inductor to provide a clean enough waveform for average zero-crossings to provide a relatively accurate speed indicator. Forget the frequency-to-voltage converters... a simple op-amp could easily detect the zero crossings and provide a useful "click" for a micro to count.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Tachometers

Quote (Spar)

All of the signals were tapped off of 1 AC line at a time
If I understand correctly, that may have been the problem.
On a delta you would have got a better signal going line to line.
Line to line has the possibility of moving the "noise" 30 degrees up on the waveform where it is less likely to cause false zero crossings.
A little load on the transformer secondary can help swamp out transients.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Tachometers

I would measure the frequency of the ripple current.

RE: Tachometers

(OP)
Wow, never thought of a DC offset.
I can't remember if I had tried line-to-line when the generator was in Delta, or line-to-ground, or both. I tried a lot of things, that what I do remember.

Here are some sample waveforms - different machines, different connections, different speeds, different loads so DO NOT compare.









www.sparweb.ca

RE: Tachometers

(OP)
Compositepro,
I've never looked at that...
...now I want to.

www.sparweb.ca

RE: Tachometers

It may be a 3 times or 6 times multiple of the base frequency Spar.
The actual multiplier is not as important as anticipating and looking for it, rather than discarding good data accidentally.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Tachometers

(OP)
Since it's of interest, here's another way to look at it.
I hooked up a microphone input (with a lot of protective resistance) to the AC power line and recorded an audio file.
http://www.sparweb.ca/Forum/Wind%20Turbine%20Outpu...

Then I processed the audio to develop the spectrum.
The frequency axis is scaled to the baseline frequency, determined by the speed of the turbine, which is 5Hz. The 60Hz is twice as intense as the 5Hz signal. Note that the cell phone doing the recording is battery powered, and the renewable energy system inverter was off at the time, so there is no ambient 60Hz going on. Using a FFT app on the phone gave a similar plot (but it was only live reading, not a recording).



To me, recording audio to process a FFT is terribly complicated - but that doesn't stop some people from getting patents on it!

But this is not the super-simple thing I came up with in the end.
I'm wondering if anyone still wants to guess another way to find the speed without filters or any other IC's.

www.sparweb.ca

RE: Tachometers

One thing to shift the complexity is to use a right-angle gear box so the power doesn't need to come through a slip ring; mount the generator in a fixed vertical axis and leave the prop to freely rotate in the horizontal plane to follow the wind. Considering a car rear differential can operate for decades without service, this would keep the need for a ladder to a minimum and not have to worry about corrosion on a slip ring. It would allow a tachometer to be used without a slip ring for that either.

However it seems like you have an electronic solution that does not require a trained clam.

RE: Tachometers

You can pretty easily use lots of different opto-isolators and a 555 or a micro to count the pulse rate. You use a unipolar isolator that only has an output on one polarity. Your windmill is putting out AC so half a line cycle the opto is conducting and half it isn't. You get about a 40% duty cycle out of it. It's also nice because very quickly, circuit-wise, you get away from the high voltage and can then use 12V or 5V or even 3V for your counting hardware.

You do have to protect the LED in the opto but that's actually very simple and extremely cheap to do.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Tachometers

Building on Keith's post.
Use a 555 and only look at pulses with a duration long enough to eliminate transients and multiple zero crossings cause by low level noise.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Tachometers

(OP)
Just to clarify, optoisolators + microcontroller was tried several times.
The spikes were strong enough to trigger the opto more than once per cycle.

It is true that at the same time, it was not filtered before getting to the optoisolator. Just a resistor and a series of diodes to try to make the trigger level a couple of volts. It wasn't enough. Filtering with the transformer may have helped, but I didn't try that until later and I'd already moved on to the LM2917.


That circuit was probably fine for axial coreless alternators that some hobbyists build. My turbine itself was very different but robust (still running today) but there were consequences, like this data below. You can see how the range of scatter changes in Star and Delta.

www.sparweb.ca

RE: Tachometers

(OP)

Quote:

One thing to shift the complexity...
Surely you're joking, 3DDave!

www.sparweb.ca

RE: Tachometers

What - do you not have a slipring?

RE: Tachometers

I have seen Spar's turbine and tower.
I was impressed.
It is well made but despite that I would prefer to take picture from a distance when the differential is raised.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Tachometers

(OP)
Me too - if anyone tried to attach an auto differential to my turbine I would stand wayyy back too. Best not to even think about it. 3DDave, sliprings are not required. They have been tried, of course, but few pass the "last more than 5 years before replacement" test. We can get into the mechanical side of it some other time.
For now, I get the feeling we're tapped out for ways to measure speed from noisy AC and at the same time sidestep any filtering.

www.sparweb.ca

RE: Tachometers

Oh - why not make it a Mack Truck differential, just for making your joke even funnier. There are plenty of small right-angle drives that would work. The example was durability, not a suggestion to use a vehicle differential.

I don't understand how you need slip rings to add a tachometer but not to get power off the unit. Unlike Bill I haven't seen it - would have been nice to see that to begin with. My fault for expecting it was a substantial unit.

RE: Tachometers

SparWeb - No filtering at all? To clarify, you input the signal directly into a microcontroller using resistors and have no software filtering?

I was confused about your single phase use comments too. Using L-G isn't a good plan unless the generator is Y connected with the center point grounded.

3DDave - Please post links to some commercial examples that use such an arrangement.

RE: Tachometers

If this is going into a micro, surely some VERY basic time-averaging of the signal can remove momentary spikes that confuse straight hardware zero-crossing detectors. It's not like the windmill can go from 0 to 100mph in an instant... there will be some rise/fall time as it speeds up and slows down, so averaging just a handful of crossings should be adequate to figure out the "instantaneous" speed.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Tachometers

The comment made was "with any filtering at all" which to me means no hardware or software filtering.

RE: Tachometers

You can drive a frequency dependent small motor from either one or three phases and then monitor the speed of the motor.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Tachometers

(OP)
Bill - that's another neat idea!
It's probably more of a power-hog than necessary but it has a brute-force beauty.

Something to say about averaging: If the baseline signal is distorted by harmonics that are multiples greater than 1 of the baseline, then all of the errors are > baseline. The average would be heavily skewed to a value greater than the baseline. Averaging only works when the error can be > or < than the baseline. That's what wrong with the chart I posted with the blue and red points. The baseline speed is about 125RPM at 0 Amps and goes up in a straight(ish) line to 400 RPM at 15 Amps. All the noise adds pulses, giving the data a seemingly higher RPM. It helped a little to select the median value in a time interval. I had measured the power curve independently using a lathe to drive the generator before everything was assembled into a wind turbine, so I do have a proper power curve for the generator to compare with.

Lionel, measuring line to line is better and that's in the schematic I used years ago - but I was so confused at the time I couldn't get good readings that I tried a lot of things including line to ground (and transformers and big capacitors and other things) just for the sake of trying it. There's a big jumble of things I tried using and failed. I accept full responsibility for being a dummy with electronics despite my determination to learn it the hard way.

To better address my comment about filtering, which was taken as a boast and not really intended that way, here's the input circuit that I'm using now. So, yes there is rudimentary filtering by tying each voltage divider to ground with a capacitor. But that's it. The three data lines (red green blue) are limited to 25mA by the resistors because the generator only delivers 30 VAC max. That's all they need to work well with my current scheme.

From here I am using a microcontroller, but I am using averaging only to slow the data stream down. There's very little need to record rotor speed 10 times per second when other parameters being measured by the datalogger need a second or two to collect a steady average.



I haven't exactly given away the key function that the microcontroller does that is making this work, yet. It's not just counting microseconds between pulses, it's doing something else that makes all the difference. I guess I'm dragging out the guessing game a bit too long. I assume you are all still game to try. By now I figure if it is obvious enough that "everybody knows", then this is the piece of info that should give it all away.

www.sparweb.ca

RE: Tachometers

(OP)
Boxcar averaging:
I hadn't heard that term but it's exactly what I do when stacking dozens of grainy photos to extract a noiseless night-time scene.

NGC891

www.sparweb.ca

RE: Tachometers

OK, more or less a type of low pass filtering. Your filtering is still there. It's just digital instead of circuitry.

RE: Tachometers

Spar. What RPM range does your tach cover?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Tachometers

We normally call that frame averaging or frame stacking, but the principle is pretty much the same. Box car averaging is typically applied to a time varying, but repetitive, signal; raw SNRs of 1:1 would not be unusual.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Tachometers

(OP)
Almost zero to almost 1 Mhz. For 2 decimal accuracy let's say 10 kHz. That's because it's a microsecond counter that's running between cycle counts. In a 4-pole machine there are 2 cycles per revolution. That would result in 1 million RPM I think. I say "almost zero" because there has to be events to actually count. After 10 seconds waiting I've programmed the timer to give up and print zero although "12 RPM" would be a valid result.

Accuracy is limited to the accuracy of the counter and the cycle-to-cycle shift in trigger timing. If you were trying to measure something really fast it would matter. I'm measuring in hundreds of RPM, so I really don't care and cannot notice.

www.sparweb.ca

RE: Tachometers

(OP)
IRStuff,
How do you determine the frame window time (either duration or repetition rate)? Do you guess with one of those analyzers until a signal appears?

www.sparweb.ca

RE: Tachometers

Powering a motor and then measuring that is simply another type of filter... analog and mechanical based, rather than digital or software. Pick your poison.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Tachometers

Never really used a box-car averager, personally, but my understanding is that there was something like a phase-locked loop that would keep hunting for the signal and the box-averager window would hunt around until the signal was actually found.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Tachometers

(OP)
This thread deserves an update, but primarily, "thank you" to all those who replied.

Your answers helped me formulate a more clear explanation of the tachometer device I was working on. Also inspired me to do more research on the matter. I was very surprised that the tacho method I've been using cannot be found in the engineering literature anywhere I look (electronics textbooks, IEEE papers, Forrest Mims, the CMOS cookbook, you name it).

RE: Tachometers

Quote (SparWeb)

I was very surprised that the tacho method I've been using cannot be found in the engineering literature anywhere I look
Quite possibly because your approach is overly complicated for the problem it is trying to solve, i.e., other solutions exist that are more simple... I run into that all of the time in my designs, and I add the knowledge to my bag of tricks.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Tachometers

I must confess most of what has already been discussed up to this point is way beyond anything I understand, so I don't even know if understand correctly what we are meant to be thinking might just be common knowledge. But here goes my twopence worth. If this turbine is charging a bank of batteries, I guess somewhere down the line there is a rectifier, just before the batteries. Would a simple measurement at the recitifier bridge provide a clean signal, the frequency of which can be easily read? Am I barking up the wrong straw completely?

RE: Tachometers

I think rectified or not, Spar's issue was the signal was just too noisy to make direct use of it in measuring frequency, hence the following "50 million ways to skin a cat" discussion where we all came at it from different perspectives (and solutions). Some basic filtering is required, no doubt about it, and the method chosen would be based mostly upon Spar's level of comfort with designing / programming / using any of the above suggestions.

Of course, I still think a simple, slow-filtered op-amp circuit with a DC offset would be the best (easiest?) solution wink

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Tachometers

One of the problems that Spar faced was the relationship between signal frequency and signal amplitude.
A simple filter that would work well at higher frequencies and signal levels would possibly filter out the signal along with the noise at lower signal levels.
A filter that worked well at lower frequencies may filter out the signal with the noise at higher frequencies.
With Spar's system, as I understand it, a zero crossing permits a one shot signal from the proceeding phase.
It matters not whether the trigger is from signal or noise zero crossing as the proceeding phase will be at a point in the wave form that is above the noise level.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Tachometers

I know I'm out of my depth here, but I'm going to stick my head above the parapet anyhow.
Would the voltage drop across a rectifier diode not provide a pretty regular pattern, varying from zero to whatever the diode characteristics produce, or would the noise still show up here also?

RE: Tachometers

That measurement appears to be clamped by the diode forward drop voltage; it's not measuring the output voltage.

RE: Tachometers

Rectified noise is still noisy.
In Spar's application, the noise amplitude at high frequencies may easily by greater than the signal amplitude at low frequencies.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Tachometers

My reasoning was that positive noise goes forwards, and negative noise backwards, similar to this method.
(It's easy for me to think outside the box since I'm not even in it) smile

RE: Tachometers

That's different, you're showing a clipping circuit, designed to reduce the amplitude of anything on Vin to not exceed the power rails by a diode drop, so any noise that's greater than Vdd or less than GND gets clipped to Vdd+Vdiode or GND-Vdiode, and the noise is otherwise intact. This is the standard approach for ESD protection on ICs

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Tachometers

(OP)
Since my original start of the thread, I've done much more investigation of the signal noise. You were very helpful because you highlighted all of the techniques I had already tried, and motivated me to dig further into why they didn't work. I really didn't do a good job characterizing the problem in my earlier posts, and having looked at it more closely, I can describe it much better now.

My turbine's base AC signal varies from 5Hz to 20 Hz. It's a 4-pole machine whose operating speed ranges from 150RPM to 600RPM.
The first distortion is that the wave isn't really sinusoidal to begin with. The next distortion is the clipping to the battery voltage, which chops the top of the sine wave. The clipping is done by a rectifier switching on and off, which introduces a variable current load on the circuit. Various terms for this, such as "commutation" or "switching" spike but these might not be good expressions for what's going on. Given that the system is 3-phase rectified, there is a 3f harmonic associated with the variable current load.

The process I used to convert the induction motor into a self-excited generator (by fastening magnets to the rotor) introduces a "slot harmonic" which ripples every N slots of the stator. This motor has 36 slots and 4 poles so the ripple is 18f harmonic. Physically this is experienced as "cogging" that you can feel turning the shaft by hand. The amplitude of this ripple is determined by the inertia of the whole system. A bare shaft turned by hand has very little inertia so the 18f harmonic is huge. Once I fasten the driving rotor to the shaft the system inertia keeps the rotational speed more steady, but the 18f doesn't really go away.

The magnets are also the reason the baseline AC wave isn't a sine. The picture below illustrates one condition at one speed (about 200RPM). The waveform changes significantly at different speeds.

I have also been able to model this noisy system. With this in mind, and referring back to the 5Hz to 20Hz operating range of frequencies, selecting a low-pass filter becomes a deep compromise. I was pleased to find a way to program the microcontroller using a method that completely ignores the noise instead. There are now no noise filters on the tachometer's inputs, nor any device like an encoder needed.



RE: Tachometers

What comes to mind now is the INTEGRATOR part of the fuel injection closed loop system on the IC engine. Just like the O2 sensor works around a 0.45V DC offset as it's baseline, is there a way to apply a DC offset to your signal, then use some sort of INTEGRATION code to measure the area above and below your DC offset baseline, and thereby detect the switching point from positive to negative, with respect to your DC offset? If this is how you're determining your rpm, I would say yes, it is already common knowlwedge and used far and wide on nearly every modern IC engine.

RE: Tachometers

Hi,

I'm not a very experienced engineer but from what I know and based on your image, just gonna throw my ideas, I can think of 2 simple things (I don't think anyone mentioned it but if otherwise sorry).

1) Wouldn't a Y-delta transformer clean out the triplens harmonic ? so you would get a clean fundamental out of the transformer and from that using some conventional method to determine the frequency.

2) Since you have access to the neutral voltage and it is not the same as the ground (battery ground) so you could measure a neutral-ground voltage and use it to clean your raw input from the generator by subtraction (or other filtering technique) ?

and I lied and just thought of another idea. Using neutral ground voltage (so the triplen harmonic) to determine frequency and then dividing it by 3 to get the speed of your motor ?

RE: Tachometers

The problem is not harmonics as much as generated noise. As I understand it, Spar is using the healthy (well above zero) portion of one phase to permit one zero crossing signal from the following phase.
Did I get that right, SparWeb?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Tachometers

(OP)
Hi,
Yes, that's about right, Bill.

bigbrain,
I did something like that, once, but the point of smoothing harmonics is to generate a tachometer signal, not deal with power conversion issues. If I cared about power conversion inefficiency, then 3-phase transformers like the variety you mention could be useful.
True enough that the neutral is not grounded, but it's even wackier than the line signals. It's similar, but the vector has 1/2 the base component, and 1x the distortions.

RE: Tachometers

(OP)
Here's a completely different approach:

1) You have 1 permanent-magnet motor of completely unspecified size or type, except that it has multiple phases.
2) The motor is turned by a variable torque that you do not regulate.
3) The motor generates electricity because it's being forced to turn and the magnets self-excite the field.
4) The power generated is used for various purposes that should not matter to the tachometer exercise.
5) You want to measure its speed at regular intervals, and accurately within 1%.
6) Next year you will have another such motor, completely different from the first, but also want to measure its speed.

What do you do? What's your budget? 10 dollars, 100 dollars, or 1000 dollars?
Attempt the 10 dollar solution:
1) You have one 8-pin DIP microcontroller, 6 resistors and handful of little wires.
2) It must work when the temperature is -40C and +40C. Outdoors.
3) You are allowed to program the microcontroller with a computer.

Can you get the 10 dollar solution to work? The parts list above costs about 6 dollars (in bulk) so you can spend the rest on a LCD.
Or do you need to spend 100 dollars to build your tachometer? Will you be spending more money (and time) re-tuning your tach?
Note that several 1000 dollar solutions were already proposed in this thread. Fun, but getting away from the point.

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