Author Topic: Spark plug tachometer  (Read 25951 times)

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Online Mechatrommer

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Re: Spark plug tachometer
« Reply #25 on: July 25, 2017, 07:08:44 am »
From research I've already done, I came up with this, which sort of works.  As far as why I chose those diodes, its just what I had lying around at the time. I don't have any particular need to use this specific circuit if you have an idea for something better.
you already have a good circuit to start with. except you want to do better filtering so you get a nice single pulse ideally. have you tried moving 1N4735 to 1N4148's cathode? and change to bigger capacitor value to get nicer pulse? and then the output A (B = ground) connected to mcu handling interrupted pin? misfire is not an issue i guess since engine is a mechanical system there is no way a piston can mistep in non-disaster condition, so you can presume that in mcu programming (any misfire can be predicted as misfire) (read averaging or natural averaging nature). another engine's trait that you can presume is its should be no more than 6000 - 10000 rpm depending on your system, so any spike/interrupt that result in rpm calculation greater than this is a false spike. so you can delay at some proper timing after an interrupt has been processed.

btw, at first thought yesterday i was about to suggest tapping from ecu digital tacho line normally available in modern consumer vehicles as someone suggested earlier, but after looking at the baja competition video, i believe tapping the spark plug line is the simplest non intrusive one can get, if ecu is not available. i built almost similar concept as yours few years ago for detecting when a camera flash is burst by using an inductor placed near the flash light.
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Online tautech

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Re: Spark plug tachometer
« Reply #26 on: July 25, 2017, 07:58:52 am »
I was hoping to be able to pick up something off the spark line because its just easier mechanical-wise, even if the circuitry is a little more complicated. I'll try tautech's and Rerouter's ideas, and if neither of those work, then I'll have to find somewhere to mount a magnet + hall sensor, or hall sensor to pick up the flywheel magnet.
Facts are most small engine shops use some sort of inductive wireless tacho for all repair and tuneup work.
A couple of typical examples:
https://www.jackssmallengines.com/jacks-parts-lookup/part/husqvarna/502711401
http://www.baileysonline.com/Chainsaw-Parts/Repair-Tools/Tachometers-Ammeter/Oregon-Wireless-Tachometer.axd
Both these units are non-contact and are used in only close proximity.

For low RPM stuff like Briggs you can also use a Treysit Sirometer, a vibro tach with a resonating wand that when adjusted to max vibration you read the RPM off the scale. Not practical for anything but hands-on use but reasonably accurate all the same.

Knock something up based around a length of ferrite rod like I suggested, shunt it to remove unnecessary amplitude and maybe limit the output with a clamp then into a counter.
Your lawnmower, motorbike or car can provide a RFI source while you get it all sorted.
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Offline Fraser

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Re: Spark plug tachometer
« Reply #27 on: July 25, 2017, 12:15:21 pm »
One question...... As this is not Formula 1 where telemetry is streamed to a pit crew / huge technical team who can tweak settings..... why do you want this logger running during competition ? I would have expected a logger such as you describe to be fitted during vehicle testing and setup. That is to say. You can mount whatever you like to the engine during testing and engine profiling. You just unbolt the sensor package and logger on competition days. Nothing 'illegal' about that.

I would fit all manner of sensors to the engine when doing testing runs. Monitoring head temperature, air temperature, exhaust gas CO content, RPM, Ignition waveform and ground speed (via a wheel sensor). The 'package' could be easily fitted for testing and designed to be easily removed when in competition. With such comprehensive data you could learn a lot about engine behaviour. The B&S engine is a simple beast and not exactly 'raced tuned' or the most reliable of ignition systems. Misfires would cost performance. Spark Plug gap is important and a decent HT level to the spark plug is essential. If your B&S still uses points to create the spark, timing them and gap setting is critical to performance. You really should be monitoring such during testing.

Correct fuel to air mixture is also a challenge with the simple B&S carburettor. The CO Monitor would give an idea of how well the carb is coping with different throttle settings. You should be allowed to tweak idle mixture on the carb as that is unique to each engine. Changing jet size is likely not permitted though.

Fraser
« Last Edit: July 25, 2017, 12:23:44 pm by Fraser »
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Offline Raj

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Re: Spark plug tachometer
« Reply #28 on: July 25, 2017, 02:25:30 pm »
Can"t attach a magnet? use optical tech
 

Offline RyersonBajaTopic starter

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Re: Spark plug tachometer
« Reply #29 on: July 25, 2017, 07:53:47 pm »
One question...... As this is not Formula 1 where telemetry is streamed to a pit crew / huge technical team who can tweak settings..... why do you want this logger running during competition ? I would have expected a logger such as you describe to be fitted during vehicle testing and setup. That is to say. You can mount whatever you like to the engine during testing and engine profiling. You just unbolt the sensor package and logger on competition days. Nothing 'illegal' about that.

I would fit all manner of sensors to the engine when doing testing runs. Monitoring head temperature, air temperature, exhaust gas CO content, RPM, Ignition waveform and ground speed (via a wheel sensor). The 'package' could be easily fitted for testing and designed to be easily removed when in competition. With such comprehensive data you could learn a lot about engine behaviour. The B&S engine is a simple beast and not exactly 'raced tuned' or the most reliable of ignition systems. Misfires would cost performance. Spark Plug gap is important and a decent HT level to the spark plug is essential. If your B&S still uses points to create the spark, timing them and gap setting is critical to performance. You really should be monitoring such during testing.

Correct fuel to air mixture is also a challenge with the simple B&S carburettor. The CO Monitor would give an idea of how well the carb is coping with different throttle settings. You should be allowed to tweak idle mixture on the carb as that is unique to each engine. Changing jet size is likely not permitted though.

Fraser

Majority of our testing is done at competition events, where we get practice/setup time on the track before the competition stages, and time between stages we can work on the car. We might only get 2-3 days of testing outside competition. The easily removable bolt-on sensor is what I'll do if I can't get it working any other way.

I'd love to do a lot of that basic engine work, but there's no point for most of it. We can adjust spark plug gap and idle speed, but that's about it. Spark is triggered by a magneto, which is bolted onto a fixed mount on the engine block. No adjustment there either.

The only meaningful engine adjustment we can make is idle speed. So we can play with that, and then adjust the centrifugal clutch appropriately.

http://bajasae.net/content/2017_BAJA_Rules_1_3_2107.pdf
See page 10/11/12 if you're curious about the rules.

The purpose of this datalogger is really two sets of measurements - suspension behaviour and CVT/gearbox behaviour.

The datalogger will record suspension movement and wheel speed for all four tires independently. It also has an accel/gyro to measure car body movement. We can use this to make suspension adjustments - we are using adjustable dampers, and can preload the springs, or change them out for springs of different stiffness without too much difficulty.

The other task is to measure wheel speed vs engine speed and throttle load. Throttle load is easy - just a potentiometer spun by the throttle cable. So is wheel speed - magnet on the wheel, hall effect sensor nearby. The accelerometer data will let us know approximately how much the wheels are slipping during acceleration. We can change CVT spring preload or even replace the CVT springs to change the gear ratios.
 

Offline larrybl

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Re: Spark plug tachometer
« Reply #30 on: July 26, 2017, 02:53:28 am »
Interesting thread, I use a Harbor freight infrared Tack to set the RPM's on these. Requires that I stick a reflective tape on the blower housing to get a reading. Most of these are twin cylinders but the plugs fire on every revolution. I have been looking for a cheep but reliable, and decent tachometer that I can add to these. The one I am thinking of trying is this https://tinytach.com/
   
 

Offline RyersonBajaTopic starter

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Re: Spark plug tachometer
« Reply #31 on: August 03, 2017, 04:58:20 am »
<Inductive tach stuff>

Got around to trying the inductive tach today. I took a ferrite core from an old radio, wrapped some wire around it, shunted the ends with a 100ohm resistor, and took some measurements. One end of the resistor was tied to the chassis as a "ground" reference. I confirmed that I had good contact to the engine block with an ohmmeter.

The following measurements were done with 10x probes, on a Rigol DS1054z.
This works nicely, I'm getting a good difference in signal across the resistor, although it seems to vary a fair bit.







Those all seem to get caught using the segmented memory. When I zoom out to try to see all the pulses, it doesn't seem to catch everything. I have a feeling this is more to do with the oscilloscope recording such a short signal on a long timescale, but I'm not sure.






Of course, these all have much too large of a signal to be useful for me. Switching the probes to 1x significantly attenuated the signal. (and yes, I made sure the oscilloscope was set to 1x mode when I switched the probes - it was set to 10x mode, as appropriate, for the above images). Signal amplitude still varied quite a bit.











Once again, I have the same issue where it doesn't seem to be catching every spark. For reference, the engine is idling at ~1200RPM, or ~20Hz. Maybe a little faster.





Am I on the right track that the "lost" signals on the 50ms timescale are not actually lost signals, but a limitation of the oscilloscope?


Any suggestions on filtering this signal?

I was thinking I could connect either end of the shunt resistor, through low pass filters, into a differential amp. Differential amp output would go into an adjustable schmitt trigger (trimmer potentiometers on the board, so I can adjust it if needed). That goes into a monostable multivibrator, which sends a clean pulse to the microcontroller.

My main concern would be keeping as much noise as possible out of my power/ground planes on the board. Everything is running at 3.3V.

I would be concerned about signals that are too large, even after the lowpass, going through the diff amp's protection diodes to the board's power/ground plane. Is this worth being concerned about or am I overthinking it?

Would two opposing zeners in parallel with the shunt resistor work? If this can be done, considering the signal pulse is ~20MHz, how do I ensure I choose zeners that are fast enough?
« Last Edit: August 03, 2017, 05:00:38 am by RyersonBaja »
 

Online tautech

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Re: Spark plug tachometer
« Reply #32 on: August 03, 2017, 06:10:01 am »
You're making this harder than it needs to be.

Cut the multi channel and Math BS, it's not needed. Too much info and it's just not needed.
We/you don't need to examine individual pulses in any detail only know that they are there and see the amplitude. The more that are on the display the more accurate the frequency measurement will be......not that it matters for other than proof of concept. A 20ms timebase setting is plenty slow enough.
No need for a chassis reference as the measurement can be made just across the winding.

Use only one channel and an amplitude setting that is sufficient to get all pulses above the trigger setting.
1200 rpm should equate to 20 Hz (sanity check)

For interfacing with a micro the shunt on the pickup coil can be changed to adjust the amplitude but you should add some way to limit the amplitude, signal diodes series'ed across the windings to get above the input threshold but little more, certainly stay less that the micro max input rating. A Schmitt trigger as a buffer will work too with similar protection.  ;)
« Last Edit: August 03, 2017, 07:33:41 am by tautech »
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Offline tron9000

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Re: Spark plug tachometer
« Reply #33 on: August 03, 2017, 08:09:44 am »
You say the rules say not modification to the engine, but what about the input to the transmission? its there any part of the transmission input coupling you can modify with a pickup sensor?
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Offline Delta

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Re: Spark plug tachometer
« Reply #34 on: August 03, 2017, 05:43:57 pm »
Take the signal from the magneto killswitch wire.

Or is this considered as a modification?
 

Offline floobydust

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Re: Spark plug tachometer
« Reply #35 on: August 03, 2017, 08:00:29 pm »
I worked on Formula SAE and it was a lot of fun.

You can try a building block approach - you need to rectify, filter out HF artifacts, clamp voltage and one-shot to prevent false-triggering from the ringing.

I would try a discrete transistor solution like this, values are out of a hat as a start. Or perhaps other members have a circuit in mind.
It's really trying to perform 4-5 functions in few parts.

R2 gives a small load to the pickup coil to dampen ringing. Adjust for good pickup coil amplitude at lowest ? (idle) speed. I don't know your coil at all, 1-10k \$\Omega\$ typical.

The pickup coil is not hard-grounded to the engine to prevent conducting RFI from the ignition system coming in to the logger, so R3 100R.

R1/C1 give a bit of HF filtering above ~5kHz. Easiest to adjust R1 there.

D1 clamps -ve excursions and Q1 turns on only on +ve excurions.

R5/C2 are a pulse-stretcher ~0.3msec. But not ideal as exponential ramp, so a second transistor might be added there. If the logger has Schmidt trigger inputs should be OK.


I would experiment with the values and see if there is hope. A textbook solution uses an instrumentation amp, comparator, one-shot but a bit complex and fragile on a baja run.
So simplicity is what I prefer to start with.
 

Offline RyersonBajaTopic starter

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Re: Spark plug tachometer
« Reply #36 on: August 05, 2017, 06:30:06 pm »
<snip>

Thanks.

Test circuit I threw together quickly out of stuff I had lying around. Why 12v? I had a car battery nearby. It was a dead battery, so actually 11v, but close enough.



Result:




Thanks. I'll play with it a little more, but that should work.


I would experiment with the values and see if there is hope. A textbook solution uses an instrumentation amp, comparator, one-shot but a bit complex and fragile on a baja run.
So simplicity is what I prefer to start with.

Still more complex than I need. Also I'm not at all concerned about fragility - its all getting soldered onto a PCB, which is then going to be in a waterproof case, mounted securely somewhere in the car.
 

Offline floobydust

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Re: Spark plug tachometer
« Reply #37 on: August 05, 2017, 07:53:41 pm »
Um that circuit is not doing what you think. Remember the input signal is AC. Your scope shows 1/2 wave because the op-amp is not biased at 1/2Vcc for -ve pulses.

No biggie, but I would have some resistance before the clamp diodes to limit the current pulses they will see. There could be enough DC resistance in your coil for this.

Right now, you get 8.64Vpk output and +/-0.7V input, but gain is sorta less. Unless scope probe was 10X but numbers should jive.


 

Offline rbm

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Re: Spark plug tachometer
« Reply #38 on: August 10, 2017, 12:38:23 am »
I work on a BMW K75 motorcycle as a hobby-interest, a family of motorcycles which is popular to customize into cafe racers.  One problem encountered by customizers with this bike is getting a good solid engine RPM signal for the aftermarket tach.  Most try to use either a spark-wire pickup or reading the RPM signal fed to Pin 16 on the OEM cluster.  The end result is usually an unstable tachometer needle that varies wildly.

Based on the circuit that Floobydust presented above, I came up with a circuit that functions excellently in this application and may be useful for the OP's baja project.  Instead of trying to condition the secondary signal picked up using a ferrite-cored coil, I am tapping into the signal sent to the primary of coil #1.   I wrote up my how-to in the following article: 

http://www.k100-forum.com/t12786-rpm-signal-conditioning-circuit-to-prevent-aftermarket-tachometer-needle-instability#151513
- Robert
 

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Re: Spark plug tachometer
« Reply #39 on: August 10, 2017, 01:01:50 am »

Based on the circuit that Floobydust presented above, I came up with a circuit that functions excellently in this application and may be useful for the OP's baja project.  Instead of trying to condition the secondary signal picked up using a ferrite-cored coil, I am tapping into the signal sent to the primary of coil #1.   I wrote up my how-to in the following article: 
It mightn't work on Briggs & Stratton as the ignition is one sealed magneto driven unit. The only possible other point (other than inductive) is the Stop wire.....but you'd need to put a scope on it to see if there's a signal that's related to rpm.
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Offline floobydust

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Re: Spark plug tachometer
« Reply #40 on: August 10, 2017, 04:23:03 am »
This Briggs engine seems designed just for this competition. It has an alternator option that you could also pick RPM off of, but I think this motor has no starter/battery?
OP can't really connect things, they are seen as "modifying" the engine.

rbm, that BMW circuit looks good, I would recommend C1 0.1uF should be 400V part.

Automotive ignition module's IGBT clamps the primary spike to ~400V. Typical 1:70-85 ignition coil that is ~30kV max. with an open-circuit on the plug wire, to protect the coil. Spark plug firing results in less.
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Offline RyersonBajaTopic starter

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Re: Spark plug tachometer
« Reply #41 on: August 14, 2017, 06:42:03 pm »
Um that circuit is not doing what you think. Remember the input signal is AC. Your scope shows 1/2 wave because the op-amp is not biased at 1/2Vcc for -ve pulses.

No biggie, but I would have some resistance before the clamp diodes to limit the current pulses they will see. There could be enough DC resistance in your coil for this.

Right now, you get 8.64Vpk output and +/-0.7V input, but gain is sorta less. Unless scope probe was 10X but numbers should jive.

Probes are 10x, but so is the scope settings, so those numbers are right. I did the single-side op-amp bias on purpose for that test.

Something is fishy here. I'm still trying to track down why the diodes aren't clamping the way they I expected. Just as a test, I did this, measuring across the diodes (probe ground on one side of diodes, probe on the other)



Results (sorry about the photos, I didn't have a USB key at the time to save from the scope):

https://imgur.com/a/Gq4G7

At a smaller timescale, it looks like generally like this:




If i had to guess, I'd guess that I'm running into a problem with the diode not switching fast enough, but IIRC, 1N4148s have a reverse recovery time of ~4ns, which is much faster than the spike I'm seeing here. I'm seeing spikes much larger than the ~0.7-1V I would expect. Or maybe the diode is acting properly, but I'm shoving too much current through it for a 0.7v drop on a 1N4148, and I should use a higher-current diode?

Next time I'm in the shop, I'll try some different resistors and see if it makes any difference, I wish I had thought of it before.

Ideas?

I could put the scope on the stop wire next time I'm in and see if I can get a signal from it, but I have no idea what the voltage on this line might be, and I don't want to blow up the front end of the scope.

@rbm: Thanks, but I can't use it, because I can't wire into the primary coil. Its sealed inside the magneto unit, which is enclosed in a solid block of plastic/epoxy, which I'd have to dissolve to get into (thus violating the "no engine mods" rule).
 

Online Ian.M

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Re: Spark plug tachometer
« Reply #42 on: August 14, 2017, 07:05:44 pm »
I wouldn't probe the stop wire with a cheap +/-30V max USB scope, but a proper scope with a 10V/div input attenuator setting and a x10 CAT II rated probe should be fine. Its *REALLY* unlikely to have high voltage to the kill switch, and if it did there would be a warning notice in the engine's installation manual.  However it wouldn't hurt to check with a DMM before scoping it
 

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Re: Spark plug tachometer
« Reply #43 on: August 14, 2017, 08:37:13 pm »
I just checked the pickup I made years ago.......a dozen turns around the ferrite rod SHUNTED with 150 \$\Omega\$.
The resistor tames the ringing and you'd add the clamping after.  ;)
Preferably close as possible to the counter.
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Offline RyersonBajaTopic starter

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Re: Spark plug tachometer
« Reply #44 on: August 14, 2017, 08:39:39 pm »
I just checked the pickup I made years ago.......a dozen turns around the ferrite rod SHUNTED with 150 \$\Omega\$.
The resistor tames the ringing and you'd add the clamping after.  ;)
Preferably close as possible to the counter.

Diodes in parallel with resistor? I'll try it.
 

Online tautech

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Re: Spark plug tachometer
« Reply #45 on: August 14, 2017, 08:49:12 pm »
I just checked the pickup I made years ago.......a dozen turns around the ferrite rod SHUNTED with 150 \$\Omega\$.
The resistor tames the ringing and you'd add the clamping after.  ;)
Preferably close as possible to the counter.

Diodes in parallel with resistor? I'll try it.
Yep.

There's a trade off with amplitude and ringing management from the shunting along with decreased sensitivity.
With scope measurement that's comparatively easy to work around as the scope will give frequency (RPM) at the trigger level setting where you can set this above the ringing to get a cleaner result.

This is why I suggested you use something at home like a lawnmower or motorbike to get the pickup part roughly optimised before moving forward.
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Offline Delta

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Re: Spark plug tachometer
« Reply #46 on: August 14, 2017, 11:50:35 pm »
Just use the bloody magneto kill switch wire will ya!

Use a resistor and Zener (or even an LED) in series to earth and hey presto.
 

Offline CatalinaWOW

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Re: Spark plug tachometer
« Reply #47 on: August 15, 2017, 12:07:37 am »
tautechs approach should work for you, but if you don't like wrapping a wire around the spark plug there are other approaches.  One has already been mentioned - detecting the rf using a cheap radio.  The other is to emulate riders and drivers who can accurately estimate RPM using their own built in acoustic sensors.

Both of these approaches work and have been implemented in the field.  Both should be immune to the mud.  But care does need to be taken to make sure you don't pick up adjacent cars.  A little thought about the effects of 1/R^2 should be all you need to cover this problem.
 

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Re: Spark plug tachometer
« Reply #48 on: August 15, 2017, 12:33:22 am »
tautechs approach should work for you, but if you don't like wrapping a wire around the spark plug there are other approaches. 
That's ^ not what I've referred to or suggested all along, instead an inductive pickup with turns on a ferrite rod.
Using a ferrite rod will impart some directionality sensitivity but this can be addressed with a rigid mount that gives best results.

Management of the signal to what the OP needs is the next step so to get clear defined pulses and then control amplitude to his counter.
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Offline CatalinaWOW

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Re: Spark plug tachometer
« Reply #49 on: August 15, 2017, 03:50:16 am »
Sorry about the confusion.  I was referring to your comment about how to process the capacitively coupled signal.
 


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