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Old 25th January 2014, 18:59   #1
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Post Custom Tachometer - Electronics related queries

I'm doing my second year of Electronics and Telecommunication at CoEP, Pune and am working on a digital tachometer system at the moment, which I need some help with.
It is more electronics oriented than automobile, but since the application is almost entirely in the motorsport field I'm posting it here.

It works on the principle of the Hall Effect. For the convenience of those who're unfamiliar with it, Hall Effect is the generation of a voltage
perpendicular to both an electric current flowing through a conductor
and an external magnetic field which is applied at right angle to the current.

So far I have the sensing part ready, it generates a pulse every time a magnet on the flywheel passes the sensor, which of course, means one rotation is complete.
Since it isn't practical to measure the rotations for 60 secs, I want to measure it for 0.5 seconds and multiply that value by 120, which will give me the RPM. I want to interface it to a 7-segment 4 digit display.
The problem is how to make the circuit do the calculation. I'm open to any uC or uP. Hoping there are electronics experts here.
Any suggestion of who to contact would also be greatly appreciated.
I plan to evolve the design and build on a small scale on an order basis when this works out.
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Old 25th January 2014, 21:37   #2
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Default re: Custom Tachometer - Electronics related queries

Considering I am in the field... I suppose I can add something to this. My viewpoint-

A. It is easier to count the number of sparks going to the cylinders than fitting a hall effect sensor to the flywheel. Because
1. Fitting that sensor would be somewhat more invasive than the simple loops around the ignition cables.
2. The flywheel needs a magnet on it.
3. The sensors I deal with have a tolerance- you may find that a jury rigged system does not operate reliably because the sensor is out of alignment or too far from the magnet.

B. Knowing the number of sparks in a given time frame, and the number of cylinders, the RPM can be calculated easily.

C. I would pick a backlit LCD or OLED display. Or even a pseudo analog gauge. 7-segment displays went out of fashion over a decade ago.

D. Many of the Atmel AVR or Microchip PIC lines should be up to the job.

E. The only drawback to counting the sparks would be that there are none in a diesel engine.

Either way, the system is still [Number of pulses/Time]*K. You might want to display a moving average though. Otherwise you will have numbers that flash so fast that they're unreadable (at least the unit's and ten's places. Which is why RPM is generally read x1000. 20 or 30 here and there makes little difference.)
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Old 25th January 2014, 22:20   #3
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Default re: Custom Tachometer - Electronics related queries

I am no electronics expert, just another student like you. As suggested by RM2488, its better you stick to a non invasive way. For a motor, you could fix a magnet in case the shaft or flywheel is accessible. In a car, its quite a tedious job to access the flywheel. Otherwise you could try mounting the magnet on a pulley of the alternator, compressor, water pump etc but in this case you have to calibrate the gear ratio or the reduction ratio in order to get the correct RPM. Instead, its better you use the ignition coil signal if its a distributor based setup.

In modern cars, AFAIK the sparkplug is immediately preceeded by its respective ignitor, and a pulse is sent from the ECU to trigger the spark. If you could do some research on wiring, you could also tap the Engine speed signal which will be available in the internal wiring mostly as an output from the ECU. Since this is a pulse width modulated signal(PWM) you could directly read it using an Arduino or a simple microcontroller. Once it is read by a microcontroller, the rest is quite easy. The PWM signal will be read as a voltage(just make sure its 0/5V) you could convert that voltage into a corresponding RPM and can drive a display. Again avoid LCDs and prefer LED displays as these can be read better in outdoor conditions compared to a backlit LCD which may have to be in its brightest setting.

Let me check with the ECU pinouts and check if this is possible. Given that almost all cars now rely upon electronic control, it's better you implement a complete electronic setup compared to trying to access the flywheel.

But keep in mind, DONT EXPERIMENT with either the electronics or the mechanicals of the car unless you are clear of what is to be done. When dealing with mUC, ensure you set the input and output ports properly. For example if you provide ECU signal to a pin in arduino at Low logic state or configured as an output, you might end up with a packed ECU. They would have designed some short circuit protection, but lets take the worst case analysis.
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Old 26th January 2014, 00:00   #4
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@arjun18, is your objective demonstrating use of a Hall Effect sensor? Or is it to make a Tachometer?

If it is the former, perhaps you should research current automotive sensor technology first. HE sensor is not the sensor technology of choice, both because it needs an extra element to be functional (the magnet), as well as unreliable sensing due to interference.

If it is the latter, preselecting sensor technology is like putting the cart before the horse. Conventional engine RPM sensing is with a reluctance sensor sensing teeth of a gear wheel mounted on the crankshaft. One would also assume your Tachometer can read RPM of engine no matter whether it is Petrol, Diesel or hybrid (so sensing sparks is not the right thing to do).

At a pinch, one can do the same by sensing the teeth of the starter gear ring (where the starter couples). The conventional crankshaft RPM sensor wheel has some indexing teeth (missing teeth, actually) - but don't bother about that for the moment.

For CPU, you can use any microcontroller that has dedicated external inputs that are coupled to an Interrupt generating mechanism on chip. You wouldn't need an OS at all, just write the software to handle Interrupts, while doing lower-priority tasks most of the time.

For clock speed, just estimate how many pulses should you be reading PER SECOND at the highest engine RPM. Since your tachometer should be rated at least 20% more than that, that should give you an idea how much time you will need to handle the top priority task - reading time between two consecutive pulses. RPM would be computed from that calculated time.

Please do remember to do exception handling, for example Plausibility Checking to obviate false readings.
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Old 26th January 2014, 13:25   #5
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Default re: Custom Tachometer - Electronics related queries

Quote:
Originally Posted by arjun18 View Post
Any suggestion of who to contact would also be greatly appreciated.
I plan to evolve the design and build on a small scale on an order basis when this works out.
https://www.google.co.in/search?q=di...ih=887&dpr=0.9

There are many circuits online and you can get them at hobby shops.
Try to make it wireless LCD than a segment lcd fisplay will look better
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Old 27th January 2014, 14:49   #6
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Originally Posted by Catalyst_delhi View Post
... Try to make it wireless LCD than a segment lcd fisplay will look better
Err... what is a "wireless LCD"?
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Old 27th January 2014, 19:18   #7
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Default Re: Custom Tachometer - Electronics related queries

Thanks for the info RM2488, I appreciate it man.
Quote:
Originally Posted by RM2488 View Post
2. The flywheel needs a magnet on it.
The car I'm building this particular circuit for does have a magnet on the flywheel.
Quote:
Originally Posted by RM2488 View Post
C. I would pick a backlit LCD or OLED display. Or even a pseudo analog gauge.
7-segment displays went out of fashion over a decade ago.
This is a BAJA SAE car, lots of mud and sunlight and dust. A 7-segment is far more readable under such conditions.
Quote:
Originally Posted by RM2488 View Post
E. The only drawback to counting the sparks would be that there are none in a diesel engine.
That's the reason I want to use a hall sensor, its quite sensitive and a tiny magnet on any rotating part connected to the engine will do the job, as mentioned by audioholic.
Quote:
Originally Posted by RM2488 View Post
Either way, the system is still [Number of pulses/Time]*K. You might want to display a moving average though. Otherwise you will have numbers that flash so fast that they're unreadable (at least the unit's and ten's places. Which is why RPM is generally read x1000. 20 or 30 here and there makes little difference.)
Yeah that's the main issue, I need help with the timing part of it, including the display duration. But this is a real issue, I hadn't given much thought to this, thanks for letting me know!
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Old 27th January 2014, 19:26   #8
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Originally Posted by audioholic View Post
If you could do some research on wiring, you could also tap the Engine speed signal which will be available in the internal wiring mostly as an output from the ECU.
It's a BAJA SAE buggy man, hardly any electronics in it, definitely no ECU. :P
That's the reason I'm using a HE sensor.
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Old 27th January 2014, 20:36   #9
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Default Re: Custom Tachometer - Electronics related queries

Quote:
Originally Posted by arjun18 View Post
A 7-segment is far more readable under such conditions.

That's the reason I want to use a hall sensor, its quite sensitive and a tiny magnet on any rotating part connected to the engine will do the job, as mentioned by audioholic.

Yeah that's the main issue, I need help with the timing part of it, including the display duration. But this is a real issue, I hadn't given much thought to this, thanks for letting me know!
You mentioned something about making and selling it, so the natural assumption would be a petrol or diesel car. Perhaps you should have led with "this is a BAJA car." The replies would have been more specific. A 7-segment is indeed more readable under bright sunlight. Not so good looking in a family car!

An alternative technology you might want to consider is optical sensing. A tiny strip of black paint/vinyl on the wheel will do the trick. Far less susceptible to noise than a hall effect sensor. I had once built something like this (I actually needed to count the number of rotations. Custom guitar pickup winding) by using a magnet and a small coil. In those days I didn't know about hall effect sensing. It worked quite well.

Regardless, I'd advise a microcontroller approach to fine tune the display timing. Ideally you'll want to update the display between 1/10th of a second to 1/2 a second, although you might have to go for an update every second depending on noise and raw delta reading. Also make sure you go for shielded cables. Hall effect sensors are usually high impedance devices. Perhaps a buffering stage between the microcontroller and the sensor.

In case mounting more magnets/ paint stripes is feasible, you could double or triple the number of readings for better resolution.
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Old 27th January 2014, 22:06   #10
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Quote:
Originally Posted by arjun18 View Post
Since it isn't practical to measure the rotations for 60 secs, I want to measure it for 0.5 seconds and multiply that value by 120, which will give me the RPM.
Measure the time between pulses (time period), calculate frequency.
Too much of filtering, and the display will lag.

Regards
Sutripta
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Old 27th February 2014, 16:38   #11
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Sorry the topic hasn't been updated recently, the BAJA event preparations were ongoing and I was busy with the event. Here's a short video of the sensing part that was taken a few months ago. I shall put up videos of the further work soon.



Also, we won the second overall award at the BAJA SAE 2014!

Last edited by arjun18 : 27th February 2014 at 16:39.
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Old 28th April 2014, 15:35   #12
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Default Re: Custom Tachometer - Electronics related queries

So it's finally here, I've been busy for the past few months with college and team related events, but here is a video of the tachometer.
To give an overview,
I'm using an arduino uno board, hall effect sensors, and a magnet mounted on the primary pulley of the CVT.
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Old 28th April 2014, 16:18   #13
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@arjun18, is your objective demonstrating use of a Hall Effect sensor? Or is it to make a Tachometer?.
derAlte, vehicles have alternators which produce alternating current. Even with a rectifier converting it to DC, there will be a "waveform". The frequency of this waveform should be directly proportional to the rpm. Once you go for baseline using an OBD II, you can program your device to detect rpm based on reading the AC component of the DC voltage, right?
Or am I missing something here.
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Old 28th April 2014, 16:53   #14
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vehicles have alternators which produce alternating current. Even with a rectifier converting it to DC, there will be a "waveform". The frequency of this waveform should be directly proportional to the rpm...
Or am I missing something here.
On a rectified signal, there would be harmonics (noise signal) introduced due to the rectification process. It's tough to get rid of what is noise vs what is not noise (the rpm input signal)
Arjun's idea to use the hall effect sensor on the CVT pulley will yield better results than trying to read the signal post rectification.

Last edited by joybhowmik : 28th April 2014 at 16:55.
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Old 28th April 2014, 16:59   #15
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... Or am I missing something here.
The amount of signal massaging one has to do on the alternator output makes it a less reliable (electrical noise), and hence costlier method. It is also indirect sensing - would need to be tailored for every vehicle (pulley sizes), and slight variable lag due to alternator being driven by belt.

The most reliable sensing method - and the cheapest - is the reluctance sensor on the flywheel teeth. It is a direct take-off from the crankshaft, the advantages being:
* Works from 0 (even if engine is not firing / creeping). The alternator based signal will not have any information in this state
* Can resolve crank-shaft angle, which facilitates computation of crankshaft acceleration even before the crankshaft goes one full rotation (this is used in fuel cutoff in engine braking and going downhill with foot off acc pedal)
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