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External Interrupts

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Started 5 years ago by Silntknight
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Silntknight
Moderator
Gbulmer, I thought about using a Hall or a reed switch, but the problem then becomes that I have to infer the stroke based on another factor, the easiest being an valve sensor. A Hall or reed will be triggered EACH time the engine makes a revolution. The intake opens once every TWO revolutions. I could use both in conjunction to provide verification, but that would make the code more complicated. Just off the top of my head it would be something like:
```
void hall() {
  if (digitalRead(valveSensor) == HIGH); {
    //interrupt stuff//
  }
}
```
This then raises the issue of timing, because it has to poll the pin for the valve's position. By that time, the valve could have closed again. Even if I place the Hall a few degrees in advance of the time when the valve opens and closes, I would still run into issues because I have a large speed range (400-4000 rpm).

Poslathian, I will do so as soon as I can. I just found out that the lab I work at has a pulse generator (I'd just have to stick a pot on it to get to 3.3V from 5V) and a scope so I can look at both my PWMs and other stuff as necessary. I'll probably just hold the switch and shake it around like it's on the engine. Otherwise, I've been looking into other solutions like a laser "tripwire" or sonar. Both have their problems, but would be MUCH more accurate if it works. The other thing I might try is mounting the switch in the crankcase, provided it is adequately shielded from the oil. There should be fewer vibrations there.
Posted 5 years ago #
gbulmer
Moderator
The Hall Sensor could trigger an interrupt.
I don't understand why it would have to be polled?
Hall sensors have no moving parts, so they deal with high speed quite well;
4000rpm = 4000/60 = 67Hz, i.e. 15mS, which is plenty slow for a Hall effect sensor. I bought some yesterday that claim a switching speed of 1.5uS, for under a $.

Dividing the number of cycles by two, i.e. only firing on alternate cycles could be handled by keeping track of what happened last time in a Boolean variable:
```
volatile bool fire = true;
//...
void sparkinterrupt() {
   if (fire( {
     // do firing stuff
   }
   fire = !fire;
}
```
Posted 5 years ago #
Silntknight
Moderator
So I had typed quite a bit until I reread your code. I thought it wouldn't work because the code wouldn't know which cycle it was on, but the fire = !fire solves that problem. I was also going to expand on your code until I realized that what you have there is really all that is needed. In plain English:
```
If it wasn't fired last time the Hall was triggered
  get the counter value
  reset the counter
Change the state of "fire"
```
Now I did think of a problem. How does the code know the initial state? What could end up happening is when I try to start the engine, the MCU doesn't know any better and it injects during the exhaust stroke. Right now the code has a 50/50 shot at working. Still, I like this better than mounting the Hall inside the crankcase and fitting a magnet to the camshaft. The flywheel already has a magnet. I'll try to think of a way to make it reliable 100% of the time.
Posted 5 years ago #
gbulmer
Moderator

How does the code know the initial state? What could end up happening is when I try to start the engine, the MCU doesn't know any better and it injects during the exhaust stroke. Right now the code has a 50/50 shot at working.

Are their any otheer bits of information that can be relied upon? Even if only to get into synch correctly.

Posted 5 years ago #
Silntknight
Moderator

The only reliable info I can think of is intake/exhaust valve positions. They both have a period that is twice the engine period, unlike all of the other components. When the intake opens, I know we are on the intake stroke. When the exhaust closes, I know we are about to start the intake stroke.

Just a thought, but could I use a solenoid (linear actuator, etc.) as a switch? Basically, the engine uses pushrods to open and close the valve, these are "extensions" on top of the push rod. I think that if I had it connected to +5V and the pin, the metal rod would be pushed against the pushrod. Then, when the rod goes up, with significantly more force than the solenoid goes down with, it changes the impedance of the circuit. It might be enough (with a resistor) to change the voltage from a HIGH to a LOW. Not sure if this would work... If not, this gave me another idea, I could wrap part of the push rod in insulating material and leave part as exposed metal. Then, all I need is two metal blocks that barely touch the pushrod so that when it is at the top of its stroke the metal blocks make contact with the pushrod metal and complete the circuit. I could use this in conjunction with a Hall to get the initial firing sequence. Basically an if loop that only runs the first 10 times the engine revolves. The kill switch would reset a counter (for the first 10 times thing I just said). I was initially concerned with the engine moving too fast for a good connection to be made, but at low speeds (idling) this *should* work.

Posted 5 years ago #
gbulmer
Moderator

Just a thought, but could I use a solenoid (linear actuator, etc.) as a switch? Basically, the engine uses pushrods to open and close the valve, these are "extensions" on top of the push rod. I think that if I had it connected to +5V and the pin, the metal rod would be pushed against the pushrod. Then, when the rod goes up, with significantly more force than the solenoid goes down with, it changes the impedance of the circuit.

Than might work, but I have no sense of how robust a solenoid is.
It might be worth an experiment to make a magnetic 'circuit', with the pushrod being part of a magnetic circuit, and in some positions completing the circuit, and in others breaking it. Then all the sensing could be outside the cylinder head.

I could wrap part of the push rod in insulating material and leave part as exposed metal. Then, all I need is two metal blocks that barely touch the pushrod so that when it is at the top of its stroke the metal blocks make contact with the pushrod metal and complete the circuit.

Again, not sure how robust a 'wrapper' would need to be. Maybe some ceramic material, the sort of material used in gas heating systems might be worth the experiment.

I think I must be missing something. Isn't the camshaft turning at the speed needed to synchronise with the induction and exhaust cycles? If it is, can you get at it, and attach a magnet for a Hall Sensor?

Posted 5 years ago #
Silntknight
Moderator

I think I must be missing something. Isn't the camshaft turning at the speed needed to synchronise with the induction and exhaust cycles? If it is, can you get at it, and attach a magnet for a Hall Sensor?

Don't worry, you definitely aren't missing anything. I just do not want to try and mount a Hall in the crankcase. In any case, on our "other" topic (I guess I should consolidate to one after this post), I posted a solution. See http://forums.leaflabs.com/topic.php?id=101&page=4
Basically what I did was say, "Screw trying to time everything, I'm just going to have one signal from the engine and make the Maple do the rest." So, then I decided to use solenoids to electronically control the valves. The beauty of the system is that everything can be controlled by one Hall on the flywheel, where there is already a magnet that I have easy access to. Even better, all I need for synchronization is phase shifts of the various PWMs, which is easy using direct counter inputs. Anyway, the other topic explains this MUCH better.
By the way, as an added benefit, I can remove the throttle plate altogether, as it creates inefficiencies. I use the intake valve and modulate how long it stays open in order to control the amount of air entering the engine. For those of you who are familiar with thermodynamic cycles, this means I do not change the initial pressure of the graph, but rather the initial volume. I go from isochoric (isovolumetric) control to isobaric, which creates more work for the same input.

Posted 5 years ago #

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