Friday, October 28, 2011

VANOS controlled by Megasquirt

My VANOS up until now has been controlled by standalone circuit which has worked well, but I've always planned on switching it to Megasquirt control.

The first step that is necessary is to upgrade to at least Alpha 11 because it has the VVT features that the current release doesn't have.

Since I already had Nitrous 1 driving my standalone circuit board, I reconfigured it to be my VVT output for Cam 1.

My first issue was that my duty cycle to hold the cam in position was close to 72%, which is twice the 35% I needed with my standalone board.  That was solved by modifying the flyback.

There's an issue with how the expansion board handles flyback.  It uses an active circuit to dump voltage through a transistor to ground.  It doesn't work...   After modifying the flyback path with a jumper to +12 the signal got better.  With Schottky diodes it got even more-better; cleaner and more efficient. 

All the diodes are tied together at their cathodes.  By connecting the cathode (striped end) to +12v where it enters the Megasquirt, the flyback energy has a direct path to dissipate.  Pad S12 is an ideal place to find +12v, I used a through-hole.  There are six diodes, four of them are visible here.. I replaced the original 1N4004 diodes with Schottky diodes, ON Semiconductor MBRS360BT3G.  DigiKey Part # MBRS360BT3GOSCT-ND.  They are $0.65 each.

Not too bad.  I like how the changes in duty cycle come in groups of 3 for changes in position.  Think of catching a faster car on the highway.. you'd have a blast of power to speed up a bunch, then idle to slow back down, then new steady-state throttle to match speed.

You can see the stiction here.  Cam takes more to get it unstuck than to keep it moving and once it stops it likes to stay stopped.

I updated to Alpha 15 a few days ago and have been constantly changing the P-I-D parameters to see where it runs best.  The Alpha 15 release has the additional option of selecting a control interval that is not synched to the cam.  With cam synch interval, the control interval changes with the rpms.  With a timed control interval it stays the same no matter what the engine speed.

Alpha 15 response.  Not that much better, but better.

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