07-30-2010, 03:51 PM
Join Date: Jul 2005
Total Cats: 68
boost overshoot arrester circuit
I have an AEM, a 5 psi wastegate can, 10 psi target, and a GT2560.
The AEM boost control algorithm is great (includes a 3D, TPS vs. RPM, boost target table), but for one thing. It has no 'D' term to help prevent overshoots. Tuning it, I either get slowish spoolup and little overshooting, or fast spoolup and massive(up to 4 psi) overshoots.
So I built an overshoot arrestor circuit. Because my wares are hard and not soft (i.e. I'm not a big software weenie), I built an analog circuit. Only took me a couple of hours lol.
The circuit cuts the power to the boost solenoid when the max boost target is gonna be hit in the next 150 ms given the rate of rise of boost. The idea is to pulse the solenoid open when overshoot is imminent.
It will not open the solenoid if the rate of rise is < 50 kPa per second. I determined this number from examining datalogs, overshoots only happen when boost rises >50 kpa/sec... and BTW I see >300 at say 5000 RPM, when I do a violent floor-lift-floor.
The circuit needs to tap into the MAP sensor, and has 2 *****:
- max boost target
- 'D' multiplier.
The latter **** simply adjusts the above-mentioned 150 ms number, from 100~300 ms.
It works pretty well after just 3 datalogged runs of tweaking. I used to get 4 psi of overshoots, I now get 0 to 0.7 psi.
Attached is the circuit's first iteration. It has been designed for the Smokorola 2.5 Bar MAP sensor. (MPX4250A)
On the left, from top to bottom, the terminals are: 12V, MAP sensor, MAP sensor ground. On the right is the output for the solenoid, the wire which normally goes to 12V. (i.e. the other wire for the solenoid goes to the ECU).
The left pot is for the D term, the right one is the max boost setpoint. The testpoint is for measuring the setpoint voltage - it has a 1:1 correspondence with the MAP sensor voltage at max boost.
I layed out this thing for SMD resistors and capacitors for compactness.