Vehicle Turbo control using op amps & PWM generator

Turbo

Hi All
Last time I looked at electronics was in the 70’s building a crystal radio set.
I am active on other forums, one of them being Patrol 4x4, so this is where this idea comes from.
I want to build a control module for the Turbo and bypass the ECU, The turbo is a Variable Nozzle Turbo that can open and close to suit the airflow and boost required. The turbo vanes are opened/closed by a pneumatic actuator which is controlled by a solenoid which is driven by a PWM signal. I have a small off the shelf PWM motor controller (12v 5 amp max) which operates the solenoid ok, but is manual and obviously doesn’t adjust appropriately to changing conditions.
After looking at a variety of ideas and doing tons of research I have come up with an idea which has evolved over time, which involves 2 primary and 2 secondary inputs:

Motor RPM using the tacho input, 0 – 5V (still to confirm)
Throttle Position Sensor (TPS), 0 – 5V
Base load +5V, to bring solenoid up to operational duty cycle range
Secondary TPS input through differentiator amp to provide a throttle change advance mechanism.

After many google searches using a variety of descriptors my understanding grew and I believe I can achieve what I want using Op Amp circuits.
I have set up an excel spreadsheet to test a variety of scenarios theoretically it all works well, but I’m not fully up to speed on electronics, it’s a long way between Crystal radio’s and Op Amps.
The main Criteria I have is that:
The circuit kept as simple as is possible.
The sensor inputs are isolated and there is no significant current drain on them and there is no risk of damage to the ECU.

• I’ve prepared a circuit plan, attached below and would like any advice, the questions I can think of immediately are:
• Voltage followers, can they be kept simple as per the unity set up or do I need to add resistors?
• Can I power the Op Amp rails with a +6 & -6V supply from the vehicle 12V supply using a basic voltage divider, and what value resistors should I use in the divider?
• Still haven’t got my head around the earths, are they all a common earth or is it multiple earths, I’ve seen virtual earths mentioned in my reading, what are they?
• What values would be appropriate for the variable resistors, VR1-VR4
• The differentiator is just the basic set up, do I need to add an additional resistor in the input and/or capacitor in parallel on the feedback resistor?

Diode 1 & 2 are in place to prevent a – ve voltage, that may arise from the under some circumstances.
I haven’t included any voltage reduction because the simulations I’ve done indicate the summed voltage will usually be significantly less than 5V and if the rails are powered at 6V + - then output will be limited by saturation anyway, Is saturation a proportional amount to rail voltage or is it a set amount irrespective of rail voltage.
The Difference amp is there to flip the range from 0 to +5V to +5V to 0 as this is the range required to feed into the PWM generator, to operate the turbo correctly.
The PWM generator I have is controlled by a potentiometer which has 3V across the contacts, wondering if it would be easier to link into it or just add more components and incorporate the PWM generator.
Sorry for all the questions, but still learning all the lingo and functionality, 2 weeks ago I didn’t know what an Op amp was.

Will come back to post schematic, gotta work out the system