[Diy_efi] ABS proportioning valve

[dan at w3eax.umd.edu]

Well, I looked it up on Howstuffworks, and according to them:

It looks like the valve has three positions: open, closed, release (one-way
allows pressure to bleed off from the brake-side).  They claim the controller
is normally open but then releases if a wheel decellerates very rapidly
until proper behavior decelleration is restored and then its re-openned.

This leaves me with a few questions:

1) Whats the purpose of closed?  It seems like you are always alternating
between release and open.  

2) Can you partially open the valve resulting in a pressure drop across it?
If thats the case then presumably you could just monitor the before and
after pressure and run a simple control loop to control the current into
the valve.

3) Is there a way to detect impending lockup before it locks up? From a
technical point of view what exactly is threshold braking?

dan