Re: [A/S] Re: Brake Controllers

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxx>]

It looks a lot like the last hydraulic controller I bought maybe 14 or
15 years ago. There's no detailed specifications there about the volume
of hydraulic fluid required to move the pressure sensor. This could be
designed only for off road or semi tractor service. In the old ones the
sensor was a brake cylinder that moved a wiper along a wire wound
resistor. Its volume was no problem on the single hydraulic brake
systems. But on the more modern dual systems, the dual master cylinder
has been customized to supply different amounts of fluid to front and
rear (when mixed drum and disk brakes) or equal amounts (when all four
wheels have the same brakes AND the same cylinder sizes or the split is
diagonal). Then the brake failure sensor compares the pressure in the
two systems. If its different, the sensor says a brake system has
failed. Changing the volume of one of those twin systems has the same
effect to the unbalance sensor AND TO THE BRAKE balance as a failed
system. For that reason vehicle makers have set a limit on the volume of
a brake controller's cylinder if mechanical. That limit on use of a
hydraulically actuated brake controller came long before the
complications of antilock brake systems.

It may be that a better way to sense hydraulic pressure is with a
pressure sensor that doesn't change volume with pressure and to hook
that to the part of the braking system that doesn't have ABS pulses.
though once the sensor has produced an electronic signal, the microchip
can do a lot of averaging to smooth out the ABS pulses for feeding to
the trailer. Though if the tow vehicle is locking up the tires, the
trailer might be doing that also and pulsing the trailer brakes may be a
more effective way to stop in a panic on a less than perfect surface.

I'm sure pressure sensors exist that would take up essentially no
hydraulic fluid, also dual sensors, that could balance sensing both
brake systems. In fact they probably are already in the antilock brake
systems and if the vehicle manufacturers would just do it, probably the
vehicle's main computer could supply a pulse width modulated signal that
would only need a power switch and a control of the average current
(depending on the braking action needed for the particular trailer in
tow) that would be as good or better than the old hydraulic controller
on the single brake. I'll have to look in my F-150 shop manual to see if
I can detect whether such brake pressure sensors are already in the
vehicle.

Gerald J.