AN2791

Opening/closing time of the INLET valves versus duty-cycle of the hold-phase

11/18

4

duty-cycle of the hold-phase

The conventional strategy adopted to drive on-off valves used in the hydraulic modulator of

ABS/ESP control unit is described in the Figure 8. The "pull-in" phase corresponds to the

maximum values of duty-cycle applied for the first part of the valve opening/closing time.

This phase guarantees the opening/closing of the valve against stiction phenomena due, for

example, to the aging of the valve, to the dirt into the brake fluid, to the stiffness change of

the valve spring and so on.

The "hold" phase corresponds to the duty-cycle value that is necessary to maintain the valve

opened/closed. Clearly this value is less than that used for the "pull-in" phase, because the

force required to overcome the static friction is greater than the force required to overcome

the dynamic one. Obviously, this kind of duty-cycle strategy is power saving too.

Figure 8.

"Pull-in"- "hold" phase duty-cycle strategy traditionally adopted to drive

on-off valves

Several tests have been carried out fixing the coil energizing frequency at 5 Hz and the time

strategy at:

●

for the first 20 ms, duty-cycle = 0.1 %

●

for the next 30 ms, duty-cycle = 90 %

●

for the last 150 ms, duty-cycle = [5:5:90] %

The different duty-cycle configurations considered are summarized on the first column of the

Table 1. In these tests, we measured the opening and closing time of the INLET valve, and,

in addition, the time in which armature-piston of the valve starts its motion. An interesting

result comes out. While the armature-piston motion and the closing time of the INLET valve

are not affected by the duty-cycle configurations, the opening time is affected. In particular,

this increases of 0.5 ms for each 5 % of duty-cycle increase of the "hold" phase.

a.

Take into account that the INLET valves of an ABS/ESP hydraulic modulator are on-off valves normally opened and

normally controlled by a current control loop. On the hydraulic modulator there are also OUTLET valves. These valves

normally closed do not require a current control loop but conventional low-side switch.