Hydraulic brake wear detection apparatus and method

A method of detecting a degree of pad wear of a brake pad of a hydraulic brake including the brake pad, a brake rotor, and a caliper housing at least partially enclosing the brake pad and brake rotor for relative longitudinal motion therebetween, the caliper housing defining a caliper cavity in selective fluid communication with a reservoir of brake fluid and with a single-acting plunger type power transmission unit via a caliper passage, the method comprising:

• providing a hydraulic brake wear detection apparatus including
• a caliper piston located at least partially within the caliper cavity and attached to a first side of the brake pad, the caliper piston including a piston cavity extending from a caliper piston rim into a caliper piston body at a location longitudinally opposite, and spaced from, the brake pad, and
• an internal piston located at least partially within the piston cavity and entirely enclosed in an internal space defined cooperatively by the piston cavity and the caliper cavity;
• selectively providing pressurized hydraulic fluid via reciprocal longitudinal motion of a plunger of the power transmission unit;
• interposing a normally open venting valve in the caliper passage hydraulically between the reservoir and both the power transmission unit and the caliper cavity;
• moving the caliper piston reciprocally longitudinally with respect to the brake rotor to selectively place a second side of the brake pad, longitudinally opposite the first side of the brake pad, into contact with the brake rotor to provide a braking action thereto during a normal non-failure braking mode of operation;
• with the hydraulic brake wear detection apparatus in a detection mode of operation at least partially having the brake pad advanced into contact with the brake rotor:
• (1) supplying a predetermined volume of positively pressurized hydraulic fluid to the caliper cavity,
• (2) closing the normally open venting valve to isolate the predetermined volume of pressurized hydraulic fluid from the reservoir of brake fluid and thus provide a closed hydraulic loop between the power transmission unit and the caliper cavity,
• (3) retracting the plunger of the power transmission unit to responsively move the internal piston in a first longitudinal direction, away from the brake rotor, under influence of negatively pressurized hydraulic fluid,
• (4) opening the normally open venting valve to allow hydraulic fluid from the reservoir to enter at least one of the caliper cavity and the caliper passage responsive to negative pressure therein, thus rebalancing the negatively pressurized hydraulic fluid within the caliper cavity to a neutral level by providing an augmented volume of hydraulic fluid within at least the caliper cavity and the caliper passage,
• (5) closing the normally open venting valve to isolate the augmented volume of hydraulic fluid from the reservoir,
• (6) advancing the plunger of the power transmission unit to responsively move the internal piston in a second longitudinal direction, toward the brake rotor, under influence of positively pressured hydraulic fluid of the augmented volume,
• (7) sensing a wear-indicating volume measurement responsive to the plunger advancement of (6),
• (8) ceasing advancement of the plunger of the power transmission unit in (6) responsive to detection of a predetermined hydraulic pressure value in at least one of the caliper cavity, the power transmission unit, and the caliper passage,
• (9) communicating the wear-indicating volume measurement to an electronic control unit, and
• (10) determining a degree of pad wear responsive to the sensed wear-indicating volume measurement.