Electro-hydraulic brake apply systems are known wherein the pressure applied to a vehicle's wheel brakes is controlled by a electronic unit that evaluates several parameters and delivers a control signal to a hydraulic modulator that sets the wheel brake pressure. A key parameter used to determine the appropriate braking pressure at the wheel brake is the driver's command, delivered as an input on the brake pedal. Generally, a pressure sensor monitors wheel brake pressure providing feedback to the control system for use in setting the braking pressure.
A typical hydraulic modulator includes a pressure generation mechanism and a means of controlling delivery of the generated pressure to the wheel brakes. This may take the form of a pump and proportional hydraulic valve, a pump with a pair of two way valves or a movable piston variable pressure chamber device. The number and arrangement of these elements included in a braking system is determined by the system layout and selected control scheme.
With a movable piston variable chamber device, a piston is driven linearly in a bore to vary the size of a pressure chamber. The pressure chamber is connected to a brake line leading to the wheel brake. For an application of braking pressure, the size of the variable chamber is reduced, to take up compliance in the system, and to increase braking pressure. The piston applies an increased force to the contained non-compressible fluid to apply the brake. To decrease braking pressure, the force on the piston is reduced and when appropriate, the size of the variable chamber is increased to release the brake. With this type of operation it is preferable to latch the piston in position for various operating requirements.
One known method of latching the piston includes the use of an electro-magnetic brake that when engaged, prevents rotation of a motor that drives the piston through an interposed drivetrain. An electromagnetic brake type latch uses frictional engagement with the motor's rotor to lock the rotor in position and thereby, prevent movement of the piston. To allow movement of the piston, the electromagnetic brake is disengaged. Engagement and disengagement of the latch is controlled by an electronic unit.