Stopping on a Dime: Electronic Parking Brake

Jan. 1, 2020
Operating a parking brake with an electronic control unit might seem extravagant at first, but it offers several advantages. An Electronic Parking Brake (EPB) can be linked to the vehicle's anti-theft system, and it can be applied automatically when
Operating a parking brake with an electronic control unit might seem extravagant at first, but it offers several advantages. An Electronic Parking Brake (EPB) can be linked to the vehicle's anti-theft system, and it can be applied automatically when placing the transmission in Park or, with a manual transmission, when removing the ignition key.

When the parking brake is built into brake calipers and the brake is applied after driving the car, the clamping force decays as the brake rotors cool down and shrink. An EPB can detect this and automatically tighten the parking brake as needed.

An EPB also can be programmed with a hill-hold function for starting out on a hill and an auto-hold function that applies the EPB after a full stop and releases it when the driver presses the accelerator. Finally, an EPB button can be placed anywhere in the cockpit, even on the steering wheel.

Two different types of EPB are in production. One uses conventional parking brake hardware, but instead of using a lever or pedal, the cables are pulled tight with an electric actuator. The actuator can be added to existing models without the need for redesigning the brake system, reducing the cost of manufacture and service parts. The actuator can be mounted anywhere on the vehicle, even outside the cabin, eliminating cable pass-through holes.

If mounted inside the vehicle, there can still be a manual release for service or in the event of electrical failure. A cable-type EPB is currently in production on BMW, Jaguar and Land Rover.

The other type of EPB uses a small electric motor built into the brake caliper. As with a window lift motor, the control unit manages the power and ground circuits to control the direction of rotation. A sprocket on the motor drives a toothed belt, like a timing belt, which in turn drives a much larger sprocket that's on a shaft aligned with the caliper piston.

The inner face of the driven sprocket is angled, like the tip of a slash-cut exhaust pipe. A swash plate rides against that angled hub, and tabs on the plate prevent it from rotating. When the motor drives the sprocket, it forces the swash plate to wobble.

On the opposite face of the swash plate is a gear with 51 teeth. It drives another gear that has only 50 teeth on its face. The wobble motion of the drive gear means only one tooth is engaged at a time, so the driven gear is forced to rotate one tooth at a time. With a 50:1 reduction ratio, the drivetrain can generate a lot of torque with an extremely small, low-powered motor.

On brake calipers with cable-operated parking brakes, the cable moves an actuating lever that turns a screw jack inside the piston, forcing the piston against the brake pad. In an EPB, the drivetrain turns the screw jack, but it operates in both directions and its range of motion is much greater. However, like a cable-operated caliper brake, it can only be used on single-piston floating calipers because it operates directly on the piston.

Although there are differences, the operation of Volkswagen's EPB system is typical for vehicles with a caliper-mounted EPB. With the ignition switch either on or off and vehicle speed below 4.4 mph (7 kph), the EPB is applied by pushing the button.

The EPB is released by pressing the brake pedal and pushing the button, but the ignition switch must be on. Holding the button down while the vehicle is moving faster than 4.4 mph (ignition on or off) activates an emergency brake program in the antilock brake system (ABS) control unit, and the hydraulic brakes will slow the car at the rate of about 12.5 feet/second/second. If the button is released above 4.4 mph, the brakes will release, but if held, the car will come to a complete stop.

When the Auto Hold button is pushed, the ABS will maintain hydraulic pressure in the calipers after a full stop until the accelerator pedal is pressed. If stopped for more than three minutes, the EPB will take over until the accelerator pedal is pressed. The Auto Hold button must be pushed again to disengage the system.

On a caliper-mounted EPB, brake pad clearance is adjusted automatically. The control unit detects the need for adjustment by monitoring the motor's current draw. That draw increases when the piston moves far enough to stretch the piston seal.

If the EPB has not been manually activated in 620 miles (1000 km), the control unit will do so when the ignition switch is turned off to check current draw.

So far, the caliper-mounted EPB is only used on more expensive vehicles, but the cable-operated EPB is more common, especially on commercial vehicles. In either system, the EPB must be deactivated to replace the rear brakes, and on caliper-mounted EPB, a scan tool is needed to retract the caliper pistons.

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