Electric parking brakes (EPB)

Introduction

The electric (or electronic) parking brake, also known as an automatic parking brake (APB), is a function offering the driver increased comfort and convenience. In addition, as the hand lever is not used, car manufacturers have more freedom of choice as to where they site the operating parts within the car.

Features such as hill- or auto-hold are also possible with an EPB. Hill-hold stops the car rolling away accidentally when standing still or setting off. Auto-hold keeps the brake pressure applied after the driver releases the pedal. If the ABS sensors detect any movement the pressure is increased. If the accelerator is pressed (or the clutch released on a manual) the brakes are let off.

There are two main systems in use as described below; namely the cable-pull and electric-hydraulic type. Both methods include a visual warning light on the dashboard. The second of these is now the most common. A third full-electric type will start to be used in the near future.

Types of EPB

 

Figure 1 Cable pull system (Source: Landrover)

Cable-pull systems

The cable pull system is simply a development of the traditional lever and cable method. As the switch is operated, a motor, or motors, pull the cable by either rolling it on a drum or using an internally threaded gear on a spiral attached to the cable. The electronic parking brake module shown as figure 1, also known as the EPB actuator, is fitted to some Range Rover and Landrover models. The parking brake can be released manually on most vehicles. After removing a plastic cover or similar, pulling a wire cable loop will let off the brake.

Electric-hydraulic caliper systems

These types are usually employed as part of a larger control system such as an electronic stability program (ESP).

Figure 2 Electric-hydraulic parking brake caliper (Source: Bosch Press)

When the driver presses the switch to activate the parking brake, the ESP unit automatically generates pressure in the braking system and presses the brake pads against the disc. The calipers are then locked in position by an electrically controlled solenoid valve. The caliper remains locked without any need for hydraulic pressure. To release the brake, the ESP briefly generates pressure again, slightly more than was needed to lock the caliper, and the valve is released. This system is show as figure 2.

Full electric drive-by-wire systems

The drive-by-wire system shown in figure 3 was developed by Continental. It uses an electric motor (3) and gearbox to apply pressure on the pads and therefore on to the disc (7). A key component is the parking brake latch. This is like a ratchet and it prevents the pressure in the piston from rotating the motor – and it therefore keeps the brakes applied.

Figure 3 Electric drive-by-wire brake caliper components. 1 Electric motor, 2 Gearbox, 3 Spindle piston, 4 Parking brake latch, 5 Brake pads, 8 Brake anchor, 7 Brake disc. (Source: Continental)

Control system

The diagram shown as figure 4 is a generic layout of an EPB system. It is from a company known as Allegro Micro Systems and they provide a wide range of motor driver, power management, and Hall-effect sensor ICs.

Figure 4 EPB general system layout (Source: Allegro Micro Systems Inc.)

Like all other similar layouts, the electric parking brake system has inputs and outputs. The control switch and wheel position sensors being the main inputs but also shown in figure 4 is a current sensor in the motor supply. This allows monitoring of motor performance and torque. The outputs are the motor and a warning lamp. On most systems there will also be a diagnostic output for reading by a scan tool either directly or via the CAN bus.

Roller testing

NOTE: Always check and follow specific instructions and specifications provided by the vehicle manufacturer.

Testing the parking brake on rollers is possible on both systems. Cable pull types can be tested much like ordinary hand or parking brakes. But there is a risk of locking the wheels. Some manufacturers have test modes – so double check!

The types with caliper motors can also be tested on rollers but the procedure is slightly different. Most of the caliper-motor systems (TRW for example) have special software incorporated in the ECU for brake testing. When the car is put on a rolling road and the rear wheels are driven by the test equipment, the ECU detects this as a test situation because the rear wheels are moving and the fronts are not. It therefore puts the system into test mode. If a multi-function display is used on the vehicle dashboard, it will display an appropriate message. The technician can then operate the handbrake-switch. The ECU applies the electric parking brake with enough force to obtain a reading on the roller brake tester. The wheels should not lock. After the test is complete the rollers are stopped and the switch is released. When the switch is activated again, the brakes are re-applied in the normal way and the wheels will be locked.

Summary

The key drivers for enhanced systems such as the EPB are increase functions, comfort and safety as well as greater freedom for vehicle designers.

The addition of electrical functions to the normal hydraulic caliper is currently (2013) the most common. However, brake-by-wire is just round the corner and development is unlikely to stop…

5 thoughts on “Electric parking brakes (EPB)

  1. I am looking for a supplier of electrically applied disc brake calipers.
    The application is special. I wish to use it as part in conjunction with a large disc as a dynamometer in an experimental wave energy machine.

    Something along the lines of the EPB you describe except that it will be immersed in seawater?
    Any thoughts Tom?

    1. Hi Joe, sounds like a fascinating project you are working on. Sorry however as I don’t have contact with suppliers. Car disc brakes work in pretty hostile environments however so may do what you need?
      Tom

  2. I need a bit of help with this EPB system, I’m on with a car build project and all the parts are from a 83/84 ford with a pinto engine, the parking brake is standard and operates rear drum brakes, but the brake pull leaver is in the way of the gear leaver when the gear leaver is in 2nd and 4th gear and can’t be moved so I’m thinking to change to electric, can anybody help me out, suggest the most simple system what’s available.

    Many thanks Roy

  3. It’s a very fascinating article but I certainly feel that the levels of electronic control in cars these days is getting way beyond the point where electronic computer controlled hand/parking brake system or the electronically controlled throttle body/plate/pedal system or fly-by-wire accelerator the level of complexity is incredible.

    I fully understand why it seems like the way to go,why it’s considered in every way superior to a throttle cable or handbrake cable. For example, it’s certainly not purely for ease of maintenance or use nor purely for reliability as the old cable system is pretty fool proof and beyond reliable.

    Ok levels of accuracy and fine grained control are reasonable particularly in.something packed with so many ECUs but is that level of control necessary for a handbrake, honestly and you can argue that it is required at the throttle to meter the fuel/air supply to incredible levels but it is often said with some basis of truth in it that the general electronic computer controlled management systems of today with their digital signalling have a bit if trouble mtching the analogue percision of a well tuned SU carburettor in as new working order.

    1. Interesting thoughts Bob, thanks for sharing. My view is that technology such as this is more about features than it is about accurate control. Hill hold, automatic application when the ignition is off for example. I’m not sure I fully agree with the SU carburettor comment relating to accurate fuel control, because modern system take far more information into account than air flow. I do remember them with affection however – must be my age 🙂

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