The secrets of brake-by-wire

Formula 1 cars have a two-pedal set-up. The throttle has about 50mm of travel, while the brake has around 10mm.

In the old days, there was a throttle cable connecting the pedal to the engine, but that was displaced by the fly-by-wire system. This year, fly-by-wire rear brakes have also been introduced as part of the major regulation changes related to the new 1.6-litre turbocharged V6 'green' engines.

To understand the brake-by-wire technology, first we need to look at the well-established fly-by-wire throttle technology:

HOW IT WORKS: FLY-BY-WIRE

The throttle pedal is now a 'torque-control pedal', and is mapped to deliver a torque level at the rear wheels. In 2014, this torque can be produced by a combination of the new 1.6-litre V6 turbocharged engine and the energy recovery systems, so is more complicated than before.

Connected to the torque control pedal is a position-sensitive potentiometer. This can be a linear or a rotary potentiometer and sends out a different level of electrical signal to the control unit as its position alters.

This signal will be processed and if, for example, fuel needs to be saved, the torque map (which is embedded in the electronic control unit) may only request electrical torque from the battery pack until it reaches the maximum available electrical torque.

After this, a command will be sent to a hydraulic control valve. This will open, sending high-pressure hydraulic fluid to an actuator connected to the engine throttle system. This actuator will open the throttle, and its position will be monitored by another potentiometer.

These electrical signals are sent back to the ECU and, when the position matches the driver's request, the hydraulic valve will hold the throttle in that position. These systems work very quickly, with a reaction time to a request in the region of 0.001 seconds.

For safety reasons, there will be a position sensor at each end of the torque-control pedal travel should anything go wrong with the pedal potentiometer. Each time the torque-control pedal is at either end of its travel, a check will be made to ensure that the systems are all still fully in agreement.

HOW IT WORKS: BRAKING BY WIRE

In contrast to the throttle set-up, the fly-by-wire rear-braking system is pressure sensitive.

A standard F1 braking system consists of four brake calipers, one on each corner of the car, and two master cylinders, one connected to the front calipers and the other to the rear. The master cylinders are connected to the calipers by Aeroquip piping. The smaller the pipe size, the slower the flow, but the stiffer the system when the driver applies the brake-pedal pressure. This pressure can be around 180kg at the brake pedal.

The brake pedal is connected to the master cylinders by the balance bar. This is a beam, the ends of which are connected to the master-cylinder pushrods, which the brake pedal pushes onto the middle of. The driver can adjust the brake balance by moving where the brake pedal pushes on this balance bar. By moving it closer to the front master cylinder he will get more front-brake pressure and vice versa. This is done with a lever mounted in the cockpit and connected to the balance bar with a cable.

The braking-by-wire system has come about because the potential harvesting (charging up the battery pack by recovering energy otherwise wasted under braking) has doubled. So there is a lot more reverse torque in the rear axle from this harvesting than in 2013.

It would play havoc with the braking balance and the stability of the car if the driver was to make any adjustments on the level of harvesting.

Sauber has suffered from brake-by-wire issues so far © LAT

To create this system, the pipe that goes between the rear master cylinder and the 'T' piece that separates the fluid flow to each rear caliper is removed. A pressure sensor is fitted to the master cylinder, measuring the pressure the driver is applying to the brake pedal and sending it to the ECU. This can range from 0bar to 100bar.

The ECU will also know what level of energy harvest the driver is requesting under braking to charge the battery. This is signalled via a dial on the steering wheel with approximately 10 positions on it. These two signals will be combined, and the rear-brake pressure needed to give the front-to-rear brake balance requested by the driver will be defined.

This pressure will be achieved by opening a hydraulic control valve, letting through high-pressure hydraulic fluid. This valve will be mounted near the rear of the car on the pipework that would normally feed the brake fluid to the rear calipers.

On this pipework there will also be a pressure sensor to monitor the newly-requested rear-brake pressure. This hydraulic valve would then shuttle back and forth to maintain the requested rear-brakeline pressure, giving the correct overall front-to-rear brake balance.

The driver's force on the brake pedal will never create a stable pressure in the master cylinders, so this hydraulic valve will always be opening and shutting to maintain the varying pressure and give a stable front-to-rear brake balance.

The driver still has the opportunity to change the front-to-rear brake balance as required. All the fly-by-wire system is doing is taking the driver's rear-brake-pressure request and reducing it by the level of negative torque the battery harvesting is requesting, to end up with the brake balance he requires for stable braking and corner entry.

This feature originally appeared in the February 20 issue of AUTOSPORT magazine

Illustrations by CRAIG SCARBOROUGH