Brake Blending

A system, used on modern, dynamically braked EMU vehicles and some locomotives, to ensure that air and dynamic braking acts in co-ordination. An electronic signal from the electric (dynamic) brake indicating the brake effort achieved is compared with the brake effort demanded by the driver or an automatic control system and will then call up additional braking from the air brake system if required. A typical set-up on a car will comprise a brake control unit which contains electronic controls and electro-pneumatic valves. Various inputs are processed in the brake control unit which then generates electronic or pneumatic outputs as necessary. Brake Demand: When a brake demand is requested by the driver (or the automatic driving control on an ATO equipped train) it is transmitted along a train wire to the brake control unit on each car. The signal can be digital or analogue providing a message either in steps or infinitely variable. The demand is then matched to a load compensation signal provided by the car suspension system. The greater the weight, the greater the brake demanded. The brake effort demand is now converted into air pressure signal and the brake is applied by sending air into the brake cylinders until it matches the signal. At the same time, a matching demand is sent to the dynamic brake controller and the traction control system will initiate dynamic braking. The system will send a "dynamic brake effort achieved" signal to the brake controller which will subtract it from the air brake demand signal and so reduce the brake cylinder pressure accordingly. Dynamic Brake: In an ideal world, the dynamic brake will be used as much as possible to reduce wear on brake pads (or blocks) and there are often circumstances when the dynamic brake will provide all the braking required. However, it is normal to leave a little air in the brake cylinders in case the dynamic brake switches off suddenly. This reduces the time taken for air pressure to restore to the demand level when dynamic braking is lost. Smoothing: Another feature of modern brake control is the "inshot". This is a small amount of air injected into the brake cylinders immediately brake is called for so that the build-up time is reduced. Braking systems are also "jerk limited"; smoothed out as they are built-up so that the passengers don't feel the cars snatch as the brake is applied. This is particularly important in the case of dynamic brakes which, if not jerk limited, have a tendency to apply sharply if the train is at speed. Fade: Once the brake is applied, the dynamic portion will have a tendency to fade as the speed, and thus the current generated by the motors, reduces. Some systems have a pre-fade control; a signal sent by the traction controller to indicate the brake is about to start fading. This gives a smoother changeover into air braking. Trailer Cars: Most types of EMU comprise a mixture of motor cars and trailers cars. As trailer cars have no motors, they do not have their own dynamic braking. They can, however, use dynamic braking on motor cars in their braking effort if that is available. In the case of a two-car pair, for dynamic demand, the motor car brake control unit will add the trailer car demand to the motor car demand. The resulting dynamic brake achieved may be sufficient to match all of the motor car demand and have some extra for the trailer. In this case, the motor car brake control unit sends a message to the trailer car to say how much of the trailer car's demand has been fulfilled by the dynamic brake. The trailer's air brake pressure can be reduced accordingly. There will be some limit on the total dynamic brake possible because of adhesion limits and this will be incorporated into the brake control calculations. If the dynamic brake is reduced for any reason, the trailer car air brakes will be reapplied first followed by the motor car brakes.