Pressure Modulation in Separate and Integrated Antiskid Systems with Regard to Safety 840467
The antiskid systems which have been on the market for some time are characterized by the fact that they are separate from the brake power-assist unit and are positioned between the master cylinder and the wheel brakes (separate configuration). At present, integrated antiskid systems are also being prepared for launching on the market. In these systems the hydraulic brake power-assist unit performs the functions of brake boosting and partly also of ABS pressure modulation.
The principles of ABS pressure modulation in separate and integrated antiskid systems are compared and questions concerning safety are discussed.
With the separate ABS (plunger system, return system) the brake circuits are closed, i.e. when braking and also during ABS operation the volume of brake fluid between the master cylinders and the wheel brake cylinders is closed and separated from the energy supply of the hydraulic brake power-assist unit. If a brake circuit fails during ABS operation, the respective master-cylinder piston advances into its end position. The energy supply is unaffected. Special monitoring of the energy supply is therefore not necessary.
With the integrated ABS there are two possible principles, one with controlled master-cylinder pistons and one with direct pressure supply into the brake circuits. The controlled master-cylinder pistons are arranged in parallel and are moved to and fro on the primary side during ABS operation by means of a control valve with the result that the pressure in the wheel brakes is modulated in accordance with the control signals from the electronic control unit. In the closed brake circuits between the master cylinders and the wheel brakes there are merely pressure-holding valves which do not remove any brake fluid from the closed brake circuits. If a brake circuit fails, the corresponding master cylinder moves to its limit stop and the energy supply is not affected, with the result that it need not be specially monitored. However, the elaborate design and the unconventional parallel arrangement of the master cylinders speak against taking this system into production.
The integrated ABS with direct pressure supply into the brake circuits reverts to the proven technique of the master cylinder with the pistons arranged in a bore. Since the control valves are between the master cylinders and the wheel brakes, brake fluid must be supplied to the wheel brakes directly from the brake power-assist unit by way of a pressure-supply valve during ABS operation. This prevents the master-cylinder pistons from extending into their end position. Thus, the brake circuits which are closed for partial braking become open brake circuits during ABS operation. The correct functioning of the pressure-supply valve must be monitored. If a brake circuit fails ABS operation must be switched off promptly so that the energy supply is not exhausted. The pressure supply must also be monitored in order to guarantee adequate braking capability under all circumstances.
Finally, it should be noted once again that, in contrast to separate systems, added attention must be paid in integrated systems to the possible faults described.