Railroad car slip prevention systems are well known in the prior art. See, for example, Japanese Utility Model Nos. 53-6170 and 59-34543; Japanese Patent No. 51-17662, and, one illustrated in FIG. 9 on page 329 of "The Articles of the Fourteenth Domestic Symposium on Cybernetics Utilization in Railways."These prior art wheel slip prevention systems are explained hereinafter with reference to FIGS. 4 and 5. In these Figures, MV1 is a first electromagnetic valve used for braking; MV2 is a second electromagnetic valve used for brake release, and KK is the wheel slip detecting system. The wheel slip prevention system inputs the axle speed of each axle and calculates the difference between each axle speed or the acceleration and deceleration and detects the slip or the recovery and controls ON and OFF of both of the electromagnetic valves MV1 and MV2.
As illustrated, CV designates the control valve which controls communication of fluid pressure, such as compressed air, according to the brake command. In addition, RV1 and RV2 designate the relay valves which amplify the flow rate of the compressed air being communicated from the above-referenced control valve CV. In the drawing FIGS. 4 and 5, "BC" is the brake cylinder and the compressed air source is designated "MR". Even though, as illustrated here, the structure of the relay valves RV1 and RV2 are a little different, they are well known and, consequently, a detailed description of such relay valves is omitted for the sake of brevity. In both FIG. 4 and FIG. 5, the brake status is illustrated in which the control valve CV communicates a predetermined amount of compressed air in accordance with a specific brake command and both electromagnetic valve MV1 and MV2 are OFF. In this case, the relay valves RV1 and RV2 supply and maintain the communication of such compressed air corresponding to the predetermined amount in the brake cylinder BC.
In the braking situation described above, when wheel slip is detected, the electromagnetic valves MV1 and MV2 are switched to ON and the compressed air in the brake cylinder BC is exhausted from the relay valves RV1 and RV2. In this manner, the brake is allowed to move to a release position. Once recovery of the axle speed is detected, which is after the wheel slip has been stopped by the brake release, the system returns to the status illustrated in FIGS. 4 and 5.
An overlapped condition can be easily maintained in the illustrated prior art railroad car braking system when the electromagnetic valve MV1 is "ON" and the electromagnetic valve MV2 is "OFF." Thus, the two electromagnetic valves MV1 and MV2 can be utilized for three conditions. Namely, these conditions are brake release, overlapping and braking, and control, i.e., readhesion control.
Nevertheless, the examples of the prior art described above with respect to FIGS. 4 and 5 have certain disadvantages associated with their use. These disadvantages are cost, size, and installation requirements, since they require two electromagnetic valves MV1 and MV2. These electromagnetic valves MV1 and MV2 are two-part, two-position valves which have an "ON-OFF" type control system. Therefore, the use of a three-point, three-position electromagnetic valve could be considered for use in this application, however, if such electromagnetic valve is an ordinary double solenoid type valve, then the electric power consumption will be almost the same as for the two electromagnetic valves MV1 and MV2. Thus power saving would be negligible.
Further prior art of interest may be found in Japanese Patent Nos. 59-4583, 60-39913, and 61-59080. Each of these patents teaches an electromagnetic valve which, although it is not used on a railroad car brake system, but instead is used in a hydraulically-controlled anti-skid system of an automobile, includes a three-part system capable of being switched into three different positions by controlling the excitation current of a single solenoid. For this reason, the use of a system incorporating these particular valves into a railroad car brake system could conceivably be considered as an alternative method of achieving the object of the present invention. However, each of these electromagnetic valves for use in an automotive application has a rather complicated construction. For example, the fixed valve seats are placed on both ends of the axial direction inside the solenoid. Furthermore, the movable valve member is installed in the movable inner core so that it can move freely by way of a guide member. These movable valve members are installed on both sides corresponding to the above-referenced pair of fixed valve seats. It should also be noted that these two movable valve members are moved by way of a spring which is positioned between them and that in the installed position of such movable valve members a predetermined clearance is of critical importance between them and the guide member. As is recognized, the structure of the internal switching valves is relatively complex.
Additionally, these three referenced electromagnetic valves are used for the pressure control of a hydraulic system and their flow is very small so that the diameter of the inlet valve seat or the outlet valve seat is small and pressure equalization in these areas is not considered. Therefore, they cannot be used as is for a railroad car which uses a pneumatic control system and has a large flow.