Patent GB-898622-A (GB)
Title: An electromagnetic brake control circuit
IPC: G06F11/08
Filed: 1959-07-14
Abstract: 898,622. Electromagnetic brakes. NATIONAL CASH REGISTER CO. July 14, 1959 [July 18, 1958], No. 31961/61. Divided out of 898,621. Class 103 (1). In an electromagnetic brake control circuit switching means is connected across a power source through the actuating winding or windings of the electromagnet, the switching means comprising one or more switching devices each provided with a control element D.C. coupled and one or more switching devices each provided with a control element A.C. coupled to a source of potential, the switching devices being simultaneously rendered conductive by the application of a signal from the source, the D.C. coupled devices thereby providing a holding current in the electromagnetic windings and the A.C. coupled devices providing an additive transient current in the windings which is effective to rapidly overcome the initial magnetic inertia of the electromagnet. Fig. 1 shows an electromagnetic brake 105 for retarding a record tape 102 and comprises one surface 147 for frictionally engaging the tape. The other friction surface is constituted by an armature 146 resting lightly on the tape and carried on a bracket 151 secured to the framework 121. When two windings 149 are energized the armature is attracted and a rapid increase in frictional engagement of the tape is effected to arrest the tape. The weight of the armature 146 applies a slight pressure to the tape which results in a slight drag which must be overcome in driving the tape during tape reading operations. This ensures that the tape will be taut when it passes a tape reading station (not shown). A spring 154 prevents bounce of the armature 146. In the brake operating circuit shown in Fig. 4, an input lead 551 to the brake operating circuit provides a path for the halt signal such that when a negative signal is applied to the lead 551 the brake coil 591 is energized under the control of the brake operating circuit and the brake 105 is caused to halt movement of the tape and hold the tape against further movement until a positive signal is applied to the lead 551 from a utilizing device. One path extends to the control electrode of valve 555 and a second path extends to a minus 300 volt D.C. terminal 557. The cathode of valve 555 is connected to earth potential and the anode of the valve 555 is connected through a resistance 558 to a plus 200 volt D.C. terminal 559. A brake coil 591 is connected between the plus 200 volt D.C. terminal 559 and the common 571 so that when valves 565 to 579 are rendered conducting a circuit is completed through the valves for energization of the brake coil 591 to operate the brake 105. The valves 565 and 566, which are D.C. coupled to the valve 555, supply about 15 milliamperes each to the brake coil 591. Since a greater force is required initially to operate the brake 105 than to maintain it in its operated condition, additional current is supplied to the brake coil 591 for a brief period by conduction of the valves 578 and 579 which are A.C. coupled to the valve 555 and which are rendered conducting for a short time and are then cut off again as the capacitor 574a charges to provide this momentarily additional current for energization of the coil 591. The combined effect of the four valves 565-579 upon the brake current can be seen in the waveform shown at 594 at Fig. 4. Considering the waveform from left to right, the current peak rises to nearly 200 milliamperes and then falls to a steady state of about 100 milliamperes.