easily be done by a skilled human operator. The circuit for automatic control is much more complex. For combined control by back-up drives, different circuits are used. Commutation of the circuits of the electrical engines themselves is done by bridge circuits (see Figure 3.14). More complex circuits are used. For example, short circuiting is eliminated by simultaneous actuation of similar arms of the bridge, which is done by the introduction of additional blocking. The arms of the bridge elements may be backed up and turned on using a voting circuit (two out of three). The choice of elements depends on the electrical parameters of the engines, primarily, on the start-up current fn. For small start-up currents (up to 2-3 A) small relays are used; for large currents remote switches are used. Circuits which control the commutating elements and logic control circuits are usually constructed according to a three-channel principle. The control circuit of a one-engine drive includes about two dozen relays, several dozen isolating and "extinguishing" diodes, and limiting resistances. Turning off the drives in given positions is done by terminal contacts which are directly in the circuits of the electrical engines or in the circuits of the control relays. The terminal contacts in the drives are usually in blocks which also contain electrically independent signals and telemetry contacts. In the electromechanical type docking device the terminals are usually sliding contacts with disks made of metal and dielectric. Their structure was improved by considering specific requirements (small mass, high reliability in general, in vibrations, vacuum, etc.). The blocks use contacts with conical and cylindrical disks (Figure 3.15). As practice and analysis using the potential failures method has shown sliding contacts have the following advantages over microswitches: 1) simplicity, minimum number of moving parts; 2) small size and mass; 3) small displacement forces (moment of friction for one pair of contacts); 4) high quality electrical contact which reestablishes itself during sliding. Contacts and metal parts of the disks are manufactured of bronze with various coatings which insure a high quality electrical contact. The dielectric parts of the disks are made of plastic, for example, polycarbonate (Deflon) which
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