As a rule, they should maintain their working capacity during failures of individual elements of the circuit; They must be protected from simultaneous issuing of two contradictory commands (for reversible engines); They should protect the source of power from short circuits in the circuit; They should provide the ability to verify the correctness of the circuit, including separate verification of back-up elements. Due to the small lifetime and absence of speed regulation the circuits of commutation and the logic circuits to control the docking device are constructed of contact elements (small relays and remote switches). It is easier to back-up circuits based on contact elements. When these circuits are backed up, as a rule, an attempt is made to insure that during a single breakdown or short-circuit failure of any element of the circuit, the circuit maintains its work capacity. The docking device uses drives with one and two electrical engines. In the latter case the engines must operate on one main gear or are kinematically connected by the gears of a differential. When a differential is used, complete redundancy is provided by satisfying three conditions: 1) the absence of dependence of the circuits of electrical power supply and commutation; 2) exclusion of failures which lead to a constant rotation of the engine after electrical power is turned on; 3) if one provides braking of the kinematic circuit of the failed electrical engine. The first condition is satisfied when one uses separate power supply buses (individual current shielding), “separated” circuits, and the use of a number of blocking devices in them. The second is satisfied by backing up individual elements of the circuit. Braking is done with a stoppage coupling. In principle, a circuit controlled manually is simple. This type of circuit is used to back up the DC engine using a three-position switch which is directly in the armature circuit, in conjunction with automatic circuit shielding. This type of circuit is reliable and flexible in its use to control the drives of one or two electrical engines. For example, for any type of failure of any element (where it is impossible to switch on or off) the drive may be turned on, turned off, or backed up; however, this requires preliminary correct diagnosis of the failure, which can
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