Dual-output bridge for bidirectional control

A circuit of controlling two oppositely directed proportional effectors by a five-arm bridge with two output signals, plus means for converting these signals into amplified power for driving the effectors, one signal per effector, said circuit comprising in detail:

• 1. a D.C. bridge which has two input terminals, as in a Wheatstone bridge, and means to power same, but which has three instead of two output terminals, two of the said three output terminals being formed by the addition of a fifth arm to a normal Wheatstone bridge, said fifth arm lying between two of the series-connected arms which normally compose one side of a Wheatstone bridge, and said fifth arm being a variable resistor whose resistance is substantially smaller than the four remaining normal arms of the bridge, the two output terminals being thus formed by the said fifth arm generating two separate output signals, the return path for both signals being the third of said three output terminals and lying on the side of the bridge opposite to the fifth arm, the voltages of the two signals differing slightly from each other but both varying together in response to a resistive sensor which composes one of the four normal arms of the bridge, the resistance of said sensor varying negatively (inversely) with the controllable parameter which it senses, the said two output signals of the bridge defining a deadband (passaband), each boundary of which corresponds to a zero voltage of one of the said two signals;
• 2. two means in cascade for amplifying the said two signals, the outputs being D.C. voltages which are proportional to the input signals, at least in the region beyond the boundaries of the deadband, the inputs of said means being connected to said two output signals of the bridge on a one-to-one basis, the said cascades starting with preamplifiers and ending with power amplifiers able to drive the said effectors from zero to full-scale power, said effectors being identified in (3), which follows;
• 3. two oppositely directed, proportional effectors, one of which operates on the aforesaid parameter to decrease that parameter, and the other of which operates on the same parameter to increase that parameter, either resultant change in the said parameter affecting, in turn, the response of the aforesaid sensor to the parameter, the inputs of said effectors being connected on a one-for-one basis to the outputs of said amplifying means in cascade, the degree of operation of either effector on the said parameter being proportional to the amplified signal which drives it, all connections from the bridge to the effectors being of such polarity and in such a manner that when the parameter, while drifting under external influence, crosses either aforesaid boundary of the deadband, that effector is selected which tends to drive the parameter back into the deadband, the degree operation of the effector being between 0 and 50 percent of full scale at said boundary;
• the width of said deadband being proportional to the resistance of said fifth arm of the bridge.