2-6 View B shows a more in-depth illustration of view A. With the wiper arms of R_{1} and R_{4} at the midpoint of travel, the voltage from the wiper arm to ground is zero volts. Therefore, zero volts would also be measured at the connection point between R_{2} and R_{3 } (the summation point). This means that the error signal is zero. With no input signal, the amplifier output is zero; therefore, the motor shaft remains stationary. Figure 2-3B.—Block diagram of a position servo. For the purpose of explanation, imagine that the R_{1}wiper arm is mechanically moved upward to a new position where a voltage between the wiper arm and ground measures +10 volts. Further measurement shows zero volts between the wiper arm of R_{4}and ground. Since R2and R_{3}are of equal values, +5 volts is measured between the sum point and ground because 5 volts is dropped across each resistor. The +5 volts at the sum point is the "error" signal. As shown in figure 2-4, (view A, view B, and view C),when no error is present, the voltage at the sum point is zero. This is because the network composed of R_{1,}R_{2}, R_{3}, and R_{4}is balanced. When the wiper of R1is moved toward +45 volts, the network becomes unbalanced as shown in view B. The left-hand side of R2becomes positive. This causes current to flow from +45 volts through R_{3}and R_{2}to the +10 volts at the left side of R_{2}. Because R_{2}and R_{3}are of equal value, the voltage drops then will be equal; therefore, the voltage at the sum point will equal +5 volts. Figure 2-4A.—Development of the error signal.

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