logic 60 to the digital to analog converter 62 for conversion to an analog form suitable for use by the servo motors the converter 62 outputs on line 64 a dc analog voltage representing the required bight position input this line connects in the automatic mode position of a switch 67 to the summing point 69 of a low level preamplifier 71 forming the first stage of a servo amplifier system the preamplifier 71 drives a power amplifier 73 which supplies direct current of reversible polarity to the electromechanical actuater 75 which in the broadest sense comprises a reversible motor to position the actuater in accordance with the input and voltage on line 64 a feedback position sensor 77 mechanically connected to the reversible motor 75 provides a feedback positional signal on line 79 indicative of the existing output position the input analog voltage and the feedback are algebraically summed at the summing point 69 to supply an error signal on line 81 the feedback signal from the position sensor is also differentiated with respect to time in a differentiator 83 and the resulting rate signal is presented on line 85 to the summing point 87 of the power amplifier 73 to modify the positional signal at that point a position sensor 77 may be any device that generates an analog voltage proportional to position and may in this embodiment be a simple linear potentiometer connected to a stable reference voltage and functioning as a voltage divider the differentiator 83 is preferrably an operational amplifier connected to produce an output signal equal to the time rate of change of the input voltage as is well known in this art the explanation of.