Improvements in or relating to servo-systems

900,550. Measuring systems. ELLIOTT BROS. (LONDON) Ltd. Aug. 8, 1960 [Aug. 6, 1959], No. 26964/59. Class 40 (1). [Also in Group XXXVI] A system for giving an output proportional to the square root of an input comprises two linear position transducers mechanically coupled to a diaplaceable member and electrically connected in series, means for supplying a reference signal to the first transducer, a comparator unit supplied with the output of the second transducer and with a signal representing a physical quantity to give an error signal representing the difference between its inputs, and means to displace the member in accordance with the magnitude of the error signal. In Fig. 1 the rate of flow of fluid in a pipe 8 is determined by measuring the pressure drop across an orifice in a device 10, which moves the core of a differential transformer 11 by an amount proportional to the pressure drop and hence to the square of the rate of flow. The output of the transformer 11 is passed to a comparator 7 (whose function is described below) and thence to a high-gain feedback amplifier 6 which incorporates a phasesensitive rectifier. The amplified signal drives a motor 5 to rotate a shaft to which are coupled the cores of two further differential transformers 1 and 2. The first of these is supplied from the same source as transformer 11 and gives an output proportional to #, the angular displacement of the shaft 4. This output is amplified and passed to the second transformer 2, whose output (proportional to #2) is fed back to the comparator 7. The signal from the conparator is the difference of its inputs, so that the shaft is set in a position where #2 is equal to the pressure signal from transformer 11 and the comparator output is zero. The value of #, which thus represents the rate of flow in the pipe 8, can be shown by a pointer on the shaft 4 or as an electrical signal taken from the input to transformer 2. The first transformer 1 is arranged to give zero output for zero angular displacement of the shaft, but the core of transformer 2 is preferably offset from this position so that a small " negative " displacement of the shaft is necessary to reduce its output to zero. This prevents a fall of sensitivity as the (positive) displacement becomes small, and also avoids instability if the shaft should move into a negative displacement position. The response of the system is linear except for low values of input, and it is often convenient to arrange that the extrap