GB900550A - Improvements in or relating to servo-systems - Google Patents

Abstract

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 #<SP>2</SP>) 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 #<SP>2</SP> 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 extrapolated linear part should pass through the origin. This can be done by angularly displacing the pointer on the shaft 4 and introducing a corresponding bias into the electrical output signal.