A capacitive transducer for the indication and control of angular position

Abstract

In the field of engineering one meets various instances where a system may involve the indication of angular position with reference to some point or axis. An obvious example is the indispensable compass in navigation which gives in angular form the position with respect to the earth’s magnetic axis, of a ship or aircraft. Moreover, in many practical applications the indication of an angular position is not of much fundamental importance, as that of a system which has at its output an angular shaft torque with the minimum of deviation from the true position as small as possible. This pre-requisite is very often met in mechanical engineering in the control of shaft angular position, (whose rotation may involve the motion of a load to some desired point) by means of a closed-loop system. But, in large systems, the two devices are required at the same time; namely, a transducer to give the indication of an angular position of an object (say for example a target) and another transducer (servomechanism) to control an output angular position to be exactly similar to that at its input. For example, the operator of a gun on board a warship, who is in the control room far away from the gun itself, receives through a radar display the position of a target. The radar display in this case is the indication of the angular position. This angular position information is then fed to an electro-mechanical equipment which tries to move the gun in the specified direction of the target. But by the time the information reaches the point of application (namely the gun) it suffers some distortion which renders the information inadequate. This situation is remedied by the use of an automatic control system which renders the information at the sending and receiving ends to be similar. In this project two attempts have been tried to sense an angular position. Both of them involve a capacitive transducer whose capacitance is a function of angular position. This capacitive electrical quantity is then used to generate periodic waveforms whose frequencies are varied by the capacitive transducers. In the first attempt it has been tried to indicate an angular position, whilst in the other, it was attempted to control an angular position. In the first attempt, the periodic waveforms are detected by means of a linear detector which translates the input sinusoidal signal into a d.c. voltage. This is then calibrated into an angular position proportional to it. In the second attempt, the control of an angular position at another distance away from the input, is accomplished by means of a position servomechanism. The input position, which may be operated manually or automatically by another interconnected system coupled to the transducer, serves as a reference for comparison in an error-measuring device when the output position is fed back to it. The error, consisting of an angular difference between the input and output shafts, actuates the servomechanism to operate in such a way as to cancel the error (bringing the output angular position in alignment with the input). The greatest advantage of this system is that the capacitive transducer is not affected by the function of its capacitance with angular position, no mater how complicated this may be.