Smiths Industries Plc

Keith R Oakey

John W Jones

A counter-pointer display of aircraft altitude is driven by a servo in accordance with a digital representation of height provided at regular intervals by an air-data computer. The content of a first register is updated regularly with the height representation, and the difference between this and the content of a second register is entered into a third register. The content of the second register is updated rapidly from a shaft encoder that is coupled to be driven together with the counter-pointer display by a stepping motor, and the content of the third register is updated in synchronism with the updating of the first register to staticize the error between the input height-representation and the output representation provided by the counter-pointer display. The stepping motor is supplied with pulses at a rate dependent on the magnitude of the staticized error value via a pulse-rate divider, the pulse-rate divider being supplied with a regular pulse train via a gate device that is operable to block passage of pulses of the pulse train to the divider. The proportion of pulses passed by the gate device and the magnitude of the voltage energization of the motor are both regulated in accordance with the magnitude of error value staticized by the third register.

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Cheltenham, United Kingdom

Keith R Oakey