1,082,722. Programmed control of machine tools. MOLINS MACHINE CO. Ltd. and D.T.N. WILLIAMSON. Feb. 18, 1964 [Feb. 21, 1963], No. 7034/63. Heading G3N. [Also in Divisions G4 and H2] Apparatus for producing mechanical movements under the control of a plurality of electric signals comprises a separate stepping motor, of the type which produces equal angular movements of a shaft in response to successive electric pulses, arranged to respond to each of the signals and a differential gearing assembly connected to receive mechanical movements generated by the stepping motors and to move an output member in accordance with the algebraic sum of these movements. In an embodiment for the control of a machine tool the electric signals are derived from tracks on a magnetic tape comprising three data tracks having a variable pulse repetition frequency and a constant rate reference track. The difference between the number of pulses recorded in a data track and the number of pulses recorded in the reference track represent in magnitude and sense a movement the tool is required to make along the corresponding movement axis while the selected length of tape is passing the reading heads. It is stated that it is preferred to provide an arrangement for checking that the stepping motors respond correctly to the signals. The checking arrangement comprises a photo-electric cell responsive to a sectored disc driven by a stepping motor and the derived pulses are used to operate a counter in opposite sense to an input from the pulses to the stepping motors. A predetermined count causes an alarm or control operation to be initiated. Alternatively, a checking signal may be computed and recorded in a further track on the tape. Alternatively, or additionally, for signals that are not subject to sudden change, the motor input signals are fed as trigger signals to a flywheel oscillator the output pulses of which is compared in a counter with the motor input pulses to detect missing stepping pulses. It is stated that a number of interconnected differential gearings may be employed and that the torsional vibrations may be absorbed by the driven shaft or by inclusion of a vibration damping coupling.
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