Patent GB-1194784-A (GB)
Title: Improvements relating to Apparatus for Controlling the Position of an Object in Dependence on a Train of Command Pulses
Inventor: WALKER DONALD FERGUSON
IPC: G01D5/36
Filed: 1968-03-02
Abstract: 1,194,784. Programmed control of machine tools. FERRANTI Ltd. Feb. 26, 1969 [March 2, 1968], No. 10253/68. Heading G3N. An apparatus for controlling the position of an object with respect to a reference structure in dependence on a train of command pulses representing increments of movement of constant length comprises digital/analogue conversion means for deriving from the train a cyclic command signal of at least three phases, each having a wavelength defined by N successive command pulses of like sense and a phase angle representing the desired position, an optical fringe counter measuring means for deriving cyclic measurement and reference signals of like frequency, which is high compared with a frequency of the command signal, so that the phase difference between measurement and reference signals represents the actual position, gating means for repetitively applying measurement or one of the reference signals to gate the command signal to derive from it a composite signal at the high frequency and of phase dependent on the phase of the command signal at that moment, a phase comparator comparing the composite signal with the other signal to derive therefrom an error signal of appropriate sense, and an actuator adjusting the position of the object to reduce the error signal. A command train of positive or negative pulses from a magnetic tape or computer is fed to digital/ analogue means 12 which includes a binary counter 13 having stages S0-S4 connected to a decoder 14 which derives a four phase signal comprising triangular waves of stepped form, each phase being applied through gates TG1-4 the outputs from which are commoned at 15 and are fed together with signals from a photo-cell 32 through filters 16, 36 squaring stages 17, 37 and edge detectors 18, 38 in opposite senses to a pulse phase comparator 21 which through a digital/analogue converter 41 controls an actuator 42 such as a motor coupled to a movable part of a machine. Secured to fixed and movable parts of the machine are optical fringe counter measuring means in the form of reference gratings 23, 25 cooperating with reference and main photo-cells 31, 32 with a common grating rotating at a speed to give a frequency high compared with the four phase signals. The signal from cell 31 passes through a phase shift amplifier 33 and squaring amplifier 34 to a phase splitting stage 35, which derives of the same frequency as the input a balanced four phase square wave output which is applied through AND gates AG1-4 to gates TG1-4. Any angular difference between the pulse trains received by comparator 21 represents the difference between actual and command position of the object e.g. a work-table, such difference being in the form of an output pulse each time one train inserts two pulses between two of the other train, hence the motor 42 is driven appropriately to nullify the error. The triangular waveforms are derived in a series of steps, smoothed to provide a more linear form. Circuits are described for producing the waveforms which may be symmetrical about a zero line or may lie wholly on one side of it. The functions of photo-cells 31, 32 may be interchanged.