Patent GB-810105-A (GB)
Title: Improvements in and connected with the control of machine tools
IPC: G05B19/33
Filed: 1956-05-08
Abstract: 810,105. Programming for machine-tools; machine-tool details. FARRAND OPTICAL CO. Inc. May 8, 1956 [Sept. 26, 1955], No. 14282/56. Class 83 (3). [Also in Group XXXVIII] A machine-tool includes a relatively movable bed and carriage, drive mechanism is adapted to shift the bed and carriage relative to each other in accordance with indexing values applied as input data to the drive mechanism and means to compensate in the relative positioning for thermal expansion, the means comprising a means to derive from a source of reference electrical signal in said drive mechanism a signal related to (1) the departure in the relative positioning of the elements from a reference relative position; (2) to the coefficient of thermal expansion of the elements; and (3) to the difference between the temperature of the elements and a reference temperature at which the bed and carriage are correctly positioned by the drive mechanism in accordance with the indexing values. As shown in Fig. 1, the carriage 6 is guided on the bed 4 by ways 7 and is moved by a lead screw 8 driven by a motor 32. The input data is supplied from a computer 22 which includes coarse and fine positioning apparatus; the thermal compensation is achieved by the apparatus indicated at 41 and 210. The lead screw 8 drives a helical potentiometer 14 which is supplied from the source 18 and the voltage from the slider of which is proportional to the position of the carriage relative to the bed. The voltage from the slider is applied to the comparison unit 26 via a switching circuit 28 where it is combined with the coarse positioning voltage from the computer 22. The resultant voltage is applied through a servo amplifier 30 to the motor 32 which is driven until the difference between the two voltages is substantially zero. The fine position of the carriage relative to the bed is measured by means of linear transformers as disclosed in Specification 801,516, [Group XXXV]. The output of the modulator 42 is a 60-cycle voltage of either one phase or the opposite phase which is applied through the switching circuit 28 to the servo amplifier 30 to drive the motor 32 in a sense to reduce the induced voltage in the winding 35 to zero at which point the carriage is correctly positioned. As shown in Fig. 1, the thermal compensation apparatus modifies the output voltage of the winding 35 which is applied to the phase detector 40 and so changes the position of the carriage relative to the bed to correct for thermal errors. The apparatus, 210 comprises a variable resistor 212 and a potentiometer 214. The potentiometer is driven from the lead screw 8 so that the position of its tap depends upon the position of the carriage. The resistor 212 and the potentiometer 214 are connected in series and supplied from the oscillator 38 with a reference voltage and an output voltage is taken from the tap 216 of the potentiometer and combined with the voltage from the winding 35 by the apparatus 41 before being passed to the phase detector 40. The resistor 212 is manually varied in accordance with the dif. ference between ambient temperature and the design temperature of the machine tool and workpiece which in this embodiment are the same; and also in accordance with the difference in value between the thermal expansion coefficient of the workpiece and machine tool. The output from the tap 216 of the potentiometer 214 when combined with the voltage from the winding 35 will correctly position the carriage. In a modification, instead of varying the error voltage from the winding 35, the thermal compensation voltage is fed into the circuit which produces the fine position command voltages and modifies these so that the carriage is correctly positioned. In a further modification, Fig. 6, the oscillator 38 supplies two potentiometers 170, 172 which are connected in series and the output voltages from the taps 174, 178 which are in phase opposition are combined and fed to a transformer 184. The tap of the potentiometer 170 is driven from the lead screw as in the embodiment shown in Fig. 1, and the tap of the potentiometer 172 is manually adjustable so as to draw from the potentiometer a voltage proportional to the distance between the origin of measurement and the point of attachment of the workpiece to the machine bed which in the first embodiment were assumed to be the same. The secondary of the transformer 184 forms two adjacent arms of a bridge circuit, the other two arms consisting of an adjustable resistor 190 and a temperature sensitive resistance 192. The temperature sensitive resistance varies in accordance with the temperature of the machine tool and the workpiece and the resistor 190 adjusts the bridge so that its output is zero at the design temperature of the machine tool and workpiece which in this embodiment must be the same. The output of the bridge is taken from the taps of two potentiometers 194, 196, one of which is adjusted in accordance with the thermal expansion coefficient of the machine tool and the other in accordance with the thermal expansion coefficient of the workpiece. The voltages from the two taps are combined together and with the error voltage from the winding 35 of the position measurement transformer and are fed to the phase detector 40 as in the embodiment of Fig. 1. In a modification of this embodiment the transformer has two separate secondary windings each of which form part of a bridge circuit and this modification enables differences in the design temperatures of the machine tool and workpiece to be allowed for in addition to differences in thermal expansion coefficient. The embodiments so far described can correct only for thermal errors within fine limits and if greater errors are to be allowed for the circuits are modified as shown in Fig. 7 in which the thermal compensation voltage is applied through an amplifier 256 to control the rotation of a motor 258. The motor 258 drives the tap of a potentiometer 254, the potentiometer being fed with a voltage from the oscillator 38. The voltage from the tap of the potentiometer is combined with the thermal compensation voltage and when this voltage balances the thermal compensation voltage so that the amplifier 256 has no output the motor stops and the position of the tap represents an angular shaft position which is proportional to the thermal compensation voltage. The motor 258 also drives the rotor of a resolver 262 which has two rotor windings and two stator windings. The two rotor windings are arranged at 90 electrical degrees apart and the two stator windings are similarly arranged at 90 electrical degrees apart and the stator windings are fed with the sine and cosine fine position voltages from the circuit shown in Fig. 4. The angular movement of the rotor relative to the stator modifies the amplitudes of the two voltages so that the resultant voltages correct for the thermal error. To correct the coarse position command signal a potentiometer 272 energized by the source 18 has its tap 274 coupled to the motor 258 to correct the total voltage supplied to the potentiometer 14. Specification 801,517, [Group XXXV], also is referred to.