| 摘要 |
1496137 Seaming non-metallic sheet material KERRY ULTRASONICS Ltd 25 June 1975 [28 June 1974] 28729/74 Heading B5K In a method of welding plastics workpieces together by the application of ultrasonic vibrational energy by an ultrasonic transducer having a working face 21 which engages one of the workpieces, the amplitude of vibration of the working face 21 is monitored by detector means 22 providing an output signal proportional to this amplitude, the output signal is compared with an experimentally determined reference signal and the application of ultrasonic energy to the workpieces is interrupted when the difference between the output signal and the reference signal attains a pre-selected datum value. As shown in Fig. 1, the ultrasonic transducer comprises a pair of piezoelectric elements 10, 12 sandwiched between metallic bodies 14, 16 and having an operating alternating voltage applied thereto at terminals 13. A tapered horn 18 is secured to the body 16 and has a working face 21. The detector means 22 comprises a strain-sensitive pick-up, e.g. a secondary piezoelectric transducer, cemented to the horn 18 near the point of maximum stress. The output voltage V 1 generated by the transducer 22 when the apparatus is in use is available at terminals 19. A voltage comparator, Fig. 2 (not shown), compares the voltage V 1 with a preset reference voltage V 0 and the output (V 0 -V 1 ) is used to switch a transistor in the circuit of a relay having a pair of contacts connected between the terminals 13 and the power supply therefor. When (V 0 -V 1 ) becomes positive the transistor is rendered non-conducting so as to cause normally open contacts of the relay to open to cut off the power supply to the transducer. A time delay circuit prevents the relay dropping out inadvertently as a result of spurious brief signals. In modifications (not shown), the detector means 22 comprises an accelerometer or a displacement monitor or the ultrasonic transducer itself is included in a motional circuit attached to its terminals and capable of providing an output signal proportional to the transducer motion. The transducer is driven under either constant-current or constant-voltage conditions. It is necessary to calibrate the apparatus to establish the value of the reference signal applicable to any particular welding operation. Preferably, the apparatus is arranged so that power is not supplied to the ultrasonic transducer until the working face 21 is in pressure engagement with the workpieces. |
