Patent GB-1416347-A (GB)
Title: Linear motors
IPC: H02P3/18
Filed: 1972-12-20
Abstract: 1416347 Looms NATIONAL RESEARCH DEVELOPMENT CORP 13 March 1973 [16 March 1972 20 Dec 1972] 12296/72 and 58693/72 Heading D1E [Also in Division H2] A linear electric motor comprises a linear motor primary member, a linear motor secondary member, switching means responsive to a control signal to disconnect energising current to the primary member and control means whereby the control signal is produced when the speed of the secondary member in the proximity of the primary member reaches a predetermined value, the control means comprising sensing means responsive to the speed of the secondary member to produce a signal the frequency of which is dependent upon the speed and comparator means responsive to the frequency signal to produce the control signal when the frequency of the frequency signal reaches a value corresponding to the predetermined value of speed. Figure 1 shows the application of the invention to brake the shuttle of a loom, the shuttle 10 forming the secondary of a linear induction motor whose primaries 12, 14 are supplied by an inverter (not shown) operating when a control signal is applied to the terminals 36, 38 of Fig. 2. Photo-transistors 20, 22 are respectively responsive to strips 16, 18 on the shuttle 10 so that as the shuttle enters the primaries 12, 14 the transistor 20 conducts to energise a relay B whose contact B2 closes to cause energisation of the primaries 12, 14, after which the alternating light and dark portions on the strip 18 produce switching of the transistor 22 at a frequency corresponding to the speed of the shuttle. When the frequency has fallen to a value corresponding to the tuned frequency of a resonant circuit 48, 50, set equal to the predetermined speed, transistors 46, 52 are operated to cut off the transistors 42, 44 of a multivibrator 34 and so de-energise a relay A whose contacts Al, A2 open to de-energise the primaries 12, 14 and thus terminate their braking effect on the shuttle 10. The latter is then stoped by a mechanical brake 28 or by the naturally-occurring friction with the sley board. Mal-operation due to dirt on the strip 18 is prevented by use of a light source illuminating and a photo cell responsive to three adjacent regions of the strip spaced apart by the pitch of the light and dark areas thereon, Fig. 3 (not shown). Alternatively magnetic sensing arrangements may be used with magnetic inserts on the shuttle co-operating with wound poles and permanent magnets, Fig. 4 (not shown) or an E-shaped core having wound poles to the centre of which is supplied a pilot tone, Fig. 5 (not shown). Figures 1 and 2 may be modified to project the shuttle 10 from the primaries 12, 14 during a picking operation of the loom, memibers 20, 28 being omitted and the transistor 22 causing the primaries 12, 14 to be switched off when the shuttle has accelerated to a predetermined speed. Further modification allows the apparatus to be used for both picking and braking. The resonant circuit may be replaced by a monostable multivibrator whose relaxation time is made equal to the pulse length of pulses derived from the transistor 22 when the light incident thereon has the required frequency. Figure 6 shows such an embodiment wherein the strip 16 is omitted and both transistors 20, 22 are responsive to the strip 18, their parallel outputs being supplied at terminal 90 to leading and trailing edge differentiators 92, 94. Operation of the circuit is described for picking, which involves the use of the members 92, 94, 96, 98, 100, 104, and for braking, which involves the use of members 92, 94, 110, 112, 114, 116, 118, 120, 122, 124, the loom timer providing a "pick enable" signal to terminal 106 and a "brake enable" signal to terminal 108. The use of the monostable multivibrator ensures that the primaries 12, 14 are turned off when the shuttle is travelling at any speed above (for picking) or below (for braking) the predetermined value. The delay device 122 prevents mal-operation at braking speeds greatly in excess of the predetermined value. The voltage of the inverter may be altered according to whether a picking or braking operation is to be performed, for example by use of the separate outputs of AND gates 104, 118 or of a signal from the loom timer. The circuit of Figure 6 may be modified to provide a control circuit, Fig. 7 (not shown) for an arrangement in which a separate inverter and control circuits are provided for each side of the loom, Fig. 8 (not shown), typical picking and braking operations being described for this modification. To prevent mal-operation due to parameters dependent on the light source intensity, ambient lighting and the distance of the photocells from the strip 18, the signal from the cells may be connected to terminal 90 via a divide-by-two circuit, so that the frequency of the signal at the terminal depends on the repetition rate of the reflective areas of the strip rather than on their width. The pitch of the reflective areas or the periods of multivibrators 96, 120 is/are correspondingly altered.