Patent GB-1459179-A (GB)
Title: Drive circuits for stepping motors
IPC: H02P8/22
Filed: 1972-12-06
Abstract: 1459179 Driving stepping motors WICKMAN MACHINE TOOL SALES Ltd 15 Nov 1973 [6 Dec 1972] 56204/72 Heading H2J A drive circuit operating a stepping motor having windings 16-19 comprises means for varying the current flow in each winding in cumulative unequal steps between zero and a maximum value, and means permitting selection of the magnitude of each incremental step independently of the other incremental steps so as to enable the incremental steps to be set independently to match the non-linear characteristic of the motor to obtain equal incremental positional steps of the motor. The apparatus comprises a counter 11 receiving command pulses and a decoder 10 supplying 4 D/A converters 24-27 with binary signals over 5 lines indicative of incremental step current values. Thus 32 different step values are possible and are selected by the decoder 10 in accordance with the known characteristics of the motor employed, 8 incremental steps being used in the transition from zero to maximum motor current and vice-versa. The control is such that two windings are energized at any one time, with one winding current remaining at its maximum value whilst that in a second winding is progressively stepped down to zero and that in a third winding is subsequently stepped up to its maximum value, Fig. 2 (not shown). The output of each converter 24-27 is applied to one input of respective differential amplifiers 28-31 to supply signals to current controllers 12-15. Feedback connections to the inverting input of each differential amplifier are provided so that each amplifier acts as a comparator of desired and actual motor currents and the outputs thereof move to reduce the difference. If the stepping rate of the motor is greater than a predetermined level, the back emf. from the coils causes amplifiers 28-31 to close switches 20-23 to connect an alternative and higher voltage supply on line 37 across the coils, diodes 32-35 being back-biased to disconnect the normal supply line 36. In a first arrangement, Fig. 3 (not shown), of two of the current controllers 12-15 for windings 16, 17, each controller comprises transistors (46, 47; 46a, 47a) whose conduction is varied under the control of amplifiers 28, 29 to maintain the winding current at a desired value. When the stepping rate exceeds the predetermined level, increase in winding current is opposed by the winding inductance to an extent that the output of amplifier 28 or 29 rises to break down a Zener diode (55) and connect the line 37 to the motor through a transistor switching circuit (56, 58, 61, 62, 63). Fig. 4 shows an embodiment which omits the supply line 37 and uses a single higher voltage supply 73, 74, by virtue of the fact that the windings 16, 18 are connected as shown in a transistor bridge whose transistors 84, 84a, 88, 88a are switched on and off to maintain the winding current at a desired value. Schmitt trigger circuits 78, 78a are respectively controlled by operational amplifiers 71, 71a and a logic circuit 72 whereby, according to the required direction of current through windings 16, 18, the transistors are switched on and off in pairs 84, 88a and 84a, 88, and one transistor (88 or 88a) of an "on" pair is maintained on whilst the other (84 or 84a) is cycled on and off to maintain the desired current value, which is increased in steps with corresponding increase of the output from amplifier 71 or 71a. Diodes 94, 94a maintain the current through the winding 16, 18 and the "on" transistor during the off periods of the "on/off" transistor.