Patent GB-1449175-A (GB)
Title: A c motor and servo system
IPC: G05B19/416
Filed: 1973-08-31
Abstract: 1449175 Automatic control of AC motors; converting KEARNEY & TRECKER CORP 31 Aug 1973 [1 Sept 1972] 41002/73 Heading G3N [Also in Division H2] In a servosystem employing an A.C. motor, particularly an induction motor to drive a movable member in accordance with a specified programme designating the velocity and distance of movement of the member, digital signals from a programmed numerical control apparatus (digital computer) are used to control the motor speed and torque; the motor driving voltage amplitude and frequency, the position of resultant stator flux, the number of stator field poles and the slip are variously controlled as a function of the instantaneous motor speed and the servo following error. In particular the frequency is selected in dependence upon actual speed and the voltage amplitude in dependence upon a predetermined relationship between motor torque and voltage amplitude. In Fig. 1 a motor 10 receives three-phase squarewave power via power transistors 12 which selectively connect a variable D.C. voltage produced from a 3-phase source 24 by a 3-phase F.W. rectifier incorporating SCR's 18. The power transistors are driven by driver transistors 16 controlled by a frequency generator 20 detailed Fig. 3 (not shown) at a frequency adjustable between 30Hz and 100Hz. The frequency generator 20 output and the firing angle of the SCR's 18 to give the required motor driving voltage amplitude are both periodically controlled by digital signals from the computer. The SCR's may be controlled directly by the computer as is described with reference to Fig. 30 (not shown) or via trigger circuits 28. Likewise the power transistors 12 may be directly controlled by the computer, the frequency generator 20 being dispensed with. An overcurrent protection circuit 38 shuts off the power transistors in the event of excessive motor current. The motor speed and servo following error are sampled during each successive 8À3 millisecs period and the digital control signals are updated each period. In one mode of operation the motor driving frequency is selected to be either 30Hz or 60Hz depending upon the sampled instantaneous motor speed. In a rapid traverse mode a variable frequency up to 90Hz is selected. The voltage amplitude is chosen so as to produce a motor torque at a value necessary to maintain the servo following error at a predetermined value. In other embodiments the digital control signals are calculated such that the motor frequency will give a constant slip, the following error being maintained as above, or alternatively so that the slip is proportional to the torque. Flow Chart of Servosystem (Fig. 10). This system employs the apparatus of Fig. 1. A pulse generator 450 produces pulses as a function of motor speed and which are accumulated in a counter 454. The generator 450 may also include a shaft position encoder. The counter contents are examined periodically and the difference X 2  detected between two successive interrogations (120 per sec.) gives the distance traversed by a slide or other mechanism coupled to the motor shaft. The signal value X 2  is fed to a summer register 458 together with a command signal X 1  from the numerical control. The summer 458 calculates the difference between X 1  and X 2  and adds the difference to a sum previously stored in an integrator 466. The content of 466 thus corresponds to the following error, i.e. the difference between the shaft position called for at any time by the NC program and the actual position indicated by signal X 2 , being the sum of the incremental errors calculated during each 8À3 millisec period. The output X 5  of the integrator is multiplied by a factor # in a scaler 470 to produce a signal X 6  which is representative of the desired/commanded speed of the motor. A summer register 476 calculates the difference between the speed command signal X6 and the actual speed signal X 2  after multiplication in a scaler 476. The output X 7  of summer 474 thus represents the incremental speed error and is fed via a variable gain lead-lag network 485, 486 and via a scaler 482 to a summer register 484. Thus the output X 12  of 484 is such as to increase the system response at low frequencies i.e. when the rate of change of the speed error signal is low. The output X 12  of 484 is fed via scaler 488 together with the actual speed signal X 2  to a summer 492 which produces a difference signal representative of the desired change in motor torque. This difference signal is fed via a scaler 498, limiter 491, function generator 500 and logic amplifier 30 to control the motor drive voltage amplitude. The frequency of the A.C. power is controlled by a unit 502 which sets counters (90), (92) Fig. 3 (not shown) during each cycle, and is chosen in dependence upon the actual speed signal X 2 . When the motor is at low speed, 30Hz is selected and when high 60Hz is selected. A unit 504 detects the sign of the output from scaler 498 and ensures that the motor is energized to rotate in the proper direction. A general purpose digital computer can be arranged to perform the operations of flow chart Fig. 10 and an alternative flow chart Fig. 12 (not shown). Flow charts for other alternative series of programs are given and various computer programs are described with reference to Figs. 14 to 25 (not shown). In addition, a computer program in machine language is given. The motor may alternatively be a synchronous type, a reluctance type, or a hysteresis type. Power Supply Circuits. (Figs. 2, 4). In Fig. 4, transformers 18 are connected to a 3-phase source. Three pairs of SCR's 154, 156 are connected to form a 3-phase FW rectifier to provide a smoothed D.C. voltage between a terminal 40 and earth. The voltage amplitude is varied by varying the firing angle of the SCR's. The SCRs 154, 158 are triggered in alternate half-cycles of the the A.C. supply, by transistors 174, 180 respectively which are controlled by pulses at terminals 186, 188 and derived from firing angle control logic circuits, Figs. 5a, 5b (not shown) or direct from the computer. The logic circuits produce from the 3- phase A.C. supply three pairs of anti-phase squarewaves Fig. 6 (not shown) which are selectively combined in logic gates and modulated with a signal from a 15, 360Hz multi-vibrator. The modulated signal is applied to counter/dividers (272) (274) to produce pulses at frequency of 120Hz for firing the SCR's. In Fig. 2, the variable voltage input from the SCRs is at 40, 42. The circuit operates to provide 3-phase squarewave power to motor 10. The power transistor circuits 12a, 12b, 12c are controlled to pass the respective positive half-cycles and 12d, 12e, 12f the negative half-cycles. Considering the circuit 12a; a squarewave control signal from frequency generator oscillator 20 Fig. 1 is applied to terminal 52 and after passing through a photo-isolator 54 alternatively turns a transistor 56 ON and OFF to control parallel power transistors 68, 70. The other circuits 12a, 12b, 12d to 12f operate similarly. Reverse current diodes 74 are provided. A registor 79 functions as a current detector for overcurrent protection. A diode 72 inhibits base current when the transistors are off.