SPEED CONTROL UNIT FOR DIRECT CURRENT ELECTRIC MOTOR WITH INTRINSIC FEED-BACK

Abstract: It is foreseen to measure the rotational speed (v) of a motor by measuring the electromotive force (e.m.f.), calculated at any moment as the difference between the feeding voltage and the voltage drop across the armature resistance (Ri), or as the value of the voltage in the instants when the current reaches the zero value. In the first embodiment, an element that calculates the value of armature resistance (Ri) at consequent instants (t1, t2) is foreseen and which then calculates the values of the electromotive force (e.m.f.) from the equation e.m.f. = Va - Ri x I. In the second and third embodiments, the value of the electromotive force is found directly by measuring the voltage (Va) during the instants in which the current (I) reaches the zero value, through a current meter followed by a current sampler and a memory. From the value of the electromotive force (e.m.f.) the rotational speed of the motor is obtained by means of the equation v = e.m.f./k, where k is a constant characteristic of the motor.

Description: ■ SPEED COSTROL ON T FOR DIRECT CORREHT ELECTRIC MOTOR WIT IHTRINSIC FEED-BACK" The present invention relates to controlling devices for direct current electric motors. There are some known feeding devices for direct current motors that comprise means for measuring the rotational speed of the motor, such as dynamo-speedometers or 5. encoders mechanically connected to the rotating organ whose speed of rotation one wants to measure. They are connected to a voltmeter indicator or to a recorder, on which the rotational speed is read; the said measuring means can furthermore be connected to one or more relays so as to obtain the stabilisation of the rotational 10* speed of the motor, according to prefixed values. These measuring means, however entail some disadvantages such as the presence of additional moments of inertia on the driving shaft, with consequent decrease in the maximum angular accelerations, increased weight and overall size of the motor, greater construction and maintenance costs of the motor. Aim of the present invention is that of obtaining a feeder for direct current motors that eliminates the said disadvantages, i.e. 5. enabling the measurement of the rotational speed and its stabilisation without dynamo-speedometers or encoders. This aim is attained by foreseeing a feeder whose electric circuit, constituted by a control unit and a supply and comprising, in some cases, electric switches, is equipped with a system for measuring 10. angular velocity (v) of the motor by using the measurement of the electromotive force, since v = e.m.f./k, where k is a constant for the motor. The measured value of the e.m.f. can be compared with a reference e.m.f. in a memory and it is used for displaying the speed of rotation (v) of the motor on an appropriate display. 15. According to a first embodiment of the present invention, the electromotive force (e.m.f.) is obtained from the equation: e.m.f. = Va - Ri x I where Va is the voltage measured at the terminals, Ri is the armature resistance of the motor and I is the absorbed current. 20. Since armature resistance Ri can vary with the temperature, it is preferably calculated by using the following equations: e.m.f. = Va - Ri x I t1 t1 e.m.f. = Va - Ri x I t2 t2 where tl and t2 are two instants of measurement, close to each 25. other. In the time interval (t1 - t2) both the armature resistance (Ri) and the electromotive force (e.m.f.) are assumed constant; the absorbed current I, instead, varies due to couple variations and to rotor commutation. It is now possible to obtain Ri and then the e.m.f. from the equation e.m.f. = Va - Ri x I. A closed loop regulation of the motor is then carried out in the known way. Φ The circuit is equipped for this purpose with a voltage amplifier, with an element for carrying out the calculation of armature resistance Ri in a short time interval (t1 - t2) , with an element carrying out the product between Ri and I and the consequent difference Va - Ri x I, with an element for comparing the electro- 1 0< motive force with the obtained force, and with an error amplifier that amplifies the difference between the two electromotive forces, and acts on the motor controls. According to a second embodiment of the present invention, the motor is fed with a pulse width modulation (p.w.m.) feeder, then 15 < voltage (Va) is measured in the instants when current (I) reaches the zero value by the effect of the pulse width modulation (p.w.m) ; the electromotive force (e.m.f.) is thus directly obtained, in this case, e.m.f. being equal to Va. To obtain the value of the speed of rotation of the motor, according to this embodiment, the circuit is 2o. equipped with an element that identifies the instants when the current is zero, and with a circuit for holding and reconstructing the e.m.f. According to a third embodiment of the present invention, a variable feeder is inserted in the circuit, producing a cyclically 25. variable voltage term, which is superimposed to the direct voltage term feeding the motor. The amplitude of this voltage cycle must be such as to include the actual value of the e.m.f. of the motor. Consequently, the absorbed current I reaches the zero value almost twice in every cycle. The e.m.f. measurement is then carried out in the instants in which the current reaches the zero value, with no 5. need for disconnecting the motor feeding circuit. By inserting a current intensity meter and a comparator into the feeding circuit, the instants in which the current reaches the zero value are easily detected and the exact value of the e.m.f., equal to the value of the voltage delivered by the feeder in the same 10. instants, can be known. The e.m.f. value, obtained as above said, can be used for the same purposes of speed control shown in the previous embodiments. The advantages of the last embodiment are mainly due to the lack of discontinuity in the motor feeding voltage and they include: 15. - almost complete cancellation of radio-electric interferences produced by the feeding circuit, due to the lack of steep edges in motor absorbed current; - better exploitation of motor windings, due to the fact that the time intervals during which the current does not flow through 20. them can be reduced as desired, independently from the motor speed; this advantage is essential to those motors that must develop the maximum torque at low speeds; - capability of the power line to recover the motor braking energy, due to the uninterrupted connection between the motor and the 25. feeding system; - the intrinsic characteristics of the cyclically variable voltage feeder easily allow the generation of bipolar voltages, thus rendering advantageous the use of this feeder for bidirectional reversible motors; - capability to set the amplitude of the cyclically variable 5. voltage term as low as desired, so limiting the instantaneous angular acceleration to values consistent with the motor application. According to a further improvement of this last embodiment, only the above detailed e.m.f. and speed measurement and visualisation 10. apparatus is realized and it is applied to a direct current motor having its own feeder and its own speed control system. It is however necessary that the feeding voltage is variable (e.g., generated by a rectifying bridge) and that the e.m.f. value is comprised within the voltage variation amplitude, so that the 15. absorbed current (I) reaches, in some instants, the zero value. The processes followed to operate the above detailed embodiments of the present invention are descripted in Claims 10, 11, 12 and 13. Preferred exemplary embodiments of the present invention are illustrated in the appended drawings, in which: 20. Fig. 1 shows an electric circuit according to a first embodiment; Fig. 2 shows an electric circuit according to a second embodiment; Fig. 3 is an embodiment of the electric circuit; Fig. 4 shows an electric circuit according to a third embodiment; Fig. 5 shows the speed measurement and visualisation apparatus, 25. already described in Fig. 4. Fig. 1 shows motor 4, having armature resistance Ri connected to supply 2 and to control unit 3 by means of switch 1. Within this circuit another circuit is paralleled; this comprises a voltage amplifier 8, an element 12 for the calculation of armature 5. resistance Ri, a multiplier 7 that carries out the product Ri x I and a memory 9. The latter stores the value of the reference e.m.f. that is compared with the value of the e.m.f. found by calculating the difference between the voltage Va and the armature voltage drop. The - difference between the two electromotive forces is 10. amplified by amplifier 10 that acts on control unit 3. The value I of current is taken at the terminals of shunt S, then it is amplified in amplifier 11 and sent to multiplier 7. Fig. 2 shows the circuit comprising motor 4, supply 2 and feed control unit 3, connected to the circuit relative to a further 15. embodiment * In fact, a synchronous current sampler 6 is foreseen which defines the instants when the current reaches the zero value. Said sampler acts on switch 15 that enables memory 13 to store the value of the electromotive force (e.m.f.) in the instants when the current reaches the zero value. In this case the equation is: Va = 20. e.f.m.. The signal given by this e.m.f. is used to display the speed of rotation v that is proportional to the e.m.f.. This is also compared to a reference e.m.f. in memory 9. The difference between the two values is amplified by amplifier 10 that acts on control unit 3. 25. Memory 13 can in particular be an integrator. Fig. 3 shows an exemplary embodiment. Motor unit 4 is connected to switches a, b, £, d, operated by feed switch control unit 20. Switches a and b are controlled according to the error voltage amplified by amplifier 10 and they carry out a pulse width modulation function (p.w.m.), whilst switches c and d are 5. controlled according to the desired direction of rotation of the motor. The added circuit comprises a synchronous, differential current sampler 6 that defines the instants in which the current reaches the zero value, pointed out by means of an appropriate output signal generated inside control unit 20; sampler 6 is 10. connected to memory 13 that reconstructs the value of voltage Va, thus, the desired e.m.f. is obtained directly since the e.m.f. is equal to Va. This is used for regulating the motor, since it is compared with a reference e.m.f. in memory 9, the difference between the two e.m.f. being amplified by amplifier 10 connected to 15. switch control unit 20. The rotational speed of the motor appears on a display 16 operated by the existing e.m.f. from memory 13. Preferably, a current limitator 13 that acts on amplifier 10 is foreseen. Fig. 4 shows a circuit comprising motor 4, having armature 20. resistance Ri, a shunt S measuring the current I, a comparator 37 which detects the instants in which the current reaches the zero value and, through the logic unit 36, enables the memory 34 to store the e.m.f. values. These values, in those instants, are equal to the voltage Va. Memory 34 can in particular be an integrator. 25. An examplary embodiment of the logic unit 36 is shown in Fig. 5. It comprises: - three pulse shapes (40) - two multiplexers (41) - one squarer (42) - one missing pulses detector (43) which are connected as shown in the figure. 5. Referring again to Fig. 4, it shows also a display 16 at the outlet of memory 34 showing the rotational speed of the motor. As in previously described embodiments, the e.m.f. value stored in memory 34 is compared with the reference value carried in memory 9 in adder 38. The error, aplified by amplifier 10, is used to drive 10. the feed control unit 39. Feeding block 39 comprises the cyclically variable oscillator 31, connected to bistable memory 32, and then to comparators 33 and to memory storing the Delta V reference value 35. Feeding block 39, driven, as already said, by amplifier 10, maintains the oscillator 15. 31 output voltage centered upon the motor e.m.f. value and reduces its oscillation amplitude within the predetermined range. If the current (I) does not reach the zero value during one or more cycles, due to a too rapid current transient, logic unit 36 generates sampling signals, corresponding to those instants in 20. which the output voltage of the oscillator (31) is at its highest or lowest value, according to the current (I) sign detected by the comparator (37). Said sampling signals enable the memory (34) to store the highest voltage output value, when the current (I) is negative, and the lowest voltage output value, when the current (I) 25. is positive. Fig. 5 shows the motor speed measurement and displaying apparatus. The motor is considered to be fed with a variable voltage by feeder A. The identifiying numbers of the various blocks an related functions are the same already detailed in Fig. 4.

Claims: C L I M S 1. Speed control unit for a direct current motor, fed by a direct voltage* power source, characterized in that the motor control circuit comprises a rotational speed (v) measuring equipment, capable of measuring the electromotive force (e.m.f.) 5. and of obtaining the rotational speed by means of the well known relationship: v = e.m.f./k where k is a known constant, characteristic of the motor; a memory (9) storing a reference e.m.f. value; an adder for comparing this 10. reference e.m.f. value with the calculated value; an amplifier (10) for amplifying the difference between the two values, which is used to operate a feed control unit (3; 20; 39) . 2. Speed control unit according to claim 1, characterized in that the rotational speed measuring equipment comprises a current 15. meter (S) followed by a synchronous sampler (6) or a comparator (37) , identifying the instants in which current (I) reaches the zero value; a second memory (13; 34) storing the values of the voltage at the terminals (Va) in those instants, in this case the e.m.f. being equal to Va. 20. 3. Speed control unit according to claim 2, characterized in that at the outlet of the memory storing the e.m.f. value (13; 34) shows the rotational speed (v) . 4. Speed control unit according to claim 2, characterized i that the memory storing the e.m.f. value (13; 34) is an integrator. 5. 5. Speed control unit according to claim 1 , characterized i that the rotational speed measuring equipment comprises a logi unit composed by a multiplier (7) and an adder, measuring the e.m.f. value by carrying out the difference between the voltage at the terminals (Va) and armature voltage drop (Ri x I) , where Ri is 10. motor armature resistance. 6 Speed control unit according to claim 5, characterized in that the rotational speed measuring equipment comprises a logic unit (12) composed by analogic multipliers and adders and a memory carrying out the calculation of the corrected value of the armature 15. resistance (Ri) at two instants (t1, t2) , identified by a clock. 7. Speed control unit according to claim 2, characterized in that the motor feed control unit (39) comprises a cyclically variable feeder, generating a voltage cycle which is superimposed to the direct voltage produced by motor power source, whose 20. amplitude is chosen so that the resulting variable voltage comprises the actual value of the e.m.f. 8. Speed control unit according to claim 7, characterized in that the cyclically variable feeder is an oscillator (31). 9. Speed control unit according to claim 8, characterized in that the motor feed control unit (39) comprises also comparators (33) a bistable memory (32) and a memory (35) storing the reference Delta V value, keeping the outlet voltage of the oscillator (31) 5. centered on the e.m.f. value stored in the memory (34) , and within a preset oscillation range. 10. Speed control process for direct current motor fed by a direct voltage power source, characterized in that the motor rotational speed (v) is obtained by measuring the electromotive 10. force (e.m.f.), obtained carrying out the difference between the terminal voltage (Va) and the armature voltage drop (Ri x I) ; where Ri is calculated at two successive instants, said e.m.f. value being then compared with a reference value and the difference between them being used to operate the motor feed control unit. 15. 11. Speed control process according to claim 10, characterized by feeding the motor with a pulse width modulation (p.w.m.) system, in which the e.m.f. is measured in those instants in which the current (I) reaches the zero value, by measuring the corresponding values of the voltage at the terminals (Va) , by means of the 20. equation: e.m.f. = Va 12. Speed control process according to claim 10, characterized by superimposing to the direct voltage feeding the motor a cyclically variable voltage term, with the resulting cyclically 25. variable voltage being comprised within a range including the e.m.f. value, in which the e.m.f. is measured in those instants (at least two i -every cycle) in which the current (I) reaches the zero value, by measuring the corresponding values of the voltage at the terminals (Va) , by means of the equation: e.m.f. = Va 5. 13. Speed control process according to claim 12, characterized in that, if the current (I) does not reach the zero value during one or more cycles, a logic unit (36) generates sampling signals directed io -the -memory (34) which was previously storing the expected e.m.f. value, said signals corresponding to those instants 10. in which the output voltage of the oscillator (31) is at its highest or lowest value, so that, according to current sign detected by the comparator (37) , the memory (34) stores the highest output voltage when the current (I) sign is negative and the lowest output voltage when the current (I) sign is positive.

Inventors: VENTURINO GIANFRANCO, VENTURINO, GIANFRANCO
Applicants: VENTURINO GIANFRANCO GSE, GSE DI VENTURINO GIANFRANCO
Application year: 1987
Main classification: H02P5
IPC codes: H02P7