A method and a circuit system for detecting the speed of rotation of a DC motor

Abstract: The method described allows the speed of rotation of a DC motor having separate exitation to be detected utilising only electrical quantities of the motor and the associated control circuitry. It provides for the detection of the armature current (Ia) and exitation current (Ie), the average armature voltage (VM) and a quantity proportional to the exitation flux (), and the formation of the product between the armature current (Ia) and the resistance (Ra) of the armature circuit to obtain the ohmic voltage drop (VR) across the armature circuit, taking the difference between the average armature voltage (VM) and the voltage drop (VR) across the armature circuit and forming the ratio between the said difference and the exitation flux () whereby to obtain a quantity proportional to the speed of rotation (n) of the motor. For application to electric traction vehicles the exitation current (Ie) is assumed as the quantity proportional to the exitation flux. A circuit system for putting the above indicated method into practice is also described.

Description: The present invention relates to DC motor drives and more particularly to a method, and to a circuit system, for detecting the speed of rotation of a separately exited DC motor. In many applications of DC motors it is necessary to know the instantaneous speed of rotation of the motor for the purpose of regulation and control. For example, in the case of electric traction vehicles for urban transport the measurement of the speed of rotation is necessary for two main purposes: to limit the speed of the motor to a maximum admissible value, and to effect a velocity feedback by comparison of the instantaneous speed of the motor with the position of the accelerator pedal for the purpose of obtaining a response to the accelerator similar to that which is obtained in vehicles driven by an internal combustion engine. In this application the measurement of the speed must be precise when close to the maximum speed, but need not be so precise in other situations: in particular, for the said feedback it is sufficient that the measurement be a continuous and monotonic function of the effective speed and that, with the vehicle stationary and the accelerator raised, the measurement should be nil. A known technique for measuring the speed of rotation of a motor makes use of an electromechanical transducer such as a tachometric dynamo or an "encoder" associated with the motor. A device of this type requires relatively delicate and, therefore, expensive operations for mechanical adjustment between the transducer and the motor, is relatively bulky because it is necessarily positioned on the axis of the motor, and in applications such as that of an electric traction vehicle in which the ambient conditions are particularly hostile due to dust, moisture, heat and vibrations, is of poor reliability or, in any event, requires frequent maintenance operations. Another known technique, utilised in drives having small motors with brushes and fixed exitation uses an indirect measurement of the speed of rotation: the back-electromotive force which develops in the armature winding due to the effect of the rotation is detected with a suitable sensor; since the back-electromotive force is proportional to the speed of rotation and to the exitation flux, and since this latter is constant, the measured quantity gives a good indication of the speed of the motor. The main disadvantage of this technique lies in the fact that the armature winding supply must be set to zero for the whole of the time the measurement of the back-electromotive force is taking place. It can be used therefore only in cases in which the inertia of the rotor is sufficiently large to make interruptions in the supply negligible. Another method of measuring motor speed is known from WO88/00771. An object of the present invention is to propose a method for detecting the speed of rotation of a DC motor having separate exitation which does not require the use of electromechanical transducers and does not require interruptions in the electrical supply to the motor. This object is achieved, according to the invention, by the fact that the method provides the following operations: detecting the armature voltage, detecting the armature current, detecting the exitation flux, forming the product between the detected armature current and a predetermined quantity corresponding to the armature circuit resistance to obtain, as a result, the value of the ohmic voltage drop across the armature circuit, taking the difference between the detected armature voltage and the value obtained for the voltage drop across the armature circuit and forming the ratio between the said difference and the detected exitation flux to obtain, as a result of this ratio, a quantity proportional to the speed of rotation. Another object of the invention is to provide a circuit arrangement for putting the above briefly-defined method into practice, which will be simple, reliable and of low cost. This other object is achieved according to the invention with a circuit system generally as described and charaterised in the fourth of the claims which follow the present description. The invention and the advantages which derive from it will become evident from the following detailed description of an exemplary and therefore in no way limitative embodiment with reference to the attached drawings, in which: Figure 1 shows in schematic form a DC motor having separate exitation and the principal functional blocks for control of its supply; and Figure 2 is a diagram showing a basic circuit system according to the invention. In Figure 1 is shown a DC motor, generally indicated M, which has an armature winding 1 and an exitation winding 2 supplied by a DC voltage source Vcc shown as a battery 3, via respective switching converters (choppers) 4 and 5. These latter are controlled in a known way by sequences of pulse signals respectively indicated Va and Vb, produced by a control unit in dependence on one or more control quantities, generally indicated Vd. Two current sensors indicated 7 and 8 are associated with the armature and exitation circuits respectively, to detect the currents Ia and Ie respectively which flow through them. A circuit arrangement 9 receives at its input various electrical quantities which characterise the operation of the motor, that is the supply voltage Vcc, the armature current Ia, the exitation current Ie and the resistance Ra of the armature circuit, and provides at its output an electrical quantity Vn which is a measure of the speed of rotation of the motor M. Before examining in detail the structure of the circuit assembly 9 it is useful to remember that the speed of rotation n of a DC motor is directly proportional to the back-electromotive force (Vcem) developed in the armature circuit and inversely proportional to the exitation flux : n ∼ Vcem  and that the back-electromotive force Vcem is substantially equal to the difference between the armature voltage VM and the ohmic voltage drop VR across the armature circuit: Vcem = VM - VR Since the voltage drop VR is given by the product of the resistance Ra of the armature circuit and the armature current Ia: VR = Ra Ia by substituting equation (3) into equation (2) and then equation (2) into equation (1) one has: n ∼ VM -Ra Ia  In the example shown in Figure 1 the armature voltage is an average voltage VM which is obtained by the product of the supply voltage Vcc and the duty cycle d.c. (the ratio between the duration of the pulse and the period) of the control signal Va of the armature chopper 4: VM = d.c. Vcc In the preferred embodiment of the invention, moreover, it is assumed that the exitation flux  is directly proportional to the exitation current:  ∼ Ie By substituting equation (5) and equation (6) into equation (4) one obtains: n ∼ d.c. Vcc - Ra Ia Ie According to the method of the invention, the speed of rotation is obtained in an indirect manner from several electrical quantities of the motor and the associated control circuitry, namely: the supply voltage Vcc, the duty cycle d.c. of the control signal Va for the armature chopper, the armature current Ia, the exitation current Ie and the resistance Ra of the armature circuit. The accuracy of detection of the speed of rotation according to relation (7) is strongly conditioned by the assumption made above about the exitation flux. As is known, in fact, the relation between flux and exitation current in a DC motor having variable exitation is not linear because of the magnetic saturation. However, in many applications, and in particular in that of vehicles mentioned at the beginning of the present description, the accuracy is sufficient because the maximum speed is obtained at an exitation which is greatly reduced with respect to the saturation value and because, in relation to the situation with the vehicle stationary and the accelerator raised, the back-electromotive force is nil so that the measurement of the speed of rotation according to the expression (7) is zero. The circuit arrangement 9 of the system for detecting the speed of rotation in a preferred embodiment of the invention is represented in Figure 2. This comprises a first circuit, enclosed in a broken line rectangle indicated 11, which produces at its output a voltage VM proportional to the average value of the armature voltage, a second circuit, enclosed in a rectangle indicated 12, which provides at its output a voltage VR which represents the ohmic voltage drop across the armature circuit, a third circuit, enclosed in a rectangle indicated 13, which provides at its output a voltage Vcem proportional to the back-electromotive force by taking the difference between the output voltages of the first (11) and the second (12) circuit, and a fourth circuit, enclosed in a rectangle indicated (14) which forms the ratio between the voltage Vcem provided by the circuit 13 and the exitation current Ie detected by the sensor 8. The output of this fourth circuit is a voltage Vn which represents the speed of rotation n of the motor. The first circuit 11 comprises two controlled electronic switches S1 and S2, for example two transistors, connected in series between the two terminals of the supply voltage source Vcc and controlled by the control voltage Va of the armature chopper 4. More particularly, one switch is controlled directly by the voltage Va and the other is controlled by the voltage Va inverted by an invertor INV1. The junction point between the two switches is connected to an integrator constituted by a resistor R1 and a capacitor C1. It can easily be demonstrated that the voltage VM which forms across the terminals of the capacitor C1 has a value substantially equal to the product of the supply voltage Vcc and the duty cycle of the control voltage Va. The second circuit 12 is a multiplier comprising an operational amplifier OP1 with a feedback resistor R2 of value proportional to the resistance Ra of the armature circuit, which receives at its input a voltage proportional to the armature current Ia. This input voltage can be obtained, as is shown, by applying a current of value Ia to a resistor R3 connected to the input of the operational amplifier OP1 via a resistor R4. The output voltage of the operational amplifier will be VR = (R2/R4) R3 Ia = kRaIa where k is a constant of proportionality determined by the values of the resistors R2, R3 and R4. The third circuit 13 is a subtracter comprising an operational amplifier OP2 with a feedback resistor R5 which receives at its input, via a resistor R6 and a resistor R7 respectively, the voltage VM at the output of the first circuit 11 and the voltage -VR obtained by inverting, with an invertor INV2, the voltage VR at the output from the second circuit 12. The output voltage Vcem, therefore, because it is proportional to the difference between the voltage VM and the voltage VR, represents the back-electromotive force which develops across the terminals of the armature winding by the effect of the rotation. The fourth circuit 14 is a divider constituted by a variable gain amplifier which receives at its input, via a resistor R8, the voltage Vcem at the output from the third circuit 13. The variable gain amplifier is constituted by an operation amplifier OP3 with a feedback network equivalent to a variable resistance. More particularly, the feedback network is constituted by a circuit comprising a capacitor C2 in parallel with the series connection of a resistor R9 and an electronic switch S3, for example a transistor, controlled by a generator G. This latter produces a continuous sequence of pulses the duty cycle of which is proportional to the exitation current Ie. The capacity of the capacitor C2 is chosen in such a way as to eliminate oscillations due to the sequence of control pulses from the electronic switch S3. This latter is controlled in such a way as to be closed during the pulses and open between one pulse and the next in the sequence. In this way the resistance Rf of the circuit with respect to DC voltage is inversely proportional to the exitation current: Rf ∼ 1/Ie. The gain of the amplifier, g = Rf/R8, , therefore, is also inversely proportional to the exitation current so that the output voltage Vn of the amplifier will be proportional to the ratio between the input voltage Vcem and the exitation current: Vn = g Vcem ∼ Vcem/Ie By assuming the exitation flux to be proportional to the exitation current and bearing in mind the above-mentioned relation (1) the voltage Vn is a quantity which represents the speed of rotation n of the motor. As can be established, the objects of the invention are achieved by the above-described circuit system. This system, in fact, makes it possible to reconstruct the speed of rotation of the motor by utilising only electrical quantities of the motor itself and its control circuit, and can be achieved using low cost circuit components such as operational amplifiers, electronic switches and a few other components of ordinary type. Naturally many variants are possible within the ambit of the same inventive concept. For example, the various circuits which constitute the system can be achieved in a substantially different manner from that represented as long as they perform the indicated functions. Moreover, the method of the invention can be put into practice by utilising digital techniques rather than analogue techniques as described.

Claims: A method for detecting the speed of rotation of a DC motor having separate excitation, characterised in that it comprises the following operations: detecting the armature voltage (VM), detecting the armature current (Ia), detecting the excitation flux (), forming the product between the detected armature current (Ia) and a predetermined quantity corresponding to the armature circuit resistance (Ra) to obtain, as a result, the value of the ohmic voltage drop (VR) across the armature circuit, taking the difference between the detected armature voltage (VM) and the value obtained for the voltage drop (VR) across the armature circuit and forming the ratio between the said difference and the detected excitation flux to obtain, as a result of this ratio, a quantity proportional to the speed of rotation (n). A method according to Claim 1, in which the detection of the excitation flux () substantially comprises detecting the excitation current, assuming that the excitation flux is proportional to the excitation current. A method according to Claim 1 or Claim 2 applied to a motor the armature voltage of which is controlled by a chopper converter functioning with a predetermined supply voltage (Vcc) and with a predetermined duty cycle, in which the detection of the armature voltage (VM) substantially comprises forming the product between the supply voltage and the duty cycle (d.c.) to obtain an average quantity (VM) which represents the armature voltage. A circuit system for detecting the speed of rotation of a DC motor with separate excitation controlled by at least one chopper converter, characterised by the fact that it comprises first circuit means (11) having the supply voltage (Vcc) of the armature chopper converter applied across two input terminals, and a control signal (VA) of the armature chopper applied to another input, the first circuit means being operable to produce at its output a first voltage value proportional to the average value (VM) of the armature voltage, a first current sensor (7) for detecting the armature current (Ia), a second current sensor (8) for detecting the excitation current (Ie), second circuit means (12) having one input connected to a first current sensor (7) and being operable to produce at its output a second voltage value (VR) indicative of the product of the armature current (Ia) and a quantity which represents the armature resistance (Ra), third circuit means (13) having at its input the first voltage value (VM) and the second voltage value (VR) and being operable to provide at its output a difference voltage (Vcem) between these two values and fourth circuit means (14) having a first input connected to the second current sensor (8) and a second input connected to the output of the third circuit means (13) and being operable to produce at an output terminal an electrical quantity (Vn) proportional to the ratio between the difference voltage (Vcem) and the excitation current (Ie), this electrical quantity (Vn) representing the speed of rotation (n) of the motor. A system according to Claim 4, in which the said fourth circuit means (14) comprise a pulse generator (G) having a control terminal connected to the said first input and being operable to produce at its output a sequence of pulses the duty cycle of which is proportional to the excitation current (Ie), a variable gain amplifier (OP3, S3) having a gain control terminal connected to the output of the pulse generator (G), and input and output terminals connected respectively to the said second input and to the output terminal of the fourth circuit means (14). A system according to Claim 5, in which the variable gain amplifier comprises an operational amplifier (OP3) and a feedback network for the operational amplifier comprising a controlled switch (S3) the control terminal of which is connected to the control terminal of the amplifier. A system according to Claim 6, in which the resistive network includes a resistor (R9) in series with the controlled switch (S1) and a capacitor (C2) in parallel with the series connection of the resistor and the switch.

Inventors: BOBBIO ATTILIO, BOBBIO, ATTILIO
Applicants: FINMECCANICA SPA, FINMECCANICA S.P.A. AZIENDA ANSALDO
Application year: 1995
Main classification: G01P3
IPC codes: G01P3, H02P7