WO1993012437A1

WO1993012437A1 - Method for determination of inductance

Info

Publication number: WO1993012437A1
Application number: PCT/FI1992/000334
Authority: WO
Inventors: Henrik Huovila
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 1991-12-10
Filing date: 1992-12-09
Publication date: 1993-06-24
Also published as: FI915799A0; FI89636B; EP0616693A1; AU3088392A; CA2125562A1; FI89636C

Abstract

A method of the invention is intented for determination of inductance by means of an electric circuit (1) subjected to the action of a control system (2). In order to determine inductance (L), a measuring system (3) is used to measure one or more parameters, such as the intensity (I), voltage (U), electromotive force and/or the like of electric current traveling in the electric circuit (1). The control system (2) includes an electric-current chopping control element (2a), such as one or more transistors or the like, whereby a power supply (I, U) operating in electric circuit (1) is used to produce in the inductive section (i) of electric circuit (1) at least two voltages (U1, U2) of substantially different intensities, depending on the chopping phase, i.e. the on-phase and the off-phase of control element (2a). Thus, according to the invention, inductance (L) is determined on the basis of voltages (U1, U2) prevailing in the inductive section (i) of electric circuit (1) and/or on the basis of electric current intensity changes (dI1, dI2) occuring therein.

Description

Method for determination of inductance

The present invention relates to a method for deter¬ mination of inductance by means of an electric circuit or a like subjected to the action of a control system, the inductance being determined by using a measuring system to measure one or more parameters, such as the intensity, voltage, electromotive force and/or the like of electric current running in the electric circuit or a like, the control system comprising at least one electric-current chopping control element, such as one or more transistors or the like, whereby an electric-current supply prevailing in the electric circuit or the like is used to generate in the electric circuit or the like or at least in its inductive section, at least two voltages of substantially dif¬ ferent intensities, depending on the chopping phase, i.e. the on-phase and the off-phase, of the control element.

A method of the invention is particularly suitable for operating and/or controlling various actuators, such as motors, valves, clutches, magnetic bearings or the like, the functions thereof being based on the deter- mination of a position of a moving actuator component, such as a valve rod, a motor rotor or a like, on the basis of an induction change occurring in an electric circuit.

A method based on the determination or the change of inductance is currently applied e.g. in the control of hydraulic and pneumatic valves. Thus, the valve is provided with a separate winding system, comprising for example a primary winding and two secondary windings. In this case, the position of a valve rod is determined by means of a metallic indicator, which is in contact with the rod and which, upon the rod movement, performs a corresponding movement between the primary and secondary windings included in the winding system. Hence, depending on the indicator position, the mutual signal amplitudes of the secondary windings differ from each other, and on the basis of this, the rod position can be determined e.g. by calculation. In principle, a method of the above type has been exploited e.g. in a so-called LVDT-sensor.

On the other hand, for example so-called brushless direct-current motors are currently provided with a so-called Hall sensor for measuring the commutating position of a motor and for controlling the motor. The Hall sensor is a magnetic-fieId sensor, which is mounted on a motor in connection with the rotor. Naturally, in order to achieve a control effect, the rotor is required to have a certain type of, e.g. asymmetric construction for alternating the magnetic field periodically, depending on the rotor position.

There is also a sensorless method developed especially for brushless permanent magnet motors for measuring the electromotive force generated by a rotating rotor. Thus, each pole of the motor is continuosuly operated basically in two successive phases, the pole operating in the first phase like a motor and in the second phase like a generator. Hence, by using in the control the voltage or electromotive force inducing in the pole operating like a generator, the commutating position of the pole can be determined for reversing its operation back to that of a motor. A problem in this type of method is that at low rotating speeds, as a result of low kinetic energy, the amplitude to be measured is also very low, which is why the method cannot be applied at speeds lower than a certain speed limit. Indeed, the equipment applying this method requires the use of e.g. separate actuating circuits at low rotating speeds of a motor, as in connection with its starting procedure or operation stoppage.

In addition to the above Hall sensors and LVDT-sensors, there are presently available also other, mainly position-measuring sensors, including for example inductive and capacitive sensors, eddy-current sensors and pulse sensors. The drawbacks of all such individual sensors can be summarized as follows: - bulky size, expensive price, generally complicated coupling electronics, increased fault possibilities, - problems associated with sensor installation, such as its tuning, susceptibility to malfunction and liability to cause trouble.

The use of conventional position-finding sensors, such as those mentioned above, always requires special fitting, tuning and other such procedures and, further¬ more, such sensors must be properly placed in an actuator, or an actuator or a part of it must be designed for the purpose. However, in several applica- tions it is not currently possible to apply methods substituting for position-finding sensors, or such methods do not even exist.

An object of the method of this invention is to provide a decisive improvement on the above drawbacks and thus to substantially raise the available prior art. In order to achieve this object, a method of the invention is primarily characterized in that inductance is determined at least during said on- and off-phases on the basis of the intensity changes of voltages and/or electric current occurring at least in the inductive section of the electric circuit or the like. The most important advantages gained by a method of the invention include simplicity and operating reliability, whereby it is possible to replace the current methods which require certain positioning and operating conditions. A "comprehensive" method of the invention can be employed in most diversified applications in a manner substantially more effective than the currently available methods, e.g. by applying modern tehcnology fully automatically. The application of a method of the invention does not significantly increase the external dimensions or necessary wiring systems of an actuator unlike most of the corresponding methods currently used. In its most simple embodiment, a method of the invention can be adapted to function on the basis of a current-supply circuit included directly in an actuator.

The invention will now be described in detail with reference made to the accompanying drawings, in which

fig. 1 shows basically a diagram of a simple electric circuit for applying a method of the invention,

fig. 2 illustrates symbols (a, b, c, d) clarifying principles of the method,

fig. 3 shows a brushless direct-current motor as one exemplary embodiment of the method,

fig. 4 shows a valve, a clutch or a like for the same purpose, and

fig. 5 shows a magnetic bearing for the same purpose. A method of the invention is intended for determination of inductance by means of an electric circuit 1 subjected to the action of a control system 2. In order to determine inductance L, a measuring system 3 is used for measuring one or more parameters, such as intensity I, voltage U, electromotive force and/or the like of an electric current traveling in electric circuit 1. The control system 2 includes an electric- current chopping control element 2a, such as one or more transistors or the like, whereby a current supply I, U operating in electric circuit 1 is capable of producing two voltages Ul, U2 of substantially dif¬ ferent intensities in the inductive section i of electric circuit l in response to the chopping phase, i.e. on-phase and off-phase of said control element 2a. Hence, according to the invention, inductance L is determined during said on- and off-phases in response to voltages Ul, U2 prevailing in the inductive section i of electric circuit 1 and/or electric-current intensity changes dll, dI2 occurring therein.

Fig. 1 illustrates in principle a simple electric circuit 1, which represents for example a circuit extending through one 6a of the coils in a single couple of electromotor poles. In coil 6a, its winding provides an inductive element L and a conductor used in the winding provides a resistive element R. Conven¬ tionally, such wirings or circuits are also provided with a diode, indicated by reference numeral 4 and facilitating in one direction the by-pass of coil 6a. As a result of the by-pass it is possible to create between the contact points X of diode 4 the inductive section i of electric circuit 1 operating as follows.

The transistor 2a serving as a control element can be used to subject the inductive section i of circuit 1 to the action of two alternating voltages, a first voltage Ul and a second voltage U2. According to one preferred embodiment of the invention, inductance L is then determined by way of calculation on the basis of the relative deviations between first voltage Ul and second voltage U2 of the electric current as well as corresponding intensity change rates dll/dt and dI2/dt of the electric current.

In a further preferred embodiment, the transistor 2a is operated at a high frequency, at least > 1 kHz, typically > 20 kHz, whereby the electric current intensities dll and dI2 change during said on- and off-phases in a substantially linear fashion. Thus, in view of the determination of inductance L, it is possible to evaluate the change of electric current intensity during on- and off-phases by means of straight lines dll and dI2, whose oppositely signed slopes correspond to intensity change rates dll/dt, dI2/dt. Hence, the relative deviation e of change rates can be determined as a difference between the slopes.

In a yet further preferred embodiment, inductance L is determined on the basis of the inverse value of a difference: dll/dt - dI2/dt between the current change rates determined from the inductive section i of electric circuit 1.

In another preferred embodiment, the inductive section i of electric circuit 1 is used to measure and/or determine a first voltage Ul, a second voltage U2 and the corresponding change rates dll/dt, dI2/dt of electric current intensity during the course of said on- and off-phases, whereby inductance L is calculably determined by applying the following equation: dll/dt - dI2/dt 1/L =

Ul - U2 Fig. 2 clarifies the application of the method by showing symbols from two electric circuits l_j , 1._j , having inductances different from each other. Figs. 2a illustrate alternating first and second voltages Ul, U2 during the on- and off-phases. Figs. 2b il¬ lustrate corresponding linear changes dll, dI2 in current intensity when using a sufficiently high frequency. Figs. 2c illustrate the rate per time dll/dt, dI2/dt of the linear changes occurring during the corresponding phases, said rates corresponding to the slopes of the straight lines shown in figs. 2b. Figs. 2d illustrate the inverse values of inductance L determined by way of calculation on the above basis. In the illustrated cases, the inductance L_{t j} of electric circuit l_{j j} is 2 x the inductance L_j of electric circuit l_j .

A method of the invention is examined hereinafter on theoretical basis, with certain allegations being used to demonstrate the validity of the above equation. A theory supporting the above equation will be briefly described hereinafter.

The basic allegation is to have such a high chopping frequency that the current intensity can be presumed to change linearly during each on- and off-phase.

For example, when the coil 6a in an electric circuit

1 as shown in fig. 1 is supplied with voltage U while the current is I and the coil resistance is R, the electric current intensity has a change rate as follows dl/dt = (U - RI) / L.

When the inductive section i of electric circuit 1 in the on- and off-phase of transistor 2a has voltages Ul and U2, the corresponding current intensity change rates will be: δ dll/dt = (Ul - IR) / L dI2/dt = (U2 - IR) / L, the resulting difference therebetween being: dll/dt - dI2/dt 1/L =

Ul - U2

Thus, as Ul and U2 are presumed to be constant, the dif erence between the derivates of current intensities * is inversely proportional to the coil inductance regardless of the transistor pulse ratio and coil resistance. Thus, the measuring result is affected neither by the change of resistance caused by the coil heating nor by the change of resistance of a clutch element.

In view of typical practical applications, it is possible to apply a method of the invention in connec¬ tion with highly diversified actuators, including electric, pressure-medium operated or the like motors, valves, clutches, magnetic bearings or the like. Thus, in order to determine inductance L, e.g. basic¬ ally as shown in fig. 1, a calculating element 5, such as a microprocessor, an analogous circuit or a like is used to provide the inductive section i of an electric circuit 1 included in an actuator 6 with measuring elements 3a, included in a measuring system 3 and fitted at least in a data transmitting communi¬ cation with said calculating element 5, for measuring and/or determining at least a change dll, dI2 in the intensity of electric current and a voltage Ul, U2 relative to time. Thus, a method of the invention can also be applied as described with the above positional sensors, whereby an electric circuit or a section thereof functioning on a simple principle of the method is adapted to operate in a particular actuator. Naturally, an electric circuit or a section thereof operating in accordance with a method of the invention can also be used to replace a sensor previously employed in said actuator.

According to what is described above, in several electrical actuators the inductance of an operating coil included therein changes in response to the position of a moving component. The operation of an actuator essentially involves a winding, such as a solenoid, a coil or a like, which means that also its current-supply circuit has a varying inductance, representing directly e.g. the position of a moving component. Hence, a particularly preferred application for a method of the invention is an actuator, whose electric circuit 1 dealing essentially with power supply is chopped by means of a transistor 2a. Thus, inductance L can be directly determined from electric circuit 1, included in an actuator and fitted with measuring elements 3a e.g. according to the above- described principle.

Figs. 3, 4 and 5 illustrate practical applications for the method carried out on the above principles.

Fig. 3 shows how the method is applied in connection with a reluctance motor representing a direct-current motor. Thus, according to the principle shown e.g. in fig. 1, the inductance L determined mathematically by means of calculating element 5 has an effect through the action of control system 2 and a control element 2b included therein on the operation of said motor in a manner that at a certain threshold value of the calculable inductance L to be determined during the rotating motion of a rotor 7, i.e. in a certain angular position of rotor 7, the polarity of a stator 8 is switched (6a, -> 6a2) for maintaining the rotating motion. Fig. 4 is a typical practical application showing e.g. a valve or a clutch, whose rod 9 is controlled by using a magnetic field generated by means of a solenoid 6a. Even in this case, a method of the invention is very simple to apply since the position of rod 9 can be determined directly from electric circuit 1 supplying current to the solenoid.

Fig. 5 illustrates yet another example of the applica- tion of a method of the invention in connection with a conventional magnetic bearing included in a moving component 10, such as a shaft or a like, said appli¬ cation corresponding in its operating principle and in its benefits to those described above.

It is obvious that the invention is not limited to the above embodiments but can be modified considerably within its basic concept even because of the simplicity and general validity of the method. Naturally, the method can be applied in a most diversified fashion in most diversified technical applications, the above description only showing a few general examples thereof. Naturally, also the equations used or the parameters used therein may be different in certain applications from those described above. Even the equation used in this context, as well as its para¬ meters, can of course be shown in very different forms indeed.

Claims

1. A method for determination of inductance by means of an electric circuit or a like subjected to the action of a control system (2) , the inductance (L) being determined by using a measuring system (3) to measure one or more parameters, such as the inten¬ sity (I) , voltage (U) , electromotive force and/or the like of electric current traveling in the electric circuit (1) or a like, the control system (2) com¬ prising at least one electric-current chopping control element (2a) , such as one or more transistors or the like, whereby an electric current supply (I, U) prevailing in the electric circuit (1) or the like can be used to generate in the electric circuit (1) or the like, or at least in its inductive section (i) , at least two voltages (Ul, U2) of substantially different intensities, depending on the chopping phase, i.e. the on-phase and the off-phase, of the control element (2a) , characterized in that inductance (L) is determined at least during said on- and off- phases on the basis of the intensity changes (dll, dI2) of voltages (Ul, U2) and/or electric current occurring at least in the inductive section (i) of the electric circuit (1) or the like.

2. A method as set forth in claim 1, whereby a control element (2a) , such as one or more transistors or the like, included in a control system (2) and operating on an electric circuit (1) or a like is used to produce at least two alternating voltages, a first voltage (Ul) and a second voltage (U2) , operating at least in the inductive section (i) of electric circuit (l) or the like, characterized in that inductance (L) is determined calculably on the basis of at least the first voltage (Ul) and second voltage (U2) of the electric current as well as the mutual deviations in the corresponding change rates (dll/dt, dI2/dt) of the electric current intensities.

3. A method as set forth in claim 1 or 2, wherein at least the inductive section (i) of an electric circuit (1) is acted upon by using a high chopping frequency (> 1 kHz) in a control element (2a) , such as one or more transistors or the like, included in a control system (2) , the intensities (dll, dI2) of electric current changing during said on- and off-phases substantially linearly, characterized in that for determination of inductance (L) the intensity change of electric current during on- and off-phases is evaluated by means of straight lines (dll, dI2) , whose oppositely signed slopes correspond to intensity change rates (dll/dt, dI2/dt) , the mutual deviation (e) of change rates being determined as a difference between said slopes.

4. A method as set forth in any of preceding claims 1 to 3, characterized in that inductance (L) is deter¬ mined essentially on the basis of the mutual deviation (e) defined by a first current change rate (dll/dt) , determined during at least one on-phase, and a second current change rate (dI2/dt) , determined during an off-phase from the inductive section (i) of the electric circuit (1) or the like, said deviation being e.g. the inverse value of the difference between those change rates.

5. A method as set forth in any of preceding claims 1 to 4, characterized in that at least the inductive section (i) of an electric circuit (1) or a like is used to measure and/or determine a first voltage (Ul) , a second voltage (U2) during said on- and off- phases as well as corresponding change rates (dll/dt, dI2/dt) of the intensity of electric current, the inductance (L) being determined calculably by using the following equation:

dll/dt - dI2/dt 1/L =

Ul - U2

6. A method as set forth in any of preceding claims 1 to 5, which is applied in connection with at least one actuator (6) , such as an electric, pressure-medium operated or a like motor, valve, clutch, magnetic bearing or a like, the action of said actuator (6) , at least during the operation thereof, being measured by means of a measuring system (3) and the action thereof during its operation being acted upon by means of a control system (2) , said measuring and control systems (2, 3) being in communication with actuator (6) through the action of one or more induc¬ tion elements (6a) , such as a coil, a winding or a like, which is in contact with at least said electric circuit (1) or the like, characterized in that, in order to determine inductance (L) by means of a calculating element (5) , such as a microprocessor, an analogous circuit or a like, the inductive section (i) of at least one electric circuit (1) in contact or associated with actuator (6) is provided with measuring elements (3a) , included in said measuring system (3) and being at least in data transmitting contact with calculating element (5) , for measuring and/or determining at least the intensity change (dll, dI2) and/or voltage (Ul, U2) of electric current relative to time.

7. A method as set forth in any of preceding claims 1 to 6, wherein the method is applied in connection with at least one actuator (6) , such as an electric, pressure-medium operated or a like motor, valve, clutch, magnetic bearing or a like, the action of said actuator (6) , at least during the operation thereof, being measured by means of a measuring system (3) and the action thereof during its operation being acted upon by means of a control system (2) , said measuring and control systems (2, 3) being in com¬ munication with actuator (6) through the action of one or more induction elements (6a) , such as a coil, a winding or a like which is in contact with at least said electric circuit (1) or the like, and whereby a control element (2a) , such as one or more transistors or the like included in said control system (2) , is adapted to act upon said actuator (6) , preferably upon an electric circuit (1) or a section thereof included in its power supply or the like, characterized in that, in order to determine inductance (L) by means of a calculating element (5) , such as a micro¬ processor, an analogous circuit or a like, directly from an electric circuit (1) or a section thereof essentially included in actuator (6) , the inductive section (i) of said electric circuit (1) included in actuator (6) is fitted with measuring elements (3a) which are at least in a data transmitting contact with calculating element (5) for measuring and/or determining at least the intensity change (dll, dI2) and/or voltage (Ul, U2) of electric current relative to time.