Title of Invention

"ANALOGUE ROTOR POSITION CONTROLLER FOR SWICHED RELUCTANCE MOTOR"

Abstract

This invention relates to a novel position using analogue electronic devices for application in a switched reluctance motor to eliminate the shaft - mounted position encoder comprising a SR motor (1) connected to a power converter (2) characterized in that said novel position sensor comprises sub-block I (15) having a summer (3) connected to said power converter for input of voltage (V) and phase current (iph), a resetting sub-block It (16) having an integrator (5), a sub-block III (17) having analog function generator (7) connected to said integrator, said function generator is connected to the negative terminal of a summing amplifier (8) for an input of current (4) and the positive terminal of said summing amplifier is connected to said power converter for an input of phase current (iph )a sub-block IV (18) having an analog divider (9) for an input current (ig) from said summing amplifier and an input of flux - linkage (?) from said integrator, said analog divider producing an output of unsaturated inductance (Lu) as an input to a low pass filter (6) and a comparator (10) of sub-block V (19) for a clock input for a ring counter (11) and buffer (12) of the sub-block VI (20), the output of which is connected to said power converter through an opto coupler (13) and a base drive (14).

Full Text

The invention relates to a novel position sensor using analogue electronic devices for application in Switched Reluctance Motors to eliminate the shaft-mounted position encoder that is presently being used. Switched Reluctance Motors are commonly used in medium power traction drives like electric cars and hoist drives like lifts, etc. However position feedback is essential for its running. The use of shaft mounted position encoders is generally common for this purpose. The main disadvantage associated with shaft mounted position encoder is that it makes the drive both delicate and clumsy as extra space is needed for the mounting of the encoder and the encoder farms part of the motor during manufacture. In some of the schemes developed till date towards elimination of shaft encoders, an energised or an unenergised phase has been fed with a high frequency diagnostic signal in order to sense the variation of inductance of a phase. Hence an extra signal source is needed and this is a disadvantage. In some other schemes, the voltages and currents of the energised phase were multiplexed and fed via an A/D card to a processor to generate the inductance data. Since the self inductance is essentially position dependent the same has been used as a measure of the rotor position but there will be errors unless correction is made for magnetic saturation, since/the inductance is dependent on the degree of magnetic saturation as well, apart from rotor position. To take care of variation of inductance with saturation the magnetisation information of the phases at different rotor positions are stored in a look-up table from which the unsaturated inductance of the phases was obtained. The rotor position was then decoded from the calculated unsaturated inductance via another look-up table and this position value is compared with a reference value to generate the turn-on instant to trigger the incoming phase. The disadvantage associated with this method is that the voltage and current information have to be processed at a very fast rate so as not to loose the position information even at high speeds. Therefore very fast 1 digital processors which are costly are absolutely essential. Also there is need to store large amount of digital data, since inductance values are to be stored as function of both rotor position as well as phase current or flux In some other schemes for elimination of shaft mounted position sensor the mutual inductance between phases is used to detect the rotor position. However the mutual inductance between phases of a Switched Reluctance Motor (SRM) is very small due to concentric nature of phase winding embracing only one stator tooth per coil and double saliency. Its maximum value is about one or two orders lower than the maximum self inductance. Methods, which depend on measurement of mutual inductance, are therefore not that reliable and robust. In some other schemes, the rate of current rise is used to detect indirectly the inductance and hence the rotor position. However, some of these schemes are restricted to low speeds of operation of the motor to avoid errors due to presence of speed-dependent emf in the windings. Thus all present schemes appear to be either complex or costly requiring fast processors OR restricted.for use.,in certain speed range OR not robust. Therefore the main object of the present invention is to propose a new indirect rotor position sensor which is devoid of errors arising due to magnetic saturation, is robust and eliminates the use of fast digital processors. To fulfill the last purpose the proposed invention uses analogue electronic devices. The invented device measures, on-line, the unsaturated inductance of the phase in an intelligent way, thus eliminating errors due to saturation. Still another object of the present invention is to propose a position sensing mechanism, which can take care of starting and reversible rotational operation. Yet another object of the present invention is to propose a position sensing mechanism in which only the powering voltage and current of an excited phase are proposed to be used to sense the present position. This will eliminate the need for extra diag- 2 nostic signal. Further object of the present invention is to propose a position sensing mechanism, which allows continuous control of switching angle within the full range of the motor. The present invention titled A novel position sensor using analogue elec tronic devices for application in a switched reluctance motor eliminates the shaft-mounted position encoder be used in conjunction with a SR motor and a Power Converter. The description of the invented device is as follows: The said novel position sensor comprises sub block-I having a summer being con-I nected to the said Power Converter for input of voltage (V) and Phase current (iPh), an integrator sub block-II having a resetting switch, a sub block-III having an analog saturation function generator connected to the output of the said integrator, the said function generator's output is is connected to the negative terminal of a summing amplifier and the positive terminal of said summing amplifier being connected to said power converter for an input of phase current (iph), a sub block-IV (18) having an analog divider for an input current (ig) from said summing amplifier output and an input flux - linkage from said integrator, said analog divider producing an output of unsaturated inductance (Lu) as an input to a low pass filter and a comparator of sub block-V for a clock input for a ring counter and buffer of the sub block-VI, the output of which is connected to said power converter through an optocoupler and a base drive. The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to non-limiting exemplary embodiments of the invention represented in the accompanying drawings. o Figure 1 : Block diagram of the proposed scheme highlighting invented novel position sensor inside the chain dotted area represented by sub block- I to VI. o Figure 2 : Circuit arrangement for flux measuring circuit. 3 o Figure 3 Circuit arrangement to generate saturation function polynomial and an analog circuit for obtaining is Vs ? characteristic o Figure 4 : Circuit arrangement for Ring Counter along with the presetting mechanism. o Figure 5(a) . Circuit diagram of integrator to obtain the ? of the ON phase. o Figure 5(b) : Circuit diagram of the resetting mechanism.) o Figure 6 . Analog circuit to represent a linear function o Figure 7 . Circuit arrangement for obtaining Lu(?) at the output of analog divider and comparator stage with Lref. o Figure 8(a) Self starting mechanism with forward and reverse running of the SR motor o Figure 8(b) : Another embodiment of self starting mechanism of the SR motor. In accordance with the present invention the mechanism uses simple linear elec tronic devices like op-amps, analogue multiplier chip (ICL 8013) and a novel diode- resistor -opamp based analogue function generator circuit as a rotor position sensor The rotor position sensor/controller of the present invention allows continuous control of turn-on angle of the in-coming phase within the full range of the designed limits of turn-on advance or delay for the motor. Figure 1 shows the layout of a medium power SR Motor drive system along with the developed mechanism. The apparatus consists of the sensing hardware, the analogue computation hardware plus the control logic for triggering of phases with auto starting capability 4 The apparatus has the following main functions. - o (a) Sensing the voltage across and the current through the energized phase via multiplexing and analogue computation of the flux level therefrom. o (b) Obtaining the unsaturated inductance from the knowledge of the flux level and current and comparing the same with a reference value to obtain the proper triggering instant of the incoming phase o (c) Controlling the logic of operation, which also has auto-starting and reversible operation features. The block diagram in Fig. 1 shows the parts marked within the outer chain dotted boundary which are novel and forms the major part of the position sensor for the SR motor. The motor (1) and the power converter (2) are parts of the existing technology, which can be precisely controlled by the present invented device. The invented device would serve to replace the conventional shaft mounted mechanical position encoder , ' which has so far been used to control the running of the SR, Motor (1). Fig. 2 shows the modification to the existing power converter (2) configuration, which would help the invented device to function properly. The modification comprises of a current sensitive shunt (RSh) to sense the ON time current as a part of the position sensing scheme. I In principle a SR motor (1) is a stepper motor running in closed-loop with position feedback. Hence it is absolutely essential to decode its position. In the present work an attempt has been made to eliminate the shaft mounted position encoder because of its well-known drawbacks. The entire analog position sensing mechanism comprising the voltage and current circuit sensing, the integrator with resetting circuit, the analog saturation function generator plus the analog divider combination and the self starting mechanism are claimed contributions of the inventors. It is clear that one must sense the voltage across and current through the presently ON phase for further processing. These voltage and current values, after being stepped down to proper levels, are 5 utilised in an integrator (5) to get the value of the flux-linkage (?) at any instant. In the present scheme as shown in Fig. 1, there is a voltage and current sensing arrangement along with multiplexing option which after being passed through proper gain stages is utilised to give a sum proportional to (Vph - iphRph) at the output of the summer (3). This is then fed to an analog integrator (5). The output of.this integrator (5) indicates the instantaneous flux level (?). This flux information is fed into an analog function generator (7). This function generator (7) outputs the current (is) drawn, (in excess of the current required for setting up m.m.f. in the air-gap) due to saturation of the iron parts. This current is subtracted from the phase current (iph) in a summing amplifier (8) the output of which (iPh - is) represents the current (ig) to set up the air-gap m.m.f. This latter current (ig) divides the flux level in an analog divider (9) giving thereby an output, which is proportional to the unsaturated inductance, of the motor phase. This output is compared with a reference level (Lref) in a comparator (10). It is noteworthy that Lu(?) is a function of position alone. Whenever Lu(?) reaches the reference value Lref a momentary high going edge is generated at the comparator output which, on being fed to the clock input of a ring counter (11) of the logic circuit, causes the presently ON phase to be turned OFF and the next phase in the rotation sequence to be turned on. The ring counter (11) is connected to the power converter (2) through a buffer (12), optocoupler (13), and base drive (14). By changing the Lref level the turn-off instant of the presently ON phase and the turn-on instant of the incoming phase may be varied. The details of the various stages of the circuit (on which the originality is claimed) described above are discussed below. o (i) The voltage and current sensing stage (shown as sub-block I (15) in block diagram) 6 Fig. 2 shows the relevant circuit for this stage. The voltages across the different phases are multiplexed with the help of transistorised multiplexing switches. In order to decrease the number of input lines the low voltage end of the voltage sensing lines is connected to the common point of the emitters of the lower switches of the main converter. This however gives a very small error (Vph + VCE = Vph) in the sensed voltages. Since the current flowing through any phase is required to be sensed during its ON time only, the arrangement of the current sensing shunt is done as shown in Fig. 2. The multiplexed voltage is then fed to an opamp connected in the differential mode The output of this opamp is - nVph. Since we can now integrate only n(Vph - iphRph) we must pass the current shunt drop Rshiph through an inverting amplifier with a gain 'a' such that The output of this stage is fed into the summer and integrator to obtain the instantaneous flux linkages as described next. o (ii) The summer and integrator with resetting circuit (shown as sub-block II (16) in block diagram) The relevant circuit for this stage is shown in Fig. 5 (a) At the input of the summing amplifier the quantities nVph and-niphRph) are fed so that at the input of the integrator we get -n(Vph - iPhRph)- Let the input resistance of the integrator to be R, and the feedback path capacitance be C,. Then the output of the integrator is given by Assuming that the mutual flux is zero (which is a valid approximation for the SR motor) so that at t = 0,?(0) - 0 tR,Ct should be properly chosen, so 7 that at low speeds of the motor the output of the integrator (5) does not go to saturation. To prevent the drift of the integrator (5) capacitor, a large resistance (large w.r.t R,) of value 68/C ? is placed across C,. At the end of the ON period of each phase the flux in the incoming phase is zero, initially (assuming no mutual flux i.e., ?(0) = 0). So the integrator (5) should also be reset to zero initial condition, after each conduction period. This is done by momentarily turning on a transistorised switch (BC368) (in series with a small resistance) across the capacitor The base pulse logic of this transistor (BC368) is fed form a monostable (74123) via proper circuit shown in Fig. 5(b). The output of the integrator (5) which represents the instantaneous flux-linkages of the presently on phase of the machine is fed to the stages described below. o (iii) Analogue ? vs is function generator (shown as sub-block III (17) in block diagram). It has been already pointed out that , does not indicate correct rotor position due to iron saturation For this reason Lph can not be used as a unique quantity indicating the rotor position. It has also been pointed out that though the phase inductance is a function of the position and the level of saturation of the machine iron, the quantityLu = , representing the air-gap inductance, is a function of rotor position alone. Hence an analog function generator has been designed which will generate the quantity is at its output when ?ph is provided at the input. This function generator is made up of network of diodes (IN4001) and precision resistors (metal film) and an opamp so as to generate the desired polynomial (saturation characteristic in this case) as is done in analog computers o (iv) The analog divider stage (shown as sub-block IV (18) in block diagram) 8 The output of the analog function generator (representing the saturation characteristic) is subtracted from the phase current to get zg = iph - is. Here ig represents the current, which is drawn to set up the air-gap m.m.f. When this signal divides ?, we get at the analog divider output a quantity which represents the unsaturated inductance Lu(?) which is purely a function of the position only. In the present case the chip used to configure that analog divider is ICL8013. The inputs of this chip are restricted within 10V. Hence the integrator output which has a maximum of 15 V, is scaled down by a factor of ?, before being fed into the analog divider (Zin) input. The signal at the Xin input of the analog divider (where the current signal ig is to be fed) is required to be of negative sign and should be between -1 V and -10 V The gains in the path of the current signal in the rest of the circuit is so set that 1A is represented by IV. However, to allow a maximum of 20A to be sensed and utilized in the divider the ig signal is passed through an inverting stage of gain 0.5 and thereafter a voltage of 1 V is added so that ig = 0 appears as -1 V at the Xim input of the divider. Also the analog divider is pretuned using the XO3, Yos and Z0s pins to give a minimum off-set in the output given by o (v) The comparator stage (shown as sub-block V (19) in block diagram) The output of the analog divider (Fig. 7) representing the unsaturated inductance of the currently ON phase LON(T), is found to contain some spurious noise signals. So it is passed through a low-pass filter with a cut off frequency of 4kHz. Interestingly these noises occur at the instant when a phase turns off. These noises can severely affect the running of the machine under heavy loads at low speed since the noise in the LON(T) signal may get compared with Lref causing turn on of the next phase at a wrong instant. Thereafter the signal is fed to a comparator at the other input of which we have a reference d.c. level Lref Whenever LON(T) equals Lref a logic HIGH 9 is generated at the comparator output. Since the comparator output works as the clock input to the ring counter of the controlling logic circuit, the HIGH pulse causes the next ring-counter output to go high while the others remain LOW. These details are discussed in the next section. As a result the next phase is turned on, which of course initially has an LON(T) value less than Lref. So the comparator output goes LOW again. Thus the comparator output is a train of narrow pulses serving as the closed-loop clock of the system The next section describes how the comparator output has been utilised to cause sequential synchronised switching of the motor phases through a logic control circuit. Logic Circuit: (Shown as sub-block VI (20) in block diagram) This section describes the mechanism of how the motor has been controlled causing it to self-start (even with load) and to run in either direction at a speed decided by the load on the motor, the angle of turn on (which is decided by the reference level Lref) and the input voltage to the d.c. bus. It also discusses how the current limiting and the flux limiting requirements (optional) can also be taken into account. The mode in which the present SR motor is operated is one where at a time only one phase is turned on. A phase is kept on in the positive torque producing region of its inductance profile for a duration, which is equal to the step angle. In the present case of the 8/6 stator/rotor combination the step angle is - 15°. So each phase remains on for a quarter of the entire phase cycle of 60°. Thus the switching logic may be controlled by a ring-counter (74164) with 4 output lines of which one remains high at a time. This is done by presetting the counter with a single "1" (at QA output) and 3 "0"s (QB, QC & QD)- The phase for which the logic signal at the output of the ring counter is HIGH is turned on. In the 10 steady state the output of the comparator (Lref &; LON(T)) serves as the clock for the ring-counter. As a result the closed-loop system runs smoothly with the switching on of the next phase taking place every 15°. However at the time of starting the motor from rest condition, LON(T) remains constant and hence no change takes place at the comparator output and the closed-loop running can not be initiated. Therefore, a separate logic for starting the motor has been developed and implemented in a novel way. The self starting mechanism is discussed below with reference to Fig. 8(a). At power on, with switch S1 in the start position, phase 4 and phase 3 are turned on. As a result the rotor aligns in a position such that it is - away from phase-1 axis and -+¦ also from pha^e-2 axis Now if switch S\ is changed over to run position with switchS2 in position, then phases 4 & 3 are turned off and simultaneously phase 2 is energised. So the rotor experiences a pull from phase-2 and tends to align with it. However as it rotates, LON(T) starts changing towards Lmar. In the process as soon as LON(T) equals Lref, a clock-pulse is generated causing the next ring-counter output to go high. With switch S2 clockwise. position we find that successive switchings cause the phases to be energised in the sequence 2-1-4-3-2 and so on. As a result the motor smoothly starts and turns inclockwise direction. Gradually the input d.c. voltage may be increased. The starting logic for reverse running of the motor has also been, included in a simple manner as follows. If the switch S2 is put incounter c1 ockwi se position before S1 is put in the RUN position then phase-1 is energised first and in a similar fashion as discussed above the switching progresses following the pattern 1-2-3-4-1 and the motor runs in the counter clockwise direction. In this way a simple scheme has been developed and successfully put into operation for self starting, forward and reverse running of the SR motor. An alternative to this method of starting is to initially run the motor in the open-loop as a stepper motor. With the help of the synchronising circuit as shown in Fig.8(b) the motor is changed over to the closed-loop mode and it easily pulls the rotor into synchronism with the closed-loop clock. The invention described hereinabove is in relation to non-limiting embodiments and as defined by the accompanying claims. -11 - WE CLAIMS 1- A novel position sensor using analogue electronic devices for application in a switched reluctance motor to eliminate the shaft - mounted position encoder comprising a Switched Reluctance (SR) motor (1) connected to a power converter (2) characterized in that said novel position sensor comprises sub-block I (15) having a summer 03) being connected to said power converter for input of voltage (V) and phase current (i ph,), the power converter being provided with a current sensing resistive shunt (Rsh ) to sense the ON time current, a resetting sub-block II (16) having an integrator (5), a sub- block III (17) having analog function generator (7) connected to said integrator, said function generator is connected to the negative terminal of a summing amplifier (8) for an input of current (is) and the positive terminal_ of said sunming amplifier is connected to said power converter for an input of phase current (iph) a sub-block (18) having an analog divider (9) for an input current (ig) from said summing amplifier and an input of flux-linkage ( ? ) from said integrator, said analog divider producing an output of unsaturated inductance (Lu) as an input to a low pass filter (A) and a comparator (10) of sub- block V (l9) for a clock input for a ring counter (11) and buffer (12* of the sub-block VI (20), the output of which is connected to said power converter through an opto coupler (13) and a base drive (14). -12- 2. A novel position sensor as claised in claim 1 wherein the said summer of said sub-block X is provided with a voltage and current sensing arrangement alongwith multiplexing option which after being passed through gain stages is utilised to give a sum proportional to n(Vph- iph Rph ) at the output of said summer. 3. A novel position sensor as claimed in claim 1 wherein said integrator of said resetting sub-block II (16) provides an instantaneous flux level (?) output for analog function generator (7) of sub-block III (l7) and analog divider (9) of sub-block IV (18). 4. A novel position sensor as claimed in claim 1 wherein said analog divider provided as output of unsaturated inductance (LU (?) for said comparator for an output far said ring counter (11) when said outpout of unsaturated inductance from said divider reach a reference level (Lref) and said ring counter of the logic circuit switches off the presently ON phase and the next phase in the rotation sequence to be turned ON. 5. A novel position sensor using analogue electronic devices for application in a switched reluctance motor to eliminate the shaft-mounted position encoder as herein described and illustrated in the accompanying drawings. -13- This invention relates to a novel position using analogue electronic devices for application in a switched reluctance motor to eliminate the shaft - mounted position encoder comprising a SR motor (1) connected to a power converter (2) characterized in that said novel position sensor comprises sub-block I (15) having a summer (3) connected to said power converter for input of voltage (V) and phase current (iph), a resetting sub-block It (16) having an integrator (5), a sub-block III (17) having analog function generator (7) connected to said integrator, said function generator is connected to the negative terminal of a summing amplifier (8) for an input of current (4) and the positive terminal of said summing amplifier is connected to said power converter for an input of phase current (iph )a sub-block IV (18) having an analog divider (9) for an input current (ig) from said summing amplifier and an input of flux - linkage (?) from said integrator, said analog divider producing an output of unsaturated inductance (Lu) as an input to a low pass filter (6) and a comparator (10) of sub-block V (19) for a clock input for a ring counter (11) and buffer (12) of the sub-block VI (20), the output of which is connected to said power converter through an opto coupler (13) and a base drive (14).

Documents:

00468-cal-1999 abstract.pdf

00468-cal-1999 claims.pdf

00468-cal-1999 correspondence.pdf

00468-cal-1999 description(complete).pdf

00468-cal-1999 drawings.pdf

00468-cal-1999 form-1.pdf

00468-cal-1999 form-18.pdf

00468-cal-1999 form-2.pdf

00468-cal-1999 form-3.pdf

00468-cal-1999 letters patent.pdf

00468-cal-1999 p.a.pdf

00468-cal-1999 reply f.e.r.pdf