# Patent

## Title
On-line measurement of an induction machine's rotor time constant by small signal d-axis current injection

## Abstract

A controller continually updates rotor time constant estimation of an induction machine by interrogating the induction machine with a small signal oscillation and monitoring the response. The small signal oscillation is injected onto the d-axis current command signal, and is generated at a frequency that represents the most recent estimate of the rotor time constant (i.e., rotor time constant equal the inverse of the frequency). The controller monitors rotor flux generated in response to the small signal oscillation, and updates the most recent estimate of the rotor time constant based on the monitored rotor flux. This process is repeated continuously to allow for the continuous updating of the rotor time constant.

## Background
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract number F33615-00-2-2002 awarded by the Air Force Research Labs of the U.S. Department of Defense.


BACKGROUND
The present invention relates to induction machines, and in particular to field oriented control (FOC) of induction machines.
Field Oriented Control (FOC) is a well-known method of controlling induction machines. In short, field oriented control transforms space vectors from a three-axis stationary reference frame (abc) to a two-axis rotating reference frame (dq). Field oriented control allows for precise control of induction machines. In particular, the d-q reference frame allows the rotor flux or direct component (d-axis component) and the torque or quadrature component (q-axis component) of a commanded current signal to be independently controlled. Thus, the rotor flux and torque produced in an induction machine can be precisely controlled.
A typical implementation of FOC uses transforms to convert from the three-axis stationary reference frame (abc) to the two-axis rotating reference frame (dq) (as applied to a three phase machine). The two-axis reference frame can be aligned with the rotor flux or, in the alternative, can be aligned with the stator flux or air gap flux. Aligning the rotating reference frame with the rotor flux allows the decoupling of the d-axis current (used for rotor flux production) and the q-axis current (used for torque production). This decoupling is the heart of FOC, and is accomplished by providing the induction machine with the correct slip frequency and stator currents (magnitude and angle).
The accuracy, effectiveness, and dynamic performance of a particular FOC scheme rest on the ability to correctly determine the required slip frequency and q-axis current vector for a desired torque and rotor flux command. Correctly determining these values depends in part on accurately estimating induction machine circuit parameters. The rotor time constant associated with the induction machine is of particular importance for these calculations and is expressed as rotor inductance divided by rotor resistance (Lr/Rr). Because resistance of the induction machine changes with the temperature of the rotor, it is not sufficient to use nominal parameters continuously for good dynamic performance. Furthermore, since the rotor is moving, it is difficult to obtain temperature measurements for direct compensation with temperature. Therefore, accurate estimation or measurement of the rotor time constant is an important aspect of field oriented control.
SUMMARY
A controller for an induction machine makes on-line measurements of a rotor time constant associated with an induction machine by injecting a small signal on the d-axis current command at a selected frequency. The controller selects the frequency of the small signal oscillation based on a most recent estimate of the rotor time constant. The rotor time constant is estimated and updated by monitoring the rotor flux generated in response to the small signal oscillation. Based on a new estimate of the rotor time constant, the frequency of the small signal oscillation is updated and injected onto the d-axis current command. In this way, the controller continually interrogates the induction machine with the small signal injection, monitors the response, and updates estimates of the rotor time constant and frequency associated with the small signal oscillation.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of signal processing operations performed by a controller that employs field oriented control and on-line rotor time constant measurement to provide command signals to an induction machine.
FIG. 2 is a block diagram of signal processing operations for measuring the rotor time constant (Lr/Rr).
FIG. 3 is a block diagram of signal processing operations employed to calculate the Fourier component of the rotor flux vector resulting from the signal injection as shown in FIG. 2.
FIG. 4 is a block diagram of signal processing operations employed to calculate the rotor time constant based on the Fourier coefficients of the rotor flux vector shown in FIG. 3.



DETAILED DESCRIPTION
FIG. 1 is a block diagram illustrating signal-processing steps performed by controller 10 for generating command signals that are provided to induction machine 12 using field oriented control (FOC). Controller 10 may be implemented by a digital signal processor (DSP) or an equivalent device capable of performing the signal processing calculations shown. In the embodiment shown in FIG. 1, and discussed throughout the application, three-phase input is provided to induction machine 12. Based on torque (quadrature or q-axis) current command signal iq* and rotor flux (direct or d-axis) current command signal id*, which represent the desired amount of torque and rotor flux to be generated in induction machine 12, controller 10 generates voltage command signals va*, vb*, and vc* that are applied, through an inverter, to the stator of induction machine 12. In addition, the embodiment shown in FIG. 1 is implemented using indirect field oriented control, although other embodiments may make use of direct field oriented control.
In particular, the signal processing steps shown in FIG. 1 illustrate a method for making on-line (i.e., during operation) closed-loop measurements of the rotor time constant Lr/Rr associated with induction machine 12. To make on-line measurements of the rotor time constant Lr/Rr, the controller injects a small signal (oscillating) input id—inject onto the d-axis current command signal id—flux* at a selected frequency, and measures the response in the monitored rotor flux estimate λrde. The selected frequency of the small signal input id—inject represents the current estimate of the rotor time constant Lr/Rr (i.e., frequency of the small signal input in radians/sec is the inverse of the estimated rotor time constant). Based on the monitored d-axis rotor flux estimate λrde generated in response to the small signal input id—inject, the present estimate of the rotor time constant Lr/Rr can be updated. Based on the updated estimate of the rotor time constant Lr/Rr, the frequency of the small signal input id—inject is updated and supplied to the induction machine. In this way, the induction machine is continuously interrogated by the small signal input id—inject, and the resulting estimate of the rotor time constant Lr/Rr is continuously updated based on the measured response to the small signal input.
Signal processing components and steps of controller 10 for controlling the operation of induction machine 12 include torque or speed regulator 14, rotor flux regulator 16, signal summer 18, decoupled current regulator and d-q/abc transform 20 (“current regulator 20”), voltage abc/αβ transform 24, current abc/αβ transform 26, current αβ/dq transform 28, voltage model rotor flux open loop observer 30, rotor flux αβ/dq transform 32, rotor time constant (Lr/Rr) estimator 34, and slip calculator 36.
In general, FOC control of induction machine 12 involves using a torque command to generate a q-axis current command signal iq* (via torque regulator 14) and a rotor flux command to generate a d-axis current command signal id* (via a rotor flux regulator 16). These commands are used to regulate the actual induction machine currents measured and transformed into iqe and ide in the decoupled current regulator making use of the electrical frequency ωe (or hysteresis current regulator) 20. Current regulation in the dq frame is well known by those skilled in the art. In this way, control of the rotor flux and torque generated in induction machine 12 is done independent of one another through the d-axis and q-axis currents.
The output of decoupled current regulator 20 is a voltage command signals vq* and vd* (not shown) in the d-q reference frame, which are then converted to voltage commands va*, vb*, and vc* and sent to the inverter 22. The transformation of the commanded voltage signals vd* and vq* from the d-q reference frame to the abc reference frame uses an the position of the electrical frequency θe. The electrical frequency ωe and the position of the electrical frequency θe are generated by slip calculator 36 and are based on inputs that include the commanded (or estimated) rotor flux λrde, measured (or commanded) q-axis current iqe, rotor speed ωr, and rotor time constant Lr/Rr. The accuracy of calculations made by slip calculator 36 depends, in part, on the instantaneous accuracy of the rotor time constant Lr/Rr of induction machine 12.
In the embodiment shown in FIG. 1, both voltages (va, vb, and vc) and currents (ia, ib, and ic) generated in the stator portion of induction machine 12 are monitored. In other embodiments, the commanded voltage signals va*, vb*, and vc* and knowledge of the DC link voltage in inverter 22 may be used instead of measuring the output voltages directly. As shown in steps 24 and 26, respectively, the monitored voltages and currents are converted to the α, β reference frame (vα, vβ and iα, iβ, respectively). The monitored currents iα and iβ are further converted to the d-q reference frame at step 28, resulting in estimated torque current iqe and estimated rotor flux current ide. As discussed above with respect to converting from the d-q reference frame to the abc reference frame and converting from the αβ reference frame to the d-q reference frame, the angular position θe corresponding to electrical frequency ωe is used. The monitored rotor flux current ide and monitored torque current iqe values are provided to decoupled current regulator 20, as discussed above.
The monitored currents iα and iβ and the monitored voltages vα and vβ are employed by voltage-model rotor flux open loop observer 30 to calculate the rotor flux λrα and λrβ. The voltage-model rotor flux open loop observers are well known in the art for calculating rotor fluxes based on voltages and currents induced in the stator. A voltage model rotor flux open loop observer is employed in this embodiment, as opposed to a current model (which is also well known in the art), because voltage flux observers are not sensitive to the rotor time constant of induction machine 12.
The estimated rotor flux λrα and λrβ are then converted from the αβ reference frame to the dq reference frame (λrqe and λrde) at step 32. Once again, the position of the electrical frequency θe is required to convert from the αβ reference frame to the dq reference frame. The d-axis rotor flux estimate λrde is used by rotor time constant estimator 34 to calculate the rotor time constant Lr/Rr that is provided to slip calculator 36. In addition, the d-axis rotor flux estimate λrde is provided to slip calculator 36.
Rotor time constant estimator 34 is discussed in more detail with respect to FIGS. 2-4. In general, rotor time constant estimator 34 uses the d-axis rotor flux estimate λrde, and in particular the small signal portion of the d-axis rotor flux estimate λrde generated in response to the small signal input id—inject injected into the rotor flux current command signal id—flux*, to estimate the rotor time constant Lr/Rr. In particular, calculating the rotor time constant based on the flux response to the small signal input id—inject is based on the fact that because induction machine 12 is an inductive load, current injected at the proper frequency (i.e., correct estimate of the rotor time constant) will result in a flux signal that lags the injected current by forty-five degrees. Detecting changes in the phase of the flux signal allows for the detection of changes to the rotor time constant Lr/Rr. In this way, induction machine 12 is continuously interrogated by a small signal input id—inject having a frequency that represents the most recent estimate of the induction machine's rotor time constant Lr/Rr.
FIG. 1 therefore illustrates a method of implementing field oriented control (in this case, indirect field oriented control, although direct field oriented control could also be employed) that makes continual adjustments to the rotor time constant estimation in order to accurately calculate the slip frequency (and therefore the position of electrical frequency θe). FIGS. 2-4 illustrate in more detail an embodiment of a method employed to make on-line measurements of the rotor time constant Lr/Rr.
FIGS. 2-4 illustrate in an embodiment of steps performed by the controller in generating the small signal input id—inject to be added to the rotor flux current command signal id—flux* and calculating the rotor time constant Lr/Rr. As will be discussed below, the rotor time constant Lr/Rr is continuously updated by injecting the small signal input id—inject into the rotor flux current command signal id—flux* and measuring the d-axis rotor flux λrde generated as a result. In particular, FIG. 2 illustrates the two stages of calculations performed by rotor time constant estimator 34, Fourier coefficient calculation of the direct rotor flux component resulting from the variable signal current injection 40 (“Fourier coefficient calculator 40”), and rotor time constant Lr/Rr estimation and variable signal injection 42 (“time constant and signal injection calculator 42”). Fourier coefficient calculator 40 takes as input the estimated d-axis rotor flux λrde, the d-axis current command signal id—flux*, and cosine and sine signals provided by rotor time constant calculator 42. Based on these inputs, Fourier coefficient calculator 40 generates Fourier coefficients Aflux—dr and Bflux—dr, which reflect changes in the rotor time constant detected by analyzing the estimated d-axis rotor flux λrde.
In one embodiment (discussed in more detail with respect to FIG. 4), the injected small signal id—inject is generated by summing the cosine and sine signals generated by rotor time constant estimator 42. The frequency of cosine and sine signals (and thus, the frequency of injected small signal id—inject formed by summing the cosine and sine signals) is based on the latest estimate of the rotor time constant Lr/Rr. As a result of summing the cosine and sine signals, the injected small signal id—inject leads the sine signal by forty-five degrees and lags the cosine signal by forty-five degrees. Because the induction machine acts as an inductive load, the resulting flux (described in more detail with respect to FIG. 3, and labeled λrd—ac) generated in response to the injected small signal id—inject will lag the injected small signal id—inject by forty-five degrees (assuming the frequency of the injected small signal id—inject represents the correct estimate of the rotor time constant) and will be in phase with the sine signal used to generate the injected small signal id—inject. The resulting flux λrd—ac (shown in FIG. 3) generated in response to the injected small signal id—inject is monitored, and changes in the phase of the resulting flux are used to detect changes in the rotor time constant. In particular, Fourier component calculator 40 generates Fourier coefficients Aflux—dr and Bflux—dr in response to detected changes in the phase of the resulting flux λrd—ac.
The Fourier coefficients Aflux—dr and Bflux—dr are provided to time constant calculator 42, and represent detected changes in the phase of the AC component of the d-axis flux λrd—ac generated in response to the injected small signal id—inject. Based on these coefficients, time constant calculator 42 uses proportional-integral (PI) control to adjust the estimated time constant Lr/Rr. In modifying the rotor time constant estimation, the frequency of the small signal input id—inject injected as part of the rotor flux current command signal id* is also adjusted. Adjustments made to the frequency of the small signal input id—inject results in continued interrogation and estimation of the rotor time constant Lr/Rr.
FIG. 3 illustrates in more detail the processing steps employed by Fourier coefficient calculator 40 to calculate the Fourier coefficients Aflux—dr and Bflux—dr. In general, the processing steps shown in FIG. 3 act to isolate the portion of the d-axis rotor flux λrde generated in response to the small signal input id—inject injected into the d-axis current command signal id—flux*. That is, the alternating current (AC) component of the d-axis rotor flux λrde is isolated. Following the isolation of the AC component of the d-axis rotor flux (labeled here as λrd—ac) changes in the angle (i.e., the phase) of the AC component due to variations in the actual rotor time constant are detected. The change in phase of the AC component is represented by the Fourier coefficients Aflux—dr and Bflux—dr. In particular, these calculations are premised on the fact that flux generated in response to the injected small signal current id—inject will lag the injected small signal current id—inject by forty-five degrees if the frequency of the injected small signal current represents the correct estimate of the rotor time constant Lr/Rr. Therefore, the flux generated in response to the injected small signal current will be in phase with the sine signal used to generate the injected small signal current id—inject.
In the embodiment shown in FIG. 3, the d-axis current command signal id—flux is converted from a current representation to a flux representation (i.e., phase shifted based on inductance Lm) at step 44. The flux representation of d-axis current command signal id—flux is compared to the monitored d-axis rotor flux λrde at step 46. Because the commanded d-axis current does not contain the small signal injection, the comparison at step 46 removes the DC component of the d-axis rotor flux λrde, leaving the AC component (denoted here as λrd—ac) of the d-axis rotor flux λrde. In particular, the AC component of the d-axis rotor flux λrd—ac includes the flux response to the small signal injection id—inject. In addition, the AC component of the d-axis rotor flux λrd—ac may include unwanted noise that is filtered in subsequent steps.
At step 48, the AC component of the d-axis rotor flux λrd—ac is compared to a reconstructed d-axis rotor flux λreconstruct. In the embodiment shown in FIG. 3, the reconstructed d-axis rotor flux λreconstruct is the product of a closed-loop system for monitoring phase changes in the AC component of the d-axis rotor flux λrd—ac. Thus, the reconstructed d-axis rotor flux λreconstruct is an ideal or clean (i.e., very little noise) signal that represents the expected rotor flux response to the small signal input id—inject injected into the commanded d-axis current id—flux*. As shown in FIG. 3, the reconstructed rotor flux λreconstruct is based on the Fourier coefficients defined by Aflux—dr and Bflux—dr and cosine and sine signals, respectively, used to generate the injected small signal id—inject. Thus, if the frequency of the small signal input id—inject accurately reflects the actual rotor time constant of induction machine 12, then the reconstructed d-axis rotor flux λreconstruct should be in phase with the AC component of the d-axis rotor flux λrd—ac (i.e., both should lag the commanded d-axis current id—flux* by 45 degrees). If the AC component of the d-axis rotor flux λrd—ac is not in phase with the reconstructed d-axis rotor flux λreconstruct, the difference in phase will result in an error signal.
For example, in one embodiment the Fourier coefficient Aflux—dr may be initialized to a value of ‘0’ and the Fourier coefficient Bflux—dr may be initialized to a value of ‘1’. The result is a reconstructed rotor flux λreconstruct that is represented by only the sine signal component used to generate the small signal injection id—inject. Because the sine signal component lags the small signal injection id—inject by forty-five degrees, the reconstructed rotor flux λreconstruct can be compared to the AC component of the d-axis rotor flux λrd—ac to detect changes in the phase of the AC component of the d-axis rotor flux λrd—ac (i.e., the AC component of the d-axis rotor flux λrd—ac will lag or lead the injected small signal id—inject by more or less than forty-five degrees).
The error signal generated by comparing the AC component of the d-axis rotor flux λrd—ac to the reconstructed d-axis rotor flux λreconstruct at step 48 is multiplied by gain value K (box 54) and further multiplied by a cosine signal and a sine signal (the generation of these signals are shown in more detail in FIG. 4), respectively, at steps 50 and 52. Multiplying the error calculated at step 48 by the cosine signal at step 50 (along with the gain constant K) results in a signal having a DC component and a frequency component. Similarly, multiplying the error calculated at step 48 by the sine signal at step 52 (along with the gain constant K) results in a signal having a DC component and a frequency component. The Fourier coefficients Aflux—dr and Bflux—dr are generated at steps 56 and 58 by integrating the DC components generated at steps 50 and 52, respectively. The Fourier coefficients Aflux—dr and Bflux—dr are provided to time constant calculator 42, which as discussed in more detail with respect to FIG. 4, uses the Fourier coefficients Aflux—dr and Bflux—dr to estimate the rotor time constant.
In addition, the Fourier coefficient Aflux—dr is multiplied with the cosine signal provided by rotor time constant estimator 42 at step 60 and Fourier coefficient Bflux—dr is multiplied with the sine signal also provided by rotor time constant estimator 42 at step 62. The resulting sine and cosine signals are summed together at step 64 to generate the reconstructed d-axis flux λreconstruct. In closed-loop fashion, the reconstructed d-axis rotor flux λreconstruct is compared with the AC component of the d-axis rotor flux such that the response to the injected small signal id—inject is continuously monitored to detect changes in the phase of the flux response to the injected small signal id—inject. In this way, the goal of the closed system is to adjust the Fourier coefficients such that the reconstructed d-axis rotor flux λreconstruct equals the AC component of the d-axis rotor flux λrd—ac generated in response to the injected small signal id—inject.
FIG. 4 illustrates how the Fourier coefficients Aflux—dr and Bflux—dr, calculated in FIG. 3, are used to modify the estimated rotor time constant Lr/Rr as well as the frequency of the small signal input id—inject.
At step 70, an arctangent operation is performed on the Fourier coefficients Aflux—dr and Bflux—dr (specifically, arctan(Aflux—dr/Bflux—dr)), with the output expressed in radians and representing the phase difference between the reconstructed d-axis rotor flux λreconstruct and the AC component of the d-axis rotor flux λrd—ac generated in response to the injected small signal current id—inject. For example, in one embodiment, if there is no difference between the reconstructed d-axis rotor flux signal reconstruct and the reference frequency sinusoidal signal (i.e., they are in phase with one another), then Aflux—dr will equal ‘0’, with the arctangent operation resulting in a zero radian difference (i.e., in phase). If there is a difference between the two signals, then the difference will be represented by the result of the arctangent operation, and converted to degrees at step 72. The resulting phase difference between the two signals, expressed in degrees, is added to a constant ‘c’, which represents a correction offset, at step 76 and provided as an input to PI controller 78.
In general, PI controller 78 adjusts the estimate of the rotor time constant based on the phase difference provided as an input to PI controller 78. Adjusting the estimate of the rotor time constant results in an adjustment of the frequency of the small signal input id—inject provided to induction machine 12. PI controller 78 adjusts the initial frequency estimate until the phase difference calculated from Aflux—dr and Bflux—dr is minimized (i.e., until the error provided to the input of PI controller 78 is driven to zero). That is, PI controller 78 selectively controls the frequency estimate Ω, which represents the frequency of the small signal current id—inject and the inverse of the rotor time constant estimate Lr/Rr, until the rotor time constant estimate Lr/Rr is brought in line with actual circuit parameters of induction machine 12.
Specifically, output of PI controller 78 is added to an initial frequency estimate at step 84 to generate the frequency estimate Ω and the rotor time constant Lr/Rr, which is calculated by taking the inverse of the frequency estimate Ω at step 86. In addition, the frequency of the small signal current id—inject is modified to equal the new frequency estimate Ω at steps 88, 90, 92, 94, and 96. The frequency estimate Ω is provided to discrete integrator 88, and wrapped angle generator 90 (which generates a sawtooth signal that is provided as a timing input to PI controller 78), and then divided into cosine and sine components at steps 92 and 94, respectively. The sine and cosine components, generated at the frequency estimate Ω are provided in feedback to Fourier component calculator 40, while the small signal input id—inject (also generated at the new frequency estimate Q and advanced by forty-five degrees) is injected into the commanded d-axis current id—flux* as shown in FIG. 1. In this way, the method described with respect to FIGS. 1-4 provides for the continual interrogation of an induction machine by a small signal input id—inject at a frequency that represents a most recent estimate of the rotor time constant. The rotor flux generated in response to the small signal input id—inject is used to modify the most recent estimate of the rotor time constant, and to modify the frequency of the small signal input id—inject for subsequent interrogations of the induction machine.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. In particular, the processing steps discussed with respect to FIGS. 1-4 illustrate one method of generating the small signal oscillation and measuring the response to detect changes in the rotor time constant. The continuous interrogation of the induction machine to generate continuous estimations of the rotor time constant may be accomplished with a variety of signal processing architectures.

## Claims

1. A method for making on-line measurements of a rotor time constant associated with an induction machine, the method comprising:
a. interrogating the induction machine with a small signal oscillation generated at a frequency selected based on a most recent estimate of the rotor time constant;
b. monitoring the response of the induction machine by monitoring a rotor flux generated in response to the small signal oscillation;
c. detecting within the rotor flux a rotor flux component generated in response to the small signal oscillation;
d. detecting a phase associated with the rotor flux component generated in response to the small signal oscillation;
e. updating the most recent estimate of the rotor time constant and the frequency of the small signal oscillation based on the detected phase of the rotor flux component generated in response to the small signal oscillation; and
f. repeating steps a-e.

2. The method of claim 1, wherein interrogating the induction machine with a small signal oscillation includes:
injecting the small signal oscillation onto a d-axis current command signal.

3. The method of claim 1, wherein monitoring the response of the induction machine includes:
monitoring voltages and currents generated in the induction machine; and
calculating rotor flux generated in the induction machine based on the monitored voltage and currents using a voltage model rotor flux open loop observer.

4. The method of claim 1, wherein updating the most recent estimate of the rotor time constant and the frequency of the small signal oscillation includes:
generating a reconstructive rotor flux having a phase that is selected to minimize a difference in phase between the reconstructive rotor flux and the rotor flux component generated in response to the small signal oscillation;
calculating a phase difference between the rotor flux component generated in response to the small signal oscillation and the reconstructed rotor flux;
providing the phase difference to a proportional-integral (PI) controller that updates the most recent estimate of the rotor time constant, the frequency of the small signal oscillation, and the frequency of the reconstructive rotor flux based on the calculated phase difference; and
modifying the phase associated with the reconstructive rotor flux based on the calculated phase difference to minimize the difference in phase between the reconstructive rotor flux and the rotor flux component generated in response to the small signal oscillation.

5. The method of claim 1, further including:
calculating a position association with an electrical frequency used for field oriented control operations based, in part, on the most recent estimation of the rotor time constant.

6. A controller for controlling operation of an induction machine based on field oriented control, the controller including:
means for controlling inputs to a stator of an induction machine based on commanded d-axis and q-axis current command signals, a calculated position associated with an electrical frequency, and monitored feedback regarding the present value of d-axis and q-axis currents in the induction machine;
means for generating and injecting a small signal oscillation onto the d-axis current command signal, the small signal oscillation generated at a desired frequency;
means for estimating d-axis rotor flux generated in the induction machine in response to the small signal oscillation injected onto the d-axis current command signal;
means for updating an estimate of the rotor time constant based, in part, on the monitored d-axis rotor flux generated in response to the small signal oscillation;
means for comparing the d-axis rotor flux generated in response to the small signal oscillation to a reconstructed d-axis rotor flux generated based, in part, on previous comparisons between the d-axis rotor flux generated in response to the small signal oscillation and the reconstructed d-axis rotor flux;
means for detecting a phase difference between the estimated d-axis rotor flux and the reconstructed d-axis rotor flux; and
means for modifying the estimation of the rotor time constant based on the detected phase difference;
means for modifying the desired frequency of the small signal oscillation based on the present estimated value of the rotor time constant; and
means for adjusting the calculated position associated with the electrical frequency used to generate the inputs provided to the stator of the induction machine based, in part, on the present estimated value of the rotor time constant.

7. The controller of claim 6, wherein the means for estimating the d-axis rotor flux generated in the induction machine in response to the small signal oscillation includes:
means for estimating a d-axis rotor flux generated by the induction machine based on monitored voltages and current generated in the induction machine; and
means for comparing the estimated rotor flux with the d-axis current command signal to isolate the d-axis rotor flux generated in response to the small signal oscillation injected onto the d-axis current command.

8. The controller of claim 6, wherein the means for modifying the estimation of the rotor time constant and the means for modifying the desired frequency of the small signal oscillation include:
a proportional-integral (PI) controller that receives as input the detected phase difference between the d-axis rotor flux generated in response to the small signal oscillation and the reconstructed rotor flux and updates the estimate of the rotor time constant, thereby modifying the desired frequency of the small signal injection and the reconstructed rotor flux, such that the detected phase difference at the input of the PI controller is minimized.

9. The controller of claim 6, wherein the means for detecting a phase difference between the d-axis rotor flux generated in response to the small signal oscillation and the reconstructed rotor flux includes:
means for calculating Fourier coefficients based on the comparison between the d-axis rotor flux generated in response to the small signal oscillation and the reconstructed rotor flux;
means for calculating the phase difference based on the calculated Fourier coefficients; and
means for generating the reconstructed rotor flux based on the Fourier coefficients.

10. A method of making on-line measurements of a rotor time constant associated with an induction machine, the method comprising:
generating a small signal oscillation at a determined frequency;
providing the small signal oscillation to the induction machine as an input;
measuring rotor flux generated by the induction machine in response to the small signal oscillation;
estimating a present value of the rotor time constant based on a position of the measured rotor flux generated in response to the input small signal oscillation;
comparing the measured rotor flux generated in response to the small signal oscillation to a reconstructed rotor flux generated based, in part, on previous comparisons between the rotor flux generated in response to the small signal oscillation and the reconstructed rotor flux;
detecting a phase difference between the estimated rotor flux and the reconstructed rotor flux;
modifying the estimation of the rotor time constant based on the detected phase difference; and
modifying the determined frequency of the small signal oscillation based on the estimated rotor time constant.

11. The method of claim 10, wherein providing the small signal oscillation to the induction machine includes:
injecting the small signal oscillation into a d-axis commanded current signal; and
generating voltage command signals that are provided to the induction machine based, in part, on the d-axis commanded current signal with injected small signal oscillation.

12. The method of claim 10, wherein measuring rotor flux generated by the induction machine in response to the small signal oscillation includes:
monitoring voltage and current generated in the induction machine; and
measuring rotor flux based on the monitored voltage and current using a voltage model rotor flux open loop observer.

13. The method of claim 10, wherein modifying the estimation of the rotor time constant includes:
providing the detected phase difference to a proportional-integral (PI) controller that modifies the estimation of the rotor time constant to minimize the phase difference.

14. The method of claim 10, wherein detecting the phase difference between the estimated rotor flux and the reconstructed rotor flux includes:
calculating Fourier coefficients based on the comparison between the measured rotor flux and the reconstructed rotor flux; and
calculating the phase difference based on the calculated Fourier coefficients.

15. The method of claim 14, wherein the reconstructed rotor flux is generated based, in part, on the calculated Fourier coefficients.

16. The method of claim 15, wherein the reconstructed rotor flux is generated based, in part, on the estimate of the rotor time constant used to generate the small signal oscillation.

17. A controller for controlling an induction machine, the controller comprising:
means for interrogating the induction machine with a small signal oscillation generated at a frequency selected based on a most recent estimate of the rotor time constant;
means for monitoring the response of the induction machine by measuring a rotor flux generated in response to the small signal oscillation;
means for comparing the rotor flux component generated in response to the small signal oscillation to a reconstructed rotor flux generated in a closed loop to minimize a phase difference between rotor flux component generated in response to the small signal oscillation and the reconstructed rotor flux;
means for calculating Fourier coefficients based on the result of the comparison between the rotor flux component generated in response to the small signal oscillation and the reconstructed rotor flux; and
means for updating the most recent estimate of the rotor time constant and the frequency of the small signal oscillation based on the calculated Fourier coefficients; and
means for providing control instructions to the induction machine based, in part, on the most recent estimate of the rotor time constant.

18. The controller of claim 17, wherein the means for monitoring the response of the induction machine includes:
a voltage model open loop observer that measures rotor flux generated by the induction machine based on monitored voltages and current in the induction machine.

19. The controller of claim 17, further including:
means for updating the reconstructed rotor flux based, in part, on the calculated Fourier coefficients.

20. The controller of claim 17, further including:
means for updating the reconstructed rotor flux based, in part, on the updated frequency of the small signal oscillation.

21. The controller of claim 17, wherein the means for updating the most recent estimate of the rotor time constant and the frequency of the small signal oscillation includes:
means for calculating a phase difference between the rotor flux component generated in response to the small signal oscillation and the reconstructed rotor flux based on the Fourier coefficients; and
means for providing the phase difference to a proportional-integral (PI) controller that updates the most recent estimate of the rotor time constant and the frequency of the small signal oscillation based on the calculated phase difference.