Abstract:
A method and apparatus is herein disclosed for determining the motor performance of an electric motor through monitoring the temperature of rotor magnets. The temperature is determined by initially heating the rotor magnets to two known temperatures and subsequently recording the temperature at each known temperature. From the known and recorded temperatures, an offset, if any, is calculated. An actual temperature is then determined from the subsequent temperatures of the magnets and the offset. The actual temperature is then used as a basis to determine the motor performance.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to the field of electric motors. More particularly, the present invention relates to determining electric motor performance using a sensed internal temperature of the motor. 
   BACKGROUND OF THE INVENTION 
   Conventional brushless direct current (DC) motors rely on the magnetic flux created by permanent magnets located on the rotor interacting with magnetic fields from the stator to generate a mechanical torque. Indeed, the output mechanical torque generated by a brushless DC motor is directly proportional to the magnetic flux density of the rotor magnets. Often, performance characteristics of a brushless DC motor are evaluated based on the output mechanical torque generated by the motor as a function of the input stator current. In many applications, it is critical to accurately determine the output mechanical torque produced by a motor for a known stator current. 
   The magnetic flux of the rotor magnets and its relationship with the magnetic fields induced by the stator current is a function of the motor temperature. It is well known that the magnetic flux density of magnetic materials (i.e., rotor magnets) decreases as temperature increases, resulting in degradation of motor performance. Herethereto, conventional approaches to this problem have been to simply recognize a performance degradation during high-temperature operation and attempt to try to regulate the ambient temperature, or to recommend only certain operating temperature conditions. 
   It would therefore be desirable to provide systems and methods for accurately sensing the temperature of the rotor magnets to provide more accurate output torque information. 
   SUMMARY OF THE INVENTION 
   The foregoing needs are met, to a great extent, by the present invention, wherein in one aspect an apparatus is provided to sense the temperature of the rotor magnets of a brushless DC motor. 
   In accordance with one embodiment of the present invention, an electric motor system is provided having an electric motor with a temperature sensor mounted inside the motor capable of measuring local temperature and a processor that utilizes a temperature signal from the temperature sensor to determine an output mechanical torque generated by the motor. 
   In accordance with another embodiment of the present invention, a centrifuge system is provided, comprising an electric motor having at least one temperature sensor, a motor shaft, and a specimen holder connected to the motor shaft a processor in communication with the temperature sensor to determine an output mechanical torque generated by the motor, 
   In accordance with another embodiment of the present invention, a method is provided for determining the output mechanical torque generated by an electric motor having rotor magnets. The method comprises the steps of sensing local temperature at a location inside the motor and calculating an output mechanical torque generated by the motor based on the determined temperature. 
   In accordance with yet another embodiment of the present invention, a system is provided for determining the output mechanical torque generated by an electrical motor having rotor magnets. The system comprises means for sensing local temperature at a location inside the motor, and means for calculating an output mechanical torque generated by the motor based on the determined temperature. 
   There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
   In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
   As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a conventional brushless DC motor. 
       FIG. 2  is a temperature vs. magnetic flux density graph for four permanent magnet materials. 
       FIG. 3  is a cross-sectional view of a brushless DC motor illustrating exemplary temperature sensor locations according to an embodiment of this invention. 
       FIG. 4  is a block diagram of an exemplary temperature sensor device. 
       FIG. 5  is a flowchart illustrating an exemplary process for determining performance characteristics of a brushless DC motor. 
       FIG. 6  is a block diagram of an exemplary centrifuge. 
   

   DETAILED DESCRIPTION 
   In some preferred embodiments, the invention provides a system and method that determines electrical motor performance using at least one sensed internal temperature of the motor. Preferably, the temperature of one or more rotor magnets is sensed. Preferred embodiments of the invention will now be described with reference to the drawing figures in which like reference numbers refer to like elements throughout. 
   FIG.  1 . is a cross-sectional view of a conventional brushless DC motor  10  having a rotor  12 , rotor magnets  14 , a stator  16 , and a motor housing  18 . An output mechanical torque is produced on the rotor  12  as a result of the interaction of the magnetic flux of the rotor magnets  14  and the rotating magnetic flux induced by the stator current. These components of brushless DC motors are well known in the art, and therefore for the purposes of this discussion is not further discussed herein. 
   Typically employed magnetic materials experience a decrease in flux density as the temperature of the magnetic material increases. Therefore, as the temperature of the rotor magnets  14  increases, the magnetic flux density Br of the rotor magnets  14  decreases. 
     FIG. 2  is a graphical illustration of the relationship between magnetic flux density and temperature over the range 0-120° C. for four permanent magnet materials commonly employed as rotor magnets in brushless DC motors. Over the operating range of most brushless DC motors, for example 0-140° C., the inverse relationship between temperature and magnetic flux density is considered to be generally linear for most permanent magnet materials. From  FIG. 2 , it is evident that ceramic ferrite  20  experiences a drop in magnetic flux density of 0.2% per degree Celsius over the range 0-120° C. Similarly, fully dense Nd 2 Fe 14 B,  22 , exhibits a drop in magnetic flux density of 0.10% per degree Celsius, where sintered SmCo 5 ,  24 , exhibits a drop in magnetic flux density of 0.045% per degree Celsius, and sintered Sm 2 Co 17    26  exhibits a drop in magnetic flux density of 0.03% per degree Celsius over the range 0-120° C. 
   Although  FIG. 2  illustrates the relationship between magnetic flux density and temperature for four permanent magnet materials commonly employed as rotor magnets in brushless DC motors, it should be appreciated that other permanent magnet materials can be used as rotor magnets as deemed suitable by one ordinary skill in the art. 
   Often, motor performance characteristics are evaluated based on the output mechanical torque τ generated as a function of input stator current I S . Moreover, in many applications, it is critical to accurately determine the output mechanical torque τ of a brushless DC motor  10  as a function of stator current I S . For example, in a centrifuge system energy calculations are based on assumed motor torque based on the input stator current I S . Calculations of acceleration rates, deceleration rates, system energy, and rotational inertia are based on an accurate estimate of the output mechanical torque. If these calculations are incorrect because of an inaccurate output mechanical torque, then the centrifuge material may be improperly centrifuged. 
   It is well known in the art that the output mechanical torque of a brushless DC motor  10  is directly proportional to I S . This relationship is expressed empirically as
 
τ=k t I s ,   (Eq. 1)
 
where the torque constant k t  of the motor  10  is a function of and directly proportional to the magnetic flux density B of the rotor magnets  14 . Therefore, because an increase in the temperature of the rotor magnets  14  causes a decrease in the magnetic flux density B r  of the rotor magnets  14 , an increase in temperature of the rotor magnets  14  causes a decrease in the value of the torque constant k t . Thus, the torque constant k t  is inversely proportional to the temperature of the rotor magnets  14 . Consequently, an increase in the temperature of the rotor magnets  14  results in a diminished output mechanical torque τ. Therefore, knowing the temperature of the rotor magnets  14  permits determination of the output mechanical torque τ, of a brushless DC motor.
 
   In order to determine the relationship between the temperature of the rotor magnets  14  and the output mechanical torque τ of a brushless DC motor  10 , a first step is to determine the maximum value of the torque constant k t  of the motor  10 . The maximum value of the torque constant k t  is the value of the torque constant k t  of the cold motor  10  operating at room temperature (20° C.). The torque constant k t  of a brushless DC motor  10  is proportionally equivalent to the voltage constant k E  of the back electromotive force (EMF) of the motor. Accordingly, the voltage constant k E  value of the motor  10  is directly related to the magnetic flux density B r  of the rotor magnets  14  and is thus inversely proportional to the temperature of the rotor magnets  14 . Therefore, once the voltage constant k E  value of a cold brushless DC motor  10  operating at 20° C. is known, a simple conversion of units yields the maximum torque constant k t , in-lbs/amp, of the motor  10 . 
   As disclosed in U.S. Provisional Patent Application 60/381,824, filed May 21, 2002 titled “Back EMF Measurement to Overcome the Effects of Motor Temperature Change”, the disclosure of which is hereby incorporated by reference in its entirety, the voltage constant k E  value of a brushless DC motor  10  is readily determined by driving the rotor with a second motor and measuring the back EMF (i.e., the voltage across two stator phases) and the revolutions per minute (RPM) of the rotor. The torque constant of the motor  10  is then easily calculated from the voltage constant k E  value of the motor  10 . 
   Equipped with the maximum torque constant k t  of a brushless DC motor  10 , the relationship between the temperature of the rotor magnets  14  and the output mechanical torque τ is readily determined. Due to the inverse relationship between magnetic flux density B r  and magnet temperature and the direct relationship between output mechanical torque τ and the magnetic flux density B r  of the rotor magnets  14 , the percent decrease in output mechanical torque τ for a brushless DC motor  10  operating with rotor magnets  14  at a particular temperature T M1 , for example, is given by
 
Δτ=( T   M   −T   M1 )·(Δ B   r ),   (Eq. 2)
 
where Δτ represents the percent decrease in output mechanical torque, T M  represents the current temperature of the rotor magnets  14 , T M1  represents the temperature at the first test point, and ΔB r  represents the percent decrease in magnetic flux density of the permanent magnet material used for the rotor magnets  14 . Using the result of Eq. 2, the percent of motor torque remaining at a particular temperature τ remaining  is then calculated from
 
τ remaining =(100−Δτ).   (Eq. 3)
 
Finally, from Eq. 1, the actual output torque τ of the motor  10  for a known stator current I S  and particular rotor magnet  14  temperature is found from
 
 τ=[ k   t(20° C.)   I   S ]·τ remaining    (Eq. 4)
 
   In a preferred embodiment of the present invention, it is possible to accurately determine the temperature of the rotor magnets  14  of a brushless DC motor  10  using sensors mounted inside of the motor  10 . FIG.  3 . is a cross-sectional view of a brushless DC motor  30  illustrating exemplary locations for mounting temperature sensors. As demonstrated by  FIG. 3 , temperature sensors  32  can be mounted on the commutation board  34  or in the motor housing  36  as close to the rotor magnets  14  as reasonably possible to obtain a relatively accurate temperature. Additionally, the temperature sensor(s)  32  maybe situated adjacent to the stator  16 , as desired. It should be appreciated by one of ordinary skill in the art that the temperature sensors  32  can be located in other positions, as according to design preferences, without departing from the scope and spirit of this invention. That is, the temperature sensor(s) may be placed at any position inside the envelope of the motor. 
     FIG. 4  depicts a block diagram of an exemplary temperature sensor circuit  40  according to this invention. A temperature sensor  42 —preferably, but not necessarily, an integrated circuit (IC) type sensor—is used to determine the local temperature at the sensor position inside the motor  10 . It should be apparent that though this preferred embodiment employs the use of an IC-type sensor to sense the rotor magnet  14  temperature, other devices capable of sensing temperature may be used as deemed suitable by one of ordinary skill in the art, such as, for example, optical, chemical, pressure, methods or schemes that are directly or indirectly capable of detecting temperature or changes in temperature. 
   In operation, if the output of the temperature sensor  42  is a digital signal, the temperature signal is passed directly to logic/decision device  46 , illustrated here as a processor. If the output of the temperature sensor  32  is an analog signal, the signal is fed to the analog-to-digital (A/D) converter  44 . The converted digital temperature signal is then passed from the A/D converter  44  to the processor  46 . The processor  46  is then used to determine the actual temperature of the rotor magnets  14 . While  FIG. 4  is discussed in the context of using digital signals or digital processing, it should be appreciated that a completely analog, hybrid, or analog-digital system may be used without departing from the spirit and scope of the invention. 
   In order to determine with a described accuracy the temperature of the rotor magnets  14  from the temperature signal relayed by the temperature sensor  42 , the offset between the local temperature sensed by the sensor  42  and the actual temperature of the rotor magnets  14  may be first determined through experimental measurements or an initial calibration or preset. To determine the offset of the rotor magnet  14  temperature from the temperature sensor  42  readings, the rotor magnets  14  may be heated to at least two different known temperatures, T M1  and T M2 , and the corresponding temperatures measured by the temperature sensor  42  T S1 , and T S2 , respectively, would be recorded. Using this data and assuming that the offset of the temperature sensor  42  readings from the rotor magnet  14  temperature exhibits a linear relationship, it is possible to accurately determine the temperature T M  of the rotor magnets  14 , for a temperature sensor  42  reading T S  using the expression
 
 T   M =[( T   M2   −T   M1 )/( T   S2   −T   S1 )]· T   S   +T   M2 −[( T   M2   −T   M1 )/( T   S2   −T   S1 )]· T   S2 .  (Eq. 5)
 
   It should be appreciated that although this embodiment uses a linear interpolation algorithm to account for the offset of the rotor magnet  14  temperature from the temperature sensor  42  reading, other algorithms, whether linear or non-linear, for determining the offset of the rotor magnet  14  temperature from the temperature sensor  42  reading may be used as deemed suitable by one of ordinary skill in the art. 
   After determining the actual temperature of the rotor magnets  14 , the processor  46  is used to determine the output mechanical torque τ for the known input stator current I S  and the determined rotor magnet  14  temperature using Eqs. 2-4. From the determined value of the output mechanical torque, the processor  46  can be used to calculate other performance characteristics of the motor  10 , including, but not limited to acceleration rates, deceleration rates, system energy of an unknown load. 
     FIG. 5  is a flowchart illustrating an exemplary process  50  for determining changes in the performance characteristics of a motor  10  according to this invention. The exemplary process  50  begins at step  52 , whereby one or a plurality of temperature sensors  42  mounted inside of the motor  10  are used to sense the local temperature at the designated sensor position(s) at step  54 . Using the reading of the temperature sensor(s)  42  at step  54 , the exemplary process  50  proceeds to step  56  whereby the actual rotor magnet  14  temperature is determined according to the process described in  FIG. 4 , or any other suitable process. Once the actual temperature of the rotor magnets  14  has been accurately determined in step  56 , the output torque of the motor  10  is calculated in step  57  based on the input stator current and the accurately determined rotor magnet  14  temperature from step  56 . The process  50  then proceeds to step  58  where performance characteristics of the motor  10  can be calculated. Such performance characteristics can include, but are not limited to acceleration rates, deceleration rates, system energy, or the rotational inertia of an unknown load. After the completion of step  58 , the exemplary process  50  may proceed to step  59  to end the process, or optionally cycle to step  54  and repeat itself periodically or aperiodically, as desired. 
   While  FIG. 5  illustrates one exemplary process for determining changes in performance characteristics of an electric motor, it should be appreciated by one of ordinary skill in the art that other processes can be employed to use the data collected by the temperature sensor  42  to determine changes in performance without departing from the spirit or scope of this invention. For example, the order of the steps in  FIG. 5  could be rearranged, the number of steps could be reduced, or additional steps could be added. 
   Furthermore, although  FIGS. 3-5  describe the use of a temperature sensor  42  to determine the temperature of the rotor magnets  14 , of a motor, it should be appreciated by one of ordinary skill in the art that the temperature sensor  42  could also be used to accurately determine the temperature of other components of the motor that affect motor performance such as, but not limited to, the stator  16  or motor housing  18  temperatures without departing from the spirit and scope of this invention. 
   In addition, while the above figures illustrate the invention as being described in the context of sensing rotor magnet  14  temperatures in a brushless DC motor  10 , it should be appreciated by one of ordinary skill in the art that the invention could also be used to accurately determine magnet temperature in other types of permanent magnet electric motors. 
   Electric motors are often used in centrifuges, such as for example laboratory centrifuges.  FIG. 6  is a block diagram of an exemplary centrifuge  60  according to this invention. The exemplary centrifuge  60  has a motor  62 , turntable shaft  64 , and centrifuge rotor  66 . The output torque generated by the centrifuge motor  60  drives the turntable shaft  64  which in turn causes the centrifuge rotor  66  to rotate. In the exemplary centrifuge system  60 , energy calculations are based on the motor torque which is calculated from sensing the rotor magnet temperature according to systems and methods according to this invention. The systems and methods described above may be used to increase the accuracy of the estimate of the output mechanical torque generated by the motor, thus increasing the accuracy of the energy calculations for centrifuge applications. Therefore, parameters such as acceleration rates, deceleration rates, system energy, and rotational inertia, are accurately determined based on the output mechanical torque generated by the motor  60 . 
   Furthermore, it should be appreciated by one of ordinary skill in the art that other uses and functions may be arrived at by utilizing the internal temperature information or flux determination. For example, if the temperature sensed by the temperature sensors is over a predetermined over temperature value, the exemplary centrifuge  60  may initiate a shutdown or recovery operation. Thus, in addition to accurately determining the mechanical torque (or other temperature affected metrics), safety considerations may be exploited. 
   The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.