Abstract:
An apparatus for determining angular positions of DC motor includes a capacitor connected in parallel with the DC motor. The current flowing through the capacitor, at any given time, is an AC ripple current responsive to a commutation event of the DC motor. The capacitor partially sources the motor current ripples during commutation and recharges itself during the off-commutation period. Since the number of commutation events per mechanical revolution is pre-determined once the DC motor is designed, the frequency of the AC ripple currents through the capacitor corresponds to the frequency of commutation, and thus a motor position of the DC motor.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to position detection and control system for a Direct Current (DC) motor. 
     2. Description of Related Art 
     Often position control of a DC motor requires feedback about the position of the motor shaft. Typically, a position sensor, such as an optical or Hall Effect encoder, or resolver is used to obtain the motor shaft position. The use of a position sensor increases the cost, size and weight of the system, and reduces the reliability and environmental compatibility of the system. For applications where the output speed of the motor is rather low, such as an actuator that consists of a DC motor and a reduction gear mechanism, a potentiometer is also commonly used to sense the position of the output shaft. This position sensing technique, however, is known to have poor position accuracy, is sensitive to environmental conditions such as temperatures, has poor durability due to the mechanical contact between the wiper and the resistive trace, and has high system cost due to additional wiring required between the motor and the controller. 
     Another known technique for obtaining motor position information is sensing the motor current directly for detecting and counting the commutation pulses as disclosed in U.S. Pat. No. 5,798,624, in which the current flowing through the lower legs of the H-bridge, same as the motor current, is monitored directly by a current sensing mechanism. The converted voltage signal of the sensed current is conditioned by using a band pass filter for extracting the commutation pulses and then fed to a pulse generator. The output of the pulse generator is then provided to a microprocessor for pulse counting to determine the motor position. Though the technique disclosed in U.S. Pat. No. 5,798,624 resolved several problems associated with the designs that use position sensors or potentiometers as mentioned above, it still suffers drawbacks. For example, it requires a special H-bridge if the sensor is located in the lower legs of the switches for capturing pulses during braking mode. Further, the system would require two sensors for bi-directional operations thereby increasing the cost of the system. If the sensor is located in the battery return, the system cannot capture commutation pulses in braking mode. In addition, the system has poor useful signal sensitivity/accuracy since the entire motor current including the main DC component is embedded in the sensor signal. The system may have pulse missing problems during start-up and stop coasting due to the use of a fixed band pass filter on the motor current signal. Further, the system may gain pulses due to brush bounces. Furthermore, since the main motor current goes through the current sensor, the system will have excessive voltage drops or power losses associated with the sensor. Also, the captured signal varies among production motors of the same design and over the life span of the same motor due to the use of current pulses associated with delayed commutation. 
     Yet another known technique for obtaining motor or actuator position information is sensing the motor terminal voltage directly for detecting and counting the commutation pulses as disclosed in U.S. Pat. No. 6,078,154, in which two high pass filters are used to capture the high frequency portion of the motor terminal voltage. The captured voltage signal is then fed through a low pass filter such that both DC component and high frequency noise in the sensed voltage signal are eliminated. The signal is further conditioned and fed to a pulse counter to determine motor position. This design solved the additional voltage drop and power loss problem that exists in U.S. Pat. No. 5,798,624. However, it still suffers significant drawbacks. The cost of the system is high due to the need for two current sources in the signal conditioning circuit and the need for a charge pump. Also, the system only works with MOSFET based H-bridge modules, not with bipolar transistors. The system may have pulse missing problems during motor start-up and stop due to the use of fixed-value high pass filters, and may have pulse gaining problems due to brush bounces. Furthermore, the captured signal may vary among production motors of the same design and over the life span of the same motor due to the use of current pulses associated with delayed commutation. 
     Still yet another known technique for obtaining motor position information is sensing the rate of change of motor current for detecting and counting the commutation pulses as disclosed in U.S. Pat. No. 6,437,533 B1. An inductor is placed at the lower side of the H-bridge to measure directly the rate of change of motor current as it flows through the lower legs of the H-bridge or through the battery return. The voltage across the inductor, L*(di/dt), is monitored, conditioned, and fed to a pulse generator circuit. The output pulse train is then provided to a microprocessor for pulse counting thereby obtaining the position of the motor. This design offers high signal sensitivity and may eliminate missing pulse problems during regenerative braking mode. However, this approach requires the use of a special H-bridge to separate the GND between the FWD and the transistor switches. The system cannot use MOSFET-based H-bridges, otherwise it will miss pulses during the braking mode. Furthermore, since the main motor current has to go through the sensing inductor, excessive voltage drops or power losses will be present if a small inductor is used. The sensed signal may vary among production motors of the same design and over the life span of the same motor due to the use of current pulses associated with delayed commutation. 
     It is therefore desirable to design a DC motor position detection and control system that will eliminate or minimize the drawbacks associated with the above-mentioned prior art systems. Preferably such a desirable system would not require special H-bridge, or motor, or additional power supplies, would not add additional voltage drops and power losses from the pulse sensing circuit, would have a high useful signal to sensed signal ratio, would not have pulse gaining problem due to brush bounces, would not have pulse missing problems during startup, regenerative braking, or stop modes of operations, would have consistent captured signals for high volume produced motors or over the life span of the same motor, and is independent of the EMI suppression designs. 
     In view of the above, it is apparent that there exists a need for an improved position detection and control system for a DC electric motor. 
     SUMMARY 
     In satisfying the above need, as well as, overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides an improved position detection system for a DC motor. 
     The position detection system eliminates shortcomings of the above-mentioned prior art systems. In accordance with the preferred embodiment of the present invention, a position detection system for obtaining motor position information includes a capacitive impedance element, shown as a capacitor, connected in parallel with the motor, and a capacitive current ripple detection apparatus. The capacitive current ripple detection apparatus captures the voltage across an impedance connected in series with the capacitor. The current ripple detection apparatus includes a signal conditioning circuit for filtering and amplifying the captured voltage signal, and a pulse generation circuit to generate a pulse train corresponding to motor communication events. The pulse train is provided to the input capture port of a microprocessor where position information is determined based on the pulse train. 
     The system provides a low cost option for obtaining the motor position information that does not require a special H-bridge, motor, or additional power supplies, by simply inserting a capacitive impedance element in parallel with the motor terminal. 
     The system provides a position sensing system that does not add excessive additional voltage drops and power losses by using capacitive sensing element in parallel with the main motor circuit such that main DC power does not go though the sensing circuit. 
     The system provides a position sensing system that will have a high useful signal to sensed signal ratio for high signal sensitivity and accuracy, which is inherently provided by the capacitor ripple currents. 
     The system provides a position sensing system that will not have pulse gaining problem due to brush bounces or pulse missing problem during startup, regenerative braking, or stop modes of operations, by inserting the sensing circuits directly across the motor terminals such that commutation events of the motor will not be missed by the sensing circuit. 
     The system provides a position sensing system that will have consistent captured signals for high volume produced motors or over the life span of the same motor and is independent of the EMI suppression designs, by not using pulses associated with delayed commutation. 
     Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematic drawing illustrating the preferred embodiment of the present invention; 
         FIG. 2  is a plot of motor current and terminal voltage associated to a DC motor with limited number of armature coils; 
         FIG. 3  is a plot demonstrating the relationship between commutation events or motor currents and current in the shunt capacitor; 
         FIG. 4  is a schematic drawing illustrating another preferred embodiment of the present invention; 
         FIG. 5  is a schematic drawing illustrating yet another preferred embodiment of the present invention; and 
         FIG. 6  is a schematic drawing illustrating yet another preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a system embodying the principles of the present invention is illustrated therein and designated at  10 . The system  10  includes a power electronics switching circuit  12 , a DC motor  14 , a capacitor  16 , and a ripple current detection apparatus  18 . A power source  20 , shown as an automotive battery, provides power to the switching circuit  12 . The switching circuit  12  is shown as an H-bridge switching circuit, the details of which will be discussed further below. The switching circuit provides voltage to drive the motor  14 . Resistor  22  and inductor  24  represent the effective resistance and inductance of the DC motor  14 , respectively. A combination of capacitor  16  and impedance  26  is in electrical parallel connection with the motor  14  and the switching circuit  12 . Further, an impedance  26  is connected in electrical series with the capacitor  16  across the motor  14 . A first node  52  of the current ripple detection apparatus  18  is connected between the capacitor  16  and a first side of the impedance  26 . A second node  54  of the current ripple detection apparatus  18  is connected to the second side of the impedance  26 . The impedance  26  may be a resistor as shown in  FIGS. 1 and 4 , or an inductor as shown in  FIG. 5 , or both as shown in  FIG. 6 . The capacitive ripple current detection apparatus  18  is in electrical communication with the impedance  26  to measure the voltage drop across the impedance  26  and thereby infer the current through the capacitor  16  and detect capacitive current ripples corresponding to motor commutation. The capacitive current ripple detection apparatus  18  is in electrical communication with a gate driver  62  to provide a feedback loop. Signals received from the capacitive current ripple detection apparatus  18  and the gate driver  62  are used to control the switching circuit  12  based on the commutation of the motor as determined by the current ripples through capacitor  16 . 
     Switching circuit  12  is an H-bridge power electronics converter, as known in the art. Though transistors ( 28 ,  30 ,  42 ,  44 ) and diodes ( 32 ,  34 ,  46 ,  48 ) are shown in  FIG. 1  and  FIGS. 4 to 6 , MOSFET power switches can be readily used to replace the combinations of transistor and diode pairs, as commonly done in the art. Switching circuit  12  has a first parallel branch including transistor  28 , transistor  30 , diode  32 , and diode  34 . Transistor  28  has a collector connected to the positive side of the power source  20  and an emitter connected to node  36 . Diode  32  is connected in electrical parallel connection with transistor  28  with the anode of diode  32  connected to node  36  and a cathode of diode  32  connected to the positive side of the power source  20 . Transistor  30  has a collector connected to node  36  and a emitter connected to the negative side of power source  20 . Diode  34  is connected in an electrical parallel connection with transistor  30  with the anode of diode  34  connected to the negative side of the power source  20  and the cathode of diode  34  connected to node  36 . Node  36  is connected to a first terminal  38  of motor  14 . The second terminal of motor  14  is in electrical communication with the second branch of the switching circuit  12 . 
     The second branch of the switching circuit  12  includes transistor  42 , transistor  44 , diode  46 , and diode  48 . The collector of transistor  42  is connected to the positive side of the power source  20  and the emitter of transistor  42  is connected to node  50 . Diode  46  is connected in electrical parallel connection with transistor  42  with the anode of diode  46  connected to node  50  and the cathode of diode  46  connected to the positive side of the power source  20 . Transistor  44  has a collector connected to node  50  and an emitter connected to the negative side of power source  20 . The diode  48  is connected in electrical parallel connection with transistor  44  with the anode of diode  48  connected to the negative side of power source  20  and the cathode of diode  48  connected to node  50 . Node  50  is in electrical communication with the second terminal of the motor  14  through resistor  22  and inductor  24 . Further, capacitor  16  and impedance  26  are connected in electrical series connection forming a branch in parallel with the motor  14  between the first and second terminal  38 ,  40 . A first node  52  of the current ripple detection apparatus  18  is connected between the capacitor  16  and a first side of the impedance  26 . A second node  54  of the current ripple detection apparatus  18  is connected to the second side of the impedance  26 . 
     To detect the ripple current through capacitor  16 , node  52  and  54  are connected to a filter and amplifier circuit  56  of the current ripple detection apparatus  18 . The filter and amplifier circuit  56  measures the ripple current through the capacitor  16  by detecting a voltage across the impedance  26 . The voltage signal is then filtered to eliminate high frequency noise. With the high frequency noise removed, the signal is amplified and the conditioned voltage signal is provided to a pulse generator circuit  58 . The pulse generator circuit  58  generates a square wave pulse train corresponding to the commutation events of the motor  14 . Though not necessary, it is preferred that the filter and amplifier circuit  56  is designed such that the pulse generator circuit does not respond to ripples associated with the post motor commutation pulses shown as reference numeral  76  in  FIG. 2  and will be explained later. The output of the pulse generator circuit  58  is provided to an input capture port of a microprocessor  60 . The microprocessor  60  counts the pulses and applies a position control algorithm that translates the pulse count information into motor position information to determine motor position. Accordingly, the motor position is provided to a feedback position control algorithm that is used to provide control signals to a gate driver  62 . The gate driver  62  actuates the switching circuit  12  based on the control signals from the microprocessor  60  providing feedback position control of the motor. 
     For a DC motor with a small number of armature coils, there are pulse ripples in both the motor current waveform  72  and terminal voltage waveform  78  as illustrated in  FIG. 2 . The pulses in the terminal voltage waveform  78  are caused by delayed commutation of the armature coils. There are two types of pulses in the motor current waveform  72 , intra commutation pulses  74  and post commutation pulses  76 . Post commutation pulses  76  are mainly “narrow” spikes and contain very low electric energy but with rich high frequency harmonics. Post commutation pulses  76  generally are not consistent among high volume motors and are not consistent over the life span of the motor. The magnitude of post commutation pulses  76  are proportional to both speed and load of the motor. Intra commutation pulses  74  contain comparable amount of electric energy as the main electric power draw and do not contain much high frequency harmonics. The magnitudes of intra commutation pulses  74  mainly depend on the speed of the motor. Generally, intra commutation pulses  74  are consistent among high volume produced motors and over the life span of the same motor. 
     The frequency, f c , of both post commutation pulses  76  and intra commutation pulses  74  are identical and are given by EQ. 1. 
                     f   c     =       p   ⁢           ⁢     n   c     ⁢     n   m       30             EQ   .           ⁢   1               
where
 
     p=number of pole pairs; 
     n c =Number of armature coils; 
     n m =motor speed in RPM; 
     Therefore, if the frequency of either of the pulses are detected, the motor speed and motor position can be determined. 
     To further promote the understanding of the operating principle of the present invention,  FIG. 3  illustrates the motor current waveform  72  and a capacitor current waveform  80  for the schematics given in  FIG. 1 . If the capacitance of capacitor  16  is sufficiently high, capacitor  16  can source the entire ripple portion of the motor current. Whereas the current draw from the power supply  20  is maintained ripple-free. During the time intervals between the commutation events, capacitor  16  will be charged by the power supply  20 . Based on this principle, the current in capacitor  16  sources the ripple or pulse currents caused by electric commutation of the motor  14  and can truly reflect the occurrence of a motor commutation event. 
     Now referring to  FIG. 4 , another configuration of the circuit provided in  FIG. 1  is provided. The circuit in  FIG. 4  is similar to the previously described circuit in  FIG. 1 , however, the connection of the current ripple detection apparatus  18  has been modified. The first node  52  of the current ripple detection apparatus  18  is connected between capacitor  16  and impedance  26 , shown as a resistor. The second node  54  is connected to the negative side of power source  20 . 
     Now referring to  FIG. 5 , another configuration of the circuit provided in  FIG. 1  is provided. The circuit in  FIG. 5  is similar to the previously described circuit in  FIG. 1 , however, impedance is shown as inductor  82 . The first node  52  of the current ripple detection apparatus  18  is connected between capacitor  16  and a first side of inductor  82 . The second node  54  is connected to the other side of inductor  82 . 
     Now referring to  FIG. 6 , the circuit provided is similar to the circuit in  FIG. 1 , however, the impedance is shown as inductor  82  and resistor  84 . Further, the connection of the current ripple detection apparatus  18  has been modified. The first node  52  of the current ripple detection apparatus  18  is connected between capacitor  16  and inductor  82 . The second node  54  is connected to the negative side of power source  20 . 
     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.