Abstract:
A power supply ( 20 ) for providing a desired current to an electric load ( 23 ) broadly comprises: a current limiter ( 21 ) for providing a desired current; a current sensor ( 28 ) for determining the actual current supplied by the current limiter and providing a feedback signal ( 30, 31 ) to the current limiter for causing the current limiter to reduce the error between the desired and the actual currents; and a test circuit ( 24 ) for temporarily changing the actual current sensed by the current sensor to a level different from the desired current for causing the current limiter to adjust the magnitude of the current provided thereby; whereby the operation of the power supply may be tested. The invention also provides an improved method of operating such a power supply.

Description:
TECHNICAL FIELD 
     The present invention relates generally to power supplies with current-limiting for electrical loads, and methods of operating same. The present invention also relates to power supplies for electric motors with a torque-limiting capability, and to methods of operating same. 
     BACKGROUND ART 
     Prior power supplies have made use of current-limiting circuitry. For example, U.S. Pat. No. 5,457,364 discloses an electric motor driver with a current-limiting mechanism. However, no known prior art power supply system has made use of an in-circuit method of rapidly testing whether the current-limiting circuitry was functioning correctly. In order to test whether the current-limiting circuitry was properly cutting back current at the appropriate value, external equipment, such as voltage and/or current probes, needed to be attached test the power supply. 
     It is also known that electric motor systems have employed mechanical torque limiters. However, such torque limiters are unreliable, take up space, are heavy, and/or are not cost effective. Further, the functionality of such systems cannot be easily verified in the field. 
     DISCLOSURE OF THE INVENTION 
     With parenthetical reference to corresponding parts, portions or surfaces of the disclosed embodiment, merely for purposes of illustration and not by way of limitation, the present invention provides power supplies for providing a desired current to an electric load that is adapted to easily self-test the current-limiting functionality of the power supply, and to methods for operating such power supplies. 
     The power supply ( 20 ) broadly comprises: a current limiter ( 21 ) for providing a desired current; a current sensor ( 28 ) for determining the actual current supplied by the current limiter and providing a feedback signal ( 30 ,  31 ) to the current limiter for causing the current limiter to reduce the error between the desired and the actual currents; and a test circuit ( 24 ) for temporarily changing the actual current sensed by the current sensor to a level different from the desired current for causing the current limiter to adjust the magnitude of the current provided thereby; whereby the operation of the power supply may be tested. 
     The current sensor may include at least one resistor ( 40 , 41 ), and may also produce more than one feedback signal ( 30 ,  31 ). 
     The current limiter may include a differential amplifier ( 45 ) which is configured to measure the difference between two feedback signals ( 30 ,  31 ). 
     The current limiter may include a first transistor ( 67 ). The current limiter may further include a second transistor ( 64 ) for driving the first transistor. 
     The current limiter may include a comparator circuit ( 54 ). 
     The current limiter may further include a reference voltage ( 15 ) as an input to the comparator circuit. 
     The current limiter may include a one-shot pulse generator ( 60 ) configured to shut off the power supply for a period of time after the actual current is found to be greater than the desired current. 
     The power supply may further include a low-pass filter configured to filter the current sensor feedback signal. 
     The test circuit may include a transistor ( 74 ), which may be adapted to rapidly ground the electrical load when the actual current is greater than the desired current. 
     The test circuit may include an operational amplifier circuit ( 85 ). The operational amplifier circuit may include a reference voltage ( 86 ). 
     The electric load may be an electric motor ( 23 ). 
     The power supply may further include a torque sensor ( 100 ) configured to measure the torque output of the electric motor and to provide a second feedback signal ( 101 ) to the current limiter for causing the current limiter to adjust its current output if the second feedback signal is not within a desired range. 
     In another aspect, the invention provides an improved method of operating a power supply ( 20 ) to an electric load ( 23 ) that has a current limiter ( 21 ) for providing a desired current, a current sensor ( 28 ) for determining the actual current supplied by the current limiter, and a feedback signal ( 30 ,  31 ) provided from the current sensor to the current limiter to allow the current limiter to reduce the error between the desired and the actual currents. The improvement broadly comprises the steps of: providing a test circuit ( 24 ); operating the test circuit to temporarily change the actual current sensed by the current sensor to a level different from the desired current; and determining that the current limiter properly adjusts in response to operation of the test circuit; thereby to test the operational integrity of the power supply. 
     The step of providing a test circuit may include the step of placing a transistor ( 74 ) in parallel with the electric load. 
     The step of providing a test circuit may include using an operational amplifier circuit ( 85 ) with feedback to control the operation of the test circuit. 
     The step of operating the test circuit may further include the steps of: providing a reference voltage ( 86 ); comparing the reference voltage to the feedback signal ( 31 ) to provide a result; and adjusting the current flowing to the current sensor as a function of the result. 
     The step of providing the test circuit may further include the step of: activating the transistor ( 74 ) of the test circuit when the actual current is greater than the desired current in order to rapidly reduce the electrical drive to the electrical load. 
     The electric load may include an electric motor, and the power supply may further include the steps of: measuring the output torque of the electric motor with a torque sensor ( 100 ); providing a second feedback signal ( 101 ) from the torque sensor to the current limiter; and adjusting the current flowing from the current limiter as a function of the second feedback signal. 
     Accordingly, the general object of the invention is to provide an improved power supply, having a testable current limiter, for an electric load, such as an electric motor. 
     Another object is to provide a current-limiting power supply which can be easily tested. 
     Another object is to provide a fault-tolerant current-limiting power supply. 
     Another object is to provide a power supply for an electric motor which limits the maximum torque delivered by the electric motor. 
     Another object is to provide a power supply for an electric motor which limits the maximum torque delivered by the electric motor, which power supply is lighter and more reliable than mechanical torque limiters would allow. 
     Another object is to provide a method of testing a power supply that has a current limiter by providing a test circuit. 
     These and other objects and advantages will become apparent from the foregoing and ongoing written specification, the drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a first embodiment of the present invention. 
         FIG. 2  is a circuit diagram of the first embodiment shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed description is an integral part. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this invention. As used in the following description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate. 
     Referring now to the drawings, and more particularly to  FIG. 1  thereof, this invention provides an improved power supply, of which a first embodiment is generally indicated at  20 . As shown, power supply  20  generally includes a voltage source  19  which provides power to a current limiter  21 . The power output  22  of the current limiter  21  provides current to a motor drive and control unit  23  and to a test circuit  24 . The status of whether current limiter  21  is actively providing power is read and indicated by the current limiter status line  35 . The test circuit&#39;s output  25  and the motor drive and control unit&#39;s output  26  are connected to the input  27  of a current sensor unit  28 . 
     The current sensor unit  28  is connected to ground  29 . All current flowing from the current limiter  21  will pass through the current sensor unit. The current sensor unit  28  will measure the total amount of current passing through it to ground, which is equivalent to the total actual current supplied by the current limiter  21 . The current sensor unit  28  will output the magnitude of the current measurement on the hi- and lo-feedback signals  30 ,  31 , respectively, which are provided to the current limiter  21 . Feedback signal  31  is also provided to the test circuit  24 . 
     The current limiter  21  compares the actual current measurement, represented in feedback signals  30 ,  31 , to a predetermined desired current level. If the actual current is greater than the desired current level, then the current limiter  21  will cut off its current output for a period of time, and will indicate this status through an overcurrent output signal  32  which connects to the test circuit  24 . Upon receiving the overcurrent signal  32 , the test circuit may help reduce the current flow from the current limiter  21  into the motor drive and control unit  23  by providing a low resistance path from the current limiter  21  to the current sensor unit  28 . The test circuit&#39;s low resistance path to ground also effectively provides the input  33  of the motor drive and control unit  23  with a path to ground, which may be important if the motor drive and control unit is a highly inductive or capacitive load. 
     The test circuit  24  also contains an input to read a run self-test control signal  34 . When active, the run self-test signal will cause the test circuit  24  to draw current from the current limiter  21 . This will cause a current greater than the desired current level to flow from the current limiter  21 , through the test circuit  24 , and through the current sensor unit  28  to ground. The test circuit  24  will ensure that the actual current flowing in the current sensor unit  28  is greater than the desired current level. The current sensor unit will measure the actual current, and indicate its magnitude on feedback signals  30 ,  31 , which will relay the information to the current limiter  21 . The current limiter will compare the actual current measurement value on the feedback signals to the desired current level and see that the actual current level is greater than the desired current level. This will cause the current limiter  21  to shut off its current output, which is indicated by the current limiter status line  35 . 
     As shown in  FIG. 2 , the current sensor unit  28  may be implemented as a first resistor  40  connected in series with a second resistor  41 , which is then connected to ground  36 . The current sensor resistors  40 ,  41  should have a low resistance, such as 1 ohm, to ensure that the current sensor unit does not dissipate much power. The voltage at the input of the first resistor is used as one of the feedback signals  30 , and the voltage at the input to the second resistor is used as a second feedback signal  31 . The feedback signals  30 ,  31  are supplied as inputs to a differential amplifier circuit  42  within the current limiter  21 . The differential amplifier circuit  42  provides a high impedance to feedback signals  30 ,  31  such that very little current is drawn by the feedback signals  30 ,  31  from the current sensor unit  28 . This ensures that the voltage on feedback signals  30 ,  31  is an accurate representation of the current that passes through the first resistor  40  and the second resistor  41  of the current sensor unit  28 . 
     The first feedback signal  30  is connected to a resistor  43  with a high resistance, such as 10K ohms, which is then connected to the positive input  44  of an operational amplifier  46 . The positive input  44  of the operational amplifier is then connected to a 1K ohm resistor  47  in parallel with a capacitor  48  which are then connected to ground. The second feedback signal  31  is connected to a resistor  49  with a high resistance, such as 10K ohms, which is then connected to the negative input  45  of the operational amplifier  46 . The negative input of the operational amplifier is then connected to a 1K ohm resistor  50  in parallel with a capacitor  51 , which are then connected to the operational amplifier output  52 . 
     The differential amplifier circuit  42  achieves several purposes. First, it provides a high impedance to feedback signals  30 ,  31  such that very low current is drawn from the current sensor unit. Second, the arrangement of the capacitors  48  and  51  make the circuit act as a low-pass filter to ensure that any high-frequency noise is removed from the feedback signals  30  and  31 . Finally, the output of the differential amplifier circuit is the difference between the voltage in the first feedback signal  30  and the second feedback signal  31  which is the voltage across resistor  40 , also labeled as Vsense  53 . Thus, the voltage at the differential amplifier circuit&#39;s output  52  represents a filtered and buffered signal measuring the current passing through the current sensor unit. 
     The output of the differential amplifier circuit  42  is connected to the threshold detector circuit  54  within the current limiter  21 . More specifically, the output of the differential amplifier circuit  42  is connected to the positive input of a comparator  55 . The negative input of the comparator is connected to the output of a voltage divider, representing the desired current level of the current limiter  21 . The voltage representing the desired current level is produced by connecting a 5 volt reference voltage across a voltage divider made up of a 1.78K ohm resistor  56  and a 1.00K ohm resistor  58 . This produces a voltage of 1.799 volts at the output of the voltage divider  15 . 
     The output  57  of the comparator  55  is connected to pull-up resistor  58 , which is then connected to a 15 volt source  59 . When the positive input of the comparator  52  is a voltage less than the negative input of the comparator, the comparator&#39;s output will be driven to 0 volts. Alternatively, when the positive input of the comparator  52  is a voltage greater than the negative input of the comparator, the comparator&#39;s output  57  will be high impedance. The pull-up resistor  58  will be able to drive the voltage at the comparator output  57  to 15 volts. 
     The threshold detector circuit&#39;s output  57  is connected to the positive edge triggered input of a one-shot pulse generator  60 . When in an untriggered state, the one-shot output  61  will be at logic-high. When Vsense  53  increases from a voltage lower than the desired current level to a voltage greater than the desired current level, the output of the threshold detector circuit  57  will change from a logic-low level to a logic-high level. This will be seen as a positive edge by the one-shot pulse generator  60 , and will cause the one-shot pulse generator  60  to change its output  61  from logic-high to logic-low for a fixed duration of time (i.e., the pulse width). 
     The output  61  of the one-shot pulse generator  60  is connected to the power switch circuit  62  of the current limiter  21 . More specifically, the output of the one-shot pulse generator  61  is connected to a 1.00K ohm resistor  63  which is connected to the base  73  of an NPN transistor  64 . The emitter  70  of the transistor  64  is connected to ground. The collector  71  of the transistor  64  is connected to a 10K ohm resistor  65 , which is connected to the gate  72  of a p-channel power MOSFET  67 . The source  68  of the p-channel power MOSFET  67  is connected to the 28 volt source  19 . The gate  72  of the p-channel power MOSFET is also connected to an 10K ohm resistor  66  which is connected to the 28 volt source. The drain  69  of the p-channel MOSFET is connected to the output  22  of the current limiter  21 . 
     When the current limiter  21  is not in an overcurrent state, the voltage at the power switch circuit input  61  is logic-high, as set by the one-shot pulse generator  60 . Because the base  73  of the transistor  64  draws very low current, the voltage at the base  73  will be nearly the same as the voltage at  61 . Thus, the voltage at the base  73  will be approximately logic-high, which will be approximately 5 v. Since the voltage at the base  73  is more than 0.7 volts greater than the voltage at the emitter  70 , the transistor  64  will be on, driving the collector  71  voltage to ground. When the collector voltage  71  approaches ground, resistors  66  and  65  make up a voltage divider between the 28 volt source  19  and ground. The voltage divider makes the voltage at the gate  72  of the p-channel power MOSFET to be 14 volts. With the gate  72  to source  68  voltage approximately 14 volts, the p-channel power MOSFET is turned on, driving the drain  69  voltage to 28 volts. The drain  69  is connected to the motor drive and control unit  23  and the test circuit  24 . When the current limiter is not in an overcurrent state, it will provide the motor drive and control unit with 28 volts. The output  61  of the one-shot pulse generator  60  is also connected to a diode  37 , which is the connected to the overcurrent signal output  32 . The overcurrent signal output is also connected to a pull-up resistor  38  which is connected to a 5 volt source. The diode  37  is oriented such that current will not flow from the one-shot pulse generator output  61  to the overcurrent signal output  32 . 
     The test circuit  24  has an n-channel power MOSFET  74  connected in parallel with the motor drive and control unit  23 . More particularly, the drain  75  is connected to output  22  of the current limiter  21 , and the source  76  is connected to the input  27  of the current sensing unit. The gate  77  of the n-channel power MOSFET  74  is connected to a pull-down resistor  78  which is connected to ground. The gate  77  is also connected to a diode  79 . The diode is oriented such that the gate voltage  77  may be driven up by a high voltage at the junction  80  at the other side of the diode. Additionally, a low voltage at the junction  80  will not be able to draw current from the pull-down resistor  78 . The junction  80  also connected to a pull-up resistor  81  which is connected to a 15 volt source. The junction  80  is also connected to another diode  83  which is then connected to the output  84  of an operational amplifier  85 . The diode  83  at the output of the operational amplifier is oriented to only allow current to pass into the operational amplifier  85 . The negative input of the operational amplifier  89  is connected to one of the feedback lines  31  from the current sensor unit  28 . The operational amplifier positive input  86  is connected to the output of a voltage divider  90 . The voltage divider is formed by a 5 volt reference voltage  91  connected to an 1.75K ohm resistor  87  which is connected to an 1.00K ohm resistor  88  which is connected to ground. It should be noted that the 5 v voltage reference connected to resistor  56  is a separate reference than voltage reference  91 . The voltage divider produces a voltage of 1.820 volts at  90 . 
     The run self-test input signal  34  is connected to a diode  92  which is then connected to the overcurrent output signal  32  at junction  93 . The diode  92  is oriented to allow current to flow from the test circuit out to the run self-test input signal  34 . At the junction  93 , the diode  92  is also connected to another diode  94 , which is then connected to the base  95  of an NPN transistor  99 . The diode  94  is oriented to allow current to flow into the base  95 . The emitter  97  of the NPN transistor  99  is connected to ground. The collector  96  is connected to pull-down resistor  81  at the junction  80 . 
     This paragraph describes the operation of the test circuit when not running a self-test and overcurrent is not en-countered. Since the run self-test signal  34  is active low, when not running a self-test, the voltage on the run self-test signal  34  will be logic-high. In this state, because of the diode  92 , the run self-test signal will not have an appreciable effect on the voltage at junction  93 . When the power source is not in an overcurrent state, the current limiter&#39;s overcurrent signal  32  will be at logic-high of approximately 5 volts due to the pull-up resistor  38 . This will drive the voltage at the base  95  of the NPN transistor  99  to a level high enough to activate the transistor  99 . When the transistor  99  is activated, its collector  96  voltage will be pulled to ground. With the collector voltage  96  pulled to ground, the voltage at the junction  80  will also be grounded. This will, in turn, make the gate voltage  77  of the n-channel power MOSFET  74  approximately 0 volts, which will turn off the MOSFET  74 . When the MOSFET  74  is off, no current will flow through the test circuit  24  from the current limiter  21  to the current sensor unit  28 . It should be noted that in this state, the voltage at the junction  80  will not be affected by the output  84  of the operational amplifier  85 . Since the transistor  99  is driving the junction  80  to ground, any voltage at the operational amplifier output  84  will not have an effect on the junction voltage  80  because of the diode  83  between the operational amplifier  85  and the junction  80 . 
     This paragraph will describes the operation of test circuit when not running a self-test and overcurrent is encoun-tered. When the current limiter  21  detects that the actual current is greater than the desired current, it will drive the overcurrent signal  32  to 0 volts. This will force the voltage at junction  93  to 0 volts regardless of the run self-test signal&#39;s value because of the diode  92 . With the voltage at  93  driven to 0 volts, there will be no current input to the base  95  of the NPN transistor  99 . This will shut off the transistor  99  such that the collector  96  will no longer be forcing the voltage at the junction  80  to 0 volts. This allows the pull-up resistor  81  to drive up the voltage at the junction  80 . A voltage divider ends up being created between the 15 volt source  82 , the pull-up resistor  81  and the pull-down resistor  78 , yielding a voltage of approximately 13 volts at the MOSFET gate  77 . This gate voltage will turn on the n-channel power MOSFET  74 , allowing the MOSFET to quickly divert the current supplied by the current limiter  21  into the test circuit  24  instead of going to the motor drive and control unit  23 . Thus, the test circuit  24  helps protect the motor drive and control unit  23  by helping the current limiter  21  cut off current flow to the motor drive and control unit  23 , and guarantees that the current limiter status output signal will change state rapidly. 
     This paragraph describes the operation of the test circuit  24  while running a self-test of the power source. A self-test of the current-limiting functionality of the power supply is initiated by making the run self-test signal  34  a logic-low of approximately 0 volts. This will also drive the voltage at junction  93  to 0 volts. With the voltage at  93  driven to 0 volts, there will be no current input to the base  95  of the NPN transistor  99 . This will shut off the transistor  99  such that the collector  96  will no longer be forcing the voltage at the junction  80  to 0 volts. The pull-up resistor  81  will attempt to drive up the voltage at the junction  80  by way of a voltage divider between the 15 volt source  82 , the pull-up resistor  81  and the pull-down resistor  78 . This voltage divider uninhibited would create a voltage of approximately 13 volts at the MOSFET gate  77 . However, the voltage at junction  80  can also be driven by the operational amplifier  85 . When the feedback voltage  31  connected to the operational amplifier negative input  89  is less than the 1.820 volts at the operational amplifier positive input  86 , the operational amplifier tries to drive its output  84  to a high voltage. This will cause a high voltage at the MOSFET gate  77  which will begin to turn the MOSFET  74  on. This will increase the current passing through the test circuit  24  from the current limiter  21  to the current sensor unit  28 , which will then increase the feedback voltage  31  produced by the current sensor unit  28 . When the feedback voltage  31  connected to the operational amplifier negative input  89  becomes greater than the 1.820 volts at the operational amplifier positive input  86 , the operational amplifier will try to drive its output  84  toward ground. Equilibrium for the operational amplifier  85  is reached when approximately 1.82 amps of current is passing though the current sensor unit  28 . This current level should be sufficient to cause the current limiter  21  to sense that the actual current provided by the current limiter  21  is greater than the desired current and that current should be shut off. A successful shut-off of current by the current limiter  21  is determined by the current limiter status line  35 . If the current limiter does not properly operate in detecting the overcurrent condition, the self-test will fail and the user will know that the system is malfunctioning from the current limiter status line  35 . 
     The function of the power system has several levels of fail-safe through redundancy built into the system. The use of two resistors  40  and  41  in the current sensor unit ensures that two component failures must occur for an overcurrent condition to go undetected. If the resistance of resistor  40  were to decrease in value due to some failure mechanism, the current limiter  21  would not cut off current at the desired current level. This is because the feedback voltage Vsense  53  will be a smaller magnitude than expected for the desired current level due to the resistor  40  being a de-creased resistance. However, the this would be detected during self-test operation of the test circuit since the test circuit uses only one feedback signal  31  which will properly read the current through resistor  41 . Additionally, if the resistance of resistor  40  were to increase in value, the current limiter  21  would shut off current at a point lower than the desired level, which is a failsafe condition. Alternatively, if resistor  41  were to fail instead of resistor  40 , the current limiter will continue to operate properly since its shut off is con-trolled by the voltage drop across of resistor  40 . 
     The use of two voltage references also adds redundancy in the current-limiting operation of the power source. If the voltage reference  56  for the current limiter were to drift to a higher value than it is supposed to, it will be detected by the operation of test circuit  24  with its separate voltage reference  91 . 
     The present invention contemplates that many changes and modifications may be made. Therefore, while the presently-preferred form of the improved power supply has been shown and described, and a number of alternatives discussed, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the spirit of the invention, as defined and differentiated by the following claims.