Abstract:
A spring return rotary actuator incorporating a DC brush commutated electric motor and a pulse width modulation drive circuit which reduces the voltage at which current is supplied to the motor once a rotation sensor senses that the actuator output shaft has stalled. The drive circuit also includes a temperature responsive feature which increases the voltage at which current is supplied to the motor in the event a sensed temperature exceeds a temperature limit.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to an actuator system of the type in which an output shaft is spring returned to a rest position and, on command, is driven to and held at a different position by an electric motor. More particularly, the invention relates to such an actuator system employing a drive circuit and method which reduces power supplied to the motor when the output shaft is stalled away from its rest position. 
     It is well known, particularly in heating, ventilating and air conditioning (HVAC) damper applications, to employ actuators of the type having an output shaft which is driven in one direction to a desired position and held in that position by an electric motor, and returned in the opposite direction to a rest position by a spring when the motor is not energized. The motor may also serve to govern the speed with which the spring returns the actuator output shaft to its rest position. 
     Depending on the type of motor used, the motor may offer more or less minimum resistance to operation of the spring return mechanism. This resistance is manifested as a torque in addition to the torque required for returning the damper or other load to its rest position which must be provided by the spring. The speed with which the motor can operate the load is determined by the power output of the motor which is transmitted to the load by a torque multiplying gear train. The resistance or load provided by the motor on the spring in returning the actuator output shaft to its rest position typically increases with increased power output capability of the motor. Thus, it is apparent that optimizing the actuator system for speed of operation and size of controlled load requires careful balancing of the motor output power capability, gear train input/output ratio and return spring strength. 
     One function of dampers in certain HVAC systems is to provide smoke and fire control. It has become a requirement that actuators used in smoke and fire control applications be capable of operation at an elevated temperature of, for example, 350° F. Operation at elevated temperature introduces additional complications and places additional demands on the actuator system. More specifically, magnetic circuit performance is generally adversely affected by elevated temperature, thus decreasing electric motor power output for a given energization voltage. A requirement for operation at elevated temperatures also places limitations on the electrical circuit design, which effectively precludes use of electronically commutated motors. Finally, elevated temperature application requirements restrict the choice of acceptable materials and lubricants, effectively precluding the use of many plastics and wick-type lubrication systems. 
     Apart from the foregoing considerations, it is desirable to minimize the energy consumed by the actuator system. In addition to reducing energy costs, this reduces the power handling requirements of circuit components which supply energization current to the motor, and reduces the power required to be dissipated by the motor, thereby permitting use of a motor of smaller size and increasing its life. 
     The applicant has achieved many of the objectives and operating characteristics indicated as desirable in the foregoing discussion by devising an actuator designed around a DC brush commutated motor. The characteristics of such a motor are used to maximum advantage by providing a unique drive circuit and method of energization which alters the average voltage at which current is supplied to energize the motor based on the actuator operating mode and environmental conditions. This approach has permitted the applicant to provide a fast acting two position spring return actuator designed to be directly coupled to a load. The actuator requires less operating power than conventional actuator designs and is capable of operation at elevated temperatures. 
     SUMMARY OF THE INVENTION 
     The present invention is an actuator system, and a drive circuit and method employed therein, the actuator including an electric motor coupled to a rotatable output member which is biased for rotation to a rest position. The motor, when energized, rotates and holds the output member at a position away from the rest position. The drive circuit includes input means for providing current in response to a command for rotation of the shaft away from its rest position, a rotation sensor operable to produce a signal indicative of whether or not the output member is rotating, and a control circuit responsive to current received from the input means and the rotation sensor signal. The control circuit is operable to supply current to the motor at a first average voltage if the output member is rotating, and to supply current to the motor at a second average voltage less than the first average voltage once the output member has stalled. The circuit may also include a temperature sensor operable to produce a signal indicative of a sensed temperature, the control circuit being responsive to the temperature sensor signal to increase voltage at which current is supplied to the motor to a third average voltage greater than the first average voltage if the sensed temperature is greater than a predetermined temperature limit. 
     The method of the applicant&#39;s invention comprises the steps of supplying electric current to the motor when positioning of the output member away from its rest position is desired, sensing whether or not the output member is rotating, controlling the voltage at which current is supplied to the motor to a first average voltage if the output member is rotating, and controlling the voltage at which current is supplied to the motor to a second average voltage less than the first average voltage once the output member has stalled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an actuator system incorporating a preferred circuit for implementing the applicant&#39;s invention; and 
     FIGS. 2A and 2B are representations of operating wave forms which occur at selected points in the circuit of FIG. 1 to facilitate understanding of the circuit design and operation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the schematic diagram of FIG. 1, reference numeral  11  identifies an actuator output member or shaft which is mounted for rotation about an axis  12 , and to which a load, such as damper or damper system (not shown) of an HVAC system may be connected for positioning in response to a load control signal provided by control signal apparatus generally identified by reference numeral  13 . Control signal apparatus  13  is schematically represented as including an alternating current source  14  in series with a switch  15 . In a representative application, and for convenience in describing the applicant&#39;s invention, current source  14  may supply current at a voltage of 24 volts AC, and switch  15  may be part of an economizer system, enthalpy sensor, air quality sensor, manually controlled switch or other apparatus which provides a switching function. The economizer system, sensor or other switching apparatus may be implemented with a microprocessor. 
     Shaft  11  is part of an actuator which includes an electric motor  16  coupled to the shaft through a torque multiplying gear train  17 . Shaft  11  is biased to rotate in a first direction by a return spring  18 . When motor  16  is not energized, spring  18  is operable to rotate shaft  11  to a rest position. Energizing motor  16  causes shaft  11  to rotate against the bias of spring  18  to an actuated position away from the rest position. The actuated position may be determined by a stop (not shown) associated with gear train  17 , shaft  11  or the load connected thereto which causes the motor, gear train and shaft to stall. In the actuator system illustrated in FIG. 1, motor  16  is a DC motor, and preferably a DC brush commutated motor. 
     When switch  15  is open, spring  18  maintains shaft  11  at its rest position. When it is desired to rotate shaft  11  to its actuated position, switch  15  is closed to provide energization current to motor  16 . The energization current is supplied from input device identified by reference numeral  20  through a conductor  21  to a first power terminal of motor  16 , and is controlled by a solid state load switch identified by reference numeral  30  connected between a second power terminal of motor  16  and a source of reference potential or ground  31 . Load switch  30 , which is shown as an N-channel power FET, is controlled by a signal supplied to its gate electrode by a control or modulation circuit  40  which receives operating power through a voltage regulator in input device  20 . Modulation circuit  40  also receives a signal indicative of whether or not shaft  11  is rotating from a rotation sensor circuit  60 . For purposes which will be described in detail hereinafter, modulation circuit  40  is also connected through a temperature sensor in the form of a temperature sensitive fusible link  80  to ground  31 . 
     Several advantages are gained by controlling the energization supplied to motor  16  at different levels depending on the operating mode (i.e., driving or holding the position of shaft  11 ) and environmental conditions (i.e., normal or elevated temperature). More specifically, when it is desired to drive shaft  11  to its actuated position under normal temperature conditions, motor  16  is energized at a level which is adequate to cause the motor to overcome the torque bias provided by spring  18  in addition to driving the load. However, the energization level is controlled to limit the motor output to a torque level which does not excessively stress the gear train, output shaft or load. 
     Once shaft  11  has stalled, the energization provided to motor  16  is reduced to a level only sufficient to ensure that shaft  11  is held in its actuated position. Conversely, when the actuator system is subjected to an elevated temperature, the energization provided to motor  16  is increased to a level sufficient to compensate for the diminished magnetic circuit performance of the motor resulting from the elevated temperature. Thus, energy usage is controlled to a level just sufficient to achieve the required actuator system performance while the stresses on mechanical components are minimized under all operating modes and conditions. 
     The present invention advantageously employs modulation circuit  40  to provide the appropriate level of energization to motor  16  indicated by the sensed operational mode and environmental conditions. Circuit  40  performs this function generally by controlling the voltage at which current is supplied to motor  16  to a first average voltage level when it is desired to rotate shaft  11  to its actuated position under normal temperature conditions, controlling the voltage at which current is supplied to the motor to a second average voltage level less than the first average voltage level once the motor has stalled, and controlling the voltage at which current is supplied to the motor to a third average voltage level greater than the first average voltage level under elevated temperature conditions. Controlling the voltage at which current is supplied to the motor is accomplished by pulse width modulating the current supplied to the motor at three different duty cycles corresponding to the first, second and third average voltage levels. 
     Turning to the specifics of a preferred implementation of the present invention, N-channel power FET load switch  30  is shown with its source electrode connected to ground  31  and its drain electrode connected to a second power terminal of motor  16 . A Zener diode  32  and a conventional diode  33  are connected in a series anode-to-anode arrangement between the power terminals of motor  16  to provide a current circulation path which limits the voltage induced across the windings of motor  16  when load switch  30  is switched to a non-conducting state. Diodes  32  and  33  also enhance the braking effect provided by motor  16  in its unenergized state on return of shaft  11  to its rest position by spring  18 . 
     Input device  20  is shown including a full wave diode bridge rectifier circuit  22  which provides unidirectional or direct current energization for motor  16 . Reference numeral  23  identifies a filter capacitor connected across rectifier circuit  22  between conductor  21  and ground  31 . 
     Rectifier circuit  22  also supplies operating power to modulation circuit  40  and rotation sensor circuit  60  through a voltage regulator including a resistor  24  and a Zener diode  25  connected in series between conductor  21  and ground  31 . Zener diode  25  may be chosen to provide a regulated five volt DC output at its junction with resistor  24 . Reference numeral  26  identifies a filter capacitor connected across the Zener diode. A resistor  27  is also connected across Zener diode  25  to provide a discharge path for certain capacitors in modulation circuit  40  and rotation sensor circuit  60 , as will be described hereinafter. Operating power is transmitted to modulation circuit  40  and rotation sensor circuit  60  through a conductor  28 . 
     Pulse width modulation of the control signal provided to load switch  30  by modulation circuit  40  is achieved by an oscillator comprising series connected inverters  41  and  42  and a feedback path including a capacitor  43  and a resistor  44  connected in series from the output terminal of inverter  42  to the input terminal of inverter  41 . Capacitor  43  is alternately charged in opposite polarities through the output terminal of inverter  42  and selected combinations of three current paths which may be of different impedances. The modulation duty cycles are determined by which current path(s) is/are active. 
     The output terminal of inverter  41  is connected to the input terminal of inverter  42  at a junction  45 , the output terminal of inverter  42  supplying the gate signal for load switch  30 . Capacitor  43  and resistor  44  are connected at a junction  46 . The current paths through which capacitor  43  is charged are connected between junctions  45  and  46 . A resistor  47  forms the first current path. A resistor  48  in series with a diode oriented to permit current flow toward junction  45  forms a second current path. A resistor  50  connected through an NPN transistor  51 , the emitter of the transistor being connected to junction  46 , forms a third current path. Transistor  51  receives its base control signal from rotation sensor circuit  60  through a resistor  52 , as will be described hereinafter. 
     For purposes of describing operation of the oscillator circuit, assume an operating mode in which the input terminal of inverter  41  has just switched to a logical low state of substantially ground potential, and is essentially isolated from signal levels in upstream parts of modulation circuit  40 , as would be the case with the diode identified by reference numeral  53  connected as shown in series with fusible link  80  between the inverter input terminal and ground  31 . Also assume that transistor  51  is being maintained in a conductive state by a suitable base control signal. 
     In such a configuration, as the output of inverter  42  switches to its logical low state, junction  46  initially goes to a corresponding low voltage. Junction  45  is at high voltage corresponding to the logical high states at the output terminal of inverter  41  and input terminal of inverter  42 . Charging of capacitor  43  then commences through resistors  47  and  50 , the collector-emitter junction of transistor being forward biased. The charging path containing resistor  48  is not active because of the reverse bias across diode  49 . Thus, charging of capacitor  43  with the described polarity and the time interval that inverters  41  and  42  remain in their present switched states is determined by resistors  47  and  50 . 
     Once capacitor  43  is sufficiently charged, thereby raising the voltage at junction  46  and producing a logical high state at the input terminal of inverter  41 , inverters  41  and  42  switch states, the output of inverter  42  switching to a logical high state. This produces a corresponding increase in the voltage at junction  46 , junction  45  being at a logical low state. Capacitor  43  then commences to charge with an opposite polarity through resistors  47  and  48 , thereby decreasing the voltage at junction  46 . Once the voltage at junction  46  and the input terminal of inverter  41  has decreased sufficiently, inverters  41  and  42  again switch states, the output of inverter  42  going to a logical low state. 
     In an exemplary actuator system embodiment, it was desired to operate motor  16  in a drive mode and under normal temperature conditions at an average voltage corresponding to an 85% duty cycle modulation level. This modulation duty cycle may be achieved by choosing the values of resistors  47 ,  48  and  50  such that capacitor  43  charges more slowly through resistors  47  and  48  than through resistors  47  and  50 , thus leaving the output of inverter  42  at a logical high state for a longer interval each cycle than at a logical low state. 
     From the foregoing description, it can be seen that terminating the base signal to transistor  51 , thus rendering it non-conducting, eliminates the contribution of the current path containing resistor  50  in increasing the voltage at junction  46 . This has the effect of maintaining the output of inverter  42  at a logical low state for a longer portion of each cycle, thus lowering the duty cycle of the gate signal supplied to load switch  30 , and reducing the average voltage at which current is supplied to motor  16 . 
     Turning to overall operation of the drive circuit shown in FIG. 1, upon closure of switch  15  to energize motor  16 , operating voltage is supplied to modulation circuit  40  and rotation sensor circuit  60  from the voltage regulator in input device  20 . This step increase in voltage is supplied to a power-up circuit comprising a capacitor  54  connected in parallel with a resistor  55  to the input terminal of an inverter  56 . The output terminal of inverter  56  is connected to the input terminal of an inverter  57 , the output terminal of which is connected to a resistor  58  at a junction from which the base control signal is supplied to transistor  51 . The step increase in voltage supplied to the power-up circuit is transmitted through resistor  55  to the input terminal of inverter  56  and results in a logical high state at the output terminal of inverter  57 . In accordance with the foregoing description, this results in the oscillator operating at a first duty cycle of, for example, less than 95%, and preferably of approximately 85%. 
     As will be described hereinafter, once output shaft  11  has stalled, rotation sensor circuit  60  forces the output of an inverter  75  to a logical low state. Capacitor  54  is then charged through diode  76 , resulting in a logical low state at the input of inverter  56 . Until shaft  11  has stalled, the input terminal of inverter  56  and output terminal of inverter  57  remain at logical high states, thereby providing a base control signal to transistor  51  which maintains it in a conducting state. In the absence of signal conditions which maintain a logical high state at the output of inverter  57 , the inverter will switch to a logical low output state, thereby rendering transistor  51  non-conducting, and decreasing the operating duty cycle of the oscillator to, for example, 40%, which corresponds to holding mode energization for motor  16 . 
     Rotation sensor circuit  60  includes a pulse generator  61  which may be implemented with a Hall effect device, such as is commercially available under Microswitch model No. SS443A, in cooperation with a magnet  62  which may be mounted near the periphery of a gear or other rotating member in gear train  17 . As shown, pulse generator  61  is mounted proximate the rotating member which carries magnet  62 , and is energized through connections to conductor  28  and ground  31 . When motor  16  and gear train  17  are rotating, magnet  62  periodically passes near pulse generator  61  causing it to produce a high output signal as illustrated in FIG.  2 A. The pulse train is carried over a conductor  63  to a junction  64  between a resistor  65  and a capacitor  66 , resistor  65  being connected between the junction and conductor  28 . Capacitor  66  is connected in series with a diode  67  between junction  64  and ground  31 , diode  67  being oriented to permit current flow from ground  31  to a junction  68  between capacitor  66  and diode  67 . Capacitor  66  produces a positive going spike at the leading edge of each pulse produced by pulse generator  61 , and attempts to produce a negative going spike at the trailing edge of each pulse, the negative going spike being clipped by diode  67  as illustrated in the waveform of FIG.  2 B. 
     The voltage waveform at junction  68  is supplied through a resistor  69  to the base electrode of an NPN transistor  70 . The base electrode of transistor  70  is biased to ground potential through a resistor  71 , and the emitter electrode of the transistor is connected directly to ground  31 . The collector electrode of transistor  70  is connected to a junction  72  between a resistor  73  and a capacitor  74 , the resistor being connected between conductor  28  and junction  72  and the capacitor  74  being connected between junction  72  and ground  31 . Capacitor  74  is biased through resistor  73  to be charged to the regulated voltage on conductor  28 . However, as long as transistor  70  continues to receive the periodic waveform of FIG. 2B resulting from rotation of output shaft  11 , the transistor periodically discharges capacitor  74 , thereby maintaining junction  72  at a logical low state. 
     The signal at junction  72  is supplied to inverter  75  whose output terminal is connected to the cathode of a diode  76 , the anode of which is connected to a junction  77  which is biased to the voltage on conductor  28  through resistor  55 . Thus, junction  77  is maintained at a logical high state as long as the output of inverter  75  is at a logical high state, which is true as long as shaft  11  is rotating. However, if shaft  11  has stalled, the pulse trains in rotation sensor circuit  60  cease and transistor  70  no longer discharges capacitor  74 , thus resulting in a logical low state at the output of inverter  75 . This forward biases diode  76  and reduces the voltage at junction  77  so that a diode  78  connecting junction  77  to the output terminal of inverter  57  is no longer forward biased. The input of inverter  56  and output of inverter  57  then go to logical low states, which renders transistor  51  non-conducting and reduces the duty cycle of the control signal to load switch  30  and the current supplied to motor  16 . 
     As previously indicated, elevated temperatures generally adversely affect the magnetic circuit performance of motor  16 . In applications in which the actuator system is used for smoke and fire control, and depending on the control system configuration, it may be desirable to move a damper connected to output shaft  11  to its actuated position under elevated temperature conditions. The applicant&#39;s circuit compensates for decreased motor efficiency under such conditions by increasing the average voltage at which current is supplied to motor  16 . This is accomplished by fusible link  80  which opens upon exposure to temperature above a predetermined temperature limit. When fusible link  80  opens, the junction between diode  53  and resistor  58  is no longer held at ground potential. Thus, if the output of inverter  57  is at a logical high state, corresponding to actuation mode energization of motor  16 , the input of inverter of inverter  41  remains continuously at a logical high state, thereby providing an unmodulated or 100% duty cycle signal to load switch  30  and supply of current at maximum average voltage to motor  16 . Thus, adequate motor output torque is maintained even under elevated temperature conditions. 
     Although a particular embodiment of the applicant&#39;s actuator system, drive circuit and method is shown and described for illustrative purposes, variations of the apparatus and method employed therein will be apparent to those of ordinary skill in the relevant art. It is not intended that the scope of coverage be limited to the illustrated embodiment, but only by the terms of the following claims.