Electricity: motive power systems – Automatic and/or with time-delay means – Responsive to thermal conditions
Reexamination Certificate
1997-07-31
2001-06-19
Ip, Paul (Department: 2837)
Electricity: motive power systems
Automatic and/or with time-delay means
Responsive to thermal conditions
C318S430000, C318S608000, C318S609000, C318S434000
Reexamination Certificate
active
06249100
ABSTRACT:
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'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.
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“HVAC Product News”, Jan. 27, 1997, product description of model AF24-PWM offered by BELIMO Aircontrols (USA), Inc.
Honeywell International , Inc.
Ip Paul
Rubow Charles L.
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