The present invention relates to motorized actuators, such as used to operate valves and dampers in a heating, ventilation, and air conditioning system; and more particularly to mechanisms for detecting when the actuator stalls.
Electrically powered actuators are commonly used to open and close valves and airflow dampers in heating, ventilation and air conditioning (HVAC) systems. A typical bidirectional actuator has an output connector that moves ninety degrees to drive the valve or damper between fully open and fully closed positions. These actuators usually include an electric motor which is connected by a gear train to the valve or damper. This allows a low torque motor to operate a relatively large load. The motor can be operated to place the valve or damper in a number of positions between the extreme open and closed limits. The voltage or current level of an analog input signal indicates the desired position.
It is important in many installations that the device operated by the actuator assume a predefined (normal) position in the event that electrical power to the actuator is lost. For example, it often is desired that an outside air damper in an HVAC duct close when electrical power is lost to prevent pipes and equipment from freezing. This return-to-normal feature is provided by a return spring which winds as the actuator moves the valve or damper into an open position, thereby storing energy in the spring. The spring unwinds as the actuator closes the valve or damper. When the motor stops, its a reduced level xe2x80x9choldxe2x80x9d current through the motor winding provides torque that when amplified by the gear train prevents the return spring energy from moving the actuator. When power is lost, a clutch decouples the motor from the gear train allowing the spring to operate the actuator.
A given actuator model is specified as providing a minimum amount of output torque. Manufacturing and component tolerances affect the actual torque produced by a particular actuator. The torque required to operate the actuator also varies as a function of temperature which varies the mechanical resistance to movement, the degree to which the return spring is wound (e.g. more torque is required when the spring is wound-up than when relaxed), and the direction of movement (i.e. whether the spring is aiding or resisting actuator motion). Therefore, in order that every actuator of a given model will meet the minimum output torque specification over its full range of motion and operating temperatures, the actuator is designed to produce a much higher torque level. As a result some actuators will have tolerances that yield an actual torque level that is greatly above the design level, for example as much as twice the specified minimum output torque.
This creates a problem in that when the device driven by the actuator reaches the end of its travel, the actuator will continue to apply force to the device until the torque rises to a level at which a stall detector trips and deactivates the actuator motor. The stall torque threshold must be set relatively high to accommodate high torque levels produced in a worst case combination of the values of the parameters affecting movement. As a consequence, a particular actuator may apply a very high torque to the driven device before shutting off, which over time can have significant adverse affects on that device and the actuator.
Therefore, it is desirable to provide a mechanism for dynamically varying the stall torque threshold as a function of the parameters that affect the torque required to operate the actuator.
The present invention is particularly suited to control an actuator that has a motor which is selectively driven in two directions by an electric current and which is coupled by a transmission to an output connector. A spring which is connected to either the motor, the transmission or the output connector, stores energy when the motor is driven in one direction and releases the stored energy when the motor is driven in another direction.
The present control technique derives a relationship between a motor stall current threshold and at least one actuator operating parameter in a group consisting of the position of the spring, the direction in which the motor is being driven, and the temperature of the actuator. The present value of each parameter of the relationship is sensed and employed using the relationship to determine a stall current threshold value. In the preferred embodiment, the relationship is expressed as a table of motor stall current threshold values stored in a memory of a controller for the actuator. The sensed values for the position of the spring, direction in which the motor is being driven, and temperature are used to address a particular entry in that table which entry then is used as the stall current threshold value.
The magnitude of electric current flowing through the motor is sensed and compared to the selected stall current threshold value. A determination is made that the motor has stalled in response to that comparison. For example, a conclusion is made that the motor has stalled when the electric current flowing through the motor exceeds the stall current threshold value.