Electricity: motive power systems – Positional servo systems – With stabilizing features
Reexamination Certificate
1999-12-09
2001-07-24
Masih, Karen (Department: 2837)
Electricity: motive power systems
Positional servo systems
With stabilizing features
C318S468000, C318S282000, C318S280000, C318S286000, C318S466000, C318S461000, C318S463000
Reexamination Certificate
active
06265843
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
MICROFICHE APPENDIX
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for controlling an actuator, and more particularly to a method for detecting the saturation status of non-synchronous incremental actuators using a variable position estimate window.
2. Description of Related Art
An actuator is a mechanical device used to move or control another device. For example, an air conditioning and heating control system may use an actuator to control a damper in an air chamber so that the damper can act as a throttle and control air flow through the chamber.
Due to their relative low cost, the current trend in the actuator industry is to use non-synchronous actuators. Non-synchronous actuators are controlled using three wires—positive, negative, and ground—whereby a controller controlling the actuator can move the actuator in a first or second direction depending upon whether a drive voltage is applied between the positive and ground wires or the negative and ground wires.
Non-synchronous actuators, however, move at different speeds in different directions, due, in part, to different loading on the actuator. Moreover, this speed-disparity is not constant; it can and often does change over time with the actuator's age, fatigue, and other factors. As a result, it is not uncommon for an actuator to move faster in a first direction than it moves in a second direction, or for an actuator, moving in a first direction for a given period of time and then in an opposite second direction for the same period of time, to not return to the same position it was in prior to moving.
As discrepancies accumulate over time, the controller's estimated positions of the actuator quickly become unsynchronized from their actual positions. And after a short period of operation, the controller controlling the actuator may estimate that the actuator is at mid-stroke when the actuator is actually at an ultimate end-stop.
One of the current methods for dealing with speed-disparity is called “overdrive,” whereby the controller continues to drive the actuator even if the controller estimates that the actuator is “saturated” (at an end-stop). If the actuator is genuinely saturated, overdriving forces the actuator to continue to drive against the end-stop, which results in heat build-up, thereby shortening the effective life of the actuator. If the actuator is not saturated, the overdrive will force the actuator to drive to an end stop, causing unacceptable control disturbances. While the tolerance for overdrive is reasonably high, the amount of overdrive needed to maintain synchronization with current non-synchronous actuators is significant. As a result, present actuators often settle at a partial stroke position when the controller estimates they are fully open or closed.
While control algorithms known in the art estimate the position of the actuator based on its “on” and “off” time and direction, these algorithms assume a constant actuation speed based on a nominal stroke time of the actuator. These algorithms are unsatisfactory for actuators with different stroke times that move at different speeds in different directions.
Furthermore, when a controller controls an actuator incrementally (i.e., moving the actuator by discrete increments in a specified direction relative to the current position of the actuator, as opposed to moving the actuator to an absolute position), the controller remains unable to accurately determine the actual position of the actuator. The controller must determine the actual position of the actuator so that it can disable movement of the actuator if the actuator is saturated. Thus, continual and accurate estimation of the position of the actuator, in anticipation of detecting a saturated condition and acting thereupon, is of paramount importance in controlled operations.
As is evident from the foregoing, a need exists for a method of minimizing the amount of error, time, and overdrive required to accurately detect the saturated condition of a non-synchronous incremental actuator.
SUMMARY OF THE INVENTION
The present inventors have recognized that the aforementioned limitations can be overcome by using first and second estimates of the position of the actuator, where between the actual position is always bound, wherein the first and second estimates are calculated based on the maximum and minimum speeds at which the actuator could be moving in each of first and second directions, and further wherein the controller ceases moving the actuator in a saturated direction once the first and second estimates converge at one of two final positions.
More specifically, the present invention comprises a method for controlling an actuator that is capable of moving in first and second directions at first and second speeds towards first and second positions, respectively. The first and second speeds are the nominal or average speeds at which the actuator travels in the first and second directions, respectively, and these speeds may be different due to the load on the actuator, the age of the actuator, etc. The method of the present invention functions identically if the speeds are, in fact, identical, if the first speed is greater than the second speed, or if the second speed is greater than the first speed.
In a preferred embodiment, the “first position” is at the end-stop for motion of the actuator in the first direction. Likewise, the “second position” is at the end-stop for motion of the actuator in the second direction.
The method of the present invention comprises the step of defining a first maximum speed and a first minimum speed for movement of the actuator in the first direction, whereby the first maximum speed is greater than the first speed and the first minimum speed is less than the first speed. Likewise, a second maximum speed and a second minimum speed are defined for movement of the actuator in the second direction, whereby the second maximum speed is greater than the second speed and the second minimum speed is less than the second speed. Alternatively, in some implementations of the present invention, the maximum and minimum speeds are the same in both the first and second directions.
During operation of the actuator, a plurality of commands are received whereby each command indicates whether to move the actuator in the first or second direction and how far to move the actuator. In response to the command, the actuator then moves in the specified direction for a specified period of time.
In response to each command, a first position estimate is derived wherein when the actuator moves in the first direction, the first position estimate is changed in response to the first maximum speed and the period of time, and when the actuator moves in the second direction, the first position estimate is changed in response to the second minimum speed and the period of time. Likewise, a second position estimate is derived wherein when the actuator moves in the first direction, the second position estimate is changed in response to the first minimum speed and the period of time, and when the actuator moves in the second direction, the second position estimate is changed in response to the second maximum speed and the period of time.
The method of the present invention comprises a next step of disabling further movement of the actuator in the first direction when the second position estimate indicates that the actuator is in the first position. Likewise, further movement of the actuator in the second direction is disabled when the first position estimate indicates that the actuator is in the second position.
As is evident from the foregoing, one object of the present invention is to minimize the amount of time required to accurately detect the saturated condition of a non-synchronous incremental actuator. It is another object of the present invention to accomplish the above objective with a minimal amount of overdrive. It is still yet another
Chen Jiade
Estill Brian T.
West Jonathan D.
Haas George E.
Johnson Controls Technology Co.
Masih Karen
Quarles & Brady LLP
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