Control method utilizing directionally based control...

Data processing: generic control systems or specific application – Specific application – apparatus or process – Mechanical control system

Reexamination Certificate

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Details

C700S056000, C318S571000

Reexamination Certificate

active

06640162

ABSTRACT:

BACKGROUND
1. Field of Invention
This invention relates to controls that are commonly used to position and control devices in various applications.
2. Description of Prior Art
There are many actuator applications, including many heating, ventilating, and air conditioning (HVAC) applications, where accurate, repeatable positioning of a device or an actuator is required. One common, simple, durable, inexpensive type of electrically driven actuator is often referred to as an incremental control type actuator. In this patent application the term “incremental control type actuator” will be used to refer to any device that is controlled based on the duration of the control signal. Incremental control type actuators often produce movement in two distinct directions based on a control signal that has two parts. One signal causes the actuator to drive in one direction, another signal causes the actuator to drive in another direction, and the absence of either signal causes the actuator to remain in the present position. An incremental control type actuator is distinct from other actuators that use a variable control signal that corresponds to a particular control value or position. Actuators that are not incrementally controlled operate based on some variable type control signal, such as variable voltage, variable current, modulated pulse width, or variable pneumatic pressure. Incremental control type actuators are often referred to by the following names:
1) floating control type actuator
2) floating point type actuator
3) position adjust type actuator
4) tri-state control type actuator
5) series 60 type actuator
6) three-wire SPDT type actuator
Although this new control method can be successfully applied to other incrementally controlled devices or final control elements (in addition to what is commonly referred to as an actuator) this discussion will focus on common actuators. There are many incremental control type actuators in use today, but the available control options are very limited. Due to the current control methods being applied to incremental control type actuators, correcting or modifying a control system that uses an incremental control type actuator often requires installing additional equipment, such as position sensors, or replacing an incremental control type actuator with an actuator that can accept another type of control signal. When an incremental control type actuator is replaced with an actuator of a different type then the controller hardware often must be changed as well, since incremental control type actuators, unlike most other types of actuators, can be controlled by inexpensive controllers that do not have analog outputs.
There is no satisfactory position control method that can be applied to incremental control type actuators without requiring the use of position switches or position sensors. Current positioning methods applied to incremental control type actuators need position switches or position sensors to operate accurately and repeatably. Because of the current methods used to position incremental type actuators control manufacturers strongly recommend the use of closed loop position control systems (systems with position feedback) anytime that the position of the final control element is of any importance. Using a position sensing devices adds to the final installed cost of any system. Beside the cost of purchasing the sensing device, there are also additional costs to install, calibrate, and setup the position sensing device. Some position switch installations are complicated and require a skilled installer. One example of closed loop position control applied to an HVAC economizer control apparatus can be seen in U.S. Pat. No. 4,462,539. It is very desirable to have a position control method that does not require the additional expense and complexity associated with closed loop systems.
One common example of open loop position control (position control without position feedback) in the HVAC area is a simple economizer controller. The damper control portion of a simple economizer controller is basically just an open loop position controller that functions to achieve the following.
1) The damper actuator is commanded fully closed when the associated space thermostat is not calling for the fan to run.
2) The damper actuator is commanded to a predetermined, adjustable position determined by a potentiometer when one of the following conditions occur.
a) The thermostat is calling for the fan to run and the outside air is too hot to be used for cooling.
b) The thermostat is calling for the fan to run and the thermostat is not calling for cooling.
3) The damper actuator is commanded to the fully open position when the thermostat is calling for the fan to run and the outside air is cool enough to be used for cooling and the thermostat is calling for cooling.
Economizer controls were developed by Honeywell, Incorporated after U.S. Pat. No. 3,979,922 was granted. The open loop damper control method outlined above is widely used as part of many economizer packages that save energy by using outside air to cool instead of relying solely on mechanical cooling. While the damper control method described above is simple and effective, all of the open loop versions require the use of an actuator that is positioned based on a modulating control signal. The open loop positioning method used in Honeywell's economizer controls requires the use of a modulating motor that operates based on a variable resistance or variable voltage signal. There is no simple open loop control method compatible with incremental control type actuators.
Incremental control type actuator are often used for two-position control where an actuator is driven to either of two extreme positions by a two-position signal. Incremental control type actuators are also used for a form of modulating control. The basic reasoning behind using an incremental control type actuator for modulating control is to use a simple controller and an associated actuator to produce a gradual, continuous action in either of two directions which continues until the controlled variable reaches the setpoint value, or the actuator reaches its travel limit. Once the setpoint is reached, the controller no longer drives the actuator and it remains in its last position until a new position is required.
The most common way that incremental control type actuators are used in heating, ventilating, and air conditioning (HVAC) systems is for a microprocessor based controller to position an actuator and its associated damper or valve as necessary to maintain the setpoint of a controlled variable. The controlled variable is often temperature or pressure. In these applications, although the actuator position is being directly manipulated, the position is not the variable being controlled. In applications where the damper or valve position is the controlled variable, a position sensor functions as a feedback device to keep track of the actuators position and the control method is classified as a closed-loop control method.
Some current open-loop control strategies that use an incremental control type actuator for temperature or pressure position control also use a microprocessor based controller to keep track of the actuator's position. The idea is that since the actuator's stroke time is known (stroke time is the time required for the actuator to move from one extreme position to another extreme position), the actuator's position can be known if the initial position is known and the time spent opening and closing is known. If an incremental control type actuator has a stroke time of 60 seconds, and an initial position of fully closed, and the actuator is driven toward the open position for 30 seconds then the controlled element is assumed to be 50% open. If the actuator is then driven toward the closed position for 15 seconds then the controlled element is assumed to be 25% open. Theoretically a microprocessor can keep performing these calculations with every movement and always know the position of the associated

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