Motors: expansible chamber type – Working member position feedback to motive fluid control – Electrical input and feedback signal means
Reexamination Certificate
2002-11-05
2004-03-16
Look, Edward K. (Department: 3745)
Motors: expansible chamber type
Working member position feedback to motive fluid control
Electrical input and feedback signal means
C091S397000, C091S41700A
Reexamination Certificate
active
06705199
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of control systems for pneumatic actuators. More specifically, the present invention relates to a servo control system that provides precise and repeatable control of a pneumatic actuator.
BACKGROUND OF THE INVENTION
Mechanical actuators are used in a variety of industrial applications to move machine elements from one position to another. There are three different ways to power the movement of an actuator, electrically, hydraulically or pneumatically. Electrically powered actuators are used in situations requiring precise control and repeatability. An electrically powered actuator such as a screw drive or belt drive system powered by a rotary servo motor system has the ability to move at different speeds and to stop at any location along the entire length of the stroke of the actuator. Unfortunately, electrically powered actuators are prohibitively expensive for applications involving moving large loads or moving loads at rapid speeds. Hydraulically and pneumatically powered actuators, on the other hand, use a fluid (oil for hydraulics vs. air or gas for pneumatics) to provide a substantial force to a piston that moves inside a chamber and is connected to the actuator. Consequently, hydraulic and pneumatic actuators can move large loads and can move those loads at rapid speeds if desired. Hydraulic and pneumatic actuators also tend to be more durable than electrical actuators.
While hydraulic actuators are well suited for many applications, their use is limited to those environments where oil can be used as part of the machine. For many applications, it is often not practical to utilize oil as the fluid to power an actuator. Pneumatic actuators also tend to be less expensive than hydraulic actuators or electrical actuators. The problem is that pneumatic actuators are much more difficult to precisely control than electrical actuators or even hydraulic actuators. Consequently, most pneumatic actuators are designed to position the actuator at only two stop positions, one at each end of the stroke of the actuator where the end of the chamber, a stopper or the like, serves to physically stop the travel of the piston, thereby positioning the actuator at one of these two stop positions. This kind of two-stop pneumatic actuator is controlled simply by supplying pressurized air to one side of the piston until the piston reaches the end of the stroke.
To control a pneumatic actuator to stop at positions other than the ends of the stroke of the actuator, the most common technique is to supply air at different pressures to both sides of the piston. Initially, this differential pressure will start the piston moving in a direction from the side with the higher pressure to the side with the lower pressure. Once the piston is moving, this differential pressure is reversed to cause the piston to stop moving. Ideally, the differential pressures can be applied to cause the piston to start and stop exactly at any desired location along the stroke of the actuator. In reality, adjusting the differential pressure to achieve the delicate balance required to precisely control the stop positions of the actuator is quite difficult due at least in part to the compressibility of the air or gas that is used as the fluid to power and control the actuator. These problems are compounded in situations involving changing loads, long stroke lengths or vertically oriented stroke directions, or in situations where the pressure of the air or gas used to power the actuator is not tightly controlled.
The most common way of adjusting the differential pressure for this kind of pneumatic actuator is by controlling a variable valve or pair of variable valves, which are sometimes referred to as proportional valves or servo valves. Examples of control systems developed for differential pneumatic actuators that use proportional valves to control the differential pressure are described in U.S. Pat. Nos. 4,481,451, 4,666,374, 4,790,233, 4,819,543, 5,154,207 and 6,003,428 and German Patent No. DE 3313 623 A1. Other variations on controlling a differential pneumatic actuator are described in U.S. Pat. No. 4,878,417 which varies the proportional flow of the fluid in response to measurements from an accelerometer, and U.S. Pat. No. 5,424,941 which uses a control system that converts differential pressure into a differential mass flow of the air that moves the piston in an attempt to minimize the problems caused by the compressibility of air.
An alternative technique for controlling the differential pressure of a pneumatic actuator is to use pulse width modulation (e.g., different widths of control pulses) to control the supply of pressurized air to both sides of the piston. Instead of turning a servo valve part way on to control the rate that air flows through the valve, pulse width modulation controls the rate by quickly turning the valve on and then off such that the average time the valve is on is equivalent to the proportional setting of a valve turned part way on for the same period of time. Examples of this pulse width modulation technique are described in U.S. Pat. Nos. 4,628,499, 4,763,560 and 4,907,493. U.S. Pat. No. 4,741,247 describes a very slow version of a pulse width modulation scheme where a series of step volumes of air are introduced into the chamber one at a time in order to move the piston a distance equal to the step volume.
Various attempts have been made over the years to address the problems caused by using air as the fluid to power a pneumatic actuator. One approach has been to use some form of a brake to assist in stopping the piston or the actuator. German Patent No. DE 2,327,387 describes an early use of an electromagnetic friction brake to stop a pneumatic actuator. This patent uses a conventional proportional servo valve to control the differential pressure. Once the actuator passes by a predetermined starting point for braking, the electromagnetic friction brake is applied intermittently to slow the piston down until it is moving at a much slower speed, at which time the brake is applied continuously to completely stop the actuator.
One of the problems with using a brake, however, is that the brake surface will wear down with repeated use and this results in variability in how accurately the system operates over time. U.S. Pat. No. 4,106,390 describes a pneumatic linear actuator where a pneumatic mechanical brake is activated in a braking cylinder separate from but connected to the piston to prevent wear directly on the piston. The pneumatic mechanical brake is only applied to stop the actuator after a three-stage series of air braking decelerations are performed by operating solenoids in response to output signals generated by a sequence generator.
Other types of brakes have also been used as part of a control system for a pneumatic actuator. U.S. Pat. No. 4,932,311 describes a pneumatic actuator having a magnetic rotary brake coupled to the piston by a ball screw shaft. A two-stage braking scheme based on a target braking speed is used to control the stopping locations of the piston. Once the piston passes the location where braking has been programmed to start, either or both an air braking arrangement and the magnetic brake may be applied at different periods along a braking process in either an intermittent or a continuous mode to keep the speed of the piston on target with a calculated braking speed. By attempting to control the speed of the actuator to match the calculated braking speed, the patent seeks to regulate the pneumatic actuator in a way that can tolerate and compensate for changes in the system, including changes in the brake.
A recent example of a controllable pneumatic actuator that uses a rotary proportional magnetic brake is described in PCT Publ. No. WO 00/53936. Both a simple control system and a sophisticated control system are described. In the simple control system, differential pressure is applied though a three-position solenoid valve to drive the pneumatic actuator until it is within a defined
Gladen Paul B.
Hochhalter Keith
Liao Chen-Fu
Leslie Michael
Look Edward K.
Patterson Thuente Skaar & Christensen P.A.
Tol-O-Matic Inc.
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