Motors: expansible chamber type – Working member position feedback to motive fluid control – Electrical input and feedback signal means
Reexamination Certificate
2001-06-15
2003-03-25
Look, Edward K. (Department: 3745)
Motors: expansible chamber type
Working member position feedback to motive fluid control
Electrical input and feedback signal means
C092S060500
Reexamination Certificate
active
06536326
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to feedback control systems for controlling a free piston machine, and more particularly relates to apparatus and methods which are added to such control systems for the purpose of avoiding damaging collisions between the free piston and cylinder end structures, such as a cylinder head, against which the piston would, under some operating conditions, collide.
2. Description of the Related Art
Many types of machines utilize expansible chamber devices, such as a piston reciprocating in a cylinder, for operating upon gases. For example, they are used for pumping, compressing or displacing gases. Predominantly, pistons oscillate in linear reciprocation within a cylinder by means of a direct mechanical connection or linkage between a driver or a load and the piston. The linkage drives or is driven by the piston along a confined path of reciprocation and most commonly is a crankshaft and connecting rod system.
However, for some applications, such as high compression ratio compressors, free piston cryocoolers and free piston Stirling engines driving alternators, there are advantages to a free piston machine for such purposes. The piston is free because no mechanical linkage confines the piston to a fixed path of reciprocation. For example, a free piston may be driven by a linear, electric motor or a free piston Stirling engine. Typically, in order to maximize the efficient use of available drive power, free piston machines are driven at their frequency of mechanical resonance. Because the pistons are unconfined, the amplitude of reciprocation, may vary under the influence of changing operating conditions. Consequently, the piston, as well as any reciprocating structures attached to it, can collide at either end of the piston stroke with physical structures at the end of or beyond the cylinder.
In such freely reciprocating machines, the amplitude and frequency of reciprocation are a function of inertia, damping, and spring and driving forces. Therefore, these machines share the common feature that, when they are overdriven or underdamped, the reciprocating parts can acquire an amplitude of reciprocation that exceeds the internal geometrical limits of the space available for the motion of the reciprocating parts. If the amplitude of reciprocation is allowed to increase indefinently, the reciprocating parts will eventually collide repeatedly at the frequency of operation with stationary structures, or even with other reciprocating parts.
In free piston machines, it is always desirable to avoid collisions which have sufficient impact to damage the machine. However, under some operating conditions it is often desirable to maximize piston amplitude or stroke in order to maximize machine performance. Consequently, it is often desirable to operate the machine at the maximum amplitude which the machine can tolerate without damage to the machine. This requires accurate control of piston amplitude and rapid response to any changes in it.
A common practice in prior art free piston machines is to control one or more of the machine parameters, which affect or are affected by the forces applied by or to the piston, such as piston drive, by means of a conventional, closed-loop, negative feedback, control system. Such systems can be either analog or digitally controlled systems. Because piston amplitude is a function of piston forces, these systems exert some control over piston amplitude and assist in avoiding collision. However, because piston amplitude is a function of piston forces and some piston forces are a function of machine loading, and because machine loading can change, sometime rapidly, under varying operating conditions, such systems often do not permit operation at maximum amplitude of piston reciprocation because they cannot control piston amplitude with sufficient precision. Under large loading, the drive force can be very large, but the piston may not approach anywhere near a collision. However, reduction of the load can result in an increase in piston amplitude, permitting a collision to occur.
In a conventional feedback control system, including those applied to free piston machines, the control system compares a measured value with its desired value to produce an actuating error signal, which is acted upon to reduce the magnitude of the error. Referring to
FIG. 1
, a controlled system
10
, such as the piston driver, for example a linear motor, is acted upon by a dynamic control element
12
having some preset forward transfer function. The control element
12
does the work of controlling the controlled system
10
. For example, the control element
12
typically is a high gain amplifier. Historically the control element was an analog circuit, but its forward transfer control function can also be performed digitally by a microcontroller or discrete logic circuitry.
A feedback element
14
, usually a sensor, applies a feedback signal to a summing junction
16
to provide the measured value. The feedback signal can be a function of piston amplitude, piston drive, piston displacement or other parameter which affects or is a function of piston amplitude.
One prior art system offering advantages in free piston machines is illustrated in U.S. Pat. No. 5,342,176, which is herein incorporated by reference. The command input
18
, also referred to as the control input or reference input, provides the set point for the feedback control system and is summed with the feedback signal at the summing junction
16
. This set point represents the desired value.
The command input may be a fixed quantity based upon the motor's electromechanical transfer constant and a particular stroke. Alternatively, it can be a variable controlled independently or by an external, physical phenomenon, or by a control circuit that seeks to maintain a particular value of that external phenomenon. For example, the control input can be the output of a summing junction receiving inputs derived from a sensor measuring a temperature or pressure affected by the free piston machine, or from a temperature or pressure set point. Of course, the control input may, in its simplest form, be a manually input adjustment.
The output
20
of the summing junction
16
provides an error signal, which is applied to the dynamic control element
12
. The error signal is the difference between the feedback signal and the reference input signal. Of course, the summing at the summing junction
16
may be performed in the more historical manner in an analog summing circuit, or alternatively it may also be performed digitally in a microcontroller or discrete logic circuitry. As known to those skilled in the art, not only may each of these elements of the control system and their signals be performed either in an analog or digital format, but hybrid systems, which include analog to digital converters and digital to analog converters, can also be constructed which utilize some of each mode. Consequently, the term “summing junction”, as well as other control system terms, is not limited to analog circuits, but include digital implementations.
For purposes of describing the invention, the term “cylinder end structure” is used to refer to a physical body at either end of the linear path of piston reciprocation with which the piston, or structures linked to and oscillating with it, can collide if its amplitude of oscillation increases excessively. Most typically, this is a cylinder head in which valves are mounted. The term “piston drive” or “drive” is the driving force or power applied to the piston to force it in its reciprocating, linear oscillation. Since piston amplitude is an increasing function of piston drive, an increase or decrease of piston drive, respectively increases or decreases the amplitude of piston oscillation if other parameters remain constant or undergo only variations which do not completely negate the change in piston drive.
It is, therefore, an object and feature of the present invention to provide a control system for a free pist
Keiter Douglas E.
Unger Reuven Z.
van der Walt Nicholas R.
Leslie Michael
Look Edward K.
Sunpower, Inc.
LandOfFree
Control system and method for preventing destructive... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Control system and method for preventing destructive..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Control system and method for preventing destructive... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3051024