Patent Grant
11466678
This invention generally relates to a linear motor compressor and associated systems and methods for gas compression operation, i.e., a natural gas vehicle home refueling appliance.
There is a rapidly developing need for natural gas vehicle (NGV) refueling stations and similar installments that require safe and cost-effective pressurization, movement and delivery of a process fluid, such as natural gas. Such installments may be used to fill vehicles, dispense process fluid, provide pressure boost stations for gas pipelines, fill energy storage systems or storage tanks, refrigeration and process fluid compression and other needs.
Existing natural gas compressors are largely based on reciprocating compressor technology, in which a rotational electric motor drives a crankshaft in a multi-piston compressor. These units suffer from high manufacturing costs, high mechanical parasitic losses, relatively high maintenance costs, and short operational lifespan between repairs. In addition, existing units are often unsuitable for specialty compressor applications where contamination or leakage of the process fluid is unacceptable. Applications requiring high purity process fluids that are compressed to elevated pressures have limited and costly options.
Linear motor compressor controller strategies have generally relied upon mechanical or pneumatic springs or electromagnetic coils to provide stability and ensure the piston has a returning force to center. As an example, U.S. Pat. No. 6,231,310, issued to Tojo et al., stabilizes its system about a central point by using a spring. A position feedback is used to oscillate about the stable position by changing the amplitude and frequency of a sinusoidal source. U.S. Pat. No. 4,750,871, issued to Curwen, stabilizes a linear motor by using external cylinders to hold the reciprocator in a centered position. External AC and DC coils are used to stabilize the system. The disclosed servomechanism is either a series of valves and ports actuated by the motion of the piston or a combination of AC and DC coils activated by position feedback.
A need therefore exists for a simple mechanical solution for a compressor station using a minimum of moving parts in a durable and robust configuration that will also satisfy the specialty compressor requirements.
Accordingly, the subject invention relates to a Free Piston Linear Motor Compressor (FPLMC), which preferably eliminates all but one major moving part and improves durability and compressor system efficiency, while significantly decreasing manufacturing costs, installation, and maintenance of gas compression, which includes but is not limited to natural gas, other hydrocarbons, hydrogen, and air.
This and other objects of the invention are addressed in one aspect of the invention by a system that includes a multi-stage dual-acting free piston driven by a linear motor. The subject arrangement is preferably used in connection with an integrated staged compressor and linear motor to result in, for example, an appliance for natural gas vehicle fueling, particularly direct fill into an unattended vehicle.
The invention further includes a control strategy that provides stability without the need for a centering force of any kind, whether mechanical or pneumatic springs or electromagnetic coils. The unique ability provided by this invention allows the piston to operate in a stable manner about any point throughout the stroke, not just about center position. With the control strategy of this invention complexity within the linear motor compressor is reduced by removing springs or additional electromagnetic coils, thus simplifying manufacturing and reducing cost and size.
The invention includes a robust free piston linear motor compressor control system that accommodates a wide range of linear motors and power system architectures. In embodiments of this invention, the linear motor compressor includes a compressor housing, a cylinder housing having a plurality of opposing compression chambers, a piston freely reciprocating within the cylinder housing, a linear electric motor positioned to reciprocate the piston, and a piston position feedback control system configured to provide adaptive current output as a function of position feedback and/or velocity feedback from the piston and/or the electric motor, to directly power and control the electric motor.
In embodiments of this invention, the control system determines motor force requirements from estimated position values and/or velocity values. An observer routine can be used to produce the position and velocity estimates from position and current measurement alone.
The control system can include a linear encoder feedback loop to track a position and/or a velocity of the electric motor or the piston. The control system then determines a current required to generate the motor force requirements as a function of the position feedback and/or the velocity feedback. The control system allows the piston to reciprocate without assistance from a mechanical spring or other centering force/mechanism.
In embodiments of this invention, the control system uses reference position values and/or velocity values for comparing to the position feedback and/or the velocity feedback to adjust current to the linear electric motor. The reference signals of the position and/or velocity may be sinusoidal or of random description.
In embodiments of this invention, a linear quadratic regulator is used to provide stable operation while minimizing state error and observing the limits of the control signal.
The controller of this invention is robust enough to handle deviations in behavior between the actual compressor through the entire range of operation and an idealized mass spring system. This has been demonstrated in simulation and hardware with a compressor driven with reluctance linear motor. It has also been demonstrated in simulation with a compressor driven with permanent magnet linear motor. It has further been demonstrated that the control is stable with a bandwidth of 20 kHz which is readily obtainable with a range of digital signal processors.
In embodiments of this invention, the system requires a power system link to be supplied with an unrestricted source to prevent instability between the link and the linear motor.
The control strategy of this invention is capable of being applied to multiple linear motor topologies for the compressor. These include permanent magnet motors, induction motors, voice coil motors, reluctance motors, and/or homopolar induction motors.
These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the following drawings.
One preferred application of the subject invention relates to refueling of natural gas vehicles. Although described in detail below with respect to NGV refueling stations, the subject invention is not limited to such applications and numerous other suitable applications achieving various pressure levels and producing various flow rates are likewise appropriate for use with the subject invention.
Natural gas refueling in a consumer or home environment is critical to the widespread adoption of natural gas vehicles and presents a unique opportunity for consumers to save significantly on the cost of fuel on a per gallon equivalent advantage over gasoline and diesel and enjoy the convenience of fueling at home. Traditional home refueling appliances (HRAs) have relied on multi-piston reciprocating compressors driven by a rotary electric motor. These systems are complicated, expensive, and have historically suffered poor reliability. The free piston linear motor compressor solves these problems by using a linear motor to drive a single, multi-stage piston, reducing complexity and part count, which improves overall reliability and simplifies manufacturing. Furthermore, efficiency of the linear motor compressor may be improved by operation at a resonant frequency with low friction coatings and reduced clearance volume losses.
To obtain an efficient linear motor design in a compressor application, it should preferably operate at a resonant frequency. This is traditionally accomplished with a mechanical spring in which the mass of the piston and the spring dictate the system's natural, resonant frequency. Heavy duty springs needed for high pressure applications can be bulky and costly, leading to a larger, heavier, and more costly compressor design. Mechanical springs are also often a wear or maintenance item resulting in interruptions in operation of the unit for routine maintenance. Mechanical springs can also be prone to fatigue and catastrophic failure, especially when operated at relatively high frequencies and high temperatures as expected with the natural gas compressor for refueling natural gas vehicles. For at least these reasons, the preferred design preferably does not use mechanical springs, instead utilizing compression chambers as a dual purpose compression chamber and gas spring. This simplifies the design by eliminating all dedicated spring-like components, and simply using the stranded gas remaining in the compression chamber as the spring, allowing for operation at resonance.
According to one preferred embodiment, the FPLMC concept, depicted in
One preferred compressor design results in four-stages of compression with compression ratios of approximately 4:1 per stage. The design assumes natural gas inlet pressures of 1 bar and has the ability to compress to at least 290 bar. This preferred compressor design operates at 15 Hz resonant frequency and has a natural gas flow rate of 60 liters per minute (˜2 standard cubic feet per minute (scfm)). The preferred compressor design is driven by a reciprocating reluctance linear motor operating on 240V, single-phase, 30 A service and capable of providing a 3,000 N compression force.
Thermal management of the linear motor and inter-stage gas are also important as reduced temperatures may further improve the overall compression efficiency of this device. Methods of heat management include forced air or water cooling to integrated heat pipes that use hermetically sealed refrigerants.
A resulting FPLMC making use of a single piston to achieve multiple stages of compression is one preferred component of the subject invention. As a result of the subject invention, a uniquely coupled electromagnetic compressor includes a fully integrated and optimized electric motor and compressor that are no longer independent.
According to a preferred embodiment of this invention shown in
A piston 30 is freely positioned within the cylinder housing 20 to reciprocate freely back and forth or up and down (any orientation is achievable) within the cylinder housing 20 thereby alternatingly charging (pressurizing) opposing compression chambers 25. A preferred arrangement of the piston 30 permits bi-directional drive and free reciprocation within the cylinder housing 20. According to one preferred embodiment, the piston 30 freely reciprocates within the cylinder housing 20 such that compression discharge from an outlet of a chamber of one side of the opposing compression chambers 25 feeds an inlet of another chamber. As described, for maximum efficiency, the piston 30 preferably operates at resonant frequency.
The plurality of opposing compression chambers 25 preferably comprise a series of stepped diameter compression chambers positioned at opposing ends of the cylinder housing 20. Alternatively, the plurality of opposing compression chambers 25 comprise compression chambers of a single diameter at opposing ends of the cylinder housing. The former embodiment may, though not necessarily, be more suited to a plurality of stages while the latter embodiment may be more suited to a single or two stage arrangement.
In this manner, compression is preferably achieved with a single primary moving part. In addition, in a preferred embodiment of this invention, the piston 30 reciprocates without assistance from a mechanical spring. A low friction coating on the piston 30 and/or cylinder housing 20 may be used in combination with a seal material optimized for a process fluid to reduce energy consumption and increase seal life.
In addition, the invention further includes a linear electric motor 35 preferably positioned in-line relative to the compressor housing 10 to reciprocate the piston 30. The linear electric motor 35 may be adapted to the cylinder housing 20 or otherwise positioned in an integrated or non-integrated manner to facilitate efficient reciprocation of the piston 30 within the cylinder housing 20. In one embodiment of this invention, the linear electric motor 35 is directly coupled to the piston 30.
According to one preferred embodiment, the linear motor compressor of the present invention may include a compressor housing 10 and/or a cylinder housing 20 that is pressurized with a process fluid. In addition or alternatively, the compressor housing 10 may include a blowdown volume 15 for depressurizing the compressor and related systems at the conclusion of the compression process. In this manner, the linear motor compressor assembly may be hermetically sealed. By hermetically sealing the compressor chambers 25 and the linear electric motor 35 in the same housing, certain hazards may be avoided when the process fluid is combustible or otherwise volatile. Sealing the relevant components permits operation at high pressures without contamination from outside sources and without risk of combustion due to sparking, arcing or other hazards that may occur depending on the installation.
According to one embodiment, the linear electric motor 35 includes a reluctance motor with dual opposing winding cores. Alternatively, the linear electric motor 35 may comprise a permanent magnet motor, an induction motor, a voice coil motor, a reluctance motor, or an alternative linear motor variant. Further, in one embodiment, the compressor system described herein may include a motor stator fully integrated within the housing. In each case, the preferred linear electric motor 35 will be robust and engineered to endure the high frequency cycles and load volumes expected for applications such as described herein.
The system may additionally be optimized with various further embodiments. For instance, according to one preferred embodiment, an integrated motor and process fluid cooling system may be utilized for heat removal. Integrated motor and interstage gas coolers may use forced air convection and require only one fan or blower.
Also a piston position feedback control system 45 with adaptive current output to minimize energy required to do work may be employed. The preferred embodiment utilizes a linear encoder feedback loop to track the position of the linear motor/piston, allowing the controller to adjust the current up or down in order to maintain an optimized frequency.
In embodiments of this invention, a control strategy provides stability without the need for a centering force of any kind, whether mechanical or pneumatic springs or electromagnetic coils. This ability allows the piston to operate in a stable manner about any point throughout the stroke, and not just about a center position. The control strategy of this invention reduces complexity within the linear motor compressor by removing springs or additional electromagnetic coils, thus simplifying manufacturing and reducing cost and size.
In embodiments of this invention, the control strategy includes position feedback to stabilize a magnetic forcer, at each instant in time using principles of automatic control. This provides improvement over, for example, using position to oscillate by controlling phase and amplitude of a sinusoidal source.
As illustrated in
These current commands are then fed to a typical PI current control block 126 to control the motor drives 132. The currents delivered to the motor 135 are measured and fed back to the current control 126 and a force estimator 128 based on the actual currents. The linear motor 135 drives the compressor and a linear encoder or potentiometer feeds back position information to an observer, in this case a Luenberger Estimator 125, to estimate position and velocity of the piston 130. The observer is also supplied with the force estimate.
This architecture readily adapts to, without limitation, reluctance, permanent magnet, induction, and/or homopolar motor linear motor variants. For example, in the case of the reluctance motor the current estimator is based on the inductance and inductance gradient as a function of position for the particular motor architecture, whereas for the permanent magnet motor the d-axis and q-axis currents of the three phase motor can be controlled to position the traveling wave, and the resultant d-axis and q-axis voltages are converted to three-phase values through a Parks transformation to gate the inverter. In the case of the reluctance motor the inverter drive is a pair of H-bridges each controlling an individual coil. In the case of the PM motor the inverter drive is a three phase bridge producing the currents to create the traveling wave. It can be seen that the control architecture is robust and readily adaptable to different linear motor types and their control.
In embodiments of this invention, the control can be set up with very little knowledge of what is actually being controlled. The actual compressor plant, motor, and drives can be replaced by a black box 140 as represented in
By assuming that the black box is a simplified mass-spring system with some potential losses, the gains for the state space controller 122 and Luenberger estimator 125 can be determined using built in routines in Matlab. The mass spring system can be represented in a linear state-space format (1), which is expanded as shown in equation (2), where the states x1 and x2 are the flotor position and velocity, respectively. The important control design parameters in equation (2) are the equivalent spring stiffness, keq, flotor mass, Mf, and viscous friction coefficient, Bd. The control force is the motor force, Fm The equivalent spring stiffness can be estimated from evaluating the peanut shaped force displacement curves from the compressor simulation, as illustrated in
The friction coefficient is typically more difficult to define. The actual friction is coulomb type friction which is not linear and appropriate for the simplified system description. One potential method to estimate a value for Bd is to select a value which will yield an equivalent energy consumption per cycle, E, for the mass spring system as observed from the actual compressor mode, i.e. the area within the force displacement loop (
For a state feedback controller, the required motor force can be calculated by multiplying the state deviations from a reference state by respective gains, K1 and K2. The behavior of a full state feedback controller will result in the system equation (4). To ensure stability in equation (4), the gains must be selected so that (A−BK) has all negative eigenvalues.
The gain values K can be selected by pole placement schemes to achieve a desired response. The issue here is that there is literally an infinite number of pole options to choose from. To simplify the task, the state feedback controller has been limited to a specific type of controller, referred to as a Linear Quadratic Regulator (LQR). LQR control is a type of optimal controller that is really a sub-optimal control. This sub-optimal control seeks to minimize the infinite horizon cost function:
J(t0)=½∫0∞(xtQx+utRu)dt (6)
where Q is a weighting matrix that penalizes deviations in the state variable and R is a weighting matrix that penalizes the amount of input used to control the system. The matrix Q must be semi-positive definite, and R must be positive definite. To get the gain values K from this cost function requires solving the Algebraic Ricatti Equation (ARE). Matlab offers a function, lqr, which will calculate the gains, ARE solution, and resulting pole locations (4), for a system defined by A and B, with weighting matrices Q and R.
Next the weighting matrices Q and R can be determined. To determine these values, the method referred to as “Byron's Rule” can be followed. First off, R is set to a value of 1. This makes R positive definite, as required. (Also, as one goes through to solve the ARE, one will notice that Q and R become a ratio, so R might as well be set to unity). For Q, Byron's rule suggests using a diagonal matrix as follows:
where δx1 and δx2 are the largest acceptable state deviations. In addition with the model parameters defined by A, these are the additional parameters for tuning the LQR controller.
A preferred method of tuning is by changing the values δx1 and δx2 which are from the previous discussion. Decreasing these values will increase the gains, i.e., less acceptable path error. It was found that gain tuning was more sensitive to δx1 than δx2. The variable δx1 can be set relatively tight to values of 1e-7 to 1e-5, while δx2 can be kept to 1 or greater. If either of these variables are set too small, very large control gains will be calculated which can drive the actual system unstable. Very large gains here will also increase the observer gains and cause computational slow down, plus possible unstable behavior.
The motor design of embodiments of this invention only measures the flotor position, which is provided by a digital encoder. For the LQR state feedback controller to work, both position and velocity must be provided. To estimate the flotor position and velocity, a Luenberger observer is employed, such as shown in
State estimates can be calculated by solving the following differential equation:
{circumflex over ({dot over (x)})}=A{circumflex over (x)}+Bu+L(ym−C{circumflex over (x)}) (8)
where C is the linear system output matrix, and ym is the measured signal, which is the encoder measurement in our case. The control input u is the motor force, Fm. For the observer, this value is not the requested motor force derived from the state space controller, but rather the calculated force based on coil current measurements and coil inductances estimated from estimated position
The observer gains are defined by the vector L. These gains are selected so that (A−LC) produces a pair of stable negative eigenvalues. As mentioned for state space controller, pole placement techniques can be used to determine the locations of these eigenvalues. For the observer to function properly with the state feedback controller, the observer should respond at least 10 times faster than the closed loop state feedback controller. Since the pole locations for the LQR controller have been determined, the L vector is calculated by placing the eigenvalues of (A−LC) to 10 times the LQR values. This solution can be accomplished using the “place” command in Matlab.
Location of the physical control hardware can be important in various embodiments of the invention. The DSP, the power electronic gate fiber optic transmitters, the position encoder power supply, and the power supplies for the current measurement transducers are desirably in one Faraday enclosure. The DSP and fiber optic transmitters desirably derive their power from a common supply. The gate leads from the fiber optic receiver cards to the transistors desirably are twisted pair shielded. The power supplies for the transistor gate circuits and any relay controls (i.e., soft start and dump link capacitor) typically should be plugged into isolation transformers. Any signals entering or leaving the Faraday enclosure desirably are passed through wave guides. Galvanic signals coming into the Faraday enclosure should be kept at a minimum. If possible, the DSP desirably has differential inputs for analog input signals.
A mechanical failsafe (not shown) may be further incorporated into the subject invention, for instance using compliant stator laminations and compressor heads to decelerate the piston during a failure mode. Ideally, in the event of an impact and control system failure, armature motion will automatically be contained in a fail-safe manner, greatly reducing the potential for damage or gas leaks.
According to a preferred embodiment of this invention, the arrangement of components as described may result in the following preferred or unique features/attributes of the invention. It is desirable for the invention to include one or more stages of compression with a single piston. Motive force is preferably supplied with a custom designed linear reluctance motor, although other motor variants such as permanent magnet, induction, and homopolar induction have also been designed.
According to one embodiment of the linear electric motor 35, a reluctance motor may include dual opposing winding cores that provide reciprocating linear motion. A reluctance motor armature, or moving part, has low losses and allows for a sealed motor housing, which can act as a receiver volume for the depressurization of the compressor.
In the preferred embodiment, compression stages are designed such that the differential pressure acting across seals is reduced by placing lower stages next to higher stages such that the pressure of the lower stage is acting on the back of the high pressure stage. This reduces the net force acting on the seal, improving seal life and durability.
Low profile valve design and unique valve locations preferably minimize a volume in the compressor which does not contribute to work. This improves the efficiency and reduces net power required for compression. The compressor cylinders may be manufactured with unique interlocking scheme to allow ease of alignment and service.
The linear motor compressor of the subject invention may further include a directly coupled compressor piston and motor armature. For example, a rigid piston or a flexible coupling may be positioned between the piston and an armature of the linear electric motor. The flexible coupling between compressor piston and motor armature as described preferably allows for independent alignment.
Resonant frequency operation preferably based on mass and dynamic gas spring may be used to increase system efficiency. As described, a dynamic gas spring preferably replaces a mechanical spring in the subject system. Advanced controls allow for operation without mechanical springs.
Advanced controls may further allow for minimal gap/volume at end of compression stroke, thus minimizing volume which does not provide useful work. In addition, such controls may enable position only control, velocity only control, or control with no external sensors (sensorless control) through active inductance measurements of the linear motor coils.
The reluctance motor as described may use laminated polygonal design to reduce cost and ease fabrication and assembly. The segments are preferably laminated in the direction perpendicular to current flow to limit losses and improve controllability. One coil preferably links all polygonal segments eliminating end turns in the individual segments and reducing losses. In addition, the segments preferably lock into a sealed stator housing. As described above, the motor is preferably vacuum pressure impregnated to provide insulation integrity. Alternatively, the reluctance motor may use a circular lamination design with similar design and benefits as described above.
As described, the resulting FPLMC system creates numerous advantages including: (1) reduces friction losses, no rotary to linear motion conversion; (2) reduces part count, uses single piston for multiple stage compression; (3) reduces differential seal pressure, increases seal life; (4) reduces moving parts, reduces maintenance; (5) control algorithm allows removal of mechanical spring typically used in linear motor compressor for resonant frequency operation; (6) reluctance motor design allows for sensorless control, eliminating additional sensors which add to cost and prone to fail; and/or (7) reduces costs and increases overall reliability of gas compressor.
Other potential markets, besides direct natural gas vehicle refueling include: (1) commercial CNG fleets where a low cost compressor could be paired with multiple vehicles for the convenience of unattended fueling; (2) assisting with NGV fast-fill dispensing to complement storage pressure equalization; (3) gas pipeline pressure boost stations; (4) hydrogen vehicles refueling; (5) air compressors for SCBA, SCUBA, and energy storage systems; (6) refrigeration and industrial gas compression; and/or (7) on-board a vehicle fuel pressure booster compression to deliver fuel to the engine of the vehicle.
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain details described herein can be varied considerably without departing from the basic principles of the invention.
This application is a continuation-in-part of U.S. patent application Ser. No. 14/536,174, filed on 7 Nov. 2014, which claims the benefit of U.S. Provisional Patent Application, Ser. No. 61/901,176, filed on 7 Nov. 2013. The co-pending parent application is hereby incorporated by reference herein in its entirety and is made a part hereof, including but not limited to those portions which specifically appear hereinafter.
This invention was made with government support under Grant No. DE-AR0000257 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
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| Number | Date | Country | |
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| Parent | 14536174 | Nov 2014 | US |
| Child | 15785963 | US |
