Abstract:
A free piston gas compressor comprising a cylinder, a piston reciprocable within the cylinder and a reciprocating linear electric motor derivably coupled to the piston having at least one excitation winding. A measure of the reciprocation time of the piston is obtained, any change in the reciprocation time is detected and the power input to said excitation winding is adjusted in response to any detected change in reciprocation time.

Description:
[0001]    This application is a divisional of copending application Ser. No. 10/293,874, entitled “Linear Motor Controller”and filed on Nov. 13, 2002. 
     
    
     
       FIELD OF INVENTION  
         [0002]    This invention relates to a controller for a linear motor used for driving a compressor and in particular but not solely a refrigerator compressor.  
         SUMMARY OF THE PRIOR ART  
         [0003]    Linear compressor motors operate on a moving coil or moving magnet basis and when connected to a piston, as in a compressor, require close control on stroke amplitude since unlike more conventional compressors employing a crank shaft stroke amplitude is not fixed. The application of excess motor power for the conditions of the fluid being compressed may result in the piston colliding with the cylinder head in which it is located.  
           [0004]    In International Patent Publication no. WO01/79671 the applicant has disclosed a control system for free piston compressor which limits motor power as a function of property of the refrigerant entering the compressor. However in some free piston refrigeration systems it may be useful to detect an actual piston collision and then to reduce motor power in response. Such a strategy could be used purely to prevent a compressor damage, when excess motor power occurred for any reason or, could be used as a way of ensuring high volumetric efficiency. Specifically in relation to the latter, a compressor could be driven with power set to just less than to cause piston collisions, to ensure the piston operated with minimum head clearance volume. Minimising head clearance volume leads to increased volumetric efficiency.  
         SUMMARY OF THE INVENTION  
         [0005]    It is an object of the present invention to provide a linear motor controller which goes someway to achieving the above mentioned desiderata.  
           [0006]    Accordingly in one aspect the invention may broadly be said to consist in a free piston gas compressor comprising:  
           [0007]    a cylinder,  
           [0008]    a piston,  
           [0009]    said piston reciprocable within said cylinder,  
           [0010]    a reciprocating linear electric motor derivably coupled to said piston having at least one excitation winding,  
           [0011]    means for obtaining a measure of the reciprocation time of said piston,  
           [0012]    means for detecting any change in said reciprocation time, and  
           [0013]    means for adjusting the power input to said excitation winding in response to any detected change in reciprocation time.  
           [0014]    Preferably said motor is an electronically commutated permanent magnet DC motor.  
           [0015]    Preferably said compressor further comprises back EMF detection means for sampling the back EMF induced in said at least one excitation winding when exciting current is not flowing, and zero crossing means connected to the output of said back EMF detection means and means for determining the time interval between output pulses from said zero crossing detection means to thereby determine the time of each half cycle of said piston.  
           [0016]    Preferably two successive half cycles of said piston operation are summed to provide said reciprocation time.  
           [0017]    Preferably means for detecting any change in said reciprocation time includes means to detect said reciprocation time from a filtered or smoothed value, to provide a difference valve and if said difference value is above a predetermined threshold for a predetermined period, said means for adjusting the power is configured to reduce the power input to said excitation winding.  
           [0018]    In a second aspect the present invention may broadly be said to consist in a method of preventing overshoot of the reciprocating portion of a linear motor comprising the steps:  
           [0019]    determining the reciprocation time of said reciprocating portion,  
           [0020]    detecting any change in said reciprocation time, and  
           [0021]    adjusting the power input to said linear motor in response to any detected reduction in reciprocation time.  
           [0022]    Preferably said reciprocating portion comprises the armature of said linear motor.  
           [0023]    Preferably said step of determining said reciprocation time includes the step of detecting zero crossings of the current in said linear motor and determining said reciprocation time from the time interval there between.  
           [0024]    Preferably said step of detecting any change in said reciprocation time includes the step of deducting said reciprocation time from a filtered or smoothed value, to provide a difference valve and if said difference value is above a predetermined threshold for a predetermined period, reducing the power input to said linear motor.  
           [0025]    In a third aspect the present invention may broadly be said to consist in a controller for a linear motor including an reciprocating portion, said controller adapted to implement at least the following steps:  
           [0026]    determining the reciprocation time of a reciprocating portion,  
           [0027]    detecting any change in said reciprocation time, and  
           [0028]    adjusting the power input to said linear motor in response to any detected reduction in reciprocation time.  
           [0029]    Preferably a reciprocating portion comprises the armature of a linear motor.  
           [0030]    Preferably said step of determining said reciprocation time includes the step of detecting zero crossings of the current in a linear motor and determining said reciprocation time from the time interval there between.  
           [0031]    Preferably said step of detecting any change in said reciprocation time includes the step of deducting said reciprocation time from a filtered or smoothed value, to provide a difference valve and if said difference value is above a predetermined threshold for a predetermined period, reducing the power input to said linear motor.  
           [0032]    To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.  
           [0033]    The invention consists in the foregoing and also envisages constructions of which the following gives examples.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    One preferred form of the invention will now be described with reference to the accompanying drawings in which;  
         [0035]    [0035]FIG. 1 is a cross-section of a linear compressor according to the present invention,  
         [0036]    [0036]FIG. 2 is a cross-section of the double coil linear motor of the present invention in isolation,  
         [0037]    [0037]FIG. 3 is a cross-section of a single coil linear motor,  
         [0038]    [0038]FIG. 4 is a block diagram of the free piston vapour compressor and associated controller of the present invention,  
         [0039]    [0039]FIG. 5 is a flow diagram showing control processors used by said controller,  
         [0040]    [0040]FIG. 6 shows a graph of compressor motor back EMF versus time, and  
         [0041]    [0041]FIG. 7 shows a graph of piston reciprocation period versus time. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    The present invention provides a method for controlling a linear motor with a number of improvements over the prior art. Firstly it has a reduced size compared to the conventional linear motor of the type described in U.S. Pat. No. 4,602,174 and thus reduces the cost. This change keeps the efficiency high at low to medium power output at the expense of slightly reduced efficiency at high power output. This is an acceptable compromise for a compressor in a household refrigerator which runs at low to medium power output most of the time and at high power output less than 20% of the time (this occurs during periods of frequent loading and unloading of the refrigerator contents or on very hot days). Secondly it uses a control strategy which allows optimally efficient operation, while negating the need for external sensors, which also reduces size and cost.  
         [0043]    While in the following description the present invention is described in relation to a cylindrical linear motor it will be appreciated that this method is equally applicable to linear motors in general and in particular also to flat linear motors see for example our co-pending International Patent Application no. PCT/NZ00/00201 the contents of which are incorporated herein by reference. One skilled in the art would require no special effort to apply the control strategy herein described to any form of linear motor. It will also be appreciated that the present invention will be applicable in any form of compressor. While it is described in relation to a free piston compressor it could equally be used in a diaphragm compressor for example, without any special modifications.  
         [0044]    One embodiment of the present invention, shown in FIG. 1, involves a permanent magnet linear motor connected to a reciprocating free piston compressor. The cylinder  9  is supported by a cylinder spring  14  within the compressor shell  30 . The piston  11  is supported radially by the bearing formed by the cylinder bore plus its spring  13  via the spring mount  25 . The bearings may be lubricated by any one of a number of methods as are known in the art, for example the gas bearing described in our co-pending International Patent Application no. PCT/NZ00/00202, or the oil bearing described in International Patent Publication no. WO00/26536, the contents of both of which are incorporated herein by reference. Equally the present invention is applicable to alternative reciprocation systems. For example while below a compressor is described with a combined gas/mechanical spring system, an entirely mechanical or entirely gas spring system can be used with the present invention.  
         [0045]    The reciprocating movement of piston  11  within cylinder  9  draws gas in through a suction tube  12  through a suction port  26  through a suction muffler  20  and through a suction valve port  24  in a valve plate  21  into a compression space  28 . The compressed gas then leaves through a discharge valve port  23 , is silenced in a discharge muffler  19 , and exits through a discharge tube  18 .  
         [0046]    The compressor motor comprises a two part stator  5 , 6  and an armature  22 . The force which generates the reciprocating movement of the piston  11  comes from the interaction of two annular radially magnetised permanent magnets  3 , 4  in the armature  22  (attached to the piston  11  by a flange  7 ), and the magnetic field in an air gap  33  (induced by the stator  6  and coils  1 , 2 ).  
         [0047]    A two coil embodiment of present invention, shown in FIG. 1 and in isolation in FIG. 2, has a current flowing in coil  1 , which creates a flux that flows axially along the inside of the stator  6 , radially outward through the end stator tooth  32 , across the air gap  33 , then enters the back iron  5 . Then it flows axially for a short distance  27  before flowing radially inwards across the air gap  33  and back into the centre tooth  34  of the stator  6 . The second coil  2  creates a flux which flows radially in through the centre tooth  34  across the air gap axially for a short distance  29 , and outwards through the air gap  33  into the end tooth  35 . The flux crossing the air gap  33  from tooth  32  induces an axial force on the radially magnetised magnets  3 , 4  provided that the magnetisation of the magnet  3  is of the opposite polarity to the other magnet  4 . It will be appreciated that instead of the back iron  5  it would be equally possible to have another set of coils on the opposite sides of the magnets.  
         [0048]    An oscillating current in coils  1  and  2 , not necessarily sinusoidal, creates an oscillating force on the magnets  3 , 4  that will give the magnets and stator substantial relative movement provided the oscillation frequency is close to the natural frequency of the mechanical system. This natural frequency is determined by the stiffness of the springs  13 ,  14  and mass of the cylinder  9  and stator  6 . The oscillating force on the magnets  3 , 4  creates a reaction force on the stator parts. Thus the stator  6  must be rigidly attached to the cylinder  9  by adhesive, shrink fit or clamp etc. The back iron is clamped or bonded to the stator mount  17 . The stator mount  17  is rigidly connected to the cylinder  9 .  
         [0049]    In a single coil embodiment of the present invention, shown in FIG. 3, current in coil  109 , creates a flux that flows axially along the inside of the inside stator  110 , radially outward through one tooth  111 , across the magnet gap  112 , then enters the back iron  115 . Then it flows axially for a short distance before flowing radially inwards across the magnet gap  112  and back into the outer tooth  116 . In this motor the entire magnet  122  has the same polarity in its radial magnetisation.  
         [0050]    Control Strategy  
         [0051]    Experiments have established that a free piston compressor is most efficient when driven at the natural frequency of the compressor piston-spring system. However as well as any deliberately provided metal spring, there is an inherent gas spring, the effective spring constant of which, in the case of a refrigeration compressor, varies as either evaporator or condenser pressure varies. The electronically commutated permanent magnet motor already described, is controlled using techniques including those derived from the applicant&#39;s experience in electronically commutated permanent magnet motors as disclosed in International Patent Publication no. WO01/79671 for example, the contents of which are incorporated herein by reference.  
         [0052]    When the linear motor is controlled as described in WO01/79671 it is possible that the compressor input power increases to a level where the excursion of the piston ( 11 , FIG. 1) results in the collision with the cylinder ( 9 , FIG. 1). When this occurs (the first collision  302 ) the piston reciprocation period  300  is reduced compared to the filtered or smoothed value  308 . More importantly because the piston period is made up of two half periods  304 ,  306 , between bottom dead centre and top dead centre, the half periods are not symmetrical. The half period moving away from the head  304  is shorter than the half period moving towards the head  306 , although both half periods are reduced in time whenever a piston collision occurs (second collision  310 ). In the preferred embodiment of the present invention the half period times are monitored and when any reduction in the half period times is detected the input power is reduced in response.  
         [0053]    It will also be appreciated the present invention is equally applicable to a range of applications. It is desirable in any reciprocating linear motor to limit or control the maximum magnitude of reciprocation. For the present invention to be applied the system requires a restoring force eg: a spring system or gravity, causing reciprocation, and some change in the mechanical or electrical system which causes a change in the electrical reciprocation period when a certain magnitude of reciprocation is reached.  
         [0054]    In the preferred embodiment of the present invention, shown in FIG. 4, back EMF detection is used to detect the electrical period of reciprocation. As already described the current controller  208  receives inputs from the compressor  210 , the back EMF detector  204  and the collision detector  206 . While in the preferred embodiment of the present invention the current controller  208 , the back EMF detector  204  and the collision detector  206  are implemented in software stored in the microprocessor  212 , they could equally be implemented in a single module or in discrete analogue circuitry. The collision detector  206  receives the electrical period data from the back EMF detector  204  allowing it to detect overshoot, or more specifically collision of the piston with the cylinder. The current controller  208  adjusts the maximum current through the duty cycle applied by the drive circuit  200  to the stator winding  202 .  
         [0055]    Example waveforms in a linear motor employing the present invention are seen in FIG. 6. The stator winding voltage is fully positive  400  for a time t on(ex)  during the beginning of the expansion stroke. With the voltage removed the current  402  decays to zero over time t off1(ex) , with the stator winding voltage forced fully negative  403  by the current flowing in the windings. For the remainder of the expansion stroke, time t off2(ex)  the winding voltage represents the back EMF  404 , and the zero crossing thereof zero velocity of the piston at the end of the expansion stroke. A similar pattern occurs during the compression stroke, rendering a time t off2(comp)  relating to the zero crossing of the back EMF  406  during compression, from which the reciprocation time can be calculated.  
         [0056]    The process the collision detector  206  uses in the preferred embodiment to detect a collision is seen in FIG. 5. Using the back EMF zero crossing data successive half period times are stored  504  and a smoothed or filtered value for each half period is calculated  500 ,  502 . These averages are summed  506  and the sum is monitored for an abrupt reduction  508 . Because of a signal noise caused for various reasons it is not safe to consider one transient reduction as indicative of a piston collision. Accordingly the variable B is preferably set at five successive cycles. The threshold difference value A is preferably set at 30 microseconds.  
         [0057]    When a collision is detected ( 510 , FIG. 5), the current controller ( 208 , FIG. 4) decreases the current magnitude. The reductions to the current and thus input power to the motor are reduced incrementally. Once the collisions stop, the current value is allowed to slowly increase to its previous value over a period of time. Preferably the period of time is approximately 1 hour. Alternatively the current will remain reduced until the system variables change significantly. In one embodiment where the system in WO01/79671 is used as the main current controller algorithm, such a system change might be monitored by a change in the ordered maximum current. In that case it would be in response to a change in frequency or evaporator temperature. In the preferred embodiment the combination of that algorithm with the present invention providing a supervisory role provides an improved volumetric efficiency over the prior art.