EP1715184B1

Movatterモバイル変換

Info

Publication number: EP1715184B1
Application number: EP06006967A
Authority: EP
Inventors: Zhuang Tian; John H. Boyd Jr
Original assignee: Fisher and Paykel Appliances Ltd
Current assignee: Fisher and Paykel Appliances Ltd
Priority date: 2005-04-19
Filing date: 2006-03-31
Publication date: 2008-03-19
Anticipated expiration: 2026-03-31
Also published as: EP1715184A1; DE602006000730T2; ATE389803T1; DE602006000730D1

Abstract

A free-piston linear compressor (1) controlled to achieve high volumetric efficiency by a controller including an algorithm (116) for ramping up input power until piston-cylinder head collisions are detected using a detection algorithm (117/118) which then decrements power input whereupon input power is again ramped up by algorithm (116). Non-damaging low energy collisions are achieved by the controller including a perturbation algorithm (119) which perturbates the input power ramp with periodic transient pulses of power to ensure piston collisions are provoked during the transient power pulses.

Description

FIELD OF INVENTION

This invention relates to a system of control for a free piston linear compressor and in particular, but not solely, a refrigerator compressor. The control system allow a high power mode of operation in which piston stroke is maximised and collisions deliberately occur.

PRIOR ART

Linear compressors operate on a free piston basis and require close control of stroke amplitude since, unlike conventional rotary 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 head gear of the cylinder in which it reciprocates.

US 6,809,434 discloses a control system for a free piston compressor which limits motor power as a function of a property of the refrigerant entering the compressor. However in linear compressors it is useful to be able to detect an actual piston collision and then to reduce motor power in response. Such a strategy can be used purely to prevent compressor damage, when excess motor power occurs for any reason or, can be used as a way of ensuring high volumetric efficiency by gradually increasing power until a collision occurs and then decrementing power before gradually increasing power again. The periodic light piston collisions inherent in this mode of operation cause negligible damage and can easily be tolerated.
US 6,536,326 discloses a system for detecting piston collisions in a linear compressor which uses a vibration detector such as a microphone.
US 6,812,597 discloses a method and system for detecting piston collisions based on the linear motor back EMF and therefore without the need for any sensors and their associated cost. This uses the sudden change in period that has been found to occur on a piston collision. Reciprocation period and/or half periods can be obtained from measuring the time between zero-crossings of the back EMF induced in the motor stator windings. The back EMF is a function of motor armature velocity and therefore piston velocity and zero-crossings indicate the points when the piston changes direction during its reciprocation cycles.
When it is desired deliberately to run the compressor at maximum power and high volumetric efficiency it is very important to ensure the collision detection system does not miss the onset of collisions as they will be a regular and expected occurrence in this mode of operation and successive collisions with increasing power will cause damage.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a control system for a free-piston linear compressor which allows for high power operation while obviating piston collision damage.
Accordingly, one aspect of the present invention provides a method of controlling a free-piston linear compressorcharacterized by:

(a) gradually increasing input power to the compressor;
(b) perturbating the power function of step (a) by superimposing transient increases in power (R_b);
(c) monitoring for piston collisions;
(d) when a piston collision is detected, decrementing said input power; and
(e) continuously repeating steps (a) to (d).

In one embodiment of a method of the present invention, the linear compressor (1) includes a free piston (22) reciprocating in a cylinder (12) driven by an electric motor having a stator (5) with one or more excitation windings (33) and an armature (17) connected to said piston, further including the step of:

supplying an alternating current to said stator winding to cause said armature and piston to reciprocate,

In another embodiment of a method the present invention, the linear compressor (1) includes a free piston (22) reciprocating in a cylinder (12) driven by an electric motor having a stator (5) with one or more excitation windings (33) and an armature (17) connected to said piston, further including the step of:

Another aspect of the present invention provides a free-piston linear compressorcharacterized by:

means to gradually increase input power to the compressor;
means for perturbating the increasing power input by superimposing transient increases in power (R_b);
means to monitor for piston collisions; and
means for decrementing said input power when a piston collision is detected.

One embodiment of a linear compressor of the present invention includes:

a cylinder (12),
a piston (17),
said piston (17) reciprocable within said cylinder (12),
a reciprocating linear electric motor coupled to said piston and having at least one excitation winding (33), and
means for obtaining an indicative measure of the reciprocation period of said piston (109), wherein
the means to monitor for piston collisions comprises means (117) for detecting any sudden reduction in said indicative measure of the reciprocation period, said reduction indicative of a piston collision with the cylinder head due to said perturbation signal, and
the means for decrementing said input power when a piston collision is detected comprises means (116) for reducing the power input to said excitation winding in response to any sudden change in reciprocation period which is detected.

Another embodiment of a linear compressor of the present invention includes:

a cylinder (12),
a piston (17),
said piston (17) reciprocable within said cylinder (12),
a reciprocating linear electric motor coupled to said piston and having at least one excitation winding (33),
means for monitoring the motor back EMF (98),
means for detecting zero-crossings of said motor back EMF (99),
means (118) for monitoring the slope of the back EMF waveform in the vicinity of said zero-crossings,
means (118) for detecting discontinuities in said waveform slope, said discontinuities indicative of a piston collision with the cylinder head, and
means (119) for perturbating said gradually increasing power input with transient increases in power, wherein:
- the means to monitor for piston collisions comprises means for detecting said discontinuities indicative of a piston collision with the cylinder head due to said perturbation signal, and
- the means for decrementing said input power when a piston collision is detected comprises means (116) for reducing the power input to said excitation winding in response to any back EMF slope discontinuity which is detected.

BRIEF DESCRIPTION OF THE DRAWINGS

One preferred form of the invention will now be described with reference to the accompanying drawings in which;

Figure 1 is a longitudinal axial-section of a linear compressor controlled according to the present invention,
Figure 2 shows a refrigerator control system in block diagram form,
Figure 3 shows a basic linear compressor control system using electronic commutation with switching timed from compressor motor back EMF,
Figure 4 shows the control system ofFigure 3 with piston collision avoidance measures,
Figure 5 shows the control system ofFigure 3 with collision control for high power operation of the compressor,
Figure 6 shows the control system ofFigure 5 including perturbation of the compressor input power according to the present invention,
Figure 7 shows a circuit for commutating current to the compressor windings, and
Figure 8 shows a graph indicative of compressor power input illustrating the perturbated ramp function high power mode (and corresponding piston collisions), together with corresponding piston expansion and compression half cycle periods, and
Figure 9 shows a linear compressor control system incorporating all of the control features ofFigures 3 to 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to controlling a free piston reciprocating compressor powered by a linear electric motor. A typical, but not exclusive, application would be in a refrigerator.
By way of example only and to provide context a free piston linear compressor which may be controlled in accordance with the present invention is shown inFigure 1.
A compressor for a vapour compression refrigeration system includes a linear compressor 1 supported inside ashell 2. Typically thehousing 2 is hermetically sealed and includes agases inlet port 3 and a compressedgases outlet port 4. Uncompressed gases flow within the interior of the housing surrounding the compressor 1. These uncompressed gases are drawn into the compressor during the intake stroke, are compressed between apiston crown 14 andvalve plate 5 on the compression stroke and expelled throughdischarge valve 6 into acompressed gases manifold 7. Compressed gases exit themanifold 7 to theoutlet port 4 in the shell through aflexible tube 8. To reduce the stiffness effect ofdischarge tube 8, the tube is preferably arranged as a loop or spiral transverse to the reciprocating axis of the compressor. Intake to the compression space may be through the head,suction manifold 13 andsuction valve 29.
The illustrated linear compressor 1 has, broadly speaking, a cylinder part and a piston part connected by a main spring. The cylinder part includescylinder housing 10,cylinder head 11,valve plate 5 and acylinder 12. Anend portion 18 of the cylinder part, distal from thehead 11, mounts the main spring relative to the cylinder part. The main spring may be formed as a combination ofcoil spring 19 andflat spring 20 as shown inFigure 1. The piston part includes ahollow piston 22 withsidewall 24 andcrown 14.
The compressor electric motor is integrally formed with the compressor structure. The cylinder part includesmotor stator 15. A co-actinglinear motor armature 17 connects to the piston through arod 26 and a supportingbody 30. Thelinear motor armature 17 comprises a body of permanent magnet material (such as ferrite or neodymium) magnetised to provide one or more poles directed transverse to the axis of reciprocation of the piston within the cylinder liner. Anend portion 32 ofarmature support 30, distal from thepiston 22, is connected with the main spring.
The linear compressor 1 is mounted within theshell 2 on a plurality of suspension springs to isolate it from the shell. In use the linear compressor cylinder part will oscillate but because the piston part is made very light compared to the cylinder part the oscillation of the cylinder part is small compared with the relative reciprocation between the piston part and cylinder part.
An alternating current instator windings 33, not necessarily sinusoidal, creates an oscillating force onarmature magnets 17 to give the armature 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 thespring 19, and mass of thecylinder 10 andstator 15.
However as well asspring 19, 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 (and temperature) varies. A control system which sets stator winding current and thus piston force to take this into account has been described inUS 6,809,434, the contents of which are incorporated herein by reference.US 6,809,434 also describes a system for limiting maximum motor power to minimise piston cylinder head collisions based on frequency and evaporator temperature.
Preferably but not necessarily the control system of the present invention operates in conjunction with the control system disclosed inUS 6,809,434.
To provide context for the linear compressor control system in the present invention a basic control system for a refrigerator is shown inFigure 2. Arefrigerator 101 incorporating anevaporator 102 and acompressor 103 is set by a user to operate at a desired cabinet temperature through a control which produces asignal 104. This causescompressor 103 to operate until the refrigerator cabinet temperature monitored bytemperature sensor 105 indicates the desired temperature setting has been attained and theerror signal 106driving control amplifier 107 falls below a given threshold. At thispoint compressor 103 is switched off. When the cabinet temperature exceeds a predetermined threshold the magnitude oferror signal 106 exceeds the predetermined value and the compressor is again turned on. This is the conventional non-linear feedback system used in refrigerators.
The control system of the present invention resides within the conventional loop described with reference toFigure 2. It receives as an input the output signal fromamplifier 107 and controls thecompressor 103 which in the present invention will be a free piston linear compressor.
The control system of the present invention operates in conjunction with the basic motor control system ofFigure 3 and preferably, although not necessarily with the system ofFigure 4. Referring toFigure 3,linear compressor 103A, which may be of the type already described with reference toFigure 1, has its stator windings energised by an alternating voltage supplied frompower switching circuit 107 which may take the form of the bridge circuit shown inFigure 7 which uses switchingdevices 411 and 412 to commutate current of reversing polarity through compressor stator winding 33. The other end of the stator winding is connected to the junction of two series connected capacitors which are also connected across the DC power supply. The "half" bridge shown inFigure 7 may be replaced with a full bridge using four switching devices. The control system is preferably implemented as a programmed microprocessor controlling the operation of thepower switching circuit 107. Theswitching circuit 107 is thus controlled by aswitching algorithm 108 executed by the control system microprocessor. The microprocessor is programmed to execute various functions or use tables to be described which for the purposes of explanation are represented as blocks in the block diagrams ofFigures 3 to 5.
Reciprocations of the compressor piston and the frequency or period thereof are detected bymovement detector 109 which in the preferred embodiment comprises the process of monitoring the back EMF induced in the compressor stator windings by the reciprocating compressor armature and detecting the zero crossings of that back EMF signal.Switching algorithm 108 which provides microprocessor output signals for controlling thepower switch 107 has its switching times initiated from logic transitions in the back EMF zerocrossing signal 110. This ensures the reciprocating compressor peaks maximum power efficiency. The compressor input power may be determined by controlling either the current magnitude or current duration applied to the stator windings bypower switch 107. Pulse width modulation of the power switch may also be employed.
Figure 4 shows the basic compressor control system ofFigure 3 enhanced by the control technique disclosed inUS 6,809,434 which minimises piston/cylinder collisions in normal operation by setting a maximum power based on piston frequency and evaporator temperature. Output 111 from an evaporator temperature sensor is applied to one of the microprocessor inputs and piston frequency is determined by afrequency routine 112 which times the time between zero crossings inback EMF signal 110. Both the determined frequency and measured evaporator temperature are used to select a maximum power from a maximum power lookup table 113 which sets a maximum allowable power P_t for acomparator routine 114.Comparator routine 114 receives as asecond input value 106 representing the power demand (P_r) required from the overall refrigerator control. Thecomparator routine 114 is used by switchingalgorithm 118 to control switching current magnitude or duration.Comparator routine 114 provides an output value 115 which is the minimum of the power required by the refrigerator P_r and the power P_t allowed from maximum power table 113.
Using just the control concepts explained with reference toFigure 4 will result in thelinear compressor 103A (when active) operating with no or minimal piston collisions in normal operation. However as disclosed inUS 6,812,597linear compressor 103A may be run in a "maximum power mode" where higher power can be achieved than with theFigure 4 control system, but with the inevitability of some piston collisions. The control system of the present invention facilitates this mode as will now be described.
Referring toFigure 5 apower algorithm 116 is employed which provides values to a another input tocomparison routine 114.Power algorithm 116 slowly ramps up the compressor input power by providing successively increasing values tocomparator routine 114 which causes switchingalgorithm 108 to ramp up thepower switch 107 current magnitude or preferably ON time duration. Power is increased to P_a + R every n cycles or piston reciprocations with P_a being the power allowed by the collision analyser (see below) and R being a power increment which defines the ramp rate. In practice usually n = 1. This ramping continues until a piston collision is detected.Collision detection process 117 is preferably determined from an analysis of the back EMF induced in the compressor windings and the technique used may be either that disclosed inUS 6,812,597, which looks for sudden decreases in piston period (Figures 8(a) and 8(b) show graphs of piston half-periods against time as mentioned below), or that disclosed inUS 10/880,389 which looks for discontinuities on the slope of the analogue back EMF signal.
Upon detection of a collision,power algorithm 116 causes a decremented value to be input tocomparator routine 114 to achieve a decrease of power.Power algorithm 116 then again slowly ramps up the compressor input power until another collision is detected and the process is repeated.
In order to maximise the probability of detecting the first collision due to increasing peak piston excursions (as continued collisions at what will be increasing power may cause damage) the effective power ramping signal provided bypower algorithm 116 is periodically pulsed every m cycles by a perturbation algorithm 119 (seeFigure 6) with an increase (R_b) in power for a very short duration. A typical valve of m might be 100. In one embodiment this is achieved by increasing the ON time ofpower switch 107 by 100µs every 1 second (seeFigure 8(c)). Shorter increases in ON times, say 50µs, could be used dependent on the collision detection system employed. This amounts to periodic application of an impulse function perturbation R_b of the ramp signal as shown inFigure 8(c), although it should be appreciated this is graph ofpower switch 107 ON time and not power as such. Every m cycles the power is increased to P_a + Rp for one cycle, that is, for one reciprocation to induce a collision if compressor power is such as to nearly be causing peak piston displacements which result in collisions with the cylinder valve gear. This low energy collision is detected and compressor input power immediately reduced by s.Rp where_s might typically be 20, thus making the provendecrement 20 times the perturbation impulse power. The ramp function resumes to gradually increase compressor power again.
Using the perturbation technique described the linear compressor can be operated at maximum power and volumetric efficiency when required with low energy non-damaging piston collisions in the certainty that continued collisions at increasing power will be avoided.
Desirably, but not necessarily the high power control methodology described is used in conjunction with control for normal operation where collision avoidance is employed as described with reference toFigure 4. A control system employing both techniques is shown inFigure 9. Here thecomparison routine 114 receives three inputs, P_r, P_t and P_a. In the system ofFigure 9 input P_a frompower algorithm 116 may be decremented by one or both of two collision detection processes 117 and 118.Process 117 looks for period change andprocess 118 looks for back EMF slope change as previously mentioned.

With such a comprehensive control system the operation may be summarised by tables I and II shown below.

Case	Situation	Description	Output
A	Normal running	Output power is the minimum of; 1- the power required by the refrigerator, Pr, 2 -the power allowed by the Collision Table, Pt or 3- the power allowed by the Collision detector, Pa.	Pr
B	Collision Avoidance	If Pr > Pt then power is held at Pt. Where Pt is a function of Running Frequency and Evaporating Pressure (or temperature, as evaporating temperature is closely correlated to pressure)	Pt
C1	Collision reaction	If a collision is detected power is decreased by about Rp	Pt - Rp or Pr - Rp

C2	Frequent collisions	If there have been more than 1 collision in the last p cycles then decrease power by n x Rp	Pt - nRp or Pr- nRp
C3	No collisions recently	If there has been no collisions in the last q cycles then increase Power by DP (this can continue until Power gets to its original value, Pt).	Pt - nRp + ΔP or Pr - nRp + ΔP
D	Safety net (only occurs for a severe collision that is undetected by the "collision detection" algorithm)	If at any time the back emf slope,S, exceeds the reference value, Sr, then the power is reduced to a minimal value, Pmin.	Pmin

Definitions

Table I - Logic for normal running of the compressor where collision avoidance is the objective.

Pr, Pa,	Power levels that are set by altering the commutation time
Pt
Rp	Power step that reduces the power level.
N	No of multiples of power change, normally n = 1
Q	No of cycles that must be collision free before Power is increased, normally p = 1,000,000
Pmin	A preset minimum power, normally about 20W

Case	Situation	Description	Output
A	Normal running	Output power is the minimum, of the power required by the refrigerator, Pr, and the power allowed by the Collision Analyser, Pa.	Pr
B	High Power	If Pr > Pa then power is increased to Pa + R every n cycles. After m cycles the power is increased to Pa + Rp for one cycle to produce a minor collision if a collision is imminent.	Pa + R or Pa + Rp
B1	Collision reaction	If a collision is detected power is decreased by about s*Rp	Pa - s*Rp

B2	Frequent collisions	If there have been more than 1 collision in the last p cycles then decrease R by δR (this can continue until R becomes a large negative number).	Pa + R - δR

B3	No collisions recently	If there has been no collisions in the last q cycles then increase R by ΔR ( this can continue until R gets to its original value).	Pa + R + ΔR
C	Safety net (only occurs for a severe collision that is undetected by the "collision detection" algorithm)	If at any time the back emf slope, S, exceeds the reference value, Sr, then the power is reduced to a minimal value, Pmin.	Pmin

Definitions

Table II - Logic for high power running where low energy collisions are inherent.

Pr, Pa	Power levels that are set by altering the commutation time
R	Power increment that defines the "Ramp Rate"
Rp	Power step that perturbates the power level to force a minor collision when
	the pump is running near its maximum stroke.
M	No of cycles between each perturbation, normally m = 100
s	Multiple that determines the power decrement after a collision, normally s=20
p	No of cycles that must be collision free before R is increased, normally p = 1,000,000
q	No of cycles during the collision count, normally q = 10,000
Pmin	A preset minimum power, normally about 20W

Preferably the collision detection algorithm is one derived from the ascertainment of a sudden decrease in piston period as disclosed inUS 6,812,597. An enhanced technique derived from this method will now be described.
The period of theoscillating piston 22 is made up of two half periods between bottom dead centre and top dead centre respectively, but neither successive or even alternate half periods are symmetrical. The half period expansion stroke when the piston moves away from the head (valve plate 5) is longer than the half period compression stroke when the piston moves towards the head. Further, because a linear compressor will often run with different periods in consecutive cycles (this becomes very significant if the discharge valve starts to leak), it is useful to separate the period times into odd and even cycles. Thus in the preferred method of piston collision detection four periods are stored and monitored; compression and expansion for the even cycles, plus compression and expansion for the odd cycles. Preferably a sudden change in either of the two shorter half cycles (compression strokes) is assumed in this method to indicate a piston collision. InFigure 8(b) typical even short cycle periods are shown whereasFigure 8(a) shows typical even expansion stroke half periods.
The process used in the preferredcollision detection algorithm 117 is to store the back EMF zero crossing time intervals fromdetector 109 for the four half periods mentioned above as an exponentially weighted moving average (ewma) to give a smoothed or filtered value for each of the first and second half periods of the odd and even cycles. Preferably, an infinite impulse response (IIR) filter is used with weightings such that the outputted latest estimate of half period time is¹/₈ of the last value +⁷/₈ of the previous estimates. These estimates are continually compared with the detected period of the most recent corresponding half cycle and the comparison monitored for an abrupt reduction. If the difference exceeds an amount "A",algorithm 117 implies a collision. A value for the threshold difference "A" may be 20 microseconds. Other thresholds could be used, especially if the perturbation impulse energy is different from that resulting from a 100µs ON time.
When a collision is detected the ON time ofpower switch 107 is reduced by (see for example transition D inFigure 8(c)) to stop further collisions. In one embodiment the ON period is reduced by 51.2 µs to produce the previously mentioned s.R_p decrement. Once the collisions stop, the ON time ofpower switch 107 is allowed to slowly increase to its previous value over a period of time (see the ramp function R inFigure 8(c)). A value for the period of time for satisfactory operation may be approximately 1 hour. Of course, power control may be achieved by controlling current magnitude or by pulse width modulation to achieve the same effect as that described.
This is the high power mode of Table II. Alternatively the ON time will remain reduced until the system variables change significantly. In one embodiment where the system inUS 6,809,434 is used as the main current control 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 a collision detection algorithm providing a supervisory role gives an improved volumetric efficiency over the prior art.
The features disclosed in the foregoing description, in the following claims and/or in the accompanying drawings may, both separately and in any combination thereof, be material for realising the invention in diverse forms thereof.

Claims

A method of controlling a free-piston linear compressorcharacterized by:
(a) gradually increasing input power to the compressor;
(b) perturbating the power function of step (a) by superimposing transient increases in power (R_b);
(c) monitoring for piston collisions;
(d) when a piston collision is detected, decrementing said input power; and
(e) continuously repeating steps (a) to (d).
A method according to claim 1, wherein the step of perturbing the power function is performed periodically.
A method according to claim 1, wherein the linear compressor (1) includes a free piston (22) reciprocating in a cylinder (12) driven by an electric motor having a stator (5) with one or more excitation windings (33) and an armature (17) connected to said piston, further including the step of:
supplying an alternating current to said stator winding to cause said armature and piston to reciprocate,
wherein the step of gradually increasing input power comprises gradually increasing the power input to said stator windings over many reciprocation periods, and
wherein the step of monitoring for piston collisions comprises obtaining an indicative measure of the reciprocation period of said piston, and detecting any sudden reduction of said indicative measure, said sudden reduction indicative of a piston collision with the cylinder head.
A method according to claim 1, wherein the linear compressor (1) includes a free piston (22) reciprocating in a cylinder (12) driven by an electric motor having a stator (5) with one or more excitation windings (33) and an armature (17) connected to said piston, further including the step of:
supplying an alternating current to said stator winding to cause said armature and piston to reciprocate,
wherein the step of gradually increasing input power comprises gradually increasing the power input to said stator windings over many reciprocation periods, and
wherein the step of monitoring for piston collisions comprises monitoring the motor back EMF, detecting zero-crossings of said motor back EMF, monitoring the slope of the back EMF waveform in the vicinity of said zero-crossings, and detecting discontinuities in said waveform slope, said discontinuities indicative of a piston collision with the cylinder head.
A free-piston linear compressorcharacterized by:
means to gradually increase input power to the compressor;
means for perturbating the increasing power input by superimposing transient increases in power (R_b);
means to monitor for piston collisions; and
means for decrementing said input power when a piston collision is detected.
A free piston gas compressor according to Claim 5, including:
a cylinder (12),
a piston (17),
said piston (17) reciprocable within said cylinder (12),
a reciprocating linear electric motor coupled to said piston and having at least one excitation winding (33), and
means for obtaining an indicative measure of the reciprocation period of said piston (109), wherein
the means to monitor for piston collisions comprises means (117) for detecting any sudden reduction in said indicative measure of the reciprocation period, said reduction indicative of a piston collision with the cylinder head due to said perturbation signal, and
the means for decrementing said input power when a piston collision is detected comprises means (116) for reducing the power input to said excitation winding in response to any sudden change in reciprocation period which is detected.
A free piston gas compressor according to claim 6 wherein said motor is an electronically commutated permanent magnet DC motor.
A free piston gas compressor according to either claims 6 and 7 wherein said means for obtaining an indicative measure of reciprocation period comprises back EMF detection means (98) for sampling the back EMF induced in said at least one excitation winding (33) when exciting current is not flowing, zero crossing detection means connected to the output of said back EMF detection means, and timing means (112) which determine the time interval between zero-crossings to thereby determine the time of each half cycle of the reciprocation of said piston.
A free piston gas compressor according to any one of claims 6 to 8 wherein said means for detecting any sudden change in reciprocation period includes averaging means which provides an average value of the times of alternate reciprocation half cycles, comparison means which compares the most recent measured reciprocation half cycle with said average value of times of corresponding half cycles to provide a difference value, and means to determine if said difference value is above a predetermined threshold for a predetermined period.
A free piston gas compressor according to any one of claims 6 to 9 wherein said power setting means is a power switching device (107) and said means (116) for controlling determines the power input to the motor by controlling the ON time of said switching device during said reciprocation period.
A free piston gas compressor according to claim 10 wherein said perturbating means (119) causes said controlling means (116) to increase the ON time of said switching device by a predetermined transient amount at periodic intervals equal to a multiple of the reciprocation period.
A refrigerator comprising a free piston gas compressor according to any one of claims 8 to 11 and an evaporator (102), said compressor including reciprocation frequency determining means (112) associated with said timing means and a temperature sensor (97) which senses the temperature at the evaporator wherein maximum compressor input power is determined as a function of frequency and evaporator temperature.
A refrigerator according to claim 12 including means (118) for monitoring the slope of the back EMF waveform in the vicinity of zero-crossings, means for detecting discontinuities in said waveform slope, said discontinuities indicative of a piston collision with the cylinder head and said means (116) for reducing power to said excitation winding also responding to detection of any back EMF slope discontinuity.
A free piston gas compressor according to Claim 5, including:
a cylinder (12),
a piston (17),
said piston (17) reciprocable within said cylinder (12),
a reciprocating linear electric motor coupled to said piston and having at least one excitation winding (33),
means for monitoring the motor back EMF (98),
means for detecting zero-crossings of said motor back EMF (99),
means (118) for monitoring the slope of the back EMF waveform in the vicinity of said zero-crossings,
means (118) for detecting discontinuities in said waveform slope, said discontinuities indicative of a piston collision with the cylinder head, and
means (119) for perturbating said gradually increasing power input with transient increases in power, wherein:
the means to monitor for piston collisions comprises means for detecting said discontinuities indicative of a piston collision with the cylinder head due to said perturbation signal, and
the means for decrementing said input power when a piston collision is detected comprises means (116) for reducing the power input to said excitation winding in response to any back EMF slope discontinuity which is detected.