GB2248491A - Ice cube making - Google Patents

Abstract

An ice cube maker has a generally upright gridded evaporator, a hot gas defrost for harvest of cubes, a storage bin below the evaporator, and a hinged cube and water curtain between the evaporator and the bin. A freeze cycle control has a temperature sensor, on the backside of the evaporator, and circuitry to count down a predetermined time after a predetermined plate temperature has been sensed. The circuit terminates the countdown if the plate temperature exceeds the predetermined temperature and restarts the countdown when the predetermined temperature is again reached. The circuit switches the refrigeration from freeze to hot gas defrost when the countdown is completed for harvest of the ice cubes. <IMAGE>

Description

A Method of Making Ice Cubes This invention pertains to a method of making ice cubes having new steps of control, and to an ice cube maker having a new and improved control for harvest and freeze cycles, and to this control for an ice cube maker.

An ice cube maker having a vertical flat plate gridded evaporator is well known and is in extensive public use. The food and beverage retailers and in particular the fast food chains and restaurants have a significant preference for this type of ice cube maker. It is commercially accepted and in many instances preferred.

The lead example of this general type of ice cube maker is made by The Manitowoc Company Inc of Manitowoc, Wisconsin, and is quite well documented in US Patent 3,430,452 of March 4, 1969.

Another example of this type of cuber is made by Mile High Equipment Company of Denver, Colorado and is documented in US Patent 4,341,087 of July 27, 1982. This patent has an extensive discussion on the merits of this general type of ice cube maker.

Despite the commercial acceptance and preference for this type of ice cube maker, there have been problems with the control of the freezing and harvest cycles, amongst other things.

Typically, this type of ice cube maker will be in a freeze cycle for 8-12 minutes and will then switch to hot gas defrost to loosen the cubes from the evaporator so that the cubes can be ejected from the evaporator. Timer controls do not work well because the specific incoming water temperature, line voltages, and ambient temperatures are unpredictable.

Many various schemes of control have been tried and found to still give problems.

Most recent examples of controls for this type of machine have been developed by Manitowoc and are disclosed in US Patent 4,480,441 of November 6, 1984 and US Patent 4,550,572 of November 5, 1985. The first is an electro-mechanical device and is thought to be commercialized but the second device has not been seen on Manitowoc products.

Other patents having ice cube makers of this general type include: 3,913,349 Johnson 3,964,270 Dwyer 3,144,755 Kattis According to the principles of the present invention there is provided a method of making ice cubes, comprising the steps of: a) initiating a freeze cycle and supplying cooled refrigerant to a refrigerant circuit on an evaporator while flowing a film of water firstly over the plate and then subsequently over ice formed upon the plate; b) sensing the temperature of the evaporator plate; c) starting a countdown of a predetermined time upon sensing that the plate is at a predetermined temperature, said temperature being the freezing point of water; d) counting down the time as long as the sensed temperature remains at or below the predetermined temperature; e) terminating the countdown if the temperature rises above the predetermined temperature; f) restarting a subsequent countdown when the sensed temperature again falls to the predetermined temperature; and g) terminating the freeze cycle and initiating a hot gas defrost cycle upon completion of a countdown.

Preferably, the pre-determined temperature is -14 3 "C. The countdown may be in the range 20-30 seconds.

The freeze cycle may continue un-interrupted until countdown is complete. Furthermore, the hot gas defrost may be initiated on countdown completion.

It may be preferable to add the step of continuing flow of water over the plate ice during countdown and terminate on countdown completion.

There may be an additional step of terminating the freeze cycle and initiating a subsequent freeze cycle upon sensing that ice falling off the evaporator plate has opened a curtain between the plate and storage bin.

The countdown may be completely repeated after restart.

In accordance with an alternative embodiment of the present invention there is provided an ice cube maker having an improved freeze cycle control, comprising of: a) an evaporator plate having a front side for freezing ice and a back side with a refrigerant circuit thereon; b) a temperature sensing thermister on the plate back side; c) a refrigerant valve operatively connected to reverse refrigerant flow to the plate from a normal freezing cycle to a hot gas defrost cycle for release of and harvest of ice frozen upon the plate; d) a freeze cycle control operatively connected to the thermister and to the refrigerant valve; and e) means in said control for 1) determining via the thermister that the plate temperature has fallen to a predetermined temperature, 2) counting down a predetermined time after the predetermined temperature has been sensed.

3) terminating the countdown if the sensed temperature goes above the predetermined temperature, and 4) switching the ice maker from the freeze cycle to the defrost cycle upon countdown completion for harvest of the frozen ice.

The countdown means may be completely reset during countdown termination or the defrost cycle.

Furthermore, the start of countdown may be adjusted.

There may be provided switch means for switching between freeze and defrost and a curtain between the plate and bin, with the switch means responsive to opening of the curtain.

FIGURE 1 is an elevational sideview, in partial section, showing the preferred embodiment of the ice cube maker of the present invention; FIGURE 2 is a similar elevational side view of the evaporator and ice curtain componentry of the structure of Figure 1 with the curtain open; FIGURE 3 is a downward looking sectional view through lines III-III of Figure 1; FIGURE 4 is an elevational sectional view through lines IV-IV of Figure 1; FIGURE 5 is a schematic of the ice maker refrigeration system, and FIGURE 6 is a logic diagram for the electrical control of the ice cube maker of Figures 1 and 5.

The principles of the present invention are embodied in and practised with the preferred embodiment of an ice cube maker such as is shown in Figures 1 and 2 and which is generally indicated by the numeral 10.

The ice maker 10 has an evaporator generally indicated by the numeral 11 which preferably has a freezing plate 12 made of a generally vertical flat metal plate having top, bottom and side flanges and an internal egg crate type matrix with vertical dividers 13 and horizontal dividers 14 dividing the freeze plate 12 into small discrete pockets for the freezing of discrete cubes. On the rear side of the freeze plate 12 is a refrigerant coil 15 which is appropriately serpentined on and over the plate 12. A temperature thermister well 16 is soldered to the rear side of the freeze plate 12 just above the vertical mid point of and on one transverse side of the freeze plate 12. The refrigerant coil 15 and thermister well 16 are spaced from each other and are both thermally enclosed within a backing of thermal insulation 17. A circulating pump 18 for the water to be frozen into ice cubes has an inlet line from a water catching reservoir 19 and an outlet line to a distributor manifold 20 mounted on the top of the evaporator 11.

A movable evaporator curtain 21 is pivotally suspended from a horizontal axis fulcrum 22. The curtain 21 and fulcrum 22 are in front of the evaporator 11 and fulcrum 22 is adjacent to and above a mid level of the evaporator 11. The curtain 21 is normally closed during freezing of ice, as is shown in Figure 1, and the curtain 21 is opened by falling ice during harvest as is shown in Figure 2. The curtain 21 retains falling water upon the freeze plate 12, and during circulation of water over the freeze plate 12 during freezing of ice the curtain 21 directs the falling water into the reservoir 19 from which the pump 18 continually recirculates the water over the freeze plate 12 during the freeze cycle. At the start of a freeze cycle, the reservoir 19 is filled with water and the fill level is controlled by a float valve 23.

The pump 18 is turned on and water from the reservoir 19 is then continuously circulated through the manifold 20 and from there downwardly by gravity over the freeze plate 12. The curtain 21 confines the falling water and directs it back into the reservoir 19. When freezing of ice is completed, the ice maker 10 switches into hot gas defrost and releases the frozen ice from the plate 12. The frozen ice looks like a waffle with individual discrete cubes being attached to each other by a thin sheet of ice frozen over the out edges of the dividers 13, 14. The falling ice forces the curtain 21 to open and the ice falls past the water reservoir 19 and into an ice bin (not shown) below the reservoir 19. The waffle ice sheet then breaks up leaving discrete cubes.

An important feature of this invention is the curtain position sensor generally indicated by the numeral 24. Movement and position of the curtain 21 are sensed and utilized to control shut-off of the ice maker 10 when the bin is full of ice cubes, and to restart the freeze cycle upon completion of a harvest of ice cubes. The curtain position sensor 24 is mounted to the ice maker 10 above the curtain 21, and the freeze plate 12 and the water manifold 20 whereby the position sensor 24 is isolated from and spaced above the moving water and ice. The curtain position sensor 24 has an electronic curtain sensor 25 which is preferably an integral U-shaped photo electric emitter and receiver (PER) having a constantly energized emitter.The sensor 25, a sensor bracket 26 and a flag fulcrum 27 are mounted to the ice maker and fixed with respect to the evaporator 11 and the curtain fulcrum 22. A movable sensor flag 28, which is preferably a first class lever, is pivotally mounted in the sensor fulcrum 27. The flag 28 is freely pivotable and is a flat piece of sheet metal having a weight 29 which by gravity biases the flag clockwise as shown in Figures 1 and 2 into normal abutment against a flag stop 30 on the bracket 26. The flag 28 has a small precisely located open sensor aperture 31 which normally is precisely registered with and which is between the emitter and the receiver of the sensor 25. The sensor aperture 31 is within the flag 28, and the aperture 31 and weight 29 jointly form a precision shutter for momentary obstruction of the beam from the emitter to the receiver of the sensor 25.The curtain 21 has a sensor cam 32 which contacts against and drives a cam follower 33 on the flag 28. The cam 32 is normally spaced from and does not contact the follower 38 and has a lost motion connection which enables the curtain 21 to flop around without effecting the sensor 25. FJhen the ice maker 10 is freezing ice, the curtain 21 is closed as shown in Figure 1 and the sensor 25 is normally transmitting from its emitter to its receiver through the flag sensor aperture 31. When the ice maker 10 releases its cubes from the freeze plate 12, the cubes fall down and force the curtain 21 open as shown in Figure 2. When the curtain 21 opens, the curtain cam 32 contacts the follower 33 and cams the flag 28 counterclockwise to the alternative position shown in Figure 2.As soon as the flag sensor aperture 31 is lifted, the opaque flag 28 obstructs the beam of the sensor 25 and the sensor provides a signal indicative of this obstruction to an ice maker control 34. The control 34 then makes the assumption that cubes are not re-established, the ice maker 10 deduces it has filled its storage bin and it shuts itself off. A self-resetting curtain timer 40 receives the signal that the curtain 21 is open and if the curtain 21 stays open for an excessive period of time, the timer 40 provides a signal to shut down the compressor 35 and other componentry. The timer 40 will provide a turn-off signal after the curtain 21 has been open too long. ;5hen the ice level in the bin falls and the curtain 21 closes and transmission of the beam of the sensor 25 is re-established, the ice maker 10 automatically starts itself.The entire curtain position sensor 24 is well above both the water and ice and is not subject to contact with the water or ice. The position sensor 24 is located far above the storage bin and it will operate regardless of what configuration of storage bin is utilized. The sensor 24 also works on very low signal voltage and current and does not bring any type of a potentially hazardous elecrical potential into the ice making and storage chambers. This sensor 24 has no springs and nothing to wear out or break. It is extremely reliable, low cost, and accessible and is easily understood by people who own, operate, repair or rely upon the ice maker 10. What electrical potential and signals are provided or made by the sensor 25, are completely isolated from contact with either ice or water.The termination of hot gas defrost function and the function of shut-off when the storage bin is filled are responsible to a clear made change of the sensor 25 from transmitting to obstructed and vice-versa.

The lost motion connection between the curtain 21 and flag 28 enables the curtain 21 to partially move without signalling ice release when the ice cubes have only partially released from the freeze plate 12. The ice maker 10 waits for the harvest completion signal until the complete sheet of ice and cubes falls off of the freeze plate 12 and substantially opens the curtain 21.

As an example, the curtain 21 will open 5 to 10 degrees before the cam 32 engages the follower 33.

The curtain 21 will then, upon dropping of the ice sheet, open a total of about 20 degrees and in the last 10 degrees of travel the flag 28 will be turned about 30 degrees. The angular mechanical motion amplification between the curtain 21 and flag 28 is at least 2:1 and preferably about 3:1 as soon as the cam 32 and follower 33 engage each other. During the mode changes from freeze to defrost and back to freeze, the compressor 35 runs continuously and there is no stop or start which greatly enhances compressor life and control component life as well as providing for increased thermal efficiency and ice production.

Another important improvement is a control system for the ice maker 10, previously identified in general by the numeral 34. The control 34 is responsive to the curtain position sensor 24 and is connected to control the water circulation pump 18, the refrigeration compressor 35, the hot gas defrost valve 36, the condensor fan 37, and other componentry as will be further described. A freeze plate temperature sensing thermister 38 is mounted in the freeze plate thermister well 16 and is operatively connected to the control 34. Within the control 34 is a self-resetting refrigeration delay timer 39, which may have either a fixed or variable delay as circumstances dictate. During freezing of ice on the freeze plate 12, the thermister 38 will electronically indicate the temperature of the freeze plate 12.The freeze plate 12 temperature has been determined to be analogous to the size of ice as a function of the thickness of the ice upon the freezing plate 12 of the evaporator 11. When the thermister 38 indicates the plate 12 temperature to be at or below a predetermined temperature, the delay timer 29 is started. If the indicated temperature remains at or below the predetermined temperature for the delay timer period, the timer 39 will complete a countdown of the delay time period and upon completion of the countdown the timer 39 will provide a signal that freezing of a batch of ice cubes has been completed. The control 34 will then switch the ice maker 10 into hot gas defrost for harvest of the ice.If during the freeze cycle, the temperature indicated by the thermister 38 merely momentarily dips down to or goes below the predetermined temperature and then returns to above the predetermined temperature, the timer 39 terminates its countdown upon the indicated temperature rising above the predetermined temperature and the timer 39 then discharges and resets itself to relative zero.

When the temperature of the freeze plate 12 subsequently falls to the predetermined temperature, the countdown will again be started. This start, terminate, erase or reset, and restart of the countdown can be done as many times as needed.

Typically and usually, it will be done only once.

When the termination and reset is done, false harvests of incomplete ice are prevented.

A predetermined temperature in the range of -17 to -11 degrees Celsius or -14i3 degrees Celsius has been found to be indicative of the proper size and quantity and thickness of ice upon the freeze plate 12 for complete harvest of a proper quantity of properly completed and sized cubes. A countdown time period in the range of 20-30 seconds has been found to prevent false or improper harvest of ice. The freeze cycle of the refrigeration system continues without interruption during the countdown period and the hot gas defrost is initiated immediately upon completion of the countdown.

The water pump 18 and water circulation over the freeze plate 12 are continued during the countdown. Upon completion of the countdown, the pump 18 and water circulation are immediately shut down and terminated concurrent with start of the hot gas defrost. The ice on the freeze plate 12 is thereby prevented from excessive sub-cool and the ice is released from the freeze plate 12 in the shortest possible time. The hot gas defrost start and the termination of the water flow over the freeze plate 12 are both done immediately upon completion of the countdown.

Figure 6 has a logic diagram of the ice maker control 34. Line power for the ice maker 10 comes in through a manually operable on/off switch 50 and through a main relay 51 to the compressor 35 and other operating components. Low voltage DC power for the control 34 and sensor 25 comes in line 42 and is taken off line power before the on/off switch 50. The signal from the freeze plate thermister 38 is fed through an adjustable potentiometer 41 and then through an amplifier 52 to the refrigeration delay timer 39. The timer 39 as previously described, has a countdown period of about 20-30 seconds. Start of the countdown period can be adjusted with the potentiometer 41 to give larger or smaller ice cubes.

Upon completion of its countdown, the timer 39 sends it signal to a harvest amplifier 53. The harvest amplifier 53 sends its signal to the hot gas defrost 36 and the water pump 18, to a control turn-on timer 54, and to a latching interlock amplifier 55. The signal to the hot gas defrost 36 simultaneously turns on the hot gas defrost 36 and turns off the water circulation pump 18. The interlock amplifier 55 feeds a signal out of a signal line 56 to a harvest period timer 57. When the amplifier 53 and interlock 55 are latched, the hot gas defrost 36 is on and the water pump 18 is off.

The signal from harvest amplifier 53 to the control timer 54 disables the timer 54 and the control amplifier 75 whereupon via signal line 66 an appropriate signal is provided to immediately disable the freeze plate temperature amplifier 52, a suction line temperature amplifier 67 and a water temperature amplifier 68 so that the control 34 does not receive signals from amplifiers 52, 67 or 68 during hot gas defrost and during initial pulldown of the subsequent freezing cycle as will be described.

When the curtain 21 is subsequently opened by falling ice, the curtain sensor 25 sends its signal that the curtain 21 has opened via signal line 58 to simultaneously unlatch the harvest amplifier 53 and the interlock amplifier 55. Immediately the hot gas defrost 36 is turned off and the pump 18 restarted and a subsequent freeze cycle is started. The signal that the curtain 21 has opened is also sent from the curtain amplifier 59 to the curtain timer 40. If the signal to the curtain timer 40 is provided for a length of time in excess of the timer 40 preset period, the curtain timer 40 sends an output signal to a full bin amplifier 60 which then provides a signal via signal line 85 to turn off the refrigeration as will subsequently be described.

The harvest period timer 57 is started simultaneously with the hot gas defrost 36. The harvest period timer 57 will have a predetermined countdown period, with a 4k2 minute countdown period being an appropriate example. The curtain timer 57 will countdown if the curtain 21 does not open and upon completion of a countdown will indicate no harvest or a faulty harvest and that something is wrong with the ice maker 10. Upon completion of a countdown, the harvest period timer 57 will send a signal to latch a faulty harvest amplifier 62 which in turn will send a signal via signal line 63 to shut off the refrigeration.

When the signal from the harvest amplifier 53 is changed upon start of the next freeze cycle, the control timer 54 is started. The control timer 54 has a countdown period that is greater than the initial pulldown time of the refrigeration system and greater than the time it takes to begin freezing ice on the freeze plate 12. A timer operatively connected between said curtain and said shut off means, said timer having means for activating said shut off means after a count down period if and when the curtain has not opened in a predetermined period of time. A preferred and an example time for countdown of the control timer 54 is about six minutes. When the control timer 54 has completed its countdown, it sends out signals by its control amplifier 75 and signal lines 66 that simultaneously enable thermister amplifier 52, 67 and 68 and therefore the thermisters 38, 64 and 65.If the suction line temperature is too high, specifically greater than 4.5 "C, a signal will then be sent by suction line thermister 64 and suction line amplifier 67 via signal line 84 to shut down the ice maker 10. The water temperature thermister 65 is positioned to sense and indicate the temperature of the water being circulated over the freeze plate 12 by the pump 18. If the indicated water temperature is then too high, for example greater than 7 OC, the water thermister 65 and water temperature amplifier 68 will send a signal via signal line 69 to cause shutdown of the ice maker 10. The amplifiers 52, 67 and 68 are disabled during hot gas defrost and pulldown, and are then enabled by the control timer 54 and amplifier 75 after the refrigeration system has stabilized in a freeze cycle.

A refrigeration condensor temperature thermister 70 senses and indicates condensor temperature to a pair of condensor amplifiers 71, 72.

The first condensor amplifier 71 is operatively connected to send a signal to turn off the condensor fan 37 if and when the condensor is too cold. The output signal line 56 of the harvest cycle latching interlock 55 is also connected to an input of the first condensor amplifier 71 so that a signal to go into hot gas defrost also turns off the condensor fan 37 and keeps the fan 37 turned off during hot gas defrost. The second condensor amplifier 72 discretely sends a signal to latching condensor amplifier 73 which in turn sends a signal via signal line 74 to shut down the ice maker 10 when the condensor temperature is too hot. The first condensor amplifier 71 will, as an example, keep the condensor fan 37 turned off if the condensor temperature is too low.

When the condensor temperature goes above 40 "C, the first amplifier 71 will cause the condensor fan 37 to turn on; and when the condensor temperature drops to below 30 OC, the first amplifier 71 will cause the condensor fan 37 to turn off. The second condensor amplifier 72 will shut down the ice maker when the condensor temperature reaches 70 "C; such a high temperature is indicative of a dirty condensor, fan motor failure, fan jammed, or plugged air inlet.

A shut down inverter 76 has an output signal line 77 operatively connected to open the main relay 51 and disable the compressor 35 and other operating components of the ice maker 10. The shut down inverter 76 inputs are connected with OR logic wherein any single input will effect shut down of the ice maker 10. A shut down signal from curtain timer 40 via signal line 85, or faulty harvest amplifier 62 via line 63, or in condensor temperature signal line 74 will cause shut down inverter 76 to open the relay 51.The suction temperature signal line 84 is connected into suction temperature inverter 78. -If the suction temperature is too high, for example above 4.5 "C, and the suction amplifier 67 has been enabled, something is wrong and a signal will be sent to the suction inverter 78 which in turn will send a shut down disable signal via signal line 79 to the shut down inverter 76.

The water temperature signal line 69 sends a signal after the water temperature amplifier 68 has been enabled, and when the water is too warm to water temperature inverter 80 which in turn sends a disabled signal to the shut off inverter 76 via signal line 81.

If the curtain 21 failed to close, the signal in line 61 is sent to a multiple ice maker full bin inverter 82, the signal from the full bin amplifier 60 is sent via signal line 85 and signal line 88 to the shut down inverter 76 for causing shut down of the refrigeration until the curtain 21 reopens. The curtain open signal is also sent via signal line 85 and connector pin 86 to any upper level ice makers (not shown) atop of the subject ice maker 10. An upper level ice maker will eventually return an upper curtain open signal via connector pin and signal line 87 to full bin inverter 82 which in turn sends a shut down signal via line 83 to the shut down inverter 76 which will effect a shut down of the subject ice maker 10 and all upper level ice makers.

Thus, a disable signal in any one of signal lines 63, 74, 79, 81, 83, 85, or 88 will cause the shut down inverter 76 to open the relay 51 and stop the compressor 35. The outputs of the inverters 76, 78, 80, 82, the condensor latching amplifier 73, the control amplifier 75, and the interlock amplifier 55 are all discretely connected to a priority encoder 89 which has its outputs connected to a decoder driver 90 which has its outputs connected to a status display 91 preferably of the digital LED type.

The status display 91 gives a visual indication of what the ice maker 10 is doing, and why. For example, the following read outs indicate the following.

INDICATED STATUS NOS EXPLANATION POSSIBLE CAUSE 0 Unit is in freeze cycle making ice, no problems.

1 Unit is in harvest cycle, ice should drop shortly, no problems.

2 Normally indicates a If "2" is shown full bin condition, but bin isn't unit off, water full, check for curtain being held individual cube open with ice. holding curtain open.

3 Unit off due to Incoming water circulating water shut off. Pump temperature not not running or pulling down to at plugged.

least 7 "C. Manual Reservoir leaking reset required. badly. Water level set too high causing premature syphoning. Sensor not insulated properly. Defect ive sensor.

4 Unit off due to Low on refrigant.

suction line not Defective refrig INDICATED STATUS NOS EXPLANATION POSSIBLE CAUSE pulling down to at erant valve.

least 4.5 "C. Manual Compressor defect reset required. ive or inefficient.

Defective power relay, won't close.

Defective start relay, won't start compressor.

Low voltage to compressor, no start.

Defective compressor valve.

Defective sensor. Sensor not insulated properly.

5 Unit off due to ice Water curtain jam not releasing from med and wont evaporator within swing open.

four minutes after Defective hot gas entering harvest valve, won't open, cycle. Manual plugged.

reset required Ice slab deformed, won't release properly.

Extremely low ambient temper ature, below 7 OC.

INDICATED STATUS NOS EXPLANATION POSSIBLE CAUSE 6 Unit is off due to Dirty condensor.

condensor Defective fan temperature climbing motor or blade.

too high. Manual Gross overcharge.

reset required. Extremely high ambient tempera ture, above 50 OC.

Defective sensor.

Decimal Indicates that all Normal time delay, point OFF sensors, except approximately condensor, are 6 minutes.

switched off for first 6 minutes of freeze cycle.

Decimal Indicates all sensors point ON have been switched on.

Decimal Indicates evaporator Normal time delay point temperature has of approximately FLASHING pulled down and unit 20 seconds before will go into harvest harvest cycle after time delay. begins.

If and when manual reset is required, the master switch 50 must be turned off for 10 seconds and then returned to "ON".

This new and improved ice maker 10 is extremely reliable and commercially effective. It is relatively simple and fool proof. It reliably harvests ice cubes and reliably starts and/or shuts itself off. When something is wrong it stops before destroying itself and it indicates what's wrong.

Although other advantages may be found and realized and various modifications may be suggested by those versed in the art, it should be understood that we wish to embody within the scope of the patentwarranted hereon, all such embodiments as reasonably and properly come within the scope of our contributions to the art.

Claims (13)

CLAIMS:

1. A method of making ice cubes, comprising the steps of: a) initiating a freeze cycle and supplying cooled refrigerant to a refrigerant circuit on an evaporator while flowing a film of water firstly over the plate and then subsequently over ice formed upon the plate; b) sensing the temperature of the evaporator plate; c) starting a countdown of a predetermined time upon sensing that the plate is at a predetermined temperature, said temperature being the freezing point of water; d) counting down the time as long as the sensed temperature remains at or below the predetermined temperature; e) terminating the countdown if the temperature rises above the predetermined temperature; f) restarting a subsequent countdown when the sensed temperature again falls to the predetermined temperature; and g) terminating the freeze cycle and initiating a hot gas defrost cycle upon completion of a countdown.

2. The method of claim 1, in which said predetermined temperature is -14 7t 3 "C.

3. The method of claim 1, in which the time of said countdown is in the range of 20-30 seconds.

4. The method of claim 1, in which the freeze cycle continues uninterrupted until countdown completion.

5. The method of claim 1, in which the hot gas defrost is initiated immediately upon countdown completion.

6. The method of claim 5, including the further step of continuing the flow of water over the plate ice during the countdown, and then terminating the flow of circulating water at the conclusion of the countdown.

7. The method of claim 1, including the further step of terminating the freeze cycle and initiating a subsequent freeze cycle upon sensing that ice falling off of the evaporator plate has opened a curtain between the plate and a storage bin.

8. The method of claim 1, in which the countdown is completely repeated after a restart.

9. An ice cube maker having an improved freeze cycle control, comprising of: a) an evaporator plate having a front side for freezing ice and a back side with a refrigerant circuit thereon; b) a temperature sensing thermister on the plate back side; c) a refrigerant valve operatively connected to reverse refrigerant flow to the plate from a normal freezing cycle to a hot gas defrost cycle for release of and harvest of ice frozen upon the plate; d) a freeze cycle control operatively connected to the thermister and to the refrigerant valve; and e) means in said control for 1) determining via the thermister that the plate temperature has fallen to a predetermined temperature, 2) counting down a predetermined time after the predetermined temperature has been sensed, 3) terminating the countdown if the sensed temperature goes above the predetermined temperature, and 4) switching the ice maker from the freeze cycle to the defrost cycle upon countdown completion for harvest of the frozen ice.

10. An ice cube maker according to Claim 9, including means for completely resetting the countdown means during a countdown termination or the defrost cycle.

11. An ice cube maker according to Claim 9, including means for adjusting the start of the countdown.

12. An ice cube maker according to Claim 9, including means for switching the ice maker from the defrost cycle to a freeze cycle, and a curtain positioned between the plate ad a storage bin, said switching means being responsive to opening of said curtain by ice falling off of the evaporator.

13. An ice cube maker according to Claim 9, in which the thermister is spaced from the refrigerant circuit.