Controls: Eye on Ice
January 22, 2010
The imperative to design more energy-efficient appliances makes this an opportune time to reexamine the traditional timed-defrost cycle used on household refrigerators and most commercial refrigeration. Timing cycles may be short or long, frequent or infrequent; based on compressor run time, or straight elapsed time. One typical defrost controller simply uses a 1,750 RPM shaded-pole motor to drive a gear train and a cam that operates a 2-form-B switch, without regard to the presence or absence of physical ice.
Unfortunately, this century-old timer technology increases both the parts count and the refrigerator’s cost to manufacture. It affects reliability by having another component with moving parts that can fail. And it causes avoidable energy losses at the user level by initiating more defrost cycles than necessary. The solution is an adaptive defrost control, which has been an elusive goal for years. There are some electronic refrigeration controls that use sophisticated algorithms to infer the presence of ice on a system’s evaporator coils, but nothing beats actually detecting the presence or absence of ice. A better solution can now be implemented using an optical ice-sensing system that has no moving parts. This particular technology, known as Ice Meister, promises to reduce manufacturing cost and parts count, enhance reliability, and save energy at the consumer level.
Moving away from timed defrost cycles is a benefit in both cool and hot weather. On a hot day with high humidity, the frequent opening and closing of a refrigerator door can cause evaporator coils to ice up rapidly. In this situation, the optical adaptive defrost control can initiate defrost cycles immediately as ice actually forms on the coil, instead of having to wait for a timed cycle.
Conversely, in cool weather with low humidity, the ice accumulation due to door openings is slower and much less. In this case, the adaptive control delays the defrost cycle until it is actually necessary. The quantified amount of energy to be saved depends upon local variables, including ambient humidity, temperature, household demographics, local customs, etc.
The Ice Meister adaptive defrost control is not an analog measurement instrument-it does it not measure ice thickness. It operates by using photons of light to sense the digital step-function phase-change of H2O molecules between their liquid and solid phases across the thermal barrier known as latent heat of fusion, which is 80 calories per gram of water.
In operation, if H2O molecules are in the liquid state, the force of gravity tends to remove them from the sensor surface, and only a thin film of H2O molecules separates the sensor surface from the ambient air. This film of H2O molecules interposes itself between the excitation signal and the receiver, and is electronically zeroed out. If, however, the H2O molecules are in the solid phase, they tend to stick together and accumulate on the sensor surface, and increase the number of molecules interposed between the optical surface and the receiver. This attenuates the excitation signal below a set threshold. It is this threshold-passing function that the system detects. Ice-formation sensing is a digital function, and has nothing to do with thickness, temperature, or time.
The ice-sensing threshold is fixed, but signal strength and amplifier gain are both adjusted with trim pots. If adjustments are set to sensitive, the adaptive defrost controller sweeps back and forth across the ice-formation threshold frequently, producing many defrost cycles of short duration each, which maintains a minimum amount of ice on the evaporator coils. If the adjustments are set to less sensitive, defrost cycles will be longer, less frequent, and maintain more ice on the coils. The adjustments are an appliance-design requirement.
Either clear ice or opaque rime ice can form on the sensor. If clear ice forms, it reacts with the sensor in one way. If opaque rime ice forms, it reacts with the sensor in another way. Either rime or clear ice will initiate a defrost cycle, but air will not. Rime ice trumps clear ice; ice of either kind trumps air.
This adaptive defrost control features a 3 mm diameter optical probe that is positioned between any two coil fins. Whenever ice begins to form on the sensor, it registers with the control electronics, which initiates a defrost cycle. The defrost cycle then melts whatever ice has formed on the coil fins and the sensor. As soon as the sensor’s ice has been melted away and removed by gravity, the defrost cycle terminates.
This adaptive technique allows ice control systems to cycle back and forth across the ice formation threshold in real time and adapt to prevailing circumstances. It initiates defrost cycles only when there is ice to be melted, for only as long as necessary, but never when there is no ice. This saves energy at the user level.
The optical technology is simple, and eliminates moving parts. It is an NASA-tested sensing method that’s used on wind power turbines and unmanned aerial vehicles. Ice Meister sensors conform to core paragraph 188.8.131.52.1 of the only published aerospace specification for in-flight ice detectors (Aerospace Standard AS 5498 from the Society of Automotive Engineers). The sensors are listed, described, and illustrated in SAE Aerospace Information Report AIR 4367A, paragraph 4.11.
Optical adaptive defrost controllers are standalone, self-contained devices with their own built-in electronics. They don’t require a microcontroller, MHz clock, or programming. The defrost controllers are configured specifically for each type of refrigerator, and designed-in as an OEM part. The 3 mm diameter probe is mounted on a bracket between any two coil fins. The back of the sensor has three pins that connect to the host system. One pin is for input power; a second is an ON/OFF digital relay control output (the host system provides an external 2-form-B relay), and the third pin is a common ground. Power consumption is <2 Watts. Input voltage in is 24 VDC, but the controller can be configured to run on virtually anything from 3.3 VDC to 120 VAC.
The output from the controller does not require signal conditioning or interpretation. The control output pin is a simple 3.3 V ON/OFF digital logic signal that goes directly to the existing defroster components. The electronics run on 3.3 VCC, so the host system must incorporate a buffering 1 form C relay to de-energize the 10 A compressor motor before energizing the 10 A heater coils. Break-before-make (form C) relays are single-pole-double-throw (SPDT); they break the first connection before making the second connection.
Because the unit is a digital design, not an analog design, there is nothing to calibrate. It’s a wide tolerance go/no-go system. If it drifts a bit one way or the other, it will not materially affect the operation. That’s because the presence or absence of ice is an optical step function that correlates to H2O being either solid or liquid. The ice to be sensed forms directly on the optical surfaces, and with great authority kills the sensor excitation signal.
An analogy might be a hot, humid summer day when a person is driving in a car, with the air conditioning blowing cold air onto the driver’s face and sunglasses. When the driver steps out of the car, the cold sunglasses instantly fog over, because water has condensed out of the air directly onto the optical surfaces. That’s the step function manner in which these ice sensors work-ice forms directly on the optical surfaces.
During aerospace testing at NASA’s Glenn Research Center in Cleveland, which has the world’s largest icing wind tunnel, it was apparent that optical sensitivity is actually in the molecular range; but for purposes of credibility, New Avionics specifies detection threshold as <0.001 in. of ice. For that reason, the sensor must be de-sensitized by incorporation of an operational amplifier.
Powered up and left unconnected, an operational amplifier generates infinite gain, because it tries to divide something by zero. But infinite gain is useless, so every practical application of an op amp incorporates negative feedback, in the form of two feedback resistors whose numeric ratio precisely sets the amplifier’s gain.
By simply controlling the ratio of two feedback resistors, the defrost controller’s op amp gain factor and relative trip level is established. As a result, there is virtually infinite range in establishing the ice sensor’s trip level, from zero to infinity. For aerospace applications, designers of aircraft generally want to detect an airplane’s earliest penetration into an icing domain so as to take the earliest remedial action. But in HVAC/R and refrigerator systems, designers may want to allow some degree of ice to form first.
By adjusting the two feedback resistors, appliance designers can exercise wide control over the sensor’s sensitivity. The maximum thickness of ice before tripping is less than the dimensions of an ice cube, but by increasing the excitation signal’s strength and increasing the op amp’s gain level, the trip level can be set to a useful level of ice.
The form factor of the controller measures 2 in. high by 0.5 in. wide; the plastic probe extends 0.5 in. in front of the plate. The back of the sensor incorporates the electronics, which are potted in two-part epoxy, and extend 1 in. back from the face plate.
All the sensor’s components are thermally matched for thermal expansion. Integrated with the refrigerator’s coils, it is not subjected to thermal shock. The sensor excitation source, sensing receiver, and electronics are all semiconductor materials, operating in a cold environment, and likely have a greater MTBF than any household refrigerator.
This ice-sensing technology now makes it feasible to implement adaptive defrost controls into new generations of refrigerators and achieve new levels of energy efficiency by eliminating unnecessary defrost cycles.
For more information, visit: www.newavionics.com