2. Description of the prior art Icing detectors for air vehicles which heat a detection surface and measure thermal heat of fusion either during melting or refreezing of the deposited ice are known. In such ice detectors, it is difficult to accurately measure heat loss due to thermal heat of fusion because there are variable losses from other sources which add or subtract from the heat of fusion and cause errors in measurement. These other losses include convective heat loss due to varying airspeed, air temperature, and air density as well as heat loss to structures supporting the ice detector. There is thus a need to provide a more accurate measure of heat of fusion as an indication of icing.

SUMMARY OF THE INVENTION In the present invention, an ice detector circuit adapts the operation of an ice detector probe or sensor to correct for variable heat losses associated with forced convection heat loss to the air flow over the sensor and conductive heat leakage to structures surrounding the sensor.

The ice detector circuit controls heating of a probe or detector for detecting ice having a melting temperature. The ice detector circuit generates an icing output indicating icing as a function of a temperature output generated by the probe. Control means in the ice detector circuit couple to the detector

for cyclically controlling a start time of the heating. The ice detector circuit further comprises measurement means coupled to the temperature output for measuring a warm-up time to a first temperature threshold below the melting temperature, and for measuring a melting time to a second temperature above the melting temperature. Comparing means in the ice detector circuit compare the melting time to a stored comparison parameter and providing the icing output. Adjusting means adjust the start time as a function of the warm-up time such that the start time is adjusted for non-icing heat loss from the detector.

In a preferred embodiment, the adjusting means further adjusts the cycling rate as a function of the melting time, and adjusts the stored comparison parameter as a function of the warm-up time.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a partial block, partial schematic diagram of a preferred embodiment of an ice detector circuit 60 coupled to ice detector probe 62 having a self-heating resistance temperature sensor ^" R^ which is disposed to receive a flow, arrow F, of air subject to icing. Temperature sensor R^ can comprise a self-heating, thin filmplatinum resistance thermometer. Ice detector 62 is mounted to a strut 64 on an aircraft, and there is some undesired thermal leakage L between the probe 62 and the strut.

Ice detector circuit 60 includes resistors R _x , R ₂ , R ₃ , R ₄ , R ₅ and R ₆ which are connected to resistance Ry to form a resistance bridge circuit 66 as shown in FIG. 1. Excitation source 68 is coupled via solid state switch 70 to excitation terminals 72, 74 of bridge 66. When switch 70 is closed by application of a control

output to line 76, the excitation source 68 energizes the bridge 66. When the bridge 66 is energized, the resistance R _v heats the probe 62 for sensing icing. Resistance R _y in probe 62, which is part of bridge 66, provides a temperature output on line 78 which is representative of temperature T of probe 62. Resistors R _l7 R ₂ , R ₃ , R ₄ , R ₅ are connected in series between the excitation terminals 72, 74 to form a resistive potential divider. The resistive potential divider has multiple taps 80, 82, 84, 86 generating potentials representing successivelyhighertemperatures thresholds in compari on to the temperature output on line 78.

A comparator 90 compares the temperature output on line 78 to the potential at tap 80 and provides an output on line 92 indicating that temperature T has reached a first threshold temperature below the melting point of ice. A comparator 94 compares the temperature output on line 78 to the potential at tap 82 and provides an output on line 96 indicating that temperature T has reached a second threshold, which is above the melting point of ice. A comparator 98 compares the temperature output on line 78 to the potential at tap 84 and provides an output on line 100 indicating that temperature T has reached a point at which substantia ly all ice is removed from probe 62. A comparator 102 compares the temperature output on line 78 to the potential at tap 86 and provides an output on line 104 indicating that temperature T is outside of a normal temperature range and that there is therefore a malfunction.

The comparator outputs on lines 92, 96, 100, 104 which indicate that successively higher temperature thresholds have been reached by the temperature T are

coupled to a measurement circuit 106. The control output on line 76 is also coupled to measurement circuit 106, providing an indication of the start of heating. Measurement circuit 106 measures the warm-up time between the start of heating and the time at which temperature T reaches the first threshold and provides an output T1TIME on line 108 indicating the warm-up time. The measurement circuit further measures the melting time between the first and second thresholds and provides an output MTIME on line 110 indicative of melting time.

Measurement circuit 106 also measures the time at which temperature T reaches the third threshold and provides an end-of-heating output on line 112 indicating when the third threshold has been reached. A controller 114 controls the control output on line 76 as a function of the end-of-heating output on line 112 and a stored adjustable parameter CYCLE RATE shown in a storage element or memory at 116. During operation, controller 114 starts a heating period by providing the control output on line 76 to enable switch 70 to connect excitation source 68 to bridge 66. Connection of the bridge circuit to the excitation source 68 causes resistor R^ to begin heating the probe 62. When temperature output T exceeds the thresholds set by the voltage divider, the comparator outputs 92, 96, 100 successively change state indicating that temperature T has reached successively higher threshold temperatures. When the temperature T reaches the third threshold, controller 114 opens switch 70 and the sensor is allowed to cool to the end of the cycle. The length of the cycle is controlled by CYCLE RATE.

Parameter adjustment circuit 118 receives outputs TITIME and MTIME on lines 108 and 110 and adjusts parameters CYCLE RATE, STRUT HEATER shown stored at 120 and ICETIME shown stored at 122 as explained in detail below in connection with FIG. 2.

Comparator circuit 124 compares the stored comparison parameter ICETIME to the sensed melting time variable MTIME and provides icing output on line 126 indicating icing. The icing output on line 126 can be coupled to a readout 128 or control device 130 controlling de-icing of aircraft surfaces.

The parameter STRUT HEATER stored at 120 controls the actuation of solid state switch 132 controlling energization of a heater 134 in strut or mounting structure 64 for probe 62.

FIG. 2 shows a flow chart or algorithm performed by parameter adjustment circuit 118. Parameter adjustment circuit 118 can comprise digital logic or a microprocessor system, and the microprocessor system can be common to other functions performed in the ice detector circuit 60, as well. The algorithm shown in FIG. 2 is an adaptive control algorithm which adapts parameters used in controlling the ice detector and the icing output to ambient conditions of heat loss and icing. The algorithm is executed once for each heating cycle of probe 62. Parameter adjustment circuit 118 starts execution of the algorithm at line 12 in FIG. 2, and updates measured times A and B with current values of TITIME and MTIME respectively as shown in block 14. TITIME is the time interval between starting heater energization and sensing the first temperature threshold. MTIME is the ice melting time interval

between sensing the first and second detection thresholds.

Time A, the time to heat the sensor to the first temperature threshold correlates with forced convection heat loss to the air flow F and conductive heat leakage L to structures (heat sinks) 64 surrounding the sensor. When the ambient temperature is lower, it takes longer to heat the sensor 62 to the first temperature threshold since the surrounding heat sinks are at a lower temperature. Time A is therefore larger when ambient temperatures are low. When there is a larger mass flow F past the sensor 62, forced convective heat loss is larger, and time A therefore increases.

The conductive and forced convective heat losses are present during the melting time interval, and introduce error into the measurement of heat of fusion of ice on the probe. Time A represents this background heat loss which is independent of heat loss due to melting ice. In decision blocks 16, 18, 20, the parameter adjustment circuit compares time A to stored time constants Al, A2, A3 in order to select different operation modes Ml, M2, M3, M4 based on the rate of background heat loss. The operation of the ice detector is thus corrected for background heat loss.

In decision block 16, the parameter adjustment circuit compares time A to time constant Al which, in a preferred embodiment is Al = 200 microseconds. If time A is less than Al, this indicates that the ambient temperature is so warm that icing will occur only very slowly, if at all, and the parameter adjustment circuit selects mode Ml in block 22. In mode Ml, the parameter adjustment circuit sets the CYCLE TIME long to allow a

long time for ice to accumulate on the sensor. In a preferred embodiment, the parameter adjustment circuit 118 sets CYCLE TIME to 8 seconds in mode Ml. In mode Ml, parameter adjustment circuit 118 adjusts stored comparison parameter ICETIME to be long to reduce the likelihood of a false indication of icing due to noise. In a preferred embodiment, ICETIME = 4 seconds in mode Ml. In mode Ml, parameter adjustment circuit sets parameter STRUT HEATER to a low level which shuts off the strut heater. After the parameter adjustment circuit adjusts parameters CYCLE TIME, ICETIME, and STRUT HEATER in block 22, the parameter adjustment circuit leaves the algorithm via lines 24, 26.

After completion of the algorithm, the CYCLE TIME, ICETIME and STRUT HEATER words selected by parameter adjustment circuit are available for use during the next heating cycle of the probe.

If time A is greater than constant Al, parameter adjustment circuit 118 next compares time A to a second constant A2 (larger than constant Al) in decision block 18. In a preferred embodiment A2 = 5 milliseconds. If time A is less than constant A2, this indicates that the ambient temperature and airflow are in a range where icing is possible, and parameter adjustment ci cuit selects mode M2 in Block 28. In mode M2, the constant ICETIME is set to a shorter time interval than it is in mode Ml, and in a preferred embodiment, ICETIME = 42 milliseconds in mode M2.

After mode M2 is selected, the parameter adjustment circuit executes decision block 30 which compares the current ice melting time B to a constant Bl which, in a preferred embodiment is Bl = 42 milliseconds. If time B is less than constant Bl, then

significant icing is not occurring and mode M2A is selected at block 32. In mode M2A, CYCLE TIME is set to 8 seconds and STRUT HEATER is set low and the strut heater is not energized. After completion of block 32, the parameter adjustment circuit leaves the algorithm via lines 34, 36, 26.

If icing time B is not less than threshold Bl, the parameter adjustment circuit advances to execute decision block 38. In decision block 38, icing time B is compared to a constant B2, and in a preferred embodiment B2 = 220 milliseconds. If icing time B is less than constant B2, then moderate icing is occurring and mode M2B is selected. In mode M2B, STRUT HEATER is set low, and the CYCLE TIME is set to a shorter time interval than it is in mode M2A in block 40. In a preferred embodiment, CYCLE TIME is set in mode M2B to 4 seconds. After execution of the instructions in block 40, the parameter adjustment circuit leaves the algorithm via paths 42, 36 and 26. if icing time B is not less than threshold B2, then there is heavy icing and the parameter adjustment circuit advances from decision block 38 to block 44. In block 44, the parameter adjustment circuit sets the STRUT HEATER to a high level and the strut heater is turned on. In mode M2C, the parameter adjustment circuit also sets the CYCLE TIME to a short interval, in a preferred embodiment CYCLE TIME = 2 to 3 seconds, to prevent ice from accumulating too thickly on the sensor. After completion of block 44, the parameter adjustment circuit leave the algorithm via paths 46, 36, 26.

Looking back now at decision block 18 discussed above, if the time A is not less than constant A2, parameter adjustment circuit advances to decision

block 20. In decision block 20, parameter adjustment circuit compares time A to constant A3. Constant A3 is preferably 50 milliseconds. If time A is less than 50 milliseconds, then there is moderate heat loss from the sensor and the parameter adjustment circuit selects mode M3 at 50. In mode M3, ICETIME is adjusted to a longer time than in mode M2 and parameter STRUT HEATER is set high. In a preferred embodiment, ICETIME - 45 milliseconds in mode M3. If time A is longer than 50 milliseconds, then the parameter adjustment circuit selects mode M4 at 48, indicating a high rate of background heat loss from the sensor, such as is encountered at low ambient temperatures. In mode M4, ICETIME is set higher than in Mode 3, and in a preferred embodiment, ICETIME = 55 milliseconds in mode 4 and STRUT HEATER is set high. After completion of block 48 or 50, the parameter adjustment circuit 118 exits the algorithm via paths 36, 26.

The strut heater is energized when ice is detected and the ambient temperature is below the setpoint. This reduces power consumption by the strut heater, extends the life of the heater and electrical parts in the strut, and increases sensitivity of the sensor to icing at near freezing temperatures by reducing heat conduction from the strut to the sensor surface. The rapid response of the strut heater to icing events keeps ice from building up on the strut.

While the invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.