Frost and dew sensor

• US 5,000,579 A
• Filed: 07/18/1990
• Issued: 03/19/1991
• Est. Priority Date: 10/20/1988
• Status: Expired due to Term

• Current Assignee: Murata Manufacturing Co Limited
• Original Assignee: Murata Manufacturing Co Limited
• Inventors: Murata, Michihiro, Takata, Eiichi, Kumada, Akira
• Primary Examiner(s): Cuchlinski, Jr., William A.
• Assistant Examiner(s): GUTIERREZ, DIEGO F
• Application Number:
  US07/553,740
• Time in Patent Office: 244 Days
• Field of Search: 374/10, 374/11, 374/28, 374/16, 73/336.5, 364/557
• US Class Current: 374/28
• CPC Class Codes: G01N 25/68 by varying the temperature of a condensing surface
• CPC Class Codes: G01N 27/18 caused by changes in the thermal conductivity of a surrounding material to be tested G01N27/20 takes precedence

Abstract

In a frost and dew sensor composed of two thermosensitive resistors, a power source and an arithmetic circuit, frost or dew is detected from the change of a differential temperature between the two thermosensitive resistors as two different currents from the power source are supplied to the respective thermosensitive resistors. In an alternative form, the sensor has a single thermosensitive resistor, which is energized as two different currents are alternately supplied from the power source so that frost or dew is detected by comparison of the time-lags between the two outputs from the thermosensitive resistor.

Claims

1. A frost and dew sensor comprising:

(a) a thermosensitive resistor capable of generating heat by itself due to a given current supplied thereto, said thermosensitive resistor having a resistance varying according to the change of temperature of said thermosensitive resistor;

(b) a power source for changing the temperature of said thermosensitive resistor periodically, said power source including a variable constant-current source for alternately supplying a heat-generating current and a comparative reference current to said thermosensitive resistor, said comparative reference current being such that it causes a temperature increase of said thermosensitive resistor to only a negligible extent with respect to the temperature increase due to the heat-generating current; and

(c) an arithmetic circuit for fetching a temperature of said thermosensitive resistor as an output voltage according to a resistance corresponding to the temperature and for generating a frost-and-dew signal according to a difference between the output voltage during the heat-generating current is being supplied to said thermosensitive resistor and the output voltage during the comparative reference current is being supplied to said thermosensitive resistor, whereby the frost-and-dew signal is issued as presumed that said thermosensitive resistor is covered with frost or dew when a differential temperature of said thermosensitive resistor in such two durations is lowered.

2. A frost and dew sensor according to claim 1, wherein said arithmetic circuit includes:

an amplifier for amplifying the output voltage of said thermosensitive resistor, during the comparative reference current is being supplied thereto, at a rate corresponding to a ratio of the heat-generating voltage to the comparative reference current to output the amplified output voltage;

a holding circuit for holding the output of said amplifier to output the same output with a delay for the duration the comparative reference current is being supplied to said thermosensitive resistor; and

a comparator circuit for first comparing the output of said holding circuit with the output voltage of said thermosensitive resistor, during the heat-generating current is being supplied, to find a differential temperature of said thermosensitive resistor between the temperature during the comparative reference current is being supplied and that during the heat-generating current is being supplied, and for secondly comparing the differential temperature with a predetermined reference value.

3. A frost and dew sensor according to claim 1, wherein said arithmetic circuit includes:

an A/D converter for converting the output voltage of each said thermosensitive resistor into digital data;

a μ

-CPU for performing a predetermined arithmetical operation with respect to the digital data, outputted from said A/D converter, to find a differential temperature at said thermosensitive resistors; and

an output circuit for generating and outputting a frost-and-dew signal according to the arithmetic result of said μ

-CPU.

4. A frost and dew sensor according to claim 2, wherein said comparator circuit includes:

a first comparator for comparing the output signal of said thermosensitive resistor, during the heat-generating current is being supplied, with the output of said holding circuit to find the difference between the two outputs; and

a second comparator for comparing the output signal of said first comparator with a reference value.

5. A frost and dew sensor according to claim 4, wherein said arithmetic circuit includes a freezing level detector circuit for comparing the output current of said thermosensitive resistor corresponding to the comparative reference current with a predetermined freezing level reference value to detect whether an ambient temperature of said thermosensitive resistor is below the freezing point.

6. A frost and dew sensor according to claim 5, wherein said arithmetic circuit includes a discriminator for comparing the output signal of said comparator circuit with the output signal of said freezing level detector circuit to discriminate whether said thermosensitive resistor is covered with frost, when the ambient temperature of said thermosensitive resistor is below the freezing point.

7. A frost and dew sensor according to claim 6, wherein said arithmetic circuit includes an amplifier for amplifying the output of said discriminator to output the amplified output;

said power source including an oscillator for producing an alternate timing to alternate the output of said variable constant-current source between the comparative reference current and the heat-generating current, said oscillator including a timing circuit for supplying a control signal to said amplifier according to the alternate timing produced by said oscillator and also for supplying the same control signal to said output circuit according to the same alternate timing;

said amplifier circuit including a switch adapted to be energized and deenergized by said timing circuit in such a manner that the output current of said thermosensitive resistor is inputted only during the comparative reference current is being supplied to said thermosensitive resistor; and

said output circuit including another switch adapted to be energized and deenergized by said timing circuit in such a manner that the output of the frost-and-dew signal to the exterior is prohibited during the comparative reference current is being supplied to said thermosensitive resistor.

8. A frost and dew sensor according to claim 7, further including a shaft carrying thereon said thermosensitive resistor, and a base supporting said shaft, said thermosensitive resistor being a thermistor.