Specimen heater and control system for accelerated weathering test apparatus

An accelerated weathering test apparatus of the type used to concentrate solar radiation upon test specimens including a heating element that transfers energy to the test specimens. A temperature sensor is operatively coupled to one of the test specimens for generating a test signal representative of the operating temperature of the test specimens. A controller for generating a temperature set point is connected to the temperature sensor and responsive to the test signal for selectively controlling a power level applied to the heating element in order to control a rate at which energy is transferred to the test specimens.

The present disclosure is directed to an accelerated weathering test apparatus of the type used to concentrate solar radiation on test specimens, and more particularly, to an accelerated weathering test apparatus with a heating element and a temperature sensor operatively coupled with a test specimen to facilitate control of the test specimen operating temperature.

Manufacturers of exterior coatings, such as paints and finishes, as well as plastics and other components which tend to degrade under exposure to solar radiation and other weathering effects, often want to know how such products will perform following years of exposure. However, such manufacturers typically require such information in a much shorter time than it would take to expose such materials to weathering effects under normal conditions. Accordingly, accelerated weathering test devices have been developed which accelerate the effects of weathering due to outdoor exposure in a much shorter time so that manufacturers need not actually wait five or ten years in order to determine how their products will hold up after five or ten years of actual outdoor exposure.

One known accelerated weathering test device is disclosed in U.S. Pat. No. 2,945,417, issued to Caryl et al. The aforementioned test device includes a Fresnel-reflecting solar concentrator having a series of ten flat mirrors which focus natural sunlight onto a series of test specimens secured to a target board measuring approximately five (5) inches wide by fifty-five (55) inches long. The Fresnel-reflecting solar concentrator directs solar radiation onto the target board area with an intensity of approximately eight suns. Both the bed which supports the mirrors of the solar concentrator, and the target board, are supported by a frame which can be rotated to follow daily movements of the sun.

A solar tracking mechanism responsive to the position of the sun, controls the operation of an electric motor that is used to rotate the test apparatus to follow movements of the sun. The axis of rotation of the test machine is oriented in a north-south direction, with the north elevation having altitude adjustment capability to account for variation in the sun's altitude at various times during the year.

Such known testing devices are also provided with an air tunnel mounted above the target board. An air deflector causes air escaping from the air tunnel to be circulated across the test specimens mounted to the target board to prevent the test specimens from overheating due to the concentrated solar radiation to which they are exposed. The amount of air is controlled by the dimension of the gap between the deflector and the specimen. A squirrel cage blower communicates with the air tunnel for blowing cooling ambient air there through. In addition, water spray nozzles are provided proximate to target board for wetting the test samples at periodic intervals to simulate the weathering effects of humidity, dew, rain, etc.

Another known accelerated weathering test device is disclosed in U.S. Pat. No. 4,807,247 issued to Robins. The aforementioned test device includes all the structure previously described above with respect to the '417 patent and further includes a system for maintaining a uniform, constant test specimen temperature during daylight hours despite variations in ambient air temperature and variations in solar radiation intensity.

The system includes a temperature sensor mounted to the target board for exposure to the concentrated solar radiation and for generating an electrical signal indicative of the temperature of the test specimen mounted to the target board. The system further includes a control mechanism electrically coupled to the temperature sensor and responsive to the electrical signal generated thereby for selectively controlling the application of electrical power to the electrical motor included within the air circulation system. In this manner, the control mechanism serves to vary the speed of the electric motor and thereby control the flow rate of cooling ambient air circulating across the target board so that the temperature of the test specimen remains constant at the desired set point.

When the sensed temperature of the temperature sensor increases, the control mechanism increases the speed of the blower motor to circulate more ambient cooling air across the target board in an attempt to lower the temperature of the test samples in the direction of the desired set point. Similarly, if the sensed temperature of the temperature sensor drops below the desired nominal temperature, the control mechanism decreases the speed of the blower to permit the test samples to warm up back to the desired set point.

The temperature control mechanism also includes a user operable adjustment mechanism, in the form of the control knob, for allowing a user to set a static, desired sensor temperature. A bypass switch is also provided for allowing the user to operate the test device in the controlled temperature-mode as described above, or in an uncontrolled mode wherein the blower motor operates at a constant speed.

Standardized testing methods have been developed for operating accelerated weathering test devices of the type described above. The American Society for Testing and Materials (ASTM) has issued standards G90, E838, D4141, D3105, D3841, D5105, E1596 and D4364 covering the testing procedures and the operating parameters for conducting such outdoor accelerated weathering tests. Other standards and appraisals have also been developed and specified by the Society of Automotive Engineers (SAE), Ford, International Standards Organization (ISO), American National Standards Institute (ANSI), Japan Industrial Standard (JIS), namely, SAE J576, SAE J1961, Ford EJB-M1J14-A, Ford EST-M5P11-A, ISO 877, ANSI/NSF 54, JIS Z 2381 and MIL-T-22085D.

Apart from outdoor accelerated weathering test devices of the type described above, other test devices are also known which utilize an artificial source of radiation to expose the test specimens. An example of such a test device is disclosed in U.S. Pat. No. 3,664,188 issued to Kockott. While such test devices have the advantage of permitting precise control over radiation intensity, temperature and humidity, such test devices have often failed to duplicate the actual light spectrum of natural sunlight to which the specimens under test will actually be exposed in everyday use. It has been acknowledged and recognized by those of skill in the art that natural sunlight and artificial sunlight test apparatus are distinct from one another and provide different sets of empirical data.

Outdoor accelerated weathering test devices of the type described above in regard to U.S. Pat. Nos. 2,945,417 and 4,807,247, have the advantage of using natural sunlight, and hence the specimens under test are exposed to the actual spectrum of sunlight. However, disadvantages of outdoor accelerated weathering test devices have been discovered.

One disadvantage of outdoor weathering devices is the difficulty in maintaining the temperature of a test specimen within a temperature range while the specimen is exposed to outdoor conditions. For example, during hot testing periods, the test specimen may have to be cooled so that a temperature thereof is maintained near a test temperature.

In contrast, during cooler testing periods, the test specimen may have to be heated so that a temperature thereof is maintained near a test temperature. Accordingly, some outdoor weathering test apparatuses include specimen heating components and control systems for controlling such specimen heating components. A temperature sensor is typically placed near the test specimen, the output of which is then provided to a control system, which controls a heater to heat the test specimen. In these outdoor weathering test apparatuses, however, the temperature sensor does not detect the actual temperature of the test specimen, but only the temperature of an object to which the test specimen is attached or the temperature on or near the heater. Accordingly, although a control system controls the output of the heater in response to the input it receives from the temperature sensor, the control system is not actually controlling the temperature of the test specimen because the output of the sensor may not be indicative of the actual temperature of the test specimen.

Therefore, there exists a need in the art for a specimen heater and its associated control system that provide control of the actual temperature of the test specimen.

DETAILED DESCRIPTION

For the purposes of promoting and understanding the principles disclosed herein, reference will now be made to the preferred embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope is thereby intended. Such alterations and further modifications in the illustrated device and such further applications are the principles disclosed as illustrated therein as being contemplated as would normally occur to one skilled in the art to which this disclosure relates.

In accordance with one principle aspect of the present disclosure, an accelerated weathering test apparatus of the type used to concentrate solar radiation upon test specimens includes a heating element that transfers energy to the test specimens. A temperature sensor is operatively coupled to at least one of the test specimens for generating a test signal representative of the operating temperature of the test specimens. A controller for generating a temperature set point is connected to the temperature sensor and responsive to the test signal for selectively controlling a power level applied to the heating element in order to control a rate at which energy is transferred to the test specimens.

Referring toFIG. 1, a prior art accelerated weathering test apparatus is designated generally by reference20and includes a pair of A-frame members22and24to support the operative portion of the apparatus. The lower ends of the A-frame members22,24are interconnected by a base member26which is operatively connected to a ground member28in order to provide azimuth rotation in the direction indicated by arrow30and elevation rotation in the direction indicated by arrow31. The elevation direction rotation accounts for periodic variation in the sun's altitude throughout the day.

Rotatively supported from the upper ends of A-frame members22,24is a mirror bed frame32which supports a plurality of flat mirrors34,36. The plurality of mirrors34,36are angled to reflect solar radiation directly impinging upon such mirrors to a target board38(seeFIG. 2).

A pair of standards40and42extend outwardly from and perpendicular to mirror bed frame32. An air tunnel44having a generally rectangular cross section is supported by the upper ends of standards40,42. Referring toFIG. 2, target board38is supported by the lower wall of air tunnel44, and a plurality of test specimens46are mounted to the target board38for exposure to the concentrated solar radiation, represented inFIG. 2by the upwardly extending arrows numbered39. The target board38may include a single specimen46or a plurality of similar or different specimens46. A squirrel cage blower assembly48communicates with one end of the air tunnel44. Squirrel cage blower assembly48includes a fan driven by an electric motor to circulate cooling ambient air through air tunnel44, represented inFIG. 2by the outwardly extending arrows numbered45. As shown inFIG. 2, air tunnel44includes a deflector50which extends for the length of target board38and causes cooling ambient air to be circulated across target board38for cooling test specimens46, represented inFIG. 2by the arrows numbered23.

Standards40,42are rotatively supported to upper ends of A-frame members22,24. A supporting shaft43(shown inFIG. 8) coincident with the axis of rotation in passing through standards40,42rotably supports that portion of the test apparatus which tracks daily movements of the sun. In order to properly position the Fresnel-reflecting solar concentrator comprised by mirrors34,36and reversible electric motor and related gear drive, generally designated by reference number54, are provided for periodically rotating the mirror bed and target board assembly to track movements of the sun. The clutch preferably couples standard40to the shaft43(shown inFIG. 8) to rotate the mirror bed frame and target board assembly while permitting manual positioning of the unit at any time to correct for any positioning errors.

A solar cell tracking unit52controls the application of electrical power to a reversible motor in order to maintain the mirror bed frame32perpendicular to incident rays of sunlight. A solar tracker may be of the type which includes two balanced photo cells and a shadowing device mounted above such photo cells for shading them. When an imbalance is detected resulting from one photo cell receiving more sunlight than the other photo cell, an electrical error signal is generated which is amplified and used to apply power to the drive motor54for rotating the unit until the photo cells are again balanced, indicating that the unit is properly positioned with respect to the sun.

Also shown inFIG. 1is a water spray nozzle assembly, designated generally by reference numeral51. As shown inFIG. 1, spray nozzle assembly51is used to periodically spray water at the test specimens to simulate dew, rain, etc.

A hinge shield or cover49is shown coupled to the air tunnel44opposite the air deflector50. A door release mechanism47is disposed on the air tunnel44for engaging and maintaining the shield in an open position. Upon release, the shield49assumes the closed position such that concentrated solar radiation reflected by the plurality of mirrors35does not reach the test specimens46.

Secured to the target board38is a feedback device (not shown) having at least one temperature sensitive component secured in heat conductive relationship therewith. Such component may be a thermistor, thermocouple, resistance temperature device, integrated circuit temperature device, semiconductor temperature device, or any other suitable device for detecting temperature of the feedback device. The feedback device may be formed from a standardized material having known thermal conductive properties or may be formed from a material similar to that of the test specimen. The temperature sensitive component may be embedded within, attached to a back surface or attached to a front surface of the feedback device. Alternatively, a non-contact optical temperature sensing device, infrared sensor, radiation thermometer, pyrometric sensor, bimetallic sensor, filled system thermometer, liquid or gas, or thermal imaging system may be used in order to determine the temperature of the feedback device. The feedback device is preferably coated with black paint to insure that the feedback device will absorb solar radiation impinging upon the area of the target board38to which the feedback device is secured. An appropriate black paint which may be used for this purpose is DUPONT DULUX Super Black High Temperature Enamel.

Referring again toFIG. 1, a controller box57houses the power and controller systems for the apparatus20. A power cable58supplies electrical power to the apparatus20for powering the electric motor54, which also actuates the fan48. A signal cable60is connected to the controller system disposed within the control box57for communication with remotely disposed devices, such as the feedback devices and input device, as will be discussed below or for communication with a central command for governing the operation of the apparatus20in accordance with the present invention.

Referring toFIGS. 2 and 3, a schematic cross sectional diagram of a typical prior art specimen heating system70is shown. The specimen heating system70includes a heating element72, a temperature sensor74embedded in the heating element72, a platen and a controller76. As shown inFIG. 3, the platen includes a pair of spaced apart plates78sandwiching the heating element72. The plates78may be constructed from aluminum or any suitable metal. The heating element72may be a wire wound silicone rubber heater. However, any suitable heating element may be used between the plates78. The heating element72receives power through the power cable82from the controller76. Accordingly, as will be described in the following, the controller76can control the heating output of the heating element72by supplying the necessary power to the heating element72through the heating element power cable82. The temperature sensor74is disposed between the plates78. The temperature sensor74provides an output signal that is indicative of the temperature near the temperature sensor74. The test signal is conveyed to the controller76through the test signal cable84.

Referring also toFIG. 4, the controller76may include a processing unit83and memory85that stores programming instructions pertaining to the control of an actual temperature of the temperature sensor74or heating element relative to a set point temperature entered by an operator. The controller may also include an analog control circuit for providing the following functionality. The controller76receives the test signal from the temperature sensor74and displays the temperature conveyed by the test signal on an actual temperature display86. The controller76also includes a set point display88, which shows the set point temperature. A first set point control button90may be disposed near the set point display88, and a second set point control button92may also be disposed near the set point display88. The first and second set point control buttons90and92, allow the operator to increase or decrease the set point temperature displayed on the set point display88. For example, as shown inFIGS. 2-4, the first set point control button90can increase the set point temperature, while the second point control button92can decrease the set point temperature. If the temperature that is displayed on the actual temperature display86is not equal or near the set point temperature that is displayed on the set point display88, the controller76will either increase or decrease the power to the heating element72to increase or decrease, respectively, the temperature sensed by the temperature sensor74. Accordingly, the controller76adjusts the temperature near the temperature sensor74to match or nearly match the set point temperature.

As shown inFIG. 2, the heating element72is attached to the outside of the target board38between the specimen46and the target board38. Because the temperature sensor74is disposed between the plates78and is near or in contact with the heating element72, the temperature that is sensed by the temperature sensor74may not reflect the actual temperature of the specimen46. In particular, the exposure surface41of the specimen46may be at a highly different temperature than the temperature that is sensed by the temperature sensor74. Accordingly, although the controller76controls the actual temperature sensed near the sensor74, the sensed temperature may not reflect the temperature that is experienced by the specimen46.

Referring toFIG. 5, a schematic cross sectional diagram of one embodiment of a specimen heating system170constructed in accordance with the teachings of the present disclosure is shown. The specimen heating system ofFIG. 5is similar to the specimen heating system70in all respects, except that, in the specimen heating system170, the temperature sensor74is embedded in the test specimen46. Accordingly, the temperature sensor74can sense the temperature inside the test specimen46and the regions of the test specimen46that surround the temperature sensor74. The noted regions may also include the internal side of the test specimen46that is attached to the heating platen72and the exposure surface41of the test specimen46. Therefore, the actual temperature display86may represent a temperature that more accurately reflects the actual temperature of the test specimen46. The temperature sensor74may be configured as a thermistor, thermocouple, resistance temperature device, integrated circuit temperature device, semiconductor temperature device, a non-contact optical temperature sensing device, infrared sensor, radiation thermometer, pyrometric sensor, bimetallic sensor, filled system thermometer, liquid or gas, thermal imaging system, or any other suitable temperature detection device.

If the temperature of the test specimen46is uniform throughout the test specimen46, the temperature sensed by the temperature sensor74reflects the temperature of the entire test specimen46. However, if the temperatures throughout the test specimen46vary locally, the temperature sensed by the temperature sensor74will be a local temperature, which may be the temperature of the test specimen near or surrounding the temperature sensor74. However, since the heating element72and plates78may cover the entire width of the test specimen46, the temperature variations inside the test specimen46may be small. Therefore, the temperature sensed by the temperature sensor74may generally reflect the temperature of the test specimen46. To provide for local temperature sensing at various points inside or on the surface of the specimen46, the temperature sensor74can be placed anywhere on or in this test specimen46.

Referring toFIG. 6, a schematic diagram of another embodiment of the specimen heating system171of the present disclosure is shown. The specimen heating system ofFIG. 6is similar to the specimen heating system70in all respects, except that, in the specimen heating system171, the temperature sensor74is disposed near or on the exposure surface41of the test specimen46. Accordingly, the temperature experienced by the temperature sensor74may likely reflect the temperature of the test specimen46at the exposure surface41or inside the test specimen46near the exposure surface41. Accordingly, when the controller76is controlling the temperature that is experienced by the temperature sensor74, the controller76may be actually controlling the temperature of the test specimens46at or near the exposure surface41.

Referring toFIG. 7, a schematic cross sectional diagram of another embodiment of specimen heating system172constructed in accordance with the teachings of the present disclosure is shown. The specimen heating system ofFIG. 7is similar to the specimen heating systems170and171in all respects, except that, in the specimen heating system172, the test specimen46does not contact the heating element72or platen78. The specimen heating system172includes a pair of standoffs96, to which a specimen frame98is attached. Accordingly, the standoffs96provide an air gap99between the specimen frame98and the heating element72or platen78. The specimen frame98is configured to hold the test specimen46. The air gap99between the test specimen46and the heating element72or platen78allows air to flow through the gap99and cool the specimen46. When heating of the specimen46is necessary, the heating element72or platen78can generate heat, which may be transferred to the test specimen46through the air gap99. Therefore, the test specimen46can be cooled when necessary through the air gap99, and can be heated when necessary by the heating element72or platen78.

In certain testing situations, it may not be necessary to heat the test specimen46, because the solar radiation may be sufficient during the day to keep the test specimen46at the set point temperature. Furthermore, during testing periods when the solar radiation is intense, the test specimen46may actually have to be cooled so that the temperature of the test specimen46remains at or near the set point temperature. Therefore, during certain testing situations, it may be necessary to cool the test specimen during the day and heat the test specimen at night. The specimen heating system172provides cooling of the test specimen46by providing the air gap99. When the heating element72is turned off, air flowing through the air gap99can cool the test specimen46.

InFIGS. 5-7, the heating element72may be disposed between the target board38and the test specimen46. Furthermore, the heating element72ofFIGS. 5 and 6directly contacts the test specimen46. However, the test specimen46may be heated by a heating element that is not near or in contact with the test specimen46. Referring toFIG. 8, a schematic diagram of another embodiment of a specimen heating system173constructed in accordance with the teachings of the present disclosure is shown. The specimen heating system173ofFIG. 8includes a heating element100that may be disposed between the mirror bed frame32and the specimen46and does not contact the specimen46. The heating element100directs heat toward the test specimen46. The heating element100may also include a reflector102that reflects the heat from the heating element100up toward the test specimen46. The controller76controls the power delivered to the heating element100by the heating element power cable104. Accordingly, the controller76can keep the temperature that is experienced by the temperature sensor74of the test specimen46(not shown) at or near the set point temperature.

The temperature sensor74that is used to sense the temperature of the test specimen46may also be a non-contact temperature sensor or an external temperature sensor. Referring toFIG. 9, a schematic cross sectional diagram of another embodiment of the specimen heating system174constructed in accordance with the teachings of the present disclosure is shown. The specimen heating system174includes a non-contact temperature sensor110that is disposed outside the test specimen46. The temperature sensor110may be any one of the known optical temperature sensors that are typically used to sense the temperature of a distant object. For example, the non-contact temperature sensor110can be an optical pyrometer that views the exposure surface41of the test specimen46. However, any suitable non-contact temperature sensor may be used outside the test specimen46to sense the temperature of the test specimen46during testing.

The disclosed specimen heating systems170-174are not limited to the particular respective embodiments shown in the foregoing. For example, the temperature sensors74ofFIGS. 5 and 7may be replaced with a non-contact temperature sensor110shown inFIG. 9and implemented as illustrated inFIG. 9of the specimen heating system170. In another example, the heating element72ofFIGS. 5-7can be replaced with the heating element100and the reflector102ofFIG. 8. Accordingly, one of ordinary skill in the art will readily appreciate that any combination of the above-described embodiments for the specimen heating system170-174are possible to achieve a desired testing result.

Furthermore, while the particular preferred embodiments have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the teaching of the disclosure. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as limitation. The actual scope of the disclosure is intended to be defined in the following claims when viewed in their proper perspective based on the related art.