Patent Application
20070051174
1. Statement of the Technical Field
The inventive arrangements relate to structures, and more particularly to methods and systems for determining the temperature of structural elements and the resulting changes to structures from temperature variations.
2. Description of the Related Art
Temperature variations in the environment are known to effect dimensional characteristics of deployed structures. While these dimensional variations can be relatively unimportant in some instances, they can have a significant effect on the performance of certain types of precision structures. This is especially true for space based deployable structures.
Space based deployable structures are especially vulnerable to dimensional variations associated with temperature changes. One reason is that such structures are often exposed to solar heating and other effects that change the temperature of the structural elements tremendously. The mechanical effects of such heating are often difficult to predict with a high degree of precision because different portions of the space deployed structure can be exposed to varying degrees of solar heating. The result is that different portion of a space structure can have very different temperatures. Another reason for this vulnerability is the relative inaccessibility of these structures. In general, it is difficult and expensive to make mechanical adjustments to space deployable structures after they have been launched into space.
Space deployed antennas can be particularly vulnerable to dimensional variations resulting from environmental temperature changes. In order to ensure peak performance, such antennas must be sized and shaped with a high degree of precision. Many types of space deployable antennas are assembled using pre-tensioned graphite cords. These long, thin cords are subject to wide variations in temperature, resulting in length variations. These length variations can distort the antenna shape, thereby degrading RF performance.
It is conceivable that compensation systems could be incorporated into deployed structures to compensate for temperature based dimensional variations of structural elements. For example, in the case of space deployed antennas, RF performance could potentially be enhanced. However, in order for such systems to operate effectively, it would be desirable to have accurate information relating to the temperature of the structural element. The temperature information for each structural component can be very useful for estimating the dimensional variation affecting that structural element.
The accepted method for determining structural component temperature usually involves the use of thermistor based sensors, a traditional sensor interface, and A/D converters. Since wide variations in temperature can occur between different portions of a single structural element, thermistor sensors are usually located at several different locations on each structural component.
Still, there are a number of difficulties associated with the use of thermistors, especially when they are used on tiny graphite cords. For example, distorted temperature readings can result from heating of the thermistor body (as compared to the temperature of the cord). Power dissipation will also occur in the thermistor, causing heating effects. Different areas of the cord are also generally at very different temperatures. The solution for achieving accurate measurement potentially requires many more thermistors than practically possible. Lastly, the use of many thermistors creates a significant potential for snagging during the deployment process as cords are extended and moved into their operating position.
The invention concerns a method for identifying a temperature induced dimensional variation in a remotely deployed structure. The method can include measuring an electrical resistance of a structural element of the deployed structure between two locations spaced apart from each other. Thereafter, the method can include predicting a dimensional characteristic of the structural element based on the measuring step. The dimensional characteristic can be a physical dimension of the structural component, such as a length or a width. Alternatively, the dimensional characteristic can be a relative change in a physical dimension of the structural component. In either case, the method can also include the step of controlling at least one variable portion of the structure in order to compensate for a temperature induced variation of the dimension characteristic.
The structural element can be selected to include any portion of a structure for which a dimensional characteristic is to be monitored or measured. For example, the structural element can be a cord. The material from which the cord is formed can be any material that exhibits useful variations in resistance as a function of temperature. For example, the method can be used with graphite cords that are commonly used in remotely deployed space structures. The method can further include selecting the structure to be an antenna structure.
According to another aspect, the invention can consist of a method for predicting temperature induced dimensional variations in structural cords in a deployable structure. For example, the structure could be an antenna and the cord could be formed of a material such as graphite. The method can begin by forming a structure that includes a plurality of cords. The electrical resistance of one or more cords in the structure can be measured to obtain information concerning their baseline resistance values at one or more known temperatures. Thereafter, the method can include predicting a dimension or a change in dimension of the cord based on the measuring step. The method can also include the step of deploying the structure to a remote environment. Thereafter, the electrical resistance of the cord can be monitored. The monitoring can allow prediction, in the remote environment, of a resulting dimension of the cord at various temperatures, or a temperature induced change of the cord dimension. Finally, the method can also include controlling at least one variable portion of the structure to compensate for the temperature induced variation of the dimension.
Viewed from a broader aspect, the method can include a process that is useful for measuring a dimensional characteristic of a structural component. In this regard, the invention can include forming an electrical connection with the structural component at two predetermined locations spaced apart from one another. Thereafter, the method can include measuring an electrical resistance of the structural component between the locations. Finally, a dimensional characteristic of the structural component can be determined based on an electrical resistance value obtained from the measuring step. The structural component can also be subjected to an environment which causes a temperature of the structural component to vary over a period of time. In that case, the value of the dimensional characteristic can be periodically determined as the temperature is varied.
The dimensional characteristic can be a physical dimension of the structural component, such as a length or a width. Alternatively, the dimensional characteristic can be a relative change in a physical dimension of the structural component. In either case, the method can also include referring to a look-up-table to cross-reference the electrical resistance value that has been measured to a predetermined dimensional characteristic of the structural component. Alternatively, or in addition to the look-up step, the determining step can include calculating the dimensional characteristic of the structural component based on a change in the electrical resistance value that has been measured.
The method can also include a calibration step. The calibration step can include measuring an electrical resistance and a dimensional characteristic of the structural component over a predetermined temperature range. Using the foregoing information, a look-up table can be generated. For example, the look-up table can relate an electrical resistance of the structural element to a dimensional characteristic of the structural component. The calibration step can occur at a pre-determined temperature or over a range of temperatures. Subsequently, the measured resistance values at various environmental temperatures can be used to predict a dimension of a structural element or a change in dimension.
According to another aspect, the invention can include a method for predicting an average temperature of a conductive structural component over an elongated length of the structural component. The method can include measuring an electrical resistance of the structural component between two locations spaced apart from each other. Finally, an average temperature of the structural component between the two locations can be predicted based on the measuring step. Using the information gained in this step, a dimensional characteristic of the structural component can be predicted based on the average temperature. For example, the dimensional characteristic can be selected from the group consisting of a length, a width, a change in length, and a change in width. According to one embodiment, the structural element can be a graphite cord. Further, the graphite cord can be included in a deployable structure prior to the measuring and predicting steps.
According to yet another aspect, the method can include identifying a temperature induced dimensional variation in a remotely deployed structure. In this instance, the method can include measuring an electrical resistance of two or more structural elements of the deployed structure between two locations spaced apart from each other on each structural element. Based on this measuring step, the method can continue by predicting a dimensional characteristic of each of the structural elements that have been measured. Using this information, the overall effect of the temperature variation on the structure can be determined. Finally, the method can include automatically compensating for the measured variations throughout the structure. The compensation process can include mechanical adjustments to the structure. Alternatively, the compensation process can involve electrically compensating for the change in the overall structure.
An example of a portion of a space-based deployable structure 100 is illustrated in
Solar heating and other effects in a space environment can have a dramatic effect on the temperature of the various structural elements 102, 104. In fact, the temperature of a structural element can even vary widely from one portion of the structural element to another. For example, this can occur when a portion of the structural element is exposed to sunlight and another portion is shaded from the sun.
Referring now to
The present invention provides a method for determining a temperature induced dimensional variation in a remotely deployed structure. In general, the method can include measuring an electrical resistance of a structural element 102, 104 of a deployed structure 200 between two locations 206, 208 on the structural element that are spaced apart from each other. For example, the two locations can be opposing ends of the structural element. Depending on the particular material of the structural element, the electrical resistance value between the two locations will change as a function of temperature. If resistance values corresponding to different temperatures are known in advance, then a temperature of the structural component can be predicted. If the temperature of the structural component can be determined in this way, then a dimensional characteristic of the element can be predicted by computational means or otherwise.
Specifically, the foregoing prediction can be accomplished by utilizing known data regarding the expansion and contraction characteristics of materials and/or specific structural components as a function of temperature. Thus, for a given change in temperature, a dimensional characteristic of the structural component can be determined. In this regard, it should be noted that the term dimensional characteristic as used herein can mean a physical dimension of the structural component, such as a length or a width. However, the term dimensional characteristic can also refer to a relative change in a physical dimension of the structural component.
In order to more fully understand the foregoing technique, it is useful to refer to the plot shown in
An advantage of the inventive arrangements is that measurement of cord resistance reports the true average temperature of the cord. In contrast, the prior art uses thermistors to report temperatures at discrete points on the cord. Testing has confirmed that graphite cord resistance varies as a result of temperature changes, and not due to changes in cord tension or other reasons. Also, the graphite cord resistance value does not affect the rate of change of resistance versus cord temperature. Further, it has been found that there is minimal hysteresis in the measured cord resistance as a function of temperature. Accordingly, the resistance at a given temperature tends to remain the same regardless of whether the cord is arriving at a given temperature after being heated or cooled.
Referring now to
For greater accuracy, the resistance between two points of a structural element can be measured at a plurality of temperatures to obtain a number of data points specific to that structural element. Thereafter, specific resistance measurements can be directly related to the temperature of the structural element. For example, in the example shown in
After the baseline data for the structural component or components has been collected in step 402, the structural element, can be deployed to a remote environment. For example, the structural element 204 can be incorporated into a space deployable structure 200 and launched into space. Thereafter, in step 404, a temperature variation can be induced into the structural element. The temperature variation can occur as a result of solar heating or from other factors present in the environment. In any case, a temperature change can occur in all or part of the structural element.
Thereafter, in step 406, the resistance between the two spaced apart locations 206, 208 on the structural element 204 can be measured in the deployed environment. Based on the resistance value measured in step 406, a temperature of the structural element 204 can be determined in step 408. The temperature can be calculated based on the measured resistance value from step 406, the known baseline resistance value at a predetermined temperature from step 402, and the typical resistance change per degree C. for the element 204. Alternatively, a look-up-table can be used to relate specific measured resistance values to corresponding temperatures as previously measured for structural element 204 under baseline test conditions in step 402. Regardless of the technique used to determine the temperature of the structural element 204, the temperature information can thereafter be used in step 410 to determine a dimensional characteristic of the structural element corresponding to a particular temperature or change in temperature relative to a baseline value.
As an alternative to first determining a temperature of the structural component, those skilled in the art will appreciate that a look-up-table can be provided which directly relates a resistance value to a dimensional characteristic of the structural element. Thus, the temperature determining step can be avoided if the dimensional characteristic data corresponding to specific temperatures is pre-calculated (e.g. prior to deployment) and has been already related to specific electrical resistance measurements in a look-up table. It should be understood that the invention is not intended to be limited to any particular method for determining dimensional characteristics of the structural components from the measured resistance data. Instead, all such methods are intended to be within the scope of the present invention.
Regardless of how the dimensional characteristic is determined in step 410, the method can include a further step of controlling at least one variable portion of the structure 200 in order to compensate for a temperature induced variation of the dimension characteristic. For example, if the structural component is a cord, then an adjusting device can be provided at one or both ends of the structural component. In
The foregoing step involves an electromechanical arrangement for physically controlling a variable portion of the structure. When the cord changes length, an adjustment mechanism 210 directly compensates to correct for that change. However, in some instances, the changes in dimensional characteristics of the structure can have effects that are of concern primarily because they alter the electrical or RF properties of the structure. This would be the case, for example, where the structure is a deployed antenna. In such instances, an alternative approach to correcting for the physical change could be a signal processing change. For example, the information relating to the change in physical dimension could be provided to a signal processing computer. The signal processing computer could implement a phase compensation algorithm to correct for the physical distortion in the antenna. Such an arrangement would be particularly useful in a phased array antenna or phased array fed reflector or lens antenna. With this approach, the mechanical deformation is not necessarily “corrected”. Instead, the physical deformation is only determined, measured, and compensated for electrically without making any actual physical geometry changes in the structure.
Referring to
As noted above, different portions of a structural element can be at very different temperatures, particularly in a space environment. In this regard, it should be noted that the temperature determined using the techniques and methods described herein will generally be an average temperature of the structural element between the two points at which resistance is measured. This averaging effect can be highly advantageous as it is more likely to permit a more accurate calculation of a temperature induced variation in a dimensional characteristic of the structural element as compared to discrete thermistor measurement techniques.
While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as described in the claims.
