Patent Publication Number: US-7221289-B2

Title: System and method for operating a loop detector

Description:
FIELD OF THE INVENTION 
   The field of the invention generally relates to methods and devices for controlling loop detectors. More specifically, the invention relates to adjusting the characteristics of loop detectors over time. 
   BACKGROUND OF THE INVENTION 
   Loop detectors function by having a circuit detect the change in inductance of a wire loop when a vehicle enters the vicinity of the loop. For instance, in some previous systems, the loop inductance changes by approximately four percent when a vehicle enters the loop. This change of inductance may be detected by sampling the change of frequency of electrical signals used to oscillate the loop. 
   Loop detectors are frequently used at traffic lights to indicate that a vehicle is present at an intersection and to initiate the changing of the light so that the vehicle can proceed through the intersection. In another example, loop detectors are sometimes placed in front of a barrier, such as a gate or a garage door. When a vehicle enters the loop, the device indicates the detection of the vehicle to a moveable barrier operator system and an action may be taken. For instance, the door or gate may be opened or closed. 
   As mentioned, loop detectors may be used in conjunction with barrier movement operators. Barrier movement operators are automated systems which are used to move a barrier with respect to an opening. Examples of barriers to be moved include garage doors, gates, fire doors and rolling shutters. A number of barrier movement operators have been sold over the years most of which include a head unit containing a motor connected to a transmission. The transmission, which may include, for example, a belt drive, a chain drive, a screw drive, gear drive or extendible arm is then coupled to the barrier for opening and closing the barrier. 
   The physical and electrical characteristics of the loop detector circuit drift with temperature and other environmental conditions. Consequently, the detection threshold becomes unreliable and must be changed if accurate responses to loop inductances are to be made. Previous loop detector systems compensated for the component drift by using a running average filter in the loop detector. However, the running average filter approach has proven inadequate to compensate for long range changes in system operation. For instance, the running averages approach often causes the loop detector to ignore a vehicle when the vehicle was on the loop for a long period of time or when a number of vehicles traversed the loop sequentially. Consequently, the gate might never be opened to allow a waiting vehicle to pass through the gate. In other circumstances, the gate might close prematurely damaging one of the string of vehicles passing through the gate. 
   SUMMARY OF THE INVENTION 
   A system for compensating for thermal drift adjusts operating characteristics of a vehicle detector to take into account the drift. The approach ensures that the detection threshold does not become outdated and that the barrier operator functions properly. 
   In many of these embodiments, the temperature of components of an operator circuit affecting the frequency of operation of a loop detector is measured. The operator includes an oscillator and the oscillator has an associated detection threshold frequency. A change in the frequency of the oscillator is compared to a detection threshold frequency to determine when a vehicle is in the vicinity of the loop detector. 
   A relationship between the temperature of the components and adjustment actions or values (that are used to adjust characteristics of the loop detector circuit) may be stored in memory. In one example, adjustment actions are used to change characteristics of an oscillator in order to maintain the oscillator frequency at a fixed value. In another example, adjustment values are used to change the detection threshold frequency of the vehicle detector. 
   The measured temperature of the components is then applied to the relationship thereby identifying an adjustment value or action. The characteristics of the loop detector are altered by the adjustment value or action. Thereafter, with the characteristics altered, the change in frequency of the oscillator is compared to detection threshold frequency to determine if a vehicle is truly present at the loop. 
   In other of these embodiments, the temperature of components of a loop detector is measured. A relationship between the temperature and a detection threshold is determined. The relationship is applied to the measured temperature to create an adjustment value. The detection threshold is adjusted by the adjustment value. A weighted average related to the threshold may also be applied to the threshold. In this case, the detection threshold is adjusted using the weighted average and the adjustment value. 
   Thus, approaches are provided to maintain an updated detection threshold in a system using a loop detector to actuate a moveable barrier. The approaches described herein avoid the problems associated with previous systems such as missed detections of vehicles in the loop and premature barrier closings of vehicles entering through the barrier. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a system for compensating for temperature drift when detecting a vehicle according to the present invention; 
       FIG. 2  is a block diagram of a system for compensating for temperature drift when detection a vehicle according to the present invention; 
       FIG. 3  is a flowchart of an approach for adjusting the detection threshold of a loop detector according to the present invention; and 
       FIG. 4  is a block diagram of a system for compensating for temperature drift in a vehicle detector according to the present invention. 
   

   Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of the various embodiments of the present invention. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings and especially  FIG. 1 , a system using a loop detector to open and close a moveable barrier is described. A wire loop  106  is placed in the ground in front of gates  102  and  104 . The loop  106  is coupled to an operator  110  via a cable  108 . The cable  108  provides a path for electrical signals representative of the inductance of the loop  106 . 
   For illustrative purposes, the description with respect to  FIG. 1  refers to moveable barriers that are gates. However, it will be understood by those skilled in the art that the moveable barrier may not only be a gate, but may be any type of barrier such as a fire door, shutter, window, garage door. Other examples of barriers are possible. 
   The operator  110  may provide circuitry for driving the loop detector  106  with an oscillator  111 . For convenience in viewing, the operator  110  is shown placed above a wall. However, it will be understood that the operator  110  may be positioned in any convenient and/or secure place, for example, behind the wall, in a building, or in the ground. The oscillator  111  may drive the loop  106  with an electrical signal having a frequency. When a vehicle  122  enters the loop  106 , the frequency of the loop  106  changes. This change in frequency may be compared to a threshold stored at the operator  110  to determine if the vehicle  122  is in the loop  106 . Alternatively, the operator  110  may supply a signal having a center frequency. Deviations from the center frequency are measured and if the deviation exceeds a threshold, then the vehicle  122  is determined to have entered the loop  106 . Other detection methods may also be used. 
   The operator  110  is coupled to the gates  102  and  104  via a wire  116 . The operator  110  determines when to open or close the gates  102  and  104 . In addition, the operator  110  may include additional functionality to receive signals from a transmitter  120 . The transmitter  120  may be carried in the vehicle  122 . The transmitter  120  may be activated by a user in the vehicle  122 . When the user activates the transmitter  120  and it is determined that the vehicle has entered the loop  106 , then the operator  110  may open the gates  102  and  104  allowing the vehicle  122  to enter. Conversely, the operator  110  may automatically close the gates  102  and  104  when it is determined that the vehicle  122  has passed through the loop  106  and it is safe to close the gates  102  and  104 . 
   The operator  110  includes a temperature sensor  112 . The sensor  112  determines the temperature of components that affect the frequency characteristics of the loop  106 . For example, the temperature of the oscillator  111  driving the loop  106  may be sensed. In another example, the air temperature within the operator  110  may be determined. A single or multiple sensors may be used to read the temperature. Temperatures of other components may also be determined. In addition, other parameters that affect the frequency (or other operating) characteristics of the loop  106  may also be measured. 
   The operator  110  also includes a memory. The memory stores the threshold, and a relationship between the temperature detected and control parameters used to adjust the loop detection circuit. For instance, the memory may store a table that indicates the adjustments to be made to the detection threshold frequency for a plurality of different temperatures. In another example, the memory may store an equation, which can be solved for a threshold value when the sensed temperature is used as the value of a variable. In still another example, the table may store oscillator adjustment parameters for different temperatures that ensure that the oscillator is driven at a constant frequency, for example, 10 Khz. Other examples of relationships between temperature and control parameters are possible. 
   The correlation between the temperature and the adjustment actions and values may be made at the time of manufacture of the operator  110 . For instance, tests can be performed to determine the adjustments for threshold for given temperature ranges and this can be programmed into memory at the time the operator  110  is manufactured. 
   In one example of the operation of the system of  FIG. 1 , the temperature of the operator  110  (including the components that affect the frequency of the loop  106 ) is measured. The temperature may be measured by any type of temperature sensing device placed within the housing of the operator  110 . 
   A relationship between the temperature of the operator  110  and parameters used to adjust the loop detection circuit is stored in memory. The relationship is applied to the measured temperature of the operator  110  to create an adjustment value or action. The characteristics of the circuit, for instance, the detection threshold frequency or the oscillator characteristics, are adjusted according to the adjustment value or action found in the table. 
   If a threshold is adjusted, a weighted average related to the threshold may also be used. In this case, the detection threshold is adjusted using the weighted average and the adjustment value. 
   Referring now to  FIG. 2 , a movable barrier operator, which is a garage door operator, is generally shown therein and includes a head unit  212  mounted within a garage  214 . The head unit  212  is mounted to the ceiling of the garage  214  and includes a rail  218  extending there from with a releasable trolley  220  attached having an arm  222  extending to a multiple paneled garage door  224  positioned for movement along a pair of door rails  226  and  228 . 
   The description with respect to  FIG. 2  refers to a moveable barrier that is a garage door. However, it will be understood by those skilled in the art that the moveable barrier may not only be a garage door but may be any type of barrier such as a fire door, shutter, window, or gate. Other examples of barriers are possible. 
   More specifically, the system includes a hand-held transmitter unit  230  adapted to send signals to an antenna  232  positioned on the head unit  212  as will appear hereinafter. An external control pad  234  is positioned on the outside of the garage having a plurality of buttons thereon and communicates via radio frequency transmission with the antenna  232  of the head unit  212 . An optical emitter  242  is connected via a power and signal line  244  to the head unit. An optical detector  246  is connected via a wire  248  to the head unit  212 . The head unit  212  also includes a receiver unit  202 . The receiver unit  202  receives a wireless signal, which is used to actuate the garage door opener. 
   An oscillator  211  within the head unit  212  is connected to a loop  210  and energizes the loop  210  with an electrical signal driven at a frequency. A threshold frequency is stored in a memory at the head unit  212 . If the frequency detected moves above the threshold frequency a vehicle is indicated to be in the vicinity of the loop  210 . 
   Temperature sensors (not shown) are present in the head unit  202  to measure the temperature of components that affect the frequency of the loop  210 , for instance, the oscillator  211 . The temperature is applied to a relationship stored at the memory of the head unit  202  to obtain an adjustment value or action. The adjustment value or action is applied to adjust characteristics of the loop detector circuit, for instance, the detection threshold or oscillator characteristics, in order to adjust for temperature variations in the components that drive and/or sense the loop  210 . 
   Referring now to  FIG. 3 , one example of an approach for adjusting the detection threshold using temperature measurements in a moveable barrier system is described. At step  302 , the system determines a relationship between a measured temperature of components that affect a characteristic of operation of a loop in a loop detector system and parameter values or actions. 
   The detection threshold represents a value that is detected above which a vehicle is believed to be in the vicinity of the loop of the loop detector. The detection threshold may be a frequency value, for instance. In another example, the detection threshold may be a current value. 
   In one example, the relationship defines the correspondence between the measured temperature and the threshold. Specifically, there may be a linear relationship between the measured temperature and the threshold. In another example, an equation may be used to represent the relationship. In still another example, the relationship may be represented in a lookup table. 
   In yet another example, the relationship defines the correspondence between temperature and parameters that keep the frequency of the oscillator at a fixed value. For instance, for temperatures from 100 to 120 degrees Fahrenheit, the gain of the oscillator may be adjusted to a first value to fix the frequency at 10 Khz. For temperatures from 120 degrees to 140 degrees, the gain of the oscillator may be adjusted to a second value to force the frequency to be 10 Khz. 
   At step  304 , the temperature of the components is measured. In addition, the temperature of anything else, for instance, the ground, that affects the characteristics of operation of the loop are measured. The temperature may be measured by any type of temperature-sensing equipment. 
   At step  306 , the measured temperature is applied to the relationship to obtain an adjustment value or action. For example, the system may perform a table look-up if the relationship is stored in a table in memory. The result of the table look-up is the identification of the adjustment value or action. In another example, if an equation is used, then the measured temperature may be applied to the equation to obtain the adjustment value. 
   At step  308 , the adjustment value or action is applied to the characteristics of the loop detector circuit. For example, if the table gives a threshold value from step  306 , this value may be added or subtracted from the threshold value. Alternatively, if the table identifies an action that adjusts the characteristics of the oscillator to maintain a constant frequency, that action is performed at step  308 . 
   At step  310 , after the characteristics of the loop detector have been adjusted, it is determined if the frequency measured at the oscillator is less than the threshold. If the answer is affirmative, then control returns to step  304  and execution continues as described above. If the answer at step  310  is negative, then at step  312  an action is taken. The action may include sending a signal to a moveable barrier operator to actuate the moveable barrier. Step  312  may take into account other information in deciding the nature of the action to take. For instance, the operator may require a code or other type of identification parameter to be received before the signal is sent to actuate the moveable barrier. 
   Referring now to  FIG. 4 , a vehicle detector  400  includes an electronics unit  402 , conductive loop  404 , and frequency detector  406 . The conductive loop  404  is a wire that is driven by the electronics unit  402 . The electronics unit  402  includes circuitry such as an oscillator  401  and capacitors to drive the conductive loop  404  at a certain frequency. The frequency detector  406  detects the frequency of operation of the loop  404 . 
   A controller  410  is coupled to the electronics  410  and to a barrier operator. A threshold detector determines when the detected frequency has passed a threshold indicating a vehicle has entered the loop. 
   Characteristics of the vehicle detector  400  are adjusted based upon a relationship that may be stored in memory. In one example, the oscillator  401  is always desired to be operated at a certain frequency, for instance, 10 Khz. A threshold, for example, 4 percent is selected for use in the threshold detector  408 . A temperature sensor  412  measures the temperature of the electronics, the air, or other components of the vehicle detector  400 . Based on the temperature, components in the electronics unit  402  are changed to ensure the oscillator  401  is driven at the pre-set frequency. The threshold used by the threshold detector  408  is not changed. When the controller  410  determines the threshold is exceeded, a vehicle is believed to be in the loop  404  and the operator can be actuated by the controller  410 . 
   In another example, the threshold of the threshold detector  408  is adjusted based upon temperature. In this case, the oscillator  401  may drift with temperature, but the threshold detection frequency is adjusted for compensation. When the controller  410  determines the new threshold detection frequency is exceeded, a vehicle is believed to be in the loop  400  and the operator can be actuated by the controller  410 . 
   Thus, a system and method allow the detection threshold of a loop detector system to be adjusted due to thermal drift and other environmental conditions. The approaches described herein avoid missed detection of vehicles at the loop as well as premature closing of barriers that can damage vehicles or other objects in the loop. 
   While there has been illustrated and described particular embodiments of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true scope of the present invention.