Hair styling iron

Abstract

A hair styling appliance has a temperature regulation controller to mitigate heat damage to the hair. The appliance includes a pair of heated jaws that can be closed to grip the hair. A power control circuit responds to an activation sensor that senses when the jaws are closed, and a temperature sensor that senses the temperature of a heating element on the jaws. The power control circuit, in response to an activation sensor output, starts monitoring the rate of change of the heating element temperature, and, in response to a rate of decrease above a predetermined level, operates the heating element at a power level lower than a steady state power level.

1 . A hair styling appliance comprising:
 at least one heating element having a heating surface for contact with hair;   a pair of jaws, wherein
 the heating element is disposed on a first of the jaws, and 
 the jaws are connected for movement between an open position for inserting hair between the jaws and removing hair from between the jaws, and a closed position in which a second of the jaws presses the hair located between the jaws against the heating surface; 
   a temperature sensor that senses temperature of the heating element proximate the heating surface;   an activation sensor that senses when the jaws are in the closed position and generates an first activation output when the jaws are in the closed position; and   a power control circuit that responds to the activation sensor and the temperature sensor, and operates the heating element at a steady state power level to maintain the temperature in a steady state, wherein
 the power control circuit includes means for determining a time rate of decrease of the temperature of the heating element, and means for monitoring the temperature and the time rate of decrease of the temperature to detect an abnormal thermal load on the heating element, and 
 the power control circuit, in response to the first activation output, starts detection of an abnormal thermal load, and, in response to detection of an abnormal thermal load, operates the heating element at a power level lower than the steady state power level. 
   
     
     
         2 . The hair styling appliance of  claim 1 , wherein the power level lower than the steady state power level is a zero power level. 
     
     
         3 . The hair styling appliance of  claim 1 , wherein
 the means for monitoring the temperature and the time rate of decrease of the temperature to detect an abnormal thermal load comprises means for comparing the time rate of decrease of the temperature that is measured to a predetermined rate of decrease representing an abnormal thermal load, and   the monitoring means detects an abnormal thermal load when the time rate of decrease that is measured is lower than the predetermined rate of decrease.   
     
     
         4 . The hair styling appliance of  claim 1 , wherein
 the means for monitoring the temperature and the time rate of decrease of the temperature to detect an abnormal thermal load comprises
 means for comparing the temperature that is sensed to a first calculated temperature, a second calculated temperature that is lower than the first calculated temperature, and a third calculated temperature that is lower than the second calculated temperature, and 
 means for comparing the rate of decrease of temperature that is measured to a first predetermined rate of decrease, a second predetermined rate of decrease that is less than the first rate of decrease and a third predetermined rate of decrease that is less than the second rate of decrease, and 
   the monitoring means detects an abnormal thermal load when the temperature that is sensed is greater than the first calculated temperature and the rate of decrease that is measured is less than the first predetermined rate of decrease, or when the temperature that is sensed is less than the second calculated temperature and the rate of decrease that is measured is less than the second predetermined rate of decrease or when the temperature that is sensed is less than the third calculated temperature and the rate of decrease that is measured is less than the third predetermined rate.   
     
     
         5 . The hair styling appliance of  claim 4 , wherein the power control circuit reduces the power level applied to the heating element by the same amount
 When the temperature that is sensed is greater than the first calculated temperature and the rate of decrease that is measured is less than the first predetermined rate of decrease,   When the temperature that is sensed exceeds the second calculated temperature and the rate of decrease that is measured is less than the second predetermined rate of decrease, or   when the temperature that is sensed exceeds the third calculated temperature and the rate of decrease that is measured is less than the third predetermined rate.   
     
     
         6 . The hair styling appliance of  claim 5 , wherein
 the means for measuring the temperature rate of decrease comprises timing means for establishing a time interval between successive measurements of rates of decrease; and   the timing means establishes first, second and third time intervals, measured from the time of a first activation output, and compares the rate rates of decreased that are measured and occur at ending of the first, second, and third time intervals to the first, second, and third predetermined rates of decrease, respectively.   
     
     
         7 . The hair styling appliance of  claim 5 , wherein
 the power control circuit stores a reference temperature corresponding to the steady state power, and   the first, second, and third calculated temperatures are calculated by subtracting predefined first, second, and third reference values from the reference temperature to define the first, second, and third predetermined rates of decrease.   
     
     
         8 . The hair styling appliance of  claim 1 , wherein
 the control circuit operates the heating element in a thermostatic mode at first power levels, that vary as a function of a temperature offset to maintain a setpoint temperature, and the temperature offset comprises difference between instantaneous temperature of the heating element and the setpoint temperature, and   the control circuit comprises means for comparing the rate of decrease to a reference rate of decrease, and, when the rate of decrease that is measured is greater than the reference rate of decrease, the control circuit operates the heating element in a boost mode at power levels higher that the first power levels, at all temperature offsets, until a rate of decrease of heating element temperature that is measured is less than the reference rate, or a heating element temperature is at a setpoint temperature corresponding to the steady state power level.   
     
     
         9 . The hair styling appliance of  claim 8 , wherein
 the activation sensor generates a second activation output when the jaws are in the open position and   the power control circuit, in the presence of the second activation output, and following operation of the heating element at the power level lower than the steady state power level, operates the heating element in the boost mode until the power level again equals the steady state power level or until detection of an abnormal thermal load recurs.   
     
     
         10 . A method of controlling power in a hair styling appliances having a heating element and a temperature sensor that senses temperature of the heating element proximate a heating element surface, a pair of jaws, wherein the heating element is disposed on a first of the jaws, the jaws are connected for movement between an open position for inserting hair between and removing hair from between the jaws, and a closed position in which a second of the jaws presses the hair against the heating element, and an activation sensor that senses when the jaws are in the closed position and generates an first activation output when the jaws are in the closed position, the power control method comprising:
 operating the heating element at a steady state power level;   when the first activation output is generated, periodically measuring the temperature of the heating element, computing a time rate of decrease of the temperature, and comparing the time rate of decrease that is computed to a predetermined rate, and   when the time rate of decrease of the temperature that is measured is less than the predetermined rate, reducing the power level to the heating element to protect against hair damage by overheating.