Temperature detecting device for image fixing apparatus of electrostatic recording apparatus

A temperature detecting device for image fixing means is used in an electrostatic recording apparatus, in which a temperature sensor element is supported on a mounting member through a slender conductive element at a small distance from the surface of and in the direction of length of the fixing means to be detected, so as to detect the temperature thereof with accuracy.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a temperature detecting apparatus or 
device for image fixing means to be used in an electrostatic recording 
apparatus. 
For example, in the electrophotographic copying machine, a toner image 
developed from an electrostatic latent image produced on a photo-sensitive 
layer is transferred to a recording or transfer sheet and subsequently 
fixed to the sheet by means of heating means such as a pair of rollers 
containing at least one heating roller. 
In the case of the standard recording or transfer sheet, the fixing 
temperature is usually in the range of 150.degree. C. to 170.degree. C. 
The range of the fixing temperature can be determined by conditions, for 
example, such as a recording speed or a thickness of the paper to be used 
and so forth. 
Unless the fixing temperature is maintained at a constant value, an 
unsatisfactory fixation and scorching of the recording sheet may occur. 
With a view to evading such disadvantages, it is common in practice that 
the temperature of the heating roller is detected and utilized for 
temperature adjustment to maintain the preset level always. 
2. Description of the Prior Art: 
In most of the hitherto known temperature control systems of the above 
type, a thermocouple is used as the temperature sensor for the fixing 
means, for example a pair of rollers as described before. 
To this end, the thermocouple is disposed in physical contact with the 
surface of the heating roller, as the result of which injuries are often 
produced to the heating roller as well as the mounting elements for the 
thermocouple. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is to provide a temperature 
detecting device for image fixing means to be used in an electrostatic 
recording apparatus including an electrophotographic copying machine which 
removes the disadvantages of the hitherto known temperature detecting 
device. 
Another object of the invention is to enhance durability and reliability of 
the temperature sensor used in the temperature detecting device. 
In view of the above and other objects which will become apparent as 
description proceeds. A temperature detecting device according to the 
invention comprises a temperature sensor element located closely to the 
surface of the fixing means, slender conductive elements connected to both 
ends of the sensor, respectively, and a mounting member for maintaining 
the sensor in a tensioned state through the conductive elements. 
The novel features and advantages of the invention will become more 
apparent from detailed description of preferred embodiment of the 
invention taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the temperature detecting device according to the 
invention denoted generally by reference numeral 1 is disposed adjacent to 
a fixing means, for example a pair of heating rollers 2 (shown only 
schematically). The sensor is constituted by a thermistor 5 tightened or 
suspended between legs 3a and 3b of a mounting member 3 by a pair of 
slender conductive elements 4 in a form of coil springs connected to both 
ends of the sensor, respectively. Further the sensor 5 is closely located 
adjacent to the surface of the fixing means 2, the gap being held at about 
1 mm in the embodiment as described hereinafter. The sensor 5 is mounted 
in the direction of the length of the fixing means 2. 
Naturally, the relation of the sensor and fixing means can be changed 
freely but above described relation is a preferable relationship. 
The conductive elements 4 are preferably made of a heat-sensitive material, 
more particularly a heat sensitive and electrical conductive material. 
The mounting member 3 is formed of a thin metal sheet material and has 
slots 3c through which the temperature detecting device may be secured to 
a stationary portion of the recording apparatus in any suitable manner. 
The end portions of the legs 3a and 3b are coverd with a heat-resistive 
insulation material 6 such as Teflon tape or the like. 
Reference numeral 7 denotes securing means for lead wires 8 of the sensor 
5, and is also made of a heat-resistive insulation material such as mica 
plate and secured to the legs 3a and 3b by means of screws or rivets. The 
lead wires 8 are connected to the respective coil springs 4 which serve 
also as electric conductors and held fixedly by the securing discs 7 in 
cooperation with notches formed in the end portions of the legs 3a and 3b, 
respectively, as can be seen from FIG. 1. 
The arrangement of the temperature detecting device according to the 
invention in which the sensor or thermistor 5 is held in a resiliently 
tensioned state by a pair of the electric conductors 4 implemented in form 
of the coil or compression springs positively prevents contact between the 
heating roller 2 and the sensor or thermistor 5 even if the conductor 4 
undergoes thermal linear expansion under the heating of the roller 2. 
Further. by virtue of the coating or the covering of the leg end portions 
with the heat-resistive insulation material, heat transfer to the 
supporting or mounting plate 3 can be significantly reduced. Formation of 
a short-circuit between the electric conductors (4 and 8) and the mounting 
plate 3 can be excluded by the function of the heat-resistive insulation 
material 6 and an insulating sleeve 9 of the lead conductor 8. It is 
preferred that the lead conductor 8 and the spring conductor 4 be secured 
to the securing disc means 7 at the junction between these conductors. 
The invention is not restricted to the arrangement described above, but 
many modifications and variations will readily occur to those skilled in 
the art without departing from the scope and spirit of the invention as 
defined in the claims.