Temperature Sensing and Control of a Fusing Roll

The invention relates to improved roller fusing apparatus for electrographic devices and more specifically to improved apparatus for controlling the temperature of the fusing surface of such apparatus.

It is well known in the art of electrographic roller fusing that the exterior surface temperature of fusing rollers must be maintained at a temperature which is high enough that proper fusing will result and offsetting will be eliminated, but not so high as to char the support material passing through the rollers, injure the roll coating, add excessive curl to the support material or cause the support material to stick to the rolls. Proper temperature control for the fusing surface has therefore presented a problem to all roller fusing devices.

The various prior art devices have attempted to control the temperature of the roller fusing surface by using thermocouples, thermistors and the like to sense directly the temperature of the exterior surface itself. In response to sensing of a temperature outside the desired fusing temperature range, the fuser heating source is appropriately controlled to return the fusing surface to the proper temperature.

Several problems have resulted from use of such surface sensing apparatus, including for example, damage to the fusing surface by the sensor, and destruction of the temperature sensor itself since the sensor is usually placed in close proximity to the path of the paper. Economically, sensors that detect surface temperature without damaging the fusing surface and yet are accurate and dependable are in many cases more expensive to manufacture and mount, than the type of sensor which could be used to sense the core of the roll.

One particularly desirable type of fusing roller employs a fusing surface of offset preventing material such as Teflon or silicone rubber. However, these offset preventing, i.e., "release", materials have a very poor heat conductivity and because of such, substantial differences in temperature between the parts of the fusing surface that do and do not contact the support medium result. Therefore it is usually necessary to sense an area of the roll surface that will contact the surface of the support medium; and this necessity amplifies the problems mentioned above and increases the possibility of paper jams and the difficulty of clearing such jams.

Some prior art devices have attempted to avoid or minimize the above-mentioned problems by sensing a thin layer of air very close to, but not touching, the exterior surface of the fusing roll. However, such devices become even more complicated and expensive.

In addition to the foregoing problems involved with surface sensors, there exists an inherent problem with respect to the overall accuracy of the control by such devices. Specifically, when the fusing surface sensed is not a good heat conductor and the sensor detects the temperature of only a discrete area of that surface, the surface sensing control devices can be "fooled" if localized hot or cold spots develop in the discrete area being sensed.

It is of course essential for proper fusing that the roll's surface be in the proper temperature range from the time the first copy sheet is contacted until the last copy sheet makes contact with the roll's surface. Therefore an additional problem has existed with respect to the heated fusing rollers in assuring that, during the period between a low (or high) temperature sensing and the time when response by the heating source has effected a return to the proper temperature, the fusing surface temperature does not move outside, i.e., undershoot (or overshoot) the proper range for acceptable fusing.

Some prior art devices have attempted to alleviate the problem of undershoot by anticipating an increase in heat loss by the fusing surface and pre-compensating or compensating for the anticipated or actual heat loss respectively. One prior art technique for compensating for an actual increase in heat loss utilizes a very precise sensing of the slight temperature drop occurring the moment which the support surface makes contact with the roll surface. This technique has utility when the heat is applied directly to the fusing surface, from the exterior of the roller; however, the sensor must be quite precise. Another technique provides an earlier anticipation of heat loss by detection of a copy sheet moving toward a radiant heat type of fusing oven. This technique provides for advance energization of the fusing oven during the time in which it takes the paper to move to the fusing oven. However, more advanced energization would be desirable, particularly with respect to internally heated fusing rollers, to avoid unacceptable undershoot of the temperature of the fusing roller's outer surface

In view of the problems outlined above it is apparent that there is need in the art for improved apparatus for sensing and controlling the temperature of the fusing surface of heated fusing rollers and for a more effective method of anticipating and compensating for a change in the heat loss from the fusing surface.

It is therefore an object of this invention to provide for such fusing apparatus, a simple and inexpensive means for sensing and controlling the fusing surface temperature.

It is another object of the present invention to provide for such fusing apparatus an improved means for anticipating a change in the heat load so as to eliminate undershoot and/or overshoot of the acceptable fusing surface temperature range.

In accordance with a preferred embodiment of the present invention, these objects along with certain other advantages of the invention are accomplished in conjunction with a fusing roller of the type having a thermally conductive core covered by an outer layer of offset preventing material by (1) internally heating the roller, (2) sensing the temperature of the internally heated conductive core rather than sensing the fusing surface temperature, (3) providing temperature control settings for the core sensor and (4) selectively switching the internal heating means into operative relation with a predetermined temperature control setting as soon as available knowledge in the apparatus indicates an increased or decreased heat loss will be forthcoming. The temperature control settings for the core sensor are selected by determination of the particular temperature differential which should exist across the cross section of a particular fusing roll to provide sufficient heat flow to compensate for the heat loss occurring at the outer surface under its various operating conditions in order to maintain a proper external release surface fusing temperature.

More specifically, in the art of heat transfer it is well known that the rate of heat flow through a material is directly proportional to the thermal conductivity of the material and the temperature differential across, i.e. the thermal gradient through, the material. Therefore it can be seen that if the fusing surface of such a roller is in equilibrium at a particular fusing temperature in the idle condition, a given temperature differential exists between the interior and exterior of the fusing roller and causes the predetermined rate of heat flow which compensates for idling heat losses e.g., to the surrounding air. Even though the idling heat losses are low, a significant temperature differential must exist particularly when the fusing roller involved includes an outer layer of release material that has a very low thermal conductivity.

Since the heat loss from the fusing surface during contact with a copy sheet and back-up roller greatly exceeds the ambient heat losses to air, it follows that during the copy operation, the compensating heat flow must be equally increased to maintain the fusing surface at the proper fusing temperature. Accordingly the temperature differential between the inner and outer surface must be significantly increased. Since the overall idling and copy run condition heat losses are generally constant for a given machine, the temperature differentials to maintain a particular fusing surface temperature in that machine can be determined for each condition.

The present invention makes use of the foregoing observations to avoid the problems presented by prior art surface sensing techniques and provides a simple and inexpensive core sensor together with either an "on-off" or a proportional controller to control the necessary temperature differential for a machine's idling, copy run or other operative conditions.

Since the present invention involves internal heating of the fusing roller there is a greater need to anticipate heat loss changes than there would be if external heating were used, to avoid temperature undershoot during the time lag while the change in heat flow effects the desired change in fusing surface temperature. If only one heat differential is utilized for copy run conditions, i.e., the optimum one for steady state copy run conditions, the need for a more advanced anticipation of heat losses increases. This invention therefore provides for a change from the idling temperature setting to the copy run setting immediately on receipt of any signal, such as a print command, which indicates copies will be forthcoming. This method of anticipation is workable whether an "on-off" or proportional controller is used because having switched to a different control point the sensor-controller will be substantially away from the "set point", causing immediate energization of the heating element. This reduces the time lag which is responsible for the undershoot and hence reduces the amplitude and duration of the undershoot.

It will be appreciated from the following detailed description that the invention disclosed herein provides an efficient, practical and quite inexpensive apparatus for controlling the surface tem380000000000000000000000000000000000000000000000000000000000000000