This invention relates to temperature measuring apparatus, and more particularly it relates to an improved temperature measuring apparatus for measuring the temperature of an electric conductor in a non-contact manner by utilizing the effect of eddy current.
A means of measuring the temperature of a metallic body by utilizing the effect of eddy current is known in the art. This means has been considered as an effective means of temperature measurement on a rolling mill line and the like, since this means has certain advantages over other temperature measuring means known in the art, such as, possibility of accurately measuring temperatures around room temperature, faster response and capability of being used in unfavorable atmospheric conditions.
However, the eddy current type thermometers which have heretofore been developed are disadvantages in that variation in the distance between a metallic body to be measured and the detecting coil affects the measured output of the meter and also causes deterioration of the temperature measuring sensitivity or the response of the measured output to temperature changes, and so on. Illustrated in FIG. 1 is a prior art apparatus of this type comprising a detecting coil 2 connected to one arm of an AC bridge 3, an oscillator 4 connected to energize the AC bridge 3, an amplifier 6 connected to another arm of the AC bridge 3, and a synchronous detection circuit 7 including a phase shifter 5 for accomplishing synchronous detection to produce a measured temperature output v. The temperature measuring characteristics of this known eddy current type thermometer may be represented by the graphs shown in FIGS. 2, 3 and 4. FIG. 2 shows the relationship between the temperature t of the metallic body 1 to be measured (the measuring distance d is constant), FIG. 3 shows the relationship between the measured output v (the temperature t is constant) and the measuring distance d between the metallic body 1 and the detecting coil 2, and FIG. 4 shows the relationship between the measuring distance d and the temperature measuring sensitivity dv/dt. As will be seen from these results, while the effect of the variation in the measuring distance on the measured output v of this prior art apparatus can be ignored as shown in FIG. 3, the temperature measuring sensitivity dv/dt is affected by the variation in the measuring distance d as shown in FIG. 4, and hence the measured output v is affected by the variation in the measuring distance.
Another type of measuring means is known in the art wherein, as shown in FIG. 5, a detecting coil 2 is connected to an oscillator 8 as the resonant element, whereby by utilizing inductance changes dependent on the temperature of the detecting coil 2, the temperature of a metallic body is counted and measured by a counter 9 in the form of a change in the oscillation frequency of the oscillator 8 in relation to the temperature as shown in FIG. 6. Also with this means, while the measuring distance d.sub.0 which will not be affected directly by variation in the measuring distance as shown in FIG. 7 may be determined by properly selecting the size of detecting coil and the value of oscillation frequency, the temperature measuring sensitivity df/dt to variation in the measuring distance is varied as shown in FIG. 8 and consequently the sensitivity is affected by variation of the measuring distance.
Thus, with the known thermometer utilizing the effect of eddy current, the temperature measuring sensitivity is invariably decreased exponentially with increase in the measuring distance d, thus giving rise to measurement errors. While still another type of measuring means has been proposed in which variation in the measuring distance is detected by a separate distance meter to automatically compensate the temperature measuring sensitivity, this type of apparatus has not been put into practical use due to the weakness of this measuring distance detection per se.