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电阻炉的保温都是采用带电接点二位控制的电子电位差计,由于它的结构简单、成本低、使用维护方便,常被用于炉温控制。其缺点是不能使炉温恒定在一个数值上,而只是在设定值附近来回波动,波动幅度一般达±10℃左右,有的甚至达几十度,以致在控温精确度要求高的场合就不能使用。那么,为什么温度波动范围会有这么大呢? 众所周知,热量传递是需要一定时间的,温度上升的快慢要看其热容量大小。一般说来,温度的上升与下降和时间的关系大体是一个指数曲线关系(图1)。从式中可见,当t=0时,Y=0;t-T时,Y=C(1-e~(-1))=C×(1-1/2.718)=63.2%·C。也就是说,当t等于时间常数T时,温度上升到稳定值的63.2%。通常用T来反映温度上升快慢的特点。在实际中,当阶跃干扰输入后,电炉通电,但炉温却并没有立刻发生变化,而是过了一段时间后,才开始缓慢上升(如图2所示)。从图中可知,OA一段时间,温度根
Resistance furnace insulation are charged contacts with two electronic potentiometer control, because of its simple structure, low cost, easy to use and maintain, often used for furnace temperature control. The disadvantage is that the furnace temperature can not be constant at a value, but only in the vicinity of the set value fluctuations, the fluctuation range of about ± 10 ℃, and some even up to dozens of degrees, so that when the temperature control accuracy demanding occasions It can not be used. So, why the range of temperature fluctuations will be so big? As we all know, heat transfer takes time, the temperature depends on the size of its heat capacity. In general, the relationship between temperature rise and fall and time is roughly an exponential curve (Figure 1). It can be seen from the formula that Y = 0 when t = 0; and Y = C (1 -e -1) = C × (1-1 / 2.718) = 63.2% · C at t-T. That is, when t equals the time constant T, the temperature rises to 63.2% of the steady value. T is usually used to reflect the characteristics of temperature rise and slow. In practice, when the step input was interrupted, the furnace was energized, but the furnace temperature did not change immediately, but only slowly after a while (see Figure 2). Can be seen from the figure, OA for some time, the temperature root