毕业设计：数字式电容测量仪的设计.doc

PAGE \* MERGEFORMAT 1 摘要 本设计是基于555定时器，连接构成多谐振荡器以及单稳态触发器而测量电容的。单稳态触发器中所涉及的电容，即是被测量的电容。其脉冲输入信号是555定时器构成的多谐振荡器所产生。信号的频率可以根据所选的电阻，电容的参数而调节。这样便可以定量的确定被测电容的容值范围。因为单稳态触发器的输出脉宽是根据电容值的不同而不同的，所以脉宽即是对应的电容值，其精度可以达到0.1%。然后在电路中加入一个由LM741以及一个电容和一个电阻构成的阻容平滑滤波器，将单稳态触发器输出的信号滤波，使最终输出电压与被测量的电容值呈线性关系。最后是输出电压的数字化，将输入到7448译码器中翻译成BCD码，输入到LED数码管中显示出来 关键词：电容，555定时器，滤波器，线性，译码器，LED数码管 目录 一、测量系统的方案设计··············································3 1.1、测量部分的系统方案设计·····································3 1.1.1、恒亚充电法测量···················································3 1.1.2、恒流充电法测量···················································3 1.1.3、脉冲计数法测量··················································· 1.2、测量信号数字化系统方案选择································3 1.2.1、利用单片机进行编程翻译··········································4 1.2.2、利用译码器进行翻译··············································4 二、单元电路的设计及原理·············································4 2.1、 电容值测量电路及原理·······································4 2.1.1、多谐振荡器电路图及工作原理·······································6 2.1.2、单稳态触发器电路图及工作原理·····································7 2.1.3、滤波器工作电路图及原理·········································· 2.2、模拟信号的处理以及数字化显示·····························9 三、系统参数设定······················································10 四、结论及谢词························································11 4.1、结论·····························································11 4.2、谢词····························································11 参考文献··································································12附表：元器件明细表······················································13 一 系统方案设计 1.1 测量部分的系统方案设计 1.1.1:恒压充电法测量 用一个电阻和电容串联，用恒压源对电容进行充电，然后根据电容充电的曲线超过某个固定电压所需要的时间，利用曲线拟合的方法测量。测量所使用的原始公式是：。可见电容的值和电压以及时间呈微分关系。用这种方法测量，时间和容值是非线性的。因此测量难度高，精度低，并且难以实现数字化。 1.1.2：恒流充电法测量。 用恒流源对电容充电，此时电容的容量和充电时间是成正比的，所以可以利用AD或者比较功能同某个固定电压比较，来实现电容测量。测量所用的原始公式是： ..所以。恒流源的电流大小是已知的，时间和电压也可以测量出来。由上面的公式即可求得电容的大小。使用这种方法来测量，精度较上一种方法有所提高，且便于操作和实现。但要使用恒流源，恒流源的的设计要求很高，且达不到测量所需要的精度要求，因此这种方法也不适用。 1.1.3：