The Small Signal and Amplitude of Alternating Current Measuring System Dan CHEN 1,a, Hao LIU 1,b,*

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1 2017 3rd International Conference on Education and Social Development (ICESD 2017) ISB: The Small Signal and Amplitude of Alternating Current Measuring System Dan CHE 1,a, Hao LIU 1,b,* 1 Beijing Institute of Fashion Technology, Beijing, China a cdan134@163.com, b gxyliuh@bift.edu.cn *Corresponding author Keywords: Electronic Design Contest, Alternating Small Signal, Frequency Measuring and Amplitude Measuring. Abstract. This thesis is based on the examination item of 2016 Beijing College-student Electronic Design Contest, thus designing the small signal and amplitude of alternating current measuring system. MSP430G2553 microcontrollers are the main engine of this system. FPGA Cyclone 1C12 is slave engine, and external circuits OPA211 circuit board as an amplifier system. Main engine receive and calculate the frequency and voltage value of OPA211 direct or alternating current small signal. And the signal connects with the input of OPA211, as well as it magnifies the small signal received more than 20 times, and the output of OPA211 connects with the corresponding pins of microcontrollers. Slave engine receive signal sent by main engine and show the numerical value through digital transistor. Introduction We make MSP430 [1] as main engine, FPGA as slave engine, and design a digital system that can measure and show voltage and frequency of alternating small signal. System s measurable scope is DC 5mV-100mV, DC 5mA-40mA, the effective value between AC 5mV-80mV (input signal frequency scope: 100Hz-5KHz), alternating current voltage frequency is between 100Hz-5KHz (input signal voltage which effective value is 20mV). The Requirements of System and Whole Design Voltage and frequency of small signal are system s two measuring numerical value, voltage value can receive by taking a sample and reading input signal through corresponding pins of main engine, and using the counter of main engine to count how times rising edge arrives in a fixed period of time, so we can obtain the signal cycle, then we can get the show frequency value by programming calculating. Lastly, sending those numerical value inputted by main engine to slave engine through UART communicates port, and slave engine show the value through digital transistors. It is known that the units of input signal s voltage and current are millivolt and milliampere. Also the scope is small, so it will be big error if input read display directly, thus, we must use amplifier system to enlarge signal before the input signal connect with main engine. Input signal: DC voltage, DC current, AC voltage, AC voltage frequency. MSP 430[2] pins read external signal by receiving voltage amplitude value and pulse signal. While the input signal is DC or AC voltage, main engine can take a sample directly. But when the input signal is DC current, there need an external circuits to transfer from the input current signal to voltage signal. When input signal is frequency, we should sampling counting impulse, and also need an external circuit to transfer from sine wave into square wave. It is means that when input signal is DC voltage, DC current or AC voltage, what main engine pins get is voltage value, but it will show different things in slave engine, so needing to distinguish what input signal need to display by different main engine s pins input signal. Check service manual of msp430g2553 we can know the internal reference voltage is 2.5V[3], and the reference voltage of sampling voltage amplitude in this system will use internal reference voltage, it is also means that the voltage system can obtain is up to 790

2 2500mV, the digital transistor in slave engine can show four numbers, main engine and slave engine communicate by choosing UART [4] communication interface. Hardware System Design All input signal are millivolt voltage signal, and measuring scope is small, so there need external circuits to enlarge input signal. Main engine internal reference voltage is 2.5V, the maximum voltage of input signal is 100mV, so we can set the enlargement factor as 20. Main engine can t identify negative half axis of alternating current signal, so there need to full wave rectifier before put into the MSP430. Amplifier and full wave rectifier circuits are as the figure 1 shown. When input signal is DC current, there can connect a resistance with 1Ω in outside part, it can transfer DC current input signal into DC voltage signal. While input signal is DC voltage or DC current or voltage, it can link to the hardware circuit U. Then when input signal is DC current, there can link input signal that be transferred to circuit U. Input terminal connect with P1.5 of MSP430 [6]. When input signal is AC frequency, there need an external circuit to transfer from sine wave into square wave, as the Figure1<2> shows. Input terminal connect with P1.0 of msp430. Figure 1. <1> Amplifying Circuit and Precision full-wave Rectifier Circuit Hardware. <2> Waveform Conversion Circuit. Software System Design Receiving value of voltage or frequency from MSP430G2553 through serial port UART [4], then display by digital transistors of FPGA [4]. In one time there can receive 8 bits data, and bit code is low 4, high 4 is needed to display the value. Read manner of FPGA is read the address of low 4 first, then read the value should be show in high 4, finally finding segment code of the value in high4 and sending it to digital transistors [5,6]. Software design flow diagram is as the Figure 2 showing. 791

3 Start DC0 Timer Source1MHZ Open ADC internal reference voltage 2.5V Set TimerB Set TimerA Set UART communication interface Timing 100US through decrement count TimerA count TAR and acquire technology pulse sum_f Set count circle temp_f=sum_f* Acquire maximum amplitude of ADC10CTL0 average_v during 500us Sum_f==0 Send average_v to average_v ==0 Send temp_f = to temp_f=>5000 value_v =average_v/1.414 Send value_v= to Send average_v to End System Testing Figure 2. MSP430G2553 Flow Chart of Software Design. When input signal is AC voltage frequency, input terminal is linked to pin U4 of external circuits, and output terminal of hardware circuits is linked to msp430 s pin P1.0. Also pin P1.4 is connected with the ground. There are data which input signal texted is frequency, as the Table 1 shows. Table 1. Input Signal is Frequency. Input frequency value F i(hz) Display frequency value F o(hz) Error δ% P.S.:average_v=0,input frequency s range is 100HZ-5000HZ,error is 792

4 When input signal is AC voltage, input terminal is linked to external circuits U2 and U4, pin P1.4 is linked to output terminal of hardware circuits which is corresponding to U2, and pin P1.0 is linked to output terminal of hardware circuits which is corresponding to U4. There are data which input signal texted is AC voltage, as the Table 2 shows. Table 2. Input Signal is AC Voltage. Input Vpp value(mv) Effective value U i(mv) Output effective value Uo(mV) Errorδ% P.S.:frequency is 1000HZ,input AC voltage s effective value s range is 100mV-1600mV,error is When input signal is DC current, input terminal is linked to external circuits U1. Picture2 s Ui is linked to output terminal Ui of hardware circuits which is corresponding to U1. Pin P1.4 is linked to output terminal Uo1 of hardware circuits which is corresponding to U1, output terminal U4 is linked to signal which frequency is more than 5KHZ. There are data which input signal texted is DC current, as the Table 3 shows. Table 3. Input Signal is DC Current. Input voltage value U i (mv) Output voltage value U o(mv) Error δ% P.S.:temp_f>5KHZ, when input signal is AC voltage and it is effective, the range is 100mV-800mV,error When input signal is DC voltage, input terminal is linked to external circuits U3, pin P1.4 is linked to output terminal Uo1 of hardware circuits which is corresponding to U3. U4 is linked to the ground, and pin P1.0 is linked to output terminal Uo2 of hardware circuits which is corresponding to U4. There are data which input signal texted is DC voltage, as the Table 4 shows. Table 4. Input Signal is DC Voltage. Input voltage value (mv) Output voltage value U o(mv) Error δ% P.S.:temp_f=0, when input signal is AC voltage and it is effective, the range is 100mV-2000mV,error Conclusion This is a digital system that can measure and display voltage and frequency value of an alternating current small signal, and the controller include msp430 which as the main engine and FPGA which as the slave engine. This system can realize some basic function which including read and display DC voltage and current, AC voltage and signal of voltage frequency. System s basic error should not over 0.1%. Acknowledgement Corresponding author of the article is Hao Liu.This research work was financially supported by Beijing Institute of Fashion Technology under HFZ /001, HFZ /003/007 and HF

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