Design of wireless meter reading system based on DSP

Traditional manual meter reading is time-consuming and laborious, and its accuracy and timeliness cannot be reliably guaranteed, which leads to the inability of relevant marketing and enterprise management software to obtain sufficiently detailed and accurate raw data. Generally, manual meter reading is done on a monthly basis, which is feasible for user metering, but not enough for deeper analysis and management decisions of relevant supply departments. With the development of large-scale integrated circuits and communication technology, in the field of electric energy measurement, digital meters have gradually replaced traditional mechanical meters with their own advantages. High-precision electric energy metering chips are the core part of digital meters. CS5460A has been widely used in various single-phase and three-phase electric energy meters and power system measurement and control fields. The degree of computer information management in power systems is getting higher and higher, and wireless meter reading systems are becoming more and more widely used.

1 Overall structure

The system uses the high-precision power acquisition chip CS5460A to collect power, and uses DSP2812 control. The processed data is transmitted through the wireless transceiver module nRF2401A to achieve wireless data transmission. The receiving module receives the data collected by the front-end acquisition system and transmits it to the host computer. The host computer can display some required electrical parameters to achieve integrated meter reading and charging. The system structure is shown in Figure 1.

2 Design of power collection system

The CS5460A is selected as the power acquisition chip, and the TMS320F2812 of TI is selected as the control chip. The CS5460A is a highly integrated A/D converter, which combines two A/D converters, high-speed power calculation function and a serial interface on a single chip. The chip is designed for accurate measurement and calculation, and can be used for the measurement of energy, instantaneous power, instantaneous voltage, instantaneous current, current effective value and voltage effective value of single-phase 2 or 3-wire power measurement equipment. The CS5460A can be connected to a low-cost bypass resistor or transformer to measure current, and can also be connected to a resistor divider or measurement transformer to measure voltage. The CS5460A has a bidirectional serial port for communication with a microcontroller and a programmable frequency output proportional to the power supply. The on-chip functions of the CS5460A also include AC or DC system calibration.

The CS5460A can operate on a single +5 V power supply and a bipolar ±5 V power supply, with a current channel input range of 50 mA or 250 mA and a voltage channel input range of 250 mV. The common-mode signal of the CS5460A is -0.25 V to VA+ when powered by a single power supply. Figure 2 is a circuit diagram of the power measurement of the CS5460A in a single-phase 2-wire system in single-power mode. This wiring method achieves isolation from the power line. Isolation is achieved through three transformers. One is an ordinary transformer used to provide the DC power supply of the CS5460A. The second is a high-precision, low-impedance transformer (commonly called a voltage transformer) with very little attenuation and phase delay under higher filtering. There is also a current transformer used to measure the power line current. A resistor is connected across the secondary pole of the current transformer to generate a current-induced voltage signal for the current channel of the CS5460A.

3 Wireless Communication System Design

nRF2401A and its peripheral circuits include the chip part, voltage regulator part, crystal oscillator part, and antenna part of nRF2401A. Voltage +5 V provides voltage for the chip; the crystal oscillator part includes Y4, C44, C45. The allowed values ​​of crystal oscillator Y1 are: 4 MHz, 8 MHz, 12 MHz, 16 MHz. This design uses a communication rate of 1 Mb/s, so a 16 MHz crystal oscillator must be selected. The inductor L1 of the antenna part is used to convert the 2.4 GHz level signal generated by the ANT1 and ANT2 pins of the nRF2401A chip into an electromagnetic wave signal, or to convert the electromagnetic wave signal into a level signal and input it into the ANT1 and ANT2 pins of the chip.

The schematic diagram of the nRF2401A communication module is shown in Figure 3. This design only uses the transceiver channel of channel 1, and channel 2 is a reserved channel. PER_UP is connected to the GPIOB4 port of DSP2812 to control the power-on of the nRF2401A chip, and GPIOB5 is connected to CE to control the chip enable. GPIOB6 is connected to CS to realize nRF2401A chip selection, GPIOB7 is connected to DR1 to control the chip channel 1 data request through DSP, GPIOB8 is connected to CLK1 to control the channel 1 clock, and GPIOB9 is connected to the chip DATA pin to realize data transmission between DSP and the module.

4. Design of power acquisition software
4.1 Program functions
The program functions include two key contents: one is the definition of the register address of the CS5460A chip and the writing of control words through the single-chip microcomputer, and the other is the reading of data.
4.2 Operation of CS5460A
The operation of CS5460A is mainly to read and write different registers and perform different operations through SPI port writing commands; the registers of CS5460A mainly include status registers, configuration registers, control registers, various calibration registers, data registers, etc. Before measuring, the registers should be set appropriately to obtain correct results.
4.3 System calibration
CS5460A provides digital calibration function. By writing the specified value to the calibration command register, the system offset calibration and system gain calibration can be realized. When performing calibration, the corresponding calibration signal must be sent to the voltage and current channels. CS5460A has a series of calibration registers. After executing a calibration command, the corresponding register will retain the correction value generated by the calibration, and these values ​​can be read out; if it is saved in an external non-volatile memory, the value is written to the corresponding register after the system is reset, and recalibration is not required. The calibration registers include DC voltage/current offset register, voltage/current gain register, and AC voltage/current offset register. Before calibration, the CS5460A must be in active state, ready to receive valid commands, and the DRDY bit of the status register must be cleared, because DRDY is used to determine whether the calibration is completed. When performing offset calibration, a zero signal should be input; when performing gain calibration, a full-scale signal should be input. After calibration is completed, measurements can be performed.