One of the key technologies in permanent magnet synchronous motor (PMSM) control systems is rotor position detection. Only after detecting the actual spatial position (absolute position) of the rotor can the control system determine the inverter's energizing method, control mode, and the frequency and phase of the output current to ensure the normal operation of the PMSM. Therefore, in PMSM control systems employing rotor field-oriented control, accurate and reliable rotor position detection is crucial for realizing vector control technology. Among commonly used rotor position sensors such as grating encoders, Hall effect sensors, and rotary transformers, rotary transformers possess outstanding advantages such as high temperature resistance, humidity resistance, good shock resistance, and strong anti-interference capability. This allows them to accurately and reliably generate rotor absolute position information, making them suitable for PMSM digital control systems and meeting the high performance and high reliability requirements of their applications. The PMSM control system mentioned in this paper uses a sine/cosine rotary transformer from Tamagawa Corporation of Japan to detect the rotor position and employs a new rotary transformer/digital converter, AD2S80A, to convert the analog signal output by the rotary transformer into a digital signal. The working principle of AD2S80A was analyzed, a position signal detection circuit was designed, and the SPI communication interface method and program example with the TMS320LF2407A digital signal processor were given. Basic Principles of Rotary Transformers A rotary transformer (resolver) is a signal element whose output voltage changes with the rotor angle. When the excitation winding is energized with an AC voltage of a certain frequency, the voltage amplitude of the output winding has a sine and cosine function relationship with the rotor angle. This type of rotary transformer is also called a sine-cosine rotary transformer. In motor control systems, rotary transformers used to measure position information are mainly sine-cosine rotary transformers. The rotary transformer used in this system is a sine-cosine rotary transformer from Tamagawa Corporation of Japan. This type of resolver consists of a rotor and stator windings, which are independent of each other. The primary and secondary coils are wound on the stator, and the rotor consists of two sets of coils 90° out of phase, using a brushless design. Figure 1 is its electrical schematic diagram. [align=center]Figure 1 Electrical schematic diagram of a resolver[/align] The specific functional relationship between the input and output voltages of the resolver is as follows: Let the rotor rotation angle be θ, and the primary coil voltage (i.e., the excitation voltage) be: er1-r2＝esin2πft where f——excitation frequency; e——signal amplitude. Then the output voltage es1-s3＝kesin2πftcosθ es2-s4＝kesin2πftsinθ where k——transmission ratio; θ——angle of rotor deviation from the origin. Let θ＝ωt, that is, the rotor moves at a constant speed, then the function curve of its output signal can be represented as shown in Figure 2. [align=center]Figure 2 Input and output waveforms of a resolver[/align] In Figure 2, the output voltage envelope signals are sinωt and cosωt. The digital converter obtains the resolver position information by detecting these two sets of output signals. Position Detection and Conversion Circuit Design [1-2] Introduction to the AD2S80A Chip: The AD2S80A is an RDC chip from AD company, part of the AD2S80 series. It features adjustable precision, high reliability, digitized status and control signals, and easy connection to microcontrollers (DSPs). It can be used for digital conversion in synchros, resolvers, and inductive synchronizers. It is available in 40-pin DIP and 44-pin LCC square packages. The AD2S80A offers several resolution options: 10, 12, 14, and 16 bits, determined by the logic states of pins SC1 and SC2. Different bandwidths and tracking rates can be obtained by selecting different external resistors and capacitors. The design of the AD2S80A conversion circuit is based on the principle of resolvers. To ensure the normal operation of a resolver, a sinusoidal excitation must be applied to its rotor. In this system, the sinusoidal excitation signal is generated by the Intersil ICL8038 chip. Based on the actual situation of the permanent magnet synchronous motor control system, a 10 kHz sine wave (i.e., the reference frequency of the AD2S80A) is selected as the excitation signal. The AD2S80A resolution is selected as 16 bits, the maximum tracking speed is 16.25 r/s, and the bandwidth is 600 Hz. Based on these performance indicators, the peripheral circuit components can be selected according to the formula. The calculation formula will not be described here. [align=center] Figure 3 shows the internal principle block diagram and peripheral circuit of the AD2S80A peripheral circuit[/align] Figure 3 shows the internal principle block diagram and peripheral circuit of the resolver/digital converter (RDC). The absolute angle measurement is achieved based on the sine signal (sin) and cosine signal (cos) introduced by the reference I/P pin, and the modulation signal input by the sin pin. As shown in Figure 3, the AD2S80A operates as a tracking converter, and the digital output can automatically track the shaft angle at the selected maximum tracking rate. Because it employs a ratio-based tracking method, the output digital angle depends only on the ratio of the input sin and cos signals, and not on their absolute values. Therefore, the AD2S80A is insensitive to changes in the amplitude and frequency of the input signal, and does not require a precise, stable oscillator to generate the reference signal. The phase-sensitive detector in the conversion loop ensures high suppression of orthogonal components in the reference signal. Its high noise suppression ratio reduces errors caused by long lines from the resolver to the converter RDC. The 16 data output lines have a tri-state output data latch function, and can be transmitted to an 8-bit or 16-bit data bus by controlling the byte select pin. Position detection module SPI communication with DSP In the control system of a permanent magnet synchronous motor, to simplify circuit design and improve the position information reading speed, the rotor position signal is transmitted to the DSP using SPI serial communication mode. However, the AD2S80A outputs 16-bit parallel data. To achieve SPI communication with the DSP, the parallel data output by the AD2S80A needs to be converted into serial data. Parallel data to serial data conversion is achieved using the 74HC165 chip. This chip can only convert 8 bits of data at a time, while the AD2S80A outputs 16 bits of data. Therefore, two 74HC165 chips need to be cascaded to achieve 16-bit parallel data to serial data output. The data transmission timing diagram of the 74HC165 is shown in Figure 4. [align=center] Figure 4 7HC165 Data Output Timing Diagram[/align] As can be seen from Figure 4, when clk inh is high, data cannot be output. When clk inh goes low, the data is shifted and output on the rising edge of the next pulse. Furthermore, when sh/ld is low, the data output from the parallel port is acquired, and when it goes high, the data is latched. Therefore, an inverter can be used to invert the input signal of clk inh before connecting it to sh/ld. When clk inh is high, the data output from the parallel port is collected into the chip. When clk inh is low, the data starts to shift and output on the next rising edge of the pulse. clk inh can be used as the chip select signal for SPI communication. SPI communication implementation When the DSP communicates with the position signal detection module via SPI, the slave mode is used. The communication clock is provided by the position detection module [3]. The clock frequency in this system is 1MHz. Figure 5 shows a schematic diagram of the SPI communication connection between the DSP and the position detection module. [align=center]Figure 5. Connection diagram between the position detection module and the DSP[/align] Since the DSP's SPI communication mode is slave mode, the DSP needs to be selected before receiving data. In this system, a low-level signal is given to the DSP via an I/O port to select it. Simultaneously, the CLKINH signal also goes low, and the 74HC165 starts serial shifting to output the acquired parallel data. After data transmission is complete, a high-level signal is given to the DSP to stop receiving data, and the 74HC165 starts acquiring the parallel data output by the AD2S80A. This achieves SPI communication between the DSP and the position detection module. One issue to note is that the data acquisition timing may be asynchronous during SPI communication; that is, the data received by the DSP may be shifted one position to the right or left compared to the actual value. To prevent data loss, the following process can be performed: Before activating the chip select signal of the DSP, the pulse signal of the 74HC165 can be detected first. After detecting the falling edge of this pulse, delay for about one pulse cycle before activating the DSP. On the next rising edge after such a pulse, the data begins to be transmitted to the DSP's SPI port. In this way, the data received by the DSP will not be shifted. Below is a reference code segment for communication between a DSP and a position detection module SPI: Setting SPI: `ldp #0e0h; set iopc3 as; spite or #0808h sacl pcdatdir ldp #0e0h splk #000fh, spiccr splk #00000010b, spictl splk #008fh, spiccr` SPI communication: `spi_wait: ldp #0e1h; detect falling edge of pulse; lacc pfdatdir and #0000000001000000b bcnd` `spi_wait, neq rpt #40; delay for one pulse period; nop; select different values ​​according to different pulses; lacc pcdatdir; gating DSP and #1111111111110111b sacl pcdatdir ldp #0e0h` `spi_rdy: bit spists, bit6 bcnd` spi_rdy,ntc ldp #pcdatdir>>7 lacc pcdatdir or #0000000000001000b sacl pcdatdir ldp #0e0h lacc spirxbuf; Output position signal ldp #6 sacl positon Conclusion This paper introduces the working principle of a rotary transformer and provides a detailed description of the high-precision rotary transformer converter chip AD280A and its peripheral circuit design. It also details the parallel-to-serial conversion of the AD280A output and its SPI communication with the high-speed digital signal processor TMS320LF2407A. This position signal detection circuit is used in a permanent magnet synchronous motor control system, forming a high-precision, high-reliability position detection unit. Actual operation results show that this method has high accuracy, small component size, strong anti-interference ability, and high reliability, making it particularly suitable for position detection systems in harsh environments and possessing high application value.