Ultrasonic motors are a new type of motor based on the inverse piezoelectric effect of piezoelectric ceramics. Compared with general electromagnetic motors, ultrasonic motors can directly output high torque at low speeds, and have fast transient response (down to the millisecond level) and high positioning accuracy (down to the nanometer level), making them very suitable for replacing traditional servo motors and stepper motors. Currently, ultrasonic motors are widely used in the autofocus systems of cameras and camcorders, and also have many applications in precision instruments and aerospace. Despite the many advantages of ultrasonic motors, their high requirements for drive signals have resulted in problems such as large circuit size and limited control performance in the drive and control circuits developed both domestically and internationally. Most studies on the control characteristics of ultrasonic motors are still based on ordinary drive circuits配套 with ultrasonic motors, making it difficult to conveniently conduct computer-based research on the control characteristics of ultrasonic motors. [img=549,237]http://www.cechinamag.com/images/Article/07b18e7f-391d-4cac-8aad-0cc2725779ad/241.gif[/img] To address the above problems, a high-performance ultrasonic motor drive and control circuit was developed using the commercially available DDS chip AD9850 and high-voltage operational amplifier PA08. This circuit can be controlled and parameter acquired via a serial interface using an educational computer, and it offers high accuracy in signal frequency and phase control. It can also automatically adjust the signal frequency under the control of a temperature sensor, and the circuit includes four reserved analog and digital universal sensor interfaces. 1 Working principle of ultrasonic motor drive and control circuit According to the working mechanism of ultrasonic motor [1], most ultrasonic motors require the relevant drive and control circuit to provide two alternating signals with the same frequency, a phase difference of 90 degrees (or adjustable), a frequency of 20kHz to 100kHz, a peak-to-peak output signal of 100 to 350V, and a power of 0.1 to 10W. In addition, since the optimal working frequency of ultrasonic motor is determined by the mechanical resonance frequency of the system, and the resonance frequency of the vibration system mainly composed of piezoelectric elements will change with the change of external parameters (such as temperature, pre-pressure, etc.), a good ultrasonic motor drive and control circuit must be able to track the change of the system resonance frequency well, so as to ensure the stable operation of the motor. 2 Basic composition and principle of the circuit The ultrasonic motor drive and control circuit mainly consists of the following 5 parts: DDS signal generation unit, signal isolation unit, signal power amplification unit, temperature sensor unit, microcontroller main control and interface control unit (see Figure 1). The DDS signal generation unit generates two independent sinusoidal signals, and the phase difference between the two signals can be adjusted arbitrarily within the range of 0 to 360 degrees. The two signals are respectively sent to a high-speed optocoupler after passing through an RC low-pass filter and a transistor emitter follower, and then amplified to 250V by a high-voltage operational amplifier to drive the ultrasonic motor. After the ultrasonic motor operates for a period of time, its temperature will change. This temperature is collected by a temperature sensor and sent to the main control microcontroller for processing. The microcontroller transmits the temperature data to the PC via a serial port. The PC, based on the control model, sends the corrected drive frequency control word back to the microcontroller, or the microcontroller corrects the DDS signal generation frequency based on the pre-stored temperature-frequency table. Considering future expansion needs, the system reserves four A/D and general-purpose I/O multiplexed ports for interfacing with photoelectric encoders, strain gauge torque sensors, etc., used to measure motor speed. The composition and function of each unit are briefly introduced below. 2.1 Microcontroller Main Control Unit The main control microcontroller is a PIC16F73A microcontroller (see Figure 2). This microcontroller operates at 5 MIPS at a frequency of 20MHz and has 4K of on-chip flash program memory. It integrates 3 timers, 1 USART (for RS232 communication), 5 8-bit A/D converters, SPI (I2C) converter, and a WDT (watchdog timer). The microcontroller controls two DDS converters (RC0-RB7, RC0-RC2, RC5) and a serial interface digital temperature sensor (using I2C bus, RC3, RC4). It also exchanges data with a PC via an RS232 serial port (RC6, RC7). Additionally, it provides 4 reserved pins (RA0-RA3) for general I/O or A/D operation, facilitating future interfacing with other sensors or controllers. [img=549,454]http://www.cechinamag.com/images/Article/07b18e7f-391d-4cac-8aad-0cc2725779ad/243.gif[/img] 2.2 DDS Signal Generation Unit The DDS signal generation unit uses two AD9850 direct digital frequency synthesizers [2] (see Figure 3, which shows one of the channels; the other channel is identical). This DDS has a high cost-performance ratio and can generate signals up to 62.5MHz, with a 32-bit frequency resolution (i.e., 0.0291Hz at a 125MHz clock), a direct phase control accuracy of 11.25 degrees, and can transmit a total of 40 bits of control words in parallel or serial mode. In order to achieve rapid frequency updates, a parallel transmission mode is used in the circuit design, so that the DDS frequency and phase control words can be updated in 5 bytes. Furthermore, to conserve bus resources, the two DDS chips share an 8-bit data line, reset line, and serial clock line, but use different data update control lines, FQ-UD. While an 11.25-degree phase control accuracy is generally sufficient for most applications, higher accuracy (around 1 degree) may be required for experimental research. This can be achieved in two ways: First, by changing the rising edge delay between the two data update lines FQ-UD. Considering the microcontroller's timing accuracy is approximately 0.2μs, this method can achieve a phase control accuracy of 3.6 degrees at a signal frequency of 50kHz. Second, by temporarily altering the frequency consistency of the two signals, making one signal's frequency a few hertz higher than the other. This method is essentially the same as the principle of phase modulation through frequency modulation in high-frequency communication, achieving a phase control accuracy better than 0.1 degrees. For example, one signal has a frequency of 50kHz, and the other 50.001kHz. At the beginning, the two paths are in the same phase. After 200μs (i.e., 10 cycles), the phase difference between the two paths becomes 10×360/50000=0.072 degrees. 2.3 Signal Isolation Unit Considering that the voltage of the signal amplification section is relatively high (about 260V), and the signal generation section also needs to interface with the computer, in order to ensure that the circuit can work completely reliably, a signal isolation unit is added between the signal generation unit and the amplification unit. This unit uses an HP2531 high-speed optocoupler as the signal transmission element. The optocoupler has a bandwidth of 3MHz and two independent optocouplers inside. It has good linearity when used as a linear optocoupler between 1kHz and 300kHz. A transistor emitter follower circuit is also added to the input end of the optocoupler to ensure that the current at the input end of the optocoupler is at the best operating point (see Figure 4). 2.4 Signal Amplification Unit This unit uses two PA08 series high-voltage operational amplifiers [3]. This chip uses an 8-pin TO3 package and can operate within a power supply range of ±15V to ±150V, with an output current of up to ±150mA. The gain-bandwidth product at 1MHz is 5MHz. In use, this chip is essentially the same as a regular op-amp, except that, considering the load characteristics of the ultrasonic motor, an ultra-fast recovery diode with a withstand voltage of over 200V is added to the circuit to protect the op-amp's output and prevent the reverse voltage generated during circuit resonance from exceeding the chip's limit voltage difference of 300V. Furthermore, the chip uses two external resistors (R7 and R8 in Figure 5) to limit the output current, thus protecting the circuit. The high-voltage power supply used in the circuit is directly generated by rectifying the 220V AC mains power, which provides operating power to the high-voltage op-amp through two 130V Zener diodes and a power resistor connected in series in the circuit. The amplitude of the output signal can be easily adjusted by changing the op-amp's gain coefficient (by adjusting potentiometer R11 in Figure 5). 2.5 Temperature Sensor Unit The temperature sensor section adopts two schemes: one is to use MAX6656[4]. This chip provides a total of 3 temperature sensors, one on the chip and two off the chip, with a temperature resolution of 0.125. In addition, this chip also provides 3 voltage monitoring channels and temperature and voltage over-limit alarms. Data transmission adopts I2C bus. The circuit realizes two-point acquisition of ultrasonic motor housing temperature by connecting two SOT23 packaged PNP transistors 2N3904 off the chip. The acquisition results are processed by the microcontroller and sent to the PC for monitoring. The other is to use LM74 temperature sensor[5]. This temperature sensor adopts SO-8 package and can be directly attached to the motor housing. The temperature sensor has a resolution of 0.0625℃, and since the temperature is directly quantized by A/D and transmitted through the SPI bus, it is not affected by the resistance of general external wires, circuit wiring, and interference. The only drawback of using this chip is that it requires 4-5 wires to be led out from the circuit board, which is less convenient than the two wires in the previous solution (actually, a 2mm thin coaxial cable was used). 3. Introduction to the Ultrasonic Motor Drive and Control Circuit of Shinshin Corporation (Japan) Shinshin Corporation's ultrasonic motor drive circuit can output two sinusoidal signals with a frequency around 50kHz and a 90-degree phase difference. The speed of the ultrasonic motor is adjusted by fine-tuning the frequency, and the motor's start and direction of operation are controlled by an external switch. Furthermore, it can achieve automatic frequency tracking within a small range through the feedback electrode reserved on the motor. Except for the power drive section and transformer, most of the circuit's components are integrated into Shinshin Corporation's custom integrated circuits. The disadvantages are that the signal phase is fixed, signal voltage adjustment is inconvenient, and the motor lacks a flexible control interface, resulting in a relatively simple motor control strategy. Since most ultrasonic motors do not have a dedicated feedback electrode, this circuit is generally only suitable for specific series of ultrasonic motors from this company. In addition, since this circuit uses a 24V DC power supply, a dedicated rectifier power supply with a power of at least 10W is required, making it inconvenient to use. [img=549,403]http://www.cechinamag.com/images/Article/07b18e7f-391d-4cac-8aad-0cc2725779ad/245.gif[/img] 4 Experimental Testing The experiment used a 20MHz active crystal oscillator as the clock source shared by the microcontroller and DDS. The circuit designed in this paper can generate two stable Sine Waves in the range of 20kHz to 300kHz, with a peak-to-peak value of up to 250V and a combined maximum output power of approximately 5W. When driving the Xinsheng Company USR30 ultrasonic motor, the output signal frequency varies between 50.1kHz and 49.2kHz with the motor temperature (the driving frequency decreases as the temperature increases), and the single-channel output current Ipp is approximately 230mA when Vpp=210V. This ultrasonic motor drive and control system has the following advantages: The system structure is simple; the power amplifier section does not require an input transformer or impedance matching circuit, resulting in high reliability; no power transformer is needed, allowing direct use of mains power, making it convenient to operate; program development is simple, allowing users to easily write control programs using various familiar programming environments (such as VC, VB, MATLAB, etc.) on a computer; high frequency and phase control accuracy, and relatively convenient signal amplitude adjustment; feedback parameters can be easily acquired through the built-in temperature sensor and attached interface; good versatility, suitable for most medium- and low-power ultrasonic motors (below 5W).