Application of single chip microcomputer in the piezoelectric ceramic transducer

Piezoelectric ceramics can produce electrostrictive effects under the action of alternating electric field. Piezoelectric ceramic ultrasonic transducers can generate vibration under the action of alternating electric field, and can generate strong ultrasonic waves during resonance. Since piezoelectric ceramics are capacitive devices in the feeding circuits of piezoelectric ceramic ultrasonic transducer,inductors and piezoelectric ceramics are often used to form an LC resonant circuit. For such LC resonant feeding circuits, the resonant frequency belongs to piezoelectric circuits. The equivalent capacitance value of the ceramic, the inductance value, the amplification factor of the transistor, the operating point of the amplifier circuit, the feedback coefficient, and the operating temperature are determined. Since the piezoelectric ceramic transducer with a nominal resonant frequency of 28 kHz has a large dispersion, its resonant frequency is generally in the range of 26-32 kHz, and the half-width of the formant is generally less than 200 Hz. Therefore, the LC resonant circuit is used as the piezoelectric ceramic ultrasonic wave. There are the following problems in the piezoelectric transducer ring: First, the circuit adjustment is difficult. 

It is necessary to adjust a plurality of parameters to make the transducer work at the resonance point, such as adjusting the working point and the feedback coefficient. Second, the component characteristics are high, such as the transistor. The magnification needs to be screened, and the matching inductance of value error should not be too large; the third is that the operation is unstable, the change of the ambient temperature will cause the resonance frequency to deviate from the resonance point, and the transducer wear causes its mass to change, so that the resonance frequency changes;These problems lead to complicated production processes of piezoelectric ceramic ultrasonic transducers, which are not conducive to mass production. The use of single-chip control technology can solve these problems very easily. The PIC16C712 microcontroller, PWM technology and frequency conversion technology are used to design the piezoelectric ceramic ultrasonic transducer feeding circuit. The scheme works well in actual production.

PIC16C712 is an 8-bit high-performance single-chip microcomputer produced by MICROCHIP company in the United States. It runs fast. When the oscillation frequency is 20MHz, one machine cycle is 200ns. There are four 8-bit A/D converters on the chip, one capture input/ compare output/PWM pulse width modulated output (ie CCP module). which shows the feeding circuit of the piezo ceramic ring ultrasonic transducer controlled by the single-chip microcomputer. The CCP module of the PIC16C712 single-chip microcomputer is set to the PWM output mode. As the oscillation signal source of the transducer, the output signal passes the TIP122 Darlington power tube. high-frequency transformer T1 output and high-frequency voltage is loaded on the piezoelectric ceramic ultrasonic transducer, so that the transducer produces oscillation, if the output frequency of the PWM signal is the piezoelectric ceramic ultrasonic transducer resonance frequency, then the primary current of the frequency transformer is the largest. The resistance of the sampling feedback resistor Rf is 0.05 ohm, and the current flowing through the primary of the high-frequency transformer T1 is converted into a voltage signal (the primary coil is operating current of the high-frequency transformer is 0.5A to 2.0A, and the differential operation is performed. The amplifier IC2 is amplified, and the filtered voltage is in the range of 0.75V to 3.0V. 

This signal is used as the feedback signal VR and is input by the RA2 pin of the PIC16C712 (this pin is the analog AN2 input).This constitutes a close loop control system. When the CCP module of the PIC16C712 is operating in PWM mode, the PIC16C712 has four special function registers TMR2, PR2, CCPR1L, and CCP1CON to control the period and pulse width of the PWM output pulse. The period of the PWM output pulse signal is determined by the following formula:PWM signal period = [(PR2) + 1] × 4 × TOSC × (TMR2 pre-divided frequency).Among them, 4×TOSC=200ns, TMR2 pre-divided frequency can be set to 1:1. By changing the value of PR2 register, the oscillation period of PWM output signal can be changed, and the signal frequency will also change.Piezoelectric ceramic ultrasonic transducer with nominal resonance frequency of 28KHz, due to the discreteness of components, its resonant frequency is distributed in the range of 26KHz to 32KHz. In order to enable the frequency of the PWM signal to lock the resonant frequency of the piezo ceramic material, the following can be used. A method of frequency sweep to determine the resonance frequency point.