Design of tire mold inner diameter measurement system based on AVR microcontroller

    Tire molds are used to mold tires, and their processing quality is very important for tire production. In order to produce good tires, high requirements must be placed on the processing quality of tire molds. The traditional processing quality detection method mainly relies on a dial indicator, and the processing quality report is obtained after manual data collection and analysis. The limitation of this method is that the operator needs to have certain work experience, and the sampling process is manually controlled, and the accuracy is affected to a certain extent. In recent years, the tire mold industry has developed rapidly with the large demand for tires, and traditional detection methods cannot meet market demand. The grating ruler is a digital displacement measurement device with a measurement range of up to tens of meters and a measurement accuracy of microns; the laser rangefinder is a non-contact measurement device that can measure the displacement of targets on irregular surfaces, but the measurement distance is small. Combining the large-scale grating ruler and the non-contact measurement laser rangefinder can realize the measurement of the target distance of the irregular surface. The grating ruler reader and the laser rangefinder are fixed on the mechanical crossbeam, and the stepper motor is used to control the movement of the crossbeam to measure the inner diameter of different layers of the mold.

    The system uses AVR single-chip microcomputer to control the stepper motor and read the grating ruler data. By receiving the control command from the host computer, the AVR single-chip microcomputer controls the movement of the stepper motor, and the digital display data and the laser controller displacement data are automatically sent to the host computer through the serial port, thereby completing the automatic measurement of the inner diameter of the mold.

    1 System composition

    The tire inner diameter measurement system based on AVR is mainly composed of AVR single chip microcomputer, host computer, grating ruler, digital display, laser rangefinder, driver, stepper motor, electronic handwheel, travel switch , etc. Its functional block diagram is shown in Figure 1.

    The microcontroller used is Atmel's ATmega16; the host computer uses the ARK3360L industrial computer produced by Advantech, which has multiple RS232 interfaces; the laser rangefinder uses the LK-G85 laser rangefinder sensor and LK-G3001V laser rangefinder controller of Japan's Keyence, with a resolution of 0.1μm, a measurement range of -15 to +15 mm, and a measurement distance of 80 mm; the grating ruler uses the KA300 series grating displacement measurement device of Guangzhou Nordson Digital Measurement and Control Equipment Co., Ltd., and the reading is realized by the company's SDS6 digital display. The final inner diameter calculation is completed after the host computer receives the laser rangefinder data and grating ruler data.

    2 System Function Analysis

    2.1 Mold inner diameter measurement method

    The tire mold is placed on a stationary work platform, and the rotating measuring platform is located at the center of the work platform. The beams and columns on the rotating measuring platform can move in both radial and vertical directions, and the laser sensor head is placed on the measuring beam. According to the different inner diameters of the tire mold, the measuring beam is moved radially, and the laser sensor head is moved into the measuring range. The distance from the laser sensor head to the inner circle of the tire mold can be measured, and then the radial movement distance of the beam is measured using a grating ruler to convert the inner diameter of the tire mold. By controlling the motor to rotate the measuring platform, the inner diameter of the tire mold can be measured one by one according to the pitch, so as to obtain the roundness of the tire mold. By moving the measuring beam vertically along the column, the inner diameter of the tire mold at different vertical heights can be measured, so as to obtain the conicity of the mold.

    The measurement system has a total of three stepper motors to control the measurement equipment to move in three directions; two grating rulers read the horizontal and vertical displacement data, and a laser rangefinder realizes non-contact displacement measurement. Its measurement structure is shown in Figure 2.

    2.2 Measurement principle and function realization

    The cross section of the tire mold tread block is shown in Figure 3: The top of the tread block is the mold bead, which has a generally small processing error and is selected as the reference. Let the distance from the bead to the center line X1 of the tread block be RT, and set this inner circle as the reference circle. The tire mold inner diameter measurement is based on the premise that the mold bead radius RT is known, and the entire measurement process is controlled by the program.

    Under the condition of ensuring the flatness and concentricity of the mold to be tested, adjust the rotary motor and the vertical displacement motor so that the laser sensor head is located at the known tire diameter position above the measured point. Adjust the horizontal displacement motor so that the horizontal distance between the laser sensor head and the tire mouth measured point is 80±0.5 mm. The laser sensor has the highest measurement accuracy at this distance.
    According to the tire mouth radius RT and the laser distance measurement value and horizontal grating ruler reading of the tire mouth measured point, the following equation can be obtained:
    
    Where XL is the laser distance measurement value, XR is the horizontal grating ruler reading, these two values ​​can be measured multiple times and averaged, and XS is the inherent value when the system is assembled and placed in the mold to be tested, that is, the distance from the center of the equipment X0 to the center of the pattern ring X1 as shown in the figure. Relative to each measured point in the vertical direction, XS is unchanged during the entire measurement process, so the following formula can be obtained:
    
    After the measuring arm moves vertically to the vertical position of the measured point of the mold to be tested, the laser sensor head is translated to 80±0.5 mm from the measured point, and then the laser distance measurement value XL' and the horizontal grating ruler reading XR' of the measured point are read. The radius of the measured point satisfies:
    

    As long as the diameter of each measured point is measured point by point and calculated according to the above method, the roundness measurement can be completed.

    During the whole measurement process, there are certain requirements for the measurement points because the patterns of the mold must be avoided. Using ATmega16 to accurately control the operation of the stepper motor can find the point to be measured, and on this basis, a handwheel controller is added to fine-tune the stepper motor to find the appropriate position before collecting data, so that the manually set measurement points can be measured.

    2.3 Equipment travel control

Travel switches     are placed at the horizontal and vertical limit positions of the measuring device to ensure safe operation. In order to reduce costs, all signals through the measuring turntable are directly connected using cables instead of electric slip rings. Therefore, the device cannot always rotate in one direction when rotating, otherwise the cable will be broken. In this design, the rotation angle does not exceed 360°. In order to distinguish between 0° and 360° of rotation, two travel switches are placed side by side at the specified positions of the measuring turntable. The rotation position is determined by judging the order of the two travel switches, and then the direction of rotation is determined.

    3.  Single chip microcomputer design

    Main idea: The host computer controls the measurement steps of the motor and sends commands to make the stepper motor measure data correctly along the design idea. At some measurement points, manual intervention is required to fine-tune the motor by turning the handwheel. The travel switch can limit the position of mechanical rotation and can also be used for reset settings.

    3.1 Stepper Motor Drive

    The operation of the stepper motor requires a stepper motor driver to convert the pulse signal from the control system into the angular displacement of the stepper motor. The speed of the stepper motor is proportional to the frequency of the pulse signal , and the step angle is proportional to the number of pulses. When the stepper motor starts, there must be a speed-up and speed-down process, and the speed-up and speed-down design is crucial. If the design is not appropriate, it will cause problems such as stalling, loss of step, and slow speed-up and speed-down processes of the stepper motor. In order to achieve speed-up and speed-down, a step-type frequency change is used to simulate the frequency linear change process. As shown in Figure 4.

    The generation of stepper motor pulses is realized by timer 1 and timer 2. After receiving the command from the host computer, the ATmega16 controller first obtains the number of steps required to rotate, and then turns on the counting function of the corresponding timer according to the corresponding command. In the control program, timer 1 is used to control two pulse outputs, and timer 2 is used to control one pulse output, thereby completing the control of three motors.

    Timer 2 uses CTC mode and match interrupt is enabled. Through match interrupt, pulse can be output at OCR2 port, and the input pulse frequency can be changed by setting the value of register OCR2 . Timer 1 uses phase and frequency correction mode, sets the values ​​of ICR1 , OCR1A and OCR1B registers respectively under different serial port commands and turns on different match interrupts, so that match pulse can be output at OC1A and OC1B ports respectively. Its output frequency control is the same as that of timer 2, except that the count upper limit value ICR1 needs to be changed. Since the three motors do not move at the same time, only one timer is in the open state at a time, and the others need to be closed.

    During program operation, a global variable is set to save the number of steps of the motor. After receiving the command from the host computer, the control program first writes the number of steps into the on-chip EEPROM before executing. Since the EEPROM data is not lost when the power is off, the data can be retrieved from the EEPROM when the system is reset after power off and then the next operation can be executed.

    3.2 Handwheel fine-tuning control

    The main function of the handwheel is to fine-tune the device to find the appropriate measuring point. The electronic handwheel has two pulse outputs, and the phase difference between the two pulses determines the rotation direction of the handwheel. The handwheel is controlled by timer 0, using the counting mode. Timer 0 is set to CTC mode, rising edge trigger, OCR0 is 1, the initial count value is 0, and the interrupt is enabled. When the external rising edge triggers, the interrupt is triggered when the count reaches 1, and the corresponding pulse output is generated in the interrupt subroutine according to the handwheel state.

    In the handwheel interrupt subroutine, the counter is first automatically cleared to wait for the next handwheel pulse. Then the forward and reverse signals and the handwheel gear status are judged. ATmega16 generates a certain number of pulses to control the stepper motor through the delay method according to the status information. The handwheel status has three directions: Z, Y, and X, and three gears: X1, X10, and X100. If the handwheel is in the X gear, a certain number of pulses are output at the corresponding port (which is also the timer pulse output port). Changing the delay size can change the output frequency, but since the maximum frequency of the delayed output pulse is determined by the crystal oscillator , the output pulse is affected to a certain extent. The pulses generated by the delay method cannot accurately control the step angle of the stepper motor, but can be used for fine-tuning.

The forward and reverse signals of the handwheel are judged     by the D flip-flop . The handwheel pulse A is used as the CLK signal, the pulse B is used as the CP signal, and the reset terminal and the set terminal are connected to a high level. When the handwheel rotates forward, the phase difference between pulse A and pulse B is positive 90 degrees, and the D flip-flop outputs a high level; if the phase difference between the reverse pulse A and pulse B is negative 90 degrees, the output is a low level.

    3.3 Serial port module

    The host computer and the single-chip microcomputer mainly use UART asynchronous communication, and the transmission and reception are processed by bytes. When the single-chip microcomputer receives commands from the host computer, it uses the UART query method to realize data reception. Its communication format is: start word + control byte + step number + end word, and the data uses the international standard ASCII code format, as shown in Table 1.

    
    During the design process, the ICC AVR compiler is used to write the microcontroller control program, and the atoi function can be used to convert the ASCII code format step number into integer data.

    The communication between the MCU and the host computer mainly uses two bytes of data to send the handwheel status information and the travel switch status information. After receiving the query command, the MCU returns the two-byte status information to the host computer. Each time after executing the stepper motor command, it also needs to send status information to the host computer and return the execution result to the host computer. Its definition is shown in Table 2.

    4 Conclusion

    In this paper, a tire mold inner diameter measurement system is designed based on AVR single-chip microcomputer, which can receive information from electronic handwheel and host computer to control multiple stepper motors in time-sharing mode and control the grating ruler reading.

    After the system equipment was installed, a standard steel plate was selected as the test target, and several test points were selected in the vertical direction. The results showed that the measurement error was within 2μm. Then a radial tire mold was selected as the test target and compared with the manual measurement data. The measurement error was within 4μm, meeting the design requirements.