Improved Design Of Nondestructive Testing Device For Korla Pear Firmness Based On Acoustic Response Method

Korla pear is one of the characteristic fruits of Xinjiang,and it has a wide cultivated areas and production.Firmness is often used as one of the important indicators to detect the differences of pear internal quality and maturity.Early,PFT-001-type non-destructive detection device for pear firmness has disadvantages of poor stability of vibration response signal and poor anti-interference performance.Moreover,the calculation accuracy of sound propagation velocity for firmness detection is lower and exist the error of manual operation in the testing process.For this situation,the design of PFT-001-type non-destructive detection device for pear firmness would be improved to improve the detection performance of this device.The main contributions of this study are as follows:(1)The design of the signal excitation and sensing device was improved by using 35BYGH250-28-type model stepper motor and CBX0804 ball screw.They were applied to drive the scaffold on the platform and then further drive piezoelectric transducers to contact pear.The distance between the excitation and the sensing point was calculated by the step angle of stepper motor and the screw pitch.Its detection accuracy reached 0.01 mm.BT-47-type pressure sensor with the sensitivity of 0.00098 N was used to motor the contact force of cantilever beam of piezoelectric transducers.When the contact force reached between 0.1 and 0.24 N,the motor stopped.Its action was controlled by outputting signal with DSP core board,and was ensured the cantilever would not be overloaded give rise to inaccurate signal acquisition.Laser switch was applied to sense whether the sample waiting for testing or not,and the detection range was from 0 to 20 m.The installed height of piezoelectric transducers and pressure sensor from the slide was set as 70 mm and 90 mm,respectively.This work was to ensure the sensing end of cantilever beam contact with the equatorial part of the sample,and to sense acoustic signal easily.The reset position of slide,with a distance of 19 mm from the equatorial part of the minimum pear,took time not more than 4.75 s to move the slide.The results could meet high detection efficiency for pears with different size.(2)The design of hardware circuit of the signal acquisition and processing was improved.The outputted offset voltage was reduced by compound circuit mode of the high-voltage amplifier circuit with a peak-amplitude of 80 V.Two chips,OP37 and PA85,were applied as the first and second operational amplifier of the high-voltage amplifier circuit.Power supply circuit with a peak-amplitude of 90 V using LM317 regulator chip was used to supply for operational amplifier.The signal conditioning circuit transferred sensing high-voltage signal into low-voltage signal by using second-order active filter circuit.This work reduced amplitude of acoustic vibration signal and acquired signal easily.The limited amplitude circuit consisted of sliding resistance,voltage stabilizer and capacitance was applied to ensure that the outputted voltage of the conditioning circuit was stably set in the range of ± 10 V.AD conversion module inputted signal with a range of ± 10 V by using AD7656-1 chip.It had six sampling channels and met the requirement of bipolar signal acquisition and multi-channel signal acquisition.The results of simulated testing for improved circuit and the debug results for manufacture of PCB board showed that actual excitation voltage with a peak-amplitude of 79.4 V and outputted voltage of the signal conditioning circuit with a range of ± 5.7 V could meet design requirements of circuit.(3)The software of signal acquisition and processing system was developed by using C language and assembly language under a condition of CCS software.The system software consists of a main program and four processing subroutine.Four processing subroutine consists of interrupt task,stepper motor control,acoustic vibration signal of pear and display of touch screen.(4)PFT-002-type non-destructive detection device was used for measuring pear firmness under three noise conditions,35 dB,75 dB and 115 dB.The detection stability was better in 75 dB noise environments because of Strong anti-interference,but poor for 115 dB noise environments.The sensitivity of PFT-002-type detection device measuring for seventy pears was 50.79% and was similar to that of TA.XT Plus texture analysis.Compared with the detection accuracy of the M + P detection system,average of the detection error was 5.88.It indicated that PFT-002-type non-destructive detection device for pear firmness had a stronger detection capability.