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1 Development status of weight sorting machinery for fruits and vegetables China is a big producer of fruits and vegetables. However, China's fruit and vegetable market has long been an extensive economy. The international market price of fruits and vegetables products is low [1]. The important reason is that the post-harvest treatment of fruits and vegetables in China is backward and the appearance quality is low. Therefore, the classification of fruits and vegetables has been closely linked with the market competitiveness of fruits and vegetables in China. This paper introduces the design of a controller, which is the core part of the sorting equipment for fruits and vegetables according to their weight. This device makes use of weighing sensors to dynamically measure fruits and vegetables to automatically classify fruits and vegetables according to their weight.
2 Fruit and vegetable weight sorting machine assembly line First, we will give a brief introduction to the basic structure and weighing mechanism of weighing fruit and vegetable weight sorter. The fruit and vegetable weight sorter consists of an upper fruit segment, a sorting sensor segment, and a divided fruit segment.
2.1 The fruit segment on the upper fruit segment is responsible for smoothly placing waxed fruits and vegetables into the fruit cup, which is driven forward by a hinge to the sensing section. This section requires fruits and vegetables to enter the sensory separation section as far as possible. Keep the operation steady to reduce the mechanical vibration at the instant of fruiting.
2.2 Sensor section sorting The sensor section consists of two parallel, integrally cast aluminum rails. The four symmetrical press feet at the bottom of the cup are pressed against the surfaces of the two guide rails and are pulled forward by the hinge. The guide rail has a rectangular groove, and the cantilever beam pressure sensor is respectively fixed in the groove, a rectangular steel plate is horizontally arranged on the sensor, the length is equal to the length of the fruit cup, the surface treatment of the steel plate is extremely smooth, the steel plate and the cast aluminum surface are in the same plane, Keep a gap with the cast aluminum to avoid data errors due to contact with the surrounding cast aluminum when the steel plate is weighed. The gap between the steel plate and the cast aluminum should not be too large to avoid vibration when the fruit cup crosses the segment. This vibration is reflected on the output signal of the sensor as a very large oscillating voltage signal, which has a great influence on the weighing measurement of fruits and vegetables. . Cast aluminum rails are machined into two low-middle-high arches with very high finish. When the fruit cup is dragged to the sensing section, the cast aluminum slope becomes higher and higher as the fruit cup moves forward, and the weighing foot of the fruit cup is lifted by the smooth cast aluminum surface steel plate, the fruit cup and the fruit and vegetable. Gravity gradually disengages from the hinge. When the fruit cup slides to the gravity sensor part, the height of the fruit cup reaches the highest point, and the gravity of the fruit cup is completely separated from the hinge in a short time. The four weighing press feet of the fruit cup are all pressed on the steel plates of the two gravity sensors on both sides. The sensor uses the gravity of the fruit cup to quickly collect data from the hinge and quickly passes through the sensor. The CPU calculates the sum of the weights of fruits and vegetables placed on the fruit cup and the fruit cup.
2.3 The fruit segment of the outlet section of the fruit outlet is immediately followed by the fruit outlet section. It arranges several fruit outlets and fruit receiving slots on the side of the conveyor belt, and each outlet is designed with a solenoid valve that controls the output. When the fruits and vegetables that have been measured in the sensor section pass through the corresponding fruit outlets, the CPU will issue a control command in time, so that the solenoid valve of the fruit outlet will be actuated, and the fruits and vegetables will thus roll into the fruit receiving groove. The more fruit you export, the finer the sorting of fruits and vegetables can be.
3 Principles of the Controller 3.1 Selection of the weighing sensor The effect of the weight sorter on measuring the weight of fruits and vegetables depends on the selection and installation of the weighing sensor. This design selects the cantilever-type load cell, which is a sensor designed using the resistance strain principle. When gravity acts on the end of the aluminum cantilever beam in the vertical direction, tensile strain is generated on the upper surface of the beam, and compressive strain is generated on the lower surface. The magnitude of the strain on the upper and lower surfaces is equal and opposite.
The strain gauges attached to the upper and lower surfaces are also stretched and shortened. A positive and negative phase change in the resistance value is obtained, and the circuit for converting the resistance value into a voltage signal is a Wheatstone bridge. Because the temperature environment of the four resistors is the same, when the ambient temperature drifts, the drift values ​​cancel each other, and the influence of the temperature change can be suppressed [2].
The bridge arm resistance consists of four strain resistors R1 to R4. USL is the excitation voltage. When the four resistors are balanced, the output of the USC is zero. When the strain resistance of the cantilever beam is unbalanced, the USC output linearly reflects the resistance deformation. Measuring the change in this voltage results in the weight of the measured object [3].
The sensor sensitivity (D) that we choose is 2±10%mV/V; recommended excitation voltage: 5V～15V (DC/AC)
Select the excitation voltage is 10V, the relationship between the excitation voltage and sensitivity can be calculated by the sensor output voltage signal should be: VOUT = VIN × D = 10V × 2mV / V = ​​20mV
This signal is relatively weak and cannot be used directly for A/D conversion. Therefore, add the signal amplifier before entering the A/D conversion. The wide range of rated capacity can meet the needs of different types of fruits and vegetables for weight sorters. Usually, the weight sorter for sorting large volumes of fruits and vegetables such as grapefruit and watermelon can be capped, and oranges, apples and the like are generally sorted. The medium size fruit and vegetable weight sorter uses 5kg capacity. Since the signal that is dynamically sensed by fruits and vegetables is an oscillating wave with a relatively large amplitude, the upper limit of this indicator should be chosen to be at least twice the upper limit of the stated fruit and vegetable in order to prevent the measured waveform from having a cutoff distortion.
3.2 Controller Block Diagram The controller includes the control board and the solenoid valve driver board in two parts. The core of the control board is a C8051F020 type CPU, in addition to a memory, A/D converter, signal amplification circuit, and opto-isolation circuit. The driver board is a spike absorption circuit of 89C51 type CPU[4], 16-channel signal power amplifier circuit and 16-way anti-magnetic valve recoil voltage. The main CPU uses the internal RAM as the data buffer and the internal FLASH as the program storage area. Because there are many intermediate variables in the measurement and calculation process that require fast read and write, and need to be retained after the power is turned off, an additional 32kB of FM18L08 ferroelectric is added. Memory. The memory address lines A0-A14 are connected to the CPU's P60-P67 and P50-P56, and the data lines D0-D7 occupy the CPU's P70-P77. The digital signal output by the optical counting sensor is connected to the CPU's P40 I/O line through an optical isolation circuit. Used to determine the exact timing for each cup to enter the weighing section.
The load cell signals of the two left and right tracks are first passed through a balanced hybrid circuit, and the weights of the left and right rails are added and synthesized. At the same time, the weak voltage signal of the millivolt level is amplified to 0V-5V. The amplified signal is an approximately damped oscillation signal with a large oscillation amplitude.
3.3 Signal Analysis and Processing We consider this signal as a periodic oscillating signal with an envelope. According to the frequency domain analysis principle of the signal, it can be known that this signal contains the oscillating carrier frequency component, including the frequency component and the DC component. The weight of fruit cups and fruits and vegetables is actually the DC component of this signal, and it is necessary to try to extract this component and then measure the value of that component. Figure 3 The method of analyzing the frequency components at a high speed through the sensor waveform is to send the digitized sensor signal to the DSP to perform FFT processing on the data and send the processed calculation result to the CPU. This can avoid the CPU operation speed. Not enough to reduce the speed of sorting. However, such an approach will increase the hardware complexity and cost of the controller and reduce the reliability of the system. If you rely on the CPU to complete the data processing, the difficulty lies in the calculation speed problem, because the weight sorter at the same time processing a fruit and vegetable, the next fruit and vegetables will arrive within 50ms, and at the same time to deal with communication, calculate the position and control of each fruit and vegetable Solenoid valve work.
In this project, a combination of analog and digital processing is used. First, the AC components caused by the oscillating carrier wave, high-frequency signal components, and uncertain vibration interference are filtered out with an analog second-order low-pass filter, and then the analog signal is digitized. , To the CPU for digital processing, this approach makes the data into the CPU greatly simplified, thus significantly reducing the pressure on the CPU resources.
Since each gap or joint on the assembly line mechanically generates a certain degree of mechanical vibration, these different parts and vibrations of different materials are reflected on the load cell as a superposition of various carrier bodies (Figure 3). It is difficult for the filter circuit to directly process the DC component. Based on this, the low-pass filtered signal is sent to the A/D converter, converted to a 16-bit digital signal [5], and sent to a 16-bit data bus of the CPU. After the low-pass filtered signal is digitized and sent to the CPU, the first step is to perform a fast Fourier transform (FFT) on the discrete signal, take out the zero-frequency amplitude in the frequency domain, and calculate the weight of the fruits and vegetables. Since the signal spectrum components passing through the second-order low-pass filter are much simpler, the collection point can be greatly reduced when performing FFT, and the conversion calculation amount is much smaller, making it possible to use this CPU for FFT. 3.4 Fruit control Because the solenoid valve drive involves a large current and high power signal, in order to prevent the resulting interference, therefore this part of the circuit and the main controller are not common ground, the optical isolation is used to reduce the electromagnetic effect between them. To the lowest. The drive circuit is designed with a spike absorption circuit for the kickback voltage at the output.
3.5 Communication system with the console After the weighing signal processing is completed, the measurement result is notified to the console in real time through serial communication. The console PC displays the progress and status statistics of the processed fruits and vegetables in the production line in real time. The CPU's two-bit serial communication line uses the ADM202E level-shifting chip to convert the TTL logic signal to an RS232 interface signal of ±12V. The serial communication port occupies the CPU's P0.0 and P0.1. The serial port implements communication between the main control board and the console system.
3.6 System Workflow After the device is started, the main CPU starts working after power-on reset, and the pipeline runs. The system first measured the number of empty cups in the first empty cup and the weight of each cup. After the fruits and vegetables are loaded, the CPU calculates the weight at the fastest speed, and then subtracts the weight of the fruit cup to obtain the actual weight of the fruits and vegetables. The true value of the weight is transmitted to the main console system machine through the serial communication channel. At the same time, the CPU uses the fruit and vegetable grading setting table of the main console to find out the fruit port number of the fruit and vegetable, set the timer time, start the timer, and send out the logic logic signal of the logic valve of the corresponding solenoid valve immediately after the arrival time of the conveyor belt. , The corresponding solenoid valve is amplified and driven to make the fruit cup turn over and the fruits and vegetables fall in the corresponding fruit outlet. In the same period of time, the CPU is not only dealing with the weighing, grading calculation and fruit control of this head fruit and vegetable, but also processing the flow of up to 16 fruits and vegetables. This real-time requirement creates a lot of pressure on the CPU hardware and software.
4 Conclusion The fruit and vegetable weight sorter controller designed based on the above principles has successfully achieved effective fruit and vegetable sorting in the fruit and vegetable sorting industry. The speed of sorting oranges can reach 20 heads/s, and the sorting weight error is less than ±3g. The product has been successfully implemented.
Although there are many kinds of fruit and vegetable weight sorter products, it is still the direction of the weight sorter design to increase sorting speed, sorting accuracy, reduce costs, and enhance the compatibility of sorting of various types of fruits and vegetables. With the development of embedded computer technology and measurement technology, weight sorters will continue to produce new products with better performance.
ã€Abstract】This paper introduces the assembly line structure of weighing fruit and vegetable weight sorter, and briefly describes the principle of the cantilever beam weighing sensor used in the weight sorter and the weight sensor selected by the weight sorter. main indicators. The working principle of a weighted fruit and vegetable weight sorter controller designed by the author was elaborated in detail, and some details were discussed about the implementation of the fruit machine and the difficulties in the use of hardware and software resources in the data processing process were pointed out. .