The suction cup device still has inconveniences such as inconvenience control, low efficiency and excessive energy consumption, which seriously hinders the practical application of the device. In this paper, the tomato picking robot vacuum chuck device with integrated vacuum generator as the core is designed, and its control strategy is determined according to the device performance test.

1 Tomato Picking Robot Vacuum Suction Device 1 1 Overall Structure The vacuum chuck device is a key part of the end effector of the tomato picking robot and consists of a vacuum system and an actuator.

The vacuum system is mainly composed of an integrated vacuum generator and a micro silent air compressor. The actuator is driven by a micro DC motor to drive the suction cup fixed to the front end of the rack through the rack and pinion drive. When the tomato picking robot is working, when the end effector reaches the predetermined position, the actuator drives the suction cup to advance, and the integrated vacuum generator uses the compressed air generated by the air compressor as a gas source to form a certain negative pressure between the suction cup and the fruit surface, so that The suction cup produces suction on the fruit, and after the fruit is sucked, it is driven back by the actuator to achieve separation of the target fruit from the adjacent fruit.

The vacuum pump unit replaces the conventional vacuum pump with an air compressor and a vacuum generator. The vacuum generator is a vacuum generating device that works according to the venturi principle. It is small in size and has no moving parts. It can be directly mounted on the end effector. The negative pressure generation and release speed is faster and easier to control, which better satisfies the tomato. The need for fruit picking.

The 186W (1/4hp) low-power air compressor is used as the air source of the vacuum generator. The air compressor is equipped with a pressure switch, which can effectively reduce the energy waste caused by the air compressor unloading.

The integrated vacuum generator has a built-in suction and blow solenoid valve, and a gas supply control solenoid valve is installed between the vacuum generator and the air compressor gas tank outlet. A digital vacuum pressure sensor is installed between the vacuum generator and the suction cup, and the vacuum pressure is fed back into the robot controller, and the feedback control of the vacuum system is realized through the gas supply solenoid valve, the suction solenoid valve and the air blowing solenoid valve. The integrated vacuum generator has a built-in check valve to ensure the reliability of the negative pressure and prevent the accidental falling off of the fruit during the suction and pulling process. At the same time, the check valve has the non-return characteristics of the check valve. It can maintain a certain negative pressure, which greatly saves the consumption of compressed air.

1 3 Control System The vacuum chuck control system is part of the picking robot control system built on the Pmac 2A PC / 104 multi-axis motion control card. The Epos position controllers of Pmac and Maxon motors have DSP digital signal processors that close the position loop, speed loop and Epos closed current loop through Pmac to form a composite servo control for more powerful and flexible motion control. The system receives the analog signal input of the vacuum pressure sensor through the 12-bit A/D converter on the Pm ac board, receives the digital signal input of the vacuum switch through the universal digital input/output interface, and supplies the air supply solenoid valve, the suction solenoid valve and the blow. The gas solenoid valve performs an output of an on/off control signal.

2 device performance test 2 1 vacuum generator supply pressure - negative pressure relationship test 2 1 1 test materials and methods start the air compressor to inflate the gas storage tank until the air compressor stops, close the air compressor, open the vacuum generator to inhale The solenoid valve, and then the gas supply solenoid valve is opened, and the corresponding change of the pressure control valve outlet pressure and the vacuum pressure sensor during the whole process of the compressed air exhaustion is recorded by the Sony T10 digital camera in the state of holding the fruit and the suction port respectively. And through the frame-by-frame playback of the video file, the relationship between the supply pressure and the negative pressure is determined.

2 1 2 Test results The results show that when the compressed air is about 400 kPa (gauge pressure), the negative pressure reaches - 89 4 kPa, and then the vacuum maintains a constant or even a slight decrease as the compressed air pressure increases.

At the same time, there is a significant pressure difference in the state in which the suction cup holds the fruit and is open. When the compressed air is 0 400 kPa, the pressure difference increases approximately linearly, while at 400 600 kPa, the pressure difference stabilizes at 28 33 kPa. This feature determines the reliable contact and suction of the fruit with the suction cup to determine the motor. The action, as well as judging the detachment of the fruit during the suction-pull process, ensures that the success rate of the device operation is critical.

2 2 suction cup pull-off force test 2 2 1 test materials and methods were tested in May 2009 at the Agricultural Equipment and Technology Laboratory of Jiangsu University. The test material is a semi-ripe tomato harvested from the vegetable base of Zhenjiang City. The average diameter of the tomato is 38 24mm. The tomato is suspended from the support by a string and connected to the HP-50 electronic dynamometer by a clamp, and the harvest robot is adjusted. The vacuum chuck and electronic dynamometer are on the same level as the fruit center. Carry out the following tests separately: (1) Pull-force-suction disc diameter relationship test Start the air compressor and open the suction valve of the vacuum generator until the vacuum pressure reaches a stable value; replace the 25-fold ripple of 20, 14, 9 mm installation Suction cup, start the motor, the rack drives the suction cup forward and sucks the fruit, pulls the fruit back at 2mm / s to return to disengage, the peak of the pull-off force is measured and recorded by the electronic dynamometer, and the test is repeated 30 times for each suction cup diameter. .

(2) Pull-negative-negative pressure relationship test starts the air compressor and opens the suction valve of the vacuum generator, and the air supply on/off solenoid valve controls the input of the compressed air of the vacuum generator. Install a 20 mm 2 5 fold corrugated suction cup and start the motor. When the rack drives the suction cup forward and sucks the fruit, pull the fruit back at 2 mm / s until it is released. Photographed by a Sony T10 digital camera, real-time recording and playback of the vacuum pressure sensor and electronic dynamometer at the moment the cup is released by frame-by-frame playback.

2 2 2 Test results and analysis The pull-out force of the suction cup is determined by the vacuum pressure and the effective area of ​​the suction. The theoretical formula is F 0 = |p |A e /1 000= |p | (D e /2)2 /1 The maximum pull-off force of F 0 in the formula, the effective area of ​​the suction of NA e, the effective diameter of the mm 2 D e suction cup, the relative pressure of mm p , the effective diameter of the kPa suction cup of different diameters and the theoretical and experimental values ​​of the pull-off force.

The test found that the larger the suction cup, the greater the pull-out force, and the actual pull-out force is slightly higher than the theoretical value. The average relative error of the test was within 2 5%, indicating good retention stability.

The pull-force-negative pressure relationship test results show that when it is at a lower vacuum (p> - 50 kPa), there is a good linear relationship between vacuum pressure and suction. F 0 = 0 198 4 |p |+ 0 943 0 The goodness R 2 is 0 993 3 and the fitted linear slope is very close to the theoretical straight line slope ( 0 201 0 ). However, at p > - 50 kPa, the actual pull-out force is slightly higher than the theoretical value, while at higher vacuum, the actual pull-off force is significantly lower than the theoretical value and the data is more discrete. Possible reason: Compared with the plane suction, the curved surface of the fruit increases the effective area A e of the actual pressure, so that the actual pull-out force is higher than the theoretical value at the lower negative pressure. The surface of the tomato is an irregular surface. When the pressure is high and the pressure is high, the problem of air leakage between the suction cup and the surface of the tomato begins to appear, which will cause the negative pressure on the edge of the suction cup to be significantly reduced, so that the actual pull-out force is lower than the theoretical value. At higher negative pressures and higher forces, the tensile deformation of the suction cup also causes the effective diameter D e of the suction cup to decrease and the air leakage at the edge, which affects the pull-out force.

2 3 vacuum sorption response time test 2 3 1 test materials and methods start the air compressor to fill the gas tank, first open the suction solenoid valve of the vacuum generator, then open the air supply solenoid valve, collected by Altai USB5935 data acquisition card Record the vacuum pressure sensor signal when the suction port is open.

2 3 2 Test results and analysis Generally, the time required for the suction port pressure to reach 63% of the final negative pressure is taken as the sorption response time of the vacuum generator.

When the vacuum generator suction valve is in the open state, the input of the compressed air is controlled by the air supply solenoid valve, and thus the suction response time includes the operation time of the air supply solenoid valve, the time when the compressed air flows through the pipeline, and the rise time of the negative pressure. Tests show that the suction response time of the gas supply solenoid valve is 107 ms, in which the supply solenoid valve operates and the compressed air flows through the pipeline for 25 ms, and the negative pressure rise time is 82 m s. The test results show that by controlling the compressed air The input can effectively meet the need to quickly generate negative pressure.

3 The control strategy of the vacuum chuck device is based on the basic test results, and the control strategy of the vacuum chuck device is determined. First, the air compressor is pre-inflated. After the robot transports the end effector to the picking position, the vision system detects the distance of the target fruit. The motor starts to advance the suction cup to the fruit, and opens the gas supply solenoid valve 8 mm away from the fruit. Negative pressure is generated; when the suction cup contacts and successfully holds the fruit, the negative pressure jumps above -80 kPa, the suction cup stops for 0 5 s; the gas supply solenoid valve is closed, the one-way valve is used to maintain the negative pressure, and the suction of the suction cup is maintained. The fruit is pulled back by the suction cup, and if the pressure sensor detects that the negative pressure drops too fast, the gas supply solenoid valve is opened again to supplement the negative pressure; until the predetermined position is reached, the suction cup stops moving, and the end effector performs subsequent clamping and separation. .

The results showed that despite the large difference in tomato fruit size, fruit stem diameter and length, the control strategy can ensure that the suction plate advances and retreats to 100 mm / s and 60 mm / s respectively, and pulls the fruit to achieve 35 mm horizontal displacement. 92% success rate. The average single operation time and air consumption are 15 s and 0 6 L, respectively, and the air compressor power can meet the needs of picking efficiency of 360/h.

However, during the test, it was found that due to the irregular surface of the fruit, there was a phenomenon that a few suckers failed to hold the fruit; when the fruit stem was short, the fruit stem was directly sucked and pulled; for the full-ripening tomato, Peel damage caused by suction may occur. The above issues need to continue to conduct in-depth research to improve and improve.

4 Conclusions (1) When the compressed air is 400 600 kPa, there is a stable pressure difference of 28 33 kPa at the suction port in the state of holding the fruit and opening, which can be used as a reliable criterion for suction and disengagement of the suction cup and the fruit. .

(2) The suction of the suction cup on the tomato fruit is positively correlated with the size of the suction cup and the negative pressure, but the actual suction force under the higher negative pressure will be significantly lower than the theoretical value. The excessively high negative pressure will greatly increase the energy consumption and the suction increase is limited.

(3) Control the input of compressed air through the air supply solenoid valve, the suction response time is only 107ms, which can realize the rapid generation of negative pressure. At the same time, the negative pressure can be maintained by the check valve, and the suction and pull can be satisfied with the extremely low air consumption. The need for fruit.

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