A method and device for synchronously controlling multiple hydraulic cylinders of a thermoforming machine

Abstract

This invention discloses a method and device for synchronous control of multiple hydraulic cylinders in a thermoforming machine, belonging to the field of hydraulic transmission control technology. Addressing the problem that the synchronous control accuracy of multiple hydraulic cylinders in a thermoforming machine is affected by uncertain factors such as the state of the hydraulic cylinders and the installation accuracy of the slide block, leading to uneven pressure on the mold and affecting product quality, a synchronous position-pressure dual-loop closed-loop nested control method is adopted. While controlling the resultant force of the ejection force of multiple independent cylinders, the position of each cylinder is kept synchronized, thereby ensuring uniform and controllable pressure on the mold and effectively guaranteeing product quality.

Description

Technical Field

[0001] This invention relates to the fields of thermoforming and hydraulic transmission technology, and specifically to a method and device for synchronous control of multiple hydraulic cylinders in a thermoforming machine. Background Technology

[0002] With the continuous development of high-tech fields such as aerospace and automotive, higher demands are being placed on the performance, structure, reliability, and efficiency of components. This has led to the development of components towards larger size, integration, more complex shapes, thinner walls, and extreme combinations of large and small geometric dimensions. These components are often used under extreme working conditions. Difficult-to-deform materials (high-strength aluminum alloys, titanium alloys, and high-temperature alloys) have seen rapid development in the aerospace and automotive fields due to their high specific strength, high pressure resistance, and corrosion resistance. However, their low elongation and poor formability necessitate the development of new precision forming technologies, namely hot forming technology.

[0003] Thermoforming involves placing a mold and raw material inside a thermoforming machine and then heating them together. The process begins by installing the mold into the thermoforming machine, heating the mold, and then, once the mold reaches a predetermined temperature, placing the part to be processed into the mold and performing hot pressing.

[0004] Traditional hydraulic presses typically use a fixed displacement pump and a proportional pump to power the main oil circuit, enabling the slide to feed and retract. Pressure is regulated by their respective proportional relief valves. This structure works well for hydraulic presses with relatively low pressure and speed precision requirements. Hot forming processes, however, demand higher pressure precision. The final pressure applied during forming must be controlled within ±2%, and the speed should be kept as stable as possible while maintaining stable pressure during the stretching process. This necessitates improvements to the traditional design to enhance the stability of pressure and speed during the forging process.

[0005] To meet the demands of high-pressure, large-workpiece thermoforming processes, a multi-cylinder hydraulic press-driven slide block is preferred. However, the synchronous control accuracy of the multi-cylinder hydraulic cylinders in the thermoforming machine slide block is affected by uncertain factors such as the state of the hydraulic cylinders and the installation accuracy of the slide block, resulting in uneven pressure on the mold and affecting product quality. To address this issue, a high-precision multi-cylinder synchronous control method needs to be developed.

[0006] Existing multi-cylinder synchronous control generally employs two methods. One method involves pre-setting the cylinder's motion trajectory, comparing the real-time movement information of the cylinders with this trajectory, and controlling the cylinder movement based on position difference feedback. The problem with this method is that there is no position comparison between the cylinders, making it impossible to guarantee consistency among them. The other method uses a master-slave cylinder control approach, comparing the position difference between the slave and master cylinders to drive the slave cylinders to follow the master cylinder's movement. The problem with this method is that if the master cylinder's movement stability is poor, it is difficult to guarantee the synchronization of the cylinders. Summary of the Invention

[0007] The technical problem solved by this application is to overcome the shortcomings of the prior art and provide a control method and device for synchronous operation of multiple hydraulic cylinders in thermoforming. This method adopts a synchronous position-pressure dual-loop closed-loop nested control mode. While controlling the resultant force of the ejection force of multiple independent hydraulic cylinders, it maintains the synchronous position of each hydraulic cylinder, thereby ensuring uniform and controllable pressure on the mold and effectively guaranteeing product quality.

[0008] The thermoforming machine's pressing process and the related actions of the hydraulic cylinders are divided into three stages: the fast descent stage, the slow descent stage, and the mold closing stage. In the fast descent stage, the slider moves away from the mold, and the hydraulic cylinder pushes the slider closer to the mold at a relatively high speed. In the slow descent stage, the slider is closer to the mold, and the hydraulic cylinder approaches the mold at a very slow speed until it contacts the mold. In the mold closing stage, the slider and mold are in close contact, and the hydraulic cylinder maintains a fixed output pressure. In the control system, the transition from the fast to the slow descent stage is determined by the fast / slow descent transition point, and the transition from the slow descent stage to the mold closing stage is determined by the pressure sensor exceeding a pressure threshold. The synchronization requirements of the hydraulic cylinders are very high in the slow descent and mold closing stages. The thermoforming multi-hydraulic cylinder control method and device of this application are mainly applied to the slow descent and mold closing stages, but can also be applied in the fast descent stage where the synchronization requirements of the hydraulic cylinders are not high.

[0009] A method for synchronous control of multiple hydraulic cylinders in a thermoforming machine includes:

[0010] S1: Each hydraulic cylinder can be individually controlled by a closed-loop position control system to obtain the pressure and position values ​​of the ejector rod of each hydraulic cylinder, obtain multiple sets of pressure and position values, and the period for obtaining the pressure and position values ​​of the ejector rod of each hydraulic cylinder is the detection period;

[0011] S2: Calculate the combined pressure and average position values ​​of multiple cylinders based on multiple pressure and position values;

[0012] S3: Set a virtual hydraulic cylinder, convert the ideal speed in the thermoforming process into a time-position value, and set the time-position value as the target position of the virtual hydraulic cylinder. Set the pressure threshold and target pressure value of the virtual hydraulic cylinder, with the pressure threshold being less than the target pressure value.

[0013] S4: Perform position-pressure dual closed-loop control on the virtual cylinder and multiple cylinders. When the resultant pressure value is less than the pressure threshold, position priority is given (position priority means that the target position of the virtual cylinder is determined according to the ideal speed in the thermoforming process). The position-pressure dual closed-loop control includes controlling the extension or retraction speed of the ejector rod of each cylinder according to the target position, so that the position value of each detection cycle approaches the average position value, so that the average position value approaches the target position.

[0014] S5: Until the resultant pressure value is greater than the pressure threshold, pressure priority is applied (pressure priority means that the target position of the virtual cylinder is determined according to the relationship between the resultant pressure value and the target pressure value. If the resultant pressure value is less than the target pressure value, the target position of the virtual cylinder increases in the next moment. If the resultant pressure value is greater than the target pressure value, the target position of the virtual cylinder decreases in the next moment). The position-pressure dual closed-loop control includes continuing step S4 until the resultant pressure value reaches the target pressure value. The cylinder enters the pressure holding stage. Under the premise that the resultant pressure value is equal to the target pressure value, the position of each cylinder is dynamically adjusted so that each cylinder approaches the synchronous position.

[0015] In step S3, the stroke limit position of the virtual cylinder is set to the limit position of the actual cylinder.

[0016] In step S3, the maximum speed of the virtual cylinder is set to the maximum speed during the slow-down phase of the thermoforming process.

[0017] In step S3, the extension direction of the cylinder ejector rod is vertically downward, and the target position of the virtual cylinder is the lowest position value among the cylinder ejector rods in step S1.

[0018] In step S3, the target pressure value is the pressure value of the thermoforming mold.

[0019] In step S2, the sum of the pressure values ​​of each cylinder's ejector rod is the resultant pressure value, and the average value of the position values ​​of each cylinder's ejector rod is the average position value.

[0020] In step S4, based on the target position, the extension or retraction speed of the ejector rod of each hydraulic cylinder is controlled so that the position value of each detection cycle approaches the average position value, and so that the average position value approaches the target position. This includes...

[0021] S41: In each detection cycle, if the obtained position value is different from the target position at that moment, add an acceleration or deceleration to the current moving speed of the cylinder ejector rod to obtain the updated moving speed, and update the target position of the virtual cylinder. The updated target position = (detection cycle × ideal speed) + the position value obtained in this detection cycle.

[0022] S42: In the next detection cycle, the new position value obtained will be compared with the updated target position;

[0023] S43: If the new position value matches the updated target position, continue to extend while maintaining the updated movement speed; otherwise, repeat steps S41-S42 until the new position value matches the updated target position.

[0024] The above settings ensure that the ejector rods of multiple hydraulic cylinders can extend at a stable and near-ideal speed, guaranteeing uniform and controllable pressure on the mold and effectively ensuring product quality.

[0025] In step S5, the hydraulic cylinders enter the pressure-holding stage, and the positions of each hydraulic cylinder are dynamically adjusted to bring them closer to position synchronization.

[0026] S51: The target position is the average position of multiple cylinders, with the position of the virtual cylinder as the average position.

[0027] S52: Based on the target position, control the extension or retraction of the ejector rod of each cylinder to make the position value of each detection cycle approach the average position value, so that the average position value approaches the target position.

[0028] S53: In each detection cycle, the resultant pressure value of the hydraulic cylinder is detected. If the resultant pressure value is greater than the target pressure value, the position value of the virtual hydraulic cylinder is reduced in the next cycle (the amount of reduction of the position value of the virtual hydraulic cylinder is determined according to the empirical value) so that the upper mold has an upward demolding tendency to reduce the resultant pressure value. Conversely, the position value of the virtual hydraulic cylinder is increased so that the upper mold has a downward mold closing tendency to increase the resultant pressure value, and finally the resultant pressure value approaches the target pressure value.

[0029] S51-S53 are repeated in each detection cycle during the pressure holding process.

[0030] A multi-hydraulic cylinder synchronous control device for a thermoforming machine includes multiple cylinders. Each cylinder is connected to a displacement sensor for acquiring cylinder position values ​​and a pressure sensor for acquiring cylinder ejector rod pressure values. Each cylinder has an independent proportional relief valve for controlling the extension or retraction speed of the ejector rod. The displacement sensors, pressure sensors, and proportional relief valves of the multiple cylinders are all connected to a controller. The controller includes a virtual cylinder module, a calculation module, and multiple control modules.

[0031] The virtual hydraulic cylinder module is used to convert speed changes during the thermoforming process into time-position values, and to set the time-position values ​​as the real-time target position;

[0032] The virtual cylinder module is used to convert speed changes during the thermoforming process into time-position values, and to set the time-position values ​​as the real-time target position. It also sets the pressure threshold and target pressure value of the virtual cylinder, with the pressure threshold being less than the target pressure value.

[0033] The calculation module is used to calculate the resultant pressure value and average position value of multiple cylinders based on the pressure value and position value of the ejector rod of each cylinder, and then send the resultant pressure value and average position value to the control module.

[0034] Each control module independently controls one hydraulic cylinder. Based on the target position, pressure threshold, target pressure value, and the combined pressure and position values ​​of multiple hydraulic cylinders, it controls the extension or retraction speed of the ejector rod of each hydraulic cylinder by adjusting the proportional relief valve. When the combined pressure value is lower than the pressure threshold, position control takes priority, making the position value of each hydraulic cylinder approach the target position. When the combined pressure value is higher than the pressure threshold, pressure control takes priority, and the combined pressure value reaches the target pressure value, keeping the position values ​​of the ejector rods of different hydraulic cylinders consistent.

[0035] In summary, this application includes at least the following beneficial technical effects:

[0036] (1) The present invention adopts a multi-cylinder real-time dynamic adaptive adjustment method. Even if the hydraulic cylinders age and the stiffness weakens or the guidance is poor, this method can still ensure a certain degree of synchronization of the multi-cylinder hydraulic platform and improve the stress condition of the hydraulic press so as to prevent it from being damaged.

[0037] (2) The multi-cylinder hydraulic system of the present invention does not require separate configuration of leveling cylinders or other synchronization mechanisms, making the system structure simpler and more reliable, and is particularly suitable for large-table thermoforming machines.

[0038] (3) The present invention uses a high-frequency response controller combined with an optimization algorithm to compensate for the dynamic performance of the hydraulic cylinder, which can achieve high control accuracy. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of a synchronous position-pressure dual-loop closed-loop nested structure.

[0040] Figure 2 Electrical block diagram of a four-cylinder synchronous system.

[0041] Figure 3 Define a single hydraulic cylinder.

[0042] Figure 4 Define the external loop.

[0043] Figure 5 Define a virtual hydraulic cylinder.

[0044] Figure 6 Set the target position of the virtual hydraulic cylinder.

[0045] Figure 7 Set the virtual cylinder speed.

[0046] Figure 8 Set the target location for the internal loop, with priority given to the location of the external loop.

[0047] Figure 9 External loop position closed-loop operation takes priority.

[0048] Figure 10 External circuit pressure closed-loop operation takes priority. Detailed Implementation

[0049] The term “exemplary” as used herein means “serving as an example, embodiment, or illustration.” Any embodiment illustrated herein as “exemplary” is not necessarily to be construed as superior to or better than other embodiments. Although various aspects of embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless specifically indicated otherwise.

[0050] This embodiment discloses a multi-hydraulic cylinder control method for thermoforming. The method is described in detail below:

[0051] A displacement sensor is installed on the ejector rod of each hydraulic cylinder to collect the cylinder position information.

[0052] Pressure sensors are installed in the rodless and rod chambers of each hydraulic cylinder to collect cylinder pressure information; alternatively, pressure sensors can also be installed on the cylinder ejector rod.

[0053] Each cylinder has an independent proportional overflow solenoid valve that controls the speed at which the ejector rod extends or retracts.

[0054] The electrical signals from the displacement and pressure sensors are introduced into the controller. The controller can be a general-purpose programmable logic controller or a dedicated controller, with a response time of less than 1ms. After calculation, the controller controls the opening and closing degree of each proportional relief valve to control the operating speed of the hydraulic cylinder, achieving multi-cylinder synchronization.

[0055] Control process such as Figure 2 As shown, taking the complete fast-fall stage, slow-fall stage, and mold-closing stage as an example, the specific description is as follows:

[0056] Step 1: Define each hydraulic cylinder as a position closed-loop control, and these closed-loop controls form an internal loop in parallel.

[0057] Step 2: Define an external loop closed-loop control, which is a position-pressure dual closed-loop control.

[0058] Step 3: Define a virtual hydraulic cylinder, which is controlled by the external loop described in Step 2. The stroke limit position of the virtual hydraulic cylinder is set to the limit position of the actual hydraulic cylinder.

[0059] Step 4: Calculate the resultant pressure value of each hydraulic cylinder as the pressure feedback value of the external circuit. The sum of the pressure values ​​of the ejector rods of each hydraulic cylinder is the resultant pressure value.

[0060] Step 5: Calculate the average position of each hydraulic cylinder as the position feedback value of the external circuit. The average position value of the ejector rod of each hydraulic cylinder is the average position value.

[0061] When the hydraulic cylinder reaches the fast / slow down transition position, and switches to slow down mode:

[0062] Step 6: Set up a virtual hydraulic cylinder, convert the ideal speed in the thermoforming process into a time-position value, and set the time-position value as the target position of the virtual hydraulic cylinder. Set the pressure threshold and target pressure value of the virtual hydraulic cylinder. The pressure threshold is less than the target pressure value, and the target pressure value is the mold pressure value.

[0063] The final target position of the virtual hydraulic cylinder is set to the lowest position value among all cylinder positions.

[0064] Step 7: Set the maximum speed of the virtual hydraulic cylinder to the slow descent speed of the hydraulic cylinder.

[0065] Step 8: Set the position of the virtual cylinder to the target position of each cylinder.

[0066] Step 9: When the pressure feedback value is less than the pressure threshold, position priority is given. Set the position-pressure dual closed-loop control to position priority mode.

[0067] Step 10, the position-pressure dual closed-loop control in position priority mode includes:

[0068] S101: In each detection cycle, if the obtained position value is different from the target position at that moment, add a positive or negative acceleration (the positive or negative acceleration is determined according to the empirical value) to the current moving speed of the cylinder ejector rod to obtain the updated moving speed, and update the target position of the virtual cylinder. The updated target position = (detection cycle × ideal speed) + the position value obtained in this detection cycle.

[0069] S102: In the next detection cycle, the new position value obtained will be compared with the updated target position;

[0070] S103: If the new position value is consistent with the updated target position, then maintain the updated moving speed and continue to extend; otherwise, repeat steps S41-S42 until the new position value is consistent with the updated target position.

[0071] Step 11: Repeat step 10 until the pressure feedback value is greater than the pressure threshold.

[0072] Step 12: Set the position-pressure dual closed-loop control to pressure priority mode. At this time, until the preset pressure value is reached and the pressure holding stage is entered, the positions of each hydraulic cylinder remain synchronized while ensuring the pressing pressure.

[0073] Specifically, it includes:

[0074] S121: The target position is the average position of multiple cylinders, with the position of the virtual cylinder as the average position of the virtual cylinder.

[0075] S122: Based on the target position, control the extension or retraction of the ejector rod of each cylinder to make the position value of each detection cycle approach the average position value, so that the average position value approaches the target position.

[0076] S123: In each detection cycle, the resultant pressure value of the hydraulic cylinder is detected. If the resultant pressure value is greater than the target pressure value, the position value of the virtual hydraulic cylinder is reduced in the next cycle to make the upper mold tend to demold upward, thereby reducing the resultant pressure value. Conversely, the position value of the virtual hydraulic cylinder is increased to make the upper mold tend to close downward, thereby increasing the resultant pressure value. Finally, the resultant pressure value is made closer to the target pressure value.

[0077] S121-S123 is repeated in each detection cycle during the pressure holding process.

[0078] The electrical principle of a typical four-cylinder thermoforming machine is as follows: Figure 2 As shown, a displacement sensor is installed on the ejector rod of each hydraulic cylinder; pressure sensors are installed in both the rodless and rod-side chambers of each cylinder; and each cylinder has an independent proportional relief solenoid valve to control the extension or retraction speed of the ejector rod. The electrical signals from the displacement and pressure sensors are fed into the controller. The controller controls the opening and closing degree of each proportional relief valve to control the operating speed of the hydraulic cylinders, achieving synchronization of multiple cylinders.

[0079] The following section uses the DELTA RMC150 controller as an example to elaborate on the synchronous position-pressure dual-loop closed-loop nested control method.

[0080] like Figure 3 As shown, each hydraulic cylinder (Axis0-3) is defined as a position closed-loop control.

[0081] like Figure 4 As shown, an external loop closed-loop control (Outer4) is defined, which is a position-pressure dual closed-loop control.

[0082] like Figure 5 As shown, a virtual cylinder (Ref5) is defined, and the limit position of the virtual cylinder's stroke is set to the limit position of the actual cylinder.

[0083] The resultant pressure value of each hydraulic cylinder is calculated as the pressure feedback value of the external circuit; the average position value of each hydraulic cylinder is calculated as the position feedback value of the external circuit.

[0084] like Figure 6 As shown, the lowest position among all cylinder positions is set as the initial value of the virtual cylinder target position.

[0085] like Figure 7 As shown, the maximum speed of each virtual hydraulic cylinder is set to the slow-down speed of the hydraulic cylinder.

[0086] like Figure 8 As shown, the position of the virtual cylinder is set as the target position of each cylinder, and the external closed loop is set to position priority mode.

[0087] like Figure 9 As shown, the external loop receives the position feedback value and assigns a new target speed to the virtual cylinder through calculation until the pressure feedback value is greater than the pressure threshold.

[0088] like Figure 10 As shown, the external closed-loop circuit is set to pressure priority mode, and the target pressure value is the mold pressure value. At this time, until the preset pressure value is reached and the pressure holding stage is entered, the hydraulic cylinders keep their positions synchronized while ensuring the pressing pressure.

[0089] A multi-hydraulic cylinder synchronous control device for a thermoforming machine includes multiple hydraulic cylinders. Each cylinder has a displacement sensor connected to its ejector rod for acquiring cylinder position values. Pressure sensors are installed in the rodless and rod-side chambers of each cylinder to collect cylinder pressure information. The pressure difference between the rodless and rod-side chambers is the ejector rod pressure value. Optionally, a pressure sensor can also be installed on the ejector rod to acquire the ejector rod pressure value. Each cylinder has an independent proportional relief valve for controlling the extension or retraction speed of the ejector rod. The displacement sensors, pressure sensors, and proportional relief valves of the multiple cylinders are all connected to a controller, which includes a virtual cylinder module, a calculation module, and multiple control modules.

[0090] The virtual cylinder module is used to convert speed changes during the thermoforming process into time-position values, and to set the time-position values ​​as the real-time target position. It also sets the pressure threshold and target pressure value of the virtual cylinder, with the pressure threshold being less than the target pressure value.

[0091] The calculation module is used to calculate the resultant pressure value and average position value of multiple cylinders based on the pressure value and position value of the ejector rod of each cylinder, and then send the resultant pressure value and average position value to the control module.

[0092] Each control module independently controls one hydraulic cylinder. Based on the target position, pressure threshold, target pressure value, and the combined pressure and position values ​​of multiple hydraulic cylinders, it controls the extension or retraction speed of the ejector rod of each hydraulic cylinder by adjusting the proportional relief valve. When the combined pressure value is lower than the pressure threshold, position control takes priority, making the position value of each hydraulic cylinder approach the target position. When the combined pressure value is higher than the pressure threshold, pressure control takes priority, and the combined pressure value reaches the target pressure value, keeping the position values ​​of the ejector rods of different hydraulic cylinders consistent.

[0093] The present application has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present application. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and implementation methods of the present application without departing from the spirit and scope of the present application, and all such modifications and improvements fall within the scope of the present application. The scope of protection of the present application is determined by the appended claims.

[0094] The contents not described in detail in this application specification are common knowledge to those skilled in the art.

Claims

1. A method for synchronous control of multiple hydraulic cylinders in a thermoforming machine, characterized in that, include: S1: Each hydraulic cylinder can be individually controlled by a closed-loop position control system to obtain the pressure and position values ​​of the ejector rod of each hydraulic cylinder, obtain multiple sets of pressure and position values, and the period for obtaining the pressure and position values ​​of the ejector rod of each hydraulic cylinder is the detection period; S2: Calculate the combined pressure and average position values ​​of multiple cylinders based on multiple pressure and position values; S3: Set a virtual hydraulic cylinder, convert the ideal speed in the thermoforming process into a time-position value, and set the time-position value as the target position of the virtual hydraulic cylinder. Set the pressure threshold and target pressure value of the virtual hydraulic cylinder, with the pressure threshold being less than the target pressure value. S4: Perform position-pressure dual closed-loop control on virtual cylinders and multiple cylinders. When the resultant pressure value is less than the pressure threshold, position takes priority. The position-pressure dual closed-loop control includes controlling the extension or retraction speed of the ejector rod of each cylinder according to the target position, so that the position value of each detection cycle approaches the average position value, so that the average position value approaches the target position. S5: Until the combined pressure value is greater than the pressure threshold, pressure takes priority. The position-pressure dual closed-loop control includes continuing step S4 until the combined pressure value reaches the target pressure value. The cylinder enters the pressure holding stage. Under the premise of ensuring that the combined pressure value is equal to the target pressure value, the position of each cylinder is dynamically adjusted so that each cylinder approaches the synchronous position.

2. The method for synchronous control of multiple hydraulic cylinders in a thermoforming machine according to claim 1, characterized in that: In step S3, the stroke limit position of the virtual cylinder is set to the limit position of the actual cylinder.

3. The method for synchronous control of multiple hydraulic cylinders in a thermoforming machine according to claim 1, characterized in that: In step S3, the maximum speed of the virtual cylinder is set to the maximum speed during the slow-down phase of the thermoforming process.

4. The method for synchronous control of multiple hydraulic cylinders in a thermoforming machine according to claim 1, characterized in that: In step S3, the extension direction of the cylinder push rod is vertically downward, and the final target position of the virtual cylinder is the position of the virtual cylinder when the resultant pressure value is equal to the target pressure value.

5. The method for synchronous control of multiple hydraulic cylinders in a thermoforming machine according to claim 1, characterized in that: In step S3, the target pressure value is the pressure value of the thermoforming mold.

6. The method for synchronous control of multiple hydraulic cylinders in a thermoforming machine according to claim 1, characterized in that: In step S2, the sum of the pressure values ​​of each cylinder's ejector rod is the resultant pressure value, and the average value of the position values ​​of each cylinder's ejector rod is the average position value.

7. The method for synchronous control of multiple hydraulic cylinders in a thermoforming machine according to claim 1, characterized in that: In step S4, based on the target position, the extension or retraction speed of the ejector rod of each cylinder is controlled so that the position value of each detection cycle approaches the average position value, and so that the average position value approaches the target position, including... S41: In each detection cycle, if the obtained position value is different from the target position at that moment, add an acceleration or deceleration to the current moving speed of the cylinder ejector rod to obtain the updated moving speed, and update the target position of the virtual cylinder. The updated target position = (detection cycle × ideal speed) + the position value obtained in this detection cycle. S42: In the next detection cycle, the new position value obtained will be compared with the updated target position; S43: If the new position value matches the updated target position, continue to extend while maintaining the updated movement speed; otherwise, repeat steps S41-S42 until the new position value matches the updated target position.

8. The method for synchronous control of multiple hydraulic cylinders in a thermoforming machine according to claim 1, characterized in that: In step S5, the hydraulic cylinders enter the pressure-holding stage. Under the premise that the resultant pressure value equals the target pressure value, the positions of each hydraulic cylinder are dynamically adjusted to bring them closer to position synchronization. This includes... S51: The target position is the average position of multiple cylinders, with the position of the virtual cylinder as the average position. S52: Based on the target position, control the extension or retraction of the ejector rod of each cylinder to make the position value of each detection cycle approach the average position value, so that the average position value approaches the target position. S53: In each detection cycle, the resultant pressure value of the hydraulic cylinder is detected. If the resultant pressure value is greater than the target pressure value, the position value of the virtual hydraulic cylinder is reduced in the next cycle to make the upper mold tend to demold upward, thereby reducing the resultant pressure value. Conversely, the position value of the virtual hydraulic cylinder is increased to make the upper mold tend to close downward, thereby increasing the resultant pressure value. Finally, the resultant pressure value is made closer to the target pressure value. S51-S53 are repeated in each detection cycle during the pressure holding process.

9. A multi-hydraulic cylinder synchronous control device for a thermoforming machine, characterized in that: It includes multiple hydraulic cylinders, each of which is connected to a displacement sensor for acquiring the position value of the hydraulic cylinder and a pressure sensor for acquiring the pressure value of the hydraulic cylinder ejector rod. Each hydraulic cylinder has an independent proportional relief valve for controlling the extension or retraction speed of the ejector rod. The displacement sensors, pressure sensors and proportional relief valves of the multiple hydraulic cylinders are all connected to the controller. The controller includes a virtual hydraulic cylinder module, a calculation module and multiple control modules. The virtual cylinder module is used to convert speed changes during the thermoforming process into time-position values, and to set the time-position values ​​as the real-time target position. It also sets the pressure threshold and target pressure value of the virtual cylinder, with the pressure threshold being less than the target pressure value. The calculation module is used to calculate the resultant pressure value and average position value of multiple cylinders based on the pressure value and position value of the ejector rod of each cylinder, and then send the resultant pressure value and average position value to the control module. Each control module independently controls one hydraulic cylinder. Based on the target position, pressure threshold, target pressure value, and the combined pressure and position values ​​of multiple hydraulic cylinders, it controls the extension or retraction speed of the ejector rod of each hydraulic cylinder by adjusting the proportional relief valve. When the combined pressure value is lower than the pressure threshold, position control takes priority, making the position value of each hydraulic cylinder approach the target position. When the combined pressure value is higher than the pressure threshold, pressure control takes priority, and the combined pressure value reaches the target pressure value, keeping the position values ​​of the ejector rods of different hydraulic cylinders consistent.

10. A multi-hydraulic cylinder synchronous control device for a thermoforming machine according to claim 9, characterized in that: The pressure sensor is located on the ejector rod of the hydraulic cylinder; or a pressure sensor is installed in both the rodless chamber and the rod chamber of each hydraulic cylinder, and the pressure difference between the rodless chamber and the rod chamber is the pressure value of the ejector rod.

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