Control method for telescopic arm of forklift

Abstract

The invention discloses a control method for a telescopic arm of a forklift. The telescopic arm comprises a first-stage arm body, a second-stage arm body, a third-stage arm body and a fourth-stage arm body which are sequentially arranged in a nested mode from inside to outside. A cylinder body of the driving oil cylinder is installed outside the first-stage arm in a hinged mode, and a piston rod of the driving oil cylinder acts on the second-stage arm in a hinged mode; the first chain wheel is mounted outside the second-stage arm; one end of the first chain is fixed at the head part of the first-stage arm, and the other end of the first chain is fixed at the tail part of the third-stage arm; according to the method, the limitation that the length is calculated only by depending on geometric multiplying power traditionally is solved, the elastic correction model containing the linear elastic coefficient and the gravity gain coefficient is introduced innovatively, the elastic elongation of the chain generated by gravity and angle changes can be dynamically calculated according to the real-time stroke of the piston rod and the real-time variable amplitude elevation angle, and the calculation accuracy is improved. Therefore, the control system can obtain arm length data closer to physical reality under dangerous working conditions of high elevation angle and long extension.

Description

Technical Field

[0001] This invention relates to a control method for a forklift telescopic boom, belonging to the field of forklifts. Background Technology

[0002] Telescopic forklifts are a versatile material handling equipment widely used in construction sites, logistics warehouses, and port terminals. Their core working component is the multi-stage telescopic boom, typically driven by hydraulic cylinders in conjunction with a chain multiplier mechanism to achieve large-scale extension and retraction.

[0003] However, multi-stage telescopic booms rely on long-distance chain drives. Chains are not absolutely rigid bodies. Under heavy load conditions, especially when the boom is at a large elevation angle (high-level stacking), the component of gravity along the boom axis will significantly increase the chain tension, causing the chain to undergo non-negligible elastic elongation. In addition, mechanical backlash will develop in the chain drive system after long-term use. When the telescopic boom switches from extension to retraction, the piston rod of the drive cylinder has already begun to retract, but due to the slack of the chain, there is often a brief static lag (idle stroke) at the end of the boom. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a control method for a forklift telescopic boom to solve the problem.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a control method for a forklift telescopic boom, wherein the telescopic boom comprises a primary boom, a secondary boom, a tertiary boom, and a quaternary boom arranged nested from the inside out; and further comprises: The drive cylinder has its cylinder body hinged to the outside of the first-stage arm, and its piston rod hinged to the second-stage arm. A first sprocket and a first chain, the first sprocket being mounted outside the second-stage arm; one end of the first chain being fixed to the head of the first-stage arm and the other end being fixed to the tail of the third-stage arm; The second sprocket and the second chain are mounted on the outside of the third-stage arm; one end of the second chain is fixed to the head of the second-stage arm and the other end is fixed to the tail of the fourth-stage arm. A third sprocket and a third chain, wherein the third sprocket is installed inside the second-stage arm; one end of the third chain is fixed to the tail of the third-stage arm and the other end is fixed to the head of the first-stage arm; A fourth sprocket and a fourth chain are provided. The fourth sprocket is installed inside the third-stage arm. One end of the fourth chain is fixed to the tail of the fourth-stage arm, and the other end is fixed to the head of the second-stage arm. The piston rod extends forward to drive the secondary arm, and the secondary arm, tertiary arm, and quaternary arm extend forward synchronously through the first sprocket, the second sprocket, the first chain, and the second chain. The piston rod retracts to drive the secondary arm, and the secondary, tertiary, and quaternary arms retract synchronously through the third and fourth sprockets and the third and fourth chains. The control method includes the following steps: Step 1: The control system receives the piston rod stroke value X in real time and collects the current luffing angle of the telescopic boom. And the direction of the action command, D; Step 2: Calculate the theoretical total length L1 of the telescopic arm under the given assumptions using the following formula: L1=K×X Where K is the geometric multiplier; Step 3: Based on the tensile characteristics of the first, second, third, and fourth chains under stress, calculate the elastic correction L2; the elastic correction L2 is positively correlated with the piston rod stroke value X and is affected by the amplitude angle. The weighted effect calculation formula is as follows: L2= ×X×(1+ × ) in, The linear elastic coefficient, This is the gravity gain coefficient; Step 4: Determine the clearance correction amount L3. When the telescopic arm is in the retracted state, it is determined that there is slack hysteresis. A non-zero clearance compensation value is introduced to compensate for the empty stroke when the piston rod has retracted but the telescopic arm has not yet followed the movement. Step 5: Calculate the real-time actual total length of the telescopic boom using the following formula: L4 = L1 + L2 + L3 Where L4 is the real-time actual total length of the telescopic boom.

[0006] Preferably, the drive cylinder extends forward to push the secondary arm, causing the first sprocket to move with the secondary arm, pulling the tertiary arm out through the first chain, the second sprocket to move with the tertiary arm, and the second chain to pull the quaternary arm out.

[0007] Preferably, the drive cylinder retracts to pull the secondary arm, causing the third sprocket to move with the secondary arm, and the third chain pulls the tertiary arm to retract. The fourth sprocket moves with the tertiary arm, and the fourth chain pulls the tertiary arm to retract.

[0008] Preferably, the geometric ratio coefficient is 3, which corresponds to the piston rod driving the second-stage arm and then driving the third-stage arm and the fourth-stage arm to extend and retract synchronously through the transmission. That is, the formula for calculating the total geometric theoretical length is: L1=3X.

[0009] Preferably, in step three, the linear elastic coefficient Defined as the cumulative elastic elongation of the first, second, third, and fourth chains corresponding to a unit piston rod stroke under horizontal working conditions.

[0010] Preferably, in step three, the gravity gain coefficient Defined as the component of gravity along the axial direction of the telescopic arm increases as the elevation angle increases, causing the first, second, third, and fourth chains to be stretched, with the actual total length exceeding the geometrically theoretical total length.

[0011] Preferably, the linear elastic coefficient and gravity gain coefficient Determined using the following two-point calibration method: First calibration point: in a horizontal state, i.e., at the variable amplitude elevation angle. =0, control the piston rod to extend to its maximum stroke. The actual length of the piston rod end is measured to be ,according to - Calculation of linear elasticity coefficient ;Right now,

[0012] Second calibration point: at the maximum elevation angle, i.e. ≥0, control the hydraulic cylinder piston rod to its maximum stroke. The actual length of the piston rod end is measured to be Based on the known linear elastic coefficient Calculation of gravity gain coefficient .

[0013] Preferably, in step four, the specific calculation logic for the clearance correction amount L3 is as follows: The control system determines the current action command direction D: If it is the extension direction: it determines that the first chain, second chain, third chain, and fourth chain are on the tension drive side and there is no transmission idle, and sets the clearance correction amount L3=0; If it is the retraction direction: it determines that the first chain, second chain, third chain, and fourth chain are on the slack release side, and sets the clearance correction amount L3=Q; where Q is the pre-calibrated total reversing clearance of the first chain, second chain, third chain, and fourth chain transmission system; during the retraction process, the positive hysteresis constant Q is added to the result.

[0014] Preferably, the telescopic arm is fully extended and slowly retracted. The instant Xt when the piston rod begins to move is recorded. The position Xm of the piston rod at the instant when the end of the telescopic arm actually begins to displace is observed and recorded. The total reversing clearance Q = |Xt - Xm| × 3 is then calculated.

[0015] Preferably, it also includes a displacement sensor for detecting the piston rod position of the drive cylinder, for sending the piston rod stroke value X to the control system, and an angle sensor for detecting the first-stage boom angle, for sending the luffing angle to the control system. .

[0016] Beneficial effects This invention overcomes the limitations of traditional length calculations that rely solely on geometric ratios. It innovatively introduces an elastic correction model that includes linear elastic coefficients and gravity gain coefficients. This model can dynamically calculate the elastic elongation of the chain due to gravity and angle changes based on the real-time piston rod stroke and amplitude angle. This allows the control system to obtain arm length data that is closer to the physical reality under dangerous conditions of high elevation angles and long extensions. Attached Figure Description

[0017] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a schematic diagram of the structure of a forklift telescopic boom according to the present invention; Figure 2 For the present invention Figure 1 Internal structure diagram. Detailed Implementation

[0018] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0019] Please see Figure 1 , Figure 2 The present invention provides a control method for a forklift telescopic boom: the telescopic boom includes a first-stage boom 1, a second-stage boom 2, a third-stage boom 3 and a fourth-stage boom 4 arranged in a nested manner from the inside to the outside.

[0020] First-stage boom 1: As a fixed base, its tail is hinged to the forklift body to support the axial sliding of second-stage boom 2.

[0021] Secondary arm 2: It is sleeved inside the primary arm 1 and can extend and retract along the axial direction of the primary arm 1 under the action of the drive cylinder 9 (piston rod).

[0022] The third and fourth arms are nested sequentially and maintain a linear motion trajectory through a built-in guide mechanism.

[0023] This telescopic boom adopts an external drive cylinder 9 and a transmission scheme consisting of a first sprocket 51, a second sprocket 61, a third sprocket 71, a fourth sprocket 81, a first chain 52, a second chain 62, a third chain 72, and a fourth chain 82. This greatly simplifies the hydraulic pipeline, but it also introduces nonlinear factors such as chain elastic deformation and mechanical clearance, which is the core problem that this control method needs to solve.

[0024] The telescopic arm is equipped with only one drive cylinder 9. The cylinder body of the drive cylinder 9 is hinged to the outside of the first-stage arm 1, and the front end of the piston rod is hinged to the second-stage arm 2. This arrangement ensures that the direct stroke of the piston rod acts only on the second-stage arm 2, that is, the extension amount of the piston rod is equal to the displacement of the second-stage arm 2 relative to the first-stage arm 1.

[0025] A first sprocket 51 is installed on the outer wall of the secondary arm 2. A first chain 52 passes around the first sprocket 51. One end of the first chain 52 is anchored to the head of the primary arm 1, and the other end is fixed to the tail of the tertiary arm 3. According to the principle of movable pulleys, when the secondary arm 2 is pushed forward by the hydraulic cylinder, the first sprocket 51 moves accordingly, forcing the first chain 52 to pull the tertiary arm 3 at twice the speed of the secondary arm 2 relative to the primary arm 1 (the speed relative to the secondary arm 2 is 1 time). A second sprocket 61 is installed on the outer wall of the tertiary arm 3. A second chain 62 passes around the second sprocket 61. One end of the second chain 62 is fixed to the head of the secondary arm 2, and the other end is fixed to the tail of the quaternary arm 4. Working principle: The drive cylinder 9 extends forward to push the secondary arm 2, causing the first sprocket 51 to move with the secondary arm 2. The first chain 52 pulls the tertiary arm 3 to extend, the second sprocket 61 moves with the tertiary arm 3, and the second chain 62 pulls the quaternary arm 4 to extend.

[0026] The third sprocket 71 is installed inside the secondary arm 2. One end of the third chain 72 is fixed to the tail of the tertiary arm 3, and the other end is fixed to the head of the primary arm 1 in the opposite direction. The fourth sprocket 81 is installed inside the tertiary arm 3. One end of the fourth chain 82 is fixed to the tail of the quaternary arm 4, and the other end is fixed to the head of the secondary arm 2. Working principle: The drive cylinder 9 retracts, pulling the secondary arm 2, causing the third sprocket 71 to move with the secondary arm 2. The third chain 72 pulls the tertiary arm 3 to retract, the fourth sprocket 81 moves with the tertiary arm 3, and the fourth chain 82 pulls the quaternary arm 4 to retract.

[0027] The telescopic boom also includes a displacement sensor for detecting the position of the piston rod of the drive cylinder 9, used to send the piston rod stroke value X to the control system, and an angle sensor for detecting the angle of the first-stage boom 1, used to send the luffing angle to the control system. .

[0028] The control method includes the following steps: Step 1: The control system receives the piston rod stroke value X from the displacement sensor and the amplitude elevation angle from the angle sensor in real time. At the same time, the control system reads the direction of the action command D from the operating handle or automatic control command (D=1 indicates extension, D=-1 indicates retraction, D=0 indicates stationary).

[0029] Step 2: Calculation of the total geometric length. This is a basic calculation based on the rigid body assumption, without considering any material deformation.

[0030] According to the telescopic arm described above, the piston rod directly drives the secondary arm 2, and each stage arm 1 increases in relative speed ratio of 1:1 through the principle of movable pulley.

[0031] Specifically, when the piston rod moves a distance X: the second-stage arm 2 moves X relative to the first-stage arm 1; the third-stage arm 3 moves X relative to the second-stage arm 2; and the fourth-stage arm 4 moves X relative to the third-stage arm 3. Therefore, the total extension of the telescopic arm is the sum of the displacements of each stage, making the calculation formula: L1=K×X Where K is the geometric expansion coefficient and L1 is the theoretical total length. In the four-stage arm structure of this embodiment, the geometric expansion coefficient is 3. This means that for every 1 meter the piston rod extends, the total length of the telescopic arm theoretically increases by 3 meters; that is: L1 = 3X.

[0032] Step 3: In actual working conditions, the first chain 52, the second chain 62, the third chain 72, and the fourth chain 82 are not absolutely rigid bodies. With the increase in extension length and the change in elevation angle, the tension on the first chain 52, the second chain 62, the third chain 72, and the fourth chain 82 will change significantly, resulting in elastic elongation. Without compensation, the actual extension length will be shorter than the theoretical length (when extended under load) or longer than the theoretical length (under specific forces).

[0033] This control method introduces an elastic correction value L2 that is positively correlated with the piston rod stroke value X (the longer the chain, the greater the total elongation), and is affected by the amplitude angle. The weighted effect.

[0034] The calculation formula is: L2= ×X×(1+ × ) Among them, the linear elastic coefficient When the boom is horizontally extended, the frictional resistance and the bending moment caused by the cantilever's own weight will generate basic tension in the chain. Linear elasticity coefficient. This reflects the linear deformation of the chain system per unit length extension under basic gravity load. Therefore, the linear elastic coefficient is... Defined in horizontal working condition ( When =0), the cumulative elastic elongation of the first chain 52, the second chain 62, the third chain 72, and the fourth chain 82 corresponding to the unit piston rod stroke.

[0035] Among them, gravity gain coefficient For when the forklift lifts goods or lifts them high without load ( >0), the components of the telescopic boom's own weight and the cargo's weight along the boom's axis will directly act on the first chain 52, the second chain 62, the third chain 72, and the fourth chain 82. With the change in amplitude... Increase As the gravity gain coefficient increases, the axial tensile force on the first chain 52, the second chain 62, the third chain 72, and the fourth chain 82 increases significantly, resulting in further elongation of these chains; This is used to quantify the additional elastic deformation caused by the change in angle. Therefore, the gravity gain coefficient... Defined as the gain effect coefficient of the axial component of gravity along the telescopic arm on the stretching of the first chain 52, the second chain 62, the third chain 72, and the fourth chain 82 when the elevation angle increases.

[0036] Step 4: In mechanical transmission structures, especially chain drives, backlash or lag effects are unavoidable. During extension: When the piston rod pushes the secondary arm 2 to extend, hydraulic thrust causes the secondary arm 2 to move forward, and the first chain 52 and the second chain 62 are taut, on the tension drive side. At this time, the chain and sprocket are tightly meshed, and there is no significant transmission idle. Therefore, if the action command direction D is the extension direction, the backlash correction amount L3 is set to 0. During retraction: When the piston rod pulls the secondary arm 2 to retract, due to inertia and friction, the tertiary arm 3 and the quaternary arm 4 will not immediately retract by gravity, but rather rely on the pulling of the third chain 72 and the fourth chain 82. At the moment of reversal, the drive-side chains (first chain 52, second chain 62) loosen, and the retraction-side chains (third chain 72, fourth chain 82) need to undergo a process from slack to tension. At this time, the piston rod has already retracted a certain distance, but the telescopic arm has not yet followed the movement, forming an "idle stroke".

[0037] Preferably, in order to compensate for this phenomenon, when the control system determines that the direction of the action command D is the retraction direction, it determines that there is slack hysteresis and introduces a non-zero gap compensation value Q, which is calculated as: L3=Q.

[0038] Where Q represents the pre-calibrated total reversing clearance of the first chain 52, second chain 62, third chain 72, and fourth chain 82. This value indicates that the lost length (e.g., 15mm-30mm) needs to be added back during the retraction calculation. In the formula L4=L1+L2+L3 of this embodiment, if L1 decreases due to the decrease of X, adding a positive value of Q can slow down the decrease rate of L4.

[0039] Step 5: The control system calculates the real-time actual total length L4 of the telescopic boom, that is: L4 = L1 + L2 + L3 Right now: L4 = 3X + [ ×X×(1+ × )]+L3 The calculation result L4 can be used for real-time height / length display on the cab screen, and can also be used as an input parameter for the forklift torque limiter (LMI) system.

[0040] To ensure that the coefficients in the formula accurately reflect the individual differences of each piece of equipment (such as chain manufacturing tolerances and assembly tightness), this control method includes a standard two-point calibration procedure, linear elasticity coefficient. and gravity gain coefficient Determined in the following ways: Obtaining the linear elastic coefficient : Control the telescopic boom to be in a horizontal position and adjust the luffing angle. =0 (allowable error ±2°), control the piston rod to extend to its maximum stroke. Record the measurements; use an external laser rangefinder to measure the actual total length of the telescopic boom's end at this point. And record it; according to the formula: L4 = 3X + [ ×X×(1+ × )]+0 (horizontally extended, =0, L3=0) Simplified to: =3 + × Thus, the solution is:

[0041] Obtain the gravity gain coefficient While maintaining the telescopic boom length or re-extending it, the luffing cylinder is controlled to raise the telescopic boom to its maximum elevation angle (usually 45° or 60°). = max.

[0042] Control the piston rod of the hydraulic cylinder to its maximum stroke (generally = Record the amplitude angle fed back by the sensor at this time. `max` refers to the actual total length of the telescopic boom along the axial direction, measured using external measurement methods. According to the formula =3 + × (1+ × ).at this time Given the quantities, the formula can be transformed into: -3 - × = × × ×

[0043] Transformed into:

[0044] In one embodiment, the telescopic arm is fully extended and slowly retracted. The instant Xt when the piston rod begins to move is recorded, and the piston rod position Xm at the instant the end of the telescopic arm actually begins to displace is observed and recorded. The total reversing clearance Q is calculated as Q = |Xt - Xm| × 3. Multiplying by 3 is because the piston rod's idle travel will be amplified by the magnification mechanism or reflected in the total length lag, depending on the observation reference. Usually, directly calibrating the total length lag is more straightforward.

[0045] Example 1 Geometric ratio coefficient K=3, linear elastic coefficient =0.002 (i.e., the chain elastic elongation is 2mm per meter of travel), gravity gain coefficient =1.5, total reversing clearance Q=15mm.

[0046] In the extended position, the operator performs high-level stacking at an elevation angle. =30° 30°=0.5), piston rod stroke value X=2000mm, command is to extend.

[0047] The total geometric length L1 = 3 × 2000 = 6000 mm.

[0048] The elastic correction amount L2 = 0.002 × 2000 × (1 + 1.5 × 0.5) = 4 × (1 + 0.75) = 4 × 1.75 = 7 mm.

[0049] At this point, due to the angle of elevation, the chain is stretched further, with a total elastic elongation of 7mm.

[0050] Since the command is to extend, and the object is on the tension side, L3 = 0mm. That is, L4 = 6000 + 7 + 0 = 6007mm.

[0051] Example 2 Unlike Example 1, the operator makes a fine adjustment, keeping the elevation angle unchanged, and the piston rod retracts. The current piston rod stroke value X = 1995 mm (retracted by 5 mm), and the command is to retract.

[0052] The total geometric length L1 = 3 × 1995 = 5985 mm.

[0053] The elastic correction amount L2≈7mm (the amount of movement is too small to be considered).

[0054] Since the instruction is to retract, it is determined that we have entered the relaxation zone, i.e., L3=Q=15mm.

[0055] That is, L4 = 5985 + 7 + 15 = 6007 mm.

[0056] In reality, due to the gaps between the first chain 52, the second chain 62, the third chain 72, and the fourth chain 82, although the piston rod retracted by 5mm (resulting in a theoretical length reduction of 15mm), the boom head may have only loosened slightly and did not actually retract. With the 15mm compensation, the displayed value remained around 6007mm, avoiding the issue of the instrument showing a significant retraction while the boom head actually remained stationary, thus improving the operating feel.

[0057] Example 3 Unlike Example 1, the operator makes a fine adjustment, keeping the elevation angle unchanged, and the piston rod retracts. The current piston rod stroke value X = 1000mm (retracted by 1000mm), and the command is to retract.

[0058] L1 = 3 × 1000 = 3000 mm.

[0059] L2=0.002×1000×(1+1.5×0.5)=2×(1+0.75)=2×1.75=3.5mm.

[0060] Since the instruction is to retract, it is determined that we have entered the relaxation zone, i.e., L3=Q=15mm.

[0061] That is, L4 = 3000 + 3.5 + 15 = 3018.5 mm.

[0062] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the scope of the invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0063] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A control method for a forklift truck telescopic arm, characterized by: The telescopic arm comprises a first-stage arm, a second-stage arm, a third-stage arm and a fourth-stage arm arranged in sequence from inside to outside; and further comprises: a driving oil cylinder, the cylinder body of which is hingedly installed outside the first-stage arm, and the piston rod of which acts on the second-stage arm; a first sprocket and a first chain, the first sprocket being installed outside the second-stage arm, and the first chain being fixed at one end to the head of the first-stage arm and at the other end to the tail of the third-stage arm; a second sprocket and a second chain, the second sprocket being installed outside the third-stage arm, and the second chain being fixed at one end to the head of the second-stage arm and at the other end to the tail of the fourth-stage arm; a third sprocket and a third chain, the third sprocket being installed inside the second-stage arm, and the third chain being fixed at one end to the tail of the third-stage arm and at the other end to the head of the first-stage arm; a fourth sprocket and a fourth chain, the fourth sprocket being installed inside the third-stage arm, and the fourth chain being fixed at one end to the tail of the fourth-stage arm and at the other end to the head of the second-stage arm; The piston rod is extended to drive the second-stage arm, and the first sprocket and the second sprocket move with the second-stage arm, the first chain and the second chain are used to pull out the third-stage arm and the fourth-stage arm. The piston rod is retracted to drive the second-stage arm, and the third sprocket and the fourth sprocket move with the second-stage arm, the third chain and the fourth chain are used to pull in the third-stage arm and the fourth-stage arm. The control method comprises the following steps: Step one: the control system receives the piston rod stroke value X of the piston rod in real time, and collects the current luffing angle of the telescopic arm and the action instruction direction D; Step two: calculate the geometric theoretical total length L1 of the telescopic arm under the assumption by the following formula: L1=K×X Wherein, K is the geometric magnification coefficient; Step three: based on the tensile properties of the first chain, the second chain, the third chain, the fourth chain under the stress state, the elastic correction amount L2 is calculated; the elastic correction amount L2 is positively correlated with the piston rod stroke value X, and is affected by the variable amplitude elevation angle The weighted influence, and the calculation formula structure is: L2= ×X×(1+ × ) wherein, is the linear elastic coefficient, is the gravity gain coefficient; Step four: determine the gap correction L3, when the telescopic arm is in the retracted state, it is determined that there is a relaxation lag, a non-zero gap compensation value is introduced to compensate for the idle stroke that the piston rod has been retracted but the telescopic arm has not yet followed the action; Step five: calculate the real-time actual total length of the telescopic arm by the following formula: L4=L1+L2+L3 Wherein, L4 is the real-time actual total length of the telescopic arm.

2. The control method of a telescopic arm of a fork lift truck according to claim 1, characterized in that: The driving oil cylinder is extended to push the second-stage arm, the first sprocket moves with the second-stage arm, the first chain is used to pull out the third-stage arm, the second sprocket moves with the third-stage arm, and the second chain is used to pull out the fourth-stage arm.

3. The control method of a telescopic arm of a fork lift truck according to claim 1, characterized in that: The driving oil cylinder is retracted to pull the second-stage arm, the third sprocket moves with the second-stage arm, the third chain is used to pull in the third-stage arm, the fourth sprocket moves with the third-stage arm, and the fourth chain is used to pull in the fourth-stage arm.

4. The control method of a telescopic arm of a fork lift truck according to claim 1, characterized in that: The geometric magnification coefficient is 3, corresponding to the piston rod driving the second-stage arm and then driving the third-stage arm and the fourth-stage arm to synchronously extend and retract, that is, the calculation formula of the geometric theoretical total length is L1=3X.

5. The control method of a telescopic arm of a fork lift truck according to claim 4, characterized in that: In step three, the linear elastic coefficient Defined as the first chain, the second chain, the third chain, the fourth chain cumulative elastic elongation corresponding to the unit piston rod stroke under the horizontal working condition.

6. The control method of a telescopic arm of a fork lift truck according to claim 5, characterized in that: In step three, the gravity gain coefficient defined as the component of gravity along the telescopic boom axis increases as the elevation angle increases, resulting in the first chain, second chain, third chain, fourth chain being stretched, the actual total length being greater than the geometric theoretical total length.

7. The control method of a telescopic arm of a fork lift truck according to claim 6, characterized in that: Linear elastic coefficient And gravity gain coefficient Determined by the following two-point calibration method: First calibration point: in horizontal state, i.e. amplitude elevation angle = 0, control piston rod to extend to maximum stroke , measure the actual length of the end of the piston rod , according to - calculate the linear elasticity coefficient ; that is, Second calibration point: at maximum elevation, i.e. ≥ 0, control the cylinder rod to the maximum stroke , measure the actual length of the end of the rod , according to the known linear elastic coefficient calculate the gravity gain coefficient .

8. The control method of a telescopic arm of a fork lift truck according to claim 1, characterized in that: In step four, the specific calculation logic of the gap correction amount L3 is: the control system judges the current action instruction direction D: if it is the extension direction: it is judged that the first chain, the second chain, the third chain and the fourth chain are on the tension driving side, there is no transmission idle stroke, and the gap correction amount L3 is set to 0; if it is the retraction direction: it is judged that the first chain, the second chain, the third chain and the fourth chain are on the relaxation release side, and the gap correction amount L3 is set to Q; wherein Q is the total gap of the first chain, the second chain, the third chain and the fourth chain transmission system which is calibrated in advance; in the retraction process, the positive value of the hysteresis constant Q is added to the result.

9. The control method of a fork truck telescopic arm according to claim 8, characterized in that: Fully extend the telescopic arm, issue slow retraction, record the moment when the piston rod starts to move Xt, observe and record the piston rod position Xm when the telescopic arm end actually starts to displace, calculate the total gap Q = |Xt-Xm| × 3.

10. The control method of a telescopic arm of a fork lift truck according to claim 1, characterized in that: Also included is a displacement sensor for detecting the piston rod position of the drive cylinder for sending a piston rod stroke value X to the control system, an angle sensor for detecting the angle of the primary arm for sending a luffing angle value to the control system .

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