Electric grab protection system and method of controlling the same

Abstract

The application relates to the technical field of crane motion control, and discloses an electric grab bucket protection system and a control method thereof, wherein the system comprises a PLC, an inductive switch, a current transformer, a grab bucket opening AC contactor and a grab bucket closing AC contactor which are electrically connected with the PLC respectively; the current transformer is used for sampling electric grab bucket current information and transmitting the information to the PLC; and the inductive switch is used for controlling whether the current transformer transmits the sampled electric grab bucket current information to the PLC. The application solves the problems of the prior art, such as the incapability of accurately controlling the opening and closing limit of the grab bucket and the poor protection effect.

Description

Technical Field

[0001] This invention relates to the field of crane motion control technology, specifically to an electric grab bucket protection system and its control method. Background Technology

[0002] For electric grabs used in general-purpose bridge cranes, the following problems often occur in manufacturing environments:

[0003] 1. During the operation of the electric grab lowering command in the cab of a general-purpose bridge crane, when the electric grab descends to the ground (vertically or on an inclined surface or over a pile of materials), if the opening command is executed for too long, the wire rope may slip into the internal drum or outer circumference of the fixed pulley of the electric hoist due to excessive looseness, causing the wire rope to jam or the reducer to be forcibly rotated, resulting in damage to the internal transmission gears. Traditional protection devices combine a rope guide and a fire limit switch, but due to the different inertia of the motors, it is difficult for the operator to accurately control the closing force of the electric grab. Therefore, this protection device is not very practical and has not been effectively used. Existing technology cannot accurately control the opening and closing limits of the grab.

[0004] 2. The electric grab bucket uses industrial sockets and plugs for power connection. During the repeated operation of the grab bucket's upward and downward commands, the industrial sockets and plugs may become loose. The motor cable, moving repeatedly on the drum, may experience broken strands in the internal copper wires after prolonged use, leading to poor power contact or phase loss, causing motor damage. Traditional protection devices are phase loss protectors, which only protect against phase loss at the upstream end, not at the downstream end. Phase loss protectors can only protect against phase loss at the upstream power supply, not at the downstream equipment.

[0005] 3. When executing the closing command of the electric grab bucket, the grab bucket picks up the material. Due to the varying hardness of different materials, it is impossible to estimate the required clamping tightness, leading to frequent overload operation of the electric grab bucket motor. During operation, the motor generates different currents depending on the material being grabbed, causing the motor to overload, which current technology cannot protect against. Traditional protection devices are thermal relays, which utilize the thermal effect of current for protection. The rated thermal stability current duration is 1 second. Prolonged overload not only accelerates the aging of the motor windings but also shortens the motor's lifespan. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, the present invention provides an electric grab bucket protection system and its control method, which solves the problems of the prior art, such as the inability to accurately control the opening and closing limits of the grab bucket and poor protection effect.

[0007] The technical solution adopted by the present invention to solve the above problems is:

[0008] An electric grab bucket protection system includes a PLC, an inductive switch, a current transmitter, an AC contactor for grab bucket opening, and an AC contactor for grab bucket closing, all electrically connected to the PLC. The current transmitter is used to sample the electric grab bucket current information and transmit it to the PLC. The inductive switch is used to control whether the current transmitter transmits the sampled electric grab bucket current information to the PLC.

[0009] As a preferred technical solution, it also includes intermediate relays KM3 and KM4. The grab bucket opening AC contactor (7) includes a KM1 coil and a KM1 contact. The grab bucket closing AC contactor (8) includes a KM2 coil and a KM2 contact. The KM1 coil, KM2 contact, and KM3 contact are connected in series to form a first control branch. The KM2 coil, KM1 contact, and KM4 contact are connected in series to form a second control branch. One end of the KM1 contact and one end of the KM2 contact are respectively connected to the two inputs of the PLC (4). The terminals are electrically connected, and the other end of the KM1 contact and the other end of the KM2 contact are both electrically connected to a 24V direct power supply; one end of the KM3 coil and one end of the KM4 coil are respectively electrically connected to the two output terminals of the PLC (4), and the other end of the KM3 coil and the other end of the KM4 coil are both grounded; one end of the circuit structure after the first control branch and the second control branch are connected in parallel is used to be electrically connected to the A phase, B phase or C phase of the three-phase motor, and the other end of the circuit structure after the first control branch and the second control branch are connected in parallel is used to be electrically connected to the neutral line.

[0010] As a preferred technical solution, it also includes an analog quantity module that is electrically connected to the PLC and the current transmitter respectively.

[0011] As a preferred technical solution, it also includes a signal transmitting module, a signal receiving module, an inductive switch, a signal transmitting module, a signal receiving module, and a PLC connection. The signal transmitting module can control the on or off signal transmission between the inductive switch and the PLC, and the signal receiving module can control the on or off signal transmission between the inductive switch and the PLC.

[0012] As a preferred technical solution, the inductive switch is a proximity switch, a limit switch, or an infrared switch.

[0013] As a preferred technical solution, the inductive switch is located inside the pulley groove of the electric grab bucket crane.

[0014] A control method for an electric grab bucket protection system, wherein when the imbalance of the three-phase motor current is greater than 10%, the PLC controls the AC contactor for grab bucket opening and the AC contactor for grab bucket closing to stop the response time of 10ms to 100ms.

[0015] As a preferred technical solution, the calculation formula for a three-phase motor current imbalance greater than 10% is as follows:

[0016] |I A -I B |≥(I A +I B +I C ) / 3*10%,

[0017] |I B -I C |≥(I A +I B +I C ) / 3*10%,

[0018] |I C -I A |≥(I A +I B +I C ) / 3*10%;

[0019] As a preferred technical solution, when the current transmitter is used to sample the electric grab bucket current, which is greater than the operating current of the electric grab bucket motor or the rated current of the electric grab bucket motor, the response time of the PLC controlling the grab bucket opening AC contactor and the grab bucket closing AC contactor is 10ms-100ms.

[0020] As a preferred technical solution, the steps include:

[0021] S1, the PLC converts the sampled three-phase current into floating-point numbers;

[0022] S2, when the absolute value of the difference between the three-phase floating-point numbers is greater than 10% of the current imbalance, the PLC controls the grab bucket opening AC contactor and the grab bucket closing AC contactor to stop working.

[0023] S3, when the timer T of the contactor reaches the set value, it controls the grab bucket to open the AC contactor or the grab bucket to close the AC contactor to reset.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] (1) The present invention effectively protects against phenomena such as tangled steel ropes and stuck steel ropes in electric grab buckets, motor overload and phase loss burnout, and gear damage in reducers. It not only reduces the failure rate and troubleshooting time, but also improves operating efficiency.

[0026] (2) This invention can quickly and accurately respond to and protect against faults such as poor contact or broken strands or phase loss in the power supply, cable connection, circuit breaker, AC contactor, and controlled motor.

[0027] (3) The present invention utilizes the internal counter of the PLC to accumulate the number of times the electric grab bucket steel rope is loose, the power supply is in poor contact and the phase is missing, and the overload is effective. This is beneficial for analyzing equipment failures through data accumulation. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the electric grab bucket protection system described in this invention.

[0029] Figure 2 This is a schematic diagram of the circuit structure of an electric grab bucket protection system according to the present invention;

[0030] Figure 3 for Figure 2 One of the magnified views of a section;

[0031] Figure 4 for Figure 2 The second enlarged view of a part.

[0032] The labels and their corresponding names in the attached diagram are as follows: 1. Inductive switch, 2. Signal transmitting module, 3. Power supply, 4. PLC, 5. Analog module, 6. Current transmitter, 7. Grab bucket opening AC contactor, 8. Grab bucket closing AC contactor, 9. Signal receiving module. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0034] Example 1

[0035] like Figures 1 to 4 As shown, in view of the problems existing in the prior art, the inventor actively thought about the problem and, after many experiments, finally devised the following solution:

[0036] 1. The rigid rebound force of the steel rope behind the pulley. When the steel rope slacks, proximity switch 1 receives a signal, which is transmitted to PLC4. After program logic calculation, PLC4 locks the grab bucket opening AC contactor 7 coil to de-energize and stop (if the coil is de-energized, the grab bucket stops operating). After the contactor stops, a reverse command (closing) is required for normal operation. This causes the proximity switch to close after the steel rope is tightened, allowing the opening command to be executed. The AC contactor (grab bucket opening AC contactor 7) coil is energized. The AC contactors executing the opening and closing have mutual control functions (grab bucket opening AC contactor 7 and grab bucket closing AC contactor 8 form mutual control), thus protecting the steel rope from slack.

[0037] As a preferred technical solution, it also includes intermediate relays KM3 and KM4. The grab bucket opening AC contactor (7) includes a KM1 coil and a KM1 contact. The grab bucket closing AC contactor (8) includes a KM2 coil and a KM2 contact. The KM1 coil, KM2 contact, and KM3 contact are connected in series to form a first control branch. The KM2 coil, KM1 contact, and KM4 contact are connected in series to form a second control branch. One end of the KM1 contact and one end of the KM2 contact are respectively connected to the two inputs of the PLC (4). The terminals are electrically connected, with the other end of contact KM1 and contact KM2 both electrically connected to a 24V direct power supply; one end of coil KM3 and one end of coil KM4 are electrically connected to the two output terminals of PLC (4) respectively, and the other ends of coil KM3 and coil KM4 are grounded; one end of the circuit structure after the first control branch and the second control branch are connected in parallel is used to connect to phase A, phase B or phase C of the three-phase motor, and the other end of the circuit structure after the first control branch and the second control branch are connected in parallel is used to connect to the neutral wire. Preferably, switch SB1 can also be connected in series in the first control branch as the sending switch for the grab opening command, and switch SB2 can also be connected in series in the second control branch as the sending switch for the grab closing command.

[0038] 2. The three-phase cable of the electric grab bucket passes through the current transmitter 6 at any position. The current transmitter 6 samples the current and transmits it to the PLC module. According to the standard GB / T1032-2012, the unbalance of the three-phase motor current is less than 10%. When the current unbalance is greater than 10% due to poor contact in any phase of the three phases, the PLC module calculates and stops the opening or closing AC contactors (grab bucket opening AC contactor 7, grab bucket closing AC contactor 8) within 50ms (the time is adjustable, but not less than 10ms), thus activating the motor protection function. This principle applies to all three-phase motors (or all three-phase electrical equipment with a current unbalance of less than 10%).

[0039] 3. The three-phase cable of the electric grab bucket passes through the current transmitter 6 at any position. The current transmitter 6 samples the current and transmits it to the PLC module. After the PLC module calculates, when the current is greater than the working current or the rated current, the action is set at 50ms (the time is adjustable, but not less than 10ms) to stop the AC contactor (grab bucket closing AC contactor 8) that is performing the closing action. Thus, it can only be reset by operating the opening action.

[0040] 4. For the currents A (phase A), B (phase B), and C (phase C), based on the motor's three-phase current taking a positive value and the imbalance less than 10%, (phase A current minus phase B current) is taken as a positive number and less than (phase A current plus phase B current plus phase C current) divided by 3 and multiplied by 10%. When it exceeds 10% and the duration is 50ms (adjustable from 10ms to 100ms, but not less than 10ms), a protection signal is output to stop the opening and closing of the AC contactors (grab bucket opening AC contactor 7, grab bucket closing AC contactor 8). The principle for the other two phase currents is the same, and the calculation formula is as follows:

[0041] |I A -I B |≥(I A +I B +I C ) / 3*10%,

[0042] |I B -I C |≥(I A +I B +I C ) / 3*10%,

[0043] |I C -I A |≥(I A +I Bx I C ) / 3*10%;

[0044] If the rated current of the motor is 11A, I A ≥11、I B ≥11、I C ≥11, when the current in one of the three phases is greater than 11A, the closed AC contactor loses power.

[0045] The PLC internal logic program: Sample the three-phase current and convert it into floating-point numbers. When the absolute value of the difference between the three floating-point numbers is greater than 10% of the current imbalance, when the delay timer T reaches the set value, the open and close Q0.1 and close Q0.2 actions are activated to protect the open and close AC contactors and stop working (the channel output of PLC4, open Q0.1 and close Q0.2 refer to the names of the electric grab action; the PLC model is S7-200CN (CPU224CN)). When there is a grab opening command or a proximity switch signal, open Q0.1 is set, and it will only be reset when the grab closes. When one or more phases of the sampled three-phase current are greater than the rated value and the duration reaches the timer T, close Q0.2 is set, and it will only be reset when the open command is given.

[0046] The specific implementation can be carried out in the following ways:

[0047] 1. Install a switch (one of proximity switch, limit switch, infrared switch, etc.) on the pulley groove of the electric grab crane to transmit signals (wireless transmitter, cable, etc.) to the PLC. (The PLC, analog module, wireless receiver, intermediate relay, and current transmitter are assembled in one electrical box.) Install the control box on the electrical cabinet of the general-purpose bridge crane driver's cab and connect a 220V power supply to power the PLC and current transmitter.

[0048] 2. Pass the three-phase power lines of the electric grab bucket motor through the holes of the current transmitter. The current transmitter 6 samples the current and transmits it to the analog module 5 of PLC4. Connect AC contactor KM2 to I0.1, KM1 to I0.2, Q0.1 to KM3, and Q0.2 to KM4.

[0049] 3. The PLC program is edited to set the protection signal control KM3 to set (on) and KM4 to set (on) according to the 10% ratio of three-phase current imbalance of the motor. The proximity switch controls KM3 to set (on), KM2 controls KM3 to reset (off), and KM1 controls KM4 to reset (off).

[0050] In this invention:

[0051] An electric grab bucket protection system mainly includes:

[0052] (1) Inductive switch 1 (proximity switch, limit switch, infrared switch, etc.);

[0053] (2) Signal transmission module (wireless signal transmission and reception, cable, etc., i.e. signal transmission module 2 and signal receiving module 9, signal transmission module 2 is preferably a wireless transmitter and signal receiving module 9 is preferably a wireless receiver).

[0054] (3) 380V to 12V power supply (i.e., power supply 3, which includes a transformer);

[0055] (4) PLC4 (can use S7-200CN CPU224CN);

[0056] (5) Analog module 5 (EM231CN can be used);

[0057] (6) Current transmitter 6;

[0058] (7) Grab bucket opens AC contactor 7;

[0059] (8) Grab bucket closing AC contactor 8;

[0060] Limit switches or proximity switches are installed at the steel rope of the pulley of the electric grab bucket crane. When the limit switch or proximity switch has a signal, it is transmitted to PLC4 by wireless remote control (transmitter and receiver) or cable. The three-phase wires of the electric grab bucket motor pass through the current transmitter 6. The current transmitter 6 samples the current and transmits it to the PLC analog module 5. After logic operation, PLC4 controls the grab bucket opening AC contactor 7 and the grab bucket closing AC contactor 8.

[0061] Preferably, when the steel rope deflects and hits a limit switch or proximity switch, the signal is transmitted to the PLC4 via wireless remote control or cable. After logic calculation, the grab bucket is opened and the AC contactor coil is de-energized to stop the operation.

[0062] Preferably, the current transmitter 6 samples the three-phase current of the electric grab bucket motor and transmits it to the PLC analog module 5. Through program calculation, it can quickly and accurately respond to protect and control the grab bucket opening AC contactor 7 and the grab bucket closing AC contactor 8 coil de-energize and stop, regardless of where there is poor contact or cable breakage and phase loss, from the power supply, cable connection, circuit breaker, AC contactor to the controlled motor.

[0063] Preferably, the current transmitter 6 samples the three-phase current of the electric grab bucket motor and transmits it to the PLC analog module 5. Through program calculation, when the current is greater than the set current or reaches the rated current of the motor, the grab bucket is controlled to close the AC contactor 7 coil to de-energize and stop.

[0064] Limit switches or proximity switches are installed at the steel rope of the pulley of the electric grab bucket crane. Once there is a signal input from the limit switch or proximity switch, it is transmitted to the PLC by wireless remote control (transmitter and receiver) or cable. At the same time, the current transmitter samples the current and transmits it to the PLC analog module. After logic operation, the PLC accurately disconnects the coil power supply of the AC contactor that performs the opening and closing action, thereby protecting the grab bucket motor.

[0065] The present invention has the following technical effects:

[0066] 1. It effectively protects against issues such as tangled and jammed steel ropes in electric grab buckets, motor overload and phase loss burnout, and gear damage to reducers, thereby reducing failure rates and troubleshooting time, and improving operational efficiency.

[0067] 2. From the power supply, cable connections, circuit breakers, AC contactors, to the controlled motor, any fault such as poor contact or broken strands / phase loss in the cable can be quickly and accurately responded to for protection. (The calculation speed of a typical PLC is within 10ms, while the inrush current time of a motor starting is within 10ms, and the tripping time of the national standard terminal leakage protection is within 100ms. Therefore, the protection output time is set within 10-100ms, with a midpoint of 50ms.)

[0068] 3. By using the PLC's internal counter, the number of times the electric grab bucket's steel rope is loose, the power supply has poor contact and phase loss, and the overload can be accumulated, which is beneficial for analyzing equipment failures through data accumulation.

[0069] As described above, the present invention can be implemented well.

[0070] All features disclosed in all embodiments of this specification, or steps in all methods or processes implied in the disclosure, may be combined and / or extended or replaced in any way, except for mutually exclusive features and / or steps.

[0071] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Based on the technical essence of the present invention, any simple modifications, equivalent substitutions, and improvements made to the above embodiments within the spirit and principles of the present invention shall still fall within the protection scope of the present invention.

Claims

1. An electric grab bucket protection system, characterized in that, Includes PLC (4), inductive switch (1), current transmitter (6), grab bucket opening AC contactor (7), grab bucket closing AC contactor (8) which are electrically connected to PLC (4). The current transmitter (6) is used to sample the electric grab bucket current information and transmit it to PLC (4). The inductive switch (1) is used to control whether the current transmitter (6) transmits the sampled electric grab bucket current information to PLC (4). It also includes intermediate relays KM3 and KM4. The grab bucket opening AC contactor (7) includes a KM1 coil and a KM1 contact. The grab bucket closing AC contactor (8) includes a KM2 coil and a KM2 contact. The KM1 coil, KM2 contact, and KM3 contact are connected in series to form the first control branch. The KM2 coil, KM1 contact, and KM4 contact are connected in series to form the second control branch. One end of the KM1 contact and one end of the KM2 contact are electrically connected to the two input terminals of the PLC (4), respectively. The other end of the KM1 contact and the other end of the KM2 contact are both electrically connected to a 24V direct power supply; one end of the KM3 coil and one end of the KM4 coil are respectively electrically connected to the two output terminals of the PLC (4), and the other end of the KM3 coil and the other end of the KM4 coil are both grounded; one end of the circuit structure after the first control branch and the second control branch are connected in parallel is used to be electrically connected to the A phase, B phase or C phase of the three-phase motor, and the other end of the circuit structure after the first control branch and the second control branch are connected in parallel is used to be electrically connected to the neutral line.

2. The electric grab bucket protection system according to claim 1, characterized in that, It also includes an analog quantity module (5) that is electrically connected to the PLC (4) and the current transmitter (6).

3. The electric grab bucket protection system according to claim 2, characterized in that, It also includes a signal transmitting module (2), a signal receiving module (9), and a connection between the induction switch (1), the signal transmitting module (2), the signal receiving module (9), and the PLC (4). The signal transmitting module (2) can control the signal transmission between the induction switch (1) and the PLC (4) to be on or off, and the signal receiving module (9) can control the signal transmission between the induction switch (1) and the PLC (4) to be on or off.

4. An electric grab bucket protection system according to any one of claims 1 to 3, characterized in that, The inductive switch (1) is a proximity switch, limit switch or infrared switch.

5. The electric grab bucket protection system according to claim 4, characterized in that, The induction switch (1) is located in the pulley groove of the electric grab bucket crane.

6. A control method for an electric grab bucket protection system, characterized in that, When the imbalance of the three-phase motor current is greater than 10%, the PLC (4) controls the AC contactor (7) to open and the AC contactor (8) to close the grab bucket to stop the response time of 10ms to 100ms.

7. The control method for an electric grab bucket protection system according to claim 6, characterized in that, The formula for calculating the current imbalance of a three-phase motor that is greater than 10% is as follows: |I A- I B |≥(I A+ I B+ I C ) / 3 10%, |I B- I C |≥(I A+ I B+ I C ) / 3 10%, |I C- I A |≥(I A+ I B+ I C ) / 3 10%。 8. A control method for an electric grab bucket protection system according to claim 6 or 7, characterized in that, When the current transmitter (6) is used to sample the electric grab bucket current, which is greater than the working current of the electric grab bucket motor or the rated current of the electric grab bucket motor, the PLC (4) controls the grab bucket to open the AC contactor (7) and close the grab bucket AC contactor (8) with a response time of 10ms-100ms.

9. The control method for an electric grab bucket protection system according to claim 6, characterized in that, Includes the following steps: S1, PLC (4) converts the sampled three-phase current into floating-point numbers; S2, when the absolute value of the difference between the three-phase floating-point numbers is greater than 10% of the current imbalance, PLC (4) controls the grab bucket to open AC contactor (7) and the grab bucket to close AC contactor (8) to stop working. S3, when the delay timer T of the contactor (7) reaches the set value, the AC contactor (7) is reset by controlling the grab bucket to open or the AC contactor (8) to close.

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