Patent Description:
An electric glue gun is an electric tool which leverages the power supplied by an electric motor to drive, via a transmission device, a push rod to extrude a glue tube, causing the glue to flow out of the tip of the glue tube, thereby implementing a filling function. Compared with manual glue guns, electric glue guns offer a higher efficiency and a more uniform glue output and are thus applied in a wide array.

After the glue inside the glue tube is completely squeezed out, a continued forward movement and extrusion of the push rod driven by the electric motor would cause an irreparable damage to the gun. In a no-load state, an excessive forward movement driven by the electric motor would also damage the gun. After the push rod moves rearward to reset, if the stroke of rearward movement exceeds a maximum stroke, it would also cause damages to the gun body. To address the above deficiencies, existing electric glue guns usually adopt a solution of manually stopping the gun based on the glue output conditions or reset conditions during use, which is highly demanding on an operator's experience and is prone to maloperation, thereby easily causing damages to the gun body. <CIT> discloses a powered dispensing tool for dispensing caulk, adhesives, or like materials. The tool comprises a main housing, a cartridge housing, a rack coupled to a plunger and a magnet coupled to the rack, wherein the plunger is movable within the cartridge housing via a transmission drivable by a motor. The tool further comprises a main control having a stroke-limit circuit in electrical communication with the motor and a Hall effect sensor so that the motor can be stopped in time during the forward movement of the rack and the plunger based on the magnetic field generated by the magnet and detected by the stroke-limit circuit to prevent the plunger from impacting the main housing. <CIT> discloses a similar tool as described in <CIT> having a stroke-limit circuit in electrical communication with a motor and that is operable to deactivate the motor to arrest powered translation of a rack in the forward direction. The stroke-limit circuit includes a switch which, when toggled to a closed configuration, supplies power to the motor and when toggled to an open configuration, does not supply power to the motor. <CIT> discloses a dispensing device with a material dispenser, whose forward movement in relation to the desired amount of material and whose backward movement in relation to the relieving pressure of the container with the material to be dispensed can be driven in a controlled manner by a motor with a detector. <CIT> discloses an extrusion apparatus having a piston whose forward and backward movement can be controlled by a motor based on the position of the piston measured by a position measuring unit, wherein the backward movement protects the damage of the piston end and the wall part of the housing accommodating the piston.

An objective of the present disclosure is to at least partially overcome the deficiencies of the prior art and in particular to provide an electric glue gun so as to effectively prevent potential damages to the gun body due to incidental excessive forward movement or rearward movement of the push rod therein, thereby overcoming the drawbacks and defects in the prior art.

To achieve this object, an electric glue gun according to the features of claim <NUM> is proposed. Advantageous embodiments are subject matter of the dependent claims. The features cited individually in the claims can be combined with one another in a technologically expedient manner and can be supplemented by explanatory facts from the description and details from the figures, with further embodiments of the invention being highlighted.

To achieve the objective above, an electric glue gun provided by the present disclosure comprises a housing, a glue tube holder arranged at a front end of the housing, and a push rod disposed on the housing and moveable forward and rearward relative to the glue tube holder. A control board, a main switch, an electric motor and a transmission mechanism are provided inside the housing. The main switch is electrically connected to the control board for controlling power on/off, and the electric motor is electrically connected to the control board to drive the push rod to move forward or rearward via the transmission mechanism. The electric glue gun further comprises a sensor switch for facilitating control of a forward/rearward movement stroke of the push rod. The sensor switch is electrically connected to the control board, such that the control board determines whether the push rod is moved to a limit position based on information feedback from the sensor switch.

Preferably, the transmission mechanism comprises a transmission gear, and the sensor switch comprises a first sensor switch corresponding to teeth on the transmission gear for detecting the rearward movement stroke of the push rod.

Preferably, the first sensor switch is an optoelectronic switch, the optoelectronic switch includes an emitting end and a receiving end, and the emitting end and the receiving end are respectively arranged at two axially opposite sides of the transmission gear and disposed in mutual correspondence.

Preferably, the transmission mechanism comprises a gear box, the transmission gear is rotatably disposed inside the gear box, the first sensor switch is provided on the gear box, and the gear box is formed with a through-hole corresponding to the first sensor switch.

Preferably, the transmission mechanism comprises a transmission shaft and an output gear which are provided in the gear box, the output gear and the transmission gear are both sleeved on the transmission shaft, transmission teeth engaged with the output gear are provided on the push rod; and/or, a support portion is provided at a front end of the gear box, a support hole is provided on the support portion, a support sleeve is provided at the support hole, and the push rod is disposed to penetrate through the support sleeve.

According to the invention, a sensing element is provided at a rear end of the push rod, and the sensor switch comprises a second sensor switch corresponding to the sensing element for feeding back the forward movement stroke of the push rod.

According to the invention, the two second sensor switches are spaced apart from each other in a front-rear direction. In an operating state, the push rod is moved forward to a first limit position corresponding to a rear one of the second sensor switches. In a non-operating state, the push rod is moved forward to a second limit position corresponding to a front one of the second sensor switches.

Preferably, the sensing element is a magnet, and the second sensor switch is a magnetic sensor switch.

Preferably, the sensing element is disposed at a top side of the rear end of the push rod and the second sensor switch is provided above the sensing element; and/or a snap groove matched with the second sensor switch is provided on an inner wall of the housing, and the second sensor switch is disposed inside the housing by fitting with the snap groove.

Preferably, a trigger corresponding to the main switch is provided on the housing. The main switch is turned on when the trigger is pressed, and the main switch is turned off when the trigger is released; and/or, a push block is provided at a front end of the push rod.

With the above technical solutions, the present disclosure offers at least one of the following advantages:.

When the gun body is in an operating state, i.e., the glue tube is installed on the glue tube holder, as the glue tube itself has a certain thickness in the front-rear direction, the push rod is moved forward till the sensing element corresponds to the rear one of the second sensor switches. At this point, the push rod reaches the first limit position, thereby avoiding continued forward movement of the push rod. Otherwise, the pressing force of the front end of the push rod would damage the glue tube holder.

If the glue tube holder is not installed with the glue tube, since there is no block by the glue tube, the forward movement stroke of the push rod will increase a little. When the push rod is moved, due to reasons such as manual operations, forward till the sensing element corresponds to the front one of the second sensor switches, the push rod reaches the second limit position. At this point, if the main switch is turned on, the control board supplies power reversely to the electric motor, and the electric motor then rotates reversely to drive the push rod to move rearward, thereby avoiding the push rod from continued forward movement. Otherwise, the pressing force of the rear end of the push rod would damage the housing.

The sensing element preferably adopts magnet, and correspondingly, the second sensor switch preferably adopts a magnetic sensor switch. By reasonably setting the specific structures of the sensing element and the second sensor switch, the magnetic sensor switch feeds back a signal to the control board when sensing the magnet. As such, the control board promptly instructs the electric motor to rotate reversely to drive the push rod to move rearward, thereby avoiding the push rod from continued forward movement. Otherwise, the pressing force of the handle at the rear end of the push rod would damage the gun body.

The sensing element is disposed at the top side of the push rod, and the second sensor switch is disposed above the push rod. By reasonably setting the specific disposition positions of the sensing element and the second sensor switch, they can better satisfy detection requirements. A snap groove is provided inside the housing. The second sensor switch is disposed in the housing by fitting with the snap groove. By reasonably setting the specific mount structure of the second sensor switch, it becomes easily assembled, and the stability of the second sensor switch can be effectively guaranteed.

According to another aspect, a method for controlling the forward movement or rearward movement of the push rod is disclosed. The method comprises the step of determining whether the push rod is moved to a limit position based on information feedback from a sensor. Further advantageous details and steps of the method arise from the disclosure of the use and functioning of the electric glue gun.

According to another aspect, an electric glue gun having a control board, a main switch, an electric motor, a transmission mechanism and a sensor adapted to execute the steps of the disclosed method for controlling the forward movement or rearward movement of the push rod is proposed. Further advantageous details and means of the electric glue gun are provided in the disclosure here and can be used to further specify the device.

In the drawings: <NUM> - housing; <NUM>- left housing; <NUM>- right housing; <NUM>-positioning convex edge; <NUM>- boss; <NUM>- snap groove; <NUM>- glue tube holder; <NUM>- push rod; <NUM>- transmission tooth; <NUM>- support sleeve; <NUM>- socket sleeve; <NUM>- push block; <NUM> - handle; <NUM>- control board; <NUM> - main switch; <NUM> - trigger; <NUM>- electric motor; <NUM> - transmission mechanism; <NUM> - gear box; <NUM>- box body; <NUM>- box cover; <NUM>- support portion; <NUM>-ring-shaped groove; <NUM> - through-hole; <NUM>- transmission gear; <NUM> - transmission shaft; <NUM> - output gear; <NUM>- magnet; <NUM>- magnetic sensor switch; <NUM>- magnetic sensor switch I; <NUM> - magnetic sensor switch II, <NUM> - substrate; <NUM> - optoelectronic switch; <NUM>- emitting end; <NUM>-receiving end.

Hereinafter, the present disclosure will be described in further detail through preferred embodiments with reference to the accompanying drawings. It needs to be understood that the oriental or positional relationships indicated by the terms "upper," "lower," "left," "right," "longitudinal," "transverse," "inner," "outer," "vertical," "horizontal," "top," "bottom," etc. are oriental and positional relationships only based on the drawings, which are intended only for facilitating or simplifying description of the present disclosure, not for indicating or implying that the devices/elements have to possess those specific orientations or have to be configured and operated with those specific orientations. Therefore, they should not be understood as limitations to the present disclosure.

It should be pointed out that the invention is not intended to be restricted by the exemplary embodiments shown. In particular, unless specifically presented otherwise, it is also possible for certain aspects of the subject matter explained in the figures to be extracted and combined with other constituent parts and findings from the present disclosure and/or figures. It should be pointed out in particular that the figures and in particular the ratios of variables illustrated are merely schematic. The same reference signs denote the same objects, and this makes it possible for explanations from other figures to be used in supplementary fashion where appropriate.

As shown in <FIG>, the electric glue gun provided in Embodiment <NUM> comprises a housing <NUM>, a glue tube holder <NUM> arranged at the front end of the housing <NUM>, and a push rod <NUM> disposed on the housing <NUM> and moveable forward or rearward relative to the glue tube holder <NUM>. A control board <NUM>, a main switch <NUM>, an electric motor <NUM>, and a transmission mechanism <NUM> are disposed inside the housing <NUM>. The main switch <NUM> is electrically connected to the control board <NUM> for controlling power on/off, the electric motor <NUM> is electrically connected to the control board <NUM> to drive, via the transmission mechanism <NUM>, the push rod <NUM> to move forward or rearward. The electric glue gun further comprises a sensor switch for facilitating control of a forward/rearward movement stroke of the push rod <NUM>, and the sensor switch is electrically connected to the control board <NUM>, such that the control board <NUM> determines whether the push rod <NUM> is moved to a limit position based on information feedback from the sensor switch.

In this embodiment, the housing <NUM> comprises a left housing <NUM> and a right housing <NUM> which are fixedly connected together, and the electric motor <NUM> is provided at the rear portion inside the housing <NUM>. To effectively switch on or off the main switch <NUM>, a trigger <NUM> corresponding to the main switch <NUM> is provided on the housing <NUM>. When the trigger <NUM> is pressed, the main switch <NUM> is turned on and the control board <NUM> is energized. When the trigger <NUM> is released, the main switch <NUM> is turned off.

The transmission mechanism <NUM> comprises a gear box <NUM> and a gear transmission mechanism disposed in the gear box <NUM>. The gear box <NUM> comprises a box body <NUM> and a box cover <NUM>. One side of the box body <NUM> is provided with an opening. The box cover <NUM> is connected to the opening side of the box body <NUM> for closing the box body <NUM>, allowing the inside of the gear box to form a relatively independent cavity. A support portion <NUM> formed by forward extension is provided on the box body <NUM>. A support hole extending in a front-rear direction through the support portion is provided in the support portion <NUM>. A support sleeve <NUM> is fixed at the support hole. The push rod <NUM> is arranged to penetrate through the support sleeve <NUM>. With the support sleeve <NUM>, the push rod <NUM> is subjected to a certain limit, thereby enhancing stability of the push rod <NUM> when moving in the front-rear direction. A front end of the support portion <NUM> is provided with a lap of ring-shaped groove <NUM>. The front ends of the left and right housings <NUM>, <NUM> are both provided with a semi-circular positioning convex edges <NUM> which are snapped into the ring-shaped groove <NUM>. With the cooperation between the positioning convex edges <NUM> and the ring-shaped groove <NUM>, the support portion <NUM> maintains fixed. The front end of the support portion <NUM> is connected with a hollow socket sleeve <NUM>. The rear end of the glue tube holder <NUM> is fixedly connected with the socket sleeve <NUM> by thread-fitting. The rear end of the glue tube holder <NUM> is hollow, and the front end of the push rod <NUM> projects into the glue tube holder <NUM>. To effectively extrude the glue tube, a push block <NUM> is fixed at the front end of the push rod <NUM>, and the push block is disposed on the glue tube holder. To allow manual adjustment of the push rod when the gun body fails, a handle <NUM> is fixedly provided at the rear end of the push rod <NUM>.

The gear transmission mechanism provided in the gear box <NUM> comprises a transmission gear <NUM>, and the transmission gear <NUM> is rotatably disposed in the gear box <NUM>. The sensor switch includes a first sensor switch corresponding to the teeth on the transmission gear <NUM>, and the first sensor switch is configured for detecting the rearward movement stroke of the push rod <NUM>. In this embodiment, the first sensor switch preferably adopts an optoelectronic switch <NUM>, and the optoelectronic switch <NUM> is fixed on the gear box <NUM> and electrically connected to the control board <NUM>. The optoelectronic switch <NUM> comprises an emitting end <NUM> and a receiving end <NUM>, and the emitting end <NUM> and the receiving end <NUM> are respectively disposed at two axial sides of the transmission gear <NUM> and are arranged in mutual correspondence. A through-hole <NUM> for avoidance is provided on the box body <NUM> and the box cover <NUM>. When the transmission gear <NUM> rotates till the teeth are located between the emitting end <NUM> and the receiving end <NUM>, the receiving end cannot receive the signals emitted by the emitting end due to obstruction by the teeth. Therefore, the control board <NUM> determines the specific rearward movement stroke of the push rod <NUM> based on the number of times of the failure of receiving the signals fed back by the optoelectronic switch <NUM>.

The transmission mechanism <NUM> drives, via toothed engagement, the push rod <NUM> to move forward or rearward. A plurality of transmission teeth <NUM> arranged in the front-rear direction are provided in the bottom part of the push rod <NUM>. The gear transmission mechanism further comprises a three-pole planetary gear structure (not shown), a transmission shaft <NUM> and an output gear <NUM>. The transmission shaft <NUM> is rotatably disposed in the gear box <NUM>. The transmission gear <NUM> and the output gear <NUM> are sleeved on the transmission shaft <NUM>. The teeth on the output wheel <NUM> are engaged with the transmission teeth <NUM> at the bottom portion of the push rod <NUM>. The three-pole planetary gear structure is disposed between the motor shaft of the electric motor <NUM> and the output gear <NUM>. When the electric motor <NUM> operates, the motor shaft drives the transmission gear <NUM> to rotate via the three-pole planetary gear structure, the transmission gear <NUM> then drives the output gear <NUM> to rotate via the transmission shaft <NUM>, and the output gear <NUM> further drives the push rod <NUM> to move forward or rearward via toothed engagement. In this embodiment, the output gear <NUM> and the transmission gear <NUM> are preferably arranged into an integral structure. The transmission ratio between the output gear <NUM> and the transmission gear <NUM> is preferably set to <NUM>:<NUM>.

To effectively control the forward movement stroke of the push rod <NUM>, a sensing element is provided at a rear end of the push rod <NUM>, the sensor switch comprising a second sensor switch corresponding to the sensing element. When the push rod <NUM> moves forward till the sensing element corresponds to the second sensor switch, the forward movement of the push rod <NUM> is in position. Then, the control board <NUM> may promptly cut off the current in the electric motor <NUM> based on the information feedback from the second sensor switch to stop the push rod <NUM> from forward movement.

In this embodiment, the sensing element adopts a magnet <NUM> and the second sensor switch adopts a magnetic sensor switch <NUM>. A recessed hole for receiving the magnet <NUM> is provided at the top side of the rear end of the push rod <NUM>. The magnet <NUM> is fixed in the recessed hole. Because the maximum stroke of forward movement of the push rod <NUM> differs somewhat dependent on whether a glue tube is installed on the glue tube holder, two magnetic sensor switches <NUM> are spaced apart from each other in the front-rear direction, including a first magnetic sensor switch <NUM> disposed at the rear and a second magnetic sensor switch <NUM> disposed in the front. To facilitate installation, two magnetic sensor switches are integrally arranged on one substrate <NUM>. The substrate <NUM> is disposed above the push rod <NUM> and electrically connected to the control board <NUM>. To install the substrate <NUM>, a boss <NUM> integrally formed with the housing <NUM> is provided on the inner wall of the housing <NUM>. A snap groove <NUM> for the edge of the substrate <NUM> to be snapped into is provided on the boss <NUM>. The substrate <NUM> is fixed in the housing <NUM> by fitting with the snap groove <NUM>.

When the gun body is in an operating state, i.e., the glue tube is installed on the glue tube holder <NUM>, as the glue tube itself has a certain thickness in the front-rear direction, the push rod <NUM> moves forward till the magnet <NUM> is vertically aligned with the first magnetic sensor switch <NUM>. At this time, the push rod <NUM> reaches the first limit position. When the gun body is in a non-operating state, i.e., the glue tube is not installed on the glue tube holder <NUM>, since there is no obstruction by the glue tube, the forward movement stroke of the push rod <NUM> increases a little. Due to causes such as manual operation, when the push rod <NUM> moves forward till the magnet <NUM> is vertically aligned with the second magnetic sensor switch <NUM>, the push rod <NUM> moves forward till the second limit position.

When the gun body is in an operating state, i.e., when the glue tube is installed on the glue tube holder <NUM>, by pressing the trigger <NUM>, the main switch <NUM> is turned on, the control board <NUM> is energized to supply power to the electric motor <NUM>. The electric motor <NUM> is energized to rotate clockwise, and the motor shaft drives, via the transmission mechanism <NUM>, the push rod <NUM> to move forward. When the push rod <NUM> moves forward till the magnet <NUM> vertically corresponds to the first magnetic sensor switch <NUM>, the control board <NUM> determines, based on the signal feedback from the first magnetic sensor switch <NUM>, that the push rod <NUM> reaches the first limit position. If the trigger <NUM> is continued to be pressed, the control board <NUM> supplies power reversely to the electric motor <NUM>. Then, the electric motor <NUM> rotates reversely, and the motor shaft drives the push rod <NUM> to move rearward via the transmission mechanism <NUM>. During the process of rearward movement of the push rod <NUM>, the optoelectronic switch <NUM> feeds back a signal to the control board <NUM> under the obstruction of the teeth on the transmission gear <NUM>. When the number of times of the optoelectronic switch <NUM> is obstructed by the teeth reaches N1, the control board <NUM> determines, based on the signal fed back from the optoelectronic switch <NUM>, that the push rod <NUM> is moved rearward to the first limit position, the control board <NUM> suspends power supply to the electric motor. Then, the electric motor <NUM> stops rotating, and the push rod <NUM> stops rearward movement. If the maximum number of times of the engagement between the transmission teeth <NUM> and the output gear <NUM> during the front-to-rear movement of push rod <NUM> reaches <NUM> times, as the transmission ratio between the output gear <NUM> and the transmission gear <NUM> is set to <NUM>:<NUM>, N1 should be <NUM> times.

The gun body is in a non-operating state, i.e., the glue tube is not installed on the glue tube holder <NUM>. When the push rod moves forward till the magnet is vertically aligned with the second magnetic sensor switch <NUM>, the push rod <NUM> reaches the second limit position. At this point, the trigger <NUM> is pressed, and the control board <NUM> determines, based on the signal feedback from the second magnetic sensor switch <NUM>, that the push rod <NUM> reaches the second limit position. If the trigger <NUM> is continued to be pressed, the control board <NUM> supplies power reversely to the electric motor <NUM>. Then, the electric motor <NUM> rotates reversely, and the motor shaft drives the push rod <NUM> to move rearward via the transmission mechanism <NUM>. During the process of rearward movement of the push rod <NUM>, the optoelectronic switch <NUM> feeds back a signal to the control board <NUM> under the obstruction of the teeth on the transmission gear <NUM>. When the number of times of the optoelectronic switch <NUM> being obstructed by the teeth reaches N2, the control board <NUM> determines, based on the signal feedback from the optoelectronic switch <NUM>, that the push rod <NUM> is moved rearward to the second limit position. The control board then suspends power supply to the electric motor. The electric motor <NUM> stops rotating, and the push rod <NUM> stops rearward movement. If the interval between the first magnetic sensor switch <NUM> and the second magnetic sensor switch <NUM> corresponds to a distance of three transmission teeth <NUM> and the maximum number of times of the engagement between the transmission teeth <NUM> and the output gear <NUM> during the front-to-rear movement of push rod <NUM> reaches <NUM> times, as the transmission ratio between the output gear <NUM> and the transmission gear <NUM> is set to <NUM>:<NUM>, N2 should be <NUM> times.

If the trigger <NUM> in the operating state is released and the push rod <NUM> does not reach the first limit position, in order to prevent glue dripping, the control board <NUM> supplies power reversely to the electric motor after the trigger <NUM> is released, then the electric motor <NUM> rotates reversely, and the motor shaft drives the push rod <NUM> to move rearward a certain distance via the transmission mechanism <NUM>. Supposing the rearward movement distance of the push rod <NUM> to prevent glue dripping is the length of one transmission tooth <NUM>, because the transmission ratio between the output gear <NUM> and the transmission gear <NUM> is <NUM>:<NUM>, then the number of teeth traversed by reverse rotation of the transmission gear <NUM> should be three, i.e., the optoelectronic switch <NUM> is blocked thrice by the teeth of the transmission gear <NUM>. The control board <NUM> determines, based on the signal feedback from the optoelectronic switch <NUM>, whether the push rod <NUM> is moved rearward to an appropriate position. When the optoelectronic switch <NUM> is obstructed thrice by the teeth of the transmission gear <NUM>, the push rod <NUM> moves rearward till the appropriate position, the control board then suspends power supply, the electric motor <NUM> stops rotating, and the push rod <NUM> stops rearward movement.

It may be understood that the specific mount position of the substrate <NUM> is not limited to the above disclosure or the drawings, and other reasonable fixation and mount structures may also be adopted, e.g., screw fixation.

It may be understood that the fitting between the sensing element and the first sensor switch is not limited to the above fitting between the magnet <NUM> and the magnetic sensor switch <NUM>, and other reasonable fitting members may also be employed.

It may be understood that the second sensor switch is not limited to the optoelectronic switch <NUM> disclosed above or illustrated in the drawings, and other sensor switches capable of detecting the number of teeth traversed by rotation of the transmission gear <NUM> may also be employed.

It may be understood that the output gear <NUM> and the transmission gear <NUM> may also adopt a separate structure, i.e., the output gear and the transmission gear are separately formed and respectively fixedly sleeved on the transmission shaft <NUM>.

It may be understood that the transmission ratio between the output gear <NUM> and the transmission gear <NUM> may also adopt another appropriate transmission ratio. The maximum number of times of the engagement between the transmission gear <NUM> and the output gear <NUM> when the push rod <NUM> moves from front to rear may also be set to another reasonable number of times based on the specific length of the push rod. Correspondingly, the specific values of N1 and N2 are determined reasonably based on the maximum number of times of the engagement with the output gear when the push rod moves as well as the transmission ratio between the output gear <NUM> and the transmission gear <NUM>.

Claim 1:
An electric glue gun, comprising a housing (<NUM>), a glue tube holder (<NUM>) arranged at a front end of the housing (<NUM>), and a push rod (<NUM>) disposed on the housing (<NUM>) and moveable forward and rearward relative to the glue tube holder (<NUM>), wherein a control board (<NUM>), a main switch (<NUM>), an electric motor (<NUM>) and a transmission mechanism (<NUM>) are provided inside the housing (<NUM>), the main switch (<NUM>) is electrically connected to the control board (<NUM>) for controlling power on/off, the electric motor (<NUM>) is electrically connected to the control board (<NUM>) to drive the push rod (<NUM>) to move forward or rearward via the transmission mechanism (<NUM>); and the electric glue gun further comprising a sensor switch for facilitating control of a forward movement stroke and a rearward movement stroke of the push rod (<NUM>), the sensor switch is electrically connected to the control board (<NUM>), such that the control board (<NUM>) determines whether the push rod (<NUM>) is moved to a limit position based on information feedback from the sensor switch, characterised in that
a sensing element is provided at a rear end of the push rod (<NUM>), and the sensor switch comprises a second sensor switch corresponding to the sensing element for feeding back the forward movement stroke of the push rod (<NUM>), wherein two second sensor switches are spaced apart from each other in a front-rear direction; in an operating state, the push rod (<NUM>) is moved forward to a first limit position corresponding to a rear one of the second sensor switches; when in a non-operating state, the push rod (<NUM>) is moved forward to a second limit position corresponding to a front one of the second sensor switches.