JPH0817624B2 - Automatic work machine height adjustment device for agricultural work machines - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is a work for adjusting the working machine to a predetermined height by ultrasonically detecting the working machine height of a working machine in a combine working machine, a tractor, etc. The present invention relates to an automatic height adjusting device.

(Prior Art) Generally, in harvesting agricultural machines such as combine harvesters,
The cutting height of the cutting unit (height with respect to the field) in order to keep the cutting height of the grain culms cut by the cutting unit as a work machine constant.
When the mowing height sensor detects the mowing height sensor, the mowing portion is always adjusted to the predetermined height by the raising / lowering request signal to the mowing portion which is output based on the detection height detected by the sensor. It is known to be equipped with a work machine height automatic adjusting device, and to use ultrasonic waves as a height sensor used for such a work machine automatic height adjusting device of a harvesting agricultural working machine.

That is, an ultrasonic wave transmitter and a receiver are attached to the mowing unit, and the ultrasonic waves emitted from the ultrasonic wave are reflected by the receiver at the receiver, and the time between them is detected to detect the mowing unit. The distance between the field and the field, that is, the height of the mowing part from the field is detected.

(Problems to be Solved by the Invention) By the way, in an agricultural work machine provided with an ultrasonic type height sensor, for example, a harvesting work machine such as a combine harvester has a mowing unit provided with a height sensor for detecting a cutting height. In addition, since a cutting blade for cutting the grain culm is provided, when driving the cutting blade, an impact sound including ultrasonic waves is emitted, and when it is used for a long period of time, It also occurs frequently.

Therefore, in an agricultural machine equipped with an ultrasonic type height sensor, when ultrasonic noise occurs due to an impact sound containing ultrasonic waves, the ultrasonic receiver receives the ultrasonic waves reflected on the field surface. May receive ultrasonic noise before,
In that case, the height sensor detects the height as if it were in a low height state.

Therefore, based on this detection, the working machine height automatic adjuster issues a lift command signal for raising the working machine, and raises the working machine by mistake, which is expensive for a harvesting agricultural working machine equipped with a cutting blade. There is a risk that

Further, the lifting request signal output based on the height detected by the ultrasonic sensor, that is, the information for raising or lowering the working machine, changes for each transmission cycle of the ultrasonic transmitter, There is a possibility that the machine suddenly switches from the ascending drive to the descending drive, and the height control of the work machine may be in the hunting state. That is, at the same time that the drive signal for driving the working machine up is switched from the on state to the off state, the drive signal for driving the working machine down may be switched from the off state to the on state.

As described above, in the conventional working machine height automatic adjusting device using the ultrasonic height sensor, there is a problem of malfunction due to ultrasonic noise and a problem of hunting. It has the drawback that it cannot be controlled.

An object of the present invention is to solve the above-mentioned problems of a work machine height automatic adjusting device provided with an ultrasonic type height sensor, avoid malfunctions due to ultrasonic noise and hunting of control, and thus, provide a good height. It is a point to provide a working machine height automatic adjusting device of an agricultural working machine that can be controlled.

(Means for Solving Problems) Therefore, according to the present invention, ultrasonic waves are periodically emitted from an ultrasonic transmitter toward a field surface, and ultrasonic waves reflected on the field surface are received by an ultrasonic receiver. Equipped with a work implement height sensor that detects the work implement height, and based on the detected height detected by this work implement height sensor, the work implement is lifted and lowered to a predetermined height by a lift request signal to the work implement. In such a working machine height automatic adjusting device for agricultural working machines, a judging means (3) for judging a raising / lowering request signal of the sensor to the working machine at predetermined time intervals, and a judging means (3). When the output of the lift request signal is determined in 3), timing is started, and when the lift request signal is determined to be canceled during the expiration of the set time period or during the time before the set time period expires The time measuring means (1) and the determining means ( 3) The timer is started after it is determined that the lifting request signal is released in step 3), and the timing is stopped when it is determined that the lifting request signal is output during the expiration of the set timing or during the timing before the set timing expires. The working machine is moved up and down after the set time period of the time measuring means (2) and the first time measuring means (1) expires, and the work machine is moved up and down after the set time period of the second time measuring means (2) expires. And a working machine lifting control means (4) for stopping the work machine are provided, while the set time period of the first and second time measuring means (1) and (2) is set longer than the transmission cycle of the ultrasonic transmitter, The setting time period of the first time measuring means (1) is set to be longer than the setting time period of the second time measuring means (2) at least longer than the determination time interval of the lifting request signal by the determination means (3). It is a feature.

(Operation) By setting the set timing period of the first and second timing means (1) and (2) to be longer than the transmission cycle of the ultrasonic transmitter, the ascending / descending request signal is continuously generated by the set timing period. Therefore, it is possible to discriminate the erroneous raising / lowering request signal due to the ultrasonic noise that occurs sporadically, and raise / lower the work implement based on only the regular raising / lowering request signal continuously output during the set time period. Controllable and also the first
The set time period of the time measuring means (1) is set to the second time measuring means (2).
By setting the time period longer than the set time period of (1) at least as long as the determination time interval of the lifting request signal by the determination means (3), a time difference can be provided for switching between the ascending drive and the descending drive in the work machine, and hunting is performed. It can be prevented.

(Example) FIG. 2 is a schematic right side view of a harvester as an example of an agricultural working machine equipped with the working machine height automatic adjusting device according to the present invention.

In FIG. 2, reference numeral (10) denotes traveling crawlers (only the right side is shown) provided on the lower left and right sides of the machine body (20), and the driver's seat (30) and the threshing device (40) are provided on the upper side of the machine body (20). ) Is installed.

On the front side of the machine body (20), a mowing section (8), which is a working machine composed of a weeding board (5), a cutting blade (6), a grain culm raising device (7), etc., is cut by a hydraulic cylinder (9). The lower end of the part (8) is attached to the body (20) so that it can be raised and lowered.
The cutting height of the cutting unit (8) can be changed.

A mounting plate (52) is erected on a mounting rod (51) of the grass plate (5) on the right side of the reaping section (8), and a working machine height sensor (S) is provided on this. Ultrasonic transmitter (11) and ultrasonic receiver (12) of this work machine height sensor (S)
Are arranged side by side in the front-rear direction of the machine body with their ultrasonic transmitting and receiving surfaces facing downward. Further, the ultrasonic waves emitted by the ultrasonic transmitter (11) are reflected on the field surface and received by the ultrasonic receiver (12).

FIG. 1 shows an electric circuit diagram and a hydraulic circuit diagram of the automatic cutting height adjusting device of the present invention.

In FIG. 1, (M) is a control section using a microcomputer, and a drive circuit (13) for operating the ultrasonic transmitter (11) of the working machine height sensor (S) at the output port (b ₁ ). ) Output is given.

The output of the drive circuit (13) is also given to the ultrasonic transmitter (1) of the work height sensor (S), and the ultrasonic transmitter (11) transmits ultrasonic waves of a predetermined frequency to the field. The ultrasonic wave receiver (12) emits an ultrasonic wave to the ultrasonic wave emitted from the ultrasonic wave transmitter (11) and reflected on the field surface.

The output of the ultrasonic receiver (12) is given to the input port (a ₁ ) of the control unit (M). A clock pulse of a clock pulse oscillator (14) including a crystal oscillator or the like is applied to the input port (a ₂ ).

The output port (b ₂ ) of the control section (M) is a solenoid (Su) of an electromagnetic switching valve (28) provided in the hydraulic circuit of the hydraulic cylinder (9) for raising and lowering the cutting section via the drive circuit (15). To
The output port (b ₃ ) is connected to the solenoid (Sd) via the drive circuit (16). When the solenoid (Su) is excited, the cutting unit (8) moves up, and when the solenoid (Sd) is excited, the cutting unit (8) moves down.

Then, the ultrasonic transmitter (11) of the work implement height sensor (S) drives the drive circuit (13) at the cycle (Ts), and the ultrasonic signal of the one-shot pulse is transmitted from the ultrasonic transmitter (12). Emit to the field. Then, the time from when the ultrasonic wave transmitter (11) transmits the ultrasonic wave to when the ultrasonic wave is received by the ultrasonic wave receiver (12) is detected, and the reaper (8) is detected based on the time difference. The cutting height is calculated, and the output of the raising / lowering request signal for determining whether to drive the cutting unit (8) upward or downward is determined based on the calculated cutting height.

The transmission period (Ts) of the ultrasonic transmitter (11) is, for example, about 26 (msec), and this period (Ts = 26
msec) is longer than this maximum time when the mowing unit (8) is at the highest point and the time required from the transmission to the reception of ultrasonic waves is the longest, and at least the receiver ( The reception according to 12) is completed within the period (Ts = 26 msec).

Then, in the automatic adjustment device configured as described above, the determination means (3) for determining the raising / lowering request signal of the sensor (S) to the reaper (8) at predetermined time intervals. When the determination means (3) determines the output of the raising / lowering request signal, the clocking is started, and when the release of the raising / lowering request signal is discriminated during the expiration of the set timing time or before the expiration of the set timing period. The first timing means (1) to be stopped and the determination means (3) determine that the lifting request signal has been released, and then start timing, and during the expiration of the set time period or before the expiration of the established time period. Second timing means (2) for stopping the timing when the output of the lifting request signal is discriminated, and the second timing means (2) for driving the working machine up and down after expiration of the set timing time of the first timing means (1). 2)
And a working machine lifting control means (4) for stopping the lifting and lowering drive of the working machine after the lapse of the counting time, while transmitting the ultrasonic wave for the set timing period of the first and second timing means (1) and (2). It is set to be longer than the transmission cycle of the instrument and the first
The set timing of the timing means (1) is set to be longer than the preset time of the second timing means (2) at least longer than the determination time interval of the lifting request signal by the determination means (3).

Specifically, the determining means (3) determines an UP request discriminator (3) for determining whether or not there is a rising request signal (hereinafter simply referred to as an UP request) to the reaper (8) from the sensor (S).
u) and a DN request discriminator (3d) for discriminating the presence / absence of a descending request signal (hereinafter simply referred to as a DN request) to the reaper (8) by the sensor (S).

The first timing means (1) is the UP request discriminator (3u).
After the output of the UP request is determined by the
An UP on-timer (1u) that stops timing when the release of the UP request is determined during the expiration of the set time period or during the time before the set time period expires, and the DN request by the DN request discriminator (3d) A DN-on-timer (1d) that starts timing after determining the output of, and stops the timing when it is determined that the DN request is canceled during the expiration of the set timing period or during the timing before the set timing period expires. ing.

The second timing means (2) is the UP request discriminator (3u).
After the release of the UP request is determined by the
An UP off timer (2u) that stops timing when the output of the UP request is discriminated during the expiration of the set timing period or during the timing before the set timing period expires, and the DN request by the DN request discriminator (3d) A DN off timer (2d) is provided to start timing after determining whether to cancel the setting, and to stop timing when the output of the DN request is determined during the expiration of the set timing period or during the timing before the set timing period expires. ing.

The working machine lifting control means (4) configures the drive circuits (15) and (16) by setting the output ports (b ₂ ) and (b ₃ ) to a high level “H” or a low level “L”. The solenoid (Su) (Sd) is energized or de-energized by turning on or off the transistor to be turned on, and the output port (b ₂ ) is turned on after the set period of the UP on-timer (1u) expires. and "H", the the UP oFF timer UP solenoid oN / oFF output means the output port after expiration of the setting time measurement period (2u) and (b ₂₎ to "L" (4u), the DN-timer of (1d) said output port after expiration of the set counting period (b ₃₎ is "H", DN solenoid on / off the output port after expiration of the setting time counting period of the DN oFF timer (2d) and (b ₃₎ and "L" And output means (4d).

On the other hand, the timers (1u) (1d) (2u) (2d) provided in the first and second timing means (1) and (2) are for timing the clock pulse from the oscillator (14). hand,
The set period of the timer (1u) (1d) (2u) (2d) is set to the transmission cycle (Ts = 26msse) of the ultrasonic transmitter (11).
The UP and DN on-timers (1u) (1d) are set at least longer than the UP and DN-off timers (2u) (2d) by the determination means (3). Discrimination time interval of lift request signal,
That is, it is set longer than the discrimination time interval of the UP request discriminator (3u). Incidentally, in the present embodiment, the set time period of the on-timer (1u) (1d) is set to 60 msec,
In addition, the set time period of the off timer (2u) (2d) is set to 30
It is set to msec.

Depending on the branch destination in the process flow described below,
There is a difference in the processing time of one flow, and the discriminator (3u)
There is a possibility that the discriminating time interval of the discriminant may also differ from the shortest discriminating time interval, but with the longest discriminating time interval of the discriminator (3u) when the processing is delayed most, the set timing period of the first timing means (1) is set. Needless to say, the second timing means (2) is set for the set timing period. Further, the discrimination time interval may be adjusted to be a constant time by a timer routine provided as a dummy. Furthermore, the processing after the discriminator (3u) may be called at regular time intervals by a timer interrupt processing or the like. Incidentally, in the present embodiment, the discrimination time interval is uniformly set to 10 msec.

The operation of the automatic adjusting device configured as described above will be described based on the flowchart shown in FIG.

In FIG. 3, a routine (P ₁ ) is an initial setting routine for automatic cutting height control, in which the output ports (b ₂ ) (b ₃ ) are set to “L” and the solenoids (Su) (Sd) are both set to non-operation. While exciting, the timers (1u) (1d) (2u) (2d) are reset and all the measured values are set to zero.

Routine (P ₂ ) is an UP processing routine that controls the output of the output port (b ₂ ) based on the PU request discrimination by the UP request discriminator (3u) and controls the drive of the rising solenoid (Su). Is.

The routine (P ₃ ) is a DN processing routine that controls the output of the output port (b ₃ ) based on the DN request discrimination result by the DN request discriminator (3d) and controls the drive of the descending solenoid (Sd). Is.

In the UP processing routine (P ₂ ), when there is an UP request, and when the output port (b ₂ ) is not “H”,
That is, only when the output port (b ₂ ) is going to be “H”, the branch routine (P ₂₁ ) measures the time of the UP on-timer (1u), and when the UP request is made, and , Only when the output port (b ₂ ) is “H”, that is, when the output port (b ₂ ) is going to be “L”, the timing of the UP off timer (2u) is measured by the branch routine (p ₂₃ ). To do.

On the other hand, in the previous determination, there is an output of the UP request, the despite not perform counting of the at branch routine (P ₂₁₎ UP ON timer (1u), in the current determination, is canceled UP request If determined, the branch routine (P ₂₄ ) resets the UP on-timer (1u) to make the measured value zero. Further, in this determination, there is no output of the UP request, despite the was performed counting the UP OFF timer (2u) in the branch routine (P _23), the output of the UP request in the current discrimination discrimination If it is done, the branch routine (P ₂₂ )
The off-timer (2u) is reset and its clock value is set to zero.

Then, in the branch routine (P _21), the UP
If it is determined that the time value of the on-timer (1u) has reached 60msec or more, set the output port (b ₂ ) to "H" and
The solenoid (Su) is excited, and the UP on-timer (1u) is reset and the measured value is made zero. On the other hand, in the branch routine (P _23), when the counting value of the UP OFF timer (2u) has determined that a more 30msec is output ports (b ₂₎ in the "L", the solenoid ( Su) is de-energized, and the UP off timer (2u) is reset to zero the time value.

Although the above description is for the UP processing routine (P ₂ ), the DN processing routine (P ₃ ) based on the DN request discriminator (3d) also has the same control as the routine (P ₂ ). However, the description thereof will be omitted.

Next, it will be described with reference to the time chart shown in FIG. 4 that the automatic adjustment device for performing the above operation can avoid malfunction due to ultrasonic noise and control hunting.

In this embodiment, the UP and DN requests determined by the transmission and reception of ultrasonic waves are settled in the next transmission cycle, as shown in FIG. That is, the UP request in the cycle is output in the next cycle. UP in cycle
For the DN request and the DN request, the state before the automatic cutting height control will be actually held, but for convenience, it is assumed to be an undetermined value. The initialization routine (P ₁ ) is executed at the time point of t = 0 in the cycle.

 Each cycle will be described below.

In the cycle, the reception voltage is detected relatively early after the ultrasonic wave transmission, and therefore the mowing unit (8) is in the low position state, and the UP request is output in the next cycle. .

UP after the first UP discrimination in the cycle and the first DN discrimination
On-timer (1u) and DN on-timer (1d) clock status,
That is, it is determined whether the time is measured or the time is stopped. In this case, since there is an UP request, the UP on-timer (1u) is in the timekeeping (however, whether the timekeeping start or the timekeeping is determined by the state in the previous cycle), while there is no DN request,
The DN on timer (1d) is stopped. In addition, UP and DN off timer (2u) (2d), the initial setting routine (P ₁ )
It goes without saying that both of the output ports (b ₂ ) and (b ₃ ) are set to “L” to maintain the time-counting stopped state from the previous cycle.

In addition, in this cycle, it is assumed that the normal reception voltage is not detected (hereinafter referred to as “missing”) and ultrasonic noise is picked up later. As a result, in the next cycle, the UP request is erroneously canceled and the DN request is output instead. The missing occurs when, for example, an ultrasonic wave hits a stump or the like and is attenuated.

Therefore, in the cycle, the UP on-timer (1u) is reset and the DN on-timer (1d) is started.

However, in many cases, the above-mentioned erroneous detection does not occur continuously. Therefore, the detection of the regular reception voltage in the cycle causes the UP request to be output again in the cycle,
When the DN request is released, the UP on-timer (1u) is started again and the DN on-timer (1d) is reset.

After that, by detecting the continuous regular reception voltage in the cycle to, more specifically, by detecting the continuous regular reception voltage for the set time period of the UP on-timer (1u), the UP on-timer in the cycle, that is, in the cycle After 60 msec has passed from the start of (1u), the output port (b ₂ ) becomes "H", and the ascending side solenoid (Su) is excited to raise the reaper (8).

Although the UP request is output in the previous cycle in the cycle, the UP on-timer (1u) is no longer clocked because the output port (b ₂ ) has already been set to “H” in the previous cycle.

According to the above procedure, when the output port (b ₂ ) is “H” and the reaper (8) is being driven upward,
If missing occurs again and noise occurs relatively later in the transmission cycle, the UP request will be canceled in the next cycle, and the DN request will be output instead. Well, output port (b ₂ )
Has no effect on.

In other words, the UP off timer (2u) and DN timer (1d) will be started in each cycle, but in the next cycle, the normal UP and DN requests based on the detection result of the regular received voltage by the cycle will be reset. Therefore, the UP off timer (2u) started by the false detection of the previous cycle.
And the DN on timer (1d) are both reset.

Then, the reaper (8) is driven to near the predetermined height,
By releasing the UP request based on the detection of the continuous regular reception voltage in the cycle and, the UP off timer (2
u) is started, the output port (b ₂ ) becomes “L” in a cycle after 30 msec has elapsed, the rising side solenoid (Su) is de-energized, and the cutting unit (8) is stopped.

It is clear that the above can eliminate both the noise (cycle) generated when the cutting unit (8) is going to be raised and the noise (cycle) generated when the cutting unit (8) is driven to rise. However, in addition, the noise (cycle) generated when the cutting unit (8) is going to be moved up and the noise (cycle) generated when it is driven downward can be eliminated by the same procedure. Needless to say.

That is, in the present embodiment, the output ports (b ₂ ) and (b ₃ ) are not immediately controlled based on the UP and DN requests from the sensor, but the UP and DN on-timers (1u) are not controlled.
(1d) and the UP and DN off timers (2u) (2d)
By setting the setting time period of each of these timers to 60 msec or 30 msec, UP and DN requests from the sensor are continuously discriminated by the amount of the set time period, and based on the consecutive discrimination result, Since the output ports (b ₂ ) and (b ₃ ) are controlled, malfunctions due to noise can be reliably eliminated.

Also, the UP and DN on-timer (1u) (1d) set time period (ie 60msec) is at least UP and DN request more than the UP and DN off-timer (2u) (2d) set time period (ie 30msec). Discrimination time interval (10)
msec), so the output port (b ₂ )
The situation in which the two outputs in (b ₃ ) are switched at the same time can be avoided, and therefore, control hunting can be surely prevented.

What happens is that even if the UP request and the DN request are reversed at the same time in the cycle, the output (b ₃ ) is in the cycle and the output (b ₂ ) is in the cycle, and they are switched in different cycles. The above two outputs (b ₂ ) (b ₃ )
During the change of, a dead zone where both are "L" is formed.

In the above embodiment, UP and DN on timer (1u) (1
d) That is, the set time period of the first time measuring means (1) is 60 mse
Also, the UP and DN off timers (2u) and (2d), that is, the set timing time of the second timing means (2) is set to 30 msec. However, these set timing periods are the ultrasonic wave transmission cycle (26 mse
Together with c), it determines the continuous discrimination time interval for noise elimination and the dead zone time of control, so it may be changed as appropriate in consideration of the frequency of noise occurrence. Needless to say.

In the above embodiment, the ultrasonic wave transmission cycle, that is, UP
And the change cycle of DN request is set to 26msec, and UP and
The DN request determination time interval is set asynchronously with the ultrasonic wave transmission cycle and is set to 10 msec, but the UP and DN request determination timing is synchronized with the ultrasonic wave transmission timing, that is, the determination time. The interval may be set to 26 msec, and the determination may be performed only once within one transmission cycle.

(Effects of the Invention) As described above, the present invention periodically emits ultrasonic waves toward the field surface from the ultrasonic transmitter, receives the ultrasonic waves reflected on the field surface by the ultrasonic receiver, and increases the working machine height. Farming work equipped with a work machine height sensor that detects the height of the work machine, and that raises and lowers the work machine to a predetermined height by a lifting request signal to the work machine that is output based on the detected height detected by the work machine height sensor. A device for automatically adjusting the height of a working machine of a machine, comprising: a judging means (3) for judging a lift request signal of the sensor to the working machine at predetermined time intervals, and the judging means (3). Start timing after determining the output of the lift request signal,
When the release of the raising / lowering request signal is discriminated during the expiration of the set timing period or during the timing before the set timing period expires, the first timing means (1) for stopping the timing and the raising / lowering request signal by the discrimination means (3). Second timing means (2) that starts timing after it is determined that the setting is released, and stops the timing when the output of the lift request signal is determined during the expiration of the set timing period or during the timing before the set timing period expires. A working machine lifting control means for driving the working machine up and down after the set time period of the first timing means (1) expires and stopping the lifting drive of the working machine after the timing period of the second timing means (2) elapses (4) is provided, while the set timing period of the first and second timing means (1) and (2) is set longer than the transmission cycle of the ultrasonic transmitter, and the first timing means (1) is provided. The set time period of the second time measuring means (2 It is characterized in that it is set to be longer than at least the set time period of the above-mentioned determination time interval of the ascending / descending request signal by the determination means (3), so that malfunctions due to noise and hunting of control can be avoided. Good height control of the machine is possible.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an electric circuit and a hydraulic circuit of an automatic adjusting device of the present invention, FIG. 2 is a schematic right side view of a harvester as an example of an agricultural working machine equipped with the device, and FIG. FIG. 4 is a flow chart for explaining the operation of the controller, and FIG. 4 is a timing chart for explaining the same operation. (1) ...... First timing means (2) ...... Second timing means (3) ...... Discrimination means (4) ...... Working machine lifting control means

Claims (1)

[Claims] 1. A height of a working machine in which ultrasonic waves are periodically emitted from an ultrasonic transmitter toward a field surface and ultrasonic waves reflected by the field surface are received by an ultrasonic receiver to detect the working machine height. A working machine height automatic adjustment of a farm work machine equipped with a sensor, which is driven based on the detection height detected by this work machine height sensor, and which raises and lowers the work machine in response to a lifting request signal to the work machine. In the apparatus, a determination means (3) for determining an up / down request signal of the sensor to the work machine at predetermined time intervals and an output of the up / down request signal by the determination means (3). The timer is started later, and the timer is stopped when it is determined that the lift request signal is released during the expiration of the set time period or during the time before the set time period expires.
Timing is started after the lifting and lowering request signal is determined to be released by the timing means (1) and the determination means (3), and the lifting and lowering request signal is sent during the expiration of the set time period or during the time before the set time period expires. Second timing means (2) for stopping the timing when the output is discriminated and the setting of the second timing means (2) by driving the working machine up and down after the expiration of the set timing of the first timing means (1). And a working machine lifting control means (4) for stopping the lifting driving of the working machine after the expiration of the time period.
The set time period of the first and second time measuring means (1) (2) is set longer than the transmission cycle of the ultrasonic oscillator, and the set time period of the first time measuring means (1) is set to the second time period.
A working machine height automatic adjusting device for an agricultural working machine, characterized in that it is set to be longer than a set time period of the time measuring means (2) at least longer than a time interval for judging the lifting request signal by the judging means (3).