GB2108677A

GB2108677A - Apparatus for measuring a distance travelled particularly lift- measuring apparatus for stackers

Info

Publication number: GB2108677A
Application number: GB08227560A
Authority: GB
Inventors: Gustav Rohr
Original assignee: Jungheinrich Unternehmensverwaltung KG
Current assignee: Jungheinrich AG
Priority date: 1981-10-14
Filing date: 1982-09-28
Publication date: 1983-05-18
Also published as: FR2515161A1; GB2108677B; DE3140795C2; FR2515161B1; DE3140795A1

Abstract

In apparatus for measuring a distance travelled, particularly a lift, individual signals from a digital angular increment detector 7 are summed up and during the measuring operation are periodically compared several times with an analog signal, which is delivered by an analog transducer 15. These summed signals are corrected in dependence on any deviation before the lift signal is delivered. For this purpose, a signal counter is provided, which has a correcting input terminal and the count of which is adapted to be periodically corrected during the operation. The analog signal is sampled after each revolution of the lifts drive mechanism rotary drive, as determined by the angular increment detector, digitized, and then compared with the counter output corresponding to the summed signals from the angular increment detector. Any deviation is used to correct this counter output. The analog transducer is designed to measure the largest possible lift. <IMAGE>

Description

SPECIFICATION Apparatus for measuring a distance travelled particularly lift-measuring apparatus for stackers This invention relates to apparatus for measuring a distance travelled, particularly liftmeasuring apparatus for stackers, particularly stackers having an extensible load carrier and a lifting mast, which apparatus comprises a lift signal generator including a transducer, and a comparator associated with the lift signal generator.

A reference made hereinafter to a signal, particularly a lift signal, includes also a displacement signal because a special adjustment is provided within a pregiven height or displacement, and means for indicating the displacement are also provided.

The disclosure includes also a tape, which is adapted to be unwound from or to be wound on a drum in dependence on the lift or distance travelled, and an electric lift signal generator, which is driven by the drum and from its output delivers a signal, particularly a lift signal, to a lift indicator. The preferred embodiment constitutes a lift-measuring apparatus.

Such lift-measuring apparatus comprising an element which can be wound up and which has been described hereinbefore and will be described hereinafter as a tape, although that term includes also a rope or a similar element of suitable material, are known.

For instance, in accordance with German Patent Specification 21 14 617, a tape or rope secured to the load carriage is unwound by means of a spring motor against a torque and a measuring tape consisting particularly of a spring tape is unwound from one drum and wound up on another drum in dependence on the rotation of the tape or rope pulley and one of said drums is connected to the drum from which the rope is withdrawn which is secured to the load carriage.

The measuring tape is provided for each height setting with two marks of non-conducting material so that a scanning of the measuring tape with sliding contacts permits the generation of a lift signal. That known apparatus involves a relatively high mechanical expenditure.

Particularly the scanning with sliding contacts is rather liable to be deranged, also by dirt, and the provision of adhesive tape in association with sliding contacts is not reliable.

For this reason it has been proposed in German Patent Specification 21 24853 to provide such apparatus with photoelectric detectors for scanning the marks on the measuring tape. But that requires also a constant quality of the properties of the surface of the measuring tape as regards reflectivity and the like.

The accuracy of these known designs is relatively low because only a limited number of marks can be accommodated if adhesive tapes are used. In those known designs a shifting of the zero point as a point of reference of a measurement is highly problematic. That problem has not been mentioned but is significant in the operation of mobile implements.

Laid-open German Application 27 09 640 discloses lift-measuring apparatus which uses also a measuring tape which consists of a perforated tape or has edge serrations. The tape is trained around a gear so that a proportional electric signal is generated, which is delivered to a pick-up. That signal can be generated by means of a driven potentiometer. The known arrangement comprises a comparator, which is programmed with a signal having a magnitude that corresponds to a desired value. The comparator compares that value with the actual value indicated by the pick-up. A command device is actuated when the desired lift has been effected.

In that known design the comparison is related to the arrival on a given level. But that known apparatus has the same disadvantages as the other known apparatus which comprise a measuring tape. In the present case the disadvantages are due to the mechanical scanning, which involves a substantial expenditure and a certain inherent accuracy. The conversion to an electric signal by a 30-turn potentiometer, as is mentioned in said publication, will involve a high expenditure, particularly if a highly accurate positioning is desired. If that desired accuracy is as high as 1%, even voltages of only a few millivolts must be processed and the insensitivity to electrical interference will now be ensured.

These scanning methods which are inaccurate and involve a relatively high expenditure have been used because particularly in the operation of fork lift trucks and other stackers having extensible load carriers the electrical and electronic equipment which is provided for the operation and control of such implements involves a strong interference by pulses.

The operation of the known apparatus is also subject to interference from the environment for instance, influences of temperature and humidity, which adversely affect the condition of the known measuring apparatus. Whereas an electric signal is generated in the known apparatus which has been discussed last, its resolving power is low.

It is an object of the invention to provide for the measurement of a distance travelled, particularly a lift, an apparatus which is of the kind described first hereinbefore and which distinguishes by a high resolving power and a low structural expenditure and has a high accuracy particularly in indicating a lift.

This invention is accomplished according to the invention in that the lift signal generator consists of an angular increment detector and the transducer is driven in synchronism with the means for driving the angular increment detector, signals generated by the transducer are adapted to be periodically sampled repeatedly into the comparator and a signal counter having a correcting input terminal (bidirectional counter) is provided for indicating the lift and is adapted to be corrected in response to a difference detected by the comparison. As a result, a periodic check or recalibration is virtually performed during the measuring operation so that an incremental signal generator can be used without a risk of an error being introduced into the measurement by interference pulses.

The reference signal is suitably triggered by an output signal of the lift signal generator so that a synchronization of the periods for the lift signal and the reference signal is ensured.

In a preferred embodiment, the lift signal consists of increment signals which represent integral fractions of the largest possible lift and the analog reference signal is continuously generated throughout the lift and is sampled in increments corresponding to an integral fraction and issued to correct the lift signal.

The above-mentioned embodiment of the apparatus for measuring a distance travelled may be used to measure deviations in either direction.

As a result, methods for a digital incremental measurement of a distance travelled can be performed with angular increment detectors which are known from machine tools. The use of such angular increment detectors is desirable because they have a high inherent accuracy and high resolving power.

In the arrangement described hereinbefore, which comprises a tape that can be wound on or unwound from a drum, which drives the electric lift signal generator, a particularly preferred embodiment comprises a combination of an angular increment detector in drive means for representing the largest possible lift or distance travelled by n revolutions of the drum, with an analog transducer, which is continuously associated with the largest lift or distance travelled and is succeeded by a circuit for periodically and repeatedly sampling from said analog transducer a signal after each revolution of the angular increment detector and for delivering that signal to a comparator, to which the signal from the angular increment detector is applied, which during each revolution exactly measures the lift, wherein the count of the signal counter is adapted to be corrected during the multiple periods in response to a deviation which has been detected. This combination results in a simple and economical and highly accurate embodiment of such apparatus.

The angular increment detector has suitably a zero signal output terminal from which a zero signal is periodically delivered in dependence on the operation of the mechanical drive means for rotating that detector, particularly after a rotation through 3600, and said zero signal triggers a circuit for sampling a reference signal from the analog transducer. This results in a synchronization with the analog transducer, which consists of a potentiometer, suitably of a rotary potentiometer. That potentiometer requires only a low accuracy as regards its linearity but its function should be highly reproducible so that an exact signal value is obtained in each position.

In this connection it is included in known manner that the angular increment detector is designed to supply signal pulses with a displacement of 900 so that a forward and backward count can be detected.

In a desirable embodiment the angular increment detector is arranged to rotate in unison with the shaft of the tape drum and the analog transducer is driven by said shaft via a speedreducing slipless transmission. In the preferred embodiment the largest lift or the largest distance travelled in the measuring range will be covered by a single scan of the analog transducer so that a predetermined value will always be sampled during the periodic reproduction. The angular increment detector need not be secured to the shaft of the tape drum. A slipless transmission may be interposed but for an accurate drive the angular increment detector is preferably secured to the shaft of the denim. The slipless transmissions consist suitably of toothed belt drives.

In another desirable embodiment of the invention the comparator is provided with a memory for storing possible correction values which are associated with deviations measured by the comparator and can be read out for delivery to the counter having a setting input terminal. The comparator is an electronic comparator, which is known per se and associated in known manner with a memory which has output terminals which can be addressed in dependence on a signal magnitude and are connected by respective connections to the counter having a correcting input terminal.

The term "counter having a correcting input terminal" includes also conventional electronic control circuits.

An electronic interpreting unit is provided, which sums up the individual signals from the angular increment detector or displacement increment detector and at the end of each period compares said signals with the analog signals sampled from the transducer, which analog signals are also digitalized in steps and by means of the memory are associated with individual values which have been summed up in a table.

To ensure a continued accuracy of the output value, the apparatus preferably comprises a screw, which is driven by the shaft of the drum and in threaded engagement with a tape pulley and has such a lead that the tape will be laid in juxtaposed helical convolutions, and the drum has such an axial length that the entire tape can be wound up in juxtaposed helical convolutions for the measurement of the largest possible lift for a load carrier along a mast.

The invention will now be described with reference to illustrative embodiments which are shown in the drawing, in which Figure 1 is a diagrammatic elevation showing apparatus according to the invention, Figure 2 is a block diagram for explaining the function, Figure 3 a block diagram for explaining the control electronics and Figure 4 a diagram for explaining the invention.

The structure shown in Figure 1 comprises a lifting frame 1, which carries a load carrier 2. A tape 3 is secured to the load carrier 2 and connected to a drum 4 and can be wound on and unwound from that drum. The shaft 5 of that drum is connected to an electric motor 6 for generating an opposite torque. The motor 6 might be replaced by a mechanical spring motor, which ensures that the tape 3 will be wound up as the load carrier is lowered. The shaft 5 carries a digital angular increment detector 7, which has three output terminals 8, 9, 10. The shaft 5 carries also a drive gear 11 of a toothed belt drive 12, which via another gear 13 drives a shaft 14.

The latter carries an analog transducer 15, which consists, e.g., of a rotary potentiometer.

Another drive gear 1 6 is secured to the shaft 5 and via a toothed belt 17 drives a drive gear 18, which is secured to a screw 19. The tape 3 or rope is trained around a pulley 20, which is in threaded engagement with the screw. The screw threads of the screw 19 have such a lead that the pulley 20 will guide the tape so that it is wound up in juxtaposed helical convolutions. This ensures an accurate relation of the measured values to the diameter of the drum. The drum 4 has such an axial length that the entire tape 3 can be stored on the drum in juxtaposed helical convolutions in a length which relates to the largest possible lift of the load carrier relative to the lifting mast.

In Figure 2, like parts are designated with like reference characters so that the description need not be repeated as far as the structure is the same. In that embodiment, an optical angular increment detector 21 is also secured to the shaft 5 and serves for a visual check. The signaldelivering electric digital angular increment detector 7 is secured to a shaft 22, which is parallel to the shaft 5 and is driven from the latter via a stipless transmission 23, particularly a toothed belt drive. That shaft 22 drives via another slipless transmission 24, preferably a toothed belt drive, a shaft 25, to which the analog transducer 1 5 is secured.The encoder 7 and 1 5 have respective output terminals 26, 27, which are diagrammatically indicated in Figure 2 and from which the signals are delivered to an electric interpreting unit, which is generally designated 28. The unit 28 comprises a comparator 29 and a signal counter 30, which has a correcting input terminal and is connected to an indicating and control unit 31. An input device 32 is associated with that indicating and control unit and can be used to preselect a predetermined lift. At least one output terminal 33 of the control unit 31 is connected to a drive motor 34 for lifting the load carrier 2.

Figure 3 shows the angular increment detector 7 having three output terminals 8, 9,10 and the analog transducer 15, which consists of a potentiometer 35.

From the two output terminals 8, 9 of the angular increment detector, signals displaced 900 are delivered to a counting device 36. The direction of the displacement depends on the sense of rotation of the angular increment detector so that 36 can detect whether the counter is rotating forward or backward and the direction of movement of the load carrier 2. The counting device is connected to the counter 30 provided with a correcting device.

The third output terminal 10 of the angular increment detector 7 delivers after each revolution through 3600 a signal pulse to a zero pulse-controlled gate 37, which controls the delivery of signals from the output terminal 38 of the transducer 1 5 and only in response to a zero pulse signal transmits an output signal to an analog-digital converter. The output signal of that converter is delivered to a controller 39, which via a lead 40 receives also the sum signal from the counter 30 having a correcting input terminal so that the comparison in the controller will indicate a deviation. A microprocessor may be used for that purpose.

In dependence on the deviation, a feedback signal is delivered via a lead 41 to the counter 30 having a correcting input terminal so that said counter is corrected in dependence on the deviation. In this manner, the drive motor 34 can be controlled via the controller 31, which is connected to the output of the counter 30. In the embodiment shown in Figure 3, the input device 32 delivers input data to the controller 39, which via a lead 42 controls a display unit 43, which may also be driven via the controller 31.

As is apparent from Figure 3, the potentiometer 35 may alternatively be a linear potentiometer, provided that it has an adequate length. The reference to an angular increment detector implies that the detection of angular increments constitutes a preferred embodiment because the increment detector may be used in conjunction with a linear actuator.

The operation is diagrammatically represented in Figure 4. The distance 44 represents a maximum lift Umax. Developments are also shown for a diagrammatic representation of the signals, namely, the distance 45 for the angular increment detector and the distance 46 for the analog transducer, which consists in the present case of a rotary potentiometer. The distance 45 is divided into n equal sections, two of which are designated 47, 48. The ends of these sections represent the positions in which the angular increment detector delivers a sampling signal or the so-called zero signal whenever the rotatable angular increment detector has performed a complete revolution of 3600.As a result, the top end of the distance 45, corresponding to Umax, has associated with it a value n x3600 with reference to such angular increment detector and an accurate count will alwavs be performed along each of said sections.

The distance 46 represents the development of the revolution of the analog transducer for the lift max i.e., a revolution of 360 of a rotary potentiometer when the lift Umax is performed.

Like the zero signals after each section 47, 48 etc., an analog signal represented, e.g., by 49 and 50 is sampled from that analog transducer. A comparator is used to check whether the signals at 47, 48 and 49, 50 correspond to a desired value, which has been stored. In case of a difference, the comparator, which is diagrammatically indicated at 51, generates a correcting signal Hnl which is applied to the correcting input terminal of the signal counter 30 and used to correct its count.

In this case the instantaneous lift at the time of each zero signal pulse will be represented by the formula u H=U x--+H,, N wherein U=tape length corresponding to one revolution of the tape drum or one revolution of the angular increment detector u=count of counter having a correcting input terminal N=number of pulses per revolution Hn=nxU or n u U N n is an integer In this way the values sampled from the analog transducer may be used for a correction of the measured lift at the time of each zero signal pulse.

The last-mentioned formula for the correcting signal shows that the potentiometer, which has been described as particularly advantageous, may be substituted by another source.

Claims

1. Apparatus for measuring a distance travelled, particularly lift-measuring apparatus for stackers, particularly stackers having an extensible load carrier and a lifting mast, which apparatus comprises a lift signal generator including a transducer, and a comparator associated with the lift signal generator, characterized in that the lift signal generator consists of an angular increment detector and the transducer is driven in synchronism with the means for driving the angular increment detector, signals generated by the transducer are adapted to be periodically sampled repeatedly into the comparator and a signal counter having a correcting input terminal (bidirectional counter) is provided for indicating the lift and is adapted to be corrected in response to a difference detected by the comparison.

2. Apparatus according to claim 1, characterized in that the transducer is an analog transducer.

3. Apparatus for measuring a distance travelled according to claim 2, comprising a tape, which is adapted to be unwound from or wound on a drum in dependence on the lift or a distance travelled, wherein the electric lift signal generator is driven by the drum, characterized by a combination of an angular increment detector in drive means for representing the largest possible lift or distance travelled by n revolutions of the drum, with an analog transducer, which is continuously associated with the largest lift or distance travelled and is succeeded by a circuit for periodically and repeatedly sampling from said analog transducer a signal after each revolution of the angular increment detector and for delivering that signal to a comparator, to which the signal from the angular increment detector is applied, which during each revolution exactly measures the lift, wherein the count of the signal counter is adapted to be corrected during the multiple periods in response to a deviation which has been detected.

4. Apparatus according to claim 3, characterized in that the angular increment detector has a zero signal output terminal from which a zero signal is periodically delivered in dependence on the operation of the mechanical drive means for rotating that detector, particularly after a rotation through 3600, and said zero signal triggers a circuit for sampling a reference signal from the analog transducer.

5. Apparatus according to claim 3 or 4, characterized in that the angular increment detector is designed to supply signal pulses with a displacement of 900 so that a forward and backward count can be detected.

6. Apparatus according to any of claims 3 to 5, characterized in that the angular increment detector is arranged to rotate in unison with the shaft of the tape drum and the analog transducer is driven by said shaft via a speed-reducing slipless transmission.

7. Apparatus according to claim 6, characterized in that the angular increment detector is secured to the shaft of the drum.

8. Apparatus according to claim 6, characterized in that the slipless transmission consists of a toothed belt drive.

9. Apparatus according to any of claims 3 to 8, characterized in that the comparator is provided with a memory for storing possible correction values which are associated with deviations measured by the comparator and can be read out for delivery to the counter having a setting input terminal.

10. Apparatus according to any of claims 3 to 9, characterized in that the angular increment detector has three output terminals, and one output terminal delivers in response to a revolution through 3600 a signal pulse to a zero pulse-controiled gate, which is connected to the angular increment detector.

11. Apparatus according to any of claims 3 to 10, characterized in that an angular increment detector is provided, which has three output terminals, two of said output terminals deliver respective signals which are displaced 900 in dependence on the sense of rotation, and are succeeded by a counting device, a signal counter having a correcting device is associated with said counting device and connected to a controller, and an input device for selecting a desired lift is associated with said controller.

12. Apparatus according to claim 3, characterized in that the shaft of the drum drives a screw, with which a pulley for guiding the tape is in threaded engagement and which has screw threads having such a lead that the tape is wound on the drum in juxtaposed helical convolutions, and the drum has such an axial length that the entire tape can be wound on the drum in juxtaposed helical convolutions in a length corresponding to the largest lift of a load carrier relative to a lifting mast.

1 3. Apparatus according to claim 1, substantially as described with reference to any of Figures 1 to 4 of the accompanying drawings.