GB2144091A

GB2144091A - Crane safety device

Info

Publication number: GB2144091A
Application number: GB08320628A
Authority: GB
Inventors: Nigel Anthony James
Original assignee: Norcros Investments Ltd
Current assignee: Norcros Investments Ltd
Priority date: 1983-07-30
Filing date: 1983-07-30
Publication date: 1985-02-27
Also published as: GB8320628D0; GB2144091B

Abstract

A safety device for a crane in which a motor (2) has an output shaft which drives a hoist barrel (4) via a speed reduction drive train (6). The motor output shaft and the hoist barrel are each provided with a digitiser (12, 14) which produces a pulse train at a rate dependent on the speed of rotation of its associated part. The number of pulses produced by the motor digitiser (12) is accumulated in a time period determined by the time taken for ten pulses to be produced by the hoist barrel digitiser (14). The accumulated number of motor digitiser pulses is compared with a preset value and if it is less than that value a relay (10) is operated to cut off the power supply to the motor (2) and apply a brake (8) to bring the hoist barrel (4) to a halt. <IMAGE>

Description

SPECIFICATION Crane safety device The present invention relates to safety devices for cranes and particularly for cranes designed to vertically lift critical loads. Such cranes are required to have the capability of safely retaining the load in the event of a system failure.

The cranes to which the present safety device is intended to be applied are of conventional type and comprise a motor which drives a hoist barrel from its output shaft through a speed reduction, hoist drive train.

The hoist barrel carries a cable. Winding or unwinding of the cable controls the movement of the load which is attached to the cable via a cable reeving arrangement. The hoist barrel may be provided with an emergency brake capable of stopping rotation of the barrel. The crane is also usually fitted with a main contactor controllable by a relay so that the power supply to the motor can be terminated. Operation of both the brake and the contactor relay will stop all crane motion in a comparatively short time.

Various types of failure can arise in such a crane system, for example interference of the load with an obstruction, fouling of the reeving arrangement, or breakage of a component of the hoist drive train can cause a system failure. In order to ensure that the load can be safely retained in the event of such a failure, it is important to detect the occurrence of such a failure as soon as possible so that immediate action can be taken before further damage is done to the crane or consequential events occur, which may result in the load being dropped.

The technical problem to be solved by the present invention is therefore the rapid detection of system failure. It has been found that many types of system failure can be detected by the presence of a mismatch between the relative rotational speeds of the motor output shaft and the hoist barrel.

The present invention accordingly provides a safety device for use with a crane having a rotatable hoist barrel on which a hoist cable is wound to moved a load during operation of the crane, and a motor having an output shaft which drives said hoist barrel in rotation via a speed reduction, hoist drive train, said device comprising means f or producing a respective pulse train at a rate dependent on the rotational speed of each of the motor output shaft and the hoist barrel, means for comparing the number of pulses produced in each train after a pre determined time interval which is short reiative to the period for a single rotation of the hoist barrel, and means for terminating crane motion if the number of pulses in the motor output shaft drive train is less than the expected value relative to the number in the hoist barrel pulse train.

The above defined safety device is advantageous in that it enables any mismatch in the drive ratio between the motor output shaft and the hoist barrel to be detected very rapidly before the barrel has had time to execute a substantial part of a revolution.

Preferably, said predetermined time interval is set by the time taken for a predetermined number of pulses to be received in the pulse train from the hoist barrel. Such an arrangement enables a particularly simple comparison to be carried out to determine whether the accumulated number of pulses from the motor output shaft is equal to or less than the expected value.

Preferably, the safety device further includes means for terminating crane motion if the rate of pulses received in the hoist barrel pulse train is greater than a predetermined value. Such an arrangement enables faults which involve the over-running of the hoist barrel to be rapidly detected and the crane stopped to prevent any further damage while safely retaining the load.

As an additional safety feature, it is advantageous to provide means for sensing the presence of a pulse in each of said pulse trains within a predetermined time period less than said predetermined time interval and means for terminating crane motion if no such pulse is received in one or both of said pulse trains. In this way failures in the means for producing the pulse train can be accommodated. Thus, if the safety device itself is not functioning properly, then the crane is prevented from being operated.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which Figure 1 is a block diagram of the safety device of the present invention, and Figure 2 is a detailed circuit diagram of the device.

A crane includes a motor 2, which drives a hoist barrel 4 of the crane in rotation via a speed reduction hoist drive train 6. The hoist barrel has a cable (not shown) wound around it which carries the load through a cable reeving arrangement (not shown). The hoist barrel is rotated in order to lift or lower the load.

The hoist barrel 4, is provided with a brake 8 which can be used to bring the barrel to a rapid halt, therefore halting the movement of the load.

Power supply to the motor 2 is controlled by motor control circuitry 2A and by a contactor circuit including a relay 10. The brake 8 is also provided with control circuitry 8A by means of which it may be applied or released.

The motor 2 and hoist barrel 4 are each provided with a rotary digitiser 1 2 and 14.

These rotary digitisers are of known design and produce a train of pulses at a rate proportional to the rotational speed of the output shaft or hoist barrel respectively. The digitiser 14 connected to the hoist barrel is arranged to produce pulses at a greater rate per revolution than the digitiser 1 2 connected to the motor shaft. The drive ratio between the motor output shaft and the hoist barrel is typically 38:1 as determined by the speed reduction drive train 6. The output of the digitiser 14 is normally "geared up" (for example by a factor of 5) relative to the output of digitiser 1 2 such that the number of pulses produced at the output of the digitisers in a given time interval will be of known ratio.

Each of these digitisers can be chosen to produce a particular number of output pulses per revolution. The output of digitiser 14 is multiplied by a calculated ratio (for example five) so as to provide output pulses at a rate much higher than the number of output pulses per revolution from digitiser 1 2.

The output of each digitiser 12, 14 is fed to a respective counter 16,18. The counter 16 consists of three cascaded counter sections, for example CMOS integrated circuits number 4518. The pulse train f rom digitiser 12 is fed directly to the first section of the counter 1 6 which, after it has received ten pulses, provides a carry pulse to the next section which counts up to ten of these carry pulses before producing a carry pulse of its own which is fed to the final section. Theref ore the first section counts "units", the second counts "tens", and the third counts "hundreds".

Each section is provided with a binary coded decimal output on four parallel lines. The outputs of the sections of counter 1 6 are each fed to a respective store section, for example, CMOS integrated circuit type number 4076.

This store made up of the three identical sections is indicated by reference numeral 20.

Each store section has four output lines on which the value stored in the section appears in binary coded decimal form.

The output from hoist barrel digitiser 14 is connected to a counter 1 8 which is adapted to count down from a preset number (for example ten) so that after it has received the preset number of input pulses it produces an output on line 22 to store 20. This input causes the output of the three sections of counter 1 6 to be stored in the respective three sections of store 20. The store sections each then produce at their output binary coded decimal lines an input to a respective section of a comparator 24. The three sections of comparator 24 may be CMOS integrated circuits type 4063.The other input to each comparator is provided by a group of four output lines from four switching elements of a miniature switch 26 which is preferably arranged in a dual-in line (DIL) package so as to be readily accommodated on the circuit board in the same manner as the CMOS integrated circuit chips also used. Four of the switching elements of the DIL switch are associated with each section of the comparator 24. By selectively closing these four switching elements any required binary coded decimal input can be provided as the other input to the associated comparator section.

A closed switching element represents a binary "one" while an open one represents a binary "zero". The DIL switch can therefore be set to any number between 0 and 999.

The use of a miniature DIL switch enables the other input to the comparator 24 to be selectively varied depending on the crane to which the safety device is to be connected or the operating mode of the device.

The comparator 24 produces an output on line 28 if the number input to the comparator from store 20 is less than the number set by the DIL switch 26. The output line 28 is connected to relay 10 and causes the relay to operate and via the motor control circuitry 2A cut off the power supply to the motor 2, and via the brake control circuitry 8A apply the brake 8.

The output of digitiser 14 is also connected via line 30 to a frequency to voltage converter 32. The converter 32 produces a voltage output on line 34 to a comparator 36. The other input to comparator 36 is provided by a potentiometer 38. The frequency to voltage converter 32 produces an output voltage at a level depending on the rate of reception of the pulses in the pulse chamber from the digitiser 14. The comparator 36 compares the voltage output on line 34 with a voltage set by the potentiometer 38. The potentiometer 38 is set to provide the other voltage input at a predetermined value which corresponds to the voltage which would be output by the converter 32 if the hoist barrel 2 were rotating at an unsafe speed. The comparator 36 produces an output if the voltage input on line 34 is greater than the voltage set by the potentiometer 38.The comparator output on line 40 is fed to relay 10 and causes this to operate to cut off the power supply to the motor 2 via motor control circuitry 2A and apply the brake 8 via brake control circuitry 8A, thereby causing all crane motions to be stopped. The overspeed detector made up of frequency to voltage converter 32, comparator 36 and potentiometer 38 may alternatively be connected to the digitiser 1 2 to trest the speed of the motor output shaft. Further a separate overspeed detector could be provided for each digitiser.

A test switch 42 is also provided to test whether the circuit is functioning correctly. In the normal operation of the circuit the switch 42 is in a position so that the output of hoist digitiser 14 is fed to counter 1 8 and along line 30. Alternatively, the switch may be placed in a position so that the output of an oscillator 44 is fed to the counter 18 and along line 30. The oscillator 44 produces an output similar to that produced by the digi tiser 14 but produces a pulse train at a rate which would cause the circuit to detect a 'mismatch' and operate the relay 10. The frequency of the oscillator 44 is intended to be compared with the motor running at its normal speed rather than at a creep speed.

As a further improvement to the circuit and the efficiency of the safety device it is possible to test the output of each of the digitisers 1 2 and 14 continuously throughout the operating period of the crane to ensure that the output includes pulses within reasonable time intervals. This may be achieved by connecting the output of each of the digitisers to a reset input of a respective timer, which may be set to a predetermined time, say 20 milliseconds. If the timers reach their preset time without receiving an input pulse from the associated digitiser at their reset input then an output is produced which is fed to the relay 10 and causes operation of the crane to be stopped.

Such a system takes account of a failure in either of the digitisers. It will be appreciated that the above described system allows for extremely rapid sensing of any system failure and can be made "fail-safe" if the safety device itself is not operating efficiently.

The described circuit operates as follows: It will be appreciated that the values given in the following description are sample values only and may be appropriately varied according to system requirements. In the following discussion it will be assumed that the hoist barrel is typically required to rotate at a speed of 3.5 r.p.m. and never greater than 3.87 r.p.m.. that the speed reduction drive train 6 produces a drive ratio between the motor output shaft and the hoist barrel of 270:1, and also that the digitiser 14 is arranged to produce 7140 pulses per revolution of the hoist barrel, whereas the digitiser 1 2 is arranged to produce 27000 pulses relative to each revolution of the hoist barrel in normal circumstances. Therefore, the ratio of the frequencies of the outputs of digitiser 1 2 and digitiser 14 will be reduced to 3.78:1.

Before operation of the crane the DIL swiching elements are set to represent the number, for example 74, which is the lowest allowable number of pulses which will be produced by the motor output shaft digitiser 1 2 if no "mismatch" is present.

At the beginning of each test period the counter 18 is set to ten and the counter 1 6 is set to zero. The counter 1 8 counts down one as each pulse is received from the digitiser 14 and after ten pulses have been received produces an output on line 22.

The counter 1 6 counts up the pulses as received from digitiser 1 2 and the accumulated total is transferred to store 20 as soon as an output is received on line 22. Therefore, the accumulated total in store 20 will represent the number of pulses received from the digitiser 1 2 during a time interval in which ten pulses have been received from the hoist barrel digitiser 14.

If the motor output shaft and hoist barrel are rotating at the correct relative speeds then in the period in which ten pulses are produced by the hoist barrel digitiser 14 there should be 37 pulses received by counter 1 6 and, accordingly, stored in a store 20, whatever the actual speeds of rotation of the motor output shaft and hoist barrel are. It is necessary to detect if there is a mismatch between the relative speeds of the hoist barrel and motor. The majority of mechanical system failures cause the hoist barrel to rotate with a greater than normal ratio to the speed of the motor output shaft. The switching elements of the DIL switch are set to represent 36.If the number of pulses produced by the digitiser 1 2 during the time taken for ten pulses to be produced by the hoist barrel digitiser 14, is less than 36 an output will be produced on line 28 from comparator 24 which will cause the relay 10 to operate and via motor control circuitry 2A cut off the power supply to the motor 2 and via brake control circuitry 8A apply the brake 8, therefore interrupting all crane motions. The load will therefore remain supported in its then position. If the input from store 20 to the comparator 24 is greater than 74 then a reset pulse will be produced to reset counter 1 6 to zero and set counter 18 to ten and a fresh testing period will commence.

With the hoist barrel rotating at 3.51 r.p.m.

the time taken for the digitiser 14 to produce ten pulses will be 24 milliseconds and this is the response time of the safety device. Under no load conditions, i.e. in the event of motor shaft breakage, the motor output shaft will tend to rotate somewhat faster. However, the ratio between the hoist barrel speed and the output shaft speed should remain constant and therefore the described circuit will operate correctly but the response time will be slightly shorter. If the barrel speed is only 0.351 r.p.m. then the response time will be correspondingly greater and of the order of 240 milliseconds. This response time is extremely fast and it virtually eliminates the possibility of any damage detrimental to the safety of the load to occur in such a short period following a system failure.

The hoist barrel overspeed detector comprising the frequency to voltage converter 32 and comparator 36 allows the rate of rotation of the hoist barrel to be monitored and the crane brought to a halt if it exceeds a predetermined value, say, 3.87 r.p.m.. The predetermined value is set by the potentiometer 38.

The overspeed detector operates as follows.

The output from the hoist barrel digitiser 14 is fed along line 30 to the frequency to voltage converter and the output voltage is fed to a comparator 36 where it is compared with the potentiometer set voltage and if it exceeds this voltage corresponding to 3.87 r.p.m. The relay 10 is operated to cause the power supply to the motor to be terminated via the motor control circuitry 2A and the brake 8 to be applied via brake control circuitry 8A. The response time of this overspeed detector is approximately 10 milliseconds.

In order to test the operation of the device, it is necessary for the motor 2 to be running at its design speed, i.e. 950 r.p.m. At such a speed it would be expected that the hoist barrel 4 would be rotating at 3.5 r.p.m. In order to carry out the test the test switch 42 is turned to the position where the oscillator 44 is connected to the counter and overspeed detector circuit in place of the output of digitiser 14. The frequency of the oscillator 44 simulates a higher speed of rotation for the hoist barrel than 3.5 r.p.m. For example, if the digitiser produces 7140 pulses per revolution, it would normally produce 41 7 pulses per second. The test oscillator 44 is therefore set to, for example, 0.45 kilohertz.

This should cause the "mismatch circuit" comprising the counters 16, 18, store 20, switch 26 and comparator 24, to operate the relay 10 but not cause the overspeed detector to operate.

Whilst a saf ety device has been disclosed which uses a predetermined number of received pulses from the hoist barrel digitiser 14, to set a time interval in which the number of received pulses from the motor output shaft digitiser is monitored, it will be appreciated that the outputs f rom both digitisers could be accumulated over a preset time interval and their relative and absolute values monitored to detect any deviation from the correct ratio or any overspeed condition. In order to achieve this a microprocessor could be used.

Claims

1. A safety device for use with a crane having a rotatable hoist barrel on which a hoist cable is wound to move a load during operation of the crane, and a motor having an output shaft which drives said hoist barrel in rotation via a speed reduction, hoist drive train, said device comprising means for producing a respective pulse train at a rate dependent on the rotational speed of each of the motor output shaft and the hoist barrel, means for comparing the number of pulses produced in each train after a predetermined time interval which is short relative to the period for a single rotation of the hoist barrel, and means for terminating crane motion if the number of pulses in the motor output shaft drive train is less than the expected value relative to the number in the hoist barrel pulse train.

2. A device as claimed in claim 1, wherein said predetermined time interval is set by the time taken for a predetermined number of pulses to be received in the pulse train from the hoist barrel.

3. A device as claimed in claim 1 or 2, further comprising means for terminating crane motion if the rate of pulses received in the hoist barrel pulse train is greater than a predetermined value.

4. A device as claimed in any one of the preceding claims, further comprising means for sensing the presence of a pulse in each of said pulse trains within a predetermined time period less than said predetermined time interval and means for terminating crane motion if no such pulse is received in one or both of said pulse trains.

5. A safety device as claimed in any one of the preceding claims, wherein said means for terminating crane motion comprises means for cutting-off power supply to the crane motor.

6. A safety device as claimed in any one of the preceding claims, wherein said means for terminating crane motion comprises means for applying an emergency brake associated with the hoist barrel.

7. A crane comprising a motor, a speed reduction hoist drive train connecting the motor to a hoist barrel, said barrel being provided with a hoist cable mounted such that rotation of the hoist barrel raises or lowers the load, respective rotary digitizers associated with an output shaft of the motor and the hoist barrel for producing pulses at a rate proportional to the rotational speed of the associated shaft or barrel, means for counting the pulses f rom the hoist barrel digitiser and producing an output siganl when the count reaches a predetermined value, means for counting the pulses from the motor output shaft digitiser and storing said count on receipt of said output signal, means for comparing said stored count with an expected value and terminating power supply to the motor of said stored counter is less than the expected value, and means for simultaneously resetting said counting means to initiate a new test period in response to said output signal, the time taken for a single rotation of the hoist barrel being long relative to the duration of a test period.

8. A safety device substantially as herein described with reference to the accompanying drawings.