Abstract:
In the anti-two block device, a non-contact measuring device detects in one form or another the distance between a boom nose and a load-bearing member supported thereby. If the distance detected is less than a predetermined threshold value, the device triggers a warning or other operation to assist an operator in preventing anti-two blocking.

Description:
This application claims benefit of provisional Nos. 60/067,815 filed Dec. 5, 1997 and provisional No. 60/069,055 filed Dec. 10, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to devices for cranes and aerial work platforms; and more particularly, to an anti-two block device for cranes and aerial work platforms using non-contact measuring and detecting devices. 
     2. Description of Related Art 
     Conventional anti-two block devices rely on physical contact between elements of the device to detect when anti-two block is imminent. Devices which rely on contact, however, have a limited life span because each physical contact serves to wear and degrade the device. Contact devices also fail because of external factors which prohibit the detection of contact. For instance, most contact devices use electrical switches, and freezing rain can prevent such switches from functioning. Furthermore, most contact devices only provide a discreet measurement. When contact occurs, this contact indicates that, for example, anti-two blocking is imminent. Such contact devices cannot supply a continuously varying real-time measurement. Contact devices also have slow response times. Accordingly, contact devices include a certain amount of error. 
     SUMMARY OF THE INVENTION 
     The anti-two block device according to the present invention includes a non-contact measuring device which detects in one form or another the distance of a load-bearing member from the nose of a boom. By comparing the detection results to one or more predetermined threshold values, the anti-two block device according to the present invention can assist an operator in preventing anti-two blocking in various ways. 
     For instance, when the distance measured by the non-contact measuring device indicates a distance between the boom nose and the load-bearing members is less than a first predetermined threshold, a warning light is driven and/or the movement of the load-bearing member is slowed. If the measured distance falls below a second predetermined threshold, then movement of the load-bearing member is stopped completely. 
     The present invention contemplates the use of numerous different non-contact measuring devices such as micro-impulse radar, a sonic measuring device, an ultrasonic measuring, an infra-red measuring device, and a laser measuring device. 
     Other objects, features, and characteristics of the present invention; methods, operation, and functions of the related elements of the structure; combination of parts; and economies of manufacture will become apparent from the following detailed description of the preferred embodiments and accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 illustrates an embodiment of an anti-two block device according to the present invention; 
     FIG. 2 illustrates another embodiment of a anti-two block device according to the present invention; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates one embodiment of an anti-two block device according to the present invention. As shown in FIG. 1, a crane boom  50  includes a boom nose  60 , and the boom nose  60  includes upper boom nose sheaves  70  and lower boom nose sheaves  30 . A cable  80  extends from the boom nose  60 , and is connected to a load bearing hookblock  40 . A hoist (not shown) controls movement of the cable  80 ; and thus, the hookblock  40 . A micro-impulse radar (MIR) system  10 , according to the present invention is mounted on a pivot plate  20 , which is attached to the shaft of the lower boom nose sheaves  30 . Because the MIR system  10  is mounted on the pivot plate  20 , the transmit and receive antennas (not shown) of the MIR system  10  always point in the direction of the hookblock  40  regardless of the elevation angle of the boom  50 . 
     As is well-known, MIR systems emit short-range ultra-wideband pulses, and detect the reflection of these pulses off of a body. Based on the reflected pulses, namely, the time delay between transmission and reception of the pulses, the MIR systems determine, among other things, the distance to the body. Besides an MIR system, the anti-two block device  10  can be any device which measures the distance to a body without relying on contact such as sonic, ultra-sonic, infra-red and laser distance measuring devices. 
     The MIR system  10  is calibrated to transmit a warning signal to the crane operator via an electric cable  90  when the hookblock  40  is within a predetermined distance of the boom nose  60 . For instance, in the embodiment of FIG. 1, the warning signal drives an indicator light  95 , and, in another embodiment, drives an indicator light and a control system (not shown) which prevents further movement of the hookblock  40  towards the boom nose  60 . 
     In an alternate embodiment, the MIR system  10  is calibrated to transmit first and second warning signals to the crane operator via the electric cable  90  when the hookblock  40  is at first and second predetermined distances, respectively, from the boom nose  60 ; the second predetermined distance being less than the first predetermined distance. The first warning signal drives a hydraulic valve (not shown) causing the line speed of the cable  80  to decrease, and the second warning signal drives the control system which prevents further movement of the hookblock  40  towards the boom nose  60 . This technique allows cable  80  to travel at the maximum line speed until the hookblock  40  reaches the first predetermined distance from the boom nose  60 . 
     A further embodiment of the anti-two block device according to the present invention is shown in FIG.  2 . In this embodiment, the MIR system  10  supplies the distance measurement, as opposed to warning signals, to a control system  15 , which includes a processor  25  connected to a ROM  26  and a RAM  27 . The control system  15  compares the distance measurement to the first and second predetermined distances. When the distance measurement from the MIR system  10  exceeds the first predetermined distance, the control system  15  output controls signals to a cable control system  35  to reduce the line speed of the cable  80  and to an indicator light  45  to warn the operator. When the distance measurement from the MIR system  10  exceeds the second predetermined distance, the control system  15  output controls signals to the cable control system  35  to stop movement of the cable  80 . 
     Instead of supplying the distance measurement to the control system  15 , the MIR system  10  can be configured to output the time delay between transmission and reception of the pulses. The control system  15  then compares the time delay to first and second predetermined time delays which correspond to the first and second predetermined distances discussed above. The control system  15  then operates in the same manner as discussed above with respect to exceeding the first and second predetermined distances depending on whether the time delay exceeds the first and/or second predetermined time delays. 
     The anti-two block device eliminates the use of physical contact to prevent anti-two blocking. Also, the anti-two block device does not include any moving parts, and is not exposed to the environment. The response time, compared to anti-two block devices based on contact, is greatly increased as well. Accordingly, the anti-two block device according to the present invention does not suffer from the problems and disadvantage of contact based anti-two block devices. 
     While the embodiments of the present invention discussed above activate indicator lights, in addition or instead of the indicator lights, warning sounds may be issued. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the invention.