Abstract:
A safety system for a machine such as a wood chipping machine. The safety system has a passive sensor that may be worn on the machine operator&#39;s wrist, a sensing coil mounted to a feed chute of the machine for detecting the proximity of the passive sensor, and control circuitry which is responsive to the sensing coil. The circuitry provides a control signal for use in stopping operation of the machine. In use, the presence of the operator&#39;s arm in the chute near to parts of the machine that can cause injury will trigger the sensing coil and cause the machine to cease operating, thus circumventing injury to the operator.

Description:
FIELD OF INVENTION 
     This invention relates to a safety system for machinery. In particular, the invention relates to safety equipment for a mobile wood chipping machine. 
     The invention will be described by way of example with reference to the use of the safety system with a mobile wood chipping machine. The system of the invention may be used with machines other than mobile wood chipping machines. 
     BACKGROUND OF THE INVENTION 
     Mobile wood chipping machines are used by workmen to chip branches and parts of trees into smaller pieces for providing mulch or to make the pieces of the tree more easily transportable to other locations or for easy disposal. Such machines have a feed chute leading into a chamber where chipping discs are present and the discs are driven to cause material introduced into the chamber to be shredded or chipped into small pieces. A pair of feed rollers are present at an end of the chute spaced from a feed opening into the chute. The feed rollers convey material placed into the chute towards the chipping discs. A safety bar extends across the chute in the vicinity of the feed opening and the bar is normally pulled away from the chute to cause a drive to rotate the feed rollers and is pushed towards the chute to disengage the drive. Serious accidents have been known to occur with the use of such machines. It is usual for workmen to wear gloves and other protective clothing, and it is possible that a workman may become snared by the material being fed into the chute in which case the workman may be unable to operate the safety bar in time or may be unable to operate the bar at all and as a consequence may be drawn into the machine and injured. Furthermore, the safety bar may malfunction. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a safety system for machinery which at least minimizes the disadvantages referred to above. 
     According to one aspect of the invention there is provided a safety system for a machine, the system having at least one passive sensor, a sensing coil associated with the machine for detecting proximity of the passive sensor and control circuitry responsive to the sensing coil for providing a control signal for use in stopping operation of the machine. 
     The passive sensor may consist of a tuned circuit. Preferably the tuned circuit consists of a coil and a capacitor connected in parallel with one another. The coil may have any suitable number of turns and the turns may be wound in a spiral to present a helical or a substantially flat-shaped circular coil. The passive sensor may be incorporated in a band normally worn or carried by the machine operator. Preferably, the band is placed around a part of the machine operator&#39;s body which might be brought into proximity to the machine. The band may be worn on the wrist of the machine operator. The operator may carry or wear more than one passive sensor. 
     The sensing coil may be configured as a spiral or as a number of turns of wire with a non-circular path. 
     The sensing coil may be placed adjacent or incorporated into the machine being used by the operator. The sensing coil may be placed adjacent those parts of the machine which are able to injure the operator. Where the system of the invention is used with a wood chipping machine, the sensing coil may be mounted to the feed chute of the machine. If necessary, the sensing coil may be insulated from the chute. The sensing coil is energised by the control circuitry and the electromagnetic field provides a control output which may then be used to cause the machine with which the safety system is associated to cease operating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A particular preferred embodiment of the invention will now be described by way of example with reference to the drawings in which: 
     FIG. 1 is a circuit diagram of a passive sensor according to an embodiment of the invention; 
     FIG. 2 is a block diagram of part of the safety system of the invention; 
     FIG. 3 is an hydraulic circuit diagram of a typical hydraulic circuit employed in a wood chipping machine; 
     FIG. 4 is a circuit diagram showing further detail of part of the safety system of the invention; 
     FIG. 5 is a detailed block diagram of a control circuit for the system shown in FIG. 4; 
     FIG. 6 is a front elevational view of a feed chute and feed rollers of a wood chipping machine with a mounted sensing coil, according to an embodiment of the invention; and 
     FIG. 7 is a side elevational view of the wood chipping machine and mounted sensing coil of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an example of a passive sensor  20 . The sensor comprises a tuned circuit consisting of a coil  21  and a capacitor  22  connected in parallel with the coil. The coil may have any suitable number of turns and it is preferred that the coil be wound in a spiral fashion so that it presents a substantially flat circular profile. The sensor may be incorporated in a wrist band and two such bands may be worn by the machine operator so that the presence of either hand of the operator within a predetermined distance form the machine may be detected. Of course the sensors need not be incorporated into a band and may be worn or otherwise carried by the operator. 
     FIG. 2 shows a block diagram of part of the safety system of the invention. The block diagram shows a battery  30  which may typically be a battery associated with a wood chipping machine  9  (see FIGS. 6 and 7) to which the safety system of the invention may be fitted. A DC to DC converter  31  receives power from the battery  30  and provides a 24 VDC output for the remainder of the circuit shown in the figure. 
     A sensor coil  32 , which comprises a coil configured in a flat rectangular shape, is located in a feed chute  10  of the wood chipping machine  9  (see FIGS.  6  and  7 ), and, as shown in FIG. 2, is coupled to sensing electronics unit  33 . An output from electronics unit  33  is used to operate a relay  34 . Relay  34  in turn allows a diverter coil  35  of the solenoid valve to be operated. This solenoid valve is part of the hydraulic circuit shown in FIG.  3 . Stop and reset switches  36  are coupled to the relay  34 . 
     FIGS. 6 and 7 show the feed chute  10 , two feed rollers  11  and a safety bar  12  of the wood chipping machine  9 . The feed rollers  11  control the feeding of branches or the like into the chipping blades of the machine  9 . The sensing coil  32  is embedded in a truncated pyramid-shaped insulator matrix  13 . The insulator matrix  13  mounts the sensing coil  32  to a wall of the chute  10  and insulates the sensing coil  32  from the wall of the chute  10 . 
     FIG. 3 shows a hydraulic pump  1 , and a reservoir  8  for hydraulic fluid is coupled to the pump. Feed valve  2 , when in the position shown, allows the pump  1  to supply fluid to safety diverter solenoid valve  3 . The valve  2  may be moved to its other position, in which case fluid from the pump is diverted to a return filter  7  and then fed back into the reservoir  8 . 
     Valve  3  includes a coil which, when energised, moves the valve from the position shown to its second position. In the position shown, fluid passes through valve  3  and is returned to the filter  7  and then to the reservoir  8  because the manual feed roller directional valve  4  is in its intermediate position. 
     The solenoid valve  3  is normally in the position shown when its associated coil is de-energised. 
     Feed roller directional valve  4  is manually operable and may be moved either to the left or to the right of its intermediate position. When valve  4  is moved to its left hand position, fluid may be supplied to flow divider/combiner  5  and by that device to feed roller hydraulic motors  6  to cause the motors to drive the feed rollers  11  in a first direction. When the valve  4  is moved into its right hand position, fluid may be supplied to the motors  6  via flow divider/combiner  5  to cause the rollers  11  to be driven in a direction opposite the first direction. 
     The safety solenoid valve  3  is normally energised and is de-energised whenever the hands of machine operator are detected close to an area within the feed chute  10  of the chipping machine  9 . When valve  3  is de-energised, it returns to the position shown in FIG.  3  and motors  6  no longer rotate and the feed rollers  11  cease their rotation. 
     FIG. 4 shows further details of the safety system of the invention. Alarm circuit  40  has the sensing coil  32  coupled to it. Coil  32  is positioned extending in a plane along the wall of the feed chute  10 . The passive sensor coils worn by the machine operator function to modify the electromagnetic field produced by coil  32  whenever the passive sensor coils are close enough to the coil  32 . This modification of the field of coil  32  is sensed by circuit  40  which provides an output as a consequence of this sensed proximity. That output is used to drive a switching circuit consisting of transistors Q 1 , Q 2  and Q 3  and a relay having a coil R 1  and a contact C 1 . With relay R 1  de-energised, contact C 1  is in the position shown. 
     An emergency stop switch SW 1  is present in series with contact C 1  and switch SW 1  is normally closed. 
     A second relay has a coil R 1  and two contacts C 2  and C 3 . Contacts C 2  and C 3  are in the positions shown when relay coil R 2  is energised. Coil R 3  is the coil of solenoid valve  3  in FIG.  3 . With FIG. 4 in the state shown coil R 3  is energised and the valve in its left hand position. 
     When coil  32  detects the proximity of a passive sensor coil, R 1  is de-energised and contact C 1  moves to its normally closed position. This de-energises coil R 2 , and contacts C 2  and C 3  move to normally closed positions and coil R 3  is de-energised. 
     A start/reset switch SW 2  is in series with emergency switch SW 1 . Once the passive sensors move away from coil  32 , coil R 1  is energised and contact C 1  returns to the position shown in FIG.  4 . Coil R 2  is de-energised and contacts C 2  and C 3  are in the opposite (NC) positions to that shown in the figure and coil R 3  is de-energised and the motors  6  (in FIG. 3) which drive the feed rollers  11  are not operating. 
     The start switch SW 2  is momentarily moved to the position opposite to that shown in FIG. 4, and thus allows coil R 2  to be energised to move contacts C 2  and C 3  into the positions shown. The coil R 3  is then energised and switch SW 2  is released and returns to the position shown. Coil R 2  is held in and coil R 3  remains energised until coil  32  once again detects the proximity of a passive sensor. 
     FIG. 5 shows a detailed circuit block diagram of a control circuit for the system of the invention. The circuit has four inputs, ANT 1 , ANT 2 , ANT 3  and ANT 4  and only one of these is used. Coil  32  (FIG. 4) has one of its ends connected to input ANT 1  and its other end is connected to ground. The circuit of FIG. 5 energises the coil  32  and also monitors that coil to detect field changes in that coil produced by the proximity of a passive sensor. An output at line  50  may be used to operate an alarm and is the output supplied to transistor Q 1  in FIG.  4 . 
     The control circuit illustrated in FIG. 5 also includes a microprocessor  51 , power supply  52 , sync comparators  53 , function switch  54 , system control  55 , alarm buzzer  56 , digitizer  57 , receiver  58 , receive switches  59  to  62 , transmitter oscillator  63 , phase splitter  64 , phase switch  65 , first transmitter  66 , second transmitter  67 , output switch  68 , and transmitter switches  69  to  72 . 
     The use of passive sensors like that shown in FIG. 1 means that no power supply need be carried by the machine operator. As the passive sensors do not require power, their function is not dependent on the presence of a power supply to them and thus give rise to operational advantages. 
     The invention has been described by way of example with reference to its use with a wood chipping machine  9 . In that example application solenoid valve  3  in FIG. 3 is necessary to ensure that drive to the feed rollers  11  is discontinued. 
     If the system of the invention were used with equipment other than the wood chipping machine  9 , a coil like coil  32 , passive sensors and other components would still be necessary but a solenoid valve  3  would not. The system of the invention would be used to control some other device to achieve activation and deactivation of the equipment being controlled.